Microtiter standard reagent A

5 g sodium bicinchoninic acid (BCA) (4,4-dicarboxy-2, 2′-biquinoline, disodium salt)

10 g Na₂CO₃•H₂O

0.8 g sodium tartrate

2 g NaOH

4.75 g NaHCO₃

Adjust the volume to 500 mL with H₂O. Stir the solution and, if necessary, adjust the pH to 11.25 using 10 M NaOH. Although the disodium salt of BCA is soluble at neutral pH, the free acid is not readily soluble even at basic pH. Microtiter standard reagent A is stable for 1-3 wk when stored in a plastic container at room temperature, and indefinitely at 4ºC.