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COD Standard Values
Posted: Saturday, February 16, 2013 3:42 PM
Joined: 2/16/2013
Posts: 1

For approximately the past 15 years we have been running daily in house COD tests on our effluent. During that time we have been using HACH COD Standard Solution 300 mg/L for QA/QC purposes as it closely matches our typical effluent values. That being said in all the years we have been using this standard it has consistently run high. One would certainly expect to see some variance in the value but I would anticipate it averaging out to close to 300. This has not been the case for us, as our results, with few exceptions, run in the 310-318 range.


In all the time we have been conducting this test we have used 3 separate HACH reactors with each reactor producing pretty well the same results. The HACH DR 2800 Spectrophotometer we currently use for reading is calibrated annually by a HACH representative and continues to produce high  standard values, even immediately following the annual calibration.


To take it a step further I recently produced my own set of standards using stock potassium hydrogen phthalate. I produced standards with concentrations of 200, 500, 1000. 1500, and 2000 mg/L respectively and read the absorbance values of each standard. I then plotted these absorbance values and produced a linear line of best fit resulting in an R2 value of 0.9996 (this was expected as the spectrophotometer should read absorbance values linear up to 2000 mg/L). I then took the absorbance of the HACH 300 mg/L standard and plotted against the line, and using the equation determined the concentration to be 312 mg/L.


Safe to say we are having difficulties pinpointing why the standard does not average closer to 300 mg/L , after all the standard claims to be a 300 mg/L standard. I am wondering if anyone else has run into a similar issue or has any perspective on what our difficulties may be?




Chad G