Abstract
Probably the best technique now available for the determination of lactic acid is that of Friedemann, Cotonio, and Shaffer 1 as modified by Friedemann and Kendall 2 and, more recently, by Friedemann and Graeser. 3 There are certain substances which interfere with the lactic acid determination; these have been listed in the original and some of the subsequent articles. Some of these substances, notably the sugars, may be removed from solution by the copper sulfate-calcium hydroxide procedure. 2 Appreciable amounts of others, however, remain in solution even after this treatment. Important examples of these substances are malic and citric acids, both of which give large yields of bisulfite-binding substances under the conditions used for the oxidation of lactic acid to acetaldehyde. In fact, by some slight modifications of the conditions it is possible to obtain very nearly a quantitative yield of acetaldehyde from malic acid (90 to 93%). 4 The product formed by the oxidation of citric acid is apparently acetone 5 and by the lactic acid method a yield of about 60% of the theoretical may be obtained. 4 However, both malic and citric acids may be quite completely removed from solution by means of basic lead acetate as follows: An amount of solution thought to contain 20-50 mg. of lactic acid and any reasonable amount of the interfering substance is placed in a 100 cc. volumetric flask, and diluted to a volume of about 50 cc. A drop of aqueous phenol red is added, then 10% NaOH is added drop by drop until the solution is neutral followed by one drop excess. A saturated solution of basic lead acetate (Haden) is added in 5 cc. portions until there is an excess; usually 5 cc. is sufficient and a large excess is to be avoided.
Get full access to this article
View all access options for this article.