[kemstudent]

I need to prepare 50 mL of "10% aqueous phosphate buffer at pH 4.9" for a procedure. I managed to find a recipe for preparing a pH 4.9 phosphate buffer -- mixing 25 mL of 0.1M citric acid and 25 mL of 0.2M Na2HPO4, then diluted to a total of 100 mL. However, I don't know how to convert this to percentage. The procedure requires a 10% buffer. How do I do the conversion?

[Momer]

Try to get 10 ml of your prepared solution then dilute it to 100, then measure the pH. I hope it could help.

Please tell me your results.

[nextpauling]

A buffer is a mixture of an acid and its conjugate base so you shouldn't be using citric acid for a phosphate buffer unless you don't mind having citric acid/citrate ion in the solution.  For a pure phosphate buffer you need to mix phoshporic acid and monobasic sodium phosphate in nearly equal proportions ( the ratio will determine the final pH).  Look up the Henderson-Haselbalch equation to find the ratio of phosphoric acid to sodium phosphate.  I'm assuming the 10% means the total mass of phosphoric acid and phosphate ion should be 10 % of the volume of water used to make the solution (a weight volume ratio).