I am preparing waste water with congo red dye but the dye is converting into violet colour after few hours. The composotion of waste water in (mg/L)- NH4NO3-176.1, NaCl- 7.0, MgCl2.6H2O- 3.4, CaCl2-. H2O-4, and K2HPO4-36.7.(Oon et al. 2020). I have added phosphate buffer (50mM) but still the colour of dye is turning into blue. what should I do so that the colour remains same throughout the experiment?