Solutions of weak acids and bases in water have hydronium ions [H3O+] and hydroxyl ions [OH–] in different concentrations.
Acidic solutions have [H3O+] ions greater than 10–7 mol L–1 and alkaline solutions have [OH–] ions greater than 10–7 mol L–1 at 25°C. A neutral solution, or pure water has [H3O+] = [OH–].
For expressing the concentration of H3O+ in a solution, the negative powers of 10 are involved. This inconvenient use of figures involving negative powers of 10 can be avoided by using a concept of pH scale. The pH of a solution is defined as the negative logarithm to the base 10 of the hydronium ion concentration.
pH = – log10 [H3O+]
The pH of acidic solutions is less than 7, that of basic solution, is greater than 7 and the neutral solutions have pH = 7.
pH is accurately measured by a pH meter, but a reasonably good estimate can be made with the help of universal indicator solution or pH paper. These have characteristic colours depending upon the pH of the solution.
Experiment: To determine the pH of the following substances by using a universal indicator solution or pH paper:
Take six test tubes and label them 1 to 6. If you are using universal indicator solution, then take 3 - 4 mL each of the test solution (about one fourth of test tube) into separate labelled test tubes.
To all the test tubes, add 4 to 5 drops of the universal indicator solution and observe the appearance of colour, if any. If you are using pH paper, add 1-2 drop each of the test solutions with the help of a dropper on a pH paper strip. Alternatively, you may dip the pH paper strips into each of the test solution. Observe the colour.
Compare the colour of the solution in the test tubes or on the pH papers with the standard colour chart to find the pH. Classify the solutions as neutral, acidic or alkaline.