Titrimetric COD Analyzer

What is the Titrimetric COD Analyzer?

A titrimetric COD analyzer determines dichromate consumption by titration with ferrous ammonium sulfate, the classical standard reference procedure. It handles coloured, turbid and high-concentration complex wastewater samples where colorimetric reading is unreliable.

Overview

The sample is refluxed with a known excess of dichromate in sulfuric acid, with silver sulfate catalyst and mercury sulfate for chloride masking, either in a closed reflux apparatus or a sealed tube. After cooling, the remaining dichromate is titrated with ferrous ammonium sulfate using ferroin or a potentiometric endpoint, and the consumed dichromate is converted into milligrams of oxygen per litre. Alternatively, the titration can be performed automatically with a burette, electrode and colour or potential endpoint detection, with results computed directly. The method is less affected by sample colour and turbidity than photometry and is suitable for COD values above the colorimetric range, although it is more labour-intensive and uses larger reagent volumes.

How does it work?

Chemical oxygen demand is determined by oxidising the sample with potassium dichromate in strong sulfuric acid at about 150 °C for two hours, usually in sealed tubes. Silver sulfate catalyses the oxidation of most organic compounds, and mercury sulfate masks chloride, which would otherwise be oxidised and inflate the result. The dichromate consumed is proportional to the oxidisable material and is measured either photometrically, from the colour change of dichromate to chromium(III) at 600 nm or by the decrease in dichromate absorbance at 440 nm, or by titration of the remaining dichromate with ferrous ammonium sulfate. Results are reported as milligrams of oxygen per litre. COD is faster than BOD and includes material that is not biodegradable, which is why it is used for discharge compliance and process control.

What are the key features?

• Titration with ferrous ammonium sulfate and ferroin indicator
• Potentiometric endpoint option for coloured samples
• Compatible with closed reflux and sealed-tube digestion
• Suitable for high COD values above colorimetric range
• Automatic burette and result computation options
• Not affected by sample turbidity or colour

What are the technical specifications?

A typical titrimetric cod analyzer configuration offers method of Dichromate reflux with titration, titrant of 0.1 mol/L ferrous ammonium sulfate, indicator of Ferroin or potentiometric, range of 50–1000 mg/L typical with 0.25 N dichromate, reflux time of 120 min and sample volume of 10–20 mL. Values are indicative of the product class; the exact figures for each configuration are confirmed on the quotation and the calibration documents supplied with the instrument.

Which applications is it used for?

The Titrimetric COD Analyzer is used in high-strength industrial effluent analysis, coloured and turbid wastewater testing, reference method verification of colorimetric results and landfill leachate and chemical process water. Across the wider COD Analyzers class, laboratories also deploy it for wastewater treatment influent and effluent monitoring, industrial discharge compliance testing and municipal plant process control.

Which standards and certifications apply?

Relevant standards and methods include ISO 6060, ISO 15705, EPA 410.4, Standard Methods 5220 D and ISO/IEC 17025. Certification candidates for this model are CE and ISO 9001 manufacturing; each is to be confirmed for the specific configuration before shipment, and certificates are supplied only where they have been issued.

COD chemistry is hazardous: dichromate is a carcinogen and strong oxidant, sulfuric acid is corrosive, and mercury salts are toxic, so digestion must be performed in sealed tubes or closed reflux systems with fume control and separate waste collection for heavy-metal-bearing residues. Chloride interferes seriously and is masked with mercury sulfate, so the expected chloride concentration must be known and the masking ratio respected; high-chloride samples may need dilution or a low-range method. Quality control requires blanks, standards, spiked samples and duplicates in each batch, and the dichromate solution should be standardised. Sealed tubes should be mixed thoroughly before reading, and the reading should be taken after the tubes cool to avoid temperature bias.

How do I select the right Titrimetric COD Analyzer?

Choose titrimetric COD when samples are coloured, turbid or too concentrated for colorimetric reading, or when you need the reference procedure for method verification. Confirm the reagent concentrations and burette accuracy suit your COD range, and plan for larger acid and dichromate usage.

How does it compare with other COD Analyzers variants?

Choose the measurement format from where the sample is taken and how the result will be used. Sealed-tube colorimetric systems with a digestion reactor are the standard for laboratories, offering ranges from 0 to 15,000 mg/L with small sample and reagent volumes. Portable colorimeters bring the same chemistry to the plant or river bank for rapid screening. Titrimetric analysis is used for high-concentration or coloured and turbid samples, where photometry is unreliable, and remains the classic reference for difficult matrices. Automated analysers with autosamplers reduce reagent handling and improve traceability where sample numbers are high. Online UV analysers provide continuous trend data for treatment control but must be correlated with the dichromate method, since UV absorbance responds to aromatic compounds rather than total oxidisable matter.