Qualitative inorganic analysis

Classical qualitative inorganic analysis is a method of analytical chemistry which seeks to find the elemental composition of inorganic compounds. It is mainly focused on detecting ions in an aqueous solution, therefore materials in other forms may need to be brought to 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, precipitation and other visible changes. Qualitative inorganic analysis was universally taught in most universities until the 1980's either in Inorganic chemistry or in Analytical chemistry classes. Since then, it has disappeared even in the curricula of Chemistry majors, due to its limited use by Chemistry professionals.

Qualitative inorganic analysis is a different branch or method of analytical chemistry which seeks to establish the elemental composition of inorganic compounds by using various analytical methods.

Physical appearance of some inorganic compounds

{| class="wikitable" border="1"

|-

!

! Salt

! Colour

|-

| 1

| MnO, MnO₂, FeO, CuO, Co₃O₄, Ni₂O₃; sulfides of Ag⁺, Cu⁺, Cu²⁺, Ni²⁺, Fe²⁺, Co²⁺, Pb²⁺, Hg²⁺, Bi³⁺, Hg, BiI₃, Bi(s), Cu(SCN)₂, Sb(s), Hg₂O(s), Cu[C(=NH)S]₂(s)

| Black

|-

| 2

| Hydrated Cu²⁺ salts, Co[Hg(SCN)₄](s),

| Blue

|-

| 3

| HgO, HgI₂, Pb₃O₄, Hg₂CrO₄(s), Ag₂CrO₄(s),

| Red

|-

| 4

| Cr³⁺, Ni²⁺, hydrated Fe²⁺ salts, Hg₂I₂(s), Cu(C₇H₆O₂N)₂(s), CuHAsO₃(s),

| Green

|-

| 5

| Hydrated Mn²⁺ salts

| Light Pink

|-

| 6

| KO₂, K₂Cr₂O₇, Sb₂S₃, Ferrocyanide, HgO, Sb₂S₃(s), Sb₂S₅(s)

| Orange

|-

| 7

| Hydrated Co²⁺ salts

| Reddish Pink

|-

| 8

| Chromates, AgBr, As₂S₃, AgI, PbI₂, CdS, PbCrO₄(s), Hg₂CO₃(s), Ag₃PO₄(s), Bi(C₆H₃O₃)(s), Cu(CN)₂(s), Ag₃AsO₃(s), (NH₃)₃[As(Mo₃O₁₀)₄](s), [SbI₆]^3-(aq),

| Yellow

|-

| 9

| CdO, Fe₂O₃, PbO₂, CuCrO₄, Ag₂O(s), Ag₃AsO₄(s),

| Brown

|-

| 10

| PbCl₂(s), Pb(OH)₂(s), PbSO₄(s), PbSO₃(s), Pb₃(PO₄)₂(s), Pb(CN)₂(s), Hg₂Cl₂(s), Hg₂HPO₄(s), Al(OH)₃(s), AgCl(s), AgCN(s), Ag₂CO₃(s), Bi(OH)₂NO₃(s), Bi(OH)₃(s), CuI(s), Cd(OH)₂(s), Cd(CN)₂(s), MgNH₄Also₄(s), SbO.Cl(s), Sb₂O₃(s),

| White

|}

Detecting cations

According to their properties, cations are usually classified into six groups.

PbCl₂ is far more soluble than the chlorides of the other two ions, especially in hot water. Therefore, HCl in concentrations which completely precipitate Hg 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.

A signature reaction of lead ions involve the formation of a yellow lead chromate precipitate upon treatment with chromate ions. This precipitate doesn't dissolve in ammonia (unlike Cu(II) and Ag(I)) or acetic acid (unlike Cu(II) and Hg(II)).

Upon treatment with concentrated sulfuric 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 SO, PO and BO. 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 and heating the solution.
• 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.

Sodium carbonate test

The sodium carbonate test (not to be confused with sodium carbonate extract test) is used to distinguish between some common metal ions, which are precipitated as their respective carbonates. The test can distinguish between copper (Cu), iron (Fe), and calcium (Ca), zinc (Zn) or lead (Pb). Sodium carbonate solution is added to the salt of the metal. A blue precipitate indicates Cu²⁺ ion. A dirty green precipitate indicates Fe²⁺ ion. A yellow-brown precipitate indicates Fe³⁺ ion. A white precipitate indicates Ca²⁺, Zn²⁺, or Pb²⁺ ion. The compounds formed are, respectively, basic copper carbonate, iron(II) carbonate, iron(III) oxide, calcium carbonate, zinc carbonate, and lead(II) carbonate. This test is used to precipitate the ion present as almost all carbonates are insoluble. While this test is useful for telling these cations apart, it fails if other ions are present, because most metal carbonates are insoluble and will precipitate. In addition, calcium, zinc, and lead ions all produce white precipitates with carbonate, making it difficult to distinguish between them. Instead of sodium carbonate, sodium hydroxide may be added, this gives nearly the same colours, except that lead and zinc hydroxides are soluble in excess alkali, and can hence be distinguished from calcium. See qualitative inorganic analysis for the complete sequence of tests used for qualitative cation analysis.

See also

• Flame test
• Bead test

References