Verifying COD Analyzer Accuracy

Chemical oxygen demand is a key indicator of organic pollution in water. Accurate COD measurements are essential for environmental compliance and process control. However, analyzer readings can drift over time due to reagent quality, optical fouling, or calibration errors. 

Verifying the accuracy of a COD analyzer requires a systematic approach using standard solutions, reference materials, and comparison with reference methods. This article outlines practical procedures to assess and maintain the reliability of COD measurements.

Standard Solution Verification

The most direct check is to measure a certified COD standard solution. Prepare a standard with a concentration close to the expected sample value. Run the analyzer according to the normal procedure and compare the reading with the certified value. If the deviation exceeds the manufacturer’s specified tolerance—typically ±10 %—the instrument requires recalibration. Standard verification should be performed before each batch of samples or at least daily.

Certified Reference Material

Using a certified reference material provides an independent check on accuracy. A reference material is a matrix‑matched sample with a known COD value, often supplied by an accredited laboratory. Analyse the reference material alongside routine samples. The measured value must fall within the certified uncertainty range. If it falls outside, the entire measurement system—reagents, digestion, and photometer—needs investigation. This check is particularly important for verifying performance after maintenance or reagent replacement.

Repeatability Assessment

Repeatability reflects the precision of the analyzer under the same conditions. Take a single sample and divide it into several aliquots. Measure each aliquot consecutively without changing any parameter. Calculate the relative standard deviation. For COD analyzers, a relative standard deviation below 5 % is generally acceptable. High variability indicates issues with sample homogeneity, reagent dispensing, or detector stability. Identifying and correcting these problems ensures consistent results.

Comparison with the Reference Method

The ultimate test of accuracy is comparison with the national reference method. For COD, the reference method is the dichromate reflux method. Collect a set of real wastewater samples covering low, medium, and high concentration ranges. Analyse each sample with both the online analyzer and the laboratory reference method, using the same sample aliquot to ensure comparability.

The relative deviation between the two results should be within ±10 %. If the deviation exceeds this limit, the analyzer’s calibration or digestion efficiency is suspect. This comparison should be performed periodically, especially for online instruments.