Understanding Cyanide Analysis
May. 16 2019
Cyanide is included on the list of contaminants regulated by many provincial jurisdictions across Canada. Although a common parameter when requesting sampling and testing, the complexity of the chemical species formed by the interaction of cyanide with other inorganics and heavy metals may cause confusion. When requesting analysis and reviewing data, it’s important to note that different forms of cyanide require different testing methods.
Cyanide is used in many areas of industrial activity, such as metal electroplating and processing, petroleum refining, and specialty chemicals and pharmaceutical production. One of the most common causes of cyanide release into the environment is its use in the mining industry for ore extract leaching processes. Cyanide is a very reactive chemical that is highly toxic to plant and animal life.
The cyanide molecule (CN) forms complexes with certain inorganics and trace metals. The form in which cyanide may be found in a particular environment is governed by equilibria which are dependent on temperature and pH. Due to the potential for simultaneous presence of multiple forms of cyanide, analysis can be challenging. As such, various methods have been developed to quantify each particular form. Since the classification of different cyanide species is very strongly dependent on pH, analytical methods have been correlated accordingly.
Free cyanide species include free ions (CN–) and hydrogen cyanide (HCN). These forms are highly bioavailable and pose the greatest risk of toxicity. From an analytical perspective, free cyanide is the sum of HCN and CN– in solution. These species are separated from other forms of cyanide by diffusion through a membrane using a solution at a pH between 6.0 and 7.0.
Weak acid dissociable (WAD) cyanide species include simple complexes, such as those formed with alkali metals, as well as some heavy metals that dissociate at a pH between 4.0 and 6.0. As shown in Figure 1 below, WAD cyanide includes simple cyanide compounds, as well as weak metal cyanide and moderately strong metal-cyanide complexes.
Total cyanide represents the total of all cyanide species, including free, WAD and strong metal cyanide complexes. This analysis is performed under strong acid conditions (pH < 2) in order to dissociate the strong metal cyanide species.
The relative stability of cyanide compounds is also illustrated in Figure 1 below. As the species becomes more stable, a progressively lower pH and increased heat and UV radiation are required for dissociation to the CN–/HCN form.
Figure 1: Least to Most Stable Forms of Cyanide
Water samples intended for cyanide analysis require immediate preservation with sodium hydroxide (NaOH) to a pH above 12.0 to prevent losses due to volatilization of HCN and biological degradation. Soil samples for cyanide analysis do not require preservation other than storage at 4-10°C prior to delivery at the laboratory.
Once sampled, the hold time for both preserved waters and soils is 14 days. Depending on what other chemical species are present in high concentrations in a particular sample, additional laboratory treatments may be required. These include the removal of interferences such as chlorine, sulphides, nitrates/nitrites, aldehydes and sugars.
Table 1: Sampling Requirements
|Minimum Sample Amount||10 g||50 mL|
|Sampling Containers||100 mL, amber glass||120 mL, HDPE or amber glass|
|Preserving Required||None||NaOH, pH > 12.0|
|Hold Time||14 days||14 days|
* Including toxicity characteristic leachate procedure and other leachates
Chemical Interferences: Thiocyanate
Thiocyanate (SCN–) is widely recognized as a potential interferent in the colorimetric analysis of cyanide. Under standard conditions, SCN– can be completely converted to cyanide. By optimizing distillation temperature, reagents, and the wavelength of the UV radiation used in the digestion process for total cyanide, Bureau Veritas has completely eliminated SCN– interference. This is verified daily by analyzing a high level of SCN– standard and ensuring that the results are less than the reporting detection limit (RDL) for cyanide.
Cyanide Analysis at Bureau Veritas
Bureau Veritas offers free, WAD and total cyanide analyses in waters, soils, leachates and air samples.
- Testing for free cyanide in air samples is performed on stack gas samples using ion chromatography; this is a single instrument method for the determination of HCN.
- The determination of free cyanide in water is performed by diffusion through a membrane at neutral pH, followed by colourimetric analysis of the captured cyanide.
- Total cyanide analysis is performed by acid distillation of the sample, followed by an ultraviolet (UV) irradiation step in order to fully dissociate strong metal-cyanide complexes. Once dissociated, the cyanide species are measured using automated colourimetry.
- WAD cyanide analysis in water and soil samples is performed by weak acid distillation of the sample with no UV radiation, followed by automated colourimetry.
It should be noted that WAD cyanide analysis at Bureau Veritas measures all simple and weakly dissociable cyanides that form HCN at a pH of 4.0. As described above, this includes free cyanide (CN–/HCN), alkali metal, and weak to moderately strong heavy metal complexes. From a regulatory standpoint, the Ontario Ministry of the Environment Conservation and Parks’ (MECP) criteria for free cyanide are based on WAD cyanide. CCME and Alberta Tier 1 criteria are based on free cyanide only. Since WAD cyanide results include the free cyanide concentration, if a WAD cyanide result is below the regulatory standard for free cyanide, then de facto the free cyanide concentration is below the regulatory standard.
Table 2: Cyanide Reporting Detection Limits
|Free Cyanide (water & leachates)||-||1 μg/L|
|WAD Cyanide||0.01 μg/g||1 μg/L|
|Total (SAD) Cyanide||0.5 μg/g||1 μg/L|