Sodium Chlorate Interference in Wastewater COD Determination: Causes, Impacts, and Effective UV Method Solutions

Chemical Oxygen Demand (COD) is one of the most critical comprehensive indicators for evaluating the degree of organic pollution in industrial wastewater and domestic sewage. It not only directly reflects the total amount of reducible substances that can be oxidized in wastewater, but also serves as an important basis for assessing wastewater treatment plant performance, discharge permit allocation, and environmental supervision. However, due to the complex and variable composition of actual wastewater, the determination process is often affected by various inorganic ions such as chloride, chlorate, nitrite, and heavy metals. Among them, sodium chlorate (NaClO₃) interference deserves particular attention.

In the traditional potassium dichromate method (GB 11914-89 or HJ 828-2017), high concentrations of sodium chlorate can cause significantly lower COD measurement results, posing major challenges to accurate evaluation of wastewater pollution load and process control. Below, we examine typical wastewater sample color changes and experimental processes.

Interference Mechanism of Sodium Chlorate on Potassium Dichromate COD Determination

Sodium chlorate itself has certain oxidizing properties under acidic conditions. However, under the strong acid, high temperature, and strong oxidizing environment of COD determination (potassium dichromate + silver sulfate catalyst), its behavior becomes complex, mainly producing the following interferences:

1. Side reactions with potassium dichromate: High concentrations of sodium chlorate can react with Cr(VI) to form dark green precipitates (possibly low-valence chromium compounds or complexes), consuming part of the oxidant. This reduces the amount of potassium dichromate available to oxidize organic matter, ultimately resulting in a lower calculated oxygen demand.

2. Optical absorption interference: The dark green substances formed after reaction exhibit strong absorption around 600 nm, overlapping with the green absorption peak of Cr³⁺. This severely interferes with colorimetric readings and further amplifies measurement deviations.

3. Synergistic effects of pH and chlorate transformation: When the initial pH of the sample is relatively high, part of the sodium chlorate is more likely to exist as chlorate ions (ClO₃⁻). During acidic digestion, these ions more readily participate in redox reactions, increasing oxidant consumption and exacerbating the negative bias.

Research data indicate that:

• When sodium chlorate concentration is <500 mg/L, interference is minimal and COD results are generally consistent with true values;

• When the concentration rises to 1000–3000 mg/L or higher, measured values may be 20%–50% lower or even more, severely compromising data reliability.

This type of interference is particularly common in wastewater containing chlorine bleaching agents, certain chemical and pharmaceutical intermediate wastewaters, and samples rich in disinfection by-products.

Technical Principles and Anti-Interference Advantages of UV COD Determination

Ultraviolet absorption (UV) COD determination is based on the characteristic absorption of organic matter at specific UV wavelengths (typically 254 nm or dual/multi-wavelength combinations). It does not rely on chemical oxidation reactions and therefore fundamentally avoids interference from inorganic oxidants such as sodium chlorate and chloride.

Core principles:

• Unsaturated bonds, aromatic rings, carbonyl groups, and other functional groups in organic molecules produce strong absorption in the ultraviolet region;

• The instrument measures absorbance at the primary wavelength (254 nm) and a reference wavelength (usually 350 nm or higher), subtracting background interference from suspended solids, turbidity, and color;

• Through calibration curves or comparison with the national standard potassium dichromate method, a conversion coefficient between absorbance and COD concentration is established to enable rapid calculation.

Based on this principle, the Nexisense UV COD online analyzer has been further optimized with the following notable features:

• Dual-wavelength compensation technology: Effectively eliminates turbidity, color, and inorganic ion background absorption, significantly improving adaptability to complex water samples;

• Reagent-free operation: No oxidants or masking agents are added during measurement, achieving zero secondary pollution;

• Ultra-fast response: Results are typically obtained within ≤60 seconds from sampling, far exceeding traditional methods;

• Maintenance-free pretreatment: Special flow path and self-cleaning design allow maintenance cycles of over 6 months;

• High repeatability: Imported core optical components and precision flow path design ensure repeatability ≤3%;

• Fully automated intelligence: Supports automatic zeroing, calibration, measurement, cleaning, maintenance, and fault recovery, suitable for unattended operation;

• Flexible measurement modes: Supports manual triggering, scheduled, and periodic measurements to meet different regulatory requirements.

These features make the instrument particularly suitable for industries where traditional methods suffer severe interference, such as chlorine-bleached wastewater, papermaking wastewater, chemical and pharmaceutical wastewater, and dyeing wastewater.

The following images show the appearance and typical on-site installation of Nexisense UV COD online analyzers.

Practical Application Scenarios and Data Reliability Verification

In on-site comparative tests conducted at multiple papermaking, dyeing, and chemical enterprises, Nexisense UV COD analyzers showed strong correlation with national standard potassium dichromate method results (R² > 0.95). Even in water samples with sodium chlorate concentrations as high as 2000 mg/L, UV method results remained stable, while traditional methods exhibited significant deviations.

The instrument also supports periodic calibration of conversion coefficients using national standard method samples, ensuring long-term data traceability and compliance with environmental monitoring stations, total load control, and discharge fee regulations.

Frequently Asked Questions (FAQ)

At what concentration does sodium chlorate significantly interfere with the potassium dichromate method?
Interference generally becomes significant above 800–1000 mg/L, and when exceeding 2000 mg/L, negative bias often exceeds 30%, requiring special attention.

Is UV COD completely unaffected by chloride ions?
UV COD shows essentially no response to chloride or chlorate ions. However, extremely high chloride concentrations may produce slight salinity background absorption, which can be effectively eliminated by dual-wavelength compensation.

Does a UV COD analyzer require frequent calibration?
It is recommended to calibrate once per month or quarter using standard solutions of known concentration or parallel samples measured by the national standard method. Daily operation is highly stable.

Is the instrument suitable for high-turbidity wastewater?
With a maintenance-free pretreatment device, it adapts well to medium turbidity (<300 NTU) wastewater. For ultra-high turbidity, coarse filtration is recommended.

Conclusion: Choosing the Right Technology to Obtain True Data

As a common component in industrial wastewater, sodium chlorate poses significant interference to traditional potassium dichromate COD determination, directly affecting the accuracy of pollution load assessment and process control. In contrast, Nexisense UV COD online analyzers, based on physical optical principles, completely avoid chemical interference and achieve fast, reagent-free, maintenance-free green monitoring.

In an era of increasingly stringent environmental requirements and widespread adoption of online monitoring, selecting advanced instruments with strong anti-interference capability and reliable data is not only a technical upgrade, but also an inevitable choice for scientific management and green development. It is hoped that more enterprises and monitoring institutions will use reliable tools to truly grasp the “pulse” of wastewater and contribute precise力量 to water pollution prevention and control.