WAD cyanide — Weak Acid Dissociable cyanide — is the sum of cyanide species in solution that release hydrogen cyanide (HCN) under weak acid conditions at pH 4.5. This fraction includes free cyanide (CN⁻ and HCN) together with the more labile metal-cyanide complexes: copper, zinc, nickel, cadmium, and silver cyanides. The stable iron-cyanide complexes (hexacyanoferrate II and III) do not dissociate at pH 4.5 and are excluded.
WAD cyanide closely tracks environmental and toxicological risk at gold-mining sites. Iron-cyanide complexes, which are excluded from WAD, are far less bioavailable than the labile species included in the WAD fraction. A WAD measurement therefore gives a more meaningful picture of actual ecological hazard than total cyanide.
In gold-mining operations, WAD cyanide is the standard compliance metric. The International Cyanide Management Code (ICMC) frames its cyanide compliance requirements in WAD cyanide terms, and most Australian and New Zealand state environmental licences follow the same convention. Many operations target WAD cyanide below 50 mg/L in active tailings pond solution as a management benchmark, with tighter limits for discharge set by site-specific environmental licences.
The most widely used analytical methods for WAD and available cyanide are OIA-1677 and ASTM D6888. Both use ligand exchange chemistry to selectively release cyanide from labile complexes, followed by gas diffusion across a hydrophobic membrane and amperometric detection. OIA-1677 was developed by OI Analytical; ASTM D6888 is the equivalent ASTM standard covering the same underlying chemistry. Continuous flow instruments such as the FS3700 Chemistry Analyser support both methods at throughputs of 20 to 60 samples per hour.
Key Points
- WAD cyanide = free cyanide (CN⁻/HCN) + labile metal complexes (Cu, Zn, Ni, Cd, Ag cyanides)
- Stable iron-cyanide complexes are excluded
- Primary compliance metric under the ICMC and most Australian environmental regulations
- Measured by ligand exchange and gas diffusion per OIA-1677 or ASTM D6888
- Always lower than or equal to total cyanide for the same sample
Relevant Standards
- OIA-1677 / ASTM D6888 (ligand exchange, gas diffusion, amperometric detection)
- APHA 4500-CN⁻ I (distillation/colorimetric)
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Frequently Asked Questions
Why is WAD cyanide used for environmental compliance instead of total cyanide?
Iron-cyanide complexes are far less toxic under environmental conditions than the species included in WAD. They do not readily break down in the stomachs of birds or fish. WAD cyanide captures the species that actually pose ecological risk, making it a more meaningful compliance metric than total cyanide.
What is the WAD cyanide limit under the ICMC?
The ICMC does not prescribe a single universal limit — signatory operations must meet applicable national laws and set their own site-specific cyanide management targets. In practice, many Australian and international operations target WAD cyanide below 50 mg/L in active tailings pond solution as a management benchmark, with tighter discharge limits set by site-specific environmental licences. Your regulator and environmental licence conditions are the authoritative source.
Can the FS3700 measure both WAD and total cyanide?
Yes. The FS3700 supports multiple cyanide methods using different chemistry cartridges — OIA-1677 and ASTM D6888 for available/WAD cyanide, ASTM D7237 for free cyanide, and ASTM D7511 or USEPA 335.4 for total cyanide, all on the same platform.