Standard reagent A

Recipe

Reagent	Amount to add	Final concentration
Sodium bicinchoninic acid (BCA) (4,4-dicarboxy-2,2′-biquinoline, disodium salt)	1.0 g	26 mM
Na₂CO₃•H₂O	2.0 g	0.16 M
Sodium tartrate•2H₂O	0.16 g	7 mM
NaOH	0.4 g	0.1 M
NaHCO₃	0.95 g	0.11 M
H₂O	to 100 mL

Stir the solution and, if necessary, adjust the pH to 11.5 with 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. Standard reagent A is stable for 1-3 wk when stored in a plastic container at room temperature, and indefinitely at 4°C.