A new approach to cyanide analysis

Engineering and Mining Journal, May 1999

The new analysis method demonstrates greater specificity for cyanide in matrices where interferences have been encountered using CA TC methods and measures cyanide at lower concentrations and offers improved precision and accuracy over currently approved CA TC methods

Cyanide is a toxic under the Clean Water Act (CWA) and is a priority pollutant as derived from the toxic pollutant list. Public and private entities subject to regulatory limits for cyanide amenable to chlorination (CATC) under the CWA, as well as CWA regulatory authorities, have indicated interference problems when the currently approved methods were used to test certain sample matrices. During the 17th Annual EPA Conference on Analysis of Pollutants in the Environment, the interference situation was publicized and a request was made for comments and suggestions for a cyanide method that would reduce or eliminate these interferences. At the time, Alpkem was working with the Univ. of Nevada-Reno Mackay School of Mines on a simplified and improved method for cyanide analysis. Alpkem began a dialog with the U.S. EPA (EPA) that developed resulted in the new cyanide method becoming a test vehicle for the Agency's Streamlining Proposal.

The new method (Method OIA-1677) is divided into two parts: 1) sample pretreatment and 2) cyanide quantification via amperometric detection. In the sample pretreatment step, ligand-exchange reagents are added to a sample. Reagents displace cyanide ions (CN-) from weak and intermediate strength metallo-cyanide complexes. In the flow-injection analysis system, a 200-uL aliquot of the pretreated sample is injected into the flow injection manifold. Hydrochloric acid converts the cyanide ions to hydrogen cyanide (HCN), which diffuses through a membrane into an alkaline solution where it is converted back to cyanide ion (CN-). The amount of cyanide ion in the alkaline solution is measured amperometrically with a silver-working electrode, silver/silver chloride reference electrode, and platinum counter electrode at an applied potential of zero volts. Current generated in the cell is proportional to the concentration of cyanide in the original sample, as determined by calibration. Analysis time is 90 sec./sample. CATC concentration is determined by chlorinating the available cyanide in the sample-using calcium hypochlorite (Ca(OC1)2), measuring the HCN using the total distillation procedure (Method 335.2), and finding the difference between the total cyanide measured before and after the chlorination.

Single-Laboratory Study. Initially, Alpkem conducted a single-laboratory validation study in order to refine the method and to demonstrate the method's specificity and selectivity. The results of that study are available from the EPA in the Report of the Draft Method OIA1677 Single Laboratory Validation Study. The study consisted of three groups of tests to establish (1) applicability of the method to various metallo-cyanide complexes, (2) the ability of method to operate in the presence of interferences, and (3) performance compared to previous methods. All tests consists of two different concentrations (0.2 and 2.0 mg/L) of 11 different metallo-cyanide complexes, or interferents, analyzed individually in triplicate.

Determining the applicability of the Method OIA-1677 to various metallo-cyanide complex yielded recoveries ranging from 97% to 104% for six of the 11 complexes (cadmium, copper, mercury, nickel, silver, and zinc). As with the currently approved methods for available cyanide, cyanide from the thermodynamically and kinetically stable complexes of cobalt, gold, and iron could not be determined. In the presence of interferents, cyanide recoveries ranged from 99-103% using this method.

Method OIA-1677 was compared to the EPA-approved CATC Method 335.1 and SM 4500 CN-I. The results showed improved recoveries and reduced relative standard deviations for Method OIA-1677 compared to both the SM 4500 CN-I and the CATC methods for selected analytes. For the mercury cyanide complexes, recovery improved from 59% for SM 4500 CN- to 99% for Method OIA-1677, and high levels of interferences in the nickel and silver determinations showed similar improvements over the CATC method. Data for zinc, cadmium, and copper were comparable among the three cyanide procedures. Again, there was no recovery and, thus, no method differences for the thermodynamically and kinetically stable cobalt, gold, or iron cyanide complexes.

Inter-laboratory Study. The inter-laboratory method validation study was performed by nine laboratories, working cooperatively as the WAD Cyanide Round Robin Group. Each laboratory analyzed an identical set of nine field samples using Method OIA-1677. These field samples were collected from nine different effluents, ranging from a publicly owned treatment works (POTW) to an industry deemed likely to contain cyanide in its effluent. Each sample was analyzed in triplicate using the FIA procedure for a total of 243 analyses (nine samples performed in triplicate by nine laboratories). Study results are detailed in the report titled The Inter-laboratory Validation of Method OIA-1677 and are available from the EPA. The purpose of the inter-laboratory study was (1) to confirm the performance of Method OIA-1677 in multiple laboratories, (2) to assess Method OIA-1677 inter-laboratory data variability, and (3) to develop Method OIA-1677 quality control (QC) acceptance criteria.