A pH buffer solution has a well defined acidity which changes only very slightly upon addition of moderate quantities of strong acid or base. The larger is the quantity of acid or base that must be added to a certain volume of a buffer solution in order to change its pH by a specific amount, the better is its action. A buffer solution is prepared by mixing a weak acid and its conjugate base in appropriate amounts in a solution. An example of a useful buffer system in aqueous solution is the phosphate system.
Your task is to prepare a phosphate buffer with properties specified by the following two conditions:
(1) pH = 7.20 in the buffer solution,
(2) pH = 6.80 in a mixture of 50.0 cm3 of the butter solution and 5.0 cm3 hydrochloric acid with a concentration of 0.100 mol dm-3 .
Chemicals and equipment Aqueous solution of phosphoric acid, sodium hydroxide solution of known concentration, hydrochloric acid (0.100 mol dm-3), solution of bromocresol green, distilled water.
Burettes, pipettes (25 and 5 cm3), Erlenmeyer flasks (100 and 250 cm3), volumetric flask (100 cm3), beaker, and funnel.
Procedure Determine the concentration of the phosphoric acid solution by titration with a sodium hydroxide solution using bromocresol green as an indicator (pH range 3.8 < pH < 5.4).
Make a buffer solution by mixing calculated volumes of phosphoric acid and sodium hydroxide solutions in the volumetric flask and filling the flask to the mark with distilled water.
Mix in an Erlenmeyer flask 50.0 cm3 of the buffer solution with 5.0 cm3 of the hydrochloric acid. Hand in your answer sheet to the referee who will also measure the pH of your two solutions and note your results. The pKa values for phosphoric acid are:
pKa1 = 1.75, pKa2 = 6.73, pKa3 = 11.50
