Urgent, urgent, urgent, determine COD by potassium dichromate method. What color is the reagent after heating? Why is the color pink after I add ferrous reagent and heat it?

Normally, it should be yellow after heating.

If the water sample has high COD, it will turn green after heating. At this time, if it is still green after adding ferrous spirit, it means that although the COD concentration is high, potassium dichromate has not been consumed and can be titrated. If the solution turns reddish brown after adding ferrous spirit, it means that the added potassium dichromate has been completely consumed, and the COD is beyond the determination range, so it is necessary to dilute the water sample for re-determination.

The upper limit of determination is COD = 700mg/L. Although titration can be carried out when it is greater than 700 mg/L, the error is large.