we use APHA 4500 SiO2 C:2017 method for silica determination. once we digest the sample and then colour develop to get the absorbance. when we keep more time ( we have to measure the absorbance after the sample cool down) absorbance value get reduced. (keep more time to cool, absorbance gradually reduced) is there any reason for that. in APHA method, it is not addressed the effect of cooling on sample colour development? please give your feedback