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 increase in 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 an)' appreciable extent. 
AVhen 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 acidimetry 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.,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. 
