Estimation of Chromium

In the analysis of chromites or of other substances containing chromium, such as leather ashes, pigments, etc., it is first necessary to obtain a solution. This is done by finely powdering the substance and heating it with a suitable flux in a crucible, preferably of nickel. Many fluxes have been employed, usually caustic alkali or alkali carbonates, but the one in most common use at present is sodium peroxide, whereby the chromium compound is rapidly converted to a chromate. Excess of alkali is removed by boiling with ammonium carbonate, which also precipitates any iron present. The filtrate is then acidified with dilute sulphuric acid and the chromium estimated either by gravimetric or volumetric methods.

When the chromium is in solution as a chromic salt, it can be precipitated completely from the boiling solution by the addition of ammonium hydroxide in the presence of ammonium salts. The precipitate may be dried at 100° C. and converted by gentle ignition to-the dark green sesquioxide and weighed as such. Ammonium sulphiqp or hydrazine sulphate may also be used as the precipitant.

The results obtained are generally high, probably due to the formation of chromic chromate during ignition, thus:

5Cr₂O₃ + 9O ⇔ 2Cr₂(CrO₄)₃.

Accurate results are obtained by igniting in hydrogen.

Chromium may also be completely precipitated as the phosphate, CrPO₄, by the addition of an alkali phosphate in presence of sodium acetate. The method gives satisfactory results with the green and violet chlorides, sulphates, and acetates, but not with oxalates. If the chromium is in solution as a chromate or dichromate, as is the case after fusion as described above, it may either be reduced to the trivalent condition and precipitated as hydroxide, or directly precipitated as an insoluble chromate. In the absence of sulphates, barium chromate is precipitated by the addition of barium acetate at the boiling-point to a solution made faintly acid with acetic acid and containing a little alcohol. After ignition the precipitate is weighed as barium chromate. If chlorides and sulphates are present only in small amount, the chromate may be thrown down by mercurous nitrate, the mercurous chromate then being converted by ignition to the sesquioxide of chromium. In the absence of chlorides the silver or lead salts may be precipitated, the latter only in absence of sulphates.

Chromium is most easily estimated in solutions containing chromate or dichromate by volumetric methods. These generally depend either on the oxidation of ferrous iron or on the liberation of iodine from potassium iodide.

In the first case, the solution is acidified with dilute sulphuric acid and a known excess of ferrous ammonium sulphate added. The excess of ferrous salt remaining in the solution is titrated with standard dichromate or permanganate, the value of the original chromate solution being obtained from the amount of iron oxidised. The method gives exact results in the presence of iron, and it is used for the analysis of ferrochrome, steel, and slags, but not in presence of nickel, cobalt, or manganese. Similar methods are employed in the presence of aluminium, zinc, manganese, or vanadium, in which the chromium is oxidised by means of ammonium persulphate, small quantities of chromium in steels and other metallurgical products being thus rapidly estimated. The electrometric method for determining the end-point in the final titration of the ferrous iron has been used with success.

The iodometric method depends on the fact that chromates interact with hydrochloric acid, liberating chlorine, which in its turn liberates an equivalent amount of iodine from potassium iodide. The iodine may then be determined by titration with standard sodium thio-sulphate. This method is largely used for chromium in whatever state of oxidation it may be, a chromium salt being first converted to chromate either by fusion with sodium peroxide or by boiling its solution with nitric acid and lead dioxide.

If iron is present results may be high, and this metal should therefore be removed by means of hydrogen peroxide before estimating the chromium; on the other hand, organic matter tends to delay the separation of iodine. The reaction is also influenced by the concentration of the acid, the presence of different salts, and by sunlight.

In chrome liquors, infusions, leather ashes, and chrome residues, the chromium is generally oxidised by one of the above methods and then estimated volumetrically.

A method, which is considered sufficiently accurate for technical purposes, for the estimation of alkali chromates and dichromates in presence of each other, depends upon titration with acid or alkali. Potassium dichromate is titrated with potassium hydroxide in presence of phenolphthalein:

K₂Cr₂O₇ + 2KOH = 2K₂CrO₄ + H₂O.

The presence of an indicator is not essential, since when neutral the colour changes from reddish yellow to greenish yellow. Potassium chromate is estimated by titration with sulphuric acid, Congo red being used as the indicator:

2K₂CrO₄ + H₂SO₄ = K₂Cr₂O₇ + K₂SO₄ + H₂O.

A further method consists in treating a solution of chromate with an excess of methylene white (the leuco base of methylene blue) in presence of hydrochloric acid. The methylene blue produced is then titrated with standard titanous chloride, the temperature being kept above 40° C. in order to sharpen the end-point. The whole process should be performed in an atmosphere of carbon dioxide. In applying this method to ferrochrome or chromium steel, the iron and chromium may be estimated together, the iron then being determined separately by ordinary methods and the chromium obtained by difference. The reaction may also be used colorimetrically.

The intensity of the yellow colour of a solution of an alkali chromate is proportional to the amount of chromate in the solution. If, therefore, a given quantity of the solution to be tested has the same tint as an equal depth of a standard solution, it is assumed that there is the same concentration of alkali chromate in both solutions. The sensitiveness is found to be greatest at concentrations between 0.004N and 0.008N with respect to the gram-atom of chromium.

Colorimetric comparisons have also been made with green chromium- chloride solutions, and it is found that, in general, the sensitiveness is similar to that of potassium chromate solutions.

When an alcoholic solution of diphenylcarbazide, containing a little acetic acid, is added to a very dilute solution of chromic acid, a violet coloration is produced. This has been made the basis of a colorimetric estimation. With more concentrated solutions the reaction gives a reddish-brown colour, and finally a brown precipitate containing about 18 per cent, of chromium.

The colour reaction with α-naphthylamine may also be used quantitatively.

As yet, little success has attended the application of electrolysis to chromium compounds, but in the analysis of chrome yellow, and of other pigments with a basis of lead chromate, the lead may be separated by the electrolysis of a solution in nitric acid, and the chromium remaining in the solution then estimated in the usual way.