Semi-micro Determination of Ferro Us Sulphide in Shales

A new method is described for determining in shales the amount of sulphides that dissolve in dilute hydrochloric acid. 1he samples are treated with hydrochloric acid under vacuum, and the hydrogen sulphide evolved is absorbed in potassium hydroxide and titrated with potassium _~'~ermanganate. The analytical results are expressed as FeS. INTRODUCTION During a study of the distribution of the sulphur content in Triassic black shales of the Sicilian basin, it has been observed that in addition to a considerable amount of pyrite (up to 6 per cent. by weight), there were present also small amounts of HCl-soluble sulphides that were easily detectable from the HEN evolved. An attempt to determine the total amount of these sulphides separately has led to the development of a new analytical procedure based on reaction of the samples with dilute hydrochloric acid under vacuum, absorption of the HzS in potassium hydroxide, and titration with potassium permanganate. In comparison with the method described by Scott (1956) this new procedure has the advantages of being more rapid and of requiring much smaller samples (only 50-150 mg as compared with l-5 g). I t can, therefore, be used in routine analysis of recent or ancient sediments even when, as often happens, only small samples are available. Moreover, preliminary treatment of the sample with hydrochloric acid enables better determination of pyrite by the nitric acid method previously described (Neglia, 1961). The filtered and washed residue after reaction with hydrochloric acid (the filter paper is not appreciably oxidized by nitric acid*) can be used directly for the pyrite determination. Interference due to sulphides, and to ferrous iron present as carbonate or silicate attacked by HC1, is thus eliminated. ANALYTICAL PROCEDURE A weighed portion of 50-150 mg of the pulverized sample is placed in flask A (Fig. 1), 50 ml of 5 per cent. potassium hydroxide solution poured into flask B and the various parts of the apparatus assembled *One no. 589 z 9 cm diameter Schleicher and Schull filter paper treated with a 32.5 per cent. by weight solution of nitric acid reduces only 0"02 ml 0"1 N KMnO4.