Soil and sediment samples were digested in HF and Si was determined by the ammonium molybdate‐oxalic acid procedure. Results were compared with three Standard Reference Materials from the United States Bureau of Standards. It was found that more than 0.05 % HF in the digests significantly suppresses Si color development. The ratio Si/Fe was more important than absolute Fe concentrations, and Si/Fe below 1 intensified Si color development. It is recommended that HF and Si/Fe should be maintained at below 0.05 % and 1, respectively.