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Cyanide Measurements: Questions and Answers
1. The ASTM D19 standard "Standard Practice for Sampling, Preservation, and Mitigating Interferences in Water Samples for Analysis of Cyanide" appears to conflict with the footnotes in the regulation.
The preservation instructions in Table II of 40 CFR 136.3 recommend several ways to measure cyanide when other chemicals are present in the sample. The instructions note that the preservation steps may not work in every circumstance. This allows analysts the flexibility to develop more useful procedures. [Footnote 6 reads "...Any procedure for removal or suppression of an interference may be employed, provided the laboratory demonstrates that it more accurately measures cyanide..."] Regulatory authorities may allow use of updated interference mitigation procedures, including the ASTM procedures, on a case-by-case basis.
2. The ASTM practice allows the samples to be collected in polypropylene whereas the final rule allows only polyethylene.
Polypropylene or polyethylene or any bottle material may be used provided that this substitution does not adversely affect the determination of cyanide.
3. The ASTM practice requires the sample to be held at room temperature for 4 hours prior to analysis. Is this addressed in the final rule or in some of the approved methods?
Yes, footnote 6 to Table II at 40 CFR 136.3 states: "... Particulate cyanide (e.g., ferric ferrocyanide) or a strong cyanide complex (e.g., cobalt cyanide) are more accurately measured if the laboratory holds the sample at room temperature and pH >12 for a minimum of 4 hours prior to analysis, and performs UV digestion or dissolution under alkaline (pH=12) conditions, if necessary. ..."