578 THE CHEMISTRY OF THE URINE. 
is first determined in the original mine ; 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 
aridity of the urine is to be called 54; if in another case it is only -7 
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.r.), 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 (epiadriurates). 
Kow 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 arid "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. N< i 
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, haematoporphyrin had happened to be our only available 
" indicator," we should have said that urine was normally an alkaline fluid ! 
The whole source of the difficult}' 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, has 
to be allowed for, due to a conversion of monohydrogen into dihydrogen phosphate in the 
process of precipitation. 
2 Cf. Hausmann, Ztschr.f Ilia. Mel. Berlin, 1896, Bd. xxx. S. 350. 
