ISO 14403:2002 is an International Standard that defines methods for the determination of total and free cyanide in water samples by continuous flow analysis. Colorimetric or amperometric techniques are both permissible under this standard.
The colorimetry of the ISO standard uses isonicotinic acid in place of pyridine and is very sensitive. The standard provides two equal options for the separation of cyanide from the sample matrix; flash distillation or gas diffusion. The standard states that total dissolved solids concentrations greater than 10,000 mg/L interfere with the flash distillation process and recommends either gas diffusion, or sample dilution if the dissolved solids concentration is high. The results of distillation or gas diffusion of samples containing less than 10,000 mg/L dissolved salts are considered equivalent.
The ISO standard does not include any interference studies or data demonstrating the performance of the method when interferences are present. The standard states that organonitrogen compounds may produce false positive results if distillation is used; the gas diffusion separation does not recover cyanide from organic compounds.
The ISO standard mentions that sulfide < 100 mg/L does not interfere, however, this contradicts the referenced paper by Berman, which found significant interferences from sulfide at concentrations greater than 10 mg/L. Other than referencing this article by Berman, the ISO standard does not include any data measuring cyanide with sulfide present.
The ISO standard does not include interference studies demonstrating the performance of the method with cyanide present or even without cyanide present. It does mention that sulfite interferes at concentrations greater than 1 mg/L, but offers no remedies of how to alleviate the interference, how to detect sulfite in samples, or whether this interference is positive or negative (the interference from sulfite causes cyanide results to be low, or even not detected).
The ISO standard considers “free” cyanide as the metal cyanide complexes that release cyanide as HCN at pH 3.8 without exposure to UV irradiation. This “free” cyanide will not recovery mercury or nickel complexes without ligand addition, nor is it clear whether the method measures cyanide complexes of copper or silver. Without support data, we can assume that the ISO standard only reports CN, HCN, cadmium, and zinc complexes as “free” cyanide. Knowing this is important because the ISO standard results for free cyanide will not be equivalent to free cyanide obtained from other techniques, such as CATC, WAD, or available cyanide, that also often report the results as “free” cyanide.
The ISO 14403:2002 standard considers “total” cyanide as the cyanide that is recovered after acidification at pH 3.8 and exposure to UV irradiation > 290nm. The standard requires recovery of cyanide from 1.0-ppm thiocyanate at less than 1 %. There is no data in the standard demonstrating that recovery from thiocyanate is less than 1%. Also, many samples will contain thiocyanate at concentrations much greater than 1 ppm. A 1% recovery of cyanide from thiocyanate at 20 ppm results in a 200 ppb false positive result.
The ISO standard requires that recovery from ferric iron cyanide complexes must be greater than 90%, however, the standards own validation study did not achieve recoveries for total cyanide greater than 90%.
The ISO standard interlaboratory study only tested drinking water, surface water, and a low carbon secondary wastewater treatment effluent. The standard did not evaluate complex matrices.
The ISO standard allows both distillation and gas diffusion. The distillation is very complex requiring heat, chilling water, and vacuum pumps. The standard is obviously not as easy for the analyst to perform as ASTM D7511.
ASTM D7511-09 is an International standard that operates similarly to other international standards, such as ASTM D6888-09, ASTM D7237-06, and ASTM D7284-08. ASTM D7511-09 was developed in conjunction with OIA1677, a USEPA method for available cyanide that was approved by the EPA in 1999. Similar to ISO 14403:2002, ASTM D7511-09 uses UV irradiation at 312 nm to dissociate cyanide as NCH from metal cyanide complexes. In environmental samples the complexes that comprise “total” cyanide are usually complexes of ferrous and ferric iron.
ASTM D7511-09 separates the HCN generated by the UV irradiation with gas diffusion and then measures the cyanide by amperometry. Amperometry is a very sensitive electrochemical technique that directly and selectively measures only the cyanide ion. Salt concentrations greater than 10,000 mg/L do not interfere.
ASTM D7511-09 mitigates sulfide interference at concentrations up to 50 mg/L. This concentration of sulfide approximates the concentration that is detected by commonly used lead acetate test strips. The ASTM also experimentally determined that concentrations greater than 50 mg/L S rapidly deplete cyanide concentrations from the time of sampling to the time of analysis. ASTM D7511-09 recommends testing for sulfide when sampling and only treating the sample if the lead acetate paper detects sulfide.
The ASTM evaluated over 20 potential interferences at two different concentrations in the absence of and in the presence of cyanide (also at two different concentrations). The results of these interference studies are summarized in the standard with the remainder available in the ASTM D7511 research report.
ASTM D7511-09 minimizes recovery of cyanide from thiocyanate to <0.1%. For instance, 20-ppm thiocyanate recovers 20 ppb or less cyanide. While still a positive interference, this recovery from thiocyanate is lower than any other total cyanide method.
The ASTM evaluated the recovery of 15 metal cyanide complexes at two different concentrations, and also compared results to two different distillation cyanide methods (EPA335.4 and ASTM D7284). Results were considered equivalent and the data is included in the standard and/or the research report.
ASTM D7511-09 evaluated 9 very complex matrices known, and demonstrated, to be problematic with conventional cyanide methods. These 9 matrices were first thoroughly tested in a single lab study, and then the same samples were distributed and evaluated in a multiple laboratory trial. This data is also available in the standard and the research report.
In summary, the ISO 14403; 2002 standard:
Did not determine the recovery of cyanide from multiple metal cyanide complexes
Did not evaluate interferences at different levels with and without cyanide present
Does not properly address sulfide interference
Does not compensate for interferences from sulfite
Did not evaluate the method using complex matrices
Recovers more cyanide from thiocyanate (if distillation is used) than ASTM D7511-09
Determined the recovery of cyanide from 15 metal cyanide complexes at 2 different concentrations, and compared the results to distillation methods.
Evaluated over 20 different potential interferences in two different concentrations with and without cyanide present.
Accurately determines cyanide in solutions containing up to 50 ppm Sulfide. Provides details on testing and treatment of samples containing higher sulfide concentrations.
Does not suffer from sulfite interference
Was evaluated in 9 very complex matrices known to be problematic with conventional cyanide methods.
Recovers less cyanide from thiocyanate than any other cyanide method.