Abstract
In an endeavor to develop a microchemical method for the determination of uric acid and purine bases it was found that uric acid was quantitatively precipitated, when in solutions as dilute as 1/10,000 by a modified Salkowski reagent. Graves and Kober, for nephelometric determinations, suspended similar silver purine precipitates in egg albumin. This procedure was found unsatisfactory. Efforts to maintain a uniform suspension by reducing the surface tension and augmenting the viscosity were also unsuccessful. In the absence of other salts it was found, however, that a silver-urate-precipitate remained uniformly suspended for one hour, which permitted satisfactory nephelometric determinations.
Technique for Determinations:
(Reagent. 40 cc. of a 5 percent ammoniacal silver nitrate solution is placed in a 250 cc. volumetric flask with 40 cc. of either 5 percent ammonium chloride or 8 percent sodium chloride. Ammonia is then added until the silver chloride is dissolved plus an excess of 10 cc., and the whole made up to volume.)
Uric acid, in a lithium carbonate solution, is delivered in amounts ranging from 0.1 to 2 mg. into centrifuge tubes, to these are added 5 to 10 cc. of the reagent. The resulting precipitate is separated by centrifugalization, the liquid drained off and the inner lip of the tube dried with filter paper. Concentrated HCl is then added drop by drop until the silver urate is entirely in solution. The addition of water to this mixture produces a turbidity which is quantitative for the amount of uric acid present. The dilution must be made in a strictly uniform way.
The most satisfactory method is to allow the desired amount of water (15, 20 or 25 cc.) to rush in suddenly from a burette.
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