Folin and Denis’s Method

This colour reaction is so sensitive that one part of uric acid in a million parts of water can be detected. Moreover, unlike the older methods which required from 75-100 c.c. of blood or more, determinations can be made with 20 c.c., and if the blood be rich in uric acid only 10 c.c. Walker Hall observes that the procedure “has many advantages and does not take up much more time than some of the qualitative methods when once the technical difficulties are overcome.” He described it as follows:—

Twenty cubic centimetres of blood are withdrawn into a wide-mouthed, tared bottle containing 0·1 gramme of finely-powdered potassium oxalate. The flask and contents are then weighed. Five times the weight of n/100 acetic acid is heated to boiling. The oxalated blood is poured into the boiling acetic acid solution, and the heating continued until the solution has begun again to boil. The mixture is filtered hot. The clear filtrate and wash waters are acidified (0·5 c.c. of 50 per cent. acetic acid) and evaporated to 3 c.c. Five drops of a 3 per cent. silver lactate solution, two drops of magnesia mixture, and ten to fifteen drops of strong ammonia hydrate are next added. The mixture is centrifugalised. The supernatant fluid is removed. To the residue five drops of freshly-saturated hydrogen sulphide water and one drop of strong hydrochloric acid are added. The tube is placed in a beaker of boiling water for ten minutes in order to remove the hydrogen sulphide. The supernatant fluid is added to 2 c.c. of a solution containing 100 grams of sodium tungstate and 80 c.c. of 85 per cent. phosphoric acid in 1,000 c.c. of water and 10 c.c. of a saturated sodium carbonate solution. The resultant blue solution is then compared with a standard uric acid solution, and the result obtained by the following formula:—

(20V)/(RW) mg. of uric acid per 100 grams blood,[20]

where 20 represents depth in millimetres of standard solution,