Fe(II)/Fe(III) speciation in iron citrate capsules by voltammetry

Significance of the topic

Iron citrate and related citrate-complexed iron formulations are widely used in food supplements, pharmaceuticals, and technical applications. Determining not only total iron but the distribution between Fe(II) and Fe(III) is crucial because oxidation state influences bioavailability, stability, and safety. Conventional bulk methods (e.g., ICP-MS) quantify total iron but do not directly differentiate oxidation states without additional preparation. Voltammetric speciation provides a direct, rapid, and robust approach to resolve Fe(II) and Fe(III) in complex citrate matrices while controlling matrix effects and minimizing sample manipulation.

Objectives and overview of the application note

The application note demonstrates a voltammetric workflow for fast speciation of Fe(II) and Fe(III) in commercially available iron citrate capsules. The goals were to establish a simple sample preparation, apply a reliable measurement protocol that compensates matrix effects, and verify accuracy and recovery versus the declared iron content of the capsules.

Methodology

The capsules (nominal 20 mg iron per capsule) were dissolved in an acidic medium to liberate iron from the citrate matrix. The dissolved sample was introduced into a vessel containing purged supporting electrolyte to avoid unwanted oxidation by dissolved oxygen. Quantification used standard addition with two separate stock solutions (Fe(II) and Fe(III)) added stepwise to the sample; this approach minimizes matrix bias and prevents interconversion during calibration by using distinct, stabilized standards for each oxidation state. Key voltammetric settings used for differential pulse/stripping analysis in Drop Mercury Electrode (DME) mode included:

• Mode: DME (Dynamic/Drop Mercury Electrode)
• Potential scan: start 0.0 V to end -1.5 V
• Sweep rate: 30 mV/s
• Peak potentials observed (method targets): Fe(II) near -0.25 V and Fe(III) near -0.80 V

The protocol employed two standard additions per measurement (one for Fe(II) and one for Fe(III)) to adapt to different concentrations and suppress oxidation artifacts during calibration.

Used instrumentation

• 884 Professional VA manual potentiostat/galvanostat equipped for the Multi-Mode Electrode (MME) workflow.
• Multi-Mode Electrode pro (mercury-based electrode capable of DME/SMDE/HMDE operation).
• viva software for instrument control, data acquisition, and evaluation.

The system supports automatic calibrator readjustment before each measurement for enhanced precision and allows modular measuring heads to change electrode types quickly.

Results and discussion

The method demonstrated accurate quantification of Fe(II) and Fe(III) in the sample. Representative outcomes from three parallel determinations gave mean concentrations of approximately 16.03 mg/L for Fe(II) and 16.51 mg/L for Fe(III). The expected total iron concentration (sum of declared species) was 31.84 mg/L while the voltammetric determination returned 32.46 mg/L, corresponding to a recovery of about 102% for total iron. These results indicate:

• Good agreement with declared iron content and acceptable analytical recovery (≈102%).
• Capability to resolve both oxidation states in a citrate-rich matrix, despite citrate’s tendency to form multiple iron complexes.
• Practical robustness of the standard-addition strategy to compensate matrix effects and potential oxidation during handling.

Potential caveats include the need to control oxygen exposure and light during sample handling to avoid artificial oxidation or reduction, and attention to electrode maintenance and replacement of mercury drops (or selection of alternative electrodes) for routine use.

Benefits and practical applications

• Direct oxidation-state-specific quantification: provides separate Fe(II) and Fe(III) values without extensive chemical separation.
• Fast and resource-efficient: minimal sample preparation and rapid measurement cycles suitable for QC environments.
• Matrix-resilient calibration: standard addition from defined Fe(II) and Fe(III) stocks corrects for citrate-related matrix effects and varying ligand speciation.
• Adaptable instrumentation: the 884 Professional VA platform and MME head allow switching electrode types and methods for broader heavy-metal trace analysis needs.

These attributes make the approach suitable for routine quality control of dietary supplements and pharmaceutical iron preparations, as well as for research studies assessing formulation stability and bioavailability influenced by iron speciation.

Future trends and potential uses

• Electrode alternatives: development and adoption of non-mercury electrodes (e.g., bismuth-based, solid-state) will improve environmental and operator safety while maintaining speciation capability.
• Method integration: combining voltammetry with separation techniques or spectroscopic detectors could increase confidence in complex matrices and enable speciation of tightly bound complexes.
• Automation and in-line monitoring: automated sample handling and inline voltammetric sensors could enable real-time monitoring during manufacturing or stability studies.
• Advanced data processing: chemometric deconvolution and AI-assisted peak identification can improve interpretation when multiple overlapping complex species are present.
• Regulatory uptake: wider validation and inter-laboratory studies can support adoption of voltammetric speciation methods for official QC workflows.

Conclusion

The voltammetric workflow using the Multi-Mode Electrode pro on the 884 Professional VA system provides a rapid, accurate, and matrix-tolerant solution for Fe(II)/Fe(III) speciation in iron citrate capsules. With proper control of oxidation conditions and the use of separate standard additions for each oxidation state, the method yields reliable quantitative results and high recovery of total iron, making it suitable for routine quality control and formulation assessment.

References

• Application Note AN-V-241: Fe(II)/Fe(III) speciation in iron citrate capsules by voltammetry, Metrohm.
• Instrument and configuration: 884 Professional VA manual for Multi-Mode Electrode (MME); Multi-Mode Electrode pro documentation (Manufacturer: Metrohm).
• Internal reference: AW VA CH-0646-032026.