48 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



and the alkaline salts, M 2 HP0 4 , in blood is approximately 1 to 5, but in 

 the urine this ratio varies according to the amount of H ion that must 

 be eliminated from the blood. In other words, a definite amount of phos- 

 phoric acid is enabled to carry variable amounts of H ion out of the body 

 by causing the amount of alkali excreted in combination with it to be- 

 come altered. For example, in the form of MH 2 P0 4 a given amount of 

 P0 4 obviously carries out more H ion than when it is excreted as 

 M 2 HP0 4 . The adjustment between these two salts is a function of the 

 kidney. We may accordingly measure the amount of alkali retained by 

 the organism by finding how much standardized alkali must be added 

 to a given quantity of urine until the reaction of the blood is obtained. 

 Since the latter value is constant, the titration can be done simply by 

 titrating the urine with an indicator such as sulphonephenolphthalein, 

 which changes tint at about P H of blood. 



A more serviceable indicator to use, however, is phenolphthalein, be- 

 cause its end point is such that when human urine just reacts neutral 

 to it that is, when the titrable acid approaches zero the C0 2 -absorb- 

 ing power of the plasma is at its maximum of 80 vols. per cent and the 

 ammonia excretion by the urine is zero (Van Slyke). It is advantageous, 

 therefore, to use this indicator, because it happens to have its turning 

 point situated for a reaction which is well to the alkaline side of neu- 

 trality, and which is reached in urine when the blood is at its maximal 

 acid-combining power and no ammonia is being used for neutralization 

 purposes. As the C0 2 -combining power of the blood decreases, there 

 should, therefore, be a proportionate increase in ammonia and in the 

 titrable acidity of the urine. 



3. Determination of Alkali Retention. Another valuable criterion of 

 the alkaline reserve is the amount of alkali required to change the re- 

 action of the urine. In health the C H of the urine varies from 

 0.000,016 N (P H = 4.8) to about 0.000,000,035 N (P H = 7.46) with a mean 

 of about 0.000,001 N (P H = 6). These extremes are rarely overstepped 

 in disease, but frequently the average is considerably different. In car- 

 dio-renal disease, for example, the mean acidity may be approximately 

 0.000,005 N (P H = 5.3), or five times the normal value, A certain de- 

 gree of acidosis is therefore common enough in this condition a fact 

 which has indicated the advisability of administering sodium bicarbon- 

 ate. It has been found that 5 grams or less of soda, given by mouth to 

 a normal person, causes a distinct diminution in tHe C H of the urine, 

 whereas in pathologic cases it may be necessary to give more than 100 

 grams before a similar effect is observed (L. J. Henderson and Palmer 15 

 and Sellards 16 ). 



This test has been found of particular value in the diagnosis of acidosis 



