Hello every body I am chemistry Technician working in environmental lab please I need your help to explain this case  for me when I analyze one of the industrial wastwater  for TOC and COD the TOC result is higher than COD please can you tell me why this is happen

Hi
I think it is beacause :in dichromate oxidation, you can not measurement aromatic carbon like benzene, toluene and etc. but in TOC measurement you can measure this type of organic carbon

The first thing I notice is that the reported COD value is right at the upper end of the detection limit for your method. Is that value from a diluted sample or was it run straight? If we presume your TOC value to be correct I would expect a COD in the 1500-2000 mg/L range. Try a 5x dilution on your sample.

I believe the Skalar TOC unit is a high temperature combustion. If so, most of the interferences and limitations are of the same nature and behave in the same way as with TOC. The only big exception would be NO_x compounds. But, I don't think we are necessarily dealing with an interference issue. I presume your blanks and standards are reading well, so we will have to presume that the instrument itself is operating properly. Does your instrument do an inorganic carbon purge before analyzing or do you do that manually prior to loading the sample? Have you verified that the acid used to remove the inorganic carbon is of sufficient strength and is still good? If you have high carbonate/bicarbonate contributions you may still be seeing residual inorganic contributions when you do your TOC reading.