Analysis of sulfides in the presence of ferric minerals by diffusion methods

Ferric minerals are commonly found in soils and sediments, especially in tropical areas. Previous studies have shown that ferric minerals in sediments can cause underestimation of acid-volatile sulfides (AVS). No reliable AVS analytical procedure has been recommended for modern sediments containing significant concentration of Fe(III). We evaluated the interference of acid-soluble ferric minerals on the analysis of sulfides using cold diffusion procedures and the effectiveness of two amendments, i.e., SnCl2 and ascorbic acid (AA), respectively, to prevent the interference. The presence of acid-soluble ferric minerals caused various degrees of AVS underestimation depending on the level of acid-soluble Fe(III) and the nature of a sample. Addition of 2 ml 1 M AA to the sample effectively protects AVS from oxidation in the presence of 0.39 mmol acid-soluble Fe(III). Addition of 1 g SnCl2 powder to the sample did not prevent the ferric interference on AVS. The presence of 0.39 mmol acid-soluble Fe(III) caused 10% and 35% loss of disulfides in a specimen pyrite, respectively, during an 18-h and a 48-h AVS diffusion procedure. The AA amendment also protected pyrite from oxidation in the presence of acid-soluble Fe(III) during a sequential diffusion procedure to separate AVS and disulfides.