MANY metals form coloured complexes with dithizone, and the successful use of this reagent for estimating one of them in a mixture demands a careful selection of organic solvent, a rigorous control of pH and other variables such as duration of shaking, nature and amount of neutral salts present, the concentration of excess dithizone, and the concentration of ‘masking' reagents capable of competitive complex formation with interfering metals1. The coloured metal complexes (yellow, red, violet, etc.) modify the colour of the green solution containing excess dithizone giving a ‘mixed-colour' extract, and by absorptiometric measurements the proportion of metal complex can be estimated with reference to calibration curves obtained by treating known amounts of metal by the standardized procedure. Such curves are commonly valid only for one initial concentration of dithizone, and error may be introduced in view of the well-known impermanence of this reagent. The theoretically obvious solution for such systems with two coloured components is to measure the transmittency at two appropriate wave-lengths ; but this involves a burdensome amount of preliminary work for, in effect, a family of calibration curves must be constructed to cover all possible variations in reagent concentration and in the amount of metal taken for analysis2.