Application Benefits 

  • Robustness - due to gas diffusion cell
  • Simplicity - for free and available cyanide, reagents are remarkably quick and easy to prepare
  • Operator workload - for total cyanide, reduced operator burden due to the use of automated in-line digestion
  • Operator safety and environmental consciousness - avoids the use of toxic reagents

 

Keywords

Cyanide, amperometric, gas diffusion, FIA, flow injection

Introduction and Principle

Cyanide analysis has long been notorious for the use of volatile, toxic reagents (pyridine, Chloramine T) in the traditional colorimetric assay. About 20 years ago, an alternative technology was developed, based on amperometric detection, which utilizes dilute acid and base solutions as the only reagents.[1] The lack of hazardous reagents and the ease of use led to the new method gaining popularity and finally being incorporated into official USEPA, ISO and ASTM methods. After receiving an official status, it has quickly become the preferred method for cyanide determination among environmental analysts, mining companies, and industrial operations.

Cyanide in sample solution is made volatile by in-line mixing with hydrochloric or sulfuric acid. The resulting solution is then directed to a gas diffusion cell where it comes in contact with a gas-permeable membrane. Volatile cyanide migrates through the membrane into an acceptor solution of sodium hydroxide. The cyanide-containing acceptor solution is directed into an amperometric detection cell where cyanide concentration is measured as an electriccurrent that cyanide generates when it contacts a polarized silver electrode. The use of gas diffusion greatly improves method robustness by minimizing detrimental effects from the sample matrix. As only gaseous components can pass through the membrane, many potential fouling agents in the sample matrix are completely excluded from the detector.

For total cyanide measurements, an in-line digestion module can automate the process of releasing cyanide from complexes with zinc, copper, cadmium, mercury, nickel, silver and iron. When the sample passes through the digestion module, it is subjected to UV radiation that breaks down the above metal-cyanide complexes, resulting in liberated cyanide.

Experimental 

Experiments were carried out using the FIAlyzer-1000 Flow Injection Analyzer equipped with an amperometric detector.

amperometric cyanide schematic

Figure 1: A schematic of the instrument setup

Flow Rates

  • C: 1.2 mL/min
  • R1: 1.2 mL/min
  • R2: 1.2 mL/min

Sample Volumes

  •  35 μL

Heater Temperature

  • 65 C

Detector Settings 

  • Excitation: 365 nm
  • Emission: 430 nm

cyanide instrument.jpg

Figure 2: FIAlyzer-1000

The instrument setup and its accompanying methods are applicable to: 

  • Free/Available cyanide determiniation by OIA-1677-09 (EPA), 4500-CN-I (Standard Methods), 14403 (ISO) or 6888-09 (ASTM). 
  • Total cyanide determination by EPA-approved D7511-12 (ASTM). This setup would require the addition of an in-line digestion module. 
  • Total cyanide determination in post-distillation samples by D7284 (ASTM). In this case, samples are first treated by manual batch distillation. 

Results

Amperometric cyanide results in FIAsoft

 Figure 3: Example calibration run

Detection Limit 3 μg CN/L
Reporting Limit 10 μg CN/L
Range Upper Limit 500 μg CN/L*
Sample throughput 40 samples/hr

Table 1: Method performance parameters

*If desired, the range can be extended to higher concentrations by decreasing the injected sample volume. 

Conclusions

The amperometric cyanide assay is a sensitive, selective and robust method for measuring cyanide in water samples. Robustness is optimized by incorporating a gas diffusion unit to isolate the amperometric detector from potential interfering components in the sample solution.

References

[1] Milosavljevic E. et al. “Rapid distillationless “free cyanide” determination by a flow injection ligand exchange method”, Environ. Sci. Technol. 29, 1995, p. 426-430.