Total cyanide is the sum of all cyanide-containing species in a sample: free cyanide (CN⁻ and HCN), WAD-range metal-cyanide complexes (copper, zinc, nickel, cadmium, silver), and the thermodynamically stable iron-cyanide complexes (hexacyanoferrate II and III, also called ferro- and ferricyanide). Because stable iron complexes do not break apart under mild acid conditions, total cyanide measurement requires a more aggressive sample preparation step than WAD or free cyanide.
To liberate cyanide from stable iron complexes, two main preparation approaches are used. The classical method subjects samples to reflux distillation under strong acid conditions (pH below 2), driving off HCN which is swept into a sodium hydroxide absorbing solution for colorimetric measurement. The alternative flow-based approach acidifies the sample and passes it through an in-line UV irradiation cell, which photolytically breaks down even the most stable hexacyanoferrate species; the liberated HCN diffuses through a gas diffusion membrane into an alkaline receiving solution for amperometric or colorimetric detection.
Total cyanide values will always be equal to or greater than WAD cyanide for the same sample. If total cyanide comes in lower than WAD on the same sample, it is a sign of a measurement error, typically an incomplete distillation.
Total cyanide is used in comprehensive waste characterisation, regulatory reporting where the full inventory of cyanide species is required, and in situations where iron-cyanide complexes may be photodegraded by sunlight in surface tailings to release free cyanide. Some national regulators require total cyanide reporting alongside WAD in environmental impact assessments.
In practice, for most gold mining compliance purposes, WAD cyanide is the controlling metric. Total cyanide becomes relevant when iron contamination of process water is significant, when characterising historical tailings, or when jurisdictional requirements specifically mandate it.
Key Points
- Total cyanide = free + WAD + stable iron-cyanide complexes
- Always equal to or greater than WAD cyanide for the same sample
- Requires aggressive sample preparation — acid reflux distillation or in-line UV irradiation — to break down iron complexes
- Less ecologically relevant than WAD for most discharge compliance purposes
- Required for comprehensive waste characterisation and some regulatory regimes
Relevant Standards
- USEPA 335.4 (distillation/colorimetric)
- ASTM D7511 (in-line UV digestion/amperometric)
- ISO 14403 (flow analysis methods for cyanide in water)
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Frequently Asked Questions
When should I measure total cyanide rather than WAD?
Use total cyanide when you need to account for all cyanide species, including stable iron-cyanide complexes. This is relevant for characterising industrial waste streams that contain hexacyanoferrate, for tailings that may be exposed to UV light (which can photodegrade iron complexes to release free cyanide), or where your regulator specifically requires total cyanide reporting.
Can total cyanide ever be lower than WAD cyanide?
No. If your total cyanide result is lower than your WAD result on the same sample, there is a measurement error. The most common cause is an incomplete distillation in the total cyanide preparation, where not all iron complexes were broken down. The full hierarchy is free ≤ WAD ≤ total.
Why does total cyanide require UV irradiation?
Some iron-cyanide complexes, particularly hexacyanoferrate II (ferrocyanide), are extremely stable and do not break down under acid and heat alone. UV irradiation — typically from low-pressure mercury lamps at 254 nm — provides the photochemical energy needed to cleave the iron-cyanide bonds and release all the cyanide as HCN for measurement. This is the principle behind in-line UV digestion methods such as ASTM D7511.