Rapid determination of phosphate and citrate in carbonated soft drinks using ion chromatography

Importance of the Topic

Phosphoric acid is widely used in cola beverages as a stabilizer, preservative and tartness modifier, while citric acid plays similar roles in many other soda formulations. Accurate quantification of these acids is critical for product quality control, cost optimization and compliance with regulatory standards. Traditional colorimetric methods are labor-intensive, time-consuming and suffer from poor reproducibility, prompting the need for more efficient analytical approaches.

Objectives and Study Overview

This application note describes the development of a rapid, reliable and rugged ion chromatography method for simultaneous determination of phosphate and citrate in carbonated soft drinks. The method aims to reduce sample injection volume, extend column life, and eliminate manual eluent and suppressor regenerant preparation by using a reagent-free IC system.

Methodology and Instrumentation

The analysis was performed on a Thermo Scientific Dionex ICS-5000+ HPIC system with high-pressure eluent generation, suppressed conductivity detection and a 0.4 µL internal injection loop. A Dionex IonPac Fast Anion IIIA guard (3×50 mm) and analytical (3×250 mm) columns were used. Elution employed a 22 mM KOH eluent generated in situ by the EGC 500 cartridge at 1 mL/min, column temperature 30 °C and detection with an AERS 500 suppressor at 55 mA in recycle mode. Sample overlap autosampler operation and reduced loop overfill ensured high throughput.

Main Results and Discussion

Seven common anions and citrate were resolved in under 8 min with clear separation of phosphate from interfering cola peaks. Phosphate calibration was linear over 200–650 mg/L (r2=1.000), while citrate required a quadratic fit over 25–250 mg/L (r2=1.000) due to incomplete dissociation at higher concentrations. Precision for retention time and peak area was below 0.1% and 0.5% RSD, respectively. Recoveries of spiked samples ranged from 100% to 105%. Ruggedness testing demonstrated stable phosphate quantification over 5 000 injections with minimal retention time drift and maintained resolution. A shortened 5 min run for phosphate-only analysis further increased throughput.

Benefits and Practical Applications

• Reduced hands-on time and operator skill compared to colorimetric assays.
• Simultaneous, accurate quantification of phosphate and citrate.
• Extended column lifetime through minimized injection volume.
• Fully automated eluent and suppressor management for continuous operation.

Future Trends and Opportunities

Advances may include integration of on-line degassing modules, expansion to other beverage matrices, coupling with mass spectrometry for broader anion profiling, and development of compact IC systems for field or high-throughput screening. Continued improvements in suppressor technology and column chemistry will further enhance resolution and speed.

Conclusion

The presented IC approach provides a fast, accurate and robust solution for routine quality control of phosphate and citrate in carbonated soft drinks. By leveraging reagent-free eluent generation and optimized column design, laboratories benefit from high throughput, reduced maintenance and superior data quality.

Instrumentation Used

• Thermo Scientific Dionex ICS-5000+ HPIC system (pump, eluent generator, detector).
• Dionex IonPac Fast Anion IIIA guard and analytical columns.
• Dionex EGC 500 KOH cartridge and CR-ATC 500 trap column.
• Dionex AERS 500 suppressed conductivity detector.

References

• Hu J, Rohrer J. Rapid determination of phosphate and citrate in carbonated soft drinks using a reagent-free ion chromatography system. Thermo Fisher Scientific Application Note 169, 2016.
• Thermo Fisher Scientific. Fast determination of phosphate and citrate in carbonated beverages using on-line degassing with the Carbonate Removal Device and a Reagent-Free IC system. Application Update 153, 2016.
• Thermo Fisher Scientific. Dionex IonPac Fast Anion IIIA column product manual.