Iodometry
Encyclopedia
Iodometry, also known as iodometric titration, is a method of volumetric chemical analysis, a redox titration
where the appearance or disappearance of elementary iodine
indicates the end point.
Free iodine is titrated against standard reducing agent such as sodium thiosulfate
solution. Usual reagents are sodium thiosulfate
as titrant, starch
as an indicator
(it forms blue complex with iodine molecules - though polyvinyl alcohol
has started to be used recently as well), and an iodine compound (iodide
or iodate
, depending on the desired reaction with the sample).The color change at the end point is usually blue to colorless.
The principal reaction is the reduction of iodine to iodide
by thiosulfate
:
A common and illustrative use of iodometry is the measurement of concentration of chlorine
in water.
Chlorine in pH
under 8 oxidizes iodide
to iodine. An overabundance of potassium iodide is added to the known amount of sample in acidic environment (pH < 4, the reaction is not complete in more alkaline pH). Starch is added, forming a blue clathrate
complex
with the liberated iodine. The blue solution is then titrated with thiosulfate until the blue color vanishes.
Two possible sources of error can influence the outcome of the iodometric titration. One is the air oxidation of acid-iodide solution and the other is the volatility of I2. The first one can be eliminated by adding an excess of sodium carbonate
in the reaction vessel. This displaces the oxygen
in the vessel by forming carbon dioxide
gas, which blows the air away, and being heavier than air, protects the mixture by remaining on top. The other error can be reduced by using an excess of iodide solution which captures liberated iodine
to form triiodide
ions, I3−.
Iodometry involves indirect titration of free iodine whereas Iodimetry involves direct titration of liberated iodine.
Redox titration
Redox titration is a type of titration based on a redox reaction between the analyte and titrant.Redox titration may involve the use of a redox indicator and/or a potentiometer.-Example:...
where the appearance or disappearance of elementary iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
indicates the end point.
Free iodine is titrated against standard reducing agent such as sodium thiosulfate
Sodium thiosulfate
Sodium thiosulfate , also spelled sodium thiosulphate, is a colorless crystalline compound that is more familiar as the pentahydrate, Na2S2O3•5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or “hypo.”...
solution. Usual reagents are sodium thiosulfate
Sodium thiosulfate
Sodium thiosulfate , also spelled sodium thiosulphate, is a colorless crystalline compound that is more familiar as the pentahydrate, Na2S2O3•5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or “hypo.”...
as titrant, starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...
as an indicator
Starch indicator
Starch is often used in chemistry as an indicator for redox titrations where triiodide is present. Starch forms a very dark blue-black complex with triiodide which can be made by mixing iodine with iodide . However, the complex is not formed if only iodine or only iodide is present...
(it forms blue complex with iodine molecules - though polyvinyl alcohol
Polyvinyl alcohol
Polyvinyl alcohol is a water-soluble synthetic polymer .-Properties:...
has started to be used recently as well), and an iodine compound (iodide
Iodide
An 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,...
or iodate
Iodate
An iodate is a conjugate base of iodic acid. In the iodate anion, iodine is bonded to three oxygen atoms and the molecular formula is IO3−. The molecular geometry of iodate is trigonal pyramidal....
, depending on the desired reaction with the sample).The color change at the end point is usually blue to colorless.
The principal reaction is the reduction of iodine to iodide
Iodide
An 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,...
by thiosulfate
Thiosulfate
Thiosulfate is an oxyanion of sulfur. The prefix thio indicates that thiosulfate ion is a sulfate ion with one oxygen replaced by a sulfur. Thiosulfate occurs naturally and is produced by certain biochemical processes...
:
- I2 + 2 S2O32− → S4O62-TetrathionateThe tetrathionate anion, S4O62−, is a sulfur oxoanion derived from the compound tetrathionic acid, H2S4O6. Two of the sulfur atoms present in the ion are in oxidation state 0 and two are in oxidation state +5. Alternatively, the compound can be viewed as the adduct resulting from the binding of...
+ 2 I-
A common and illustrative use of iodometry is the measurement of concentration of chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
in water.
Chlorine in pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
under 8 oxidizes iodide
Iodide
An 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,...
to iodine. An overabundance of potassium iodide is added to the known amount of sample in acidic environment (pH < 4, the reaction is not complete in more alkaline pH). Starch is added, forming a blue clathrate
Clathrate compound
A clathrate, clathrate compound or cage compound is a chemical substance consisting of a lattice of one type of molecule trapping and containing a second type of molecule...
complex
Complex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
with the liberated iodine. The blue solution is then titrated with thiosulfate until the blue color vanishes.
Two possible sources of error can influence the outcome of the iodometric titration. One is the air oxidation of acid-iodide solution and the other is the volatility of I2. The first one can be eliminated by adding an excess of sodium carbonate
Sodium carbonate
Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...
in the reaction vessel. This displaces the oxygen
Oxygen
Oxygen 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...
in the vessel by forming carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
gas, which blows the air away, and being heavier than air, protects the mixture by remaining on top. The other error can be reduced by using an excess of iodide solution which captures liberated iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
to form triiodide
Triiodide
In chemistry, triiodide can have several meanings. Triiodide primarily refers to the triiodide ion, I3−, a polyatomic anion composed of three iodine atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium triiodide, ...
ions, I3−.
Iodometry involves indirect titration of free iodine whereas Iodimetry involves direct titration of liberated iodine.