. The pH scale We usually deal with acid / base solution in the concentration range - to M - . To express the strength of such low concentrations, Sorensen introduced a logarithmic scale known as the pH scale. The term pH is derived from the French word ‘Purissance de hydrogene’ meaning, the power of hydrogen.
pH of a solution is defined as the negative logarithm of base of the molar concentration of the hydronium ions present in the solution. pH = - log [H O ] .....( . ) The concentration of H O + in a solution of known pH can be calculated using the following expression. [H3O + ]= -pH (or) [H3O + ]= antilog of (-pH) .....( .
) Similarly, pOH can also be defined as follows pOH = - log [OH - ] .....( . ) As discussed earlier, in neutral solutions, the concentration of [H O ] + as well as [OH ] + is equal to - M at C . The pH of a neutral solution can be calculated by substituting this H O + concentration in the expression ( . ) pH = log [H3O + ] = log - lo g = + ( )= [ log = ] (- )(- ) Similary, we can calculate the pOH of a neutral solution using the expression ( .
), it is also equal to . The negative sign in the expression ( . ) indicates that when the concentration of [H O ] increases the pH value decreases. For example, if the [H O ] increases from to to10 M - - , the pH value of the solution decreases from to .
We know that in acidic solution, [H O ] > [OH ] - , i.e., [H O ] > - . Similarly in basic solution [H O ] < . So, we can conclude that acidic solution should have pH value less than and basic solution should have pH value greater than . .
. Relation between pH and pOH A relation between pH and pOH can be established using their following definitions pH=-log [H3O + ] .....( . ) pOH = - log [OH - ] .....( . ) Adding equation ( .
) and ( . ) XII U8-Ionic XII U8-Ionic - - - - Figure . The pH scale pH + pOH = log [H3O + ] log [OH - ] log [H3O + ]+log [OH - ] pH+pOH = log [H3O + ][OH - ]