Quick Reference
Process control (gold leaching): WAD or Available cyanide (OIA-1677)
Environmental discharge: WAD cyanide
Drinking water: Free cyanide
Waste characterisation: Total cyanide
Keep reading for the chemistry, method selection, and common pitfalls.
Three different cyanide values on one lab report. Which one matters? The answer depends entirely on what you're trying to achieve, and getting it wrong has real consequences for compliance, process efficiency, and wildlife protection.
The Chemistry: A Quick Refresher
The cyanide ion (CN⁻) readily bonds with metals to form complexes of varying stability. Depending on pH and metals present, cyanide exists in three categories:
Free cyanide is the cyanide ion (CN⁻) and hydrogen cyanide (HCN). Below pH 9.3, most exists as volatile, toxic HCN gas. This is why gold operations maintain pH above 10.5.
Weak acid dissociable (WAD) cyanide includes free cyanide plus complexes that break apart under mildly acidic conditions (pH 4 to 6). These form with zinc, copper, cadmium, nickel, and silver.
Total cyanide is everything: free, WAD, and stable iron cyanide complexes that require strong acid, heat, and UV irradiation to break apart.
Why Does This Matter? Three Real-World Scenarios
Scenario 1: Gold Mill Process Control
In a carbon-in-leach circuit, you need to know whether you've got enough cyanide to dissolve gold. Total cyanide includes iron cyanides that don't participate in leaching. Free cyanide alone doesn't capture weak metal-cyanide complexes that release cyanide during the process.
What you need is available cyanide, functionally similar to WAD cyanide. Methods like USEPA OIA-1677 and ASTM D6888 were developed for this purpose.
The problem: traditional titration methods can overestimate available cyanide by tens of percent when copper is present. If you're dosing based on inflated titration results, you're wasting money on reagent that isn't doing anything useful.
Scenario 2: Environmental Discharge Compliance
Most environmental regulations specify WAD or free cyanide, not total, because iron cyanide complexes are far less toxic under environmental conditions.
The International Cyanide Management Code sets 50 mg/L WAD cyanide as the threshold for wildlife protection. The EU Mining Waste Directive is stricter: mines permitted after May 2008 typically must keep WAD cyanide below 10 mg/L.
Using total cyanide for compliance would be overly conservative. Using free cyanide alone might understate risk if significant WAD complexes are present.
Scenario 3: Wildlife Protection at Tailings Dams
Wildlife mortality at tailings facilities correlates strongly with WAD cyanide. Birds and bats are exposed when WAD complexes dissociate in the acidic environment of their stomachs.
Field data shows significantly reduced mortality when WAD cyanide stays below 50 mg/L. Free cyanide measurements won't capture this risk—an operation might have low free cyanide but substantial copper-cyanide complexes that become toxic after ingestion.
The Measurement Methods: Picking the Right Tool
For Free Cyanide
Measured by gas diffusion at neutral pH (~6), where free cyanide converts to HCN gas and diffuses across a membrane. No distillation, heat, or UV required.
Key methods: ASTM D7237, ISO 17690
Best for: Drinking water compliance, immediate toxicity assessment.
For WAD Cyanide
Uses weak acid conditions (pH 4 to 6) to release cyanide from metal complexes. Distillation methods take hours. Gas diffusion with ligand exchange takes minutes and eliminates many interferences.
Key methods: USEPA OIA-1677 / ASTM D6888 (ligand exchange), APHA 4500-CN-I (distillation)
Best for: Environmental compliance, wildlife protection, gold mill process control.
For Total Cyanide
Requires strong acid (pH <2), heat (~125°C distillation), and UV irradiation to break iron cyanide complexes.
Key methods: USEPA 335.4 / APHA 4500-CN-C, ASTM D7511 / USEPA OIA-1678
Best for: Comprehensive waste characterisation.
Common Pitfalls
Copper Interference
Silver nitrate titration responds to both free cyanide and copper-cyanide complexes—in high-copper ore, it can overestimate available cyanide by 2x or more. Gas diffusion amperometry (OIA-1677) sidesteps this by measuring only cyanide that diffuses as HCN.
Sample Handling
Cyanide samples are unstable. Essential rules: preserve immediately with NaOH to pH 12+, chill to 4°C, analyse within 14 days (24 hours if unpreserved), and test for oxidising agents and sulphides before preservation.
Choosing the Right Measurement: A Decision Framework
| Situation | Recommended Measurement | Why |
|---|---|---|
| Gold mill process control | WAD or Available cyanide (OIA-1677) | Measures cyanide that actually participates in gold leaching |
| Tailings dam discharge | WAD cyanide | Correlates with wildlife toxicity risk |
| Drinking water compliance | Free cyanide | Measures immediately bioavailable, toxic form |
| Cyanide destruction verification | WAD cyanide | Confirms removal of environmentally significant species |
| Waste characterisation | Total cyanide | Comprehensive accounting of all cyanide species |
| Real-time process optimisation | Available cyanide (continuous analyser) | Enables tight process control, reduces reagent costs |
The Business Case for Accurate Measurement
Getting cyanide analysis right isn't just about regulatory compliance. There's a genuine financial case for precision.
A mid-sized gold mill using continuous online cyanide analysers instead of periodic grab samples and titrations can typically achieve:
- 10 to 15% reduction in cyanide consumption through tighter process control
- Improved gold recovery by maintaining optimal cyanide concentrations
- Reduced detoxification costs by not over-treating discharge streams
- Lower risk of compliance penalties from accurate regulatory reporting
When cyanide costs several dollars per kilogram and a typical operation uses thousands of tonnes annually, a 10% reduction translates to hundreds of thousands of dollars in savings.
The Practical Bottom Line
For gold mill process control and environmental compliance, WAD or available cyanide measured by gas diffusion amperometry offers the best combination of accuracy, speed, and relevance. Getting this right protects your workers, your wildlife, your compliance record, and your bottom line.
Frequently Asked Questions
Can I use silver nitrate titration for process control?
It depends on your ore. Titration works reasonably well for low-copper ores but can overestimate available cyanide by 2x or more when copper is present. If you're processing copper-gold ore, gas diffusion methods (OIA-1677) give more accurate results.
How quickly do I need to preserve cyanide samples?
Immediately is the goal. Cyanide degrades through volatilisation, oxidation, and microbial action. Preserve to pH 12+ with NaOH, chill to 4°C, and analyse within 14 days. Unpreserved samples should be analysed within 24 hours.
Why do regulations specify WAD cyanide instead of total?
Iron cyanide complexes (included in total but not WAD) are far less toxic under environmental conditions—they don't readily break apart in an animal's stomach. WAD cyanide captures the species that actually pose ecological risk.
What's the difference between WAD and available cyanide?
They're functionally similar for most purposes. "Available cyanide" typically refers to methods like OIA-1677 that measure cyanide available for gold leaching. The results correlate closely with WAD measurements but may differ slightly depending on the metal-cyanide species present.
Walker Scientific supplies cyanide analysers for laboratory, field, and process applications across Australia and New Zealand. Explore our range:
- FS3700 Chemistry Analyser — Laboratory continuous flow analysis
Contact us to discuss which measurement approach best suits your operation.
Further Reading
- International Cyanide Management Code
- USEPA Method OIA-1677: Available Cyanide by Ligand Exchange and Amperometric Detection
- ASTM D6888: Standard Test Method for Available Cyanide with Ligand Displacement and Flow Injection Analysis
- Australian Government Leading Practice Handbook: Cyanide Management
