Marcu 31, 1923] 
NATURE 
435 

degree inversely proportional to the so-called specific 
inductive capacity of the medium. Water has one of 
the highest specific inductive capacities of all known 
substances, so that in it the attraction between two 
ions is relatively small: hence in water the ions may 
separate more effectively than in other solutions, and 
watery solutions are found to show the phenomena of 
electrolytic dissociation to an exceptional degree. Now 
water is a solvent of unique importance, partly because 
of its common occurrence, partly because it dissolves 
so many other bodies, and especially because, without 
exception, all biological phenomena occur in media 
which are essentially solutions or suspensions in water. 
Hence the study of the electrolytic dissociation of 
bodies dissolved in water is of quite peculiar interest, 
especially in physiology. 
Now water itself is capable of electrolytic dissocia- 
tion, though only to a small degree. In pure water at 
22° C., eighteen parts in ten thousand million, that is, 
one ten-millionth part of one gram molecule per litre, 
is broken up into hydrogen (H") and hydroxyl (OH’) 
ions, the * denoting the positive and the ’ the negative 
charge. Such a very small degree of dissociation is 
f little importance in pure water: its insignificance 
presumably due to the smallness of what we have 
alled—to cover our ignorance—the dynamic forces 
tending to separate H,O into H* and OH’. In solu- 
tions, however, especially in solutions of acids and 
alkalies (that is, of bodies capable, by their own dis- 
ciation, of yielding one of the ions of water, H* or 
HH’), even this small dissociation of water into its 
ions may become of preponderant importance. 
It is obvious that the ions of the solvent itself, if 
present in appreciable amount, might be expected to 
lay a special réle in the behaviour of a solution: 
there is, however, a very real interest in the study 
of the hydrogen ion, in view of modern theories of the 
electrical constitution of matter. Atoms are supposed 
to possess a positive nucleus, with a charge equal to 
some multiple of the elementary negative charge on 
an electron, with layers of electrons circulating round 
the nucleus in stable orbits. The simplest atom of 
is hydrogen, with a positive nucleus of unit ele- 
entary charge and a single negative electron revolving 
round it: remove this negative electron from a dis- 
solved hydrogen atom, and we are left with a singly 
charged positive nucleus—next to the electron the 
simplest of all known natural bodies. In mobility, 
in combining power, in general dynamic effectiveness, 
this dissolved elementary unit might be expected to 
be, and actually proves to be, an agent of quite peculiar 
importance. 
Expressing concentrations, in gram molecules (or 
ions) per litre, by means of brackets, it is found that 
at 22° C. in pure water, 
[H"][(OH’]= 10-4. 
This is the law of chemical mass action, which, in such 
a dilute solution as water is of its own ions, is accurately 
obeyed. Now in pure water there is no other agent 
capable of carrying electricity, and since the water 
itself cannot carry an appreciable resultant charge the 
positive and negative charges must balance one 
another, and therefore 
[H"]=[OH’]= 10-7, 
NO. 2787, VOL. 111] 









































If, however, we dissolve in the water another substance 
supplying one of the ions of water, for example, hydro- 
chloric acid (HCl), which we may regard as being 
almost totally dissociated into its ions H’ and Cl’, to 
a concentration (say) of one gram molecule per litre, 
then the equation above is entirely upset : the hydrogen 
lon concentration [H"], or c.H as we shall often call it, 
has now become unity instead of 1077, so that the 
hydroxyl ion concentration [OH’] is now only 10-4, 
Even this, expressed in actual molecules, is an astonish- 
ingly large number : there are about 6 x 103 molecules 
in a gram molecule, so that even in normal hydro- 
chloric acid there are six million hydroxyl ions per 
cubic centimetre. Clearly, even a strong solution of 
acid contains an appreciable number of hydroxy] ions. 
If, conversely, we dissolve caustic soda to make a 
normal ” solution, instead of hydrochloric acid, then 
[OH'] becomes unity and [H"*] becomes 10-4. We 
may make up different strengths of acids or alkalies in 
which the hydrogen and hydroxyl ion concentrations 
Acid. 
{H’). [OH’). Alkali. (HJ. [OH’). 
N I ro714 ro-%4 I 
N/1o to-! to7}8 N/1o Lose to-1 
N/roo to-? to-#? N/to0o ror™ to-? 
N/rooo 10-8 tons N/1000- 10-14 To>8 
N/1oooo 10-4 noe N/1o000 10-10 10-4 
may be calculated as in the accompanying table. It is 
usual to consider only the hydrogen ion concentration : 
the hydroxyl ion concentration may always be calculated 
from it, by dividing the quantity & in the equation 
fH"J[OH']=k by [H']. At 22° C., k=107™, but it 
varies slightly with temperature. Now [H’], or c.H, 
may change enormously from one solution to another, 
say from 10~1*to 1, that is, one hundred million million 
times: hence it is impossible to represent the full 
possible range of variation of ¢.H in a single diagram, 
and since it is often necessary in physical chemistry to 
show the relations of c.H graphically, it has become 
customary to express the hydrogen ion concentration 
in terms of logarithms. The logarithm of ro-!4 is — 14, 
and of 1 is o, so that log c.H can be represented, over 
almost the entire possible range, by numbers lying 
between o and —14. To avoid, further, the use of 
negative numbers the negative sign is understood, and 
the symbol #.H (or its variants P,,, P,, etc.) is defined 
by the expression p.H=—log c.H. In this way, at 
22° C., if p.H=7 the solution is neutral, if ~.H be less 
than 7 the solution is acid, if .H be greater than 7 the 
solution is alkaline; and a decrease of ~.H means an 
increase in hydrogen ion concentration. 
This system of nomenclature has certain obvious 
advantages if used with discretion: not seldom, how- 
ever, it lends itself to obscuring the fact that the real 
agent at work is the actual hydrogen ion concentration 
¢.H ; it is difficult enough even for the expert to picture 
a quantity in terms of its negative logarithm, and it 
leads to confusion and suspicion on the part of the 
inexpert and beginner. For most of the phenomena 
of biology, moreover, which occur within a narrow 
range of c.H, it is quite unnecessary : for example, in 
physiology, apart from a few cases of secretion, the 
important range of ¢.H in the body is from 10~? to 10-8, 
and it is better when possible to deal with the hydrogen 
ion concentration in multiples (or decimals) of 10~7, 
and to use the ».H notation only when the total range 
