Classical
qualitative inorganic analysis is a method of
analytical chemistryAnalytical chemistry is the study of the chemical composition of natural and artificial materials. Properties studied in analytical chemistry include geometric features such as molecular morphologies and distributions of species, as well as features such as composition and species identity...
which seeks to find
elementalA 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. The term is also used to refer to a pure chemical substance composed of atoms with the same number of protons.Common examples of elements...
composition of inorganic compounds. It is mainly focused on detecting
ionAn ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge...
s in an
aqueousWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
solutionIn chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent. Gases may dissolve in liquids, for example, carbon dioxide or oxygen in water. Liquids may dissolve in other liquids. Gases can...
, so that materials in other forms may need to be brought into this state before using standard methods. The solution is then treated with various
reagentA reagent or reactant is a substance or compound consumed during a chemical reaction. Solvents and catalysts, although they are involved in the reaction, are usually not referred to as reactants....
s to test for
reactionsA chemical reaction is a process that leads to the transformation of one set of chemical substances to another. They are studied by chemists under a field of science called chemistry. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, often coming about...
characteristic of certain ions, which may cause color change, solid forming and other obviously visible changes.
Qualitative inorganic analysis is that branch/method of analytical chemistry, which seeks to establish elemental composition of inorganic compounds through various reagents.
Physical appearance of inorganic salts
| Sr No. |
Salt |
Colour |
| 1 |
MnO, MnO2, FeO, CuO, Co3O4, Ni2O3; sulphides of Ag+, Cu+, Cu2+, Fe2+, Co2+, Pb2+, Hg2+ |
Black |
| 2 |
Hydrated Cu2+ salts |
Blue |
| 3 |
HgO, HgI2, Pb3O4 |
Red |
| 4 |
Cr3+, Cr6+, Ni2+, hydrated Fe2+ salts |
Green |
| 5 |
Hydrated Mn2+ salts |
Light Pink |
| 6 |
KO2, K2Cr2O7, Sb2S3, ferricyanide Ferricyanide is the name for the anion [Fe6]3−. Its systematic name is hexacyanoferrate ion. The most common salt of this anion is potassium ferricyanide, a red crystalline material that is used as an oxidant in organic chemistry.[Fe6]3− consists of an...
s |
Orange |
| 7 |
Hydrated Co2+ salts |
Reddish Pink |
| 8 |
Chromate Chromates and dichromates are salts of chromic acid and dichromic acid, respectively. Chromate salts contain the chromate anion, CrO, and usually have an intense yellow color. Dichromate salts contain the dichromate anion, Cr2O, and usually have an intense orange color...
s, AgBr, AgI, PbI2, CdS |
Yellow |
| 9 |
CdO, Fe2O3, PbO2, CuCrO4 |
Dark brown |
Detecting cations
According to their properties, cations are usually classified into six groups. Each group has a common reagent which can be used to separate them from the solution. To obtain meaningful results, the separation must be done in the sequence specified below, as some ions of an earlier group may also react with the reagent of a later group, causing ambiguity as to which ions are present. The division and precise details of separating into groups vary slightly from one source to another; given below is one of the commonly used schemes.
1st analytical group of cations
1st analytical group of cations consists of ions that form insoluble
chlorideThe chloride ion is formed when the element chlorine picks up one electron to form an anion Cl
−...
s. As such, the group reagent to separate them is
hydrochloric acidHydrochloric acid is the solution of hydrogen chloride in water. It is a highly corrosive, strong mineral acid and has major industrial uses. It is found naturally in gastric acid....
, usually used at a
concentrationIn chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent.To...
of 1–2 M. Concentrated HCl must not be used, because it forms a soluble complex ion ([PbCl
4]
2-) with
Pb2+Lead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
. Consequently the
Pb2+Lead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
ion would go undetected.
The most important cations in 1st group are
Ag+Silver is a chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
,
Hg
{{Unreferenced|date=July 2009}}
Classical qualitative inorganic analysis is a method of analytical chemistryAnalytical chemistry is the study of the chemical composition of natural and artificial materials. Properties studied in analytical chemistry include geometric features such as molecular morphologies and distributions of species, as well as features such as composition and species identity...
which seeks to find
elementalA 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. The term is also used to refer to a pure chemical substance composed of atoms with the same number of protons.Common examples of elements...
composition of inorganic compounds. It is mainly focused on detecting
ionAn ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge...
s in an
aqueousWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
solutionIn chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent. Gases may dissolve in liquids, for example, carbon dioxide or oxygen in water. Liquids may dissolve in other liquids. Gases can...
, so that materials in other forms may need to be brought into this state before using standard methods. The solution is then treated with various
reagentA reagent or reactant is a substance or compound consumed during a chemical reaction. Solvents and catalysts, although they are involved in the reaction, are usually not referred to as reactants....
s to test for
reactionsA chemical reaction is a process that leads to the transformation of one set of chemical substances to another. They are studied by chemists under a field of science called chemistry. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, often coming about...
characteristic of certain ions, which may cause color change, solid forming and other obviously visible changes.
Qualitative inorganic analysis is that branch/method of analytical chemistry, which seeks to establish elemental composition of inorganic compounds through various reagents.
Physical appearance of inorganic salts
| Sr No. |
Salt |
Colour |
| 1 |
MnO, MnO2, FeO, CuO, Co3O4, Ni2O3; sulphides of Ag+, Cu+, Cu2+, Fe2+, Co2+, Pb2+, Hg2+ |
Black |
| 2 |
Hydrated Cu2+ salts |
Blue |
| 3 |
HgO, HgI2, Pb3O4 |
Red |
| 4 |
Cr3+, Cr6+, Ni2+, hydrated Fe2+ salts |
Green |
| 5 |
Hydrated Mn2+ salts |
Light Pink |
| 6 |
KO2, K2Cr2O7, Sb2S3, ferricyanide Ferricyanide is the name for the anion [Fe6]3−. Its systematic name is hexacyanoferrate ion. The most common salt of this anion is potassium ferricyanide, a red crystalline material that is used as an oxidant in organic chemistry.[Fe6]3− consists of an...
s |
Orange |
| 7 |
Hydrated Co2+ salts |
Reddish Pink |
| 8 |
Chromate Chromates and dichromates are salts of chromic acid and dichromic acid, respectively. Chromate salts contain the chromate anion, CrO, and usually have an intense yellow color. Dichromate salts contain the dichromate anion, Cr2O, and usually have an intense orange color...
s, AgBr, AgI, PbI2, CdS |
Yellow |
| 9 |
CdO, Fe2O3, PbO2, CuCrO4 |
Dark brown |
Detecting cations
According to their properties, cations are usually classified into six groups. Each group has a common reagent which can be used to separate them from the solution. To obtain meaningful results, the separation must be done in the sequence specified below, as some ions of an earlier group may also react with the reagent of a later group, causing ambiguity as to which ions are present. The division and precise details of separating into groups vary slightly from one source to another; given below is one of the commonly used schemes.
1st analytical group of cations
1st analytical group of cations consists of ions that form insoluble
chlorideThe chloride ion is formed when the element chlorine picks up one electron to form an anion Cl
−...
s. As such, the group reagent to separate them is
hydrochloric acidHydrochloric acid is the solution of hydrogen chloride in water. It is a highly corrosive, strong mineral acid and has major industrial uses. It is found naturally in gastric acid....
, usually used at a
concentrationIn chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent.To...
of 1–2 M. Concentrated HCl must not be used, because it forms a soluble complex ion ([PbCl
4]
2-) with
Pb2+Lead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
. Consequently the
Pb2+Lead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
ion would go undetected.
The most important cations in 1st group are
Ag+Silver is a chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
,
Hg
{{Unreferenced|date=July 2009}}
Classical qualitative inorganic analysis is a method of analytical chemistryAnalytical chemistry is the study of the chemical composition of natural and artificial materials. Properties studied in analytical chemistry include geometric features such as molecular morphologies and distributions of species, as well as features such as composition and species identity...
which seeks to find
elementalA 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. The term is also used to refer to a pure chemical substance composed of atoms with the same number of protons.Common examples of elements...
composition of inorganic compounds. It is mainly focused on detecting
ionAn ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge...
s in an
aqueousWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
solutionIn chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent. Gases may dissolve in liquids, for example, carbon dioxide or oxygen in water. Liquids may dissolve in other liquids. Gases can...
, so that materials in other forms may need to be brought into this state before using standard methods. The solution is then treated with various
reagentA reagent or reactant is a substance or compound consumed during a chemical reaction. Solvents and catalysts, although they are involved in the reaction, are usually not referred to as reactants....
s to test for
reactionsA chemical reaction is a process that leads to the transformation of one set of chemical substances to another. They are studied by chemists under a field of science called chemistry. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, often coming about...
characteristic of certain ions, which may cause color change, solid forming and other obviously visible changes.
Qualitative inorganic analysis is that branch/method of analytical chemistry, which seeks to establish elemental composition of inorganic compounds through various reagents.
Physical appearance of inorganic salts
| Sr No. |
Salt |
Colour |
| 1 |
MnO, MnO2, FeO, CuO, Co3O4, Ni2O3; sulphides of Ag+, Cu+, Cu2+, Fe2+, Co2+, Pb2+, Hg2+ |
Black |
| 2 |
Hydrated Cu2+ salts |
Blue |
| 3 |
HgO, HgI2, Pb3O4 |
Red |
| 4 |
Cr3+, Cr6+, Ni2+, hydrated Fe2+ salts |
Green |
| 5 |
Hydrated Mn2+ salts |
Light Pink |
| 6 |
KO2, K2Cr2O7, Sb2S3, ferricyanide Ferricyanide is the name for the anion [Fe6]3−. Its systematic name is hexacyanoferrate ion. The most common salt of this anion is potassium ferricyanide, a red crystalline material that is used as an oxidant in organic chemistry.[Fe6]3− consists of an...
s |
Orange |
| 7 |
Hydrated Co2+ salts |
Reddish Pink |
| 8 |
Chromate Chromates and dichromates are salts of chromic acid and dichromic acid, respectively. Chromate salts contain the chromate anion, CrO, and usually have an intense yellow color. Dichromate salts contain the dichromate anion, Cr2O, and usually have an intense orange color...
s, AgBr, AgI, PbI2, CdS |
Yellow |
| 9 |
CdO, Fe2O3, PbO2, CuCrO4 |
Dark brown |
Detecting cations
According to their properties, cations are usually classified into six groups. Each group has a common reagent which can be used to separate them from the solution. To obtain meaningful results, the separation must be done in the sequence specified below, as some ions of an earlier group may also react with the reagent of a later group, causing ambiguity as to which ions are present. The division and precise details of separating into groups vary slightly from one source to another; given below is one of the commonly used schemes.
1st analytical group of cations
1st analytical group of cations consists of ions that form insoluble
chlorideThe chloride ion is formed when the element chlorine picks up one electron to form an anion Cl
−...
s. As such, the group reagent to separate them is
hydrochloric acidHydrochloric acid is the solution of hydrogen chloride in water. It is a highly corrosive, strong mineral acid and has major industrial uses. It is found naturally in gastric acid....
, usually used at a
concentrationIn chemistry, concentration is the measure of how much of a given substance there is mixed with another substance. This can apply to any sort of chemical mixture, but most frequently the concept is limited to homogeneous solutions, where it refers to the amount of solute in the solvent.To...
of 1–2 M. Concentrated HCl must not be used, because it forms a soluble complex ion ([PbCl
4]
2-) with
Pb2+Lead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
. Consequently the
Pb2+Lead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal, also considered to be one of the heavy metals. Lead has a bluish-white color when freshly cut, but tarnishes to a dull grayish color when exposed to air...
ion would go undetected.
The most important cations in 1st group are
Ag+Silver is a chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
,
Hg{{suMercury , also called quicksilver or hydrargyrum , is a chemical element with the symbol Hg and atomic number 80...
, and Pb
2+. The chlorides of these elements cannot be distinguished from each other by their colour - they are all white solid compounds. PbCl
2 is soluble in hot water, and can therefore be differentiated easily. Ammonia is used as a reagent to distinguish between the other two. While AgCl dissolves in ammonia (due to the formation of the complex ion [Ag(NH
3)
2]
+), Hg
2Cl
2 gives a black precipitate consisting of a mixture of chloro-mercuric amide and elemental mercury. Furthermore, AgCl is reduced to silver under light, which gives samples a violet colour.
PbCl
2 is far more soluble than the chlorides of the other two ions, especially in hot water. Therefore, HCl in concentrations which completely precipitate Hg{{su|b=2|p=2+}} and Ag
+ may not be sufficient to do the same to Pb
2+. Higher concentrations of Cl
− cannot be used for the aforementioned reasons. Thus, a filtrate obtained after first group analysis of Pb
2+ contains an appreciable concentration of this cation, enough to give the test of the second group, viz. formation of an insoluble sulfide. For this reason, Pb
2+ is usually also included in the 2nd analytical group.
This group can be determined by adding the salt in water and then adding dilute hydrochloric acid. A white precipitate is formed, to which
ammonium hydroxideAmmonium hydroxide , also known as ammonia water, ammonical liquor, ammonia liquor, aqua ammonia, or aqueous ammonia, is a solution of ammonia in water...
is then added. If the precipitate is insoluble, then Pb
2+ is present; if the precipitate is soluble, then Ag
+ is present, and if the white precipitate turns black, then Hg{{su|b=2|p=2+}} is present.
Confirmation test for lead:
- Pb2+ + 2 KI → PbI2 + 2 K+
- Pb2+ + K2CrO4 → PbCrO4 + 2 K+
Confirmation test for Silver:
- Ag+ + KI → AgI + 2 K+
- Ag+ + K2CrO4 → Ag2CrO4 + 2 K+
Confirmation test for dimeric mercury ion:
- Hg{{su|b=2|p=2+}} + 2 KI → Hg2I2 + 2 K+
- Hg{{su|b=2|p=2+}} + NaOH → HgO + 2 Na + H2O
2nd analytical group of cations
The
2nd analytical group of cations consists of ions that forms acid-insoluble
sulfideA sulfide is a chemical compound containing sulfur in its lowest oxidation number of −2.- Properties :...
s. Cations in the 2nd group include: Cd
2+,
Bi3+Bismuth is a chemical element that has the symbol Bi and atomic number 83. This trivalent poor metal chemically resembles arsenic and antimony. Bismuth is heavy and brittle; it has a silvery white color with a pink tinge owing to the surface oxide. Bismuth is the most naturally diamagnetic of all...
,
Cu2+Copper is a chemical element with the symbol Cu and atomic number 29.It is a ductile metal with very high thermal and electrical conductivity. Pure copper is rather soft and malleable and a freshly-exposed surface has a pinkish or peachy color...
,
As3+Arsenic is the chemical element that has the symbol As, atomic number 33 and atomic mass 74.92. Arsenic was first documented by Albertus Magnus in 1250. Arsenic is a notoriously poisonous metalloid with many allotropic forms, including a yellow and several black and grey forms...
, As
5+,
Sb3+Antimony is a chemical element with the symbol Sb and atomic number 51. A metalloid, antimony has four allotropic forms. The stable form of antimony is a blue-white metalloid. Yellow and black antimony are unstable non-metals...
, Sb
5+, Sn
2+, Sn
4+ and Hg
2+. Pb
2+ is usually also included here in addition to the first group.
The reagent can be any substance that gives S
2− ions in such solutions; most commonly used are
H2SHydrogen sulfide is the chemical compound with the formula H
2S. This colorless, toxic and flammable gas is partially responsible for the foul odor of rotten eggs and flatulence....
(at 0.2-0.3 M),
AKTThioacetamide is an organosulfur compound with the formula C2H5NS. This white crystalline solid is soluble in water and serves as a source of hydrogen sulfide in the synthesis of organic and inorganic compounds...
(at 0.3-0.6 M). The test with the sulfide ion must be conducted in the presence of dilute HCl. Its purpose is to keep the sulfide ion concentration at a required minimum, so as to allow the precipitation of 2nd group cations alone. If dilute acid is not used, the early precipitation of 4th group cations (if present in solution) may occur, thus leading to misleading results. Acids beside HCl are rarely used. Sulfuric acid may lead to the precipitation of the 4th group cations, while nitric acid directly reacts with the sulfide ion in the reagent, forming colloidal sulfur.
The precipitates of these cations are almost indistinguishable, except for
CdSCadmium sulfide is a chemical compound with the formula CdS. Cadmium sulfide is yellow in colour and is a semiconductor. It exists in nature as two different minerals, hexagonal greenockite and cubic hawleyite. Cadmium sulfide is a direct band gap semiconductor and has many applications for...
which is yellow. All the precipitates, except for
HgSCinnabar, sometimes written cinnabarite, is a name applied to red mercury sulfide , or native vermillion, the common ore of mercury. The name comes from the Greek - "kinnabari" - used by Theophrastus, and was probably applied to several distinct substances. Other sources say the word comes from the...
, are soluble in dilute nitric acids. HgS is soluble only in
aqua regiaAqua regia or aqua regis is a highly corrosive, fuming yellow or red solution, also called nitro-hydrochloric acid. The mixture is formed by freshly mixing concentrated nitric acid and concentrated hydrochloric acid, usually in a volumetric ratio of 1:3 respectively...
, which can be used to separate it from the rest. The action of ammonia is also useful in differentiating the cations. CuS dissolves in ammonia forming an intense blue solution, while CdS dissolves forming a colourless solution. The sulfides of As
3+, As
5+, Sb
3+, Sb
5+, Sn
2+, Sn
4+ are soluble in yellow
ammonium sulfideAmmonium hydrosulfide is the chemical compound with the formula SH. It is the salt derived from the ammonium cation and the hydrosulfide anion. The salt exists as colourless, water soluble, micaceous crystals. The compound is encountered mainly as a solution, not as the solid...
, where they form
polysulfidePolysulfides are a class of chemical compounds containing chains of sulfur atoms. The two main classes of polysulfides are anions with the general formula Sn2−. These anions are the conjugate bases of the hydrogen polysulfides H2Sn...
complexes.
This group is determined by adding the salt in water and then adding dilute hydrochloric acid followed by hydrogen sulfide. Usually it is done by passing hydrogen sulfide over the test tube for detection of 1st group cations. If it forms a reddish brown or black precipitate then Bi
3+, Cu
2+, Hg
2+ or Pb
2+ is present. Otherwise, if it forms a yellow precipitate, then Cd
2+ or Sn
4+ is present; or if it forms a brown precipitate, then Sn
2+ must be present; or if a red orange precipitate is formed, then Sb
3+ is present.
To distinguish between ions in the black or reddish brown precipitate, it is first boiled in diluted HNO
3. If it is insoluble, then Hg
2+ is present. If it is soluble, then Cu
2+, Hg
2+ or Pb
2+ may be present; sulfuric acid is then added to the resulting solution. If a white precipitate forms, then Pb
2+ may be present; if no precipitate is formed, then a new solution is made by adding an excess of ammonium hydroxide in the original salt solution. A resulting blue color indicates the presence of Cu
2+, and a white precipitate indicates bismuth. Otherwise, if the precipitate is insoluble then Hg
2+ is present.
To distinguish between ions in the yellow precipitate, an excess of NaOH is added to the original salt solution to form a white precipitate. The test tube is then shaken, and if the white precipitate dissolves, then Sn
4+ is present; otherwise, Cd
2+ is present.
Confirmation test for lead:
- Pb2+ + 2 KI → PbI2 + 2 K+
- Pb2+ + K2CrO4 → PbCrO4 + 2 K+
Confirmation test for copper:
- 2 Cu2+ + K4[Fe(CN)6] + CH3COOH → Cu2[Fe(CN)6] + 4 K+
- Cu2+ + 2 NaOH → Cu(OH)2 + 2 Na+
- Cu(OH)2 → CuO + H2O (endothermic)
Confirmation test for bismuth:
- Bi3+ + 3 KI (in excess) → BiI3 + 3 K+
- BiI3 + KI → K[BiI4]
- Bi3+ + H2O (in excess) → BiO{{su|p=+}} + 2 H+
Confirmation test for mercury:
- Hg2+ + 2 KI (in excess) → HgI2 + 2 K+
- HgI2 + 2 KI → K2[HgI4] (red precipitate dissolves)
- 2 Hg2+ + SnCl2 → 2 Hg + SnCl4 (white precipitate turns gray)
3rd analytical group of cations
3rd analytical group of cations includes ions that form sulfides which are insoluble in
basicIn chemistry, a base is most commonly thought of as an aqueous substance that can accept hydrogen ions. Bases are also the oxides or hydroxides of metals. A soluble base is also often referred to as an alkali if hydroxide ions are involved. This refers to the Brønsted-Lowry theory of acids and bases...
solution. The reagents are similar to these of the 2nd group, but separation is conducted at pH of 8–9. Occasionally, a
buffer solutionA buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of acid or base is added to it. Buffer solutions are used as a means...
is used to ensure this pH.
Cations in the 3rd group are, among others:
Fe2+Iron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
, Fe
3+,
Al3+Aluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13. It is not soluble in water under normal circumstances....
, and
Cr3+Chromium is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable...
.
The group is determined by making a solution of the salt in water and adding
ammonium chlorideAmmonium chloride is, in its pure form, a clear white water-soluble crystalline salt of ammonia. The aqueous ammonium chloride solution is mildly acidic. Sal ammoniac is a name of natural, mineralogical form of ammonium chloride...
and ammonium hydroxide.
The formation of a reddish brown precipitate indicates Fe
3+; a gelatinous white precipitate indicates
Al3+Aluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13. It is not soluble in water under normal circumstances....
; and a green precipitate indicates Cr
3+ or Fe
2+. These last two are distinguished by adding sodium hydroxide in excess to the green precipitate. If the precipitate dissolves, Fe
2+ is indicated; otherwise, Cr
3+ is present.
4th analytical group of cations
The fourth group of cations include
Zn2+Zinc , also known as spelter, is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
,
Ni2+Nickel is a chemical element, with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. It is one of the four ferromagnetic elements at about room temperature, other three being iron, cobalt and gadolinium...
,
Co2+Cobalt is a hard, lustrous, gray metal, a chemical element with symbol Co and atomic number 27. Although cobalt-based colors and pigments have been used since ancient times for making jewelry and paints, and miners have long used the name kobold ore for some minerals, the free metallic cobalt was...
, and
Mn2+Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
. This group is determined by the addition of ammonium chloride, ammonium hydroxide, and hydrogen sulphide gas to the solution of the salt. A colored precipitate indicates Mn
2+; a white precipitate indicates Zn2+; and a black precipitate indicates either Ni2+, if the color of the original solution is green, or Co2+ otherwise.
5th analytical group of cations
Ions in 5th analytical group of cations form carbonateIn chemistry, a carbonate is a salt or ester of carbonic acid, characterized by the presence of the carbonate ion, or a carbonate functional group O=C2....
s that are insoluble in water. The reagent usually used is (NH4)2CO3Ammonium carbonate is the commercial salt, formerly known as sal volatile or salt of hartshorn. Ammonium carbonate is used when crushed as a smelling salt. It can be crushed when needed in order to revive someone who has fainted...
(at around 0.2 M), with a neutral or slightly basic pH. All the cations in the previous groups are separated beforehand, since many of them also form insoluble carbonates.
The most important ions in the 4th group are Ba2+Barium is a chemical element. It has the symbol Ba, atomic number 56, and is the fifth element in Group 2. Barium is a soft silvery metallic alkaline earth metal. It 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...
, Ca2+Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust...
, and Sr2+Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
. After separation, the easiest way to distinguish between these ions is by testing flame colour: barium gives a yellow-green flame, calcium gives orange-red, and strontium, deep red.
6th analytical group of cations
Cations which are left after carefully separating previous groups are considered to be in the sixth analytical group. The most important ones are Mg2+Magnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
, Li+Lithium 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 three. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
, Na+, K+Potassium is the chemical element with the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash...
and NH{{suThe ammonium cation is a positively charged polyatomic cation of the chemical formula NH. It has a formula weight of 18.05 and is formed by the protonation of ammonia...
. NH{{su|b=4|p=+}} gives a brown colored precipitate with Nessler's reagentNeßler's reagent is a reagent named after Julius Neßler and is used to detect small amounts of ammonia. It is a 0.09 mol/L solution of potassium tetraiodomercurate in 2.5 mol/L potassium hydroxide. A yellow coloration indicates the presence of ammonia: at higher concentrations, a brown...
; the rest of the ions are distinguished by flame color: lithium gives a red flame, sodium gives bright yellow (even in trace amounts), potassium gives violet, and magnesium, bright white.
1st analytical group of anions
The 1st group of anions consist of CO{{suIn chemistry, a carbonate is a salt or ester of carbonic acid, characterized by the presence of the carbonate ion, or a carbonate functional group O=C2....
, HCO{{suIn inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
, CH3COO−An acetate, or ethanoate, is either a salt or ester of ethanoic acid.In chemistry, the abbreviation Ac refers to the acetyl group. The anion and the functional group may be written as −OAc and AcO−, or OAc respectively...
, S2−, SO{{suSulfites are compounds that contain the sulfite ion SO . The sulfite ion is the conjugate base of sulfurous acid. Although the acid itself is elusive, its salts are widely used.-Structure:...
, {{chem and NO{{suThe nitrite ion is NO2−. The anion is bent, being isoelectronic with ozone. More generally, a nitrite compound is either a salt or an ester of nitrous acid.-Examples:* nitrous acid, HNO2...
. The reagent for Group 1 anions is dilute hydrochloric acid (HCl) or dilute sulfuric acid (H2SO4).
- Carbonates give a brisk effervescence with dilute H2SO4 due to the release of CO2, a colorless gas which turns lime water
Lime water is the common name for saturated calcium hydroxide solution. Its chemical formula is Ca2. Since calcium hydroxide is only sparsely soluble, i.e. ca. 1.5g per liter at 25°C, there is no visible distinction to clear water. Attentive observers would notice a slightly earthy...
milky due to formation of CaCO3. The milkiness disappears on passing the lime water through an excess of the gas, due to formation of Ca(HCO)3.
- Acetates give the vinegar-like smell of CH3COOH when treated with dilute H2SO4. A blood red colouration is produced upon addition of yellow FeCl3, due to formation of iron(III) acetate.
- Sulphides give the rotten egg smell of H2S when treated with dilute H2SO4. The presence of sulphide is confirmed by adding lead(II) acetate
Lead acetate is a chemical compound, a white crystalline substance with a sweetish taste. It is made by treating litharge with acetic acid. Like other lead compounds, it is toxic. Lead acetate is soluble in water and glycerin...
paper, which turns black due to the formation of PbS. Sulphides also turn solutions of red sodium nitroprussideSodium nitroprusside is the inorganic compound with the formula Na2[Fe5NO]·2H2O. This red-coloured salt, which is often abbreviated SNP, is a potent vasodilator.-Properties:...
purple.
- Sulfites produce SO2 gas, which smells of burning sulphur, when treated with dilute acid. They turn acidified K2Cr2O7 from orange to green.
- Nitrites give reddish brown fumes of NO2 when treated with dilute H2SO4. These fumes cause a solution of potassium iodide
Potassium iodide is an inorganic compound with formula KI. This white salt is the most commercially significant iodide compound, with approximately 37,000 tons produced in 1985. It is less hygroscopic than sodium iodide, making it easier to work with...
(KI) and starchStarch or amylum is a polysaccharide carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds.Starch is produced by all green plants as an energy store and is a major food source for humans....
to turn blue.
2nd analytical group of anions
The 2nd group of anions consist of Cl−The chloride ion is formed when the element chlorine picks up one electron to form an anion Cl
−...
, Br−A bromide ion is a bromine atom with charge of −1.Compounds with bromine in formal oxidation state −1 are called bromides, and each individual chemical in this class can be called a bromide, as well...
, I−An iodide ion is an iodine atom with a −1 charge. Compounds with iodine in formal oxidation state −1 are called iodides. This can include ionic compounds such as caesium iodide or covalent compounds such as phosphorus triiodide. This is the same naming scheme as is seen with chlorides...
, NO{{suIn inorganic chemistry, a nitrate is a salt of nitric acid with an ion composed of one nitrogen and three oxygen atoms . In organic chemistry the esters of nitric acid and various alcohols are called nitrates.-Chemical properties:...
and C{{suAn oxalate is the deprotonated, charged form of oxalic acid or an ester of oxalic acid. The oxalate anion has the chemical formula C2O42−, also written...
. The group reagent for Group 2 anion is concentrated sulfuric acid (H2SO4).
After addition of the acid, chlorides, bromides and iodides will form precipitates with silver nitrateSilver nitrate, also known as lunar caustic, is a soluble chemical compound with chemical formula AgNO
3. This compound is a versatile precursor to many other silver compounds, such as those used in photography. Comparatively, it is far less sensitive to light than the halides...
. The precipitates are white, pale yellow, and yellow, respectively. The silver halides formed are completely soluble, partially soluble, or not soluble at all, respectively, in ammonium hydroxide (NH4OH) solution.
Chlorides are confirmed by the chromyl chlorideChromyl chloride is a chemical compound with the formula CrO2Cl2. This compound is an opaque dark blood-red liquid at room temperature and pressure. It is tetrahedral, somewhat like SO2Cl2...
test. When the salt is heated with K2Cr2O7 and concentrated H2SO4, red vapours of chromyl chloride (CrO2Cl2) are produced. Passing this gas through a solution of NaOH produces a yellow solution of Na2CrO4Sodium chromate is a yellow solid chemical compound used as a corrosion inhibitor in the petroleum industry, a dyeing auxiliary in the textile industry, as a wood preservative. and as a diagnostic pharmaceutical in determining red blood cell volume....
. The acidified solution of Na2CrO4 gives a yellow precipitate with the addition of (CH3COO)2PbLead acetate is a chemical compound, a white crystalline substance with a sweetish taste. It is made by treating litharge with acetic acid. Like other lead compounds, it is toxic. Lead acetate is soluble in water and glycerin...
.
Bromides and iodides are confirmed by the layer test. A sodium carbonate extract is made from the solution containing bromide or iodide, and CHCl3Chloroform is the organic compound with formula CHCl
3. This colourless, sweet-smelling, dense liquid is a trihalomethane. It is also considered somewhat hazardous...
or {{chemCarbon 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...
is added to the solution, which separates into two layers: a brown colour in the {{chem|CHCl|3}} or {{chem|CS|2}} layer indicates the presence of Br−, and a violet colour indicates the presence of I−.
Nitrates give brown fumes with concentrated H2SO4 due to formation of NO2. This is intensified upon adding copper turnings. Nitrate ion is confirmed by adding an aqueous solution of the salt to FeSO4 and pouring concentrated H2SO4 slowly along the sides of the test tube, which produces a brown ring around the walls of the tube.
Upon treatment with concentrated sulfuric acid, oxalates yield colourless CO2 and CO gases. These gases burn with a blue flame and turn lime water milky. Oxalates also decolourise KMnO4 and give a white precipitate with CaCl2.
3rd analytical group of anions
The 3rd group of anions consist of SO{{su, PO{{su and BO{{su. They react neither with concentrated nor diluted H2SO4.
- Sulphates give a white precipitate with BaCl2 which is insoluble in any acid or base.
- Phosphates give a yellow crystalline precipitate upon addition of HNO3 and ammonium molybdate.
- Borates give a green flame characteristic of ethyl borate
Triethyl borate, also called boron triethoxide or boric acid, triethyl ester, is a colorless, flammable liquid with the formula B3. It is an ester of boric acid and ethanol....
when ignited with concentrated H2SO4 and ethanol.
Modern techniques
Qualitative inorganic analysis is now used only as a pedagogical tool. Modern techniques such as atomic absorption spectroscopyIn analytical chemistry, atomic absorption spectroscopy is a technique for determining the concentration of a particular metal element in a sample...
and ICP-MSICP-MS is a type of mass spectrometry that is highly sensitive and capable of the determination of a range of metals and several non-metals at concentrations below one part in 10
12...
are able to quickly detect the presence and concentrations of elements using a very small amount of sample.