The pH Scale

pH scale is a set of numbers from 0 to 14. It is used to identify neutral substance, or indicate the acidity/alkalinity of a substance.

Acids have pH values less than 7. The smaller the pH value, the more acidic the substance is. Concentration of hydrogen ions increases as the pH of the substance decreases.

Alkalis have pH values more than 7. The bigger the value, the more alkaline the substance is. Concentration of the hydroxide ions increases as the pH of the substance increases.

Neutral substances have pH of 7.

Measurement of pH

The pH of a solution can be measured with Universal Indicator or with a pH meter.

Universal Indicator

pH of a substance can be approximately measured using a Universal Indicator. A Universal Indicator comes in the form of a solution or pH paper.

In the form of solution

Drip a few drops of Universal Indicator into the solution. Compare the colour with the pH chart (as shown in the featured picture).

In the form of pH paper (using for soil testing)

Dip the pH paper into the solution. Compare the result with the pH chart.

Colour of Universal Indicator

in strong acids – red,
in weak acids – yellow/orange
in neutral solution – green
in weak alkali – blue
in strong alkali – purple

pH meter

pH can be measured accurately using a pH meter. It is an electrical method of measuring the pH of a solution. It consists of a pH probe connected to a meter. pH of the substance is measured by dipping the probe into the solution. The meter shows the pH either on a scale or digitally.