Environ. Sci. Technol. 1997, 31, 52-57
Method Comparison and Evaluation for the Analysis of Weak Acid-Dissociable Cyanide JOHN R. SEBROSKI* AND RICHARD H. ODE Bayer Corporation, Environmental Testing Services, State Route 2 North, New Martinsville, West Virginia 26155
This study compared and evaluated three methods to determine weak acid-dissociable cyanide. An emphasis was placed on data quality using a performance-based approach to overcome the problems associated with cyanide analyses. The three methods compared were Standard Methods, 18th ed., Method 4500 CN- I (macro distillation, colorimetric finish); Bayer Method SA-61A (steam distillation at pH 4.5, ion selective electrode finish); and Bayer Method SA-100 (FIA or flow injection analysis, gas diffusion separation, and amperometric detection). The study demonstrated method detection limits, recoveries of cyanide species, and use of ligand exchange reagents to improve selected cyanide species recoveries. Potential interferences were examined with the FIA procedure to demonstrate that the method would be applicable to industrial wastewater samples subject to NPDES regulations. This performance-based approach led to a successful means to measure weak acid-dissociable cyanide in an industrial effluent where other approved methods have failed.
Introduction Many industries are required to report cyanide to the U.S. Environmental Protection Agency or to their residing state agency as part of the NPDES or SPDES (National or State Pollution Discharge Elimination System) permit monitoring program as required by the Clean Water Act. Several state agencies and the EPA have recognized the need to regulate free cyanide in lieu of total cyanide. The term “free cyanide” is considered the sum of cyanide anions (CN-) and hydrogen cyanide (HCN), which are the two most toxic cyanide species (1). Of course, what the EPA actually denotes as free cyanide are those cyanide species that are readily dissociated into the free form. The only free cyanide method that is currently approved for NPDES monitoring is the cyanide amenable to chlorination (CATC) procedure (2). The procedure requires two analytical determinations (total cyanide and cyanide after chlorination) and is subject to many interferences that have been well documented (3-6). In fact, the ASTM states that it is beyond the scope of the test method to describe procedures for overcoming all of the possible interferences that may be encountered (4). Additionally, nickel and mercury cyanide species are reported to have incomplete recoveries and are a function of concentration, i.e., at lower concentrations, a higher percentage of cyanide is dissociated (5, 6). Another method documented by ASTM and Standard Methods for the Examination of Water and Wastewater, 18th ed., is the weak acid-dissociable (WAD) procedure (3, 4). This method is gaining acceptance in states that include (but not * Corresponding author telephone: (304) 455-4400; fax: (304) 455-5134; e-mail:
[email protected].
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limited to) Texas and Pennsylvania. This method is not subject to as many interferences and only requires one analytical determination. While this method is an improvement over the CATC procedure, interferences present in the sample matrix can still pose problems. A deficiency of the WAD procedure is the incomplete recovery of mercury cyanide species when concentrations are greater than 20 µg/L CN(5). It has been suspected that the former procedures described can create cyanide during the harsh distillation process, yielding a positive interference in some matrices. This study compared and evaluated three methods to determine weak acid-dissociable cyanide. An emphasis was placed on data quality using a performance-based approach to overcome the problems associated with cyanide analyses. The three methods compared were as follows: (a) Standard Methods, 18th ed., Method 4500 CN- I, manual distillation at pH 4.5, colorimetric finish (3); (b) Bayer Method SA-61A, steam distillation at pH 4.5, ion selective electrode finish (7), and (c) Bayer Method SA-100, flow injection analysis (FIA), gas diffusion separation, and amperometric detection (8). The study evaluated method detection limits, recoveries of cyanide species, and use of ligand exchange reagents to improve selected cyanide species recoveries. Potential interferences were examined with the FIA procedure to determine if the method would be applicable to a broad spectrum of matrices such as POTW (publicly owned treatment works) effluents, industrial effluents, groundwater wells, and surface waters. The goal of the study was to determine the optimum method to measure weak acid-dissociable cyanide in industrial wastewater samples subject to NPDES or SPDES regulations. Toxicity studies have shown that only 10.69 and 5.6 parts per billion (ppb) free cyanide is chronically toxic to freshwater and marine water aquatic life, respectively (9); therefore, a method detection limit of less than 5.6 ppb is required. In order for the method to be acceptable, complete recoveries of WAD cyanide species are necessary. Conversely, the less toxic cyanide species such as potassium ferrocyanide and potassium ferricyanide should not be recovered as WAD cyanide. Preferably, the method could be automated and would not be subject to any interferences.
Experimental Section Procedures and Apparatus. Method 4500 CN- I. Samples were refluxed for 1 h in a macrocyanide distillation apparatus (Kontes Catalog No. 479100-000) at pH 4.5 in the presence of zinc acetate as described in the method. The pH was adjusted with an acetate buffer solution. Distillates were collected in 1.25 N NaOH and analyzed colorimetrically with pyridine-barbituric acid reagent at 578 nm in a 1-cm cell utilizing a HACH DR 3000 spectrophotometer. Method SA-61A. This method, developed at Bayer Corporation’s Environmental Testing Services (ETS) Laboratory in 1994, uses the same reagents and standards as described in Method 4500 CN- I. The only difference in sample preparation was that a 10-min distillation was performed on a steam distillation apparatus typically used for ammonia analyses. The apparatus utilized was a BUCHI 322 steam distillation unit. The distillate was collected into 50 mL of 1.25 N NaOH solution and diluted to 250 mL total volume prior to analysis. The distillates were analyzed with an Orion Model 94-06 cyanide ion-specific electrode and Model 90-02 double junction reference electrode connected to an Orion Model 520 mV meter. Calibrations were performed daily and plotted on a semi-logarithmic graph. Method SA-100. This method is a modified version of a flow injection analysis (FIA) system that was developed by
S0013-936X(96)00016-8 CCC: $14.00
1996 American Chemical Society
the University of Nevada, Reno, and Perstorp Analytical Environmental for the precious metal mining industry (6, 10). Ligand exchange reagents were added as a sample pretreatment to free the cyanide from metal-cyanide complexes. Various ligand exchange reagents were examined during this study. The samples were injected into the FIA system (Perstorp Model 3202 CNSolution cyanide analyzer and a Tecator 5027 autosampler) where the samples were acidified online with 0.12 M HCl. Hydrogen cyanide was separated through a polypropylene, hydrophobic, gas diffusion membrane with 0.1 µm pore size (Perstorp Analytical PN A0015200 or Gelman Sciences PN M5PU025) and collected into a 0.1 M NaOH acceptor stream. The determinative step was done with an amperometric flow cell and detector. Two flow cells were utilized during the study: the Perstorp Analytical Model 3202 stock flow cell and a Dionex flow cell (Perstorp Analytical PN 002298). The method parameters were as follows: 200 µL injection loop, 150 s cycle time, 130 s injection time, peak height evaluation, nonlinear calibration, and 0.0 V applied potential. Calibrations were performed daily, and calibration verification/updates were performed every 10 samples.
Reagents and Standards Reference Solutions. Stock reference solutions of potassium cyanide were prepared at approximately 1000 mg/L as CNin sodium hydroxide and standardized by titration as described in Method 4500 CN- D (3). Several other cyanide species were prepared at approximately 1000 mg/L as CN-. Soluble cyanide species Hg(CN)2 (Aldrich Catalog 20,814-0), K3Fe(CN)6 (Aldrich Catalog 24,402-3), K4Fe(CN)6 (Aldrich Catalog 22,768-4), and K2Ni(CN)4 (Pfaltz and Bauer Catalog N05040) were directly weighed and dissolved in 0.01 M NaOH. The cyanide species CuCN (Aldrich Catalog 21,630-5), AgCN (Aldrich Catalog 18,453-5), Zn(CN)2 (Aldrich Catalog 25,6498), and Hg(CN)2 were stoichiometrically added to potassium cyanide in 0.01 M NaOH to form the following cyanide species in solution: [Cu(CN)4]3-, [Ag(CN)2]-, [Zn(CN)4]2-, and [Hg(CN)4]2-. Intermediate solutions at 100 mg/L were prepared in 0.01 M NaOH. All cyanide solutions were stored in amber glass bottles under refrigeration at 4 °C. Quality Control Samples. An EPA-certified quality control sample (amenable and total cyanide) was prepared as 125 µg/L free cyanide and 250 µg/L total cyanide (half of which was free cyanide). The quality control solution was prepared from 5.00 mL each of QC SPEX-CYN ampules 1 and 2 (available from Spex Industries Inc., Edison, NJ) per liter. Additionally, single blind quality control samples were prepared from ERA Lots 9959 and 9960 (available from Environmental Resource Associates, Arvada, CO) as per ERA instructions. All quality control solutions were prepared daily and preserved with NaOH to a pH > 12. Laboratory-fortified blanks and laboratory blank samples were analyzed on a routine basis for each analytical method under evaluation. Refer to each analytical method for additional quality control requirements (3, 7, 8). Ligand Exchange Reagents. Various ligand exchange reagents were prepared to evaluate their effectiveness at improving certain cyanide species recoveries. EDTA (ethylenediaminetetraacetic acid, Fisher Scientific Catalog BP118500), EGTA ([ethylenebis(oxyethylenenitrilo)]tetraacetic acid, Fisher Scientific Catalog 02783-100), and CDTA ((()-trans1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid monohydrate, Aldrich Catalog 31,994-5) were prepared as 0.1 wt % solutions in 0.01 M NaOH. Dithizone (diphenylthiocarbazone, Fisher Scientific Catalog D90-10) was prepared as a 0.01 wt % solution in 0.01 M NaOH and was subjected to sonication prior to use. TEP (tetraethylenepentamine, Aldrich T1,150-9) and Tiron (4,5-dihydroxy-1,3-benzene disulfonic acid disodium salt monohydrate, Aldrich Catalog 17,255-3) were prepared as 0.1 wt. percent solutions in laboratory-
FIGURE 1. Method detection limits (MDLs) were determined at the 99% confidence interval as described in 40 CFR Part 136 Appendix B. The reliable detection limit (RDL) and limit of quantitation (LOQ) are also shown for each analytical method. grade water. All ligand exchange reagents were added at 100 µL/10-mL sample, except dithizone which was added at 500 µL/10-mL sample.
Results and Discussion Method Detection Limits. According to Standard Methods, 18th ed., the colorimetric method is capable of a lower detection limit in the range of 5-20 µg/L (3). As a performance indicator, method detection limits were determined for Method 4500 CN- I, Method SA-61A, and Method SA-100 as described in 40 CFR Part 136 Appendix B (12). At least seven replicate analyses were performed at concentrations between 1 and 5 times the MDL. The MDL was calculated to be the standard deviation × the student’s t value. The concentration at which there is an “acceptably low risk” of making a false error and failing to detect an analyte which is present at that concentration is defined as the RDL (reliable detection limit) by Keith (13). The RDL was calculated as twice the MDL. The LOQ (limit of quantitation) can be calculated as approximately three times the limit of detection or MDL (14). The LOQ is defined as the level above which quantitative results may be obtained with a specified degree of confidence (13). The MDL, RDL, and LOQs are reported for each analytical procedure in Figure 1. The detection limits for Method SA100 are reported with the FIA instrument’s stock flow cell. Later, the MDL was determined to be considerably lower (0.317 µg/L) with the Dionex flow cell installed on the FIA system. Method SA-100 (FIA, gas diffusion, amperometric detection) was the only procedure with an LOQ lower than the 5.6 µg/L marine toxicity level. Cyanide Species Recovery Study. Cyanide solutions of KCN, K2Ni(CN)4, [Cu(CN)4]3-, [Ag(CN)2]-, [Zn(CN)4]2-, Hg(CN)2, K4Fe(CN)6, and K3Fe(CN)6 were prepared at 100 µg/L and analyzed in triplicate by Methods 4500 CN- I, Method SA61A, and Method SA-100. No ligand exchange reagents were added to determine initial method deficiencies. The data and RSD values are reported in Table 1. The less toxic iron cyanide complexes were not detected significantly above the method blank values in any of the three methods examined; however, the WAD cyanide species of mercury(II) cyanide was only partially recovered. A mean recovery of only 7.36% was observed for potassium nickel cyanide with the FIA method. Ligand Exchange Evaluation. The use of chelating agents was investigated to enhance the nickel and mercury(II) cyanide recoveries in the FIA system. The decomposition of the cyanide complexes can be aided by a ligand exchange reaction. The displacement can be generally depicted by the following equation (5): M(CN)n + yL ) M(L)y + nCN where
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TABLE 1. Cyanide Species Recoveries: No Ligand Exchange Reagentsa % recovery cyanide species 100 ppb as CN-
Method 4500 CN- I
Method SA-61A
Method SA-100
[Ag(CN)2][Cu(CN)4]3[Zn(CN)4]2Hg(CN)2 K2Ni(CN)4 KCN K4Fe(CN)6 K3Fe(CN)6 method blank
99.7 (4.52) 101 (1.20) 98.1 (1.23) 38.3 (6.47) 96.2 (2.59) 98.1 (1.71)
