Anal. Chem. 1996, 68, 2890-2896
A Renewable-Reagent Fiber-Optic Sensor for Measurement of High Acidities Kevin J. Kuhn* and James T. Dyke
Los Alamos National Laboratory, CST-15, Los Alamos, New Mexico 87545
A renewable-reagent fiber-optic HNO3 sensor was developed for HNO3 measurement in the 0.1-10.0 M range. The HNO3 sensor employs a tubular Nafion cationexchange membrane to extract acid species from an external HNO3 sample into an internal flowing reagent solution. In high-concentration HNO3 samples, incomplete HNO3 dissociation results in a significant concentration of neutral HNO3 species in addition to protons. As both neutrals and protons are potentially membranepermeable species, various reagent compositions were tested to examine the contributions of both acid transfer mechanisms. Continuous reagent flow limited internal acid accumulation and transferred reagent to the sensor optical detection cell. All reagent compositions included cresol red as a colorimetric indicator, which was measured within the sensor detection cell. Careful fiber-optic alignment provided sufficient light throughput in a backscatter illumination mode to allow use of a photodiode array detector for visible spectral acquisition. The use of Ca2+ as a reagent countercation produced notable reductions in HNO3 sensor response to interferent cations and temperature changes. Sensor measurement of HNO3 samples in the tested concentration range produced average relative standard deviations of less than 0.4%. Control over reagent flow rate should allow for extension of the HNO3 sensor measurement range to 16.0 M HNO3. Various chemical processes involving actinides1 require maintaining HNO3 concentration at a constant level in the range 2.08.0 M. In addition, efforts to recover the HNO3 component from process waste must produce HNO3 concentrations greater than 5.0 M to be reusable in actinide processes. Prior to reuse, recovered HNO3 concentration must be measured to specify dilution factors necessary for optimized actinide processing. Thus, the deployment of an on-line analyzer to monitor HNO3 concentration in these processes is desirable for obtaining efficient actinide processing, reducing acid consumption, and minimizing operating costs. A number of spectroscopic approaches to monitoring HNO3 concentrations up to about 10.0 M have been described.2-5 The most successful on-line high acid monitoring5 approach to date involves immobilization of an acid-sensitive matrix on the surface (1) Cleveland, J. M. The Chemistry of Plutonium; American Nuclear Society: La Grange Park, IL, 1979. (2) Carey, W. P.; DeGrandpre, M. D.; Jorgensen, B. S. Anal. Chem. 1989, 61 (15), 1674-1678. (3) Carey, W. P.; Jorgensen, B. S. Appl. Spectrosc. 1991, 45 (5), 834-838. (4) Garrison, A. A.; Martin, M. Z. AT-Process, J. Process Anal. Chem. 1995, 1 (2), 95-98. (5) Martin, M. Z.; Garrison, A. A. AT-Process, J. Process Anal. Chem. 1995, 1 (2), 127-131.
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of an optical element of a transmission cell. Contact of the thin immobilized layer with the sample matrix produces a colorimetric response. The primary problem with this approach has been isolating the thin-layer absorbance from the high-absorbance background inherent in the acid matrix. In general, methods involving immobilized chemistries are subject to problems with inadequate path lengths, path length instability due to matrix swelling, reagent photolability, and reagent leaching. The renewable-reagent fiber-optic sensor concept has been described as a valid sensor development approach for a number of analytes.6-12 The renewable-reagent sensor is configured to allow control over parameters that influence the sensitivity, selectivity, and dynamic range of the sensor measurement. This report describes the development of a renewable-reagent fiberoptic sensor for measuring HNO3 in the 0.1-10.0 M concentration range. The renewable-reagent sensor approach employs a membrane sampling element for matrix normalization prior to colorimetric detection. THEORY OF OPERATION The renewable-reagent fiber-optic HNO3 sensor employs a semiselective Nafion membrane to sample permeable acid species from HNO3 solutions. Membrane-permeable species are transferred to an internal reagent solution in contact with the sensor side of the membrane. A colorimetric acid-base indicator is included in the reagent to monitor the accumulation of acid. Continuous reagent flow transfers the colored reagent from the membrane sampling volume into an optical detection cell where color development is measured using fiber optics for illumination and light collection. The renewable-reagent HNO3 sensor response is determined by a number of factors including sample HNO3 speciation, membrane permeability, reagent composition, and reagent flow rate. Successful deployment of the renewable-reagent sensor rests with retaining control over reagent conditions on the sensor side of the membrane to influence the sampling and detection of HNO3 species. To more fully understand renewable-reagent HNO3 sensor operation, the dependence of sensor response on each of these factors is examined below. (6) Luo, S.; Walt, D. R. Anal. Chem. 1989, 61 (2), 174-177. (7) Lieberman, S. H.; Inman, S. M.; Stromvall, E. J. Anal. Chim. Acta 1989, 217, 249-262. (8) Berman, R. J.; Christian, G. D.; Burgess, L. W. Anal. Chem. 1990, 62 (19), 2066-2071. (9) Berman, R. J.; Burgess, L. W. Proceeding of SPIE, Chemical, Biochemical and Environmental Fiber Sensors II 1990, 1368, 25-35. (10) Varineau, P. T.; Duesing, R.; Wangen, L. E. Appl. Spectrosc. 1991, 45 (10), 1652-1655. (11) DeGrandpre, M. D. Anal. Chem. 1993, 65 (4), 331-337. (12) Lin, Z.; Burgess, L. W. Anal. Chem. 1994, 66 (15), 2544-2551. S0003-2700(96)00242-9 CCC: $12.00
© 1996 American Chemical Society
HNO3 Speciation. The composition of concentrated HNO3water mixtures has been described by Che´din.13 Within the 0.110.0 M concentration range, the equilibrium dissociation of HNO3 varies considerably according to the reaction
HNO3‚nH2O ) H+ + NO3- + nH2O where the undissociated acid can exist as either a free acid molecule (n ) 0) or as a hydrate (n ) 1, 3). As acid speciation is a relevant factor in the HNO3 sensor response, we need to describe the various acid species using consistent terminology. The three undissociated acid species will be collectively referred to as neutral HNO3 species as their lack of charge determines the mechanism of permeation through an exchange membrane. The dissociated acid will be referred to as a proton. When the discussion concerns the overall acid content of a solution without regard to speciation, the term HNO3 or HNO3 sample will be used. HNO3 sample speciation is important to sensor function as both neutrals and protons are potentially membrane-permeable species. Concentrations of each distinct HNO3 species determine the magnitude of concentration gradients that define the driving force for acid permeation. The total concentration of neutral HNO3 species varies from essentially 0 to 6.0 M over the 0.1-10.0 M HNO3 sample concentration range. The proton content of HNO3 samples increases to a maximum of 4.3 M (at 7.7 M HNO3) and then decreases in more concentrated HNO3 samples. Membrane Permeability. The structure of Nafion consists of a network of hydrophilic sulfonate cation-exchange sites which reside in a hydrophobic polymer matrix. Nafion membranes exhibit considerable porosity and tend to swell in aqueous solutions as water penetrates into the pores. Neutral acid species permeate the Nafion membrane via diffusion through the network of membrane pores. Thus, neutral acid permeation through the membrane is governed by characteristic diffusion coefficients and concentration gradients across the membrane. Protons permeate Nafion by hopping from site to site through the network of cation-exchange sites. Proton permeation through the membrane depends on effective proton permeability and on the proton concentration gradient across the membrane. The cation-exchange mechanism of proton transport requires that a countercation permeate through the membrane in the opposite direction to maintain charge neutrality. The effective proton permeability defines the proton permeation rate through Nafion for a given countercation. Charge interaction with Nafion exchange sites reduce the countercation permeation rate such that high-affinity cations permeate more slowly. Reagent Composition. Operation of the HNO3 sensor with a neutral internal reagent allows only neutral HNO3 species to permeate through the membrane and accumulate within the reagent. Addition of countercations to the reagent activates cation exchange as a mechanism for proton transport while the diffusional mechanism of neutral acid transport remains active. To examine both mechanisms of HNO3 transport, the sensor was tested using neutral- and countercation-containing reagents. Reagent Flow Rate. Concentration gradients and HNO3 species permeation rates through the membrane determine the rate of acid transfer into the reagent. However, the actual concentration of HNO3 accumulated in the reagent is variable due to control of the reagent flow rate. In practice, flow rates were (13) Che´din, J. J. Chim. Phys. 1952, 49, 109-125.
Figure 1. Renewable-reagent fiber-optic HNO3 sensor design: (a) source fiber optic, (b) collection fiber optic, (c) reagent delivery capillary, (d) reagent drain capillary, (e) Teflon tubing, and (g) tubular Nafion membrane.
selected that produced internal sensor HNO3 accumulations within the transition range of the cresol red indicator. EXPERIMENTAL SECTION Reagent Solutions. A 2.51 × 10-4 M cresol red14 stock solution was prepared by dissolution of the solid powder in deionized water. Stock dilution with deionized water produced a neutral sensor reagent solution which contained 5.0 × 10-5 M indicator. Stock dilution and nitrate salt addition to deionized water produced cation reagent solutions which contained 5.0 × 10-5 M indicator and 50.0 mM Na+ or Ca2+. Nitric acid sample solutions spanning the concentration range from 0.1 to 10.0 M were prepared by addition of concentrated HNO3 to appropriate volumes of deionized water. Interferent samples were prepared by adding incremental concentrations of NaNO3, Ca(NO3)2, or Al(NO3)3 to 1.0 M HNO3 samples. Sensor Construction. The renewable-reagent fiber-optic HNO3 sensor design is depicted in Figure 1. A key feature of this design is the separation of the sample accumulation region from the optical detection cell which isolates the optical measurement from the effects of membrane dynamics (i.e., swelling). HNO3 was sampled into and accumulated within a 0.50 µL reagent volume defined by a length of 330 µm i.d. tubular Nafion cationexchange membrane (TT-020, Perma Pure Inc., Toms River, NJ). Reagent renewal within the HNO3 accumulation region occurred via a 100 µm i.d. reagent delivery capillary (Polymicro Technologies Inc., Phoenix, AZ) inserted into the tubular membrane and terminated just above the sealed distal end. Reagent flow continuously transferred reagent from the accumulation region into the optical detection cell. The sensor detection cell was build within the 3.1 µL internal volume of a length of 1/32 in. i.d., 1/16 in. o.d. Teflon tubing. A 300 µm core source fiber optic (HCS, Ensign-Bickford Optics Co., Avon, CT) extended nearly the length of the detection cell. The source fiber end face was buried in a (14) Jorgensen, B. S.; Nekimken, H.; Sellon, D. Los Alamos National Laboratory internal document, Report on Indicators for Optical High Acidity Sensor, 1995.
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plug of HNO3-resistant optical epoxy (EP21HTAR, Master Bond, Hackensack, NJ) which served to isolate the source fiber from reagent compositional changes and enhance backscatter illumination of the detection volume. The flow of reagent from the acid accumulation region into the optical detection cell was routed around the epoxy plug which was centered in the internal diameter of the Teflon tubing. A 400 µm core collection fiber optic was aligned on-axis and offset 0.7 cm behind the source fiber end face. Reagent waste was removed from the detection cell via 250 µm i.d. capillary tubing. Instrumentation. The HNO3 sensor reagent delivery capillary was connected to a remote syringe pump (Amica 5000, Hamilton Co., Reno NV) which delivered reagent at an average volumetric flow rate of about 22 µL/min. The source fiber optic was illuminated with the focused output of a 100 W tungstenhalogen light source (Model 77501, Oriel Corp., Stratford, CT). Source light captured by the sensor collection fiber was delivered to an f/2.2 spectrograph (100S, American Holographic, Littleton, MA) and dispersed onto a 1024 element photodiode array detector (Instaspec III, Oriel Corp.). Integration times between 3.0 and 3.3 s were typical for the thermoelectrically cooled array detector operated at +5.0 °C. Pump flow rate and photodiode array data collection were controlled via computer, and sensor data analysis was performed in MATLAB (The MathWorks, Inc., Natick, MA). Experimental Procedure. Renewable-reagent HNO3 sensor response dynamics were characterized by monitoring sensor 518 nm absorbance every 5.0 s (0.2 Hz) before, during, and after an exposure to 6.0 M HNO3. Visible sensor absorbance spectra were calculated from a reference taken with the sensor exposed to deionized H2O and sample scans taken after 9 min of exposure to a given HNO3 sample. Temperature effects on membrane transport were reduced by maintaining reference and sample solution temperatures at 25.0 °C. Experiments conducted to examine the sensor response to 1.0 M HNO3 with incremental additions of interfering cations were not temperature controlled. Data Analysis. Steady-state sensor differential absorbance spectra [∆A ) -log(I/IR)] were generated by comparison of a reference scan (IR) with intensity scans recorded in HNO3 samples (I). The average absorbance at 600 nm was used as a correction for changes in intensity not specific to the sensor response. Discrete absorbance values were extracted from a 3 nm bandpass average centered around the 518 absorbance maximum. Combining absorbance information at 518 nm and 432 nm as a method for throughput correction and signal-to-noise enhancement was explored; however, the additional noise superimposed on absorbances below about 440 nm actually reduced the signal-to-noise value of the calculated difference. Cresol Red Indicator. The differential absorbance behavior of cresol red as an indicator for HNO3 in the 0.5 mM to 0.2 M range was characterized using a spectrophotometer. The neutral sensor reagent with the indicator in its base form was used as the measurement reference (IR). Conversion to the indicator acid form produced spectra exhibiting a decrease in 432 nm absorbance and an increase in 518 nm absorbance with increasing HNO3 content. An isobestic point was observed at 474 nm. A theoretical line generated from a consideration of the indicator dissociation constant, pKa ) 1.3,15 and a measured 518 nm extinction coefficient provided a good fit to calibration points. (15) Dean, J. A. Lange’s Handbook of Chemistry, 14th ed.; McGraw-Hill: New York, 1994; p 8.115.
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Figure 2. Renewable-reagent HNO3 sensor response dynamics for exposure to a 6.0 M HNO3 sample at 25.0 °C: (a) neutral, (b) sodium, and (c) calcium indicator reagent. Table 1. Renewable-Reagent HNO3 Sensor Path Length Estimates from a Comparison of 0.1 cm Path Length Cuvette Absorbances and Sensor Absorbances at 432 nm
reagent composition 5.0 × 10-5 M cresol red 5.0 × 10-5 M cresol red, 50.0 mM NaNO3 5.0 × 10-5 M cresol red, 50.0 mM Ca(NO3)2
0.1 cm cuvette (AU)
HNO3 sensor (AU)
HNO3 sensor path length (cm)
0.1149 0.1167
0.5672 0.5538
0.4936 0.4746
0.1159
0.5254
0.4533
HNO3 Sensor Path Lengths. Absorbance path lengths for the HNO3 sensor were estimated by a comparison of cresol red reagent absorbance measured in the spectrophotometer with the sensor optic cell absorbance using deionized water as a reference. As the salt content of the various reagent solutions differed, sensor absorbances were measured for each reagent to account for refractive index induced changes in absorbance path lengths. Differences in the refractive indexes of the various reagents impose different boundary conditions on the exposed signal fiber optic. Boundary condition variations define differences in the range of illumination angles collected by the signal fiber. As each illumination angle has a distinct path length through the optical detection cell, changes in the range of angles collected by the signal fiber should reflect the effective path length difference. Estimates of sensor path length based on relative absorbance measurements for the three reagent compositions of interest are given in Table 1. Measurements indicated that the sensor absorbance path length decreased as the refractive index of the reagent solution increased. The path length decrease is expected as the increase in refractive index reduces the angular collection range (numerical aperture) of the signal fiber.16 RESULTS AND DISCUSSION HNO3 Sensor Response Dynamics. Guidance in selecting suitable sensor HNO3 exposure and recovery times was obtained by characterization of sensor response dynamics (Figure 2). In general, sensor response profiles exhibited a gradual increase in absorbance over the entire acid exposure time. The continuous increase in sensor absorbance was due to the non-steady-state (16) Palais, J. C. Fiber Optic Communications; Prentice-Hall, Inc.: Englewood Cliffs, NJ, 1984.
Figure 3. Renewable-reagent HNO3 sensor visible absorbance spectra for HNO3 samples in the 0.1-10.0 M concentration range for the neutral internal reagent case. Differential spectra were acquired using 5.0 × 10-5 M cresol red as a reference: (a) 0.0, (d) 0.5, (e) 1.0, (f) 2.0, (g) 3.0, (h) 5.0, and (k) 10.0 M HNO3.
Figure 4. Renewable-reagent HNO3 sensor visible absorbance spectra for HNO3 samples in the 0.1-10.0 M concentration range for the calcium internal reagent case: (a) 0.0, (b) 0.1, (e) 1.0, (f) 2.0, (g) 3.0, and (l) 10.0 M HNO3.
diffusional transfer of solvated HNO3 (i.e., protons) out of the stagnant reagent layer at the membrane interface and into the flowing reagent. In practice, absorbances obtained after 7 min in 6.0 M HNO3 were considered to be at steady state as the absorbance changed only 1.0% over the next 5 min of acid exposure. With this in mind, it was convenient to defined the steady-state sensor absorbance as an average of absorbances collected during the last minute of acid exposure (11th-12th min). Response times were reported as the time required in acid for the sensor to register 90% of the steady-state absorbance value. Baseline recovery times were identified with the time necessary for the sensor absorbance to reach 0.5% of the steady-state absorbance value. Sensor dynamics were expected to vary depending on the composition of the internal reagent phase. Thus, 6.0 M HNO3 sample response profiles were collected for operation of the sensor with various reagent solutions. As the sensor absorbance path length changed with reagent composition, the absorbance curves in Figure 3 were path length normalized prior to presentation. The fastest sensor response and baseline return times were obtained for sensor operation with a neutral reagent solution. Under neutral reagent operation, the sensor response time was 35 s and the baseline return required less than 8.5 min. In the neutral reagent sensor, internal accumulation of acid occurred via permeation of neutral acid species through the Nafion membrane. As neutral acid species did not interact with membrane charge sites, the rate of HNO3 transport across the membrane was determined by diffusion. Addition of Na+ or Ca2+ to the indicator reagent produced an increase in sensor response and recovery times. The Na+ reagent response time of 65 s and recovery time of 24 min were the longest observed. Intermediate values of 55 s and 16 min for sensor response and recovery times, respectively, were obtained for use of the Ca2+ reagent. The longer cation reagent response and recovery times accompany the introduction of proton exchange as another mechanism for acid transport across the membrane. The charge interaction between membrane exchange sites and protons produce slower rates of permeation relative to neutrals. Slower proton permeation rates through the Nafion membrane provide a plausible explanation for the longer sensor response and return times. The use of Ca2+ as an exchangeable cation in the reagent produced slightly faster response times and significantly faster
recovery times than did Na+. Differences in sensor cationexchange response times tended to be minimized as effective Ca2+ and Na+ permeation rates were enhanced by the comparatively fast proton permeation rate in order to maintain charge neutrality within the membrane. At cation concentrations of 50 mM, the faster sensor recovery times obtained using Ca2+ as a countercation appeared to emphasize the importance of cation charge in displacing protons from the membrane. Although Na+ is more mobile in Nafion, the displacement of one proton per cation compared to two for Ca2+ could account for the observed difference in recovery times. Sensor HNO3 Absorbance Spectra. Visible absorbance spectra were collected to characterize the renewable-reagent HNO3 sensor response to calibration samples in the range from 0.1 to 10.0 M HNO3. Fiber-optic coupling provided sufficient light throughput to allow sensor absorbance measurements down to about 410 nm. The full set of calibration samples were tested for sensor operation under the three distinct reagent conditions. In general, sensor spectra exhibited characteristic cresol red absorbance features as observed in spectrophotometer spectra. Acid accumulation within the sensor produced a decrease in 432 nm absorbance, an increase in 518 nm absorbance, and an isobestic point at 474 nm. Sensor absorbance spectra for measurement of HNO3 samples in the neutral reagent case are given in Figure 3. Absorbance spectra describing the sensor response to HNO3 samples in the 2.0-10.0 M range reproduced the cresol red absorbance features described above. The 0.1-1.0 M HNO3 samples did not exhibit 432 nm absorbance minima or share an isobestic point with the higher acid spectra. However, the sensor 518 nm absorbance response to these lower concentration HNO3 samples was found to be very reproducible. An offset in the absorbance baseline due to reagent refractive index variations was apparent at nonabsorbing wavelengths longer than 575 nm. The increase in refractive index resulted from the accumulation of neutral acid species within the internal reagent phase. Reagent compositions including either 50 mM Na+ or Ca2+ produced a similar series of sensor absorbance spectra for exposure to HNO3 samples. Figure 4 presents absorbance spectra describing the sensor response to 0.1-10.0 M HNO3 samples for the Ca2+ reagent case. Characteristic absorbance features of the cresol red indicator are clearly reproduced in the sensor response spectra. The additional mechanism of cation exchange produced Analytical Chemistry, Vol. 68, No. 17, September 1, 1996
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Figure 5. Renewable-reagent HNO3 sensor 518 nm absorbance responses for HNO3 samples in the 0.1-10.0 M concentration range: (+) neutral, (×) sodium, and (O) calcium reagents. Table 2. Renewable-Reagent HNO3 Sensor 518 nm Absorbance Measurement Reproducibility for a 6.0 M HNO3 Sample Concentration
reagent 5 × 10-5 M cresol red 5 × 10-5 M cresol red, 50 mM NaNO3 5 × 10-5 M cresol red, 50 mM Ca(NO3)2
sample av abs [HNO3] (518 nm) (M) (AU) 6.0 6.0 6.0
0.7201 0.7312 0.7381
std (AU) 0.0017 0.0019 0.0025
larger sensor absorbance magnitudes for each tested HNO3 sample relative to the neutral reagent response. Again a baseline absorbance offset was observed indicating a change in reagent refractive index. However the replacement of Ca2+ with H+ produced an apparent reduction in reagent refractive index which tended to offset the refractive index increase associated with neutral acid permeation. Sensor HNO3 Response Curves. Characteristic 518 nm sensor absorbances for each tested HNO3 sample are given in Figure 5 for the three distinct reagent conditions tested. Prior to plotting, 518 nm absorbances were multiplied by appropriate path length ratios to produce normalized absorbance values for each reagent condition. The neutral reagent response curve exhibited smaller incremental increases in absorbance at lower HNO3 sample concentrations (e2 M) than did cationic reagent curves. At higher HNO3 sample concentrations, sensor measurement sensitivity decreased significantly for all three reagent conditions. The decrease in sensor sensitivity at high HNO3 sample concentrations would seem to reflect operation of the sensor near a limit in the usable acid indicator range. Evidence supporting an alternative explanation for the reduction in sensor sensitivity will be provided via a consideration of internal acid accumulation in the next section. Despite the reduction in sensitivity at higher HNO3 concentrations, sufficient sensitivity was obtained to identify HNO3 sample concentration over the full 0.1-10.0 M range in all three reagent cases. An estimate of HNO3 sensor response reproducibility was calculated as the standard deviation in absorbances collected from six consecutive measurements of 6.0 M HNO3. Again, the reproducibility was tested for each individual reagent composition of interest. The resulting values, given in Table 2, correspond to relative standard deviations of 0.24%, 0.26%, and 0.34% for sensor operation employing a neutral, Na+, and Ca2+ reagent, respectively. 2894 Analytical Chemistry, Vol. 68, No. 17, September 1, 1996
Figure 6. Internal acid accumulation within the renewable-reagent HNO3 sensor with neutral HNO3 species concentration in the sample: (+) neutral, (×) sodium, and (O) calcium reagents.
Internal Acid Accumulation. To provide insight into the relationship between neutral HNO3 species concentration in the sample and the internal accumulation of acid, the data presented in Figure 5 were transformed. Sensor absorbances (Figure 5, y-axis) were converted to internal acid concentrations using characteristic quantities obtained from the spectrophotometer calibration. The compilations of Che´din12 provide the necessary information to convert HNO3 sample concentrations (Figure 5, x-axis) to neutral acid species concentrations. The resulting neutral reagent acid accumulation curve (Figure 6) indicates that the sensor was unable to maintain initially high neutral acid permeation levels across the tested HNO3 sample concentration range. Rather than exhibiting internal acid accumulations that continued to increase proportionally (Fick’s law) with neutral acid sample concentration, the sensor ability to extract neutral acid species appeared to approach a limit. The small increases in neutral acid permeation in concentrated HNO3 samples are consistent with the earlier observation of a reduction in sensor absorbance sensitivity. Nafion shrinkage in concentrated HNO3 environments with an accompanying reduction in membrane porosity could restrict neutral acid permeation. The additive effect of cation exchange to neutral acid permeation can be observed by plotting Na+ and Ca2+ reagent acid accumulations against the neutral acid concentration (Figure 6) of HNO3 samples. Including the cation-exchange mechanism of acid transport does not alter the accumulation curve shape. However, an increase in internal acid accumulation was evident as Na+ and Ca2+ reagent curves were offset above the neutral reagent curve. The offsets indicated that different cationic reagents produced specific increments of acid accumulation which added to a variable amount of accumulated neutral acid. When a Na+ internal reagent was used, cation exchange gave an increase of 0.014 M in accumulated acid for each tested HNO3 sample. Cation exchange with Ca2+ produced an acid increment of 0.023 M compared to that of neutral acid permeation alone. The invariant cation-exchange increment resulted from the chosen combination of low reagent cation concentration (50 mM) and fast reagent flow rate (22 µL/min.). This combination effectively limits the steady-state countercation content in the membrane, which controls the concentration of exchangeable cation. The charge difference between Ca2+ and Na+ countercations provides Ca2+ the ability to exchange for twice as many protons for equivalent membrane phase concentrations. The actual difference
Table 3. Measured Percent Change in Renewable-Reagent HNO3 Sensor 518 nm Absorbance for 1.0 M HNO3 Samples with Varying Concentrations of Interfering Cations reagent [cation] (M)
sample [HNO3] (M)
interferent, [NaNO3] (M)
% ∆abs (518 nm)
interferent, [Ca(NO3)2] (M)
% ∆abs (518 nm)
interferent, [Al(NO3)3] (M)
% ∆abs (518 nm)
none none none none none 0.05 Na+ 0.05 Na+ 0.05 Na+ 0.05 Na+ 0.05 Na+ 0.05 Ca2+ 0.05 Ca2+ 0.05 Ca2+ 0.05 Ca2+ 0.05 Ca2+
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
0.250 0.500 0.750
17.56 41.16 55.39
0.250 0.500 0.751
0.22 4.52 8.43
27.12 40.66 59.74 80.82 98.01 1.27 4.86 9.33 14.74 22.85
1.89 1.98 5.94
16.72 40.67 56.27 74.11 99.59 4.10 5.81 10.23 14.34 19.78 2.44 5.72 9.60 9.95 17.63
0.083 0.167 0.251 0.375 0.500 0.083 0.167 0.251 0.375 0.500
0.250 0.500 0.751
0.125 0.250 0.375 0.500 0.750 0.125 0.250 0.375 0.500 0.750 0.125 0.250 0.375 0.500 0.750
Table 4. Measured Percent Change in Internal Reagent Acid Accumulation for the Renewable-Reagent HNO3 Sensor Operating in HNO3 Samples Maintained at 25.0, 30.0, or 35.0 °C sample [HNO3] (M)
∆ temp (°C)
reagent
% ∆abs (518 nm)
% ∆[HNO3]
reagent
% ∆abs (518 nm)
% ∆[HNO3]
2.0 2.0 5.0 5.0 8.0 8.0
25 to 30 25 to 35 25 to 30 25 to 35 25 to 30 25 to 35
neutral neutral neutral neutral neutral neutral
9.01 15.49 4.32 6.19 2.71 5.18
12.7 21.7 8.2 11.9 6.0 11.8
calcium calcium calcium calcium calcium calcium
2.32 4.51 1.73 2.94 1.39 1.45
4.3 8.3 3.9 6.8 3.7 3.8
Figure 7. Renewable-reagent HNO3 sensor percent absorbance change for added amounts of Na+ and Ca2+ charge to 1.0 M HNO3 samples: (+) neutral, (×) sodium, and (O) calcium reagents.
was less, probably due to the slower effective permeation rate of Ca2+ relative to Na+. Sensor Cation Interferent Responses. Interference experiments were performed to identify reagent phase compositions which would minimize the sensor sensitivity to interfering sample cations. As these experiments were conducted without temperature control, interference curve slopes from various tested reagent and cationic interferent combinations were compared. Results for Al3+, Ca2+, and Na+ as interfering cations in 1.0 M HNO3 samples are give in Table 3. The tabulated percent changes in 518 nm absorbance produced by Ca2+ and Na+ interferents are presented in Figure 7 versus cation sample normality (N ) n[Mn+]). The measurement of interferent samples revealed that sample cation identity was of little importance in describing sensor interferent sensitivity. Rather, interference responses tended to
fall along lines of similar slope identified by the reagent solution composition. To confirm that the interference responses were scaling proportionally to cation normality rather than nitrate concentration, CaCl2 and Ca(NO3)2 interference responses were compared. Despite differences in individual response magnitudes, the sensitivities were similar. The sensor sensitivity to cationic interferences was significantly reduced by the addition of an exchangeable cation to the reagent. The neutral reagent response described a change of about 63% in sensor absorbance per normal unit of interferent cation. The HNO3 sensor interferent sensitivity was reduced to about 16% and 12% when Na+ and Ca2+, respectively, were included in the reagent. The resistance to variations in cation transport in Nafion cation-exchange membranes by including relatively high affinity cations in the receiver solution has been described.17,18 Sensor Temperature Responses. The renewable-reagent HNO3 sensor temperature sensitivity was tested by measuring the absorbance of a subset of HNO3 samples at 25.0, 30.0, and 35.0 °C. Results from sensor temperature studies for neutral- and calcium-containing reagents are summarized in Table 4. The observed trend was an increase in sensor absorbance with increasing temperature for each HNO3 sample tested. In general, differences in neutral and Ca2+ reagent absorbance calibration curve shapes preclude a direct comparison of sensor absorbances to describe relative sensor temperature sensitivities. Therefore, a comparison of relative percent changes in sensor acid accumulation calculated from measured absorbances is provided in Table 4. Changes in acid accumulation indicated that temperature changes had a significant effect on the sensor HNO3 response in (17) Cox, J. A.; DiNunzio, J. E. Anal. Chem. 1977, 49 (8), 1272-1275. (18) Cox, J. A.; Twardowski, Z. Anal. Chem. 1980, 52 (9), 1503-1505.
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reagent. Normalized absorbance magnitudes were nearly identical for HNO3 samples in the 0.1-3.0 M concentration range. In the 5.0-10.0 M HNO3 sample range, sensor B exhibited slightly larger absorbances than sensor A.
Figure 8. Comparison of renewable-reagent fiber-optic HNO3 sensor 518 nm absorbance response for HNO3 samples in the 0.110.0 M concentration range: (+) sensor A; (×) sensor B.
both reagent cases. However, the use of the Ca2+ reagent produced internal acid accumulations which were notably less affected by changes in sample temperature. Sensor Response Curve Comparison. The usefulness of the renewable-reagent sensor beyond a controlled laboratory setting hinges on the ability to create multiple devices that respond in a predictable and calibratable fashion. In fabricating HNO3 sensors, a series of well-defined assembly steps were followed to minimize variations in optical and flow geometry. A second HNO3 sensor (B) fabricated via the same procedure but without regard for component dimensions was tested to compare sensor response characteristics. The original sensor (A) and sensor B exhibited significant differences in tubular membrane lengths (0.6-1.0 cm) and absorbance path lengths (0.49-0.63 cm). Sensor response dynamics were quite similar for the two sensors. Response times were obtained for both sensors within 35 s of 6.0 M HNO3 exposure. After HNO3 sample removal, sensor B returned to baseline about 4 min before sensor A. Calibration curves (Figure 8) were generated from visible absorbance spectra to compare sensor responses to the full range of HNO3 samples. Similar indicator transition curves were produced by both sensors operating with a neutral internal
2896 Analytical Chemistry, Vol. 68, No. 17, September 1, 1996
CONCLUSIONS Fiber-optic illumination and collection in the described renewable-reagent HNO3 sensor design provided sufficient light throughput to allow broad-band visible absorbance measurement down to about 410 nm. The gain in signal-to-noise ratio and inherent throughput correction expected from combining absorbance information about the 518 nm maximum and 432 nm minimum was not realized due to greater relative short wavelength noise. Instead, a subtraction correction of the 518 nm absorbance using a nonabsorbing wavelength (600 nm) removed apparent absorbances caused by refractive index induced throughput variations. This approach produced good measurement stability as sensor HNO3 sample absorbances exhibited an average relative standard deviation of less than 0.4%. Sensor operation using cationic internal reagents under the specified conditions produced proton-exchange responses that were insensitive to HNO3 concentration. The differential sensor response to HNO3 samples was the result of neutral acid permeation. The inclusion of exchangeable cations in the reagent solution stabilized Nafion membrane transport properties. The permeability of Nafion to acid species was significantly less affected by interferent cations present in HNO3 samples when Na+ or Ca2+ was included in the reagent compared to the neutral reagent case. A Ca2+ countercation containing reagent also reduced the sensor sensitivity to variations in HNO3 sample temperature. Control over reagent flow rate in the renewablereagent design should allow for extension of the sensing range up to concentrated (16.0 M) HNO3. Received for review March 12, 1996. Accepted May 21, 1996.X AC960242A X
Abstract published in Advance ACS Abstracts, July 15, 1996.
