Protactinium is a
chemical elementA chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
with the symbol
Pa and
atomic numberIn chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...
91. It is a dense, silvery-gray metal which readily reacts with
oxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, water vapor and inorganic
acidAn acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
s. It forms various
chemical compoundA chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s where protactinium is usually present in the
oxidation stateIn chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
+5, but can also assume +4 and even +2 or +3 states. The average concentrations of protactinium in the Earth's crust is typically on the order of a few parts per trillion, but may reach up to a few parts per million in some
uraniniteUraninite is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2, but also contains UO3 and oxides of lead, thorium, and rare earth elements...
ore deposits. Because of the scarcity, high radioactivity and high toxicity, there are currently no uses for protactinium outside of scientific research, and for this purpose, protactinium is mostly extracted from spent
nuclear fuelNuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
.
Protactinium was first identified in 1913 by Kasimir Fajans and Oswald Helmuth Göhring and named
brevium because of the short
half-lifeHalf-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of the specific
isotopeIsotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
studied, namely protactinium-234. A more stable isotope of protactinium was discovered in 1918, and therefore the name was changed to protoactinium and then to protactinium in 1949. The new name meant "before
actiniumActinium is a radioactive chemical element with the symbol Ac and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902...
" and reflected the fact that
actiniumActinium is a radioactive chemical element with the symbol Ac and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902...
is a product of radioactive decay of protactinium.
The longest-lived and most abundant (nearly 100%) naturally occurring
isotopeIsotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
of protactinium, protactinium-231, has a
half-lifeHalf-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 32,760 years and is a decay product of
uranium-235- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
. Much smaller trace amounts of the short-lived
nuclear isomerA nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons . "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the other possible excited nuclear states...
protactinium-234m occur in the decay chain of
uranium-238Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
. Protactinium-233 results from the decay of
thoriumThorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
-233 as part of the chain of events used to produce
uranium-233Uranium-233 is a fissile isotope of uranium, bred from Thorium as part of the thorium fuel cycle. It has been used in a few nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 160,000 years....
by neutron irradiation of thorium-232. It is an undesired intermediate product in thorium-based
nuclear reactorA nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
s and is therefore removed from the active zone of the reactor during the breeding process. Analysis of the relative concentrations of various uranium, thorium and protactinium isotopes in water and minerals is used in
radiometric datingRadiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
of
sedimentSediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s which are up to 175,000 years old and in modeling of various geological processes.
History
In 1871,
Dmitri MendeleevDmitri Ivanovich Mendeleev , was a Russian chemist and inventor. He is credited as being the creator of the first version of the periodic table of elements...
predictedProfessor Dmitri Mendeleev published the first Periodic Table of the Atomic Elements in 1869 based on properties which appeared with some regularity as he laid out the elements from lightest to heaviest....
the existence of an element between
thoriumThorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
and
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
. The actinide element group was unknown at the time. Therefore, uranium was positioned below
tungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
, and thorium below zirconium, leaving the space below tantalum empty, and until the 1950s
periodic tableThe periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...
s were published with this structure. For a long time chemists searched for eka-tantalum as an element with similar chemical properties as tantalum, making a discovery of protactinium nearly impossible.
In 1900,
William CrookesSir William Crookes, OM, FRS was a British chemist and physicist who attended the Royal College of Chemistry, London, and worked on spectroscopy...
isolated protactinium as an intensely radioactive material from uranium; however, he could not characterize it as a new chemical element and thus named it uranium-X. Crookes dissolved uranium nitrate in
etherEthers are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'. A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether"...
, the residual aqueous phase contains most of the and . His method was still used in the 1950s to isolate and from uranium compounds. Protactinium was first identified in 1913, when Kasimir Fajans and Oswald Helmuth Göhring encountered the isotope
234Pa during their studies of the decay chains of
uranium-238Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
: → → → . They named the new element
brevium (from the Latin word,
brevis, meaning brief or short) because of its short half-life, 6.7 hours for . In 1918, two groups of scientists, led by
Otto HahnOtto Hahn FRS was a German chemist and Nobel laureate, a pioneer in the fields of radioactivity and radiochemistry. He is regarded as "the father of nuclear chemistry". Hahn was a courageous opposer of Jewish persecution by the Nazis and after World War II he became a passionate campaigner...
and
Lise MeitnerLise Meitner FRS was an Austrian-born, later Swedish, physicist who worked on radioactivity and nuclear physics. Meitner was part of the team that discovered nuclear fission, an achievement for which her colleague Otto Hahn was awarded the Nobel Prize...
of
GermanyGermany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
and
Frederick SoddyFrederick Soddy was an English radiochemist who explained, with Ernest Rutherford, that radioactivity is due to the transmutation of elements, now known to involve nuclear reactions. He also proved the existence of isotopes of certain radioactive elements...
and John Cranston of
Great BritainGreat Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
, independently discovered another isotope of protactinium,
231Pa having much longer half-life of about 32,000 years. Thus the name
brevium was changed to
protoactinium as the new element was part of the decay chain of uranium-235 before the actinium (from =
protos meaning
first,
before). For ease of pronunciation, the name was shortened to
protactinium by the
IUPACThe International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science . The international headquarters of IUPAC is located in Zürich,...
in 1949. The discovery of protactinium completed the last gap in the early versions of the periodic table, proposed by Mendeleev in 1869, and it brought to fame the involved scientists, particularly Meitner.
Aristid von GrosseAristid von Grosse was a German nuclear chemist. During his work with Otto Hahn, he got access to waste material from radium production, and with this starting material he was able in 1927 to isolate protactinium oxide and was later able to produce metallic protactinium by decomposition of...
produced 2 milligrams of Pa
2O
5 in 1927, and in 1934 first isolated elemental protactinium from 0.1 milligrams of Pa
2O
5. He used two different procedures: in the first one, protactinium oxide was irradiated by 35 keV electrons in vacuum. In another method, called the
van Arkel–de Boer processThe crystal bar process was developed by Anton Eduard van Arkel and Jan Hendrik de Boer in 1925. This process was the first industrial process for the commercial production of pure ductile metallic zirconium. It is used in the production of small quantities of ultra-pure titanium and zirconium...
, the oxide was chemically converted to a
halideA halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
(
chlorideThe chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
,
bromideA bromide is a chemical compound containing bromide ion, that is bromine atom with effective charge of −1. The class name can include ionic compounds such as caesium bromide or covalent compounds such as sulfur dibromide.-Natural occurrence:...
or
iodideAn iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
) and then reduced in a vacuum with an electrically heated metallic filament:
- 2 PaI5 → 2 Pa + 5 I2
In 1961, the British Atomic Energy Authority (UKAEA) produced 125 grams of 99.9% pure protactinium by processing 60 tonnes of waste material in a 12-stage process, at a cost of about 500,000 USD. For many years, this was the world's only significant supply of protactinium, which was provided to various laboratories for scientific studies.
Oak Ridge National LaboratoryOak Ridge National Laboratory is a multiprogram science and technology national laboratory managed for the United States Department of Energy by UT-Battelle. ORNL is the DOE's largest science and energy laboratory. ORNL is located in Oak Ridge, Tennessee, near Knoxville...
in the US is currently providing protactinium at a cost of about 280 USD/gram.
Occurrence
Protactinium is one of the rarest and most expensive naturally occurring elements. It is found in the form of two isotopes –
231Pa and
234Pa, with the isotope
234Pa occurring in two different energy states. Nearly all natural protactinium is protactinium-231. It is an
alpha emitterAlpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
and is formed by the decay of uranium-235, whereas the
beta radiatingBeta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...
protactinium-234 is produced as a result of uranium-238 decay. Nearly all uranium-238 (99.8%) decays first to the
234mPa isomer.
Protactinium occurs in
uraniniteUraninite is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2, but also contains UO3 and oxides of lead, thorium, and rare earth elements...
(pitchblende) at concentrations of about 0.3-3
partsIn science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement...
231Pa per million parts (ppm) of ore. Whereas the usual content is closer to 0.3 ppm (e.g. in
JáchymovFor other places called Joachimsthal, see Joachimsthal Jáchymov . compl: "Sant Joachim's Sthal" is a spa town in north-west Bohemia in the Czech Republic belonging to the Karlovy Vary Region. It is situated at an altitude of 733 m above sea level in the eponymous St...
,
Czech RepublicThe Czech Republic is a landlocked country in Central Europe. The country is bordered by Poland to the northeast, Slovakia to the east, Austria to the south, and Germany to the west and northwest....
), some ores from the
Democratic Republic of the CongoThe Democratic Republic of the Congo is a state located in Central Africa. It is the second largest country in Africa by area and the eleventh largest in the world...
have about 3 ppm. Protactinium is homogeneously dispersed in most natural materials and in water, but at much lower concentrations on the order of one part per trillion, that corresponds to the radioactivity of 0.1 picocuries (pCi)/g. There is about 500 times more protactinium in sandy soil particles than in water, even the water present in the same sample of soil. Much higher ratios of 2,000 and above are measured in
loamLoam is soil composed of sand, silt, and clay in relatively even concentration . Loam soils generally contain more nutrients and humus than sandy soils, have better infiltration and drainage than silty soils, and are easier to till than clay soils...
soils and clays, such as
bentoniteBentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium , sodium , calcium , and aluminum . Experts debate a number of nomenclatorial...
.
In nuclear reactors
Two major protactinium isotopes,
231Pa and
233Pa are produced from thorium in nuclear reactors; both are undesirable and are usually removed, thereby adding complexity to the reactor design and operation. In particular,
232Th via (
n,2
n) reactions produces
231Th which quickly (half-life 25.5 hours) decays to
231Pa. The last isotope, while not a transuranic waste, has a long half-life of 32,760 years and is a major contributor to the long term radiotoxicity of spent nuclear fuel.
Protactinium-233 is formed upon neutron capture by
232Th. It further either decays to uranium-233 or captures another neutron and converts into the non-fissile uranium-234.
233Pa has a relatively long half-life of 27 days and high
cross sectionA cross section is the effective area which governs the probability of some scattering or absorption event. Together with particle density and path length, it can be used to predict the total scattering probability via the Beer-Lambert law....
for neutron capture (the so-called "neutron poison"). Thus instead of rapidly decaying to the useful
233U, a significant fraction of
233Pa converts to non-fissile isotopes and consumes neutrons, degrading
the reactor efficiencyNeutron economy is defined as the ratio of an adjoint weighted average of the excess neutron production divided by an adjoint weighted average of the fission production....
. To avoid this,
233Pa is extracted from the active zone of thorium
molten salt reactorA molten salt reactor is a type of nuclear fission reactor in which the primary coolant, or even the fuel itself is a molten salt mixture...
s, during their operation, so that it only decays to
233U. This is achieved using several meters tall columns of molten
bismuthBismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...
with lithium dissolved in it. In a simplified scenario, lithium selectively reduces protactinium salts to protactinium metal which is then extracted from the molten-salt cycle, and bismuth is merely a carrier. It is chosen because of its low
melting pointThe melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
(271 °C), low vapor pressure, good solubility for lithium and actinides, and immiscibility with molten
halideA halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
s.
Preparation
Before the advent of nuclear reactors, protactinium was separated for scientific experiments from uranium ores. Nowadays, it is mostly produced as an intermediate product of
nuclear fissionIn nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
in thorium high-temperature reactors:
- The times are half-lives.
Protactinium metal can be prepared by reduction of its
fluorideFluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
with
calcium fluorideCalcium fluoride is the inorganic compound with the formula CaF2. This ionic compound of calcium and fluorine occurs naturally as the mineral fluorite . It is the source of most of the world's fluorine. This insoluble solid adopts a cubic structure wherein calcium is coordinated to eight fluoride...
,
lithiumLithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
or
bariumBarium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
at a temperature of 1300–1400 °C.
Physical and chemical properties
Protactinium is an
actinideThe actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...
which is positioned in the
periodic tableThe periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...
to the left of
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
and to the right of
thoriumThorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
, and many of its physical properties are intermediate between those two actinides. So, protactinium is more dense and rigid than thorium but is lighter than uranium, and its melting point is lower than that of thorium and higher than that of uranium. The thermal expansion, electrical and thermal conductivities of these three elements are comparable and are typical of "poor metals". The estimated
shear modulus of protactinium is similar to that of
titaniumTitanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
. Protactinium is a metal with silvery-gray luster that is preserved for some time in air. Protactinium easily reacts with oxygen, water vapor and acids, but not with alkali metals.
At room temperature, protactinium crystallizes in body-centered
tetragonalIn crystallography, the tetragonal crystal system is one of the 7 lattice point groups. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base and height .There are two tetragonal Bravais...
structure which can be regarded as distorted body-centered cubic lattice; this structure does not change upon compression up to 53 GPa. The structure changes to face-centered cubic (
fcc) upon cooling from high temperature, at about 1200 °C. The thermal expansion coefficient of the tetragonal phase between room temperature and 700 °C is 9.9/°C.
Protactinium is
paramagneticParamagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
and no magnetic transitions are known for it at any temperature. It becomes
superconductiveSuperconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
at temperatures below 1.4 K. Protactinium tetrachloride is paramagnetic at room temperature but turns
ferromagneticFerromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
upon cooling to 182 K.
Protactinium exists in two major
oxidation stateIn chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
s, +4 and +5, both in solids and solutions, and the +3 and +2 states were observed in some solid phases. As the electron configuration of the neutral atom is 7s
26d
15f
2, the +5 oxidation state corresponds to the low-energy (and thus favored) 5f
0 configuration. Both +4 and +5 states easily form
hydroxideHydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
s in water with the predominant ions being Pa(OH)
3+, Pa(OH)
22+, Pa(OH)
3+ and Pa(OH)
4, all colorless. Other known protactinium ions include PaCl
22+, PaSO
42+, PaF
3+, PaF
22+, PaF
6–, PaF
72– and PaF
83–.
Chemical compounds
| Formula |
color |
symmetry |
space group In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
|
No |
Pearson symbol The Pearson symbol, or Pearson notation, is used in crystallography as a means of describing a crystal structure, and was originated by W.B. Pearson. The symbol is made up of two letters followed by a number. For example:* Diamond structure, cF8...
|
a (pm) |
b (pm) |
c (pm) |
Z |
density, g/cm3 |
| Pa |
silvery-gray |
tetragonal In crystallography, the tetragonal crystal system is one of the 7 lattice point groups. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base and height .There are two tetragonal Bravais...
|
I4/mmm |
139 |
tI2 |
392.5 |
392.5 |
323.8 |
2 |
15.37 |
| PaO |
|
rocksalt |
Fmm |
225 |
cF8 |
496.1 |
|
|
4 |
13.44 |
| PaO2 Protactinium oxide is a chemical compound with the formula PaO2. The black oxide is formed by reducing Pa2O5 with hydrogen....
|
black |
fcc |
Fmm |
225 |
cF12 |
550.5 |
|
|
4 |
10.47 |
| Pa2O5 Protactinium oxide is a chemical compound with the formula Pa2O5. When it is reduced with hydrogen, it forms PaO2....
|
white |
|
Fmm |
225 |
cF16 |
547.6 |
547.6 |
547.6 |
4 |
10.96 |
| Pa2O5 |
white |
|
orthorhombic |
|
|
692 |
402 |
418 |
|
|
| PaH3 |
black |
cubic |
Pmn |
223 |
cP32 |
664.8 |
664.8 |
664.8 |
8 |
10.58 |
| PaF4 |
brown-red |
monoclinic |
C2/c |
15 |
mS60 |
|
|
|
2 |
|
| PaCl4 |
green-yellow |
tetragonal In crystallography, the tetragonal crystal system is one of the 7 lattice point groups. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base and height .There are two tetragonal Bravais...
|
I41/amd |
141 |
tI20 |
837.7 |
837.7 |
748.1 |
4 |
4.72 |
| PaBr4 |
brown |
tetragonal |
I41/amd |
141 |
tI20 |
882.4 |
882.4 |
795.7 |
|
|
| PaCl5 |
yellow |
monoclinic In crystallography, the monoclinic crystal system is one of the 7 lattice point groups. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal length, as in the orthorhombic system. They form a rectangular prism with a...
|
C2/c |
15 |
mS24 |
797 |
1135 |
836 |
4 |
3.74 |
| PaBr5 |
red |
monoclinic |
P21/c |
14 |
mP24 |
838.5 |
1120.5 |
1214.6 |
4 |
4.98 |
| PaOBr3 |
|
monoclinic |
C2 |
|
|
1691.1 |
387.1 |
933.4 |
|
|
| Pa(PO3)4 |
|
orthorhombic |
|
|
|
696.9 |
895.9 |
1500.9 |
|
|
| Pa2P2O7 |
|
cubic |
Pa3 |
|
|
865 |
865 |
865 |
|
|
| Pa(C8H8)2 |
golden-yellow |
monoclinic |
|
|
|
709 |
875 |
1062 |
|
|
Here
a,
b and
c are lattice constants in picometers, No is space group number and
Z is the number of
formula unitA formula unit in chemistry is the empirical formula of an ionic or covalent network solid compound used as an independent entity for stoichiometric calculations. It is the lowest whole number ratio of ions represented in an ionic compound...
s per unit cell;
fcc stands for the
face-centered cubicIn crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
symmetry. Density was not measured directly but calculated from the lattice parameters.
Oxides and oxygen-containing salts
Protactinium oxides are known for the metal oxidation states +2, +4 and +5. The most stable is white pentoxide Pa
2O
5, which can be produced by igniting protactinium(V) hydroxide in air at a temperature of 500 °C. Its crystal structure is cubic, and the chemical composition is often non-stoichiometric, described as PaO
2.25. Another phase of this oxide with orthorhombic symmetry has also been reported. The black dioxide PaO
2 is obtained from the pentoxide by reducing it at 1550 °C with hydrogen. It is not readily soluble in either dilute or concentrated
nitricNitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
,
hydrochloricHydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
or
sulfuric acidSulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
s, but easily dissolves in
hydrofluoric acidHydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
. The dioxide can be converted back to pentoxide by heating in oxygen-containing atmosphere to 1100 °C. The monoxide PaO has only been observed as a thin coating on protactinium metal, but not in an isolated bulk form.
Protactinium forms mixed binary oxides with various metals. With alkali metals
A, the crystals have a chemical formula APaO
3 and perovskite structure, or A
3PaO
4 and distorted rock-salt structure, or A
7PaO
6 where oxygen atoms for a hexagonal close-packed lattice. In all these materials, protactinium ions are octahedrally coordinated. The pentoxide Pa
2O
5 combines with rare-earth metal oxides R
2O
3 to form various nonstoichiometric mixed-oxides, also of perovskite structure.
Protactinium oxides are
basicA basic oxide is an oxide that shows basic properties in opposition to acidic oxides and that either*reacts with water to form a base; or*reacts with an acid to form a salt.Examples include:*Sodium oxide which reacts with water to produce sodium hydroxide...
; they easily convert to hydroxides and can form various salts, such as
sulfateIn inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
s,
phosphateA phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
s,
nitrateThe nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
s, etc. The nitrate is usually white but can be brown due to
radiolyticRadiolysis is the dissociation of molecules by nuclear radiation. It is the cleavage of one or several chemical bonds resulting from exposure to high-energy flux...
decomposition. Heating the nitrate in air at 400 °C converts it to the white protactinium pentoxide. The polytrioxophosphate Pa(PO
3)
4 can be produced by reacting difluoride sulfate PaF
2SO4 with phosphoric acidPhosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...
(H3PO4) under inert gas atmosphere. Heating the product to about 900 °C eliminates the reaction by-products such as hydrofluoric acidHydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
, sulfur trioxideSulfur trioxide is the chemical compound with the formula SO3. In the gaseous form, this species is a significant pollutant, being the primary agent in acid rain. It is prepared on massive scales as a precursor to sulfuric acid.-Structure and bonding:Gaseous SO3 is a trigonal planar molecule of...
and phosphoric anhydride. Heating to higher temperatures in an inert atmosphere decomposes Pa(PO3)4 into the diphosphate PaP2O7, which is analogous to diphosphates of other actinides. In the diphosphate, the PO3 groups form pyramids of C2v symmetry. Heating PaP2O7 in air to 1400 °C decomposes it into the pentoxides of phosphorus and protactinium.
Halides
Protactinium(V) fluoride forms white crystals where protactinium ions are arranged in pentagonal bipyramids and coordinatedIn chemistry and crystallography, the coordination number of a central atom in a molecule or crystal is the number of its nearest neighbours. This number is determined somewhat differently for molecules and for crystals....
by 7 other ions. The coordination is the same in protactinium(V) chloride, but the color is yellow. The coordination changes to octahedral in the brown protactinium(V) bromide and is unknown for protactinium(V) iodide. The protactinium coordination in all its tetrahalides is 8, but the arrangement is square antiprismatic in protactinium(IV) fluoride and dodecahedral in the chloride and bromide. Brown-colored protactinium(III) iodide has been reported where protactinium ions are 8-coordinated in a bicapped trigonal prismatic arrangement.
Protactinium(V) fluoride and protactinium(V) chloride have a polymeric structure of monoclinic symmetry. There, within one polymeric chain, all the halide atoms lie in one graphite-like plane and form planar pentagons around the protactinium ions. The coordination 7 of protactinium originates from the 5 halide atoms and two bonds to protactinium atoms belonging to the nearby chains. These compounds easily hydrolyze in water. The pentachloride melts at 300 °C and sublimates at even lower temperatures.
Protactinium(V) fluoride can be prepared by reacting protactinium oxide with either bromine pentafluorideBromine pentafluoride, BrF5, is an interhalogen compound and a fluoride of bromine. It is a strong fluorination reagent.It melts at −61.30 °C and boils at 40.25 °C. BrF5 finds use in oxygen isotope analysis. Laser ablation of solid silicates in the presence of bromine pentafluoride releases O2 for...
or bromine trifluorideBromine trifluoride is an interhalogen compound with the formula BrF3. This toxic, colourless, and corrosive liquid is soluble in sulfuric acid but explodes on contact with water and organic compounds. It is a powerful fluorinating agent and an ionizing inorganic solvent...
at about 600 °C, and protactinium(IV) fluoride is obtained from the oxide and a mixture of hydrogen and hydrogen fluorideHydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
at 600 °C; a large excess of hydrogen is required to remove atmospheric oxygen leaks into the reaction.
Protactinium(V) chloride is prepared by reacting protactinium oxide with carbon tetrachlorideCarbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...
at temperature of 200–300 °C. The by-products (such as PaOCl3) are removed by fractional sublimation. Reduction of protactinium(V) chloride with hydrogen at about 800 °C yields protactinium(IV) chloride – a yellow-green solid which sublimes in vacuum at 400 °C; it can also be obtained directly from protactinium dioxide by treating it with carbon tetrachloride at 400 °C.
Protactinium bromides are produced by the action of aluminum bromide, hydrogen bromideHydrogen bromide is the diatomic molecule HBr. HBr is a gas at standard conditions. Hydrobromic acid forms upon dissolving HBr in water. Conversely, HBr can be liberated from hydrobromic acid solutions with the addition of a dehydration agent, but not by distillation. Hydrogen bromide and...
, carbon tetrabromide or a mixture of hydrogen bromide and thionyl bromideThionyl bromide is the chemical compound SOBr2. It is less stable and less widely used than its chloride analogue, thionyl chloride. It is prepared by the action of hydrogen bromide on thionyl chloride, a characteristic reaction where a stronger acid is converted to a weaker acid:Thionyl bromide is...
on protactinium oxide. An alternative reaction is between protactinium pentachloride and hydrogen bromide or thionyl bromide. Protactinium(V) bromide has two similar monoclinic forms, one is obtained by sublimation at 400–410 °C and another by sublimation at slightly lower temperature of 390–400 °C.
Protactinium iodides result from the oxides and aluminum iodide or ammonium iodideAmmonium iodide is the chemical compound NH4I. It is used in photographic chemicals and some medications. It can be prepared by the action of hydroiodic acid on ammonia. It is easily soluble in water, from which it crystallizes in cubes. It is also soluble in ethanol...
heated to 600 °C. Protactinium(III) iodide was obtained by heating protactinium(V) iodide in vacuum. As with oxides, protactinium forms mixed halides with alkali metals. Among those, most remarkable is Na3PaF8 where protactinium ion is symmetrically surrounded by 8 F– ions which form a nearly perfect cube.
More complex protactinium fluorides are also known such as Pa2F9 and ternary fluorides of the types MPaF6 (M = Li, Na, K, Rb, Cs or NH4), M2PaF7 (M = K, Rb, Cs or NH4) and M3PaF8 (M = Li, Na, Rb, Cs), all being white crystalline solids. The MPaF6 formula can be represented as a combination of MF and PaF5. These compounds can be obtained by evaporating a hydrofluoric acid solution containing these both complexes. For the small alkali cations like Na, the crystal structure is tetragonal, whereas it lowers to orthorphombic for larger cations K+, Rb+, Cs+ or NH4+. A similar variation was observed for the M2PaF7 fluorides, namely the crystal symmetry was dependent on the cation and differed for Cs2PaF7 and M2PaF7 (M = K, Rb or NH4).
Other inorganic compounds
Oxyhalides and oxysulfides of protactinium are known. PaOBr3 has a monoclinic structure composed of double-chain units where protactinium has coordination 7 and is arranged into pentagonal bipyramids. The chains are interconnected through oxygen and bromine atoms, and each oxygen atom is related to three Pa atoms. PaOS is a light-yellow non-volatile solid with a cubic crystal lattice isostructural to that of other actinide oxysulfides. It is obtained by reacting protactinium(V) chloride with a mixture of hydrogen sulfideHydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
and carbon disulfideCarbon disulfide is a colorless volatile liquid with the formula CS2. The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical non-polar solvent...
at 900 °C.
In hydrides and nitrides, protactinium has a low oxidation state of about +3. The hydride is obtained by direct action of hydrogen on the metal at 250 °C, and the nitride is a product of ammonia and protactinium tetrachloride or pentachloride. This bright yellow solid is stable to heating to 800 °C in vacuum. Protactinium carbide PaC is formed by reduction of protactinium tetrafluoride with barium in a carbon crucible at a temperature of about 1400 °C. Protactinium forms borohydrides which include Pa(BH4)4. It has an unusual polymeric structure with helical chains where the protactinium atom has coordination number of 12 and is surrounded by six BH4– ions.
Organometallic compounds
Protactinium(IV) forms a tetrahedral complex tetrakis(cyclopentadienyl)protactinium(IV) (or Pa(C5H5)4) with four cyclopentadienylA cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups . Based on the type of bonding between the metals and the cyclopentadienyl]] moieties, cyclopentadienyl complexes are classified into the following three categories: a) π-complexes, b) σ-complexes, and c) ionic...
rings, which can be synthesized by reacting protactinium(IV) chloride with molten Be(C5H5)2. One ring can be substituted with a halide atom. Another organometallic complex is golden-yellow bis(π-cyclooctatetraene) protactinium, Pa(C8H8)2, which is analogous in structure to uranoceneUranocene U2 is the most notable cyclooctatetraenide of the f elements and one of the first organouranium compounds to be synthesized. Uranocene is a member of the actinocenes, a group of metallocenes incorporating elements from the actinide series...
. There, the metal atom is sandwiched between two cyclooctatetraene1,3,5,7-Cyclooctatetraene is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature...
ligands. Similar to uranocene, it can be prepared by reacting protactinium tetrachloride with dipotassium cyclooctatetraenide1,3,5,7-Cyclooctatetraene is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature...
, K2C8H8, in tetrahydrofuranTetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...
.
Isotopes
Twenty-nine radioisotopes of protactinium have been discovered, the most stable being 231Pa with a half-lifeHalf-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 32,760 years, 233Pa with a half-life of 27 days, and 230Pa with a half-life of 17.4 days. All of the remaining isotopes have half-lives shorter than 1.6 days, and the majority of these have half-lives less than 1.8 seconds. Protactinium also has two nuclear isomerA nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons . "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the other possible excited nuclear states...
s, 217mPa (half-life 1.2 milliseconds) and 234mPa (half-life 1.17 minutes).
The primary decay mode for isotopes of protactinium lighter than (and including) the most stable isotope 231Pa (i.e., 212Pa to 231Pa) is alpha decayAlpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms into an atom with a mass number 4 less and atomic number 2 less...
and the primary mode for the heavier isotopes (i.e., 232Pa to 240Pa) is beta decayIn nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
. The primary decay productIn nuclear physics, a decay product is the remaining nuclide left over from radioactive decay. Radioactive decay often involves a sequence of steps...
s of isotopes of protactinium lighter than (and including) 231Pa are actiniumActinium is a radioactive chemical element with the symbol Ac and atomic number 89, which was discovered in 1899. It was the first non-primordial radioactive element to be isolated. Polonium, radium and radon were observed before actinium, but they were not isolated until 1902...
isotopes and the primary decay products for the heavier isotopes of protactinium are uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
isotopes.
Applications
Although protactinium is located in the periodic table between uranium and thorium, which both have numerous applications, owing to its scarcity, high radioactivity and high toxicity, there are currently no uses for protactinium outside of scientific research.
Protactinium-231 arises from the decay of uranium-235 formed in nuclear reactors, and by the reaction 232Th + n → 231Th + 2n and subsequent beta decayIn nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
. It may support a nuclear chain reaction, which could in principle be used to build nuclear weaponA nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
s. The physicistA physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
Walter Seifritz once estimated the associated critical massA critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...
as 750±180 kg, but this possibility (of a chain reaction) has been ruled out by other nuclear physicists since then.
With the advent of highly sensitive mass spectrometersMass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...
, an application of 231Pa as a tracer in geology and paleoceanographyPaleoceanography is the study of the history of the oceans in the geologic past with regard to circulation, chemistry, biology, geology and patterns of sedimentation.- Source of information :...
has become possible. So, the ratio of protactinium-231 to thorium-230 is used for radiometric datingRadiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
of sediments which are up to 175,000 years old and in modeling of the formation of minerals. In particular, its evaluation in oceanic sediments allowed to reconstruct the movements of North Atlantic water bodies during the last melting of Ice AgeAn ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
glacierA glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
s. Some of the protactinium-related dating variations rely on the analysis of the relative concentrations for several long-living members of the uranium decay chain – uranium, thorium and protactinium, for example. These elements have 6, 5 and 4 f-electrons in the outer shell and thus favor +6, +5 and +4 oxidation states, respectively, and show different physical and chemical properties. So, thorium and protactinium, but not uranium compounds are poorly soluble in aqueous solutions, and precipitate into sediments; the precipitation rate is faster for thorium than for protactinium. Besides, the concentration analysis for both protactinium-231 (half-life 32,750 years) and thorium-230 (half-life 75,380 years) allows to improve the accuracy compared to when only one isotope is measured; this double-isotope method is also weakly sensitive to inhomogeneities in the spatial distribution of the isotopes and to variations in their precipitation rate.
Precautions
Protactinium is both toxic and highly radioactive and thus all manipulations with it are performed in a sealed glove box. Its major isotope 231Pa has a specific activity of 0.048 CiThe curie is a unit of radioactivity, defined asThis is roughly the activity of 1 gram of the radium isotope 226Ra, a substance studied by the pioneers of radiology, Marie and Pierre Curie, for whom the unit was named. In addition to the curie, activity can be measured using an SI derived unit,...
/gram and primarily emits alpha-particles of the energy 5 MeV, which can be stopped by a thin layer of any material. However, it slowly decays, with the half-life of 32,760 years, into 227Ac, which has a specific activity of 74 Ci/gram, emits both alpha and beta radiation, and has a much shorter half-life of 22 years. 227Ac, in turn, decays into lighter isotopes with even shorter half-lives and much greater specific activities (SA), as summarized in the table below.
| Isotope |
231Pa |
227Ac |
227Th |
223Ra |
219Rn |
215Po |
211Pb |
211Bi |
207Tl |
| SA (Ci The curie is a unit of radioactivity, defined asThis is roughly the activity of 1 gram of the radium isotope 226Ra, a substance studied by the pioneers of radiology, Marie and Pierre Curie, for whom the unit was named. In addition to the curie, activity can be measured using an SI derived unit,...
/g) |
0.048 |
73 |
3.1 |
5.2 |
1.3 |
3 |
2.5 |
4.2 |
1.9 |
| Decay |
α |
α, β |
α |
α |
α |
α |
β |
α, β |
β |
| Half-life Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
|
33 ka |
22 a |
19 days |
11 days |
4 s |
1.8 ms |
36 min |
2.1 min |
4.8 min |
As protactinium is present in small amounts in most natural products and materials, it is ingested with food or water and inhaled with air. Only about 0.05% of ingested protactinium is absorbed into the blood and the remainder is excreted. From the blood, about 40% of the protactinium
deposits in the bones, about 15% goes to the liver, 2% to the kidneys, and the rest leaves the body. The biological half-life of protactinium is about 50 years in the bones, whereas in other organs the kinetics has a fast and slow component. So in the liver 70% of protactinium have a half-life of 10 days and 30% remain for 60 days. The corresponding values for kidneys are 20% (10 days) and 80% (60 days). In all these organs, protactinium promotes cancer via its radioactivity. The maximum safe dose of Pa in the human body is 0.03 µCi that corresponds to 0.5 micrograms of 231Pa. This isotope is 2.5 times more toxic than hydrocyanic acid. The maximum allowed concentrations of 231Pa in the air is 3 Bq/m3.
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