Qualitative inorganic analysis

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. 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

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. NH₄⁺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.

Confirmation test for Lead ion:

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

Confirmation test for Silver ion:

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

Confirmation test for Mercury(I) ion:

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

2nd analytical group of cations

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 hydrogen sulfide (at 0.2-0.3 M), thioacetamide (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 polysulphide 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

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, Cr³⁺ is indicated; otherwise, Fe²⁺ is present.

4th analytical group of cations

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

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 brick red, and strontium, crimson 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⁺, 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, colorless (although magnesium metal burns with a bright white flame).

1st analytical group of anions

The 1st group of anions consist of CO2−
3, HCO−
3, CH₃COO⁻, S²⁻, SO2−
3, S
2O2−
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 an excess of the gas through the lime water, due to formation of Ca(HCO₃)₂.

Acetates give the vinegar-like smell of CH₃COOH when treated as 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

The 2nd group of anions consist of Cl⁻, Br⁻, I⁻, NO−
3 and C
2O2−
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: an orange 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, at the junction of the two liquids caused by the formation of Fe(NO)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

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 of BaSO₄ 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

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.