Hi, 

First prepare 0.2 M monobasic phosphate solution and 0.2 M dibasic phosphate solution. You can refer to the attached document, refer No. 12. 

Hope this will help you. 

Hi there,

Prepare 0.1M KH2PO4 and  0.1M K2HPO4 then using a pH meter and magnetic stirring add the K2HPO4 solution to the KH2PO4 until the desired pH. As pK of the second acidic function of phosphoric acid is around 6.8 (ie. below the desired pH7) the added volume of K2HPO4 should be larger but close to the one of KH2PO4 (pH should be pK if both volumes are identical).

Prepare 800 mL of distilled water in a suitable container.

Add 20.209 g of Sodium phosphate dibasic to the solution.

Add 3.394 g of Sodium phosphate mono basic to the solution.

Adjust solution to final desired pH using HCl or NaOH

Add distilled water until volume is 1 L.