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Substitution of chemicals for cyanide analysis

Can I substitute lead nitrate for lead carbonate as sulfide precipitating agent? Will the addition of sulfamic acid counteract the nitrate that may interfere due to the above reaction? I am working on my undergraduate dissertation and would like anything that may of help.

Many thanks.

Arnold Tanoy
- Iloilo City, Iloilo Province, Philippines
2003

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Lead and all other base metal salts should not be used in an attempt to remove sulfide prior to cyanide analysis. The lead sulfide formed rapidly reacts with cyanide forming thiocyanate and a significant loss in cyanide recovery. Cadmium and zinc can form insoluble cyanide complexes with iron that will not be recovered.

The best alternative if sulfide is present is to dilute to less than 50 ppm S and analyze within 48 hours using ASTM D6888 as the determination step. Use this determination step even if cyanide is distilled.

Nitrate should not be added since it will react in the distillation with thiocyanate and organic compounds generating cyanide. This occurs even with sulfamic acid added.

Also, be careful with preservation since it has been repetitively demonstrated that use of NaOH can generate cyanate if samples have been chlorinated. However, it has also been demonstrated that under different circumstances preservation with NaOH can cause complete loss of CN.

William Lipps
- College Station, Texas
June 21, 2008