References

     Sorenson table of buffer mixtures
Na2HPO4Solution (mL) KH2PO4 Solution (mL) pH
0.25 9.75 5.288
0.5 9.5 5.589
1.0 9.0 5.906
2.0 8.0 6.239
3.0 7.0 6.468
4.0 6.0 6.643
5.0 5.0 6.813
6.0 4.0 6.979
7.0 3.0 7.168
8.0 2.0 7.381
9.0 1.0 7.731
9.5 0.5 8.043