pH Scale & Indicators



phscale

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┬áthe 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.

IndicatorColour in strongly acidic solutionpH at which colour changeColour in strongly alkaline solution
Methyl orangeRed4Yellow
Litmus paperRed7Blue
PhenolphthaleinColourless9Red
Screened methyl orangeRed4Green

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:

universal indicator colour

 

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).


Mini Chemistry

Administrator of Mini Chemistry. If you spot any errors or want to suggest improvements, please contact us. Looking for guest writers.

Leave a Comment