H + ions NH + (aq) + H O ( ) NH OH(aq) + H + (aq) Ammonium hydroxide is a weak base ( K b = . × – ) and therefore remains almost unionised in solution. This results in increased of H + ion concentration in solution making the solution acidic. Thus, the pH of NH Cl solution in water is less than .
Consider the hydrolysis of CH COONH salt formed from weak acid and weak base. The ions formed undergo hydrolysis as follow: CH COO – + NH + + H O CH COOH + NH OH CH COOH and NH OH, also remain into partially dissociated form: CH COOH CH COO – + H + NH OH NH + + OH – H O H + + OH – Without going into detailed calculation, it can be said that degree of hydrolysis is independent of concentration of solution, and pH of such solutions is determined by their p K values: pH = + ½ (p K a – p K b ) ( . ) The pH of solution can be greater than , if the difference is positive and it will be less than , if the difference is negative. Problem .
The p K a of acetic acid and p K b of ammonium hydroxide are . and . respectively. Calculate the pH of ammonium acetate solution.
= . . BUFFER SOLUTIONS Many body fluids e.g., blood or urine have definite pH and any deviation in their pH indicates malfunctioning of the body. The control of pH is also very important in many chemical and biochemical processes.
Many medical and cosmetic formulations require that these be kept and administered at a particular pH. The solutions which resist change in pH on dilution or with the addition of