We have seen that concept of ionic product of water enables us to classify solutions as acidic, basic, or neutral by specifying the H3O+ ion concentration. For expressing the H3O+ ion concentration a logarithmic scale was devised by P.L. Sorensen (1909). This scale is called pH scale The pH of a solution may be defined as negative logarithm of the hydronium ion concentration in moles per litre.
Thus, mathematically, pH may be expressed as
pH = – log [H3O+]
From the above relation, the concentration of [H3O+] can be written as[H2O+] = 1 x 10 -pH
This enables us to give alternative definition of pH as the negative power to which 10 must be raised in order to express the hydronium ion concentration of the solution.
PH = – log [H3 O+] = – log [ 1 x 10 -7] = 7
For acidic solutions, [H3Q+] concentration is more than 1 X w -1 mol dm-3. Therefore, pH of acidic solution is less than 7. For basic solutions, the pH value is greater than 7. The range of pH is from 0 to 14. The solutions having pH between 0 and 2 are strongly acidic, those with pH between 2 to 4 are moderately acidic while others having pH between 4 to 7 are weakly acidic. Similarly, the solutions having pH value between 7 to 10 are weakly basic, those having pH 10 to 12 are moderately basic whereas others which have pH range between 12 to 14 are strongly basic.
The complete range of [H3O+] and pH has been illustrated in Fig. 25.1
Fig. 25.1. Range of pH and [H3O+]
The approximate pH of a solution can be determined with the help of pH papers. pH papers have different colours in solutions of different pH. A pH paper can determine pH of a solution with an accuracy of about 0.5. However, for accurate measurement of pH (up to accuracy of 0.001 units) pH metres are used.
The approximate pH of some common substances are given in Table 25.4.
Table 25.4. The pH of Some Common Substances
Acid-base indicators or pH indicators are substances which undergo change in colour with pH. These indicators can be used to find whether the solution is acidic or alkaline.
Some common acid base indicators are: litmus, phenolphthalein, methyl orange and methyl red.
The colour of an indicator undergoes an observable change over a range of pH. Therefore, these indicators cannot be used to find the exact pH of the solution.