Determination and measured value of hydrogen peroxide in soft drinks containing ascorbic acid

In the determination of hydrogen peroxide in soft drinks containing ascorbic acid by using the oxygen electrode method, it was necessary to adjust the sample to the optimum pH for catalase with phosphate buffer. Various methods for mixing the sample with phosphate buffer were studied with 11 kinds of commercial soft drinks. Two of those methods were as follows: (i) the sample was put in the sample cell of the apparatus with the oxygen electrode after purging dissolved oxygen from the phosphate buffer; (ii) the sample was mixed with phosphate buffer and dissolved oxygen was purged from this sample solution in the cell. The released oxygen was then measured after addition of catalase. By the former method, hydrogen peroxide was not found in any samples. The latter gave hydrogen peroxide contents of 0.2-4.7μg/ml. The reason for this was that hydrogen peroxide was formed by oxidation of ascorbic acid before the addition of catalase during mixing of the sample with phosphate buffer.