A rapid, precise method for simultaneous measurement of ferrous and ferric iron concentrations in drainage and other waters is reported. The determination of both ferric and total iron concentrations are made in the same solution at different light wavelengths, which significantly reduces the error of the analytical procedure. The method is based on the colorimetric measurement of red-colored ferric-sulfosalicylate complex formed at low pH. After the addition of ammonia which leads to the increase in pH, the 5-sulfosalicylic acid (SSA) forms a yellow complex with all the iron ions in solution. The wavelengths of maximal light absorbance for both these complexes were determined. They were 500 and 425 nm, respectively. The relationship between both the ferric and total iron concentrations and the light absorbance obeys Beer's law under the conditions studied. The optimal ratio and volumes of the reagents used were determined. The effect of the ions usually found in mine drainage waters along with iron ions was studied. The cultivation media for the iron-oxidizing bacteria Thiobacillus ferrooxidans and the bacterial mass itself were found to have no effect on the iron determination. It was found that the color complexes were stable for a long time (at least 48 h). (C) 2002 Elsevier Science Ltd. All rights reserved.