Properties of Uric Acid
When pure, uric acid is white in colour and crystallises in rhombic form. In contrast to urea it is very insoluble, but much less so in blood serum than in distilled water, viz., ⅟₄₀₀₀₀ of water as opposed to ⅟₁₀₀₀ parts of plasma. It yields with alkalies two series of salts, viz., the biurate or mono-basic, and the so-called neutral or bi-basic urate, the latter of which is much more soluble. In water the mono-basic urate forms a colloidal solution from which the crystalline salt gradually precipitates.
The greater solubility of uric acid in blood plasma was, by Garrod and Haig, attributed to the alkalinity of the plasma. But it must be recalled that the earlier workers in this sphere judged of the alkalinity of the plasma by its reaction to litmus, a crude procedure as compared with the use of phenol-phthalein, and Frankel’s electro-potential measurements. Working with these as criteria, it has been shown that blood is normally alkaline in only a minority of cases, and indeed, according to Flack and Hill, the plasma is in reality neutral.
In the urine uric acid and the urates are held in solution by the neutral phosphates. This because the decomposition of the urates into uric acid by the acid salts of the urine is inhibited by the di-sodium phosphate present therein. Its maintenance in solution is possibly also reinforced through the influence of other constituents in the urine, notably, the urinary pigments and sodium chloride.