The direct colorimetric estimation of reducing sugars and other reducing substances with tetrazolium salts.

Since Kuhn & Jerchel (1941) showed that tetrazolium salts were reduced by bacteria, yeasts and germinating seeds, 2:3:5-triphenyltetrazolium chloride or bromide has come into prominence as a viability testing reagent (Cottrell, 1947) and for vital staining (Mattson, Jensen & Dutcher, 1947). Its specific advantages for this purpose and for the purposes which follow lie in its Eo value of -008 V. (Jerchel & Mohle, 1944) and, more especially, in the fact that it is colourless in the oxidized form and an intense cherry red colour in the reduced form. This latter peculiarity gives it a measure of uniqueness among organic compounds.* In oxidized form the compound is soluble in water; after reduction, which with sugars only takes place in alkaline solution, it becomes insoluble in water but soluble in many organic solvents. Recently Mattson & Jensen (1950) have described its use in a method for the estimation ofreducing sugars. The advantage ofsuch a method is obvious as the substitution of a direct colorimetric method for titration or for an indirect colorimetric or gravimetric method can lead to improved accuracies and a saving of time. Apart from a general investigation of the properties and behaviour of the reagent, four possible specific applications were chosen-estimation of reducing sugars, of blood sugar, of cysteine and of ascorbic acid. Before any specific methods could be elaborated for the colorimetric measurement of reducing substances it was necessary to establish the general characteristics and stoicheiometrical relations of the reagent in both oxidized and reduced forms.