pH

A solution may be neutral, acidic or alkaline.

K_w = 1 × 10^-14 at 298 K

To express the acidity or alkalinity of a solution, it is sufficient to express only the H₃O⁺ ion concentration.

Sorensen , in 1909 , suggested a convenient method of expressing the H₃O⁺ ion concentration in terms of pH.

The symbol has been taken from the Danish word potenz de hydrogen ion which mean power of hydrogen ion.

pH of a solution is defined as negative logarithm of hydrogen ion concentration.

pH scale

The pH scale range is taken as 0 to 14 for practical purpose.

Relationship between pH and pOH

Relationship between K_a and K_b or pK_a and pK_b

log k_w = log k_a + log k_b

– log k_w = –log k_a – log k_b

–log k_a – log k_b  = – log 10^-14

pK_a + pK_b = pK_w = 14

Limitation of pH scale

The pH value of the solutions do not give the exact idea of their relative strength

Measurement of pH

Accurate measurement of pH of a solution is done with the help of an instrument , called pH- meter.

Approximate pH can be determined with the help of pH papers which show different colours when dipped in solution of different pH.