Anal. Chem. 1983, 55, 1677-1682
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Determination of Total Cyanide in Thiocyanate-Containing Wastewaters Nicholas J. Csikai* a n d A. J. B a r n a r d , Jr. Research & Development, J. T. Baker Chemical Company, Phillipsburg, New Jersey 08865
At hlgh acidities, thiocyanate reacts wlth nitrate or other oxidants to produca free cyanlde. For thiocyanate-contalnlng wastewaters, a total cyanide procedure is reported based on use of EDTA at pH 4.0, acetate buffered, to dlsrupt the complexes of cadmlirm, chromium, copper, Iron, nlckel, and rlnc. NRrlte Interference is obvlated by adding sulfamic acid to the sample. Sulfldos are removed from the sample by lnltlal treatment with cadmium carbonate and, If necessary, from dlstliiates wlth aadmlum nitrate.
The determination of total cyanide is an important test in assessing wastewaters. The term “total cyanide” implies the sum of free cyanide present in a sample and produced in a sample treatment that disrupts complex metal cyanides. Wastewaters from operations producing inorganic thiocyanate salb, or wing thcsm, can contain thiocyanate at a concentration of 5 pg/mL (ppnn) or more and yet seldom have a “true” total cyanide vdue as high as 0.1 ppm. This study has focused on reliably determining the total cyanide value for such wastewaters; the findings, however, should be broadly applicable. The common approach to determining total cyanide in wastewaters is to make the sample highly acidic with sulfuric acid, heat under reflm while bubbling air through the solution, and absorb the evolved hydrogen cyanide in alkaline solution (1-4). Sulfuric acid is used alone or with additives intended to aid in disrulpting metal cyanides, such as magnesium chloride, magnesium chloride with mercuric chloride, cuprous chloride, and silver nitrate. To attain a highly acidic medium, tartaric acid (5,6) and phosphoric acid (7,B) have also been used. In less acidic media, complexing agents have been used to displace metals from cyanide complexes, thereby releasing cyanide. For tlhis purpose, EDTA (i.e., ethylenediaminetetraacetate) has been reported in 4 M phosphoric acid, alone (9-11) or with citric acid (9, 10) present, in citric acid (12), a t p H -4 (13, 14), and for a nondistillation procedure in alkaline medium (15). While this study was under way, use of 4,5-dihydroxy-m-benzenedisulfonic acid (Tiron) at pH 4.5 was reported, with tetraethylenepentaamine added to disrupt mercury cyanides (16). At low cyanide levels, photometry is widely applied and is usually based on reacting cyanide with chloramine T to form cyanogen chloride, which combines with pyridine and a cyclic amine to form a dye (1-3). Potentiometry with a cyanideselective electrode has come into use (2, 13, 15, 17-19). The most important interferent in determining total cyanide in wastewaters is sulfide, which evolves as hydrogen sulfide and compromises most analytical finishes (1-4). This interference can be obviated by treating the caustic-stabilized samples before distillation (1-3) or the absorbing liquid (18-20) with a sulfide precipitant and filtering. Precipitants recommended include lead and cadmium carbonates, lead acetate, cadmium nitrate, and bismuth citrate. Permanganate oxidation is also used. Many studies using a high-acidity procedure report thiocyanate as a serious interferent (4, 16, 20-22). In such a medium, the decomposition of thiocyanate in the absence of
oxidants leads to volatile carbonyl sulfide, which is converted to sulfide on absorption in an alkaline liquid (20). Less recognized is the conversion of thiocyanate at high acidities in the presence of a strong oxidant to free cyanide with oxidation of the contained sulfur to oxysulfur anions (cf. ref 23). In this situation, the total cyanide value will be elevated to the extent that thiocyanate is converted to cyanide in the heating-evolution process. A less acidic medium appeared attractive since under such conditions thiocyanate is relatively stable and many oxidants, including nitrate, are weaker. The use of ligand-displacement by EDTA under such conditions was therefore investigated. Interference by thiocyanate was avoided in a recent study by ligand displacement a t pH 4.5 (16). In a practical heating-evolution process, enough acid must be present to ensure complete volatilization of cyanide, as hydrogen cyanide, within 1or 2 h. In the procedure evolved, EDTA is used at pH 3.5 to 4.5 with heating to displace cyanide from metal complexes. This pH range is attained and maintained in a closed distillation system. The caustic-stabilized wastewater sample is initially treated with cadmium carbonate to remove any sulfide. If sulfide is detected in the absorbing liquid, this liquid is treated with cadmium nitrate and filtered. Sulfamic acid has a favorable effect in the heating-evolution process by obviating minor interference due to the reaction of trace nitrite with organic nitrogen compounds, present in some wastewater samples, or with the EDTA added, to yield free cyanide. In this paper, this new displacement procedure is evaluated with synthetic samples and thiocyanate-containing wastewaters. Comparison is made with the high-acidity procedure.
EXPERIMENTAL SECTION Apparatus. Use a suitable distillation apparatus (1-4). Ground glass joints are preferable; if rubber stoppers are used, wrap with polyfluoroethylene tape. A combined magnetic stirrer-heating mantle is convenient. Use class A volumetric ware. Reagents. All reagents are “Baker Analyzed” reagent grade or equivalent. Water is deionized or distilled. Cyanide working standard solutions were prepared by accurate dilution of a 1000 ppm K,Fe(CN), stock solution, standardized iodometrically and stored in the dark, and a 1000 ppm KCN stock solution standardized by a silver titration. These working solutions were made 0.2 M in NaOH. Acetate buffer was prepared by adding 54 g of NaCzH3Oz.3Hz0and 125 mL of CH,COOH to water, diluting with water to 1000 mL and adjusting to pH 4.0 with NaOH or CH3COOH. Phosphate buffer, pH 3.1, was prepared by dissolving 138 g of NaHzP04.Hz0 in 900 mL of water, adding 70 mL of CH3COOH, and diluting with water t o 1000 mL. The pyridine-barbituric acid reagent and the chloramine T solution were prepared and stored as in ref 1. K,Cr(CN), was purchased from Universal Specidties Co., and K,Co(CN), and CuCN from Aldrich Chemical Co., Inc. Solutions of Ni(CN):-, Cu(CN)Z-,Cd(CN):-, and Zn(CN)42-were prepared by mixing KCN and the corresponding metal ion in appropriate amounts. Procedure. Sample Preservation and Sulfide Removal. Transfer at least 600 mL of the wastewater to a well-rinsed screwcap polyolefin bottle and stabilize by adding 5 mL of 50% sodium hydroxide solution. (This sample can be stored for several weeks without significant change in the total cyanide content.) To remove sulfide, transfer 600 mL of the well-mixed sample to
0003-2700/83/0355-1677$01.50/00 1983 American Chemical Society
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a 1-L beaker containing a magnetic stirring bar. Add 0.2 g of cadmium carbonate. Stir for only a few minutes. If a yellow precipitate forms, repeat the cadmium carbonate addition until no yellow precipitate is observed. Filter promptly through paper and collect 500 mL in a graduated cylinder. Evolution of Cyanide. Transfer this filtered portion to a 1-L distillation pot. Add several glass beads unless magnetic stirring is used. To the absorption bottle, add 50 mL of 1 M sodium hydroxide and then water, if necessary, to a depth that ensures complete scrubbing of the air stream. Connect the absorber bottle to the condenser outlet tube. Start cooling water through the condenser. Connect the distillation pot to the condenser. Apply vacuum to the absorber outlet and adjust so that 2 to 4 bubbles/s enter the liquid in the pot. Remove the air inlet tube, add to the pot 2 g of sulfamic acid, and swirl the assembly until the solid dissolves completely. Then add 5 g of solid EDTA, disodium salt, dihydrate, and swirl until most of it dissolves. Replace the air inlet tube, add 4-5 drops of methyl red solution (100 mg of indicator in 100 mL of reagent alcohol), wash down the tube with water, and then add, while swirling the assembly, portions of 2.5 M sulfuric acid until the solution color just changes from yellow to red. Then add 1 M sodium hydroxide dropwise until the solution just turns orange. Now add 50 mL of pH 4.0 acetate buffer and swirl to mix. (The mixing steps can be facilitated by use of a combined magnetic stirrer-heating mantle; also, instead of swirling the assembly, the air flow can be increased to facilitate mixing and then be readjusted.) Heat the mixture to boiling, but do not leave unattended during the initial heating to ensure that the air flow is maintained and that liquid neither backs up into the air inlet tube or the absorber inlet nor emerges from the outlet of the absorber bottle. Reflux for 2 h with vapor rising to three-fourths of the cooled length of the condenser. Stop the heating, but continue air flow for at least 15 min. Transfer the contents of the absorber to a 250-mL volumetric flask and dilute to the mark with water rinsings from the absorber and connecting tubes. If the cyanide content is not to be assessed within a few hours, transfer the solution to a well-cleaned screwcap polyolefin bottle. Cadmium Salt Treatment of the Absorbing Liquid. Transfer about 10 mL of the diluted absorbing liquid to a small beaker and add a few drops of 3% cadmium nitrate tetrahydrate solution. If a yellow precipitate or solution forms, treat 75 mL of the diluted absorbing liquid with 0.5 mL of the cadmium nitrate solution. Heat, if necessary, to initiate sulfide precipitation and cool; filter through paper (Whatman No. 42 or equivalent). Test a portion of the filtrate with cadmium nitrate solution for the completeness of sulfide removal. Photometric Determination of Cyanide. Pipet 10.0 mL of the clear, sulfide-free, diluted absorbing liquid into a 100-mL volumetric flask. Add 40 mL of 0.2 M sodium hydroxide. (For a 500-mL volume of the wastewater sample, if the total cyanide content is expected to be greater than 0.8 ppm, use 5.0 mL of the diluted absorbing liquid and add 45 mL of 0.2 M sodium hydroxide.) In 100-mL volumetric flasks, prepare a blank (50 mL of 0.2 M sodium hydroxide) and also a 10-pg cyanide (KCN) standard diluted to 50 mL with 0.2 M sodium hydroxide. To each of the three flasks, add 15 mL of pH 3.1 phosphate buffer (pH is now 5.5-6.5) and 2.0 mL of chloramine T solution and mix. After 1-2 min, add 5.0 mL of pyridine-barbituric acid reagent, mix, dilute with water to mark, and mix thoroughly. Allow at least 8 min for color development and within 30 min read the absorbance of the sample and standard against the blank in a 10-mm cell at 580 nm. For the standard, the reciprocal sensitivity (Le., micrograms of CN/absorbance) should best agree within 1 5 % of that for the established calibration line; otherwise repeat the determination. Calculate the total cyanide content of the sample, in parts per million, from either the absorbance of the 10-pg standard or the calibration curve. For the calibration curve, pipet 0, 1.0, 2.0, 5.0, 10.0, and 15.0 mL of a working cyanide (KCN) standard solution (1pg/mL) into 100-mL flasks and 0.2 M sodium hydroxide to give a volume of 50 mL. Continue as for a sample. Plot the net absorbance vs. micrograms of cyanide. Calculate the reciprocal sensitivity. Prepare a new curve whenever a new batch of pyridine-barbituric acid reagent is introduced.
Validation. Distill two standards (50 and 200 pg of cyanide, added as ferricyanide), diluted to 500 mL in the manner of a sample, and determine cyanide in the absorbing liquid. The values should agree within *lo% of the amounts taken. Alternatively, add measured volumes of the ferricyanide working solution to a wastewater sample and determine the recovery of cyanide using the flask and heating-evolution assembly in which the unspiked sample was just treated. The recovery should be greater than 90% and preferably greater than 95%. Potentiometric Determination of Cyanide. For standardization, place in a 100-mL beaker 50 mL of deionized water and 1.0 mL of 10 M sodium hydroxide, bring to about 25 "C, and stir magnetically at a rate that does not create a vortex. Insert a cyanide electrode (silver iodide-silver sulfide, Orion 94-06 or equivalent) and a reference electrode (Orion 94-02 doublejunction or equivalent), connected to an expanded scale pH meter. After 5 min of mixing, record the millivolt value. In succession, add 0.05, 0.05, 0.40, and 0.50 mL of a cyanide (KCN) standard (0.1 mg/mL) and then 0.40 and 0.50 mL of the stock cyanide (KCN) solution (1mg/mL), and record the equilibrium millivolt value. The seven readings correspond to 0,0.005,0.010,0.050,0.100,0.50, and 1.00 mg of cyanide. On three-cycle semilogarithmic paper, plot the millivolt values (linear scale) vs. milligrams of cyanide taken and draw a straight line through the points. Prepare a new curve each time a series of samples is to be run. For the absorbing liquid from the treatment of wastewater samples, test for sulfide; if present, add 3% cadmium nitrate tetrahydrate solution, heat if necessary to initiate sulfide precipitation, cool, and filter through paper. Place a portion of the sulfide-free absorbing liquid in a beaker, bring to 25 "C, insert the electrodes, stir, and record the equilibrium millivolt value. Read the milligrams of cyanide from the calibration plot. RESULTS AND DISCUSSION Thiocyanate with Nitrate as an Interference. Thiocyanate in highly acidic medium is converted to hydrocyanic acid by the action of nitrate, which oxidizes the contained sulfur to sulfate or other oxyanions (cf. ref 23). The highly acidic conditions in usual procedures for total cyanide in wastewaters favor converting thiocyanate to cyanide if oxidants are present. (Total conversion of 1.0 ppm of thiocyanate yields 0.46 ppm of cyanide.) Typical results for water and wastewater samples are given in Table I. The total cyanide values when nitrate is present correspond to 5-95% conversion of thiocyanate to free cyanide in the sample treatment. This finding is also shown by further results for wastewaters given in Table 11. The magnesium chloride added in the high acidity procedures does not decrease this thiocyanate interference, as has been stated ( 4 ) . Thiocyanate values were obtained by an iron(II1)-thiocyanatephotometric procedure (ref 2, p 330). Use of Silver Additive. For electroplating wastewaters, Rohm and Davidson (24) use a high-acidity procedure in which silver nitrate is added to promote disruption of metal cyanides. We attempted this approach, substituting silver sulfate t o avoid adding nitrate ion. With aqueous samples containing thiocyanate, alone or with nitrate, satisfactory results were obtained with the thiocyanate interference markedly reduced. For example, total cyanide values for solutions containing 10 to 50 ppm of thiocyanate and 100-1000 ppm of nitrate and no cyanide were 0.5 to 12 ppm; with silver sulfate present (magnesium chloride omitted), the values were less than 0.1 ppm. In contrast, when this approach was applied to thiocyanate-containing wastewaters, the thiocyanate interference persisted, even when silver was added in greater amount. Addition of a Reductant. In the high-acidity procedure, attempts were made to obviate the interference of thiocyanate when oxidants are present by adding a reducing agent. Reductants examined included ascorbic acid and sulfamic acid, which have been suggested for total cyanide procedures ( 1 , 3 , 4 , 2 5 ) ,and also sodium borohydride with either boric anhydride (26, 27) or cobalt(I1) chloride (27). The reductant
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Table I. 'Thiocyanate Interference in High-Acidity Procedure for Total Cyanide When Nitrate Present, Deionized Water and Wastewater Samples sample naturlea DIW DIW DIW DIW DIW DIW DIW DIW
ww A ww 24 ww 13 WWB ww :( wwD ww II ww I? ww (3 a
total cyanide found, ppm with sulfamic acid not added added
thiocyanate present, ppm
nitrate present, ppm
0 10 10 10 50 50
200 0 100 100 200 1000 0 200
