CH. XII.] ESTIMATION OF TITRATABLE ACID. 277 



Method. 



Measure 20 cc. of the urine into tubes (2), (3) and (6). 



Measure 20 cc. of the buffer P H = 7*42 into (i). 



Measure 20 cc. of the buffer P H = 7-47 into (5). 



Place about 20 cc. of water into (4). 



Add 10 to 15 drops of 0-02 per cent, phenol red to (i), (3) and 

 (5), using a dropping pipette (fig. 5) and adding exactly the same 

 amount to each tube. 



Mix the contents of the tubes by smartly rotating them be- 

 tween the palms of the hands. 



Titrate (3) with the standard soda. A precipitate of earthy 

 phosphates may appear and the colour as seen through Y gradually 

 approaches that seen through X. Let the amount of soda added 

 be (a) cc. 



To (i) and (5) add (a) cc. of distilled water from a burette or 

 pipette. 



To (2) and (6) add (a) cc. of the standard soda. Read the 

 burette. 



Complete the titration of (3) until the colour as seen through 

 Y is intermediate between that seen through X and Z. The tubes 

 must be spun just before the observation is made to ensure an equal 

 distribution of any precipitate. Let the amount of soda required 

 for this operation be (b) cc. 



Calculation. 



20 cc. of urine require (a) + (b) cc., say (A) cc. of the soda, 

 which is (c) times Normal. 



So 100 cc. require (A) x 5 cc. of the soda. 



So 100 cc. contain (A) x 5 x 10 x (c) cc. of o-i N. acid. 



NOTES. i. It is important that the examination be made in fresh 

 urine. Should this be impossible a little toluol should be added to the speci- 

 men to prevent the ammoniacal fermentation of the urea. 



2. Should the urine be so concentrated that a precipitate of urates 

 separates out, the urine may be diluted with an equal volume of distilled 

 water and the mixture gently warmed till it clears. It is then cooled under 

 the tap and the estimation made as described above, allowance being made for 

 the dilution in the final calculation. 



3. By a similar method, enzyme solutions, digestion mixtures, etc., can 

 be brought to any desired hydrogen-ion concentration. Suitable buffer 

 solutions and indicators can be prepared according to the directions given 

 in pages 22 to 28. 



