Diabetes: its pathological physiology / by John J.R. Macleod.

  • Macleod, J. J. R. (John James Rickard), 1876-1935.
Date:
1913
Catalogue details

Licence: In copyright

Credit: Diabetes: its pathological physiology / by John J.R. Macleod. Source: Wellcome Collection.

    22/252
    either by weighing (Pfluger-Allihn method, 1) or by titration (Bertrand method, 2*), which is the most accurate method for simple solutions of dextrose, we cannot depend on the results, because of the property which certain constituents of urine possess of dissolving cuprous oxide. In the titration methods of Pavy and Purdy, this source of error is in part surmounted by adding to the reagent either glycerine or ammonia, which com- pletely dissolves the cuprous precipitate, and therefore makes it possible to determine, with tolerable accuracy, when all of the cupric salt has been reduced. These methods are not of sufficient accuracy for measuring the reducing power of normal urine, for the most serious objection to their use depends on the employment of a fixed equation for calculating how much dextrose a certain amount of reduction corresponds to. This is inaccurate, because no allowance is made for the fact that the degree of reduction, produced by a given amount of dextrose, is considerably greater when there is an excess of cupric salt present than when there is only a small amount. Not only should there always be an excess of the alkaline cupric solution present, but specially prepared tables should be em- ployed for calculating from the degree of reduction what amount of dextrose is present. Such tables are used in the Pfhiger- Allihn and Bertrand processes. One of the substances in urine which redissolves some of the cuprous oxide is ammonia, which is partly produced by the action of the alkali in the copper solution. In all the above methods the copper solution contains a large amount of caustic alkali, and this, besides producing ammonia when the reagent is boiled with urine, has also the property of destroying some of the dextrose (see p. 16). To avoid these effects of the alkali, Bang (3) has introduced a titration method in which the copper solution contains sulphocyanides and car- bonates, but no caustic alkali. When reduction occurs, cuprous sulphocyanide is formed, and goes into solution, the degree to which reduction has occurred being then determined by titrating with a standard solution of hydroxylamin, which reduces the unreduced cupric salt. From specially prepared tables the * In the Bertrand process, the cuprous oxide precipitate is dissolved in a solution of ferric sulphate in sulphuric acid. During the process of solution, an amount of ferrous salt becomes produced, which is equivalent to the amount of cuprous oxide. The ferrous salt is then measured by titrating with a standardized solution of potassium permanganate. It is the most reliable method for simple solutions of sugar.