Field determination of trace molybdenum in river-water samples by a visualcatalytic method

A sensitive visual colorimetric method has been developed for the semiquant itative field determination of trace molybdenum. The molybdenum-catalyzed o xidation of ascorbic acid at pH 3.2 in the presence of o-phenylenediamine w as used as the indicator reaction, which produces yellow quinoxaline deriva tives. After a fixed reaction time, the reaction is stopped by readjusting pH to 1 with hydrochloric acid. For the visual determination, the color int ensity of the final solution (10 ml) for a water sample is compared to that of a color standard solution containing 0.04 mu g (4 mu g l(-1)) of Mo-VI prepared by the same procedure. Two handmade cells of the same size with 10 -, 20-, 30- and 40-mm light paths are used in the color comparison for a wi de determination range (0.005-0.2 mu g). The intensity of the color standar d is held constant by the adjustment of the reaction time, for example, 10 min at 25 degrees C, with the use of a simple relationship between the reac tion time and the field temperature. Molybdenum down to 1 mu g l(-1) in a 4 -ml river-water sample was determined without any special instrument. Analy tical performances were evaluated and compared with those obtained by the s pectrophotometric measurements. The application to field survey has reveale d the distribution of molybdenum concentration along the river streams and a polluted point. (C) 1999 John Wiley & Sons, Inc.