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Acid commentary on pH

One often hears the comment "If it ain't broke, why fix it?". A thoughtful and beautifully written piece [1] on the lack of understanding of pH tells us this is broke, and that it needs fixing. Try and answer these simple questions:

Question 1

If the pH of a solution is 7.40, what is the new pH:
(a) if the hydrogen ion concentration doubles? (hint - the logarithm of 2 is 0.3).
(b) if the hydrogen ion concentration increases 10-fold?

Question 2

If the hydrogen ion concentration of a solution is 40 nmol/L, what is the new hydrogen ion concentration:
(a) if the hydrogen ion concentration doubles?
(b) if the hydrogen ion concentration increases 10-fold?
The answers to the first question are 7.1 and 6.4. If you can't work out the answers to the second, then you are beyond help. For you to check it out, the Table and Figure give the relationship between hydrogen ion concentration in nmol/L and pH.
Relationship between hydrogen ion concentration and pH
Hydrogen ion concentration (nmol/L) pH
20 7.70
25 7.60
30 7.52
35 7.45
40 7.40
45 7.35
50 7.30
55 7.26
60 7.22
65 7.19
70 7.15
75 7.12
80 7.10
85 7.07
90 7.05
95 7.02
100 7.00
126 6.90
158 6.80

Clever medical students

When these questions were put to 430 medical students about to take their final pathology examinations, only 10% could get the answers to question 1 correct. Now don't forget that these are medical students who have, nowadays, to get virtually all A grades at A level, usually including mathematics, and that to get into medical school a Nobel prize or two helps. So if only 10% of them can get it right, what hope is there for the non-anaesthetists among us. The students found the answers to the second question trivial.


Most respondents to an external quality scheme (148/157) report pH rather than hydrogen ion concentration, though rather fewer (72%) of 22 medical schools do so. So pH lives on in most UK hospitals even if understood by few new doctors (and maybe even older ones?).

Veil of mystery

The point the paper makes, as others have made before, is that pH is an inverse logarithm, and is counter-intuitive. It is hopeless trying to make corrections against changes in carbon dioxide concentrations without complex algorithms. A nice turn of phrase likens pH to a "veil of mystery diverting attention from the leading character within the drama of acid-base homeostasis, namely the hydrogen ion concentration".


It is not as if this is a new finding, and the authors show how the consequences of poor understanding of pH, and arguments about measurement and description of pH and hydrogen ion concentration have been rehearsed before. But nothing has been done. Bandolier got the correct answers to question 1, but gets a severe headache when asked to think about acid-base homeostasis. It has become a subject best avoided. If that is because we can't understand pH, and there is a better and more easily understood method using hydrogen ion concentrations in nmol/L, then it's time for a change. There's no point in having evidence if people can't use it. We've dumped odds ratios in favour of NNTs, so why not dump pH in favour of nmol/L?


  1. J Hooper, WJ Marshall, AL Miller. Log-jam in acid-base education and investigation: why make it so difficult? Annals of Clinical Biochemistry 1998 35: 85-93.

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