Does the temperature of the test vial affect results in Hach Method 8000 for Chemical Oxygen Demand (COD)?

Document ID

Document ID TE3733

Version

Version 4.0

Status

Status Published

Published Date

Published Date 25/01/2018
Question
Does the temperature of the test vial affect results in Hach Method 8000 for Chemical Oxygen Demand (COD)?
Summary
The effects of varying temperature for the test vials used in Method 8000 for Chemical Oxygen Demand (COD)
Answer
Hach Method 8000 for COD in wastewater is approved by the United States Environmental Protection Agency (USEPA) in Title 40, Chapter I, Subchapter D, Part 136.3 of the Code of Federal Regulations (CFR), and it also obeys the operating principles of the Standard Methods for the Examination of Water and Waster (Standard Methods) 5220 D ( COD via Closed Reflux, Colorimetric Method). In both procedures, temperature is of vital importance.

In Method 8000, steps 10-11 instruct that 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 Method 8000 requires the vial be allowed to cool to room temperature (generally equated to be 15-30 °C / 59-86 °F), and it is highly recommended to do so in a tube rack. Allowing the digested tubes to cool while inside a DR instrument’s round vial compartment causes damage by melting the rubber/plastic 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 ( Mercuric Chloride and unoxidized Chromium) and ensure that the optical path is clear.

While both the Hach and Standard Methods COD methods require cooling vials to room temperature before analysis, the expected results on not-yet cooled COD vials is not published. In order to stay compliant to the USEPA's CFR requirements it is strongly advised to allow COD vials to return to room temperature prior to colorimetric analysis.

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. The following NIST-traceable Laboratory Control Standards for COD are offered:
 
Catalog Number/Order Code 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