ANALYSIS OF TISSUES 581 



The combined filtrate and washings are acidified with 5 c.c. of 50 per 

 cent, acetic acid and evaporated in a porcelain basin to about 3 c.c. This 

 liquid is poured into a centrifuge tube and the basin washed out with two 

 portions of 2 c.c. of 'i per cent, lithium carbonate, solid matter being loosened 

 with a rubber-tipped stirring rod. 



The liquid in the centrifuge tube is treated with 5 drops of silver lactate 

 solution, etc., and treated as described on p. 561, but after the hydrogen sul- 

 phide has been boiled off, the contents are poured into a beaker and the in- 

 side of the tube carefully washed without disturbing the precipitate with about 

 4 c.c. of water ; 2 c.c. of the uric acid reagent, and 10, 15 or 20 c.c. of sat- 

 urated sodium carbonate solution, depending on whether dilution is made to 

 25, 50 or 100 c.c., are added. This can be judged by inspecting the standard 

 made previously with i mgm. of uric acid and 2 c.c. reagent diluted to 100 c.c. 

 The solution can be filtered if necessary before comparing the colours. 



The result is given in mgm. uric acid per 100 c.c., or 100 gm., of blood by 



. 20 V 



the formula ^ , 



where 20 is depth in mm. of the standard, 

 where R is depth in mm. of the unknown, 

 where V is volume of dilution 1(25, 50 or 100), 

 where W is weight of blood taken. 



Estimation of Creatinine (p. 562). 



In collecting the blood for this determination excess of oxalate must be 

 avoided as potassium picrate may be precipitated. It should be measured out 

 from a 20 per cent, solution of which 10 drops are enough for 30 c.c. of blood. 



i o or 20 c.c. of blood are placed in a 50 or 100 c.c. measuring flask, or better 

 into a 50 or 100 c.c. stoppered measuring cylinder; the vessel is filled to the 

 mark with saturated picric acid solution 1 and the contents shaken a few times. 

 About i gm. of picric acid is added and the contents are shaken for 5 minutes. 

 This gives a saturated solution of picric acid. The mixture is put into centri- 

 fuge tubes and solution and precipitate are separated. 



If more blood be available, double the quantities can be taken and the pre- 

 cipitate filtered off. 



The filtrate is compared with a solution of -2 mgm. of creatinine in 100 c.c., 

 prepared by taking i mgm. of creatinine from the standard and diluting it to 500 

 c.c. in a measuring flask with saturated picric acid solution. This solution can 

 be kept for several determinations. 



10, 15 or 20 c.c. of the filtrate are compared with the same volume of this 

 dilute standard by adding i c.c. of 10 per cent, sodium hydrate to each and 

 allowing the colour to develop for 10 minutes. No further dilution is necessary. 

 The most accurate way of measuring out the alkali is by drops, which should be 

 determined for 5 c.c. and a fifth of this number taken. 



The standard may be set at 10, 15 or 20 mm. 



standard reading 

 reading ot unknown = m ^ m ' of eatmme per 100 c.c. of blood. 



1 Freshly prepared solutions of pure picric acid must be used (Folin and Doisy, J. Biol 

 Chem., 1917, 28, 349. 



