Detection of Chromium

Chromium compounds when reduced on charcoal leave a dark green residue; if the substance be first mixed with sodium carbonate, the fused mass obtained gives a yellow solution with water, the tint of which deepens on addition of sulphuric acid; in the presence of zinc, zinc chromate, which is insoluble in the alkaline solution, is formed. On heating a chromate in a dry tube with a little potassium hydrogen sulphate, a red or green liquid may be formed with evolution of oxygen. Some chromates evolve oxygen when heated alone. Ammonium dichromate decomposes on heating, giving off nitrogen, while the sesquioxide is left as a bulky green powder,

(NH₄)₂Cr₂O₇ = Cr₂O₃ + N₂ + 4H₂O.

Some undecomposed ammonia also escapes.

If a trace of chromate or dichromate be heated in an opaque bead of fusion mixture on a platinum wire, a yellow bead results; a chromium salt will give the same result if the bead is touched with a minute quantity of sodium peroxide. When heated with borax on platinum wire in an oxidising flame, chromium compounds impart to the bead a yellow or dark red colour, which on cooling becomes yellowish green; if heated in the reducing flame an emerald green bead is obtained.

The presence of chromium in solution and its state of oxidation may be indicated by the colour due to its ions; thus Cr^••• is green, CrO₄'' yellow, and Cr₂O₇'' orange. The solutions of chromous salts are generally deep blue. When hydrogen sulphide is passed into a solution of chromate in presence of an acid, the colour changes to green and sulphur is precipitated:

2K₂CrO₄ + 10HCl + 3H₂S = 4KCl + 2CrCl₃ + 8H₂O + 3S.

A similar change takes place with sulphurous acid, but no sulphur is precipitated:

2K₂CrO₄ + 3H₂SO₃ + 2H₂SO₄ = 2K₂SO₄ + Cr₂(SO₄)₃ + 5H₂O.

If an excess of hydrogen peroxide be added to a solution containing a chromate, made slightly acid with sulphuric acid, and the mixture then shaken up with a few c.c. of ether, a blue ethereal layer is formed above the aqueous solution. The colour is due to the formation of perchromic acid, and the reaction affords a useful test for identifying chromates in presence of sulphuric acid. If the ether used is free from alcohol, 1 part of potassium chromate in 40,000 parts of water can be detected. Another sensitive test for chromates and dichromates is to add a trace of a-naphthylamine to the solution and acidify with tartaric acid, when an intense blue colour results.

Solutions of chromic salts give a bluish- or whitish-green gelatinous precipitate of chromic hydroxide with ammonia, slightly soluble in excess of the reagent, forming a bluish-pink solution, but completely precipitated on boiling. Sodium or potassium hydroxide gives the same precipitate, which dissolves in excess of either reagent, giving a bright green solution; complete precipitation only takes place on prolonged boiling with a large excess of water. The presence of nonvolatile organic compounds interferes with the precipitation and may entirely prevent it. Ammonium sulphide produces the same precipitate, insoluble in excess:

Cr₂(SO₄)₃ + 3 (NH₄)₂S + 6H₂O = 2Cr(OH)₃ + 3 (NH₄)₂SO₄ + 3H₂S.

Chromium may also be precipitated as chromium phosphate, CrPO₄.

A chromium salt may be converted to a chromate by means of such oxidising agents as sodium peroxide, hydrogen peroxide, potassium persulphate, or an alkali hypochlorite or hypobromite. If sodium peroxide is used, it is added to a cold solution of the chromium salt, and the mixture then boiled until the effervescence ceases. The green or purple colour changes to yellow.

2Cr(OH)₃ + 3Na₂0₂ = 2Na₂CrO₄ + 2NaOH + 2H₂O.

If the oxidation is carried out with a hypochlorite or hypobromite, chlorine or bromine water is added to a cold solution of the chromic salt, which has previously been rendered alkaline with sodium or potassium hydroxide, and the mixture is then boiled.

2Cr(OH)₃ + 3NaOCl + 4NaOH = 2Na₂CrO₄ + 3NaCl + 5H₂O.

Excess of the halogen may be removed by acidifying with sulphuric acid and again boiling.

Since chromates and dichromates are powerful oxidisers, warming with concentrated hydrochloric acid results in the liberation of chlorine. In presence of a chloride these salts may be detected by heating with concentrated sulphuric acid, when chromyl chloride, CrO₂Cl₂, distils over as a greenish-yellow vapour, which condenses to a reddish-brown liquid.

When analysing a substance containing a chromate, the latter is reduced by hydrogen sulphide with precipitation of sulphur; it is therefore precipitated as hydroxide in the iron group. To avoid the sulphur precipitate, the reduction may be performed with sulphurous acid, excess being expelled by boiling before passing in hydrogen sulphide. The precipitated hydroxides of iron, aluminium, and chromium are treated with water and sodium peroxide, when the aluminium and chromium pass into solution as aluminate and chromate respectively. The presence of the latter is seen by the yellow colour, and can be confirmed by the precipitation of an insoluble chromate from the neutral or slightly acid solution. For example, silver nitrate gives a brick-red precipitate of silver chromate, insoluble in acetic acid but soluble in nitric acid and in ammonia; a soluble lead salt precipitates yellow lead chromate,1 soluble in excess of caustic alkali; barium chloride gives light yellow barium chromate, soluble in mineral acids but insoluble in acetic acid; dark red mercurous chromate may also be precipitated.

To detect a chromium salt in the presence of a chromate, the mixture may be boiled with sodium carbonate and filtered; chromium hydroxide is precipitated from the chromium salt, while the yellow colour of the chromate is seen in the filtrate.

A chromate may be detected in presence of a dichromate in several ways. If the precipitation of barium chromate by excess of barium chloride leaves an acid solution, a dichromate is indicated. Excess of methylene blue gives a precipitate with dichromates, and if a chromate is present a further precipitate is obtained on adding dilute sulphuric acid to the filtrate. A concentrated solution of manganese sulphate added to a boiling solution gives a blackish-brown precipitate if a chromate is present; while by mixing the hot solution with an equal volume of a boiling solution of sodium thiosulphate, a distinct turbidity, or even a brown precipitate of chromium sesquioxide, is produced by a dichromate.

Free chromic acid in a solution of a dichromate may be detected by the liberation of iodine from potassium iodide.