Fosfaatbuffer

The phosphate buffer system is vital in urine and intracellular fluid.

It consists of sodium dihydrogen phosphate and sodium hydrogen phosphate.

Since sodium dihydrogen phosphate is a weak acid, it doesn't dissociate much in acidic and neutral solutions.

However, when a strong base, such as sodium hydroxide, is introduced into the solution, sodium dihydrogen phosphate dissociates to give up hydrogen ions. These ions bind with hydroxide ions, forming water molecules. 

At the same time, sodium hydrogen phosphate is also formed.

Since this process converts sodium hydroxide into the weak base sodium hydrogen phosphate, the pH of the solution rises only modestly.

Sodium hydrogen phosphate does not dissociate under alkaline conditions.

But, when a strong acid like hydrochloric acid is added to the solution, sodium hydrogen phosphate takes up the hydrogen ions, forming sodium dihydrogen phosphate.

Since this process converts HCl into the weak acid sodium dihydrogen phosphate, the pH of the solution is reduced only slightly.