1024
Anal. Chem. 1980, 52, 1024-1028
(2) J. Karube, T. Mitsuda, T. Matsunaga, and S. Suzuki, J . Ferment. Techno/., 55. 243 (1977). (3) I.Karube, T. Matsunaga, and S. Suzuki, J . Solid-Phase Biochem., 2, 97 (1977). (4) K. Matsumoto, H. Seijo, T. Watanabe, I. Karube, and S. Suzuki, Anal. Chim. Acta, 105, 429 (1979). (5) T. Matsunaga, I . Karube, and S. Suzuki, Anal. Chim. Acta, 9 9 , 233
11 9 7 A I _,. (6) M. Hikuma, T. Kubo, T. Yasuda, I. Karube. and S. Suzuki, Biotechnol.
(8) M. Hikuma, H. Suzuki, T. Yasuda, I. Karube, and S. Suzuki. Eur. J. Appl. Microbial. Biotechnol., 8 , 289 (1979). (9) M. Alexander, "Introduction to Soil Microbiology", John Wiley & Sons,
New York, 1961, pp 272-292. ( 1 0) Japanese Industrial Standard Committee, Testing Methods for Industrial Waste Water, JIS K 0102, p 36, 1974. (11) Ref. 10, p 33.
~
Bioeng., 21, 1845 (1979). (7) M. Hikuma, T. Kubo, T. Yasuda, I.Karube, and S.Suzuki, Anal. Chim. Acta, 109, 33 (1979).
RECE1\'ED
for review October
303
1979. Accepted March 12,
1980.
Pneumatoamperometric Determination of Parts-per-Billion Dissolved Species by Gas Evolving Reactions P. R. Gifford and Stanley Bruckenstein" Chemistry Department, State University of New York at Buffalo, Buffalo, New York 14214
Pneumatoamperometry is applied to a new method for trace analysis of aqueous solutes in which the solute of interest is reacted to form a volatile electroactive product which is flushed from solution using an electroinactive gas. The gas stream passes over one surface of a hydrophobic gas-porous electrode, where the electroactive species is electrolyzed at constant potential, giving a current response proportional to the initial amount of the solute. Detection limits for the solutes tested are: Hg(I1) (5 ppb), As(II1) ( 3 ppb), I- (6.5 ppb), and 103- (0.5 ppb).
T h e r e is a continuing concern about the impact of trace levels of pollutants on the environment. This concern has led to the establishment of maximum allowable limits of hazardous substances for drinking water supplies, such as 0.005 mg L-' for mercury a n d 0.1 mg L-l for arsenic ( I ) . T h e high toxicity a n d low allowable limits for these and other toxic species illustrate the need for sensitive analytical methods. Numerous articles describing methods for these and related analyses have appeared in recent years and are reviewed biannually ( 2 - 4 ) . Where applicable, conversion of a solute to a volatile species by appropriate reactions prior to measurement, e.g., by some form of spectroscopy, has been found to be an excellent technique. This approach is widely used for determinations of gaseous hydride-forming elements, and for Hg, by atomic absorption spectrometry (AAS) ( 5 , 6). T h e approved method for mercury analysis is based on the method of Hatch a n d Ott (7) and involves reduction of mercury ions in solution to metallic Hg, followed by evolution of Hg vapor which is then determined by AAS. Using a 100-mL sample, the method gives a detection limit of 0.2 p g L-' H g (0.2 p p b ) (8). I n the standard method for arsenic, inorganic arsenic is reduced to arsine which is passed into a n absorber tube containing silver diethyldithiocarbamate to form a red complex for photometric measurement. T h e minimum detectable amount by this method is 1 pg As (9). T h e ASH?vapor has also been isolated by collecting it in a balloon reservoir or cold trap. The collected ASH, is then analyzed by AAS. Detection limits of about 1 ng As are reported for this method (10, 1 1 ) . 0003-2700/80/0352-1024$0 1 O O / O
Iodide is normally present in only trace quantities in natural waters and thus serves as an indicator of seawater intrusion. T h e standard method for I- determination utilizes iodide's ability to catalyze the reduction of ceric ions by arsenious acid. Iodide is determined by the bleaching of the ceric color, and the method has a detection limit near 0.2 pg I- for a 10-mL (20 ppb) sample (9). We describe a new application of hydrophobic gas-porous electrodes, in particular the gold gas-porous electrode (Au GPE), for the determination of trace solutes in aqueous samples. First, the solute is reacted to form a volatile electroactive product. This product is then flushed from solution by purging with an electroinactive gas and passed over one side of the Au GPE, whose potential is set to be on the limiting current region of the electroactive species, and a recording made of electrode current vs. time. Such recordings show peak currents that are proportional to the concentration of the trace solute. We denote the use of a porous electrode structure to determine the concentration of an electroactive species present in the gas phase as pneumatoamperometry in the case of potentiostatic control and current measurement. [The prefix pneumato is derived from Greek and pertains to gas, air, and vapor.] T h e potential of this new technique is tested with four analytes: Hg(II), As(", I-, and IO3-. Hg(1I) was reduced to mercury, As(II1) to AsH3, and 1- and IO3- reacted with each other to form Is. All volatile products were determined by oxidation a t the Au G P E . After the reviewers comments of this paper were received, a paper by D. D. Nygaard (12) was published describing the determination of mercury by: (1) reduction of Hg(I1) with stannous, ( 2 ) purging with air or nitrogen and (3) oxidizing the mercury in the purging gas phase at a Clark-type electrode (13). His procedure for mercury is the same in principle as the one we describe.
EXPERIMENTAL Instrumental and Apparatus. A Princeton Applied Research Model 173 potentiostat equipped with a Model 179 digital coulometer and a Model 178 electrometer probe (Princeton Applied Research Corp., Princeton, N.J.) was employed in this study. To reduce high frequency current noise present in the potentiostat and cell, a 0.0024-pF capacitor was connected between the auxiliary and reference electrodes. Also a 50 kR resistor was inserted c' 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
1025
P-* Figure 2. Sampling train for determination at an Au GPE
Flgure 1. Gold gas-porous electrode (Au GPE) assembly. A: Gas inlet; B: contact entrance; C: standard taper for electrochemical cell; D: gold contact wire; E: Gore-Tex membrane treated with Au resinate. Angle is to remove any gas bubbles that might form during potential cycling; F: heat-shrinkable tubing; G: gas outlet
in series with the auxiliary electrode. Current data were recorded on a Bristol millivolt strip-chart recorder Model lPH560-51-T46-T82 (The Bristol Co., Waterbury, Conn.) which was modified with a resistor divider network to allow recording voltages from 1 mV to 10 V full-scale. The input signal to the recorder was filtered through an RC filter of R = 1 M R and C = 0.22 pF to 1.0 wF. The electrochemical cell consisted of a Au GPE, a coiled gold wire auxiliary electrode, and a saturated calomel electrode (SCE) reference. The supporting electrolyte was either 0.2 M or 1.0 M H2S04. The SCE was isolated from the Luggin capillary by a closed ground glass stopcock to minimize chloride contamination due t o leakage of the SCE. The stopcock formed the junction between the supporting electrolyte and the saturated potassium chloride. All potentials are reported vs. the SCE. Construction of A u GPE. The working electrode assembly (Figure 1) consists of a Au GPE, gas inlet, and gas outlet. The Au GPE is constructed using the method developed by previous workers in our laboratory (14). The Au GPE is constructed from a porous nonwetting membrane, a piece of 0.004-inch Gore-Tex polytetrduoroethylene sheeting (W. L. Gore and Associates, Inc., Elkton, Md.) onto which gold resinate solution (Engelhard Industries, East Newark, N.J.) is heat-cured. Contact to the electrode face is made through a gold wire pressed against a gold contact pad painted on the side of the electrode membrane and held in place by wrapping the end of the electrode assembly with Teflon tape. The end of the electrode is then covered with heat-shrinkable tubing to prevent solution leakage into the interior of the porous electrode assembly. Prior to use, the electrode is potential cycled at a rate of 20 mV s-l for 8 h between oxygen and hydrogen evolution. +1.6 \.' to -0.4 V vs. SCE, in supporting electrolyte until a reproducible current-potential curve is obtained. Reagents a n d Solution. All reagents were of AR grade and used without further purification. Solutions were prepared with water obtained from a Milli-Q Reagent Grade Water System (Millipore Corp., Bedford, Mass.). Nitrogen supplied as boil-off from liquid nitrogen (Union Carbide, Linde Division) was used as the purging gas. The flow rate, 4OC-800 mL min-', was controlled using a flowmeter (Union Carbide, Linde Division). Samples and/or reagents were introduced into the reaction vessel with a 25-pL syringe (Precision Sampling Corp., Baton Rouge, La.) or a 10-pL syringe (Hamilton Co., Reno, Nev.). Glassware was periodically cleaned in a 50-50 volume mixture of concentrated nitric and sulfuric acids followed by extensive rinsing with reagent grade water. Procedures. General. The apparatus used for determinations a t a Au GPE is shown in Figure 2. The nitrogen purging gas is controlled through a flowmeter and bubbled through a coarse frit
sealed into the side near the bottom of the reaction vessel. The reaction vessel, in which the volatilization reaction occurs, consists of a borosilicate glass vessel with an approximate volume of 20 mL. 4 three-way stopcock between the flowmeter and the reaction vessel may be used to divert the N2 from the reaction vessel. Depending on the kinetics of the volatilization reaction, N2 may or may not be allowed to bubble through the solution in the reaction vessel continuously. A side port fitted with a rubber septum allows introduction of sample and/or reagent via a microliter syringe. The purging gas exits through the outlet tube a t the top of the reaction vessel. A trap precedes the Au GPE gas inlet to prevent carryover of solution droplets to the gas phase side of the Au GPE membrane. D e t e r m i n a t i o n of M e r c u r y . Dilute mercury solutions were prepared fresh weekly by dilution of 0.01 M Hg(C10J2 stock solution with 0.2 M H2S04. The reductant solution, 2 % w/v SnC12.2H20in 1.0 M H2S0,, was prepared fresh every few days. The electrochemical cell supporting electrolyte was 0.2 M H2S04. The Au GPE was potentiostated a t +0.975 \.' for Hg determinations. Prior to each measurement, the electrode was pretreated electrochemically. It was oxidized a t +.l.35 V for 60 s, reduced at -0.20 V for 60 s, then set a t +0.975 V for 60 s. This waiting period of 1 min was sufficient for the residual current to become negligible. This pretreatment was necessary to maintain the current response. The cause of the time dependent decrease of the Hg response is unknown. Five milliliters of the Sn(I1) solution were placed in the reaction vessel. Nitrogen was continuously bubbled through the reaction vessel at 400 mL min-'. Samples of Hg(I1) were added to the reaction vessel through the septum using a microliter syringe and the resulting charge and/or current due to the electrooxidation of the generated Hg vapor was recorded. Experiments were performed at room temperature. Determination of Arsenic. Arsenate imd arsenite solutions were prepared from AR grade As20Rand NaAs02, respectively, in saturated potassium bitartrate (KHTar) buffer solution. The reductant solution, 4% w/v NaBH, in 0.1 M NaOII, was prepared fresh daily. The electrochemical cell supporting electrolyte was 1.0 M H,SO,, The Au GPE was initially potential cycled between -0.2 and 1.50 V for 1 h and then was potentiostated at f1.35 V until a stable residual current of 1-2 F A was obtained. The potential cycling step was performed only at weekly intervals, and there was no significant loss in sensitivity observed over this time period at constant potential. Five milliliters of KHTar buffer were placed in the reaction vessel and the desired portion of As(II1) solution was added with a microliter syringe. After the cell contents were mixed by bubbling gas at a rate of 500 mI, m k ' , g.ss flow through the vessel was stopped by diverting the nitrogen to the atmosphere and 25 FL of 4% NaBH, was added through the rubber septum. Two minutes later. the gas flow through the reaction vessel was resumed at a rate of 500 mL min-' and the Au GPE response was recorded. Between determinations, the reaction vessel was emptied, rinsed with reagent grade water, and fresh KHTar buffer was added. Experiments were performed a t room temperature. Determination of Zodide. Stock solutions of iodide were prepared from NaI dried at 105 "C for 1 h. The oxidizing solution was 0.05 M K I 0 3 in 0.1 M H2S04. The electrochemical cell supporting electrolyte was 1.0 M H2S04. The Au GPE. after potential cycling as described above for arsenic determination, was potentiostated at +1.35 V until a stable
1026
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
400
I
8
LC-
i
-"O.l"
14
12
I
'0 E, (volts1
08
J
I
5t
06
Figure 3. Partial current-potential curve in 0.2 M H,SO, at Au GPE with and without deposited Hg. Potential scan rate = 12 mV s-', Arrows indicate direction of potential scan. Dashed lines show current due to Hg oxidation at the Au W E . S o l i line shows current for 0 2 M H,SO, supporting electrolyte at the Au GPE
residual current of 1-2 pA was attained. Five milliliters of iodate solution were placed in the reaction vessel and nitrogen was bubbled continuously at a rate of 800 mL min-'. Portions of iodide solution were added to the iodate solution through the rubber septum using a microliter syringe and the resulting Au GPE response was recorded. Experiments were performed at room temperature. Determination of Iodate. The reducing solution for IO3- deM I- in 4 M HC10,. The electrochemical terminations was cell supporting electrolyte was 1.0 M H2S04. A water-jacketed reaction vessel was thermostated at 30 "C by circulation of water from a thermostated bath (Forma Scientific Inc., Marietta, Ohio). The Au GPE was pretreated electrochemically as in the case of the iodide determination and was potentiostated at +1.35 V until a stable residual current of 1-2 PA was attained. Five milliliters of reducing solution were placed in the reaction vessel and nitrogen was bubbled continuously at a rate of 800 mL min-'. Portions of 103- solution were added to the reaction vessel through the rubber septum using a microliter syringe and the Au GPE response was recorded.
RESULTS AND DISCUSSION Determination of Mercury. T h e Hg(I1) determination is based on the reaction Hg(I1)
+ Sn(I1)
-
Hg
+ Sn(1V)
(1)
T h e volatile Hg is swept from the reaction vessel by K2to the Au G P E where the Hg is electrooxidized t o Hg(I1) and gives a current response proportional to the initial amount of Hg(I1). T o determine the appropriate potential for Hg oxidation, current-potential curves were recorded for Hg(I1) solutions at a n Au G P E (Figure 3). In this experiment Hg(I1) was added to the supporting electrolyte. At a potential of +0.975 V, a stripping peak due to the oxidation of previously reduced Hg to Hg(I1) occurs. Thus, this potential was chosen for the oxidation of Hg in t h e gas phase. Au G P E charge and current responses were characterized with respect to purging gas flow rate over the range of 100 to 500 m L min-' (Figure 4) using 965 ng Hg(I1). As the N2 flow rate is increased, a sharper and higher current peak which tails more rapidly is observed. At flow rates greater than 250 m L min-', the delay between sample injection and charge or current response is no more t h a n several seconds. Porous electrodes of the construction type exemplified by the Au G P E show a current response which is limited a t high gas flow rates because transport of the electroactive species
Figure 5. Plot of peak current vs. amount of analyte. (0)Hg(II), slope = 10.7 f 0.5 nA/ng Hg(I1); intercept = -0.44 f 0.07 FA. (0)As(III), slope = 24.9 f 0.3 nA/ng As(II1): intercept = 0.62 f 0.06 FA. (A) I-, slope = 1.98 f 0.07 nA/ng I-; intercept = -52.5 f 0.4 nA
through the porous membrane becomes rate determining. Typically, about 10% of the electroactive material in the gas phase is electrolyzed a t the beginning of this "limiting" current region. At lower N2 flow rates, the peak current is smaller because transport in the gas phase becomes rate determining. However, the fraction of electroactive material in the gas stream that is electrolyzed at the Au GPE becomes larger as the gas flow rate decreases because its gas phase concentration gradient extends further into the gas phase at lower flow rates. T h e flow rate dependence shown in Figure 4 is characteristic of pneumatoamperometry a n d pneumatocoulometry a t the Au G P E a n d is not unique to Hg determinations. On the basis of these studies, a gas flow rate of 400 mL min-' was chosen for the pneumatoamperometry of Hg. This flow rate gives a sharp, well-defined current peak and short response times. Problems due to vigorous bubbling of the reductant solution made higher flow rates undesirable. T h e current response for Hg vapor electrooxidation was recorded vs. time for a series of Hg(I1) concentrations and the peak current was taken to be the difference between the extrapolated base-line current and the maximum current response. T h e peak current values were plotted vs. amount of Hg(II), and gave a straight line with slope of 10.7 f 0.5 nA/ng Hg(I1) for 40 to 400 ng Hg(I1) (Figure 5 ) . Linear plots are obtained for Hg(I1) samples u p to 2 F g Hg(I1) (the upper limit of this study). Reproducibility is f570 over this concentration range. A detection limit of 25 ng Hg(I1) in 5 mL of reactant solution (5 ppb) was obtained. Determination of Arsenic. This determination is based on the reduction of arsenious with sodium borohydride to produce volatile arsine. This reaction is nonstoichiometric a n d requires a large excess of NaBH, which rapidly decomposes to form hydrogen. Equation 2 summarizes this situation.
ANALYTICAL CHEMISTRY, VOL. 52. NO. 7 , JUNE 1980
As(II1) + NaBH,
H+
AsH3 + H2 + products
1027
(2)
T h e arsine is swept from solution by the purging gas stream to the Au G P E where it is electrooxidized to give the measured current response. T h e reduction of arsenic compounds to ASH, is p H dependent and has been examined in detail by Braman and co-workers (15). They report, as the optimum conditions, p H 1-2 for As(V) reduction and p H -4 for As(II1) reduction. Reduction of As(V) in phosphoric acid at p H 2 gave a much lower response t h a n that obtained for comparable As(II1) concentrations. This difference in response for As(II1) and As(V) has been previously observed and appears to be due to the reduction kinetics of the As(V) species (16). Therefore, all subsequent work involved determination of As(II1). A K H T a r buffer of p H 3.6 was used for As(II1) determinations. T h e determination of As(V) would require a pre-reduction step to obtain the sensitivity reported below for As(II1). As shown in Equation 2, H 2 is produced owing to NaBH, hydrolysis in acid media, both in the absence of arsenic and concurrent with ASH, generation. This H 2 is partially electrooxidized a t the Au GPE, giving rise to a background response. Gold is a particularly poor electrode material for the oxidation of hydrogen under our conditions, and only a minute fraction of all the hydrogen produced is detected. However, notwithstanding this desirable electrochemical situation, a major aspect of this study was to minimize H 2 background response even more while maximizing the As(II1) response. Optimal response for As(II1) is obtained by: (1)interrupting gas flow (by-passing the nitrogen around the cell) prior to addition of NaBH, to the sample, (2) adding NaBH,, and ( 3 ) waiting long enough for the reaction to proceed before resuming gas flow. We found that adding NaBH, while purging with N2 produced a significant decrease in As(II1) response as compared to using the optimal technique given in the steps above. This was established by injection of NaBH4 to the reacted sample again, and determining the additional current response above the background. T h u s the 2-min waiting period in step 3 above was found to be sufficient. A satisfactorily low and reproducible background response of 0.62 f0.06 yA for H 2 produced from excess NaBH, was obtained using K H T a r buffer and 25 pL of 4% NaBH,. T h e current response for ASH, electrooxidation was recorded vs. time for a series of As(II1) concentrations. The peak current was plotted vs. amount of As(II1) and gave a straight line having a slope of 24.9 f 0.3 nA/ng As(II1) for 0 to 400 ng As(II1) (Figure 5). Linearity is obtained from 0 to 500 ng As(II1). At higher As(II1) levels, a negative deviation from the line is observed. Ten determinations of 300 ng of As(II1) gave a standard deviation of h3%. A detection limit of 25 ng As(II1) (5 ppb) was obtained. Determination of Iodide. T h e iodide determination is based on the reaction:
f 100nA
I
I
-
6Hf
+ IO3- + 51-
-
31,
+ 3 H20
(3)
which proceeds rapidly and quantitatively in the presence of excess iodate in acid media (17). T h e volatile I2 is then electrooxidized to IOy a t the Au G P E to obtain a current response. Using the procedure given in the Experimental section, the current response for I2 electrooxidation was recorded for a series of I- concentrations. Peak currents were plotted as a function of the amount of 1- and gave a straight line over the range studied (0-3 pg I). For 30 to 200 ng I-, a slope of 1.98 f 0.07 nA/ng I- was obtained (Figure 5 ) . In a series of ten determinations of 275 ng I-, a standard deviation of f4% was obtained. A detection limit of 32.5 ng 1- (6.5 ppb) was obtained a t a current noise level of 10 nA on the residual current of 1-2 FA.
Time (minutes)
Figure 6. Current-time response for IO3-. Curves A-E correspond to 0, 2 , 5, 20, and 30 ng IO3', respectively
Determination of Iodate. It is possible to determine IO3by reaction with I- followed by electrooxidation of I,. Our interest in this study lies not only in the determination of trace amounts of iodate, but in studying the problems of determining, by pneumatoamperometry, iodine produced by reaction with other oxidizing agents. Since there are many such oxidizing agents, the ability to determine trace iodine produced by oxidation of iodate provides the basis of a generally useful technique for other oxidants. There are significant problems in using Reaction 3 for I03-determinations. First, the concentration of 1- must be sufficiently low to avoid formation of appreciable amounts of nonvolatile I,-. Secondly, kinetic problems can arise because of the low IO3-.a n d I- levels employed. These are manifested by low Au G P E currents a n d broad peaks. These problems can be minimized by using a high concentration of acid and carrying out Reaction 3 above room temperature. Optimization of competing requirements led to using 1 X M I- in 4 M HC10, and carrying the reaction out a t 30 "C. Under these experimental conditions, good peak currents are obtained for ppb levels of IO< (Figure 6 ) . A plot of peak current. vs. amount of IO3- is linear over the concentration range studied (0 to 1 yg IO
