A solution may be neutral, acidic or alkaline.
Kw = 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.
The pH scale range is taken as 0 to 14 for practical purpose.
Relationship between pH and pOH
Relationship between Ka and Kb or pKa and pKb
log kw = log ka + log kb
– log kw = –log ka – log kb
–log ka – log kb = – log 10-14
pKa + pKb = pKw = 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.