Abstract
The titrimetric method for the determination of blood sugar proposed by Maclean 1 in 1919 has been used with success in a number of researches both abroad and in this country. Its simplicity, accuracy and the small amount of blood required recommend it to those who prefer titrimetric to colorimetric quantitative determinations. Briefly, the principle of the MacLean method is:
1. Removal of the proteins by heating oxalated blood with acidified sodium sulphate, adding colloidal iron, and filtering.
2. Boiling, under standardized conditions, an aliquot part of the filtrate with a known amount of an alkaline copper solution containing potassium iodide and potassium iodate.
3. Titration of the iodine liberated upon acidification with standard thiosulphate.
Since, under the conditions defined by MacLean, the cuprous salt reduced by the sugar is oxidized by the iodine liberated, the excess iodine titrated with thiosulphate is inversely proportional to the amount of reduction. Empirical factors can, therefore, be determined converting the volume of thiosulphate used into concentration of glucose.
It was found, however, that by MacLean's method the determinations of sugar in freshly drawn blclod to which no anticoagulant had been added gave values distinctly higher than those determined on the same blood which had been oxalated,
This fact led to a comparison of sugar determinations made on pure glucose solutions with and without the addition of oxalate, with and without the use of colloidal iron. The amount of oxalate required to prevent coagulation was about 2 mg. per c.c. It was found that the presence of the oxalate resulted in too low sugar values, i.e., too much thiosulphate was used in the titration, only in case colloidal iron was used. Experiments in which the amount of colloidal iron was progressively increased from 0 to 3 c.c. showed that increasing amounts of thiosulphate were required for titration.
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