Dissolve in double-distilled water to give a ten times solution. Adjust the solution to pH10, at which point it turns yellow. Then boil it until it becomes colourless. It is then cooled to room temperature, reset to pH10, and boiled again. This cycle is repeated until the pH remains at 10 after boiling (usually 5 times). The volume is adjusted and the solution is stored at 4°C for up to two weeks.