Measuring the acidity of an aqueous solution

© Andrey Kiselev (Fotolia) analyze
© Andrey Kiselev (Fotolia)

Certain compounds when dissolved in water (H_2O) will cause it to dissociated into constituent hydrogen (H^^+) and hydroxide (OH^^-) ions. Solutions with more hydrogen ions than hydroxide ions are acidic; those with more hydroxide than hydrogen are alkaline.

Acidity and alkalinity are quantitatively defined by pH. A solution with a pH of less than 7.0 is acidic and more than 7.0 is alkaline. A solution with a pH equal to 7.0 is called neutral.Inorganic compounds such as hydrogen chloride, nitrogen dioxide and hydrogen sulfide and organic compounds including acetic acid and citric acid all form solutions of with a pH of less than 7.0 when dissolved in water.

The metals lithium, sodium and potassium all react violently in water to form solutions with a pH of more than 7.0. This is why they are called alkali metals.

Colour change and electrical conductivity are the two main ways of measuring pH.

A variety of natural and synthetic dyes in aqueous solution will change colour depending on the concentration of hydrogen and hydroxide ions. These dyes are called indicators and include methyl orange, phenolphthalein and litmus. A blue litmus solution will turn red in the presence of an acid; this solution will change back to red with the addition of and acid.
The most useful indicator, called Universal Indicator is a mix of phenolphthalein and other synthetic dyes. Very strong acids cause Universal Indicator to turn red; weak acids give an orange or yellow colour. A neutral solution with a pH of 7 turns green, while increases in alkalinity yield first blue and then green.

Chemical indicators are given an approximate measure of acidity; however pH can be determined far more accurately using electrical conductivity.

Both hydrogen and hydroxide ions in a solution conduct electricity. An electrical pH sensor is calibrated using a solution of known pH, usually 7.0, called a buffer solution. Any solution with a voltage less than the buffer will be acidic; a solution with a voltage greater than the buffer will be alkaline.

pH meters which work using this principle can measure acidity to several decimal places.