Key Pitfalls in Blank Tests for COD Analyzers

Performing a blank test is essential for reliable Chemical Oxygen Demand (COD) measurement. However, several issues can compromise the blank value and, consequently, the accuracy of sample results. Below are the main problems encountered when running a blank with a COD analyzer.

1. Reagent Contamination

The digestion solution (containing potassium dichromate, silver sulfate, and mercury sulfate) and the sulfuric acid reagent may already contain trace organic impurities or reducing agents. Even low-level contamination elevates the blank absorbance, leading to systematic overestimation of COD.

2. Purity of Dilution Water

High-quality distilled or deionized water is mandatory. Residual chlorine, organic leachates from tubing, or ammonia can produce a false positive blank. Water with a COD value above 2 mg/L is unsuitable for precise trace-level analysis.

3. Dirty Digestion Vessels

Glass vials or cuvettes not thoroughly cleaned—especially if previously used for high-COD samples—retain organic films. Incomplete rinsing with chromic acid or strong oxidizing agents can cause blank readings to drift unpredictably.

4. Cross Contamination from Air

During sample preparation, volatile organic compounds (e.g., solvents, alcohol vapors) in the laboratory air may absorb into the blank mixture. Even dust particles carrying organic matter can raise the blank signal.

5. Incomplete Digestion or Side Reactions

If the digestion temperature or duration deviates from the standard method (e.g., 150?°C for 2?h), partial oxidation of refractory organics in reagents may occur inconsistently. Chloride interference (even in the blank) can be problematic if mercury sulfate is insufficient.

6. Instrument Baseline Drift

Photometric COD analyzers rely on a reference blank to zero the absorbance at 600?nm (or lower wavelength for dichromate reduction). Aging of the lamp, fluctuations in detector sensitivity, or bubbles inside the cuvette cause baseline shift, making the blank appear non-zero.

7. Operator Errors

Using a dirty pipette tip, mixing reagents in the wrong order, or failing to cap vials tightly—all allow evaporation or contamination. Even fingerprints on the cuvette optical surface affect transmission.