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pH scale

In 1909, Soreson devised a scale known as pH scale on which the strength of acid solutions as well as basic solutions could be represented by making use of hydrogen ion concentration. Sorenson linked the hydrogen ion concentrations of acid and base solutions to the simple numbers 0 to 14 on the pH scale.
pH of a solution is defined as the negative logarithm of the hydrogen ion concentration in moles per litre.
Mathematically: pH = log10
In pure water [H+] = 1.0 × 10-7
∴ pH of pure (neutral) water = - log (10-7) = 7
Thus, the pH value of pure water is equal to 7.
Acid Solutions
We know that all acidic solutions have H+ ion concentration greater than 1.0 × 10-7. The H+ ion concentration in an acidic solution may be 10-5, 10-6, 10-4, etc.
Consider an acidic solution whose H+ ion concentration = 10-6
∴ Its pH = - log (H+) = - log(10-6) = 6
Clearly, pH values of all acidic solutions will be less than 7.
Basic Solutions
All basic solutions have H+ ion concentration less than 10-7. If the solution is basic, its OH- ion concentration will be more than 10-7. By knowing the concentration of OH- ions, the concentration of H+ ion can be calculated.
[H+] = 
[H+] = 10-7 or pH = 7, the solution is neutral.
[H+] > 10-7 or pH < 7, the solution is acidic.
The complete pH scale is given below:


It may be noted that (i) solutions having pH between 0 to 2 are strongly acidic: (ii) solutions having pH between 2 to 4 are moderately acidic: and (iii) solutions having pH between 4 to 7 are weakly acidic.
Similarly, solutions have pH value
  1. between 7 and 10 are weakly basic;
  2. between 10 and 12 are moderately basic, and
  3. between 12 and 14 are strongly basic.
Importance of pH Value
  1. pH has a great importance in agriculture. Soil is often tested to determine whether acidic or basic fertilisers are required for a particular crop.
  2. It has a great importance in biochemical reactions such as digestion of food, etc. Human blood has pH 7.4. If it changes by 0.2 pH units, death results.
  3. Large number of qualitative and quantitative analysis is carried out at definite pH values.
    For example, pH must be adjusted between 0.4 to 0.6 for the complete precipitation of cations of group II.
  4. Food preservation also needs a definite pH value.

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