Can COD be measured while the vial is still hot?

Document ID

Document ID TE227

Published Date

Published Date 04/30/2019
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Question
Can COD be measured while the vial is still hot?
Summary
Cooling vials properly for COD analysis
Answer
It is not recommended to measure the vials before letting them cool to room temperature. The measurement may be inaccurate and a hot reagent vial could damage the digital instrument.

Hach Method 8000 for COD in wastewater is approved by the United States Environmental Protection Agency (USEPA) in 40 CFR 136.3, and it also obeys the operating principles of Standard Methods for the Examination of Water and Wastewater (Standard Methods) 5220 D (COD via Closed Reflux, Colorimetric Method). In both procedures, temperature is of vital importance. In Hach Method 8000, steps 10-11 say to let the vials cool in the reactor for approximately 20 minutes to 120 °C or less prior to inverting them several times while still warm. Standard Methods justifies this mixing as a way to combine condensed water and dislodge insoluble matter. 

Step 12 of the Hach Method requires the vial be allowed to cool to room temperature, and it is highly recommended to do so in a tube rack. Empirical feedback from Hach’s Research and Design group proved that letting the tubes cool while inside a DR instrument’s round vial compartment can easily melt the rubber rings used on the vial stabilizers; they are designed to handle temperatures of approximately 80 °C and below. Standard Methods also requires cooling sample vials to room temperature naturally and justifies this gradual cooling as a way to discourage precipitate formation, settle any suspended matter (unoxidized Chromium and Mercuric Chloride) and ensure that the optical path is clear. While both reference COD methods require cooling vials to room temperature before analysis, the expected results on not-yet cooled COD vials is not published.

So long as the COD vial remains unopened after digestion and kept out of direct sunlight (strong UV light), its color remains stable for an indefinite long period of time. Color stability varies by case-to-case, and it is recommended to measure a reacted vial containing Laboratory Control Standard periodically. Keep it reserved in a cool, dark room, and observe for any color variances at 350 nm for ultra-low-range vials, 420 nm at low-range vials, and 620 nm for high-range and high-range-plus vials. Hach offers the following NIST-traceable Laboratory Control Standards for COD:
 

Product # Control Standard Concentration Bottle Volume
1218629 300 mg/L 200 mL
1218649 300 mg/L 500 mL
2672629 800 mg/L 200 mL
2253929 1000 mg/L 200 mL

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