. HOW STRONG ARE ACID OR BASE SOLUTIONS? We know how acid-base indicators can be used to distinguish between an acid and a base. We have also learnt in the previous section about dilution and decrease in concentration of H + or OH – ions in solutions.
Can we quantitatively find the amount of these ions present in a solution? Can we judge how strong a given acid or base is? We can do this by making use of a universal indicator, which is a mixture of several indicators. The universal indicator shows different colours at different concentrations of hydrogen ions in a solution.
A scale for measuring hydrogen ion concentration in a solution, called pH scale has been developed. The p in pH stands for ‘ potenz ’ in German, meaning power. On the pH scale we can measure pH generally from (very acidic) to (very alkaline). pH should be thought of simply as a number which indicates the acidic or basic nature of a solution.
Higher the hydronium ion concentration, lower is the pH value. The pH of a neutral solution is . Values less than on the pH scale represent an acidic solution. As the pH value increases from to , it represents an increase in OH – ion concentration in the solution, that is, increase in the strength of alkali (Fig.
. ). Generally paper impregnated with the universal indicator is used for measuring pH.