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A buffer or a buffer solution undergoes only a slight change in pH when adding H+ or OH- in the solution.In other words we can say that it resists the change in pH.
The phosphate buffer contains This buffer system consists
of dihydrogen phosphate ions (H2PO4-) as hydrogen-ion donor (acid) and hydrogen phosphate ions (HPO42-)
as hydrogen-ion acceptor (base). These two ions are in equilibrium with each other as indicated by the
chemical equation below.
H2PO4-(aq) + H+(aq) + HPO42-(aq)
If additional hydrogen ions are added, they are consumed in the reaction with HPO42-,
and the equilibrium shifts to the left. If additional hydroxide ions enter the added, they react with H2PO4-, producing
HPO42-, and shifting the equilibrium to the right.
Thus we can say that phosphate buffer needs both the Na2HPO4 and NaH2PO4 in the solution to resist the change in pH.
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Nov 15th, 2015
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