# pH Scale & Indicators

Show/Hide Sub-topics (Acids, Bases & Salts | O Level)

The pH scale (as shown in the figure above) is used to measure the acidity or alkalinity of an aqueous solution. The pH scale is numbered between 0 to 14.

For reference, the equation for pH is given by:

$$\text{pH} =-\text{lg} [\text{H}^{+}]$$

pH is given by the negative logarithm of the concentration of hydrogen ions ($[\text{H}^{+}]$). You can see that pH depends on the concentration of $\text{H}^{+}$ only and not on the strength of the acid.

A neutral solution has a pH value of 7. (E.g. pH of pure distilled water) This means that the concentration of $\text{H}^{+}$ ions is equal to the concentration of $\text{OH}^{-}$ ions.

An acidic solution has a pH value of less than 7. (E.g. pH of sulphuric acid)

• This means that the concentration of $\text{H}^{+}$ ions is more than the concentration of $\text{OH}^{-}$ ions.
• The smaller the pH value, the more acidic the solution is.
• A solution with a pH value of 2 is more acidic than a solution of pH value of 3.

An alkaline solution has a pH value of more than 7. (E.g. pH of aqueous sodium hydroxide)

• This means that the concentration of $\text{H}^{+}$ ions is less than the concentration of $\text{OH}^{-}$ ions.
• The bigger the pH value, the more alkaline the solution is.
• A solution with a pH value of 10 is more alkaline than a solution of pH value of 9.

### Indicators

Indicators are organic substances that have different colours in acidic and in alkaline solutions.

• Each indicator has an acidic colour, an alkaline colour and a pH value at which it changes colour.

A table of common indicators are shown below.

Indicator Colour in strongly acidic solution pH at which colour change Colour in strongly alkaline solution
Methyl orange Red 4 Yellow
Litmus paper Red 7 Blue
Phenolphthalein Colourless 9 Red
Screened methyl orange Red 4 Green

A Universal Indicator (or pH Indicator) gives different colours at different pH numbers. The colours at the various pH numbers are shown in the figure below:

### pH Value & Agriculture

Most plants grow best when the pH value of the soil is about 6.5.

• Plants do not grow well in soil that is too acidic or too alkaline.
• Most types of soil are slightly acidic or alkaline with pH values between 6 to 8.

Soil can become too acidic from excessive use of chemical fertilizers and from acid rain.

Excess acid in soil can be neutralized by adding lime (calcium hydroxide).