EPA-600/4-77-034A July 1977 Environmental Monitoring Series DETERMINATION OF TRACE METALS IN EFFLUENTS BY DIFFERENTIAL PULSE ANODIC STRIPPING VOLTAMETRY Environmental Monitoring and Support Laboratory Office of Research and Development U.S. Environmental Protection Agency Cincinnati, Ohio 45268 ------- RESEARCH REPORTING SERIES Research reports of the Office of Research and Development, U.S. Environmental Protection Agency, have been grouped into nine series. These nine broad cate- gories were established to facilitate further development and application of en- vironmental technology, Elimination of traditional grouping was consciously planned to foster technology transfer and a maximum interface in related fields. The nine series are: 1. Environmental Health Effects Research 2. Environmental Protection Technology 3, Ecological Research 4. Environmental Monitoring 5. Socioeconomic Environmental Studies 6. Scientific and Technical Assessment Reports (STAR) 7. Interagency Energy-Environment Research and Development 8. "Special" Reports 9. Miscellaneous Reports This report has been assigned to the ENVIRONMENTAL MONITORING series. This series describes research conducted to develop new or improved methods and instrumentation for the identification and quantification of environmental pollutants at the lowest conceivably significant concentrations. It also includes studies to determine the ambient concentrations of pollutants in the environment and/or the variance of pollutants as a function of time or meteorological factors. This document is available to the public through the National Technical Informa- tion Service. Springfield, Virginia 22161, ------- EPA-600/4-77-034 July 1977 DETERMINATION OF TRACE METALS IN EFFLUENTS BY DIFFERENTIAL PULSE ANODIC STRIPPING VOLTAMETRY by James T. Kinard Benedict College Columbia, South Carolina 29204 Grant No. R803490-01-0 Project Officer Morris E. Gales, Jr. Environmental Monitoring and Support Laboratory Cincinnati, Ohio 45268 ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY OFFICE OF RESEARCH AND DEVELOPMENT U.S. ENVIRONMENTAL PROTECTION AGENCY CINCINNATI, OHIO 45268 ------- DISCLAIMER This report has been reviewed by the Environmental Monitoring and Support Laboratory, U.S. Environmental Protection Agency, and approved for publication. Approval does not signify that the contents neces- sarily reflect the views and policies of the U.S. Environmental Protection Agency, nor does mention of trade names or commercial pro- ducts constitute endorsement or recommendation for use. ii ------- FOREWORD Environmental measurements are required to determine the quality of ambient waters and the character of waste effluents. The Environmental Monitoring and Support Laboratory-Cincinnati conducts research to: o Develop and evaluate techniques to measure the presence and concentration of physical, chemical, and radiological pollutants in water, wastewater, bottom sediments, and solid waste, o Investigate methods for the concentration, recovery, and identification of viruses, bacteria, and other microbiological organisms in water; conduct studies to determine the response of aquatic organisms to water quality. o Conduct an Agency-wide quality assurance program to assure standardization and quality control of systems for monitoring water and wastewater. There is an ever-increasing interest in the use of instrumental methods to analyze water and waste samples, whether the resulting data are to be used for research, surveillance, compliance monitoring, or enforcement purposes. Accordingly, the Environmental Monitoring and Support Laboratory has an on-going methods research effort in the development, evaluation, and modification of instrumental methods. This particular report pertains to the evaluation of differential pulse anodic stripping voltametry. The method has potential routine appli- cation for the analysis of trace metals in surface waters and domestic and industrial wastes. Dwight G. Ballinger, Director Environmental Monitoring & Support Laboratory Cincinnati iii ------- ABSTRACT This investigation was designed to appraise the applicability of differential pulse anodic stripping voltametry to the determination of trace metals in a wide variety of industrial and domestic effluents. Fifteen (15) types of effluents of different industrial processings were employed, providing a rather comprehensive matrix base for asses- sing the potential for practical utilization of the technique. The results revealed that the technique is highly sensitive and that practical application to total metal determination requires care- ful analytical work and a quality of sample digestion with which present state-of-the-art digestion procedures cannot comply. However, when the technique is coupled with a closed-system acid digestion process, zinc, cadmium, lead, nickel, antimony, bismuth, copper, thallium, tin and in- dium can be determined individually and simultaneously in groups at concentrations ranging from the sub-parts-per-billion to the parts-per- million levels. A procedure for providing low blank buffer and electro- lyte systems was tested, and the efficiency for the entire process, including digestion, sample transfer and analysis, was found to range from 93 to 100%. Quantitative data for samples of complex matrices were in good agreement with flame atomic absorption spectrophotometric results for each element investigated. From 40 to 50 previously digested samples with 4 to 5 elements per sample can be analyzed each 8 hour working day (corresponding to 160 to 250 individual determinations) at about one-third the costs for flame atomic absorption spectrophotometry. This report was submitted in fulfillment of Grant Number R803490- 01-0 by Benedict College, Columbia, South Carolina, under the sponsor- ship of the U. S Environmental Protection Agency. Work was completed as of February 1, 1977. IV ------- CONTENTS Foreword iii Abstract iv Figures vi Tables vii Acknowledgment viii I. Introduction 1 II. Conclusions 3 III. Recomnendations 4 IV. Development of Digestion Procedures 5 V. Evaluation of Reagent Purification Procedures 7 VI. Development of Qualitative and Quantitative Methods - 10 VII. Evaluation and Application of Analytical Methods 21 VIII. Comparison of Differential Pulse Anodic Stripping 25 Voltametry and Flame Atomic Absorption Spectro- photometry for Effluent Analysis IX. Discussion 31 References v ------- FIGURES Number 1 Current-potential curves for DPASV in purified and unpurified sodium acetate . 9 2 Reproducibility of DPASV for simultaneous stripping of zinc, cadmium, lead and copper in 0.5 F sodium fluoride 19 3 Peak current versus concentration for the simul- taneous stripping of Zn, Cd, Pb, Bi and Cu 20 4 Atomic absorption calibration curves for tin, zinc, and bismuth 26 5. Atomic absorption calibration curves for cadmium and lead 27 6 Dependency of reduction potential on atomic radius. ... 33 7. Separation of copper and bismuth signals for DPASV using acetate ion 35 8 Current-potential curves for the simultaneous stripping of Cd, Pb, Cu and Bi in the presence of ammonia and carbonate 36 VI ------- TABLES Number Page 1 Trace Metal Contamination in Different Batches or Recrystallized Sodium Acetate 8 2 Types of Effluents and the Number of Samples Analyzed Through this Study 22 3 Trace Metals in a Mixture of Industrial Effluents and Precision Data 23 4 Recovery Data for the Simultaneous Determination of Zinc, Cadmium and Lead in Industrial Mixture II. . 23 5 Results from Application of DPASV to a Variety of Effluents 24 6 Comparison of DPASV and Flame AAS Data for Metals in Effluents 28 7 Comparison of Sensitivities, Detection Limits and Linearity Range of Trace Metal Determination for DPASV and Flame AAS 28 8 Comparison of Start-up Costs for DPASV and Flame AAS. . 29 9 Advantages and Disadvantages of DPASV and Flame AAS . . 30 10 Correlation Between Reduction Potentials and Electronic Configurations of Metal Ions 32 VI1 ------- The author would like to express his thanks to the personnel of the Environmental Monitoring and Support Laboratory for their support of this effort and to Mr. Clyde Bishop and Dr. Willie Ashley for administrative guidance. He is especially grateful to Mr. Morris Gales, Jr. for his technical assistance and his engagement in useful scientific discussions. Thanks also to Nathaniel Harry and Kenneth James for their research assistance. viii ------- SECTION I INTRODUCTION This investigation comprised an appraisal of the applicability of differential pulse anodic stripping voltametry (DPASV), using the hang- ing mercury drop electrode, to the identification and determination of trace metals in a wide variety of industrial and domestic effluents. Its scope also encompassed the development and evaluation of sample pre- treatment and reagent purification procedures to be used in conjunction with a highly sensitive technique. Problems associated with day-to-day individual determination of zinc, cadmium, lead, copper, bismuth, thallium, tin, indium, nickel and antimony were developed. Since flame atomic absorption spectrophotometry (AAS) is the most widely used technique for routine determination of metals in water and wastewaters, a phase of this research constituted a comparative study of AAS and DPASV for analysis of effluents. Advantages and disadvantages of both techniques were explored, and cost data based on present-day economics were generated. This report then, is not just a compilation of data to demonstrate the technical elegence of DPASV, but is a prescription for its appli- cation to routine analysis of effluents with complex matrices and ranging from domestic to that of the metals plating industry. Experimentation proceeded in several phases. The first phase involved (a) development of sample digestion procedures; (b) evaluation of reagent purification procedures; and (c) development of methods for identification of metals. Phase two constituted: (a) development of methods for the quantitative determination of metals and (b) application of these methods to the analysis of effluents. In phase three, a com- parative study of flame AAS and DPASV was conducted for determining metals in the same set of effluent samples. Advantages and disadvantages of both techniques were assessed. Several questions surfaced prior to the commencement of this study and, perhaps, are expressive of the scientific curiosity that gave birth to it. These are: What metals can be identified and determined by DPASV and at what levels of detection; can a generalized procedure be given for all possible metals or groups of these metals in all types of effluent samples; and, what advantages and disadvantages exist for total metal determination? Each phase of this investigation was designed to, in some ------- way, provide an answer to each of the above questions. The basic principle of anodic stripping voltametry has been well presented by Barendrecht (1). It is basically a two-step process that involves a preconcentration phase and a stripping phase. Preconcentra- tion is achieved by cathodically reducing the metal ion(s) of interest at a constant potential that is 300 or 400 millivolts more negative than the polarographic half-wave potential, Ei, of the ion of interest or of the ion with the most negative Ei value. The solution is usually agitat- ed or the electrode is rotated during this first phase. Following pre- concentration a rest period of about 20 to 30 seconds is allowed for quiescent conditions to develop. Anodic stripping is then initiated by applying a potential that is a linear function of time. In DPASV, this excitation function consists of a fixed-height potential pulse occurring at regular intervals and superimposed on a slowly varying linear potential ramp. Characteristics of the current-potential signal such as the peak current ( ip ) and the half-peak potential ( Ep/2 ) were tested for their analytical utility in effluents. Theoretically, *p is a linear function of the cell concentration and Ep/2 i-s characteristic of the metal being oxidized. Values for these parameters for a metal depend upon the proper- ties of a particular electrolyte system —such as its pH, ionic strength and complexing capability —and on the electrode system. The indicator electrode used throughout this study was a hanging mercury drop electrode (HMDE). The experimentally controllable parameters are rate of stirring, mercury drop size, preconcentration time ( ^d ), preconcentration potential ( ^ d ), scan rate (V ), the solution volume and the temper- ature. ------- SECTION II CONCLUSIONS It is concluded from this study that trace amounts of zinc, cadmium, lead,bismuth, copper, thallium, tin, indium, antimony and nickel can be identified and determined in industrial and domestic effluents by DPASV following digestion in a closed system. This system and technique, with regard to total metal determination, is highly competitive with AAS in terms of precision and accuracy, and is superior to AAS in terms of sensi- tivity, detection limits and costs. Although the process requires sample pretreatment, it is a non-destructive technique, and qualitative and quantitative analyses can be carried out on a sample simultaneously for as many as five elements. ------- SECTION III REOCMffiNDATIQNS The Investigator recomnends that: 1. DPASV with the closed acid digestion system be employed as an alternative method to AAS for routine quantitative determination of total trace metals such as zinc, cadmium, lead, bismuth, copper, thallium, tin and indium in industrial and domestic effluents; 2. This technique and system be employed as the most feasible method for the simultaneous identification of these metals at trace and ultratrace levels in effluents; and 4. The Chelex-100 ion exchange technique be employed as the most feasible procedure for rapid removal of trace metals from electro- lyte systems. ------- SECTION IV DEVELOPMENT OF DIGESTION PROCEDURES A number of digestion procedures involving wet and dry ashing were explored and found to be inadequate for the sensitive technique of DPASV. Trace amounts of organic material and potential complexing species were found to have a pronounced effect on the position ( EL^ ) an^ size ( xp or q, the number of coulombs) of the current potential'signal for a particu- lar metal. Continuous boiling of samples in an oxidizing acid system may have been adequate for destroying the organic material, but led to considerable metal contamination and loss of volatile metal constituents. This evidence dictated the need for a non-contaminating, closed digestion system that could be subjected to high temperatures when charged with a concentrated oxidizing acid solution. The Parr Acid Digestion Bomb, Model No. 4745 (Parr Instrument Co., Moline, Illinois) appeared to be a prime candidate. It consisted of a Teflon (Trademark of E. I. Dupont Co., Delaware) cup and cover and a stain- less steel encasement with a spring-loaded closure and screw cap. This general purpose bomb withstood temperatures up to 15CPC and pressures up to 1200 psig. The furnace used to perform digestions was a Lab Heat Muffle Furnace (Blue M Electric Co., Blue Island, Illinois). Procedures that were developed for the digestion of industrial and domestic effluents using the Parr Acid Digestion Bomb are given below. (1) Weigh to the nearest 0.1 mg a previously leached and rinsed Teflon cup. (This step may be omitted if the sample does not contain a high percentage of particulates and the concentra- tion can be more appropriately expressed as weight of analyte/ volume of effluent); (2) Pipet from 0.020 to 10.0 ml of the thoroughly mixed sample into the Teflon cup (The sample size will depend upon the expected level of analyte in the sample); (3) Weigh Teflon cup with sample to the nearest 0.1 mg (This step may be omitted if step #1 is omitted); ------- 4. Add 0.5 to 3 ml of high purity nitric acid to the Teflon cup (if the total volume of liquid in the Teflon cup is less than 3 ml add enough distilled-demineralized water to bring the final volume to approximately 3 ml); 5. Place Teflon cup in the encasement and tighten screw cap by hand; 6. Place bomb in a furnace that has been preheated to 160OC; 7. Allow time for thermal equilibrium to be attained and adjust the temperature to 160°C; 8. Heat from 1 to 8 hours or more (depending upon the complexity of the sample matrix) at 160+5°C; 9. Allow bomb to cool and transfer sample to a volumetric flask (10, 25, or 50 ml capacity). Dilute to the mark with the ap- propriate buffer system for determining the analyte of interest (see SECTION VI - DEVELOPMENT OF QUALITATIVE AND QUANTITATIVE METHODS). ------- SECTION V EVALUATION OF REAGENT PURIFICATION PROCEDURES The technique of DPASV has the capability of detecting a number of metals at the nanogram/ml and sub-nanogram/ml levels. To exploit this capability required the use of electrolytes and buffer systans of high purity and quality. Reagent grade chemicals contained high levels of impurities (see curve B of figure 1) and were not suitable for methods using DPASV for analytical measurements. High purity versions of most of these reagents were not available through the market and/or were highly expensive. For salts, repeated reerystallization was attempted but was found to be laborious and, in most cases, inadequate (see Table 1). Electrolytic purification was found to be adequate but required the use of a potentiostat and a purification period of 24 hours per liter of electrolyte. A procedure employing the sodium form of Chelex 100 (BIO • RAD Laboratories, Richmond, California) was evaluated and found to be suita- ble. The material is an ion exchange resin that has the capability of removing copper, lead, zinc and other heavy metals from aqueous solutions. Curve A in Figure 1 reveals its effectiveness in removing contaminants from a 1M sodium acetate solution after just one pass through a column. Two passes gave a blank solution with virtually no detectable amounts of metal contaminants. ------- TABLE 1. TRACE METAL CONTAMINATION IN DIFFERENT BATCHES OF RECRYSTALLIZED SODIUM ACETATE Metal concentration (ng/ml)* Batch no. 1 2 3 4 Zinc 148 166 76.0 17.9 Cadmium 5.69 3.12 2.39 2.30 Lead 96.5 43.2 46.8 49.6 Copper 36.4 37.8 61.3 66.8 * Concentration values are for 2 F sodium acetate solutions prepared from different batches of recrystallized sodium acetate. The following procedures for preparing, using and regenerating the column gave good results in our laboratory. 1. Prepare approximately 20 ml of a saturated solution of disodium or tetrasodium ethylenediaminetetraacetate. 2. Warm on a hot plate and added enough Chelex 100 (Na - form, 100 to 200 mesh). 3. Develop column (A 50 ml buret with a plug of glass wool near the outlet will be adequate). 4. Wash extensively with distilled - deminerialized water (about 10 bed volumes). 5. Adjust the pH of the solution of interest to a value above 7. 6. Pass solution through column twice. A flow rate of about 4 ml Imin/cm2 is adequate. To regenerate the column pass through 2 bed volumns of high purity IF HNCg, 6 bed volumes of distilled- demineralized water, 2 bed volumes of IF NaOH, and 6 bed volumes of water. ------- ae. oc, D U IF solution after 1 pass through Chlex—100 IF untreated solution Bi Zn Curve B I -i.i -0.1 -0.9 -0.7 -0.5 -0.3 POTENTIAL VERSUS SCE, volts Figure 1. Current potential curves for DPASV in purified and unpurified sodium acetate. ------- SECTION VI DEVELOPMENT OF QUALITATIVE AND QUANTITATIVE METHODS The anodic stripping data were generated by use of a Model 174 Polarographic Analyzer (Princeton Applied Research Corporation, Princeton, New Jersey) that was connected to a Model 2000 Omnigraphic X - Y Recorder (Houston Instruments, Houston, Texas). This instrument was operated in the differential pulse mode and the current output was always in the microampere range. The electrochemical cell consisted of a glass bottom and a self- mounting plastic top that was purchased from Princeton Applied Research Corporation. The reference electrode was a saturated calomel electrode (SCE) with a salt bridge attached. A disc of porous Vycor (Corning Glass) silicated glass, attached at the end of the isolated electrode tube and held in place by a Teflon (Dupont) sheath, served as the salt bridge tube between the isolated electrode and the solution in the electrolysis cell. This cavity was filled with the same electrolyte system that was used for the test solution. A spiral of platinum wire served as the isolated electrode. The sparge tube consisted of a two-way Teflon stopcock, a glass tube and a tapered Teflon tip (Princeton Applied Research Corpo- ration) . The sparge tube was connected to a train assembly that consisted of a wash bottle containing a concentrated disodium ethylenediaminetetra- acetate solution, a wash bottle containing a vanadous chloride solution and a tank of high purity nitrogen gas. Vanadous chloride solution served as an oxygen scrubber while the disodium EDTA served to remove trace metals from the gas stream. Ultrex grade nitric acid (J. T. Baker Company, Phillipsburg, New Jersey), high purity metals and demineralized - distilled water were used to prepare the standard solutions. All solutions were stored in poly- ethylene containers. The containers were leached with 6 M nitric acid and rinsed thoroughly with demineralized-distilled water before using. Each blank was prepared by adding to a volumetric flask (10, 25 or 50 ml) the same amount of HNO3 as used for digestion, a volume of de- mineralized-distilled water equal to that of the digested sample and diluting to the mark with the appropriate electrolyte system. 10 ------- QUALITATIVE APPROACH The blank was transferred to the electrolysis cell and was purged for 5 minutes with nitrogen. The nitrogen stream was then diverted over the solution by turning the stopcock to the appropriate position on the sparge tube. A deposition potential was selected and the mercury drop was adjusted to give a size that corresponded to 2 divisions/drop on the micrometer head of the HMDE. Stirring was initiated. The instrument was switched to "Cell" concurrently to activating a timing device. Thirty (30) seconds were allowed to pass after stirring was terminated, and strip- ping was initiated at 10 mv/s. Preconcentration and stripping were re- peated to check for reproducibility. An aliquot of a standard solution of a metal ion (if *-d of 30 seconds to 3 minutes were used and a blank volume of 25 ml was in the cell, 20 - 50 of 10 - 20 ppb of standard) was added to the cell and the solution was purged again. Stirring precon- centration, and stripping were conducted under the same conditions, and the spiked curve was obtained. Other metals were added sequentially and DPASV was conducted to assess their ddetectability in the blank and in the presence of potentially interfering analytes. QUANTITATIVE APPROACH Once an assessment was made to identify the metals that were detect- able in a particular electrolyte system, the pH and ionic strength of that system were varied separately to ascertain conditions for optimizing the current-potential signal. A modified blank was prepared and the linearity between ip and C, was investigated for each metal. Considerable time was saved by preparing a combined standard (composed of all of the detectable metals). Preconcentration and stripping were performed following each addition of standard. The following are methods that were developed through this study. 11 ------- ANALYTICAL METHOD NO. 1 SIMULTANEOUS IDENTIFICATION AND DETERMINATION OF ZINC, CADMIUM, LEAD, BISMUTH, AND COPPER IN EFFLUENTS Analytes: Zn, Cd, Pb, Bi, and Cu Matrix: Industrial or Domestic Effluent Electrolyte System: Ed= -1.4 V Range: Zn Cd Pb Bi Cu Precision: (% rela- tive stan- dard devi- ation) Accuracy: 0.5 0.08 0.10 0.32 0.50 Zn Cd Pb Acetate buffer, PH=5.5-5.8 115 ng/ml 120 ng/ml 200 ng/ml 210 ng/ml 118 ng/ml + 2.7 + 2.2 + 2.4 Zn and Cd Pb Bi and Cu Cu Bi 2.6 2.3 100 +_ 6% 100 + 4% 100 + 8% 1. Procedure 1.1 Leach all glassware in 6M HNO3 for 5 hours and rinse thoroughly with demineralized - distilled water. 1.2 Transfer digested sample from Teflon cup to a 25 ml volumetric flask. Add the appropriate amount of sodium acetate to acid solution to form a buffer of pH = 5.5 to 5.8. If the acid concentration is high in the digested sample, increase the con- centration of sodium acetate or evaporate the solution slowly to drive off most of the acid and take up digestate in sodium acetate. Adjust pH to 5.5 - 5.8. 1.3 Transfer the solution to the cell and purge for 5 minutes. 1.4 Set the Ed to ~ solution. V. Divert the stream of nitrogen over the 12 ------- 2. 1.5 Stir solution and preconcentrate for 30 seconds to 5 minutes. 1.6 Turn off stirrer and allow 30 seconds for quiescent conditions to develop. 1.7 Initiate scan at 10 mv/s and scan to the desired potential of termination. 1.8 Repeat steps 1.4 - 1.7. 1.9 Spike with from 10 - 100 of combined standard. Try to use a spike size that will give signals that are about twice the size of the original signals. (Step 1.9 is not necessary if a cali- bration curve is used) . 1.10 Purge and repeat steps 1.4 - 1.7. Calculation C = ii v Cs/ (i2 v + (i2 - %) V) Where Cc = original cell concentration Cs = concentration of standard solution used for spiking il = peak height in original cell solution ±2 = spiked peak height v = volume of standard solution added as spike V = original sample volume 3 . Interferences In interferes with Cd and Tl interferes with Pb. Long preconcentration times and high concentration of analytes may cause a Cu - Zn intermetallic to form. This intermetallic strips at the same potential as that of Cu and enhances the Cu signal while lowering the Zn signal. It may be eliminated by using a shorter preconcentration time or diluting the sample solution. 13 ------- ANALYTICAL MEfflCD NO. 2 SIMULTANEOUS IDENTIFICATION AND DETERMINATION OF ZINC, INDIUM, THALLIUM, BISMUTH AND COPPER IN EFFLUENTS Analytes: Zn, In, Tl, Bi, and Cu Matrix: Industrial or Domestic Effluent Electrolyte System: Acetate buffer, pH=5.5-5.8 Ed=-1.4V Range: Zn, Bi and Cu (same as Method 1) In 0.3 ng/ml 150 ng/ml Tl 0.10 ng/ml — 200 ng/ml Precision: Zn, Bi and Cu (same as Method 1) (% Relative Standard Deviation) In + 3.0 Tl + 2.6 Accuracy: 96 +_ 6% 1. Procedure (Same as Method 1) 2. Calculations (Same as Method 1) 3. Interferences Cd interferes with In and Pb interferes with Tl. (See Method 1) ANALYTICAL METHOD NO. 3 DETERMINATION OF LEAD AND THALLIUM IN EFFLUENTS Analytes: Pb and Tl Matrix: Industrial or Domestic Effluent Electrolyte System: Acetate buffer, pH 5.5 - 5.8. for total Pb plus Tl. Add 0.33 g of EDTA salt for Tl determination in the presence of Pb. 14 ------- Ed = -0.8 V Range: Pb 0.10 200 ng/ml PI 0.10 200 ng/ml Precision: Pb +_ 2.4 (% Relative Standard Deviation) Tl + 2.6 Accuracy: 95+6% 1. Procedure Follow procedure in Method 1 to determine the sum of Pb and Tl in acetate buffer. Add EDTA salt and repeat the procedure to determine the amount of Tl present. 2. Calculations Subtract the concentration of Tl from the sum of the concen- tration of Tl and Pb. 3. Interferences There are no known interferences. ANALYTICAL METHOD NO., 4 IDENTIFICATION AND DETERMINATION OF INDIUM IN EFFLUENTS Analyte: In Matrix: Industrial or Domestic Effluent Electrolyte System: 0.1 MNI^CNS - 0.015 M C6 H3 (OH)3 (pyrogallic acid) Ed = -0.8 Range: 0.05 ng/ml 170 ng/ml Precision: +2.7 (% Relative Standard Deviation) 15 ------- 1. Procedure (Same general procedure for DPASV) 2. Interferences (There are no known interferences. ANALYTICAL METHCD NO. 5 DETERMINATION OF TIN IN EFFLUENTS Analyte: Sn Matrix: Industrial or Domestic Effluent Electrolyte System: 5N HC1 % =-0.8 V Range: 0.5 160 ng/ml Precision: +_ 3.8 (% Relative Standard Deviation) Accuracy: Undetermined (Low level blank was difficult to obtain) 1. Procedure Digest sample according to the Parr bomb digestion procedure. Carefully evaporate sample to near dryness, without heating too rapidly. Take up the digestate in 5N HC1. 2. Calculations (Same as Method No. 1) 3. Interferences Pb interferes with the determination of Sn. ANALYTICAL METHCD NO. 6 DETERMINATION OF ANTIMONY IN EFFLUENTS Analyte: Sb Matrix: Industrial or Domestic Effluent Electrolyte System: 1M H Cl 16 ------- Ed =-0.8 V Range: 1 200/ug/ml Precision: +_ 4.2 (% Relative Standard Deviation) Accuracy: Undetermined (Low level blank was difficult to obtain) 1. Procedure Same as Method No. 5 except the digestion is to be taken up in INHC1 2. Calculations (Same as Method 1) 3. Interferences No apparent interferences ANALYTICAL METHOD NO. 7 DETERMINATION OF NICKEL IN EFFLUENTS Analyte: Ni Matrix: Industrial or Domestic Effluent Electrolyte System: airmonia-tartrate Ed =-1.0 V Range: 0.8 150ng/ml Precision: +_ 4.0 (% Relative Standard Deviation) Accuracy: 91 +_ 6% 1. Procedure Same as Method No. 5 except the digestate is taken up in -^, arrmonia - .5M tartrate. 2. Calculations Same as Method No. 1 17 ------- DETECTION LIMITS AND PRECISION The operational definition of detection limit used by the investi- gator was the minimum detectable amount of analyte that produces a signal significantly different from zero. A large number (n>10) of blanks were analyzed and the standard deviation for each element present was calculated. The detection limit in each case was calculated as D.L. > ns Where ns and n^ were the number of determinations performed on the sample and blank, respectively. The parameter t was taken from the t - Distri- bution table (2), its value depended upon the degrees of freedom ng + n^ - 2 at the 95$> probability level. Precision data were acquired by analyzing a large number (n 10) of samples of the same effluent and determining the % relative standard deviation. Since the method of standard additions were used to acquire quantitative data the reproducibility was the same as the precision. Typical reproducibility data are shown in Figure 2. DEPENDENCY OF PEAK CURRENT ON CONCENTRATION Calibration curves are prepared from peak-current versus concentra- tion data. Figure 3 gives a set of typical calibration curves. These were determined while simultaneously stripping Zn, Cd, Pb, Cu and Bi in acetate buffer. The method of standard additions gave results that were not significantly different from results acquired by use of the calibra- tion curves. Of course, use of the standard additions method required that there is a linear relationship between ip and C. In the application of these methods the calibration data were treated by the method of least squares . 18 ------- 1.0 -0.8 -0.6 -0.4 -0.2 POTENTIAL VERSUS SCE, volts Figure 2. Reproducibility of DPASV for simultaneous stripping of cadmium, lead, and copper in 0.5F sodium fluoride. zinc, ------- Cu Bi Cd Zn Pb £ 200500 100 k. D Q. E 2 160 400 80 120300 60 - OL OL Z> u < 80 200 40 - 40 100 20 - 20 40 60 80 100 Pb 40 80 120 160 200 Zn and Bi 10 20 30 40 50 Cu and Cd CONCENTRATION, nanograms/ml Figure 3. Peak current versus concentration for the simultaneous stripping of Zn, Cd, Pb, Bi and Cu. (Acetate buffer of PH 5.481, td=3Q S, Ed= -1.400V, M.S.= 3) ------- SECTION VII EVALUATION AND APPLICATION OF ANALYTICAL METHODS The methods have been applied to the identification and determination of trace metals in a variety of effluents. The various types and the number of samples studied in this laboratory are listed in Table 2. This represents a broad spectrum of effluent matrices that were tested, and the conclusions presented in the first part of this report are derived from a comprehensive matrix data base. A mixture constituting various amounts of practically all types of effluents listed in Table 2 was developed and analyzed. The results along with precision data are given in Table 3. A second mixture composed of effluents and tap water was developed and analyzed. The results along with the recovery data are shown in Table 4. This recovery reflects the total efficienty for the digestion procedure, sample transfer, and analytical measurement. Since it was acquired by the use of a real sample of perhaps the most complex matrix available it is a good representation of the applicability of the entire procedure for DPASV, provided careful analytical work is conducted. Table 5 contains a listing of some results that were obtained for some of the different effluent types. The Egg Processing and Steel Mill sample required the greatest amount of time for digestion (10 hours). 21 ------- TABLE 2. TYPES OF EFFLUENTS AND NUMBER OF SAMPLES ANALYZED THROUGH THIS STUDY Effluent Type Further description No. of Samples Studied Textile & domestic Raw sewage Printing industry and domestic waste Meat Packing Aluminum products Alumnium products Paper mill Paper mill Egg treatment Steel mill Steel mill Textile Slaughter house Metals plating Metals plating Soft drink Synthetic Fibers Dyeing and Finishing Chicken Farm Waste from corrugated box manufac- turing facility also. Influent to oxidation pond. Domestic waste. 60% industrial and 40% domestic. Raw waste. Different locations. Collected from pond-after oil separation. Raw. After pH adjustment. Raw waste. Effluent from clarifier. Raw waste. Raw waste. Refined. Raw. Raw. Raw. Refined. Raw. Raw. Raw. Raw 6 10 5 3 6 3 5 2 3 25 18 10 2 6 2 3 2 1 3 22 ------- TABLE 3 TRACE METALS IN A MIXTURE OF INDUSTRIAL EFFLUENTS AND PRECISION DATA CONCENTRATION, ug/ml Element Zn Cd Pb Bi Cu Sample #1 17.2 20.4 27.4 14.50 12.0 Sample #2 21.9 20.4 27.9 4.61 13.6 Sample #3 21.8 20.6 29.1 4.28 12.4 Sample #4 32.4* 18.3 27.2 4.13 12.7 Standard Deviation Mg/mL + 2.60 + 1.33 + 0.85 ± °-18 + 0.68 * Apparent contamination. Rejected this datum in computing the standard deviation. TABLE 4. RECOVERY DATA FOR THE SIMULTANEOUS DETERMINATION OF ZINC, CADMIUM AND LEAD IN INDUSTRIAL MIXTURE II Naturally Occurring Element C«g/ml) Zn 25.4 26.1 Cd 4.42 5.76 Pb 8.34 8.95 Added C«g/ml) 4.44 4.92 4.25 5.24 2.54 4.61 Found C«g/ml) 33.9 29.5 8.63 11.65 11.4 14.0 Recovery (%) 113.6 95.1 97.5 106 105 96.8 23 ------- TABLE 5. RESULTS FROM APPLICATION OF DPASV TO A VARIETY OF EFFLUENTS Concentration, ng/ml Effluent Type Textile Printing Lake Marion Egg Processing Steel Mill Zn 0.90 2.30 30.61 268 848 X103 Cd 11.0 1.5 114 10.3 XLO3 Pb 16.9 7.8 121 93.4 711 X103 Cu 24.1 2.43 193 420 X103 Bi 3.72 11 X103 24 ------- SECTION VIII COMPARISON OF DIFFERENTIAL PULSE ANCDIC STRIPPING VOLTAMETRY AND FLAME ATOMIC ABSORPTION SPECTRCHOTCMETRY FOR EFFLUENT ANALYSIS Flame atonic absorption spectrophotometric data on metals in effluents were acquired by using a Model 503 Atomic Absorption Spectro- photometer (Perkin-Elmer Corporation, Norwalk, Connecticut). For most samples a triple slot burner head was used and the deuterium arc back- ground corrector was activated. New single element hollow cathode lamps of short shelf life were used in all cases except for the determination of copper. This element was determined while using aCo-Cu-Fe-Mn- Mo multielement lamp. An air - acetylene flame was employed for all samples and final readout data were acquired while operating in the concen- tration mode with integration times of 3 and 10 seconds. Refrigerated samples were allowed to reach thermal equilibrium with the laboratory atmosphere prior to analysis. Effluents that were dif- ficult to aspirate because of a large percentage of particulates were filtered. No sample pretreatment was conducted with exception of the addition of nitric acid (approximately 5 ml of HMOs/ liter of effluent) immediately following sample collection for the purpose of preservation. Figures 4 and 5 are typical calibration curves obtained by flame AAS. In most cases linearity complied with the specifications given in reference (2). Table 6 contains some of the quantitative data obtained from the comparative analyses of effluents by DPASV and flame AAS. The DPASV results were ascertained by employment of the methods developed through this study. The working solution that resulted after application of the Parr acid digestion bomb technique and dilution contained levels of metals that ranged from 10 ng/ml to 400 ng/ml. 25 ------- Zn 5.0 Bi Sn 50 4.0 40 O 9 3.0 30 LU 2.0 20 1.0 10 ! I Sn —-D Zn —-•*-— Bi 10 1 20 30 2 3 40 4 50 Sn,Bi 5 Zn CONCENTRATION, micrograms/ml Figure 4. Atomic absorption calibration curves for tin, zinc, and bismuth. ------- NJ Pb Cd 50 30 10 Pb / —o- Cd —&— Pb I Cd 50 Pb Figure 5. Atomic absorption calibration curves for cadmium and lead. 5 10 15 20 10 20 30 40 CONCENTRATION, ------- TABLE 6. COMPARISON OF DPASV AND FLAME AAS DATA FOR METALS IN EFFLUENTS Concentration, (ug/ml) Sample no. and effluent type Mixed sample #12 Steel Mill #4028 TABLE 7. Detection DPASV* Element Cng/ml) Zn 0.5 Cd 0.08 Pb 0.10 Bi 0.32 Cu 0.50 Tl 0.10 In 0.30 Element DPASV Zinc 0.72 Cadmium 20.6 Lead 29.1 Copper 19.2 Bismuth 4.28 Cadmium .07 Lead 6.97 Copper .654 Bismuth 11.6 AAS 0.80 20.9 31.5 17.3 2.51 0.07 5.86 0.57 12.9 Percent Difference 10 1.4 7.62 9.90 41.4 0 15.9 12.8 10.1 COMPARISON OF SENSITIVITIES, DETECTION LIMITS AND LINEARITY RANGE OF TRACE METAL DETERMINATION FOR DPASV AND FLAME AAS Limit Sensitivity Linearity Range AAS DPASV AAS DPASV** AAS Cng/ml) (ng/ml/nA) (ng/ml/lfebs)(ng/ml)(ng/ml) 2 5 5 1 30 2 50 8 5 4 200 3 50 2 18 25 500 400 90 500 700 1,000 2,000 20,000 2,000 5,000 20,000 50,000 28 ------- The good agreement that exists between the results for most metals suggests that DPASV competes well with the present widely accepted technique, AAS, for metal determination. It was observed in most cases throughout this comparative study that the Parr bomb digestion - DPASV results were higher than the flame AAS data. The consistency in this trend together with the accuracy of the DPASV methods suggests that the Parr bomb digestion technique and the sensitivity of DPASV provide for the combined total determination of metals in the solution and particulate phases of an effluent sample. The rather poor agreement between the results of the two techniques for bismuth in mixed sample #12 is apparently due to the fact that level of this metal is close to its lower detection limit for AAS. The accuracy of the DPASV method for Bi at the 100 - 200 ng/ml (the range of concen- tration of Bi in the working solution for which this sample was prepared). Table 7 lists sensitivity, detection limits, and linearity (upper limit), data for both techniques. Table 8 lists the items that constitute start-up costs for each method. It appears that DPASV is superior to AAS in terms of detection limits, sensitivity, and cost (see Table 9). The precision for the two techniques is comparable. TABLE 8. COMPARISON OF START-UP COSTS FOR DPASV AND FLAME AAS Estimated Cost ( $ ) Category DPASV AAS Basis Instrumentation 5,200 16,000 Needed Accessories 1,750 1,490 six lamps and HMDE Set Up burner head six capillaries 4 cells and 3 bombs Reagents 350 450 and supplies (start-up) SPECIAL fan and venting 160 $7,300 $18,000 29 ------- TABLE 9. ADVANTAGES AND DISADVANTAGES OF DPASV AND FLAME AAS Advantages and Disadvantages Category DPASV ASS Superior technique 1. Start-up costs 2. Annual Maintenance 3. Technical skill required 4. Instrumental Tune-up Estimated to be 1/2 about 2/5 the costs for AAS Have used instrument for 3 years at zero maintenance cost Highly sensitive experienced technician Ininediate use 5. Counter space Very compact 6. Supportive supplies and equipment 7. Sample preparation 8. Precision 9. Accuracy 10. Detection Limits Must digest completely Depends on purity of reagents and preconcen- tration time $1,000 (routine) Must tune-up for optimum performance Requires about 4 times the space for DPAS Little or none DPASV DPASV AAS 11. Sensitivity Very sensitive DPASV DPAV DPASV DPASV Comparable Comparable DPASV DPASV 30 ------- SECTION IX DISCUSSION DPASV AND CHEMICAL PERIODICITY IB IIB IIIA IVA VA +0.103 V Cu- -1.0043 V Zn -0.5936 V Cd -0.580 V In -0.5778 V Tl -0.4779 V Sn -0.3620 V Fb +0.080 V Bi A number of interesting correlations have been made between the DPASV characteristics of these elenents and their chemical periodicities. The number that is written above each elenent in the above portion of the periodic Table is the standard reduction potential (3) of the most stable form of the metal ion in aqueous acid media. For comparative purposes, these potentials are given.'in reference to the SCE. Table 10 gives the electronic configuration of each ion along with its reduction potential. 31 ------- TABLE 10. CORRELATION BETWEEN REDUCTION POTENTIALS AND ELECTRONIC CONFIGURATIONS OF METAL IONS Metal ion Cu+2 Bi+3 Pb+2 Sn +2 Tl +1 In+3 Cd+2 Zn+2 Outer Electronic Configuration 4s23d23d23d13d13d1 6s25d106p06p°6p0 6s25d106p°6p06p° 5s24d105P05p°5p0 6s25d106p°6p°6p0 5so4d105po5p05po 5s04d105P05p°5P0 4s°3d104p04p°4p0 Reduction potential + 0.103 + 0.080 - 0.3620 - 0.3779 - 0.5778 - 0.580 - 0.5936 - 1.0043 Since the reduction potential is related to the free energy change (spontaneity) for the corresponding half-cell reaction it may be looked upon as a quantity related to the energy necessary to produce the neutral atom or amalgam. Zn+2 requires the greatest amount of energy to be reduced, and this apparently is related to the scheme that two electrons must be placed in the somewhat shielded 4s empty orbital. Similarly, Cd+2 ion has all 4 d orbitals filled, shielding the 5s empty orbital. But the reduction potential for Cd+2 is over 400 mv more positive than the value for Zn+2 ion. The outer orbitals for Cd+2 are for principal levels 4 and 5 while those for Zn+2 are 3 and 4. This may account for the con- siderable difference in the reduction potentials of the two ions. The entire order of the reduction potentials of the metal ion can be explained based on their electronic configuration. Notice the order by which the ions occur in Table 10 and the order by which they exist in the periodic chart. When Cu+2 is excluded there appears an interesting correlation between the reduction potentials of these ions and their periodic place- ment. Another interesting correlation was made between the electronic densities (4) of the neutral atoms and their stripping positions on the current-potential curves. A typical case is shown in Figure 6. 32 ------- to •5 Q 1.40- 1.20- -1.0 -0.8 -0.6 -0.4 -0.2 0.0 +0.2 POTENTIAL VERSUS SCE, volts Figure 6. Dependency of reduction potential on atomic radius. ------- Separation of Bi and Cu The sensitivity of the signal for Tl in acetate buffer is approximately twice that of the acetate buffer-EDTA solution, 2.42 nA/ng/ml and 1.35 nA/ng/ml, respectively, with the *d = 10 seconds. The detection limits for Tl are 3.25 ng/ml and 1.58 ng/ml for acetate buffer and acetate buffer - EDTA solution, respectively with a *d = 10 seconds. The Ep/2 value for Pb + Tl in acetate buffer is -0.505 V versus SCE while a value of -0.520 V versus SCE exists for Tl in acetate buffer - EDTA solution. Through a separate study performed in this laboratory, it has been determined that the diffusion coefficient of Tl+1 is twice that of_Pb+2. Tl+1 gives virtually the same iD/C value (6.22 X 1011 nA/M) as Pb+z (6.28 X IQll nA/M) when run separately in acetate buffer. Thus in acetate buffer, the amount of electricity or total charge (coulombs) involved in the stripping of a certain concentration of Tl+l should be the same as the one for Pb+2 under the same experimental conditions. In the determination of the sum of the concentrations for Tl and Pb (which have the same EL/2 it does not matter which of the two metal ion standards is used for the spike. EDTA completely complexes the lead to form a stable species when it is added to the acetate buffer system. Tl gives a good signal in the presence of EDTA and can be readily determined in this new medium. Separation of Bi and Cu A number of workers have experienced difficulty in determining bismuth in the presence of copper. Figure 7 shows how acetate is used to separate the signals for bismuth and copper. Greater consistency in the data was obtained when the decaying portion of the wave was used to determine ip for copper. A different electrolyte system that provides for the simultaneous determination of Bi and copper is O.IENK^ Cl - Ammonium chloride and potassium carbonate separately do not lead to the separation of Cu and Bi. Figure 8 reveals how the combination of the two effect the needed separation. Curves A and B represent the stripping for a nonpurged and purged solution - in 0.1FNH4 Cl. Curve C is obtained after traces of Pb, Cd and Cu have been added. Curve D was run after the NHj Cl solution was made 0.23 F in KgCOs- The addition of carbonate ion causes all species to be stripped at more negative potentials. Peaks 1 and 2 in curve C become 1 and 2, respectively, in curve D while 3 splits into two peaks. Curve E is a repeat run, and the reproducibility is very good. Lead and copper standards were added before running curve F and we can see that Cu is component C^ while C2 is Bi, just the reverse order as occurs in acetate. Notice that the front part of the wave for Bi has decreased since the addition of Cu. This forces one to read the decaying current for Bi, which gives good reproducibility, even when the solution is spiked with copper. 34 ------- z U4 Q£ oe. ID u in Ii;I~ Curves A and B - Duplicate curves in solution of 2.5 ml of nitric acid diluted to 25 ml with 2M sodium acetate. C, D and E - Repeated runs after addition of 6.8 grams of sodium acetate. F - Run after addition of another 6.8 grams of sodium acetate. F I I -0.6 0.0 -0.4 .0.2 POTENTIAL VERSUS SCE, volts Figure 7. Separation of copper and bismuth signals for DPASV using acetate ion. ------- OL 3 U I I Curve A - Unpurged 0.1 F NH4CI Curve B - purged 0.1F NH4Cl Curve C - Spiked with Cd, Pb and Bi Curve D - Added K7CO3 Curve E - Repeat run Curve F - Spiked with Pb and Cu. Cd -0.8 -0.6 -0.4 POTENTIAL VERSUS SCE, volts -0.2 Figure 8. Current potential curves for simultaneous stripping of Cd, Pb, Cu and Bi in the presence of ammonia and acetate. 36 ------- Summary - The technique of DPASV is highly sensitive and requires an experienced analyst to apply the associated methods for determining a number of metals. The investigation has attempted to focus on those elements that are relatively ubiquitous, difficult to determine by other methods, are known toxicants or have questionable toxicity, and are likely to be more prevalent in effluents than other metals. Although through this investigation elements such as antimony, tin, and nickel have been explored using a number of electrolyte systems, this report does not pre- clude the extension of this list of metals to a wider spectrum. 37 ------- 1. Barendrecht, E., Review article on "Stripping Voltametry", Electroanalytical Chemistry, Vol. 2, ed. Bard, J. A., Marcel Dekker, Inc., New York, New York, p. 53 (1967). 2. Analytical Methods for Atomic; AbsorptIon Spectrophotometry, Perkin-Elmer Corporation, Norwalk, Connecticut (1973). 3. Latimer, W. M., Oxidation Potentials, 2nd ed., Prentice-Hall, Inc., New York, New York (1952). 4. Sanderson, R. T., Chemical Periodicity. Reinhold Publishing Corporation, New York, New York (I960). 38 ------- TECHNICAL REPORT DATA (Please read Instructions on the reverse before completing) 1. REPORT NO. EPA-600/A-77-034 2. i. RECIPIENT'S ACCESSIOI»NO. 4. TITLE AND SUBTITLE DETERMINATION OF TRACE METALS IN EFFLUENTS BY DIFFERENTIAL PULSE ANODIC STRIPPING .VOLTAMETRY 5. REPORT DATE July 1977 issuing date 6. PERFORMING ORGANIZATION CODE 7. AUTHOR(S) James T. Kinard 8. PERFORMING ORGANIZATION REPORT NO. 9. PERFORMING ORGANIZATION NAME AND ADDRESS Benedict College Columbia, South Carolina 29204 10. PROGRAM ELEMENT NO. 1HA-322-JBA-027 11. CONTRACT/GRANT NO. R803490-01-0 12. SPONSORING AGENCY NAME AND ADDRESS Environmental Monitoring and Support Laboratory-Gin., Office of Research and Development OH U.S. Environmental Protection Agency Cincinnati, Ohio 45268 13. TYPE OF REPORT AND PERIOD COVERED 14. SPONSORING AGENCY CODE EPA/600/06 15. SUPPLEMENTARY NOTES 16. ABSTRACT Differential pulse anodic stripping volametry (DPASV) was evaluated to determine its applicability to industrial and domestic effluents. The results show that trace amounts of zinc, cadmium, lead, bismuth, copper, thallium, indium, antimony, tin and nickel can be determined individually and simultaneously. A procedure for providing low blank buffer and electrolyte systems was tested. The efficiency for the entire process, including digestion, sample transfer and analysis, was found to range from 93 to 100%. Pulse anodic stripping voltametry was found to be superior to atomic absorption in terms of sensitivity, detection limits and cost. 17. KEY WORDS AND DOCUMENT ANALYSIS DESCRIPTORS b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group Volumetric Analysis, Digesters, Cadmium, Metals, Copper, Bismuth, Antimony, Indium Water Analysis, Lead (metal), Nickel, Tin, Thallium, Zinc, Containers Anodic Stripping 07B 18. DISTRIBUTION STATEMENT RELEASE TO PUBLIC 19. SECURITY CLASS (This Report} TTMPT 21. NO. OF PAGES 47 20. SECURTTV~CLASS (This page) UNCLASSIFIED 22. PRICE EPA Form 2220-1 (9-73) 39 *UA 60VBNMO(rPmKTIK60FFIC£! 1977- 7S7-OS6/6470 ------- |