Individual standard stock solutions containing each color were prepared by dissolving 100 mg unpurified colorant in 100 mL distilled deionized water. The solutions were kept in dark flasks. The working standard solutions of each color were prepared by appropriate dilution of stock solutions with water to give concentrations between 0.10 and 60 mg L−1, taking into consideration the purity of the colorants. The mixed standard solutions containing all colorants at concentrations between 0.10 and 10 mg L−1 were also prepared by mixing and dilution of appropriate aliquots from standard stock solution of each substance. All solutions were stored at 4 ◦C in the dark and were stable at least for 2 months.