Question

the initial pH of the phosphate buffer was 6.05. I added HCL, it started decreasing to 3.75. Then on a separate bottle I added NaOH in the phosphate buffer increased to 11.06. Why would the buffer decrease when adding HCL and increase when NaOH is added?

Answer 1

On adding HCl in more quantity, since HCl is strong acid it undergoes complete dissociation, to produce more number of H⁺ ions which increases the concentration of H⁺ ions. Hence it results in decrease in pH.

HCl (aq.) ---------------> H⁺ (aq.) + Cl^- (aq.)

pH = - Log[H⁺]

Similarly,

On adding NaOH in more quantity, since NaOH is strong base it undergoes complete dissociation, to produce more number of OH^- ions which decreases the concentration of H⁺ ions. Hence it results in increase in pH.

NaOH (aq.) ---------------------> Na⁺ (aq.) + OH^- (aq.)