How do the calibration curves vary for each of the COD reagents?

Document ID

Document ID TE233

Version

Version 2.0

Status

Status Published

Published Date

Published Date 27/05/2017
Question
How do the calibration curves vary for each of the COD reagents?
Summary
Explanation of the calibration curves for different COD ranges
Answer
The Dichromate Chemical Oxygen Demand (COD) test measures the oxygen equivalent of the amount of organic matter oxidizable by potassium dichromate in a 50% sulfuric acid solution. The end products of the COD reaction include carbon dioxide, water, and various states of the chromium ion. The reaction is always from Cr6+ to Cr3+. The different Hach reagents listed below will determine the amount of chromium in a particular valence state. Either the amount of reduced chromium (chromic ion), or the amount of unreacted dichromate, is measured colorimetrically.

The Hach Ultra Low Range COD test is the lowest range and highest sensitivity COD test available. Results are measured at a wavelength of 350 nm. The maximum sensitivity is at 345 nm, but the test measurement is made at 350 nm for instrumentation considerations. The calibration line for this test has a negative slope. The amount of hexavalent chromium remaining after digestion is measured and it decreases as the COD concentration increases.

Hach’s High Range and High Range Plus COD are the highest test ranges available from Hach Company. The chemistry and calibration data are identical for both tests. High Range Plus COD is designed to eliminate the dilution step normally required for COD samples which have concentrations from 1500 mg/L up to 15,000 mg/L. When the High Range Plus vial is manufactured, dilution water is added directly to the High Range COD Reagent to accomplish a 1 to 10 dilution when 0.20 mL of sample is added to the reagent vial. The results are measured at 620 nm, and the calibration line has a positive slope. The amount of trivalent chromium is measured, and its concentration increases as the COD concentration increases. One observes the appearance of the green chromium ion, which is a product of the reaction between the reagent and any COD, so the scale is not reversed. The instrument is measuring the amount of green color produced.