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RECEIVED for review May 29,1975. Resubmitted July 22,1977. Accepted October 18, 1977. This work was supported by the National Science Foundation.
Determination of Total Arsenic at the Nanogram Level by High-speed Anodic Stripping Voltammetry Phillip H. Davis," Gerald R. Dulude, Reginald M. Griffin, Wayne R. Matson, and Eric W. Zink Environmental Sciences Associates, Inc., 45 Wiggins Ave., Bedford, Massachusetts 0 1730
A method for the analysis of total arsenic at nanogram levels has been developed employing high-speed anodic stripping voltammetry (ASV) of arsenic at a gold-film electrode (gold plated on pyrolytic graphite). Samples are wet ashed to near dryness with a mixture of HN03, HC104 and H2S04, then reconstituted with a 37 % HCI solution containing 1 % Cu2CI2. ASCI, is volatilized from solution using a special apparatus. The AsCI,-HCI vapors are collected in 4 mL of deionized water and the resulting solution is analyzed for its arsenic content by high-speed ASV (analysis time 140 s) with a semi-automated ASV instrument capable of providing a direct digital readout of arsenic concentration. Development and optimization of analytical methodology are presented. Precision of the method is good (RSD = f7% at 50 ng As) and accuracy, as determined by analysis of National Bureau of Standards standard reference materials (SRM), is excellent. The method Is free of interferences from the anions and cations present in the SRMs analyzed. 0003-2700/78/0350-0137$01.00/0
The widespread use of arsenical compounds in industry and the concern over arsenic toxicity and possible carcinogenicity have prompted the development of numerous methods for quantitative arsenic analysis through the years. T h e list of methods encompasses "wet chemistry" gravimetric and volumetric techniques as well as costly and sophisticated modern instrumental methods such as inductively-coupled plasma emission and neutron activation analysis. An excellent review of methods for the determination of arsenic has been presented ( I ) . Nearly all currently employed methods for trace arsenic analysis require some prior isolation and/or preconcentration of arsenic from the sample matrix. None of the methods (with the possible exception of Whitnack and Brophy's (2) single sweep polarographic procedure or McKinney and Mack's ( 3 ) flameless atomic absorption technique) can really be considered rapid techniques. Arsenic analysis by arsine generation-atomic absorption has been semi-automated by C 1977 American Chemical Society
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Fiorino e t al. ( 4 ) ,but this procedure still requires considerable operator supervision a n d t h e instrumentation is relatively complex a n d costly. Spectrophotometric (colorimetric) methods are t h e most widely employed procedures for arsenic determination. This is true primarily because of their apparent simplicity and their low cost. Again, however, these methods are limited in terms of sensitivity, and require considerable skill and attention on t h e p a r t of t h e analyst. T h e limited sensitivity of t h e colorimetric methods is a detriment in t h e case of analysis of biological samples where sample size a n d preparation time are often limiting factors. T h e recent NIOSH recommendation to OSHA for a proposed standard for arsenic in air in t h e work environment states t h a t “inorganic arsenic shall be controlled so t h a t no worker is exposed t o a concentration of arsenic in excess of 0.002 mg (2.0 Kg) per cubic meter of air as determined by a 15-minute sampling period” (5). This proposed standard will make analysis by t h e now accepted silver diethyldithiocarbamate method essentially impossible since t h e practical working range of this method is about 1-20 wg of arsenic. Quantities of arsenic collected on filters or other sorbents under t h e conditions specified by NIOSH will be only a few nanograms at t h e 2 1 g / m 3 level. Analysis by t h e arsine generation-atomic absorption method (NIOSH Method P&CAM No. 139) is reliable only a t levels of arsenic above a b o u t 0.2 kg (6). One electroanalytical procedure which has been demonstrated to be a very sensitive analytical technique for trace element determination is anodic stripping voltammetry (ASV) (7-13). Kaplin e t al. (14, 1 5 ) ,appear to have pioneered t h e work on this technique for trace arsenic analysis. American investigators who have employed ASV for arsenic analysis are few. Forsberg (16, 17) determined arsenic by ASV and differential pulse anodic stripping voltammetry (DPASV) using a gold wire electrode of small area. T h e detection limit of both ASV and DPASV was 0.02 n g / m L (0.02 ppb) in pristine solutions using a plating time of 20 min. Sulek, Zink, a n d Dulude (18) used a gold film plated on wax-impregnated graphite to determine arsenic by ASV. These workers reported that their method gave results for arsenic in wet ashed fish which agreed favorably with those obtained by hydride evolution-atomic absorption spectroscopy a n d neutron activation analysis. All of t h e techniques described above are subject to varying levels of interference which can become significant in real samples. This article describes a new anodic stripping voltammetric procedure for t h e determination of nanogram levels of arsenic(II1). T h e electroactive form of arsenic under t h e conditions employed for anodic stripping voltammetry is the As(II1) ion. Prior t o analysis, As(V) ions must be reduced to As(II1). As with all other methods for determination of arsenic, t h e analysis of very low levels of arsenic by ASV may be subject t o interferences from materials in t h e matrix solution, especially iron(I1) and copper(I1). Thus, a scheme for t h e separation of arsenic from its matrix is highly desirable. A scheme which simultaneously accomplishes both goals is t h e distillation of arsenic trichloride (AsC13) from a solution of cuprous chloride (Cu2C12)in concentrated hydrochloric acid (HC1). Since reduction and distillation are carried out in essentially a single process in a single piece of apparatus we have elected to term t h e procedure “reductillation”. Volatilized AsC13 and HC1 are collected in deionized water and the resulting solution is analyzed for arsenic by anodic stripping voltammetry.
EXPERIMENTAL Apparatus. The anodic stripping voltammeter employed for
all arsenic ASV measurements was an Environmental Sciences Associates, Inc., Model 3010A Trace Metals Analyzer (TMA). This instrument utilized a staircase potential scan stripping scheme. Osteryoung et al. (1%21), have developed a theoretical treatment of the staircase anodic stripping scheme and have presented conclusive evidence of its advantages over differential pulse anodic stripping voltammetry (DPASV). Staircase stripping effectively resolves the faradaic stripping current component from the total analytical signal. The Model 3010A TMA is capable of integrating the peak shaped stripping current-time curve and providing a direct digital readout of the integrated signal. Use of this latter feature became important, as it will be seen later. A HewlettPackard Model 7015A X-Y recorder was used to record arsenic stripping signals. Areas of some arsenic stripping current-voltage curves were measured using a Kent Zero Setting Polar Planimeter. The electrode used in this study was the ESA 3010/3010A High Sensitivity Electrode Assembly (ESA Part No. 3600-01). Figure 1 is a schematic diagram of the High Sensitivity Electrode Assembly. The working electrode for arsenic ASV consisted of a thin film of gold electroplated on the upper, inner surface of the pyrolytic graphite tube. This was the only active portion of the electrode since all other surfaces were coated with a nonconducting epoxy material. The useable lifetimes of gold electrodes prepared in this manner ranged from days to weeks. No treatment of the electrode was necessary between analyses except for a single rinse with a blank solution (7 M HCl) following analysis of a sample with a relatively high arsenic content (ca. 100 ng As) to minimize carry-over of solution. When the electrode became insensitive (Le,,sensitivity to arsenic decreased substantially), it was simply wiped free of gold and replated. Preparation of the gold film will be presented in more detail below. The surface area of the active portion of the High Sensitivity Electrode was approximately 3 cm2, The analytical solution was stirred by a Teflon stirring propeller and shaft which extended down through the electrode structure and rotated about its longitudinal axis. This stirring geometry was highly efficient and thus led to rapid electrodeposition of material from solution. The reference electrode was a Ag/AgCl (saturated NaC1) electrode isolated from the analyte solution by a porous Vycor plug. The reference compartment was f i e d with saturated sodium chloride and a few NaCl crystals to maintain saturation. The counter electrode was a platinum wire which was also separated from the sample-matrix solution by a porous Vycor plug. The counter electrode compartment was filled with 0.1 F N2H4.2HCl. The gold film was plated on the inner, upper surface of the High Sensitivity Electrode following the instructions of the electrode manufacturer (22). Basically, 1 mg of gold as AuC13 (100 ,uL of lo-’g Au/mL stock solution) was plated onto the graphite substrate from a matrix of 0.5 F HC1 at a plating potential of -0.15 V vs. Ag/AgCl (saturated NaC1) reference electrode. It was found that slow plating of gold from quiescent solution gave a reproducibly smooth, yellow plate of gold as opposed to a brownish, amorphous gold plate when gold was deposited from a stirred solution. The glass apparatus developed for the reductillation process is shown schematically in Figure 2. The injection port and nitrogen inlet were outfitted with rubber septums. A Teflon tube forced through a small puncture in the rubber septum and terminated on the opposite end by a Kel F female luer fitting was connected to a supply of nitrogen gas. A length of platinum wire inside the Teflon tubing, of a diameter nearly equal to the i.d. of the Teflon tube, acted as a flow restrictor for the nitrogen. An optimal nitrogen flow rate was reproducibly obtained by operating at 10 psig nitrogen pressure. The “Spanish Dungeon” trap acted to prevent physical carry-over of fine droplets produced by the bubbling action of the nitrogen. The reductillation head was outfitted with a female 24/40 S ground glass joint which matched with the joint of the digestion reductillation tube. The ASV analysis cell/collection tube was a 75 mm X 85 mm electrochemical cell (ESA Part No. 3010-41). The analysis cell contained 4 mL of deionized water which served to trap the AsC1, and HC1 vapors carried out of the reductillation apparatus by the flow of nitrogen gas.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
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blotor S h a f t
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Figure 1. Schematic diagram of the high-s
itivity electrode
Reagents. Deionized water used in this study was produced by passing tap water through a train of commercial Culligan brand carbon filters and ion-exchange beds. The particular sequence in this case was a Culligan carbon filter, a Culligan “duo-bed“ consisting of cation-exchange resin bed and an anion-exchange resin bed followed by a “uni-bed” conditioner, a combined anion-cation exchange resin bed. Finally, a 5-pm filter removed fines and other particulates. A stock solution of lo-’g Au/mL was prepared by dissolving 1 g of high purity (99.99%) gold (Research Organic/Inorganic Chemical Corp.) in a minimum of aqua regia. An excess of hydrochloric acid was added and the solution was gently deaerated with N2 to purge residual chlorine from the solution. The gold solution was then diluted to 100 mL with deionized water. A stock solution of g As(III)/mL was prepared by dissolving 1.320 g of reagent grade As203 (Fisher Scientific) in 5 mL of 10 F NaOH, adding 25 mL of 37% HC1, and diluting to 100 mL with deionized water (23). Standard solutions of g As(III)/mL (stable for a t least 1 month) and lo4 g As(III)/mL (stable for a t least 1 week) were prepared from the g As(III)/mL solution as needed. A stock solution of g As(V)/mL was prepared by dissolving 4.164 g of reagent grade Na2HAs04.7H20(Mallinckrodt) in 40 mL of deionized water acidified with 0.5 mL of 95% H,SO, (G. F. Smith Chemical Co., Columbus, Ohio, double distilled from Vycor) and diluting the solution to 100 mL with deionized water. Working standard solutions of and g As(V)/mL were prepared as required from the lo-* g As(V)/mL stock solution.
Standardized sodium hydroxide (Mallinckrodt) was used in titrations of HC1 solutions. Potassium hydrogen phthalate was purchased from Fisher Scientific as were sodium chloride for the reference electrode compartment and hydrazine dihydrochloride for the counter electrode compartment Nitric acid, perchloric acid, and sulfuric acid used in digestion of samples were obtained from G. F. Smith Chemical Co. Hydrochloric acid was reagent grade (J. T. Baker) and did not contain sufficient levels of Sb, Bi, Cu, Hg, or Fe to give detectable signals under the conditions used. The 95% cuprous chloride (Ventron Corp.) reagent was found to contain a significant amount of arsenic which contributed t o high blank readings. Therefore, a procedure for removing arsenic from cuprous chloride stock solutions was devised. Basically, the procedure employed was to boil a 10% (w/v) solution of Cu2C12in concentrated ( 3 7 % )HC1 in the presence of a quantity of “electrolytic purified” grade copper metal powder (Fisher Scientific). During the process, the solution acquired a pale greenish yellow color as Cu2+ions were reduced to Cut ions. Likewise, As was removed from the solution as the volatile AsC13. The solution, along with the copper metal, was stored in a bottle containing a Teflon coated stirring bar. Exposure of the solution to air caused a darkening of the liquid due to formation of Cu2+ ions, but the solution could be cleared orice again by placing the container with the stirring bar on a magnetic stirrer for several minutes. A clear or slight straw-colored appearance of the solution indicated it was ready for use. The reagent should not be used when it is dark and opaque.
Figure 3. Typical arsenic ASV signals E, = -0 150 V vs Ag/AgCI (satd NaCI), T = 2 mtn, Y = 100 m V / s , matrix 5 mL of 7 M HCI
Table I. Effect of HCI Concentration on Arsenic Stripping Current Signal Morphologya Figure 2. Schematic diagram of the reductillation apparatus. Digestion-reductillation tube: 85 mm X 20 mm borosilicate glass tube outfitted with 24/40male ground glass joint; Spanish dungeon trap: 2-4 mm in diameter; condenser-delivery arm: 0.d. 8 mm tapered to -2 mm; ASV analysis cell: 75 mm X 18 mm; nitrogen delivery tube: 0.d. 8 mm, 2-mm opening at bottom
Repeated opening of the bottle and exposure of the Cu2C12 solution to air will accelerate the slow dissolution of Cu metal in the bottle. This can result in a slowly increasing blank due to arsenic present in copper metal. The use of a closed system with provisions for dispensing the liquid while maintaining a nitrogen atmosphere over the liquid would lessen this problem. The use of a higher purity copper metal should also ameliorate the situation. Procedure. Reductillation was carried out in a simple glass apparatus (ESA Part No. 3600-10 and 3600-11) after the addition of the necessary reagents. In those cases in which analyses were performed on real samples, such as NBS standard reference materials, the samples were first wet ashed to near dryness with a mixture of HN03:HC104:H2S04(23:23:1). The exact amount of acid mixture used depended on the nature of the sample matrix and sample size. In cases where only inorganic arsenic was reductillated, an aliquot of the appropriate arsenic stock solution was dispensed into the digestion-reductillation tube without wet ashing. Following sample preparation, 6 mL of concentrated (37%) HCl and 0.7 mL of 10% Cu&12 solution were added to the As(V) residue in the digestion-reductillation tube. The tube was then fitted to the reductillation head (Figure 2). The apparatus was placed into a reductillation-digestion block (ESA Part No. 3600-13-a block of aluminum specially drilled to accept 4 reductillation units) and the block was heated to a temperature of 105 OC. Care was exercised to maintain the block at this temperature since the reductillation process removed heat from the block. The outlet tip of the condenser-delivery arm was simultaneously immersed deeply into a 4.0-mL volume of deionized water in the Model 3010A ASV analysis cell/collection tube. After 1 min (to allow time for the solution in the reductillation tube to heat), the N2 flow at a rate of ca. 144 cm3/min (regulated by adjusting the pressure of the N2 inlet to about 10 psig) was initiated and continued for 11 more minutes. At the end of the 12-min reductillation period, the condenser-delivery tube was removed from the collection liquid while the reductillation apparatus was simultaneously removed from the heating block and the nitrogen flow was discontinued, in that order. After the addition of 0.5 mL of concentrated (37%) HC1 to the contents of the collection-analysis cell, to adjust its final volume to 5 mL and its HCl concentration to 7.3 M, the collection solution was directly analyzed on the ESA Model 3010A TMA for its arsenic content. Except where stated otherwise, arsenic ASV analyses were accomplished using a deposition potential of 4.150 V vs. Ag/AgCl
Peak Peak Concn current, potential, HC1, M Eps, V PA 4 5 6
7 8
273 286 364 331 313
t0.190 +0.186 +0.175 +0.170 +0.160
Peak width at Peak half-height, mV area 80 186 70 191 52 175 48 168 46 175 Av= 179 * 9
a 200 ng As(II1) in 5 m L of HC1 solution, potentials vs. Ag/AgCl (saturated NaCl), peak area measured by polar planimeter, El = - 0.15 V vs. Ag/AgCl (saturated NaCl), u = 100 mV/s; T = 2 min
(saturated NaCl), a plating time ( T ) of 2 minutes, and a stripping potential scan rate ( u ) of 150 mV/s. Also, unless otherwise indicated, quantitation was by standard additions and measurement of peak heights to minimize the affects of other variables of the system. All values for variables given above (e.g., volumes. flow rates, potentials, etc.) are optimal values determined in the course of this investigation. Justification of the choices will be presented in the next section. RESULTS AND DISCUSSION A r s e n i c S t r i p p i n g C-V C u r v e s at the G o l d F i l m E l e c t r o d e . Arsenic is not amenable to analysis by anodic stripping at t h e hanging mercury d r o p electrode or t h e mercury film electrode because of t h e insolubility of arsenic in mercury. The best electrode material found to date is gold. In this study, a thin film of gold plated on a pyrolytic graphite substrate served as the working electrode. Use of this electrode in conjunction with a supporting electrolyte of hydrochloric acid resulted in analytically useful stripping signals such as those shown in Figure 3. T h e spikes on either side of t h e stripping peaks are integration markers indicating t h e zone within which t h e signals are integrated by t h e Model 3010A TMA. These signals were sharp, well defined peaks whose peak potentials did not shift significantly with increasing arsenic concentration. These signals were responsive to a number of measurement parameters, some of which are presented below. A r s e n i c S t r i p p i n g Peak Morphology. T h e morphology of t h e arsenic stripping peak was found t o be a function of the concentration of HC1 in the matrix solution as illustrated aptly in Table I. T h e stripping peak potential (E,) shifted cathodically with increasing HC1 concentration while the peak width at its half-height, W l j z(an indication of electrochemical
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
141
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Figure 4. Calibration plot of As(II1) in 7 M HCI (0-1000 ng). E, = -0.150 V vs. Ag/AgCI (satd NaCI); T = 2 min; v = 100 mV/s; matrix, 5 mL of 7 M HCI
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of 7 M HCI
reversibility), approached a minimum. T h e peak height varied considerably with HCl concentration, t h u s indicating t h e advisability of using the standard additions method when one uses peak height to quantitate signals. However, the d a t a clearly showed that t h e area of t h e stripping peak a t each concentration of HC1 was constant within experimental error. Since it was t h e peak area which was measured by t h e 3010A T M A when it was used in t h e direct digital readout mode, small changes in HC1 concentration among several analyte solutions were insignificant. Analytical Response. T h e Model 3010A T M A provided a direct digital display of arsenic concentration in any units desired (ng, ppm, pg, etc.) through calibration of t h e ins t r u m e n t with a n As(II1) standard. Figure 4 illustrates t h e analytical range and sensitivity of the anodic stripping method. T h e digital response in "nanograms arsenic" was linear between 0 a n d 500 ng As(II1). T h e detection limit of anodic stripping is a function of several parameters, including the deposition time ( T ) and the rate of mass transport of reducible ion to the electrode surface. This latter point will be discussed in more detail below. Thus, by using shorter deposition times (e.g., 1 min or 30 s), it was possible to extend the range of the method to higher arsenic concentrations, but a corresponding decrease in analytical detection limit was observed. T h e extended range was a consequence of the fact t h a t a smaller fraction of arsenic was deposited on the finite surface of t h e electrode. Unlike true metals such as lead or copper, arsenic will not electrochemically deposit on itself once t h e gold surface has been completely covered with arsenic because of t h e nonconducting nature of arsenic (16). This phenomenon in all probability accounts for the small break in the calibration plot a t arsenic levels greater t h a n 500 ng when a 2-min deposition time was used. High Speed Anodic Stripping. T h e primary disadvantage of anodic stripping to date has been the rather long analysis times necessitated by long deposition (pre-concentration) times. Typical deposition times found in the literature for t h e hanging mercury drop or the mercury film electrodes, in conjunction with electrolysis cells of 10-100 m L volume, are 5-30 min. As it was pointed out previously. the analytical detection limit of anodic stripping analysis is directly dependent on the m o u n t of material deposited at the electrode surface and consequently on the rate of mass transport to the electrode surface. T h e special cell geometry with its highly efficient stirring action in combination with the rapid staircase stripping scheme has made high speed anodic stripping of ppb levels of arsenic a reality. Total analysis times were 90-140 seconds. T h e rate of arsenic deposition onto the electrode is ) the illustrated graphically in Figure 5. T h e half-time ( t l j 2of high sensitivity electrode-cell assembly was approximately 1.6 min. T h e t1,2was constant for 8, 20, and 120 ppb arsenic in 6 M HC1. A deposition time of only 5 min resulted in accumulation of about 88% of t h e arsenic (111) solution.
Reductillation. The efficiency of the reductillation process was influenced by t h e following variables: t h e flow rate of nitrogen through the reductillation apparatus, t h e reductillation time, t h e concentration of Cu2C12in the HCl matrix solution, heating block temperature, concentration and volume of HC1 in the reductillation tube, and the volume of deionized water in t h e collection/analysis cell. All of these variables can be rigorously controlled, however, as indicated by t h e precision of t h e reductillation process (vide infra). T h e effect of each of these variables was evaluated by holding all other variables constant. For example, in evaluation of t h e effect of the concentration of HC1 on reductillation efficiency, no AsC13 was reductillated unless t h e concentration of HCl in the reductillation vessel was greater t h a n 8 M. Even a t 10 M HC1, only '75% of t h e arsenic was reductillated, but 100% efficiency was achieved with 12 M ( 3 7 % )HC1. Likewise, the volume of HCl in the reductillation vessel significantly affected the arsenic reductillation efficiency. Whereas only 5 m L of HC1 was barely enough to cover t h e end of the nitrogen delivery tube thus giving a recovery of about 90%, the use of 10 mL of HC1 resulted in only about 50% recovery. This was probably a consequence of less efficient volatilization of AsC13 from the ,solution by the nitrogen within the 12-min reductillation time frame. Consequently, t h e optimal HC1 volume in t h e reductillation vessel was set a t 6 mL. T h e volume of deionized water in t h e collection-analysis cell was limited on the upper end to a value of about 4 m L by the physical geometry of the electrode/cell assembly. This will be discussed in greater detail in the next section. However, a volume of water in t h e collection cell less t h a n 3 m L was found to give only about 60% arsenic recovery. This is probably a result of t h e inability of s,mall volumes of water to completely capture t h e AsC13 vapo'rs at t h e nitrogen flow rate used (ca. 144 cm3/min). For inorganic As(V) standards in concentrated HC1, a Cu2Clz concentration of 0.002% in t h e reductillation tube produced only a 53% recovery of arsenic. However, a 10-fold increase in Cu2C12concentration resulted in 10070 recovery of As. Concentration increases of Cu2C12from 0.02 to 1.0% in the reductillation vessel did not produce any change in arsenic recovery. Consequently, a n optimal Cu2C12concentration of 1% was chosen to allow for consumption of Cu+ ions by reducible materials in a real sample. Approximately 1% Cu2Cl2resulted from adding about 0.7 mL of the 10% Cu2C12 stock solution to 6 m L of 37% HC1 used to reconstitute wet digested samples. The effects of other controllable parameters are summarized in Tables I1 and 111. An optimum temperature for the heating block (ESA P a r t No. 3600-13) during reductillation of samples was found to be 105 "C. T h e evaporation of HC1 a.nd water from t h e re-
ANALYTICAL CHEMISTRY, VOL. 50, NO 1, JANUARY 1978
142
'l'able 11. Effect of Nitrogen Flow Kate on kductillationU P l o w rdte
'
c 111 / 111i 11
n y Ab Kecol. eredb
Recovery,
89
89 86 93 Y5
30 9U 14U BOU
CF
/G
66 93 95
' I,,
- U . 1 5 U V \I. Ag/ AyCl ( a a t u r d e d NaCl), r 2 150 iriV s - ' , aiialysia by i r i e w m m e n t of peak h e l y h t a (staiidrtrd additions), heating block temp. = 103 'C, reductillation time = 1 2 niin (includcb 1 inin without nittug:eii f l o w ) , ~ o l u m eut 1 2 M HCI - 4.5 m L ; Volume uf 1U% Cu,Cl, - U.5 niL, Volume deionized water 111 collecniiii, I
100 ng h ( V ) taken fur analysis.
tion tube = 4 mL. __
I _ _ _ -
-
'l'able 111. Effect of Time on the Kecovery of Reductillationa
Reductillation time, niin
ng As recovered
5
18
15 20 30
Y'7 95 101 106
1u
liecot c r y , c/L 48 97 95 101
106
Conditions same as fur Table 11; N, flow rate = 144 cm3/min. Table IV.
Efficiency of Reductillation of Arsenic(III)a
As( 111) taken, p g 0.010 0.100 0.200 0.400 0.600
1.00 10.00 40.00 100.00 400.00 800.00 1000.00
Dilution for ASV
As found,
Recovery,
!Jg
%
Direct Direct Direct Direct Direct 1:102
0.0105
105 91 89 103 104 130 95 91 103 94 101 99
1:102 1:102 1:103
1:103 1:104 1:10'
0.091 0.177 0.411 0.622 1.30 9.5 36.3 103 377 805 985
Ei = - 0.15 V vs. Ag/AgCl (satd NaCl); 7 = 2 min; v = 1 0 0 mV sec-'; Reductillation conditions are given in text.
ductillation apparatus removed considerable heat from t h e heating block. Thus, it was necessary to adjust the hot plate setting upward from its initial equilibrium setting while reductillating samples. Also, a 1-min period of heating without passing nitrogen through t h e system in order to allow time for t h e solution in t h e reductillation apparatus to heat was found t o be beneficial in reducing the time required for quantitative reductillation. C o l l e c t i o n of AsC13/HC1 V a p o r s . T h e ultimate utility
uf the reductillatioii prucedure for the separation-reductiui~ of arsenic depended not oiily u n the efficieiicj JIKI prwiaion of the volatilizatioii uf arsenic, but also on t h e etticiency uf collectiuii of the AsCl~{ vapors by t h e deioiiized Lbattr i n the ASV analysis cell. Likewise, the sensitivity aiid precisiuri of the ASV nieasuremciit itself was dictated by cunditioiis of the ineasureineiit matrix (Le., i l l this caae, the HCI sulutiori pruduced by bubbling " 2 1 vapur through the deiuiiized $$\.dter in the cullection cell) ab indicated in Table I. Heme, it was Iieccasarj t u acquire some knowledge of (1) the chdnge in vulunie uf t h e 4 inL of deionized water in the w u r s e uf reductillatiw and ( 2 ) t h e change in cuncentrations o! HC'I in the collection solution during reductillation. 'The arcrage change in volume during reductillation as determilied Ly duplicate run5 uii 5 sepdrate samples (i.e., a tutal of 10 ruris) u'aa 0.48 giving an average final volunie in the a i d y a k cell of 4.48 mL. T h e average concentratiuii of HC1 in the analysis cell at the completion of reductillatiun was found tu be 6.81 M as determined by titration of the aolutioii against staiidardized NaOH. T h e geumetry of t h e electrode arrangement dictated that the vulume of solutiun in the analysis cell cuuld be no less t h a n 5 niL. Therefore, a volume adjustment was necessary. Addition of 0.5 mL of deionized water to the 4.48 mL of solution in the analysis cell gave a filial HCI concentration of 6.1M. Referring to Table I, it can be seen that the width of the arsenic stripping peak is nearly stabilized at an HC1 concentration of 7 M. In order for the Model 3010A T M A t o integrate t h e arsenic stripping peak and provide a digital display of arsenic concentration, it was necessary that t h e stripping peak width be relatively constant since t h e instrument could integrate only within a specified "window" width. Therefore, by adding 0.5 mL of concentrated (12 M) HC1 to the 4.48 m L of collection volume, the average resulting HC1 concentration was 7.3 M, a n ideal level for stable arsenic stripping signals. E f f i c i e n c y of R e d u c t i l l a t i o n . In order to apply t h e reductillation-AS\' technique to diverse samples containing varying amounts of arsenic, the efficiency of the reductillation of several levels of arsenic was examined. Reductillation efficiency was determined by reductillating quantities of arsenic from 10 ng to 1mg. Dilutions of the collection solution before ASV analysis with 'iM HC1 were made where necessary. T h e d a t a (Table IV) show t h e excellent collection efficiency of 4 mL of deionized water over t h e 100000-fold range of arsenic levels studies. P r e c i s i o n of R e d u c t i l l a t i o n - A S V P r o c e d u r e . T h e precision of t h e overall reductillation-ASV technique was evaluated by running 4 replicate analyses a t various levels of arsenic. T h e results of this study are tabulated in Table V. T h e data show that t h e precision is quite respectable in light of the relatively high blank observed in this case. The majority of the arsenic in t h e blank came from t h e cuprous chloride reducing agent. Work is presently under way in our laboratory to further refine t h e procedure for purifying the Cu2C12of arsenic (vide supra).
Table V. Precision of the Reductillation Procedurea* A4V) taken, ng 0 10 50 200 4 00 600
As found, ng 6, 5, 9, 8 11, 19, 19, 20 43, 50, 48, 49 191,200,191,191 336, 351, 332, 384 510, 506, 533, 528
a Based on 4 replicate analyses. ditions are given in text.
Av ng As, blank corrected
S.D.,
R. S.D.,
Recovery,
0
%
%
-
i 1.8
+ 26
-
10 41 186 344 512
t4.2 i3.1 i4.5 I .23 i 13
f 25
100 82 93 86 85
Ei= - 0.15 V vs. AgiAgCl ( z ;atd NaC1); i= 2 min; v
i7 12
+7 +3 = 100 m V
s-I;
Reductillation con-
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
Table VI. ASV Analysis of Arsenic in Standards and Collaboratively Studied Samples ASV Certified value or Sample results, group average, Sample designation ppm As PPm As Orchard Leaves NBS 10.14 10.0 * 2.0 SRM ~ 1 5 7 1 NBS Bovine Liver SRM 2 1 5 7 7 0.057 0.055 Collaborative "Control" 0.006 0.011 Ketchup Collaborative "Spiked 0.12 0.11 Ketchup + 0.13 PPm E.P.A. ~1 U.S. E.P.A. 0.024 0.026 E.P.A. = 2 U.S. E.P.A. 0.102 0.109 0.147 0.154 E.P.A. 3 3 U.S. E.P.A. E.R.A. Commercial 0.097 0.110 Standard Diluted 1:10 Diluted 1 : l O Diluted 1 : l O
0.0025 0.010 0.014
0.0026 0.011 0.015
Table VII. Selectivity of Volatilization of AsC1, in the Absence of the Reducing Agenta As( V )
taken, rJg
2 0.100 0 2 a
CONCLUSIONS T h e effectiveness of t h e reductillation-ASV procedure for measurement of total arsenic at the nanogram level in various sample types has been demonstrated. The method is relatively rapid, requiring only 12 min for reductillation and a 2-min ASV analysis time. In the latter stages of this study, the per sample reductillation time was significantly decreased by reductillating multiple samples in the same 12-min interval. Excluding sample wet ashing time, samples can be routinely analyzed for arsenic a t the ppb level iit a rate of about 12 per hour when four reductillations are accomplished simultaneously. T h e procedure was found to be as accurate, precise, a n d reliable a t t h e nanogram level as the more accepted colorimetric techniques are a t the microgram and milligram level. This technique has been successfully applied to t h e routine analysis of arsenic on air filters and in biological specimens (24-26).
LITERATURE CITED
s1762
E.P.A. sl E.P.A. -2 E.P.A. +3
143
As( 111) taken, rJg
Arsenic recovered as As(III), rJg
0
0.044 0 0.100 0.107
0 0.100
0.100
Conditions same as for Table IV.
Accuracy of Reductillation-ASV Procedure. T h e accuracy of t h e reductillation-ASV technique was evaluated by analyzing several samples of "known" arsenic content (Table VI). Among these samples were NBS Standard Reference Materials (SRM), E P A water standards, and a ketchup sample which was analyzed in conjunction with a round-robin arsenic analysis. T h e samples were wet digested to near dryness with a mixture of HN03:HC104:H2S04 (23:23:1), reconstituted with concentrated HC1 (6 mL) and 10% Cu2C12(0.7 m L ) , reductillated, and analyzed by ASV. T h e agreement between values derived by the reductillation-ASV technique and the accepted or target values for the standards was excellent. I n addition, t h e freedom from interference from numerous cations and anions is evident since rather high levels of potential interferants are present in these standards. Separation and Determination of As(II1) and As(V). Table VI1 illustrates very well the high volatility of AsC1, in HC1 compared t o t h e nonvolatile AS(V) species. T h e small amount of arsenic detected in t h e collection solution from As(V) samples was probably due to an As(II1) impurity in the NaHAs04. T h i s dramatic difference in t h e volatility of the two inorganic arsenic ions suggests t h a t it should be feasible to effect a measurement of t h e As(III)/As(V) ratio in some types of samples, especially water samples. As(II1) could be determined by distillation of the sample in the absence of the Cu2C12,and As(V) could then be determined by a difference between total arsenic concentration (determined on a second aliquot of t h e sample) and t h e concentration of As(II1).
(1) Y. Talmi and C. Feldman, "The Determination of Traces of Arsenic: A Review", in "Arsenical Pesticides", E. A. Woolson, Ed., American Chemical Society, Washington, D.C., 1975. (2) G. C. Whitnack and R. G. Brophy, Anal. Chim. Acta, 48, 123 (1969). (3) G. L. McKinney and P. A. Mack, "More on the Analysis of Arsenic and Selenium in Environmental Samples", presented at the 12th ACS Midwest Regional Meeting, October 1976. (4) J. A. Fiorino, J. W. Jones, and S. G. Capar. Anal. Chem., 48, 120 (1976) (5) "Criteria for a recommended standard ....Occupational Exposure to Inorganic Arsenic, New Criteria--1975", U.S. Department of Health, Education, and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, 1975. (6) Statement of Edward J. Baier, Deputy Director of NIOSH at the OSHA Informal Hearing on Proposed Standard for Occupational Exposure to Inorganic Arsenic, September 8, 1976. (7) I.Shain in "Treatise on Analytical Chemtstcy", Part I, Vol. 4, I.M. Kotthoff and P. J. Elving, Ed., Interscience, New York, N.Y., 1967. (8) E. Barendrecht in "Electroanaiytical Chemiistry: A Series of Advances", Vol. 2, A. J. Bard, Ed., Marcel Dekker, New York, N.Y., 1967. (9) H. Monien, Chem.-Ing.-Tech., 42, 657 (1970). (10) H. Moniem, Chem.-1ng.-Tech., 43, 6613 (1971). (11) W. D. Ellis, J . Chem Educ.. 50, A131 1,1973). (12) W. Kemula and 2 . Kublik in "Advances, in Analytical Chemistry and Instrumentation", Vol. 2. C. N. Reilley, Ed.. Interscience, New York, N.Y., 1963. (13) R. Neeb, "Inverse Polarographyand Vohammetry", Verhg Chemk?, Munich, Germany, 1969. (14) L. F. Trushina and A. A. Kaplin, Zh. Anal. Khim., 25 1616 (1970). (15) A. A. Kaplin. N. A. Veits, and A. G. Stromberg, Zh. Anal. Khim., 28, 2192 (1973). (16) G. Forsberg, Ph.D. Dissertation, University of Missouri, 1975. (17) G. Forsberg, J. W. O'Laughlin, R . G. Megargle, and S R. Koirtyohann, Anal. Chem., 47, 1586 (1975). (18) A. M. Sulek, E. W. Zink, and G. R. Dulude, 89tb Annual Meeting, Association of Official Analytical Chemists, Washingtcsn, D.C., October 1975, Paper No. 69. (19) J. H. Christie, U. Eisner, and R. A. Osteryoung, "Staircase Voltammetric Stripping Analysis from Thin-Film Mercury Electrodes", presented at the 27th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March, .l976, Paper No. 129. (20) J. H. Christie and R. A. Osteryoung, Anal. Chem., 48, 869 (1976). (21) U. Eisner, J. A. Turner, and R. A. Osteryoung, Anal. Chem., 48, 1608 (1976). (22) Environmental Sciences Associates, Inc , Bedford, Mass , Method No. TMA-12. (23) L. Meites, Ed., "Handbook of Analytical Chemistry", 1st ed., McGraw-Hill, New York, N.Y., 1963, pp 3-55: (24) P. H. Davis et al., "Total Arsenic Analysis of Blood, Urine, and Air by Hiah SDeed Anodic Striooina Voltammetrv". American Industrial Hvaiene Confeience, May 22-2i, 7977, New Orleans, La., Paper No. 56 (25) P H. Davis et al., J . A m . Ind. Hyg. Assoc , in press. (26) Environmental Sciences Associates, Bedford, Mass., Methods TMA-13, 14, 15, and 16
RECEIVED for review July 15, 1977. Accepted October 25, 1977. We acknowledge the National Institute of Occupational Safety a n d Health, NIOSH Contract No. HSM 99-72-107, for financial support during the initial phase of the investigations necessary for t h e development of the method.
