Detection and Estimation of Antimony

Antimony compounds, when heated on charcoal in the blowpipe flame, are reduced to metal (particularly if previously mixed with fusion mixture), but the metal volatilises and burns to antimony trioxide. By depositing the oxide on a glazed porcelain surface, adding a spot of silver nitrate, and blowing a current of gaseous ammonia on to the silver nitrate a black stain is obtained. Alternatively, if the white incrustation which remains on the charcoal is moistened with ammonium sulphide solution, a deep orange stain is produced.

When a current of hydrogen sulphide is passed through a solution containing antimony ions, an orange-coloured precipitate of antimony sulphide is obtained, the composition of which varies according to the state of oxidation of the antimony. This precipitate is more soluble than the corresponding arsenic sulphide, but less so than tin sulphide. It is soluble in alkalis and alkali sulphides (including ammonium polysulphide), and reprecipitated from solution by the addition of acid. Arsenic and tin sulphides behave similarly, but antimony may be separated from these elements and identified by one of the following methods: -

(a) Marsh's test is applied, and the evolved gases passed into a neutral solution of silver nitrate. The precipitate is filtered off, washed, and treated with concentrated hydrochloric acid to dissolve any silver antimonide formed. Antimony can then be identified in this solution by dilution and treatment with hydrogen sulphide.

(b) Antimony and tin sulphides are soluble in concentrated hydrochloric acid, but arsenic sulphide is not appreciably so. If a piece of platinum foil and a little zinc are placed in this solution the presence of antimony is indicated by the formation of a black stain on the platinum, which does not disappear on removal of the zinc.

(c) Arsenic sulphide may be separated as in (b) by treatment with concentrated hydrochloric acid; antimony and tin are detected in the solution by making use of the different solubilities of their sulphides in the acid.

(d) Arsenic and antimony may be detected in an acid solution containing an excess of tin by the addition of a little stannous chloride, followed by sodium bisulphite or sulphurous acid a drop at a time. The hydrogen sulphide thus liberated first precipitates arsenic trisulphide, and afterwards antimony trisulphide. Under these conditions no tin sulphide is precipitated.

An important test for antimony is Marsh's test, which is carried out in a manner similar to that employed for arsenic. Stibine, which is produced in this test, is decomposed at a lower temperature than arsine, and the antimony mirror is usually formed in front of the constriction in the heated tube; this antimony mirror is not readily soluble in bleaching powder solution. (It is important that the bleaching powder solution should be freshly prepared, as old solutions always contain some chlorite, in which the antimony deposit is soluble.) The deposits produced by arsenic and antimony in this test may be identified individually by passing a current of hydrogen sulphide through the heated tube in the reverse direction to that taken by the gases evolved from the generating flask; a yellow deposit indicates arsenic, while an orange-red deposit indicates antimony. Further confirmation may be obtained by passing a current of dry hydrogen chloride through the tube; the antimony sulphide is removed while the arsenic sulphide remains. Marsh's test for antimony is sensitive to 0.0002 mg.

Antimony compounds are readily hydrolysed, yielding, with cold water, insoluble basic salts. If boiling water is used, hydrolysis sometimes proceeds a further stage with the formation of antimonic acid or hydrated antimony oxides. The insoluble residues are, in general, redissolved on acidifying.

The more difficultly soluble antimony salts may be dissolved in liquids containing tartaric acid or tartrates, the resulting solutions being stable only when neutral or alkaline; on acidifying, insoluble hydroxides or hydrated oxides are produced.

Antimonious compounds may be distinguished from antimonic compounds by the action of hydriodic acid, or potassium iodide in acid solution; iodine is liberated by antimonic compounds but not by antimonious compounds.

The method most usually adopted for the estimation of antimony is by precipitation of the trisulphide. The sulphide, which is precipitated from a cold, acid solution (the solution being raised to boiling towards the end of the precipitation), is thoroughly washed, dried and converted to black crystalline antimony trisulphide (by heating in a current of carbon dioxide), in which form it is weighed. The black, crystalline product may be produced direct if the precipitation is carried out in a hot solution rendered strongly acid with hydrochloric acid.

Antimony may also be estimated as tetroxide by precipitating first as trioxide, oxidising this precipitate with nitric acid and igniting the residue carefully at 800° C.

In the presence of sodium potassium tartrate, and in faintly acid solution, pyrogallol will precipitate antimony quantitatively.

Potassium Bromate Method

In this method, tervalent antimony is oxidised to quinquevalent by the action of potassium bromate: -

KBrO₃ + 3SbCl₃ + 6HCl = 3SbCl₅ + KBr + 3H₂O

The substance containing antimony is dissolved in concentrated hydrochloric acid to which bromine has been added. Excess of bromine is rerrfoved by boiling, and the solution reduced by the addition of sodium sulphite solution. The hot solution is then titrated with standard potassium bromate solution until the colour of methyl orange is destroyed. The presence of calcium, magnesium and ammonium salts in quantity tends to give high results, while excess of copper obscures the end-point. If copper is present in any quantity, it is advisable to remove it before estimating the antimony. Solutions containing antimony in the quinquevalent condition may be completely reduced by mercury in the presence of 3 to 4N HCl. The reduction should be carried out in an atmosphere of carbon dioxide. The antimony may then be estimated in such solutions by the bromate method.

Potassium Iodide Method

In this method the sample is dissolved in hydrochloric acid to which a little potassium chlorate has been added. Excess of chlorine is removed and potassium iodide added; the liberated iodine is then titrated with a standard solution of sodium thiosulphate.

Iodine Method

The solution is made in hydrochloric acid; tartaric acid is added and the mixture neutralised with sodium carbonate. The solution is now made faintly acid, a saturated solution of sodium bicarbonate is added and the mixture is titrated with standard iodine solution.

Other volumetric methods have been suggested involving the use of the following solutions: potassium permanganate, potassium dichromate, cerium sulphate and titanium chloride. Chloramine-T (sodium p-tohienesulphoneehloramide) may also be employed either for potentiometric or visual titration of antimony. A hydrochloric acid solution is used, with, in the case of visual titration, methyl red as indicator.

Electrolytic Titration of antimony may be carried out in a boiling solution in dilute hydrochloric acid containing hydroxylamine hydrochloride. The initial current should be 3.20 amperes at a pressure of 2 volts.

The quantitative electrodeposition of antimony from acid solutions presents considerable difficulties, but the method has been employed for the estimation of the metal.2 Solutions of sodium or ammonium thioantimonite or thioantimonate are more usually employed. The presence of poly sulphides interferes seriously with the deposition, but their influence may be minimised by the addition of sodium sulphite or potassium cyanide.