Will sample throughput time exceeding 30-45 seconds as listed in method 10067 effect the chemical oxygen demand (COD) result?

Document ID

Document ID TE4020

Published Date

Published Date 27/05/2017
Question
Will sample throughput time exceeding 30-45 seconds as listed in method 10067 effect the chemical oxygen demand (COD) result?
Summary
Sample throughput time in chloride removal cartridges for COD method 10067
Answer
Low COD results are obtained on KHP based COD standards if the sample throughput time using the Chloride removal cartridges (Catalog Number/Order Code 2661825) exceeds the 30-45 second time-frame given in the method.

This occurs when the liquid "locks up" the cartridge due to entrained air and/or surface wetting and has to be forced through by increasing the vacuum to initiate flow.

Since KHP is relatively easy to oxidize, it is reasonable that some percentage of it and other similar organics can be lost through oxidation in the cartridge causing low results.

The same work shows that using higher vacuum with faster sample throughput still removes sufficient chloride to meet the method design criteria of 90% minimum removal at a 1000 mg/L chloride level. Simultaneously, the KPH recovery moves to the 95 - 100% recovery range required by the design criteria.

In summary, it is recommended to immediately increase the vacuum until the sample begins too flow and then maintain the vacuum at that level until the full vacuum step that follows if "lock up" occurs.