Free Cyanide Analysis in National Pollutant Discharge Elimination System Wastewater Samples

Hydrogen cyanide (HCN) is a water soluble compound with high toxicity. When it comes into contact with mucous membranes, such as gills, and skin of the aquatic animals, it is easily absorbed and reaches the bloodstream.

In the U.S., Section 304 (a) of the Clean Water Act mandates that the U.S. Environmental Protection Agency (EPA) periodically update and publish the ambient water quality criteria (AWQC). These criteria indicate the maximum contaminant concentrations for marine and freshwater environments based on empirical toxicity data.

The freshwater fish species rainbow trout (Oncorhynchus mykiss) was established to be the most sensitive to cyanide. Free HCN concentration in surface waters can be in the range of 0.005 to several mg/L. The acceptable chronic and acute AWQC limits for free cyanide in freshwater as established by the EPA are 5.2 and 22 µg/L, respectively.

ASTM D 7237-10

The method for calculating free cyanide concentration in effluents and aqueous wastewater described in ASTM D 7237(4) involves measuring the quantity of free cyanide ions (CN- and HCN) which dissociates from metal-cyanide complexes at a pH of 6-8, which is a representative pH range for aquatic environments.

ASTM D 7237-10 employs the technology and instrumentation similar to that specified in ASTM D 6888(5). However, it involves gentle conditions, e.g. pH 6-8 buffer versus H2SO4 or HCl, and does not employ ligand displacement reagents.

ASTM D 7237-10 can be applied to measure aquatic free cyanide and free cyanide in natural waters, wastewater effluents, and saline waters in the range of 2-500 µg/L or parts per billion.

Regulatory Status of ASTM D 7237-10 Within the U.S.

In April 2012, a Methods Update Rule (MUR) was signed by the U.S. EPA Administrator. This rule approves the application of novel analytical techniques to test the pollutants in wastewater under the Clean Water Act.

The final MUR published in the Federal Register in May 2012 comprised of six ASTM methods that included the preservation, sampling, and analysis of available, free, and total cyanide species. ASTM D 7237-10 was also included in those six methods.

ASTM D 7237-10 can now be used by facilities with National Pollutant Discharge Elimination System (NPDES) permits to test wastewater samples for the purpose of reporting regulatory compliance.

Principle of Operation

Figure 1 depicts a flow diagram illustrating the free cyanide analysis performed as outlined in the ASTM D 7237-10 standard. During the analysis, a sample that is injected into a carrier stream is mixed with a phosphate buffer at a pH of 6 to measure the free cyanide.

To measure aquatic free cyanide, the pH is maintained to be 6-8. HCN gas existing in the sample gets diffused across a hydrophobic membrane and reaches a basic acceptor solution.

Here, it is converted into CN- and is transported to a flow cell of an amperometric detector. The converted cyanide ions react with a silver electrode to generate current that is proportional to the concentration of cyanide ions.

The response of the detector is exhibited as a peak. The height of the resulting peak is proportional to the concentration of cyanide found in the sample.

Flow diagram for free cyanide analysis by ASTM method D 7237- 10.

Figure 1. Flow diagram for free cyanide analysis by ASTM method D 7237- 10.

Instrumentation for Cyanide Analysis by ASTM D 7237-10

The design and performance attributes required for a flow injection analysis (FIA) instrument to carry out the gas-diffusion amperometry method is defined by ASTM D 7237-10.

The design attributes for this method require the following: an amperometric detector including a silver working electrode, an AgCl reference electrode, a gas diffusion manifold including a hydrophobic membrane, and a stainless steel or platinum counter electrode.

All the requirements specified in ASTM D 7237-10 are satisfied by OI Analytical’s Flow Solution™ 3700 cyanide analyzer, which is a modular and compact laboratory instrument.

OI Analytical Flow Solution™ 3700 cyanide analyzer for ASTM D 7237- 10.

Figure 2. OI Analytical Flow Solution™ 3700 cyanide analyzer for ASTM D 7237- 10.

Request more information about the Flow Solution™ 3700 cyanide analyzer

Method Performance

Table 1. outlines the performance specifications for the free cyanide analysis carried out as outlined in the ASTM Method D 7237-10, using the Flow Solution™ 3700 cyanide analyzer.

Table 1. Performance specifications for ASTM D 7237-10

Range 2.0 µg/L – 500 mg/L
Throughput 30 samples/hour
Precision at 5 µg/L <1%RSD
50 µg/L <1%RSD
500 µg/L <1%RSD
Method Detection Limit (MDL) 0.1 µg/L

This information has been sourced, reviewed and adapted from materials provided by OI Analytical.

For more information on this source, please visit OI Analytical.

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