Free cyanide is the sum of the cyanide ion (CN⁻) and hydrogen cyanide (HCN) dissolved in solution. These are the simplest, most mobile cyanide species and the most acutely toxic to living organisms. At typical environmental pH, the proportion of HCN rises as pH decreases: the crossover point (where HCN equals CN⁻) is approximately pH 9.2 (the pKa of HCN). At pH 8, HCN already accounts for roughly 94% of free cyanide; at pH 7, essentially all of it is HCN. Because HCN is uncharged and crosses cell membranes more readily than CN⁻, lower-pH waters pose greater acute hazard at any given free cyanide concentration.
Free cyanide is a subset of WAD cyanide, which in turn is a subset of total cyanide. At the same site and in the same sample, free cyanide will always be less than or equal to WAD cyanide.
In gold processing circuits, free cyanide is the active leaching species. Monitoring free cyanide concentration in the leach tank allows operators to maintain the optimal reagent level for gold recovery without over-dosing, which wastes reagent and creates additional waste treatment burden downstream.
For environmental and drinking water assessment, free cyanide is used when the immediate acute toxicity fraction is of interest rather than the broader compliance metric. WHO and many national drinking water guidelines specify free cyanide limits, not WAD limits.
The historically common method is silver nitrate titration (Liebig-Dénigès). However, this method is not specific to free cyanide in complex matrices and can overestimate results when copper-cyanide complexes are present. Gas diffusion methods per ASTM D7237 are more selective, operating at approximately pH 6 so that labile metal-cyanide complexes do not dissociate and interfere with the measurement.
Key Points
- Free cyanide = CN⁻ (cyanide ion) + HCN (hydrogen cyanide)
- Most acutely toxic and bioavailable cyanide species
- Always a subset of WAD cyanide, which is a subset of total cyanide
- HCN is the predominant form below pH 9.2 (the pKa of HCN); at pH 7 essentially all free cyanide is HCN
- Active leaching species in gold-processing circuits
- Measured by silver nitrate titration or gas diffusion (ASTM D7237, ~pH 6)
Relevant Standards
- ASTM D7237 (gas diffusion/amperometric, ~pH 6, selective for free cyanide)
- APHA 4500-CN⁻ (titrimetric and colorimetric)
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Frequently Asked Questions
What is the difference between free cyanide and WAD cyanide?
Free cyanide is only the cyanide ion (CN⁻) and hydrogen cyanide (HCN). WAD cyanide includes free cyanide plus the more labile metal-cyanide complexes (copper, zinc, nickel, cadmium, silver) that also break down and release HCN at pH 4.5. WAD is the broader measure.
Why is free cyanide important in gold processing?
HCN and CN⁻ are the active species in gold leaching. They react with gold particles to form soluble gold-cyanide complexes that can be recovered downstream. Maintaining the right free cyanide concentration in the leach circuit is critical for efficient gold recovery.
Is silver nitrate titration accurate for free cyanide?
Not in complex matrices. Liebig-Dénigès titration works reasonably well for clean solutions but overestimates free cyanide in the presence of copper-cyanide complexes, which are common in gold mine process waters. Gas diffusion methods (ASTM D7237) are more selective and give more accurate results.