Qualitative inorganic analysis

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Classical qualitative inorganic analysis is a method of analytical chemistry which seeks to find elemental composition of inorganic compounds. It is mainly focused on detecting ions in an aqueous solution, 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 reagents to test for reactions characteristic of certain ions, which may cause color change, solid forming and other visible changes.[1]

Qualitative inorganic analysis is that branch or method of analytical chemistry which seeks to establish elemental composition of inorganic compounds through various reagents.

Physical appearance of inorganic salts[edit]

Sr No. Salt Colour
1 MnO, MnO2, FeO, CuO, Co3O4, Ni2O3; sulfides 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 blue
5 Hydrated Mn2+ salts Light Pink
6 KO2, K2Cr2O7, Sb2S3, ferricyanides Orange
7 Hydrated Co2+ salts Reddish Pink
8 Chromates, AgBr, As2S3, AgI, PbI2, CdS Yellow
9 CdO, Fe2O3, PbO2, CuCrO4 Dark brown

Detecting cations[edit]

According to their properties, cations are usually classified into six groups.[1] 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. This happens because cationic analysis is based on the solubility products of the ions. As the cation gains its optimum concentration needed for precipitation it precipitates and hence allowing us to detect it. 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[edit]

1st analytical group of cations consists of ions that form insoluble chlorides. As such, the group reagent to separate them is hydrochloric acid, usually used at a concentration of 1–2 M. Concentrated HCl must not be used, because it forms a soluble complex ion ([PbCl4]2-) with Pb2+. Consequently the Pb2+ ion would go undetected. NH3 is also included in ZERO group of cation (according to NCERT textbooks). The most important cations in 1st group are Ag+, Hg2+
2
, and Pb2+. The chlorides of these elements cannot be distinguished from each other by their colour - they are all white solid compounds. PbCl2 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(NH3)2]+), Hg2Cl2 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.

PbCl2 is far more soluble than the chlorides of the other two ions, especially in hot water. Therefore, HCl in concentrations which completely precipitate Hg2+
2
and Ag+ may not be sufficient to do the same to Pb2+. Higher concentrations of Cl cannot be used for the before mentioned reasons. Thus, a filtrate obtained after first group analysis of Pb2+ 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, Pb2+ 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 hydroxide is then added. If the precipitate is insoluble, then Pb2+ is present; if the precipitate is soluble, then Ag+ is present, and if the white precipitate turns black, then Hg2+
2
is present.

Pb2+ + 2 KI → PbI2 + 2 K+
Pb2+ + K2CrO4 → PbCrO4 + 2 K+

Confirmation test for Silver:

Ag+ + KI → AgI + K+
2Ag+ + K2CrO4 → Ag2CrO4 + 2 K+

Confirmation test for dimeric mercury ion:

Hg2+
2
+ 2 KI → Hg2I2 + 2 K+
2 Hg2+
2
+ 2 NaOH → 2 Hg
2
O + 2 Na+ + H2O

2nd analytical group of cations[edit]

The 2nd analytical group of cations consists of ions that form acid-insoluble sulfides. Cations in the 2nd group include: Cd2+, Bi3+, Cu2+, As3+, As5+, Sb3+, Sb5+, Sn2+, Sn4+ and Hg2+. Pb2+ is usually also included here in addition to the first group. Although these methods refer to solutions that contain sulfide (S2-), these solutions actually only contain H2S and bisulfide (SH-). Sulfide (S2-) does not exist in appreciable concentrations in water.

The reagent used can be any substance that gives S2− ions in such solutions; most commonly used are H2S (at 0.2-0.3 M), AKT (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, whereas nitric acid oxidises the sulfide ion in the reagent, forming colloidal sulfur.

The precipitates of these cations are almost indistinguishable, except for CdS, which is yellow. All the precipitates, except for HgS, are soluble in dilute nitric acid. HgS is soluble only in aqua regia, 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, whereas CdS dissolves forming a colourless solution. The sulfides of As3+, As5+, Sb3+, Sb5+, Sn2+, Sn4+ are soluble in yellow ammonium sulfide, where they form polysulfide 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 Bi3+, Cu2+, Hg2+ or Pb2+ is present. Otherwise, if it forms a yellow precipitate, then Cd2+ or Sn4+ is present; or if it forms a brown precipitate, then Sn2+ must be present; or if a red orange precipitate is formed, then Sb3+ is present.

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+
+ 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[edit]

3rd analytical group of cations includes ions that form hydroxides which are insoluble even at low concentrations. The reagents are similar to these of the 2nd group, but separation is conducted at pH of 8–9. Occasionally, a buffer solution is used to ensure this pH.

Cations in the 3rd group are, among others: Fe2+, Fe3+, Al3+, and Cr3+.

The group is determined by making a solution of the salt in water and adding ammonium chloride and ammonium hydroxide. Ammonium chloride is added to ensure low concentration of hydroxide ions.

The formation of a reddish brown precipitate indicates Fe3+; a gelatinous white precipitate indicates Al3+; and a green precipitate indicates Cr3+ or Fe2+. These last two are distinguished by adding sodium hydroxide in excess to the green precipitate. If the precipitate dissolves, Fe2+ is indicated; otherwise, Cr3+ is present.

4th analytical group of cations[edit]

The fourth group of cations include Zn2+, Ni2+, Co2+, and Mn2+. Of these, Zinc salts are colourless, Manganese salts are faint pink or colourless, and Nickel and cobalt salts may be brightly coloured, often blue-green. The precipitate, washed in water is reacted with extremely dilute hydrochloric acid. This precipitates nickel salts, if any. The supernatent liquid is filtered and reacted with excess of Sodium Hydroxide. This precipitates any Manganese salts. Hydrogen sulphide is passed through the supernatent liquid. If a white precipitate forms, Zinc is present.

5th analytical group of cations[edit]

Ions in 5th analytical group of cations form carbonates that are insoluble in water. The reagent usually used is (NH4)2CO3 (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 5th group are Ba2+, Ca2+, and Sr2+. 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[edit]

Cations which are left after carefully separating previous groups are considered to be in the sixth analytical group. The most important ones are Mg2+, Li+, Na+ and K+. All 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.

Detecting anions[edit]

1st analytical group of anions[edit]

The 1st group of anions consist of CO2−
3
, HCO
3
, CH3COO, S2−, SO2−
3
, S
2
O
2−
3
and NO
2
. 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 limewater milky due to formation of CaCO3 (carbonatation). The milkiness disappears on passing the lime water through an excess of the gas, due to formation of Ca(HCO3)2.
  • 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.
  • Sulfides give the rotten egg smell of H2S when treated with dilute H2SO4. The presence of sulfide is confirmed by adding lead(II) acetate paper, which turns black due to the formation of PbS. Sulfides also turn solutions of red sodium nitroprusside purple.
  • Sulfites produce SO2 gas, which smells of burning sulfur, when treated with dilute acid. They turn acidified K2Cr2O7 from orange to green.
  • Thiosulfates produce SO2 gas when treated with dilute acid. In addition, they form a cloudy precipitate of sulfur.
  • Nitrites give reddish brown fumes of NO2 when treated with dilute H2SO4. These fumes cause a solution of potassium iodide (KI) and starch to turn blue.

2nd analytical group of anions[edit]

The 2nd group of anions consist of Cl, Br, I, NO
3
and C
2
O2−
4
. The group reagent for Group 2 anion is concentrated sulphuric acid (H2SO4).

After addition of the acid, chlorides, bromides and iodides will form precipitates with silver nitrate. 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 aqueous ammonia solution.

Chlorides are confirmed by the chromyl chloride 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 Na2CrO4. The acidified solution of Na2CrO4 gives a yellow precipitate with the addition of (CH3COO)2Pb.

Bromides and iodides are confirmed by the layer test. A sodium carbonate extract is made from the solution containing bromide or iodide, and CHCl3 or CS
2
is added to the solution, which separates into two layers: a brown colour in the CHCl
3
or 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, caused by the formation of Fe(NO)2+
.[2]

Upon treatment with concentrated sulphuric acid, oxalates yield colourless CO2 and CO gases. These gases burn with a bluish flame and turn lime water milky. Oxalates also decolourise KMnO4 and give a white precipitate with CaCl2.

3rd analytical group of anions[edit]

The 3rd group of anions consist of SO2−
4
, PO3−
4
and BO3−
3
. They react neither with concentrated nor diluted H2SO4.

  • Sulfates 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 when ignited with concentrated H2SO4 and ethanol....

Modern techniques[edit]

Qualitative inorganic analysis is now used only as a pedagogical tool. Modern techniques such as atomic absorption spectroscopy and ICP-MS are able to quickly detect the presence and concentrations of elements using a very small amount of sample.

See also[edit]

References[edit]

  1. ^ a b E. J. King "Qualitative Analysis and Electrolytic Solutions" 1959, Harcourt, Brace, and World, New York.
  2. ^ C. Parameshwara Murthy (2008). University Chemistry, Volume 1. New Age International. p. 133. ISBN 81-224-0742-0.