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Measuring pH

Measuring pH means measuring the acidity or alkalinity of an aqueous sample. The pH scale runs from 0 (most acid) to 14 most alkaline (basic) with 7.0 being the pH of pure water (neutral). The scale is logarithmic so for every unit change (eg from 6 to 5) there is a tenfold change in acidity.

The pH measurement of fresh water is usually in the range 6.5 to 8.2 but pH can be affected by catchment geology, industrial runoff and sewage. Changes to pH outside this range have a profound effect on life in the water and no life will exist at the extremes of the pH range (with the exception of a few hardy bacteria).

pH is usually measured in the field but for this virtual laboratory the samples have been brought to the laboratory for measurement.

The measure of pH is based on the concentration of hydrogen ions (H+) in water.

Water may break down into H+ and OH- ions as shown in the following reaction:

H2O H+ + OH-

Water contains both H+ and OH- ions (hydroxide) and it is the balance between these ions that creates the pH scale.

Ions pH Solution
Evenly balanced H+ = OH- 7 Neutral
More H+ than OH- 0 to 7 Acidic
More OH- than H+ 7 to 14 Alkaline

The pH scale is logarithmic (in fact the 'p' in pH means 'the negative log to the base 10') and for each unit change there is a tenfold change in acidity:

pH = -log (concentration of H+ ions)

eg from pH 6 to pH 5 means a tenfold INCREASE in acidity
     from pH 8 to pH 9 means a tenfold DECREASE in acidity.

Pure water has a pH of exactly 7 but water or aqueous (aqueous means water containing) solutions in nature are rarely exactly pH 7.

Move the arrow on the pH scale below to each red mark to find the pH of common substances.

Move the arrow to each red mark on the scale to see substances with particular pH's.


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