I use colorimetric closed reflux-method.
I am ussing at this moment kit vials, I use Ultra high range reagents, the detection range is form 200ppm-15.000ppm, I blend my sample, I know that my COD is arround 70ppm so I add an aliquote of KHP STD solution to reach ~300ppm, I take 0.2ml of spike sample, put it in the cap vial (sorry for my inglish)add 0.5g HgSO4 and digest it at 150C 1.5hr, also I run a blank with detilled water and NaCl (27gr/L).
At the end my blank and my sample are green!
The salinity of my sample is ~26gr/L.
I decided to prepare my own reagents Dichromate (0.25N), AgSO4, HgSO4, H2SO4, but they are in costums, I hope to get it this week but my fear is to have the same problem. I do not want to say the name of the company, but his procedure says that the reagent work for seawater, I can not obtain good ressult.
Should I develop the titration with Mohr Salt?
I can not work with open reflux I don't have the facility.
But i Will try to run my samples at same way with reagent prepared by my selft, but before that any advice is wellcome.