of hydrogen ion. In dilute solutions (< . M), activity of hydrogen ion (H + ) is equal in magnitude to molarity represented by [H + ]. It should be noted that activity has no units and is defined as: = [H + ] / mol L – From the definition of pH, the following can be written, pH = – log a H+ = – log {[H + ] / mol L – } Thus, an acidic solution of HCl ( – M) will have a pH = .
Similarly, a basic solution of NaOH having [OH – ] = – M and [H O + ] = – M will have a pH = . At °C, pure water has a concentration of hydrogen ions, [H + ] = – M. Hence, the pH of pure water is given as: pH = –log( – ) = Acidic solutions possess a concentration of hydrogen ions, [H + ] > – M, while basic solutions possess a concentration of hydrogen ions, [H + ] < – M. thus, we can summarise that Acidic solution has pH < Basic solution has pH > Neutral solution has pH = Now again, consider the equation ( .
) at K K w = [H O + ] [OH – ] = – Taking negative logarithm on both sides of equation, we obtain –log K w = – log {[H O + ] [OH – ]} = – log [H O + ] – log [OH – ] = – log – p K w = pH + pOH = ( . ) Note that although K w may change with temperature the variations in pH with temperature are so small that we often ignore it. p K w is a very important quantity for aqueous solutions and controls the relative concentrations of hydrogen and hydroxyl ions as their product is a constant. It should be noted that as the pH scale is logarithmic, a change in pH by