Redox titration of vitamin C in orange juice using OMNIS

Importance of the Topic

Vitamin C (ascorbic acid) is a key antioxidant in food products, playing an essential role in human nutrition and food quality. Accurate quantification of vitamin C is critical for quality control in the food and beverage industry to ensure nutritional labeling compliance and product consistency.

Objectives and Study Overview

This application note presents a rapid and precise method for determining vitamin C in orange juice using the OMNIS titration system. The study aims to demonstrate automation of sample handling, reagent dosing, and endpoint detection to achieve reproducible results with minimal manual intervention.

Methodology and Instrumentation

No extensive sample preparation is required. A 50 mL aliquot of orange juice is directly pipetted into the titration vessel. Glyoxal (2 mL, 40% w/w) is added automatically and allowed to react for 5 minutes to eliminate interferences. Sulfuric acid (5 mL, 1.0 mol/L) is then dosed to acidify the medium. Titration is performed potentiometrically with 0.01 mol/L iodine as titrant, using a double platinum‐sheet electrode. The OMNIS setup comprises:

• Sample Robot S with Pick&Place and 2-channel pump modules
• OMNIS Titrator and Dosing Module (2× 5 mL and 1× 20 mL cylinders)
• Double Pt-sheet electrode and analog measuring module
• Rod stirrer and OMNIS software license

Key operational parameters include an applied polarisation current of 1.0 µA, start volume 7.5 mL, stirring rate 8, 50 µL dosing increments, and endpoint detection with a 30 mV criterion.

Main Results and Discussion

Under the stated conditions, five replicate determinations yielded an average vitamin C concentration of 303.8 mg/L with a relative standard deviation of 0.44%. The low variation demonstrates high precision. The combination of glyoxal treatment and acidification effectively suppresses interference from other oxidizable compounds, ensuring selectivity toward ascorbic acid.

Benefits and Practical Applications

The automated OMNIS method offers:

• Minimal manual handling and sample preparation
• High throughput through programmable dosing and robotics
• Robust endpoint detection via potentiometry
• Reproducible and traceable results, ideal for QA/QC in beverage production

Future Trends and Opportunities

Advancements may include integration of inline sample delivery for continuous monitoring, adoption of microfluidic platforms to reduce reagent consumption, and application of machine learning to optimize titration curves. Expansion to other antioxidants and food matrices can broaden the method’s applicability.

Conclusion

The described OMNIS potentiometric titration offers a reliable, automated, and accurate approach for vitamin C analysis in orange juice. Its high precision and low interference make it a valuable tool in routine food quality laboratories.

Reference

1. Application Note T-162: Redox titration of vitamin C in orange juice using OMNIS, Metrohm.