Division 4.2 Buffer substate.

Dissolve 1,5 g of disodium p-nitrophenylphosphate in sodium carbonate-bicarbonate buffer (4.1) and dilute to 1 000 ml in a volumetric flask with buffer (4.1).

This solution is stable if stored in a refrigerator (≤ 4 ^oC) for one month but a colour control test should be carried out on such stored solutions — see 6, precaution number 3.