532 Analyst, August, 1968, Vol. 93, $9. 532-534 The Determination of Iron(I1) Sulphide in Soil in the Presence of Iron(II1) Oxide BY G. PRUDEN AND C. BLOOMFIELD (Rothamsted Experimental Station, Harpenden, Herts.) Hydrogen sulphide is partially oxidised by iron(II1) when soils containing sulphide and iron(II1) oxide are acidified. Satisfactory recoveries of hydrogen sulphide are obtained by using a solution of tin(I1) chloride in hydrochloric acid to decompose the sulphide. IN studying the reduction of sulphate in waterlogged soil it was necessary to determine the amount of iron(I1) sulphide that was produced as one of the reaction products. Attempts to determine this as the iron(I1) iron liberated when the soil was treated with a solution of a mercury(I1) salt were unsuccessful. The difficulties of this method are aggravated by the need to correct for soluble iron compounds, also formed during anaerobic incubation, and the determination of this quantity is uncertain, particularly under the conditions imposed by the requirements of the main experiment.The only alternative seemed to be to determine iron(I1) sulphide as hydrogen sulphide liberated on acidification. As iron(II1) oxide would also be present in the reaction mixture, we determined the extent to which this would interfere, according to the reaction 2Fe3+ + S2- = 2Fe2+ + S, with calcium sulphide as the source of sulphide. The apparatus used is illustrated in Fig. 1. Fig. 1. Details of apparatus 0 SAC and the authors.PRUDEN AND BLOOMFIELD 533 METHOD REAGENTS- Ammoniacal zinc sulphate solution-Dissolve 50 g of zinc sulphate, ZnSO,.?H,O, in 250ml of water.Add 250ml of concentrated ammonia solution, allow to stand overnight in a stoppered container and filter. Potassium iodate solution, 0.1 N. Potassium iodide solution, 1 per cent. w/v, apueous-Freshly prepared. Hydrochloric acid (1 + 1 v / v ) . PROCEDURE- About 0.1 g of calcium sulphide and various amounts of hydrated iron(1I-I) oxide were weighed into the conical flask (Fig. 1) and the apparatus flushed with nitrogen. About 25 ml of dilute hydrochloric acid were added from the dropping funnel, and the liberated hydrogen sulphide swept out with a gentle stream of nitrogen. When the initial vigorous effervescence had ceased, the contents of the reaction flask were heated to boiling.The hydrogen sulphide evolved was absorbed in about 5 ml of the ammoniacal zinc sulphate solution diluted with 25 to 30 ml of water. When all of the hydrogen sulphide was expelled, 2 to 3 ml of potassium iodide were added to the receiver, followed by starch solution and by dilute hydrochloric acid to dissolve the precipitate of zinc sulphide ; the hydrogen sulphide thus liberated was titrated immediately with 0.1 N potassium iodate to the first appearance of a permanent blue colour. TABLE I THE EFFECT OF IRON(III) OXIDE ON THE RECOVERY OF HYDROGEN SULPHIDE Sulphide taken, mg 30.8 28-3 29-7 28.8 28-5 28-8 29.7 28.7 28.8 Iron(II1) oxide added, mg 20 60 140 200 400 900 50* 1 oo* 500* Sulphide recovered, mg 30.6 26-9 22.1 17.5 14.4 13.9 24.9 21.7 15-7 Recovery, per cent.99.4 95.1 74.4 60-8 50.5 48-3 88-2 76-9 55.6 * Added as subsoil. The iron(II1) oxide used to obtain the results given in Table I was a synthetic goethite that dissolved readily in dilute acid, and it might therefore have caused a bigger proportional error than the less readily soluble iron(II1) oxide of a soil. However, when subsoil from the Rothamsted farm was substituted for the laboratory preparation, the loss of sulphide was not appreciably smaller. The error caused by iron(II1) oxide is thus considerable; the failure of Chaudhry and Cornfield1 to recover more than 60 per cent. of sulphide added to air-dried soil can presumably be ascribed to this effect. Because of the lack of any method for determining the iron(I1)-to-iron(II1) ratio in a reduced soil it is impossible to generalise, but the few determinations we have made of the sulphide contents of field samples suggest that the largest iron( 111) oxide-to-sulphide ratios in Table I are probably those most relevant to actual soils.As the concentration of iron(II1) oxide decreases during the anaerobic incubation of a soil, the extent of the error would vary throughout our experiments, and the results obtained by this method would be quite meaningless. Attempts to limit the interference by adding metallic zinc or tin to the sulphide before acidification failed, but by adding tin(I1) chloride to the acid used to decompose the sulphide (about 2 g per 25 ml), the loss of sulphide was largely overcome (Table 11). When water was added to the sulphide - iron(II1) oxide mixture, satisfactory recoveries were obtained when the concentration of the acid used to decompose the sample was increased to give a final value of about 5 N.534 PRUDEN AND BLOOMFIELD TABLE I1 THE RECOVERY OF HYDROGEN SULPHIDE IN THE PRESENCE OF IRON(III) OXIDE WITH TIN(II) CHLORIDE - HYDROCHLORIC ACID Sulphide taken, Iron(II1) oxide taken, Sulphide recovered, Recovery, mg mg m g per cent.30.6 45 30.4 99-5 29.2 100 28.9 99.0 29.2 200 29.2 100.0 29.4 900 28.6 97.5 28.7 60* 28.4 99.0 The effect of the tin(I1) chloride reagent on other sulphur compounds present, or likely to be present, in the incubated reaction mixture was tested. Thiosulphate was reduced quantitatively to sulphide, but the effect on sulphate and elemental sulphur was negligible. Pyrite reacts slowly, thus causing a positive error. It is unlikely that pyrite is formed in our experiments, but, as it is often the chief sulphur compound in sulphidic soils, it could cause serious error when dealing with a natural soil. In testing sulphate, sulphur and pyrite, the hydrogen sulphide was absorbed in sodium hydroxide solution and titrated with 0*001 N mercury(I1) acetate, with dithizone as indicator.2 The tin(I1) chloride reagent has been used for several years in our work on sulphate reduction. The iodate method described above is unsuitable for the amounts with which we are concerned; instead, hydrogen sulphide is absorbed in 25 ml of 0.05 N hypochlorite solution (0.4 N with respect to sodium hydroxide), and the excess of hypochlorite determined iodimetrically . REFERENCES * Added as subsoil. 1. 2. Chaudhry, I. A., and Cornfield, A. H., PI. Soil, 1966, 25, 474. Archer, E. E., Analyst, 1956, 81, 181. Received November 23rd, 1967