576 THE CHEMISTRY OF THE URINE. 



of the acid products of metabolism what will be the effect on the dis- 

 tribution of bases ? It has been shown experimentally, that if to a 

 mixed solution of mono- and di-hydrogen phosphates, a mineral acid 

 (such as sulphuric acid) be added, in quantity not greater than is 

 equivalent to the bases present in the monohydrogen form, no free acid 

 is afterwards found in solution; but there will be an increase in the 

 dihydrogen phosphates at the expense of the monohydrogen phosphates 

 in proportion to the amount of acid added. Not only is this true of 

 sulphuric acid ; it has been shown that all the weaker acids or acid salts 

 which are liable to reach the urine from the circulation (e.g. hippuric 

 acid or acid oxalates) are able, when added to a solution of the mixed 

 phosphates, to remove base from the monohydrogen form, and so to 

 produce almost an equivalent increasein the acid phosphates. So long, 

 therefore, as both these types of phosphate exist side by side (and they 

 are always found together in acid urine), we can assume that the 

 acidity of the fluid is due to the acid phosphate, and practically to that 

 alone. The simultaneous existence of the monohydrogen form will 

 be seen to be a guarantee of this, as it will have to disappear by inter- 

 change of bases, before any other urinary constituent can begin to exert 

 its own proper acidity to any appreciable extent. 



When the urine reacts alkaline to litmus, the alkalinity may under 

 different circumstances be due (1) to excess of basic phosphates, (2) to 

 carbonates of the fixed alkalies, or (3) to ammonium carbonate. 



Determination of the acidity. 1 It is, as we have seen, not difficult 

 to assign the acidity of the urine to its proper cause ; but when we 

 endeavour to discover a method by which to estimate the degree of 

 acidity, and especially a mode in which to express its value numeric- 

 ally, we meet with considerable difficulties. 



In the case of a fluid the acidity of which is due to a strong acid, 

 capable of forming stable salts with the alkalies, the ordinary methods 

 of acidirnetry yield a determinate result, and the estimation of acidity 

 is one of the simplest operations in chemistry. We have but to note 

 the amount of a standardised solution of alkali which is sufficient 

 exactly to neutralise the acid present, and the point of neutralisation is 

 given sharply and exactly by the colour change which occurs in the 

 presence of one of many available indicators. In urine, owing to the 

 unstable phosphate equilibrium, and the presence of other salts which 

 influence the result, the process is much less determinate. To litmus, 

 as already stated, a dihydrogen phosphate, e.g. NaH 2 P0 4 , is acid, while 

 Na 2 HP0 4 and Na 3 P0 4 are alkaline ; but no mixture of these salts can 

 be found which is, strictly speaking, neutral to litmus paper. 



If we start with a urine acid to litmus and gradually add alkali, 

 we at last reach a point when the fluid shows a paradoxical behaviour. 

 It makes red litmus paper tend to blue, and blue paper tend to red, 

 inducing in fact a somewhat violet colour in both. It reacts at once 

 acid and alkaline. This occurs when the monohydrogen phosphates, 

 which during the addition of alkali are gradually increased at the 

 expense of the dihydrogen salts, have come to bear a certain proportion 

 to the latter. 



Many urines exhibit this so-called amphoteric reaction without the 



1 I have discussed this subject at what may seem disproportionate length, but the pro- 

 blem involved illustrates well the complexity of chemical conditions in the urine ; and 

 much has been written upon it of late on what I venture to believe are erroneous lines. 



