Potentiometric determination of cyanide

Importance of Cyanide Determination

The accurate measurement of cyanide is critical for environmental safety, industrial quality control and regulatory compliance. Due to its high toxicity, even low concentrations (0.05 mg/L) can be lethal to aquatic life. Routine monitoring in electroplating baths, wastewater streams and natural water bodies ensures process control and protection of ecosystems.

Objectives and Study Overview

This bulletin presents a comprehensive potentiometric titration method for quantifying cyanide over a wide concentration range. It outlines protocols for high-concentration electroplating baths, moderately contaminated wastewater and trace cyanide in natural waters. The goal is to demonstrate reproducible titration conditions, sample preparation steps and calculation procedures adapted to each matrix.

Instrumentation Used

• Metrohm Titrino series (models 702 SET/MET, 716 DMS, 736 GP, 751 GPD, 785 DMP) or 726 Titroprocessor with 700 Dosino/685 Dosimat
• Magnetic stirrer (2.728.0040)
• Exchange units (6.3014.223 and 6.3014.213)
• Silver titrode with Ag₂S coating (6.0430.100) and electrode cable (6.2104.020)

Methodology and Sample Preparation

• High-concentration plating baths: Mix sample with NaOH, titrate with 0.1 M AgNO₃ and detect the first equivalence point potentiometrically.
• Wastewater (1–100 mg/L CN⁻): Adjust sample volume and titrant strength (0.01 M AgNO₃), then titrate under similar conditions.
• Trace analysis in water (≤0.5 mg/L): Pre-treatment by volatilization of HCN at pH 4 with an air stream; absorb HCN in NaOH, then titrate with low-strength AgNO₃ (0.002 M or 0.0002 M) using extended conditioning and drift control.
• Total cyanide determination: Acid digestion with HCl in the presence of Cu(I) and Sn(II), followed by HCN stripping and potentiometric titration of the absorbate.

Main Results and Discussion

The method delivers linear response across three orders of magnitude; detection limits down to 0.01 mg/L were achieved for trace samples. Titration curves exhibit well-defined inflection points. Blank values and titrant titer must be determined with each batch. Parameter optimisation (signal drift, equilibration time, pause intervals) enhances precision and repeatability.

Benefits and Practical Applications

• Rapid and accurate quantification in plating baths for Zn, Cd, Cu, Pb, Ag industries.
• Effective monitoring of cyanide destruction processes in wastewater treatment.
• Trace-level analysis in drinking and surface waters for environmental surveillance.
• Adaptable titrant concentrations and sample preparation protocols for diverse matrices.

Future Trends and Possibilities

Advances in miniaturised potentiostats and microfluidic sample handling may enable on-site real-time monitoring. Coupling with automated sample pretreatment systems and integrating sensor arrays could further improve throughput and sensitivity. Development of ion-selective electrodes tailored for cyanide may offer direct measurement alternatives.

Conclusion

Potentiometric titration with AgNO₃ provides a versatile, robust and sensitive approach for cyanide determination across a broad concentration range. Proper sample preparation, careful blank and titer determination and optimised titration parameters ensure reliable results for industrial and environmental applications.

Reference

• Metrohm Application Note T-22: Cyanide in alkaline plating baths for cadmium, copper, lead or zinc. Metrohm Ltd., Herisau.
• DIN 38405, part 13: Anions (Group D). Determination of cyanides.
• DIN 38405, part 14: Anions (Group D). Determination of cyanides in drinking, low-contaminated ground and surface water.