Spectrophotometric determination of water colour in hazen units

Colour is a useful index of dissolved humic substances in water. The reference method for colour determination is a visual comparison of samples with reference solutions of hexachloroplatinate, the Hazen method. The precision of this method is poor in relation to common drinking water criteria. A spectrophotometric method with better precision that gives results on the same scale as the Hazen method is described in this paper. Filtration is crucial before spectrophotometric measurement. We found no effect of filtration on true colour. The method has been validated through interlaboratory comparisons. Forty laboratories using this method obtained the same mean values as 20 laboratories using visual judgment. The standard deviations of spectrophotometric measurements were half the values of visual determinations.