1020
Anal. Chem. 1980, 52, 1020-1024
(31) N. Conroy, K. Hawley, W. Keller, and C. Lafrance, in "Proceedings of the First Speclalty Symposium on Atmospheric Contribution to the Chemistry of Lake Waters'' Int. Assoc. Great Lakes Research, Sept. 28-Oct. 1, 1975, pp 146-165. (32) E. Nieboer, D. H. S. Richardson, and F. D. Tornassini. &yologist, 81. 226 (1978). (33) D. H. S. Richardson and E. Nieboer in "Cellular Interactions in Symbiotic and Parasitic Associations", C. B. Cook, P. W. Pappas, and E. D. Rudolph, Eds. Proceedings, Fifth Annual Colloquium, College of Biological Sciences, Ohio State University, Ohio State University Press, Columbus, Ohia. 1980. (34) D H.' S.-ichardson, E. Nieboer, P. Lavoie, and D. Padovan, New Phflol , 82. 633 11979). (35) A. 'E, MaAell and R. M. Smith, "Critical Stability Constants" (4 volumes), Plenum Press, New York and London, 1977. (36) L. G. Sill& and A. E. Martell, Stability Constants of Metal-ion Complexes", Chem. SOC.(London) Spec. Pub/., No. 17 (1964): Suppl. No. 1, Chem. SOC.(London) Spec. Pub/., No. 25 (1971). (37) N. E. Milad, S. E. Morsi, S.T. Soliman, and L. M. N. Seleem, Egypt. J . Chem. 1SDec. Issue. Pub. 1974). 101-6 (1973): Chem. Abstr.. 81. . 422279 '(1'974), and 62, 145909f(1975). G. A. Heath and G. Hefter, J , Electroanal. Chem., 84, 295 (1977). R. Kalvoda, W. Anstine, and M. Heyrovskf, Anal. Chirn. Acta, 50, 93 (1970). F. C. Anson and R. S. Rodgers, J . Electroanal. Chern., 47, 287 (1973). F. C. Anson, J. 8.Flanagan, K. Takahashi, and A . Yamada, J . Electroanal. Chem., 67, 253 (1976). I.
(42) (43) (44) (45) (46) (471 .
I
(48) (49)
G. V .
Prokhorova, L. K. Shpigun, and E. N. Vinogradova, Zh. Anal. Khim., 27, 780 (1972). H. B. Mark. Jr., and C. N. Reilley, J . Electroanal. Chern., 4, 189 (1962). H. B. Mark, Jr., and C. N. Reiliey, Anal. Chern., 35, 195 (1963). H. 6.Mark, Jr., J . Electroanal. Chem.. 7 , 276 (1964). E. Itabashi. J . Electroanal. Chem., 60, 285 (1975). H. Zachariasen. I. Andersen. C. Kostd. and R. Barton. Clin. Chern.. 21. 562 (1975). D. Mikac-Devib, F. W. Sunderman, Jr., and S. Nomoto, Clin Chern., 23, 948 (1977). C. J. Flora, "Determination of Nanogram Quantities of Nickel by Differential Pulse Polarography at a Dropping Mercury Electrode and Selected Applications", M.Sc. Thesis, Laurentian University, May 1979.
RECEIVED for review April 23, 1979. Resubmitted February 15, 1980. Accepted February 15, 1980. Financial assistance is gratefully acknowledged from Falconbridge Nickel Mines Limited, Falconbridge, Ontario, and the Inco/United Steelworkers Joint Occupational Health Committee, Copper Cliff, Ontario. One of the authors (C.J.F.)records his appreciation for the award of a National Research Council of Canada Postgraduate Scholarship.
Ammonia Electrode with Immobilized Nitrifying Bacteria Motohiko Hikuma, Tatsuru Kubo, and Takeo Yasuda Central Research Laboratories, Ajinomoto Co., Inc., 7 Suzuki-cho, Kawasaki-ku, Kawasaki, 210, Japan
Isao Karube" and Shuichi Suzuki Research Laboratory of Resources Utilization, Tokyo Institute of Technology, Nagatsuta-cho, Midori-ku, Yokohama, 227, Japan
The ammonia electrode consisted of immobilized nitrifying bacteria and an oxygen electrode. When a sample solution was pumped into the flow system of the sensor, the electrode current decreased until a steady state after 8 min. Measurement could be also made with a 3min pumping period with lfflle loss of sensitivity. Calibration plot of the current difference vs. concentration of ammonia was linear up to 1.3 mg/L. The sensor was applied to wastewater and good agreement was obtained with the conventional method (f6YO). The sensor could be used for more than 2 weeks and 1400 assays.
T h e determination of ammonia in a sample solution is important in various fields such as medical, environmental and industrial process analyses. An ammonia-gas electrode consisting of a combined glass electrode and a gas permeable membrane is usually used for this purpose. In this case, the determination must be performed under strong alkaline conditions (above p H 11). Furthermore, volatile compounds such as amines sometimes interfere in the determination of ammonia. Recently many microbial sensors consisting of immobilized microorganisms and an oxygen probe have been developed (1-8). As previously reported, the concentration of substrate can be determined from the respiration activity of the immobilized microorganisms which is directly measured by the oxygen probe attached ( I , 6). A nitrifying bacterium Nitrosomonas sp., utilizes ammonia as a sole source of energy and oxygen is consumed by the respiration as follows; 2NH3 + 302
Nitrosornonas sp.
2HN02
+ 2H20
0003-2700/80/0352-1020$01 .oo/o
Therefore, ammonia may be determined by the microbial sensor using immobilized nitrifying bacteria such as N i t r o somonas europaea and a n oxygen probe. In this study, nitrifying bacteria were immobilized by being entrapped between two membranes, a porous acetylcellulose membrane and the Teflon membrane of the oxygen electrode. T h e microbial sensor was applied t o the determination of ammonia in wastewaters. EXPERIMENTAL Materials. Chloramphenicol was purchased from Sankyo Co. Other reagents were commercially available analytical reagents or laboratory grade materials. The wastewaters were obtained from a fermentation factory. Culture of Microorganisms. Activated sludges containing nitrifying bacteria were obtained from a fermentation factory. About 200 mL of the activated sludge (MLSS 5000 ppm) was inoculated into 2.5 L of the culture medium containing 0.0670 (NHJ2S04,0.05% K2HP04,50 ppm MgS04,4 ppm CaCl,, 20 ppm FeC13,and 25 g of powdered CaC03 (30-pm average diameter, a support for nitrifying bacteria) (9). The mixture was placed in a vessel as shown in Figure 1 and the bacteria were cultured under aerobic conditions (1/2 VVM aeration) for more than four months at room temperature (20-30 "C). The pH of the culture medium was controlled at 8.2 by adding 1 N Na2C03. The fresh culture medium of the same composition described above except for 0.6% (iVHJ2S04employed was fed at a rate of 200 mL/day. On the other hand, nitrifying bacterium, Nitrosomonas europaea ATCC 19718, was cultured in a 2-L flask containing 500 mL of culture medium (pH 8.2-8.4) for 25 days at 25-30 "C. The culture medium employed was the same composition described above except for 0.3% (NH,),S04 employed (sterilized at 120 "C for 30 min). Then, 0.05 ppm cresol red was added as a pH indicator. Calcium and magnesium salts were sterilized separately to avoid precipF 1980 American Chemical Society
-
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7 , JUNE 1980
1021
cz&-i+i+ 3 LB -
1A
u Figure 3. Schematic diagram of the sensor system. ( I ) Sampler, (2) peristaltic pump, (3) water jacket, (4) flow cell, (5) magnetic stirrer, (6) microbial electrode, (7) recorder. (A) Buffer solution (3.9 mL/min), (B) air (250 mL/min), (C) hot water (30 "C),(D) waste
Figure 1. Schematic diagram of the cultivation system for nitrifying bacteria from activated sludges. (1) Basin (2.5-L liquid volume), (2) baffles, (3) air diffuser, (4) culture medium (powdered calcium carbonate was suspended), (5) pH controller, (6) solenoid valve. (A) waste, (B) air (1.3 Llmin), (C) culture medium (0.2 L/d), (D) 1 N Na,C03 -
y/
I
C
2 3
0
I
I
0
4
5
d $6
a
4
i
10
i
9
8
7
Figure 2. Scheme of the microbial electrode for ammonia determination. (1) Aluminum anode, (2) electrolyte, (3) platinum cathode, (4) insulator, (5) rubber ring, (6) Teflon membrane, (7) vinyl patch (spacer), ( 8 ) immobilized microorganisms, (9) acetylcellulose membrane, ( 10) nylon net
itation. When colors of the indicator in the broth changed pink to yellow by nitrite produced by bacteria, sterilized potassium carbonate solution (1 N) was added to the broth keeping the pH of the broth constant. I m m o b i l i z a t i o n of t h e M i c r o o r g a n i s m s . Two mL of the culture broth containing bacteria or 50 mL of the culture broth of N . europaea was filtered through the porous acetylcellulose membrane (Millipore Co., Type HA, 0.45-pm pore size, 25mm diameter, and 150-pm thickness) with a vinyl patch (14-mm o.d., 6-mm i.d., and 50-pm thickness). The bacteria were retained on the acetylcellulose membrane as shown in Figure 2. A s s e m b l y of t h e M i c r o b i a l Electrode. The scheme of the microbial electrode is illustrated in Figure 2. An oxygen probe (Denki Kagaku Keiki Co., Model 3021) consisted of a Teflon membrane (50-pm thickness), a platinum cathode, an aluminum anode, and a saturated potassium chloride electrolyte. The porous membrane retaining the microorganisms was carefully attached on the Teflon membrane of the oxygen probe so that the microorganisms were trapped between the two membranes and covered with Nylon net and fastened with rubber rings. Apparatus. Figure 3 shows a schematic diagram of the system which consisted of a jacketted flow cell (3-cm diameter, 4-cm height, and 5-mL liquid volume) with microbial electrode, a magnetic stirrer (1000 rpm), a peristaltic pump (Technicon Model I), an automatic sampler (Toyo Kagaku Sangyo Co., Model SC-160 FA), and a recorder (Yokogawa Electric Works Co., Model
10
I 1 20 30
Time (rnin)
Figure 4. Response curves of the microbial electrode sensor when 9.6 mL of standard solutions were pumped for 12 min. The experiments were carried out under the standard conditions described in the text
ERB-6-10 or Model LER-12A). Procedure. The temperature of the flow cell was maintained at 30 f 0.2 "C by passing warm water through the jacket. A buffer solution containing 0.01 M sodium borate and 20 ppm chloramphenicol was saturated with dissolved oxygen and was transferred to the flow cell at a flow rate of 3.9 mL/min together with 250 mL/min of air. When the current of the electrode reached a steady-state value, a sample was injected into the flow cell at a flow rate of 0.8 mL/min for 3 min or 12 min with 12-min or 30-min intervals. In this paper, the concentrations of sample reported are those in the flow cell. D e t e r m i n a t i o n of A m m o n i a . Ammonia was determined by distillation-acidimetry (10). RESULTS AND DISCUSSION Response of t h e Sensor. Figure 4 shows typical response
curves of the sensor using immobilized nitrifying bacteria. T h e current a t zero time was obtained with the buffer solution and showed the endogenous respiration level of t h e immobilized microorganisms. When the sample solution containing ammonia was injected into the system for 1 2 min, it permeated through the porous acetylcellulose membrane and was assimilated by the immobilized nitrifying bacteria. Consumption of oxygen by the bacteria began and caused a decrease in dissolved oxygen around the membrane. As a result, t h e current of the electrode decreased markedly with time until a steady state was reached. T h e steady-state current is obtained within 8 min. I t indicates that the consumption of oxygen by the microorganisms and that diffused from a sample solution to the membrane were in equilibrium. When sufficient quantity of the bacteria is immobilized in the electrode, the current of the electrode for an ammonia solution depends mainly on the rate of diffusion of ammonia from the sample solution to the immobilized bacteria. Therefore, steady-state
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
1022
Table I. Influence of pH on Current Difference of the Microbial Electrode current difference ,Q PH 8.0
______
remarks Steady-state current was not obtained.
8.5
0.302
9.0
0.284
P A
Standard solution containing 0.88 m g / L ammonia was employed for experiments.
1 32 mg/L
0.4
0
30
60
T i me (min)
Figure 5. Response curve of the microbial electrode by the pulse method. Standard solutions (2.4 mL) were pumped into the system for 3 min with 15-min intervals. The experimental conditions were the same as Figure 4 except for sample volume employed
0 4
Table 11. Influence of Temperature on Current Difference of the Microbial Electrode current temperature, difference,a "C
P A
30
0.280
35 38 40
0.320
remarks
0.335 __.
inactivated
h
-b 0 3
Standard solution containing 0.88 m g / L ammonia was employed for experiments.
2
Experimental and were harvested whenever they were required. Response of the electrode for ammonia did not change with harvesting time of nitrifying bacteria. Calibration. Figure 6 shows calibration curves of the microbial sensor when ammonia sulfate solution was employed for the experiments. A linear relationship was observed between the current difference and concentration of ammonia below 1.3 mg/L by both the steady-state and the pulse method. The minimum concentration for determination was 0.05 mg of ammonia per liter. The reproducibility of the current difference was examined using the same sample. The current difference was reproducible within 4% of the relative error when a standard solution containing 1.32 mg/L of ammonia was employed. The standard deviation was 0.025 mg/L in 10 experiments by the pulse method. Influences of pH and Temperature. As the respiration activity of microorganisms depends on p H and temperature, their influences on the current output of the microbial sensor were examined. The standard solation (0.88 mg/L, ammonia) and sodium borate buffer solution (0.01 M, the p H was adjusted by 1N HC1) were employed for experiments. As shown in Table I, the current difference of the microbial electrode for the standard solutions was almost constant between p H 8.5 and 9.0. On the other hand, the steady-state current was not obtained from the microbial sensor below p H 8.5 (when the electrode was contacted with the standard solutions, the current of the electrode decreased markedly and then increased gradually to the initial current). This result suggested that the immobilized nitrifying bacteria were inactivated below p H 8.5 with nitrite produced by fermentation from ammonia. Therefore, the buffer solution of p H 9 was employed for the sensor. The influence of temperature on the current output of the electrode was examined. As shown in Table 11, the current difference obtained from the electrode for the standard solutions was almost constant from 30 to 38 "C. However, the current difference was decreased above 40 "C, because the bacteria in the membrane were inactivated by heat. T h e microbial sensor does inherently not respond to organic substances such as glucose. However, when the microbial electrode was used for a long time, it responded to glucose. This might be caused from the growth of contaminated microorganisms around the immobilized nitrifying bacteria. In order to prevent the contamination, various reagents such as
Q: 1
L
-z 0 2 7;)
* C
5
01
0
Figure 6. Calibration curves of the microbial electrode. The determination was carried out by the steady-state method (0)and by the pulse method (e).The experimental condltions were the same as those described in Figures 4 and 5
current depended on the concentration of ammonia. When the buffer solution was only transferred to the flow cell, the current of the microbial sensor finally returned to its initial level within about 8 min. T h e time required for the determination of ammonia was long by the steady-state method. Therefore, t h e pulse method (a certain volume of a sample solution was injected for certain time) was employed for the determination. Figure 5 shows response curves of the microbial sensor by the pulse method under the same conditions described in Figure 4 except for the sample volume (2.4 mL) and injection time (3 min) employed. In this case, the current difference (the current difference between the initial and the minimum) reached 80% of that obtained by the steady-state method. The assay can be done within 4 min and the electrode recovery time is about 8 min by the pulse method. The total time required for the assay of ammonia was 30 min by the steady-state method and 12 min by the pulse method. The pulse method was, therefore, employed for actual assays of ammonia. On the other hand, the microbial electrode using immobilized Nitrosomonas europaea was employed for experiments. The results were similar t o those obtained by the electrode using nitrifying bacteria isolated from activated sludges. However, the current output of the electrode using N . europaea decreased to 5070 of its initial value with 50-h assays. This result indicated that the activity of N . europaea rapidly decreased with repeated uses. Therefore, nitrifying bacteria isolated from activated sludges were used for further work. They were continuously cultured as described in the
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
1023
Table IV. Response of the Microbial Electrode Sensor to Various Salts Ammonia
r I*--
+
3
‘c)
+-
2
u O’ 0
~
Glucose , - ~
-.,
:c~*.--:x:T
~
,*
~.._ ~-~:
0
2
~
a
6
4
4
KH~PO,
100
KCl NaCl
100 100 100
MgSO,
10
Figure 7. Selectivity and reusability of the microbial electrode (0) KH,PO,-Na,CO, (0.05 M, pH 9), ( 0 )KH,P0,-Na,C03 (0.05 M, pH 9) 20 mg/L chloramphenicol, and ( 0 )Na,B,O, (0.01 M, pH 9) 20
+
mg/L chloramphenicol were employed for the buffer solution, respectively. The microbial electrode was successively used for more than 6 days and standard solution (0.88 mg/L as NH,) and glucose (170 mg/L) were applied to the experiments. The experiments were carried out under the standard conditions except for buffer solution employed Table 111. Response of the Microbial Electrode Sensor to Varidus Compounds
ethyl alcohol glutamic acid acetic acid sodium nitrite
1.7 17
0 0.010
170 1.7 17 170
0.104
1.7
0 0.014
urea
monomethyl amine diethyl amine triethyl amine monoethanol amine a
17
0.270
- 0.01 -0.01 -0.01
0.01-0.09a
0.010 0.090
0.187
170 170
0.137
170 170
0 0
0 0
concentration of ammoniaQ conven- micro- relational bial tive meth- elec- differod, mg/ trode, ence, L mg/L $7,
sample A waste water B
current difference, M A
0.88 170 170 170 170
glucose
0.078 0.028
Table V. Comparison between Ammonia Concentration Determined by Conventional Method and That by the Microbial Electrode Sensor
+
ammonia
MA
0.280
~~
Time( d )
composition of sample, mg/L
current difference,
composition of sample, mg/L NH, 0.88
0
After the same sample was injected 1 0 times for
1 0 0 h.
antibiotics (chloramphenicol, nystatin), and antiseptic (sodium benzoate), or a respiration inhibiter (sodium azide) were added to the buffer solution. Chloramphenicol was suitable for this purpose. Then, various buffer solutions containing chloramphenicol were employed. Figure 7 shows the current difference obtained from the microbial sensor, when the standard solution (0.88 mg/L as ammonia) and glucose solution (170 mg/L) were repeatedly injected into the system. As shown in Figure 7, the sensor did not respond to glucose and the output current was stable for a long time (70% of its initial value after 10 days), when sodium borate (0.01 M) containing chloramphenicol (20 mg/L) was employed. Therefore, nitrifying bacteria were not inhibited, but most of contaminated bacteria were strongly inhibited by chloramphenicol added to sodium borate buffer solution. This buffer solution was used for further works. Selectivity. The selectivity of the microbial electrode for ammonia is shown in Table 111. The sensor scarcely responded to compounds such as glucose, ethyl alcohol, glutamic acid, acetic acid, and ethyl amines, but slightly responded to nitrite, urea, and monomethyl amine. However, the method is specific to ammonia when the concentration of interference substance is the same level. Influence of Salts. The influence of salts on the activity of microorganisms is well known. Therefore, the influence
c
D
623 702
660’ 670’
47
45; 171
170
’
6 -5 -4
remarks BOD, 1 7 0 0 m g / L
1
2 140 5 6
Original concentration. Dilution factor, 100. Dilution factor, 10. Dilution factor, 20. of salts such as potassium phosphate monobasic, potassium chloride, sodium chloride, and magnesium sulfate on the current of the electrode was examined. As shown in Table IV, the sensor responded slightly to potassium phosphate monobasic, potassium chloride, but the increase of the electrode current was only 0.08 p A for 100 mg/L of KH,PO, (1 mg/L of KH,PO, corresponds to 0.003 mg/L of ammonia). Application and Reusability of the Microbial Sensor. The microbial electrode was applied to the determination of ammonia in wastewaters of a fermentation factory. T h e concentration of ammonia and the BOD of the sample solutions were determined by the distillation-acidimetry and JIS (Japanese Industrial Standard) method ( I O , I I ) , respectively. As shown in Table V, a good agreement was obtained between the ammonia concentration determined by the conventional method and that obtained by the microbial sensor. T h e relative difference between the two methods was less than 6%. The results were not affected by organic compounds which were determined by the conventional BOD method. The microbial sensor could be used for more than two weeks and 1400 assays. The electrode current difference decreased gradually; for example, the current difference diminished to 50% of its initial value after 10-day use. Therefore, occasional calibration of the electrode was required. For example, two kinds of standard solutions (0.44 and 1.32 mg/L of ammonia) were injected every 10 samples (2 points calibration). In this case, the variation of the successive calibration curves was negligibly small (within 0.3%). The microbial electrode was preferably stored in an ice box (4“C)being inserted in 50 mg/L of ammonia sulfate solution (pH adjusted to 8 with 0.1 N Na,CO,). The response of the electrode did not degrade for more 1 han 30 days.
ACKNOWLEDGMENT T h e authors are grateful to T. Kiya, Y. Sstkata and K. Mitsugi, Central Research Laboratories, Ajinonioto Co., Inc., for their helpful advice and encouragement during this study. LITERATURE CITED I. Karube. T. Matsunaga, S. Mitsuda, and S. Suzuki. Biofechnol. Bioeng., 19, 1535 (1977).
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).
There 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 and 0.1 mg L-l for arsenic ( I ) . The high toxicity and 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 and 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 ppb) (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. The 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.] The 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 GPE. 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
