Determination of 4-carboxybenzaldehyde in terephthalic acid by polarography

Significance of the topic

4-Carboxybenzaldehyde (4-CBA) is a key impurity in the production of terephthalic acid, impacting polymer quality and industrial processes. Reliable detection of 4-CBA at the low microgram-per-gram level is essential for quality control and regulatory compliance in polyester manufacturing.

Objectives and overview of the study

This application bulletin outlines a rapid polarographic method for quantifying 4-CBA in terephthalic acid. The goal is to demonstrate a simple sample preparation, achieve detection in the lower ppm range, and validate precision and accuracy using standard addition.

Methodology

The procedure involves:

• Weighing 5 g of terephthalic acid and forming a slurry with 40 mL ultrapure water.
• Adjusting pH by adding 15 mL of 25 % ammonia solution with gentle heating; clearing the solution with minimal NaOH if required.
• Cooling, diluting to 100 mL with water, and adding 3 mL ammonium buffer (pH 9.6).
• Performing differential pulse polarography with standard addition: 10 mL of sample solution is measured, and known increments of 4-CBA are added to establish calibration.

Used instrumentation

• VA instrument capable of Multi-Mode Electrode operation and differential pulse measurement.
• Working electrode: Multi-Mode Electrode pro with mercury drop capillary (DME).
• Reference electrode: Ag/AgCl in 3 mol/L KCl.
• Auxiliary electrode: Platinum rod.
• Supporting electrolyte: Ammonia buffer (1 mol/L NH4Cl, 2 mol/L NH3, pH 9.6).

Main results and discussion

The method yields a linear polarographic response for 4-CBA around ‑1.20 V. In an example determination, a terephthalic acid sample (5 g) produced a concentration of 9.530 µg/g ± 0.010 µg/g (0.11 % RSD). Detection limits reach the lower ppm range, and precision is maintained through replicate standard-addition measurements. The dropping mercury electrode provides stable, reproducible peak currents suitable for routine analysis.

Benefits and practical applications of the method

• Minimal and straightforward sample preparation without organic solvents.
• High sensitivity and low detection limit for trace-level impurities.
• Short analysis time supports high-throughput quality control.
• Standard addition compensates for matrix effects in complex samples.

Future trends and potential applications

While mercury electrodes offer excellent performance, environmental and safety concerns encourage development of alternative electrodes (e.g., solid microelectrodes or mercury-film electrodes). Automation and coupling with flow systems or microfluidic platforms could enhance throughput. The approach may be extended to other benzaldehyde derivatives and related aromatic impurities in petrochemical or polymer industries.

Conclusion

The described differential pulse polarographic method enables fast, precise quantification of 4-carboxybenzaldehyde in terephthalic acid down to microgram-per-gram levels. Its simplicity, reproducibility, and sensitivity make it well suited for routine industrial quality control.