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ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
Amperometric Membrane Electrode for Measurement of Ozone in Water John H. Stanley and J. Donald Johnson* Department of Environmental Sciences and Engineering, School of Public Health, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514
An amperometric membrane electrode has been developed for the Selective measurement of molecular ozone in water. The membrane electrode system consists of a gold cathode, a double junction reference electrode (10 % KNO,/SCE), a potassium sulfate electrolyte (0.5 M, pH 3.7), and a Teflon membrane (3.3 X lo-' cm thickness). A current sensitivity of 0.484 yA*(mg/L)-'*(crn)-' was observed at an applied voltage of +0.6 V (vs, SCE) at 22 O C . A detection limit of 62 pg/L Is predicted d twice the observed residual current. Less than 2 % interference was observed from bromine, hypobromous acid, chloride dioxide, hydrogen peroxide, trichloramine, and hypochlorous acid.
A drawback t o the use of ozone as a chemical disinfectant and/or oxidant for water treatment is the lack of a selective and reproducible analytical technique for monitoring residual ozone concentrations. Present methods are influenced by the chemical instability of the ozone molecule in water and are subject t o interferences from oxidizing agents that may be present. Amperometric membrane electrodes have been used successfully for dissolved oxygen and halogen analysis ( I , 2 ) . More recently, t h e use of these electrode systems for the measurement of molecular ozone dissolved in aqueous solution has been demonstrated (3-5). This work indicates that membrane electrodes possess the required detection sensitivity and potentially the selectivity for the measurement of molecular ozone. Also, membrane electrodes may be particularly suitable for ozone analysis since in situ measurements are possible. A new amperometric membrane electrode design has been developed for the measurement of ozone in water. This design has evolved from the original amperometric ozone electrode proposed earlier by Dunn and Johnson (3) and voltammetric studies of the ozone reduction process to optimize the electrolytic media and applied voltage. A homogeneous Teflon membrane was chosen in place of the microporous film from the earlier design because the diffusion transport properties of the former aid in the selective measurement of gaseous molecules like ozone. Operational characteristics of the membrane electrode and potential interferences in water treatment have also been examined. EXPERIMENTAL Apparatus. A Delta Scientific (now National Sonics Cor-
poration) model no. 8324 HOC1 amperometric electrode has been modified using an Orion model No. 09-02 double junction reference electrode and a Teflon membrane (Delta Scientific No. 824110) cm thickness (See Figure 1). A 0.38-cm2gold button of 3.3 X serves as the working cathode. The electrochemical circuit is completed using a 0.5 M potassium sulfate supporting electrolyte (pH 3.7, 0.1 M NaOAc/HAc buffer system). The membrane is applied over the gold surface and a capillary f i i of electrolyte. The membrane is held in position using Teflon tape and a plastic ring collar. The electrolyte layer between the cathode and the membrane is replenished through wicks located around the electrode tip leading to an electrolyte reservoir in which 0003-2700/79/0351-2144$01 .OO/O
the reference anode is immersed. When the membrane is mounted over the cathode, a primary concern is the avoidance of tears or creases in the membrane which affect diffusion processes and the reproducibility of the current response. Voltammetric studies were conducted using a three-electrode arrangement consisting of a gold-plated E. H. Sargent model no. S-30421platinum hook working electrode, an identical platinum hook auxiliary electrode, and a saturated calomel reference electrode. The rotating gold electrode was mounted in an E. H. Sargent model no. S-76985 synchronous rotator (600 rpm). Voltages were impressed and current was monitored using a McKee-Pedersen Instruments MP-1502 Electroanalyzer equipped with an MFE model 715 X-Y Recorder. The electrode potential was monitored using a Fisher Scientific model no. 420 pH/ion meter. Ozone was produced from oxygen using a W. R. Grace model LG-2-L1 Corona generator. The generator output was varied from 0 to 1.5% by wt in O3 by controlling the applied voltage across the electrical discharge tube and the oxygen gas flow rate. Amperometric titrations were conducted using the above equipment with the Sargent rotating platinum hook electrode, synchronous rotator, and a saturated calomel reference electrode. A voltage of +0.2 V (vs. SCE) was applied to the working electrode. Procedures. All reagents and test solutions were prepared using distilled and deionized water. Chemical reagents were prepared according to standard procedures (6) with Fisher ACS Certified, Malinckrodt, or Baker Analyzed chemical reagents. Two buffers were used, pH 6.0 (0.1 M KH2P04,Na2HPO4),and pH 3.7 (0.1 M HAC, NaOAc). Current-potential relationships were developed at 5 "C a t an electronic sweep rate of 25 mV (min)-' in the anodic then the cathodic direction. Ozone concentration measurements were made a t 300- to 500-mV intervals to normalize the current measurements, expressed as pA (mg/L)-', for ozone losses during the experiment. Voltammograms were also developed in oxygen saturated electrolyte solutions to establish the base-line current response. Amperometric measurements were made with the membrane electrode positioned in a cylindrical Plexiglas contactor (13 cm x 6 cm). Ozone was contacted in the test solution with a circular glass fritted diffuser inserted through the top plate of the Plexiglas reactor. The contactor was placed on a magnetic stirrer to accomplish stirring a t the electrode tip. After ozonation and complete mixing was accomplished, a steady-state current response was achieved. After observing this current for approximately 30 s, the current response was recorded as a sample was collected for analysis. Water column and electrode temperature were controlled using an external Plexiglas jacket. Temperature equilibrium was attained after approximately an hour. Current potential relationships were also developed using the Delta Scientific electrode in a two-electrode system in the Plexiglas contactor. These voltammograms were developed in a manner similar to the three-electrode studies except the stirring rate was 1000 rpm. The desired electrolyte was ozonated directly in the contactor and samples were again periodically analyzed to account for ozone loss. The iodometric procedure (6, 7) was chosen as the reference technique for ozone analysis in the absence of interferences. End-point detection for direct and back titration procedures using either sodium thiosulfate or phenylarsine oxide was accomplished amperometrically. Oxidants considered possible interferences were prepared by known procedures (8,9) and analyzed by standard methods (6). 0 1979 American
Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979 Cable to Instrument-
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/
Orion 90-02 Double Junction Reference Anode (10% K N 0 3 / S C E )
\ B
APPLIED VOLTAGE ( V o l t s vs SCE:l
1 -PIastlc
Teflon (33 xMembrone 10.3 cm thickness)
Collor
Flgure 2. Current-voltage relationship at the rotating gold electrode in 0.05 M H2S0, (pH 1.3) and 1.6 M KCI (pH 6.0). Solid lines are for ozone reduction current and dashed lines are residual current
Gold Cathode ( 0 38 c m 2 surface ore01
IOC
Figure 1. Schematic view of the membrane electrode system
I
The membrane electrode was lowered into a concentrated solution of the particular oxidant and the current response was noted. Identical measurements were made with the membrane applied over the cathode and with the membrane removed.
RESULTS AND DISCUSSION A principal concern was improving the selectivity of the membrane electrode for molecular ozone in the presence of other oxidants. Along with the selectivity, a linear and reproducible current response with a detection sensitivity comparable to or exceeding that observed with other designs (3-5) was desired. Several design factors were examined in an effort to optimize both the current sensitivity and selectivity of the system. Design Features. Dunn (3)used a potassium chloride (1.6 M, pH 6.0) supporting electrolyte and measured reduction currents a t an applied voltage of +0.35 V (vs. Ag/AgCl). Voltammograms developed in this electrolyte illustrate that the application of more anodic voltages is limited to the potential range less positive than +0.4 to 0.5 V (vs. Ag/AgCl) because the observed current is affected by a competing oxidation reaction at more positive voltages. This results in a reduced current response from ozone and a more significant residual current signal. Gaur and Schmid (10) first observed this oxidation current in the presence of chlorine, which they report results from the anodic dissolution of the gold cathode due to the formation of a gold chloride complex. Voltammograms developed in studies with other electrolytes ( 4 , 11, 12) are limited by oxygen evolution a t much more positive voltages. The use of one of these electrolytes would allow for the application of more anodic voltages. Since the use of a more positive voltage a t the gold cathode restricts electron flow, selectivity is improved because reduction reactions occur only for the most powerful oxidizing agents like ozone. The choice of a supporting electrolyte thus directly affects the selectivity of the system for ozone. This was confirmed by studying the reduction of ozone in several electrolytic media: potassium chloride (1.6 M, pH 6.0), sulfuric acid (0.05 M, pH 1.3), and perchloric acid (0.1 M, pH 1.2). As is illustrated in Figure 2, the use of chloride electrolyte restricts the accessible anodic potential range at which the ozone reduction processes can occur when compared with the use of sulfate electrolyte. Reduction currents for ozone were
-
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-40' ' ' I' I O 0 9 0 8 0 7 0 6 0 5 06 0 3 0 2 0
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APPLIED VOLTAGE IVoIts v s S C E I
Figure 3. Current-voltage relationship at the rotating gold electrode in 1.6 M KCI (pH 6.0) and 0.5 M KCI (pH 3.7). Solid lines are for ozone reduction current and dashed lines are residual current
observed up to an applied voltage of +1.05 V (vs. SCE) in sulfuric acid. In the potassium chloride media, reduction currents were observed a t potentials at or below +0.65 V (vs. SCE) only. A similar result was observed in perchloric acid. In addition, the increased stability of the O3molecule in the acidic media resulted in a greater reduction current response and a more electrochemically reversible process. The residual current was also much less significant in the sulfate media even a t the more anodic voltages. A low and reproducible residual current is essential for maximizing the current detection sensitivity of the system. The effect of the pH of the supporting electrolyte on the reduction current was examined more closely using sulfate and chloride media. In Figure 3, an oxidation current is again observed in the more acidic chloride media. Figure 4 shows improved electrochemical reversibility and an increase in the magnitude of the reduction current of ozone in sulfate media a t lower p H values. Based on these results, potassium sulfate (0.5 M, p H 3.7) was selected as the supporting electrolyte. The effect of chloride on the reduction current response also required the
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ANALYTICAL CHEMISTRY, VOL. 51,
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NO. 13, NOVEMBER
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1
0
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0
4
8
12
16 20 24 28 32 36
TiME ( s e c ) - 40
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APPLIED VOLTAGE ( V o l t s vs SCE]
Figure 4. Current-voltage relationship at the rotating gold electrode in 0.05 M H2S04(pH 1.3), 0.5 M K2S0, (pH 3.7), and 0.5 M K,S04 (pH 6.0). Solid lines are for ozone reduction current and dashed lines are residual current
-10
Figure 6. Response time to step change from 0 to 1 mg/L ozone at stirring rate of 1000 rpm 10
c
l
o /
( m g / l 03) II
I O 0 9 0 8 0 7 0 6 0 5 0 4 0 3 0 2 01
APPLIED VOLTAGE ( V o l t s v s
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Flgure 5. Current-voltage relationship at b e uncovered Delta gold button cathode in 0.5 M K,SO, (pH 3.7). Soli line is for ozone reduction current and dashed line is residual current
use of a double junction reference electrode. For amperometric or steady-state measurements with the new modified Delta electrode system, an applied voltage of +0.6 V (vs. SCE) was selected based on the voltammogram developed with this two-electrode system in Figure 5 . The current response observed with this stationary electrode was less ideal than the voltammetric relationship obtained with the rotating gold electrode. But the current response is a more realistic appraisal of the steady-state membrane electrode performance. The selection of an applied voltage of +0.6 V (vs. SCE) represents a compromise between additional selectivity (by the application of more anodic voltages) and a concomitant loss of current range and sensitivity. Application of the membrane over the cathode to complete the design further reduces the current sensitivity of the system but defines the steady-state current and greatly improves selectivity. Current Sensitivity. A typical current response of the membrane electrode system at f0.6 V (vs. SCE) is 0.484 with a standard deviation of f0.007 pA (mg/L)-' (cm)-2and an r value of 0.995 at 22 "C in an ozone test solution a t pH 4.9. Residual currents at this voltage range from -2 to -15 nA. A detection limit of 62 pg/L is predicted a t twice the residual
Figure 7. Current sensitivities at 13 OC (O),22 at +0.6 V (vs. SCE)
OC
(O),and 32 OC (0)
current of 15 nA. The high correlation and low residual current shows the current response is linear and reproducible. The current response is only slightly affected by stirring. A 98% current response is observed at a stirring rate of 600 rpm with a 2.5-cm stirring bar located 1.4 to 1.6 cm below the electrode tip. Since the effect is highly dependent on membrane structure and thickness, the use of this Teflon membrane is advantageous since only minimal stirring is required to achieve the maximum current response. Response Time. The response time of the membrane electrode to a step change in the dissolved ozone concentration is illustrated in Figure 6. A 95% response was observed in 18 s when the electrode was transferred from air saturated water to a sample containing approximately 1mg/L of ozone. The response time is dependent on the membrane permeability for ozone and the thickness of the electrolyte film. The initial lag in the response has been observed before (13) but no explanation is given for this experimental peculiarity. Temperature Effects. The current response of the system is highly dependent on temperature as illustrated in Figure 7. Current sensitivities (4) range from 0.316 to 0.992 pA (mg/L)-' (cm)-2at 13 and 32 "C with an average variation at normal temperatures of 5.8% per "C. Since the current response of a given membrane at steady-state conditions is solely dependent on the rate of transport from the test solution
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
3.0
-13.5
2.5
-14.0
-14.5
E
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1.5
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wastewaters were excluded from the study based on their observed lack of polarographic behavior (8, 15). Trichloramine, hypobromous acid, bromine, hypochlorous acid, hydrogen peroxide, and chlorine dioxide exhibited less than a 2% interference in the measurement of ozone. With the membrane removed, the response to all the compounds except hydrogen peroxide increased. Presumably, the membrane selectivity prevents the transfer of polar species such as hypochlorous acid in favor of gaseous molecules like chlorine dioxide. However, only those compounds which are powerful oxidants are capable of being reduced at the cathode surface even though diffusion through the membrane is possible.
ACKNOWLEDGMENT
-15.0
The authors gratefully acknowledge the assistance of R. Smart in obtaining reference material. I .o
3.2
3.3
3.4
3.5
(TEMP ( O K ) 1-1 x 10-3 In Pm (a)vs. Flgure 8. In $ (0)vs. l / T ( K ) X
LITERATURE CITED
-15.5
l/T(K) X
low3
at 13, 22, and 32 OC
Table I. Current Response to Interferences
7% interference in measurechemical PA ment of compound pA (mg/L)-' (mequiv/L)-' ozonea current response
% Br HO C1 NCl, H A c10,
5 3 3 2 0 5
x x x x
10-4 10-4 10-5 10-5
0.080 0.048
1.8
0.001 2 x 10-4 0
0.03
1.1
0.01
-
x 10-3 0.068 1.5 a Based o n current response to ozone at 22 "C = 0.184 uA (mg/L)-' = 4.416 uA (meauiv/L)-' to the cathode, the high temperature coefficient is mainly attributable to the permeability characteristics of the membrane. If the permeability coefficient is directly proportional to the current sensitivity, $ should also exhibit a similar temperature dependence. In Figure 8, the temperature dependence of the experimental current sensitivities and membrane permeabilities to ozone is demonstrated to be in adherence with the classical laws governing an activated diffusion process as first suggested by Mancy (14). Interferences. The current response of the membrane electrode to several oxidants considered to be possible interferences was examined as shown in Table I. Other oxidizing and reducing agents commonly found in water and
(1) I.Fatt, "Polarographic Oxygen Sensor", CRC Press, Cleveland, Ohio, 1976. (2) J. D. Johnson, J. W. Edwards, and F. K. Keeslar, .I. Am. Water Works Assoc., 70, 341 (1978). (3) J. F. Dunn and J. D. Johnson, "Ozone Ampermetric Membrane Electrode" in "Second International Symposium on Ozone Technology", Ozone Press International, Jamesville, N.Y., 1976, pp 132-141. (4) R. SMlt and K. H. Mancy, mpublished repot?, Deparbnent of Envionmental and Industrial Health, University of Michigan, Ann Arbor, Mich., 1977. (5) J. H. Stanley and J. D. Johnm, "A Mdecuhr Ozone Membrane Electrode", Paper presented at the 175th National Meeting of the American Chemical Society, Anaheim, Calif., March 1978. (6) American Public Health Associah, "Standard MeWcds for the Examination of Water and Wastewater", 13th ed., Washington, D.C., 1971, pp 107-143, 27 1-275. (7) 0. L, Flamm, Environ. Sci. Technof., 11, 978 (1977). ( 8 ) F. K. Keeslar, "A Selective Amperometrlc Membrane Electrode for Measurement of Chlorine and Bromine Residuals in Water", Technical Report, Submitted to Dept. of EnvironmentalSciences and Engineering, Unlversity of North Carolina, 1975, ESE publication no. 605. (9) J. R. Knechtel, E. G. Janzen, and E. R. Davis, Anal. Chem., 50, 202 (1978). (10) N. Gav and G. M. Schmid, J. Elecrroanal. Chem. Interfacis/ Ektrochem., 24, 279 (1970). (11) D. C. Johnson, D. T. Napp, and S.Bruckenstein, Anal. Chem., 40, 482 (1968). (12) A. A. Rakov and V. I. Veselovskii, RussianJ . Phys. Chem., 35, 1134 (1961). (13) A. Berkenbosch, Acta Physiol. Pharmacol. Neerl., 14, 300 (1967). (14) K. H. Mancy, D. A. Okun, and C. N. Reilley, J . Nectroanal. Chem., 4, 65 (1962). (15) D. T. Sawyer, R. S.George,and R. C. Rhodes, Anal. chem., 31, 2 (1959).
RECEIVED for review September 11, 1978. Accepted August 20, 1979. This work was supported by a grant to J.D.J. from the Environmental Protection Agency (Program IBC-611). This paper was presented a t the 175th National Meeting of the American Chemical Society, Division of Environmental Chemistry, Anaheim, Calif., March 12, 1978, from a thesis submitted to the Graduate School, University of North Carolina, Chapel Hill, N.C., in partial fulfillment of requirements for a Master of Science in Environmental Sciences. ESE Publication No. 534.
