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Portrait of pH

Science · Chemistry

pH

1909

Chemistry — acids and bases

pH is the logarithmic measure of how acidic or alkaline a solution is: the negative base-10 logarithm of the hydrogen ion activity. It was introduced in 1909 by the Danish chemist Søren Sørensen, working at the Carlsberg Laboratory in Copenhagen on the chemistry of brewing, where small changes in acidity ruined batches of beer. Because the scale is logarithmic, each unit is a tenfold change in hydrogen ion concentration — a fact that does most of the work in questions about acid rain and ocean acidification. In water at 25 °C the familiar 0-to-14 range follows from the ionic product of water being 1.0 × 10^-14, which also puts neutrality at 7; at other temperatures neutrality sits elsewhere.

How to spot them

  • ▸The negative logarithm to base 10 of hydrogen ion activity
  • ▸Introduced at the Carlsberg brewery laboratory in 1909
  • ▸A scale from 0 to 14 in water at 25 °C, where each step is a factor of ten
  • ▸Universal indicator runs red through green to violet across it
  • ▸Blood is held between 7.35 and 7.45; the sea has fallen from about 8.2 to 8.1

Key facts

Introduced in 1909 by Søren Sørensen at the Carlsberg Laboratory in Copenhagen, for the chemistry of brewing (1909)

Published in his 'Enzymstudien II'; he first wrote it with a subscript, and it was revised in 1924 to the modern electrochemical definition.

Defined as the negative base-10 logarithm of the hydrogen ion activity

Loosely taught as −log10[H+], the concentration form, which is a good approximation in dilute solution.

A change of one unit on the scale means a tenfold change in hydrogen ion concentration

So a value of 3 is a hundred times more acidic than a value of 5.

Runs from 0 to 14 in water at 25 °C because the ionic product of water is 1.0 × 10^-14

Strong acids and superacids can go below 0, and concentrated alkalis above 14.

Universal indicator turns red at the acid end of it and violet at the alkaline end

A mixture of dyes; litmus, the older test, is extracted from lichens.

Ocean acidification has pushed the sea's value down from about 8.2 before industrialisation to about 8.1 today

A drop of 0.1 is roughly a 30 per cent rise in hydrogen ion concentration.

Nobody is certain whether Sørensen's lower-case initial stood for potenz, puissance or potential

He published in Danish, German and French and never explained; one theory is that he simply labelled his two electrode solutions p and q.

Key ideas

  • It is a measure of hydrogen ion activity, not of acid strength or concentration. A concentrated weak acid and a dilute strong acid can give the same reading, which is why titration, not a meter, tells you how much acid is present.
  • Buffer solutions — a weak acid with its conjugate base — resist change by mopping up added acid or alkali, which is how blood is held in a range of a tenth of a unit.

Quick-fire facts

  • •Søren Peter Lauritz Sørensen ran the Carlsberg Laboratory's chemistry department from 1901 to 1938.
  • •Stomach acid is about 1.5–3.5, lemon juice about 2, black coffee about 5, blood 7.35–7.45, seawater about 8.1, household bleach about 13.
  • •Human blood outside 6.8–7.8 is generally fatal; acidosis and alkalosis describe drifts within that.
  • •The glass electrode, developed from 1909 onwards and made practical by Arnold Beckman's 1934 meter, is how it is measured in practice.
  • •Phenolphthalein is colourless in acid and pink above about 8.3; methyl orange turns at about 3.7.
  • •Fluoroantimonic acid, the strongest known superacid, is around 10^19 times stronger than concentrated sulfuric acid.
  • •'Neutral' acid rain is not 7 but about 5.6, because atmospheric carbon dioxide dissolves to give carbonic acid.
  • •Common misconception: 7 is only neutral at 25 °C. Pure water at 50 °C is neutral at about 6.6, because more of it has ionised — it has not become acidic.

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