Purification of activated charcoal for decolorizing soil extracts

Activated charcoal is purified by boiling it twice with concentrated hydrochloric acid and then leaching with sodium chloride. The prepared charcoal is practically free from phosphate extractable by customary soil extractants. The use of 0.1 g. or less of this charcoal for decolorizing 25 ml. of filtered soil extracts is advised. The adsorption of ions by the treated charcoal from solutions in distilled water and 0.002N‐H2SO4 was investigated. Ammonia, nitrate, phosphate, potassium, calcium, magnesium and manganese were adsorbed to various extents from aqueous solutions. In acid media, the adsorption of ammonia was negligible; that of phosphate, potassium, magnesium or manganese was insignificant and that of nitrate was further intensified. Addition of charcoal at the rate of 0.1 g. per 25 ml. removed all colour from 2.5% acetic acid extracts and Morgan extracts of ordinary field soils, but did not successfully decolorize 1% citric acid extracts.