KHP solutions degrade over time but 10 times within a short period sounds unlikely.  Did you check the preparation method?

Thank you Dr. Paul Milham.

I dissolved 0.85 g of oven dried (in 110 C for 2 hrs) KHP in 500 ml of distilled water to prepare a 2000 mg/l of COD solution. I also tried another solution with non-oven dried KHP of 0.85g dissolved in 500 ml. Both samples showed similar COD values when measured by TNP-10 and the values were around 150- 350 mg/l.

Can KHP has degraded over time while been stored in the lab for few years?

I don't believe that KHP degrades significantly then stores as a dry powder for a few years. You properly have another error in the analysis. You should order a new portion KHP or find another defined chemical in your chemicals cupboard and test if the method or the standard is wrong.

Thank you for your replies. 

I tried with another KHP powder sample in lab and got the similar results. Is there any thing strange about the method that I have mentioned above?

Thank you very much!

Dear Mark Nowosad,

Thank you!

But I wonder why I cannot open the link you sent.

Thank you Mark Nowosad!

I went through the documents you sent and I prepared the COD standard solution exactly same as described in those. Only difference is I used a different analyzer for the measuring the COD value.

I haven`t  found changes in values of COD with p.a. drug over time.

It is possible you have another problem.

In my experience you should prepare standard solution of different concentration of potassium hydrogen pthalate . Then checked by titration method as well as analyzer.i think titration method would be more accurate.