Need chemistry help to explain why phosphate buffer needs both Na2HPO4 and NaH2PO4 in order to resist change in pH

Anonymous
timer Asked: Nov 15th, 2015
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Question Description

Explain why phosphate buffer needs both Na2HPO4  (sodium dibasic phosphate) and NaH2PO4   (sodium monobasic phosphate)   in order to resist changes in pH?

Tutor Answer

psumanrec
School: Purdue University

Thank you for the opportunity to help you with your question!

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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