None of the published methods for the determination of total sulphur in soil has proved to be completely satisfactory in combining accuracy, sensitivity, speed and ease of manipulation.
Methods based on the oxidation of sulphur to sulphate are tedious'*2 and when sodium peroxide is used difficulties arise due to frothing and spitting. Such methods involve extraction steps and often considerable analytical skill is required to recover all the sulphate.
Methods based on reduction of the soil with metallic iron+5 or metallic magnesiurn'll' ancl estimation of the sulphide formed, avoid many of the difficulties associated with oxidation methods but are time-consuming. An additional difficulty encountered by the authors was that samples of iron obtained from various sources containecl approsimately 70 ,ug S/g, and as soils often contain less than 50 pg S/g it can be appreciated that mcthocls employing iron (e.g. LITTLER, 4.5 g Fe per 0.5 g soil) containing this amount of sulphur lack precision because of the high blank.
In this paper moclifications of a reduction mcthocl, and an oxidation method employing sodium bicarbonate are describecl and the results obtained by these two methods are compared with those given by a published oxidation methods. The bicarbonate method as applied to soil was extended to cover the determination of total sulphur in plant material, and the results obtainecl were compared with those derived by other published methodsueio.
Reagents
All chemicals used were of analytical reagent quality. Reducing mixtzrre. Boil 500 ml of hydriodic acid (sp. gr. 1.7). 250 ml of formic acid (go%), and 125 ml of hypophosphorous acid (50%) in a 1-1 flask under reflux for I h. Pass nitrogen gas through the boiling solution to remove any hydrogen sulphide produced. This mixture was prepared as suggested by JOHNSON AND ARICLEY~~ except that a longer boiling period is recommended.
Zinc acetate-sodium acetate sohction. Dissolve 50 g of zinc acetate and 12.5 g of Am!. G/rim. llctn, 27 (1962) 158-164