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The principal difficulty which obtains in the use of Fehling's solution is the determination of the end-point of the reaction, that is, the point at which the copper has been just completely reduced. Heretofore, we have usually relied on our judgment as to whether the blue color had entirely disappeared from the more or less clear supernatant fluid after the precipitate had settled.
This, I contend, is not a reliable method of determining the end-point. We can often demonstrate the presence of unreduced copper in what is apparently a colorless or rather "blueless" supernatant fluid. Several means of fixing the end-point have been suggested, but they are all more or less cumbersome or time-consuming, especially for ordinary clinical work.
The method which I propose has proved very satisfactory and has the merit of being simple, rapid and fairly accurate. The principle on which it depends is that of separating the
Hunter JW, Taylor K. A NEW METHOD OF USING FEHLING'S SOLUTION. JAMA. 1912;LIX(6):441. doi:10.1001/jama.1912.04270080123016