578 THE CHEMISTRY OF THE URINE. 



is first determined in the original urine ; that existing as monohydrogen 

 phosphates is then removed by precipitation with barium chloride, and 

 that present as acid phosphates is finally determined in the filtrate. 1 



But how exactly are we to express the urinary acidity in terms of 

 the results so obtained ? 



Some recent writers have denoted the acidity by the figure express- 

 ing simply the ratio of acid phosphates to total phosphates. 2 If the 

 P 2 5 in the former be (say) 54 per cent, of the total P 2 5 , the relative 

 acidity of the urine is to be called 54 ; if in another case it is only 27 

 per cent., the acidity is to be considered as half that in the first case. 



Such a procedure seems to be wholly misleading. If of two 

 specimens of urine one contains twice as much acid phosphate as the 

 other, but at the same time twice the amount of the monohydrogen salt, 

 the acidity, expressed in the above manner, will be the same in each case. 



Such urines will certainly not behave as if of equal acidity, nor will 

 they indicate the same acid production within the body. 



We may here illustrate what we mean by the expression " behave as if of 

 equal acidity." One of the most important results of a high grade of acidity 

 is a tendency for the urine to deposit its uric acid in the free condition. In a 

 later section, dealing with the urates (q.v.\ the mechanism of this separation 

 will be discussed. We shall find that one essential step in the process con- 

 sists in the conversion of certain less acid urates (biurates) into more acid 

 urates (quadriurates). 



Now it is the acid phosphates which bring this change about, by removing 

 base from the first form of urate, themselves becoming, of course, converted 

 pari passu into more basic phosphates. But the latter, as they increase in 

 quantity, tend to yield back the base to the quadriurates, so that a point is 

 possible when the whole system will be in equilibrium. The less acid the 

 urine, the sooner is this point reached. A little consideration will show that 

 the " degree of acidity," from this point of view (and it is an important aspect), 

 will be a function both of the absolute amount of the acid phosphates, and of 

 the ratio they bear to the total phosphates. But we are hardly in a position 

 to express the acidity quantitatively in terms of these two factors, because we 

 do not know precisely at what stage the urates and phosphates are in equi- 

 librium. It is probable, in fact, that the point of equilibrium is different for 

 each of the diverse changes which may occur in the urine, as a result of its 

 acidity, just as it is different for the colour change in diverse indicators. No 

 more striking instance of the relativity of the phenomena involved could be 

 given than a fact we shall discuss under the head of the pigments. Urinary 

 hsematoporphyrin is always found in the so-called alkaline form ; and if we 

 add to any normal urine either neutral or acid haematoporphyrin, we find 

 that it immediately assumes the alkaline form. Equilibrium in this case is 

 only attained when base has been transferred to the pigment from the acid 

 phosphate. If, then, heematoporphyrin had happened to be our only available 

 " indicator," we should have said that urine was normally an alkaline fluid ! 



The whole source of the difficulty we have been discussing is found 

 in the fact that the terms " degree of acidity " or " degree of alkalinity " 

 are unscientific, though convenient, modes of expression. With increase 

 of knowledge, they will be replaced by expressions denoting the actual 



1 For the principles of this determination, see p. 633. An error of some 3 per cent, lias 

 to be allowed for, due to a conversion of monohydrogen into dihydrogen phosphate in the 

 process of precipitation. 



2 Of. Hausmann, Ztschr. f klin. Med., Berlin, 1896, Bd. xxx. S. 350. 



