Direct Detection of Acidity, Alkalinity, and pH with Membrane Electrodes

Letter pubs.acs.org/ac

Direct Detection of Acidity, Alkalinity, and pH with Membrane Electrodes Gastón A. Crespo, Majid Ghahraman Afshar,† and Eric Bakker* Department of Inorganic, Analytical and Applied Chemistry, University of Geneva, Quai Ernest-Ansermet 30, CH-1211 Geneva, Switzerland S Supporting Information *

ABSTRACT: An electrochemical sensing protocol based on supported liquid ionselective membranes for the direct detection of total alkalinity of a sample that contains a weak base such as Tris (pKa = 8.2) is presented here for the first time. Alkalinity is determined by imposing a defined flux of hydrogen ions from the membrane to the sample with an applied current. The transition time at which the base species at the membrane−sample interface depletes owing to diffusion limitation is related to sample alkalinity in this chronopotentiometric detection mode. The same membrane is shown to detect pH (by zero current potentiometry) and acidity and alkalinity (by chronopotentiometry at different current polarity). This principle may become a welcome tool for the in situ determination of these characteristics in complex samples such as natural waters. starts to flatten owing to diffusion into the membrane, a potential change at a transition time indicates the depletion of these ions near the membrane. Under ideal circumstances (when diffusion is the dominant mode of transport), the square root of this transition time is proportional to the labile and therefore titratable, ion concentration. The chronopotentiometry methodology was originally introduced with solid electrodes25,26 and was mainly utilized for fundamental characterizations such as the determination of diffusion coefficients27 and, with ion-selective electrodes, of membrane concentrations.28 The protocol appears to be particularly promising as a readout principle for ion-selective membrane electrodes since these membranes possess adequate selectivity for the extraction of just one type of analyte ion. Such chronopotentiometric detection was originally proposed with membranes that contained no ion-exchanger29−32 and was recently made significantly more useful with permselective membranes for the detection of heparin/protamine33 as well as total and free calcium in blood.34 In contrast to the systems described above, a permselective membrane is here explored in a chronopotentiometric sensing mode to establish a flux of protons from the membrane into the sample. This process protonates any available base in the sample and hence gives information on total alkalinity. Since such permselective membranes also function as pH electrodes in zero current potentiometry, and as acidity detectors in chronopotentiometry when interrogated with reversed current polarity, a multipulse sequence is explored here to see if all three quantities, alkalinity, acidity, and pH, can be determined with a single membrane.

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nowledge of total acidity and alkalinity is important for food quality, disease prevention, and environmental monitoring.1−4 Total acidity is a measure of the total hydrogen ion concentration in the sample that is titratable with strong base, while alkalinity is defined in complete analogy (see references for extended definition of alkalinity).5,6 While pH can be directly determined by potentiometry with ion-selective electrodes,7 the determination of total alkalinity and acidity requires titrimetric approaches.8 These quantities have therefore so far not become directly accessible in the unperturbed sample with chemical sensor technology, which makes them very difficult to assess directly in labile environments where sampling may perturb local equilibria. Traditionally, a measured volume of titrant required to achieve neutralization is used for volumetric analysis. If, on the other hand, the titrant can be delivered by electrochemical control, a direct detection in a chemical sensor format could be envisioned.9,10 Coulometric and amperometric delivery of ions across ion-selective and/or permselective membrane electrodes have indeed been reported.11−14 While such bulk sample titrations promise to be very accurate, they also necessitate the use of thin layer sampling compartments, fluidic systems or require precise aliquots and seem therefore less attractive for continuous measurements in unmodified samples in applications such as environmental monitoring.15,16 More recently, localized titration approaches were introduced that generate the titrant in situ and monitor the chemical perturbation either at the same electrode or at a fixed distance from the generating electrode.17−24 In perhaps the simplest and most promising strategy, an imposed current across a liquid membrane ion-selective membrane electrode forces the extraction of ions (protons) from the sample into the membrane while the membrane potential is monitored at the same time. As the concentration gradient of the analyte ion in the aqueous stagnant layer next to the membrane electrode © 2012 American Chemical Society

Received: October 3, 2012 Accepted: November 10, 2012 Published: November 11, 2012 10165

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Letter

EXPERIMENTAL SECTION

Reagents and Solutions. Potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (KTFPB), tetrakis(4chlorophenyl)borate tetradodecylammonium salt (ETH 500), chromoionophore I, 2-nitrophenyloctylether (o-NPOE), tris(hydroxymethyl)aminomethane (Tris), acetic acid, sodium acetate, sodium chloride, sodium hydroxide (1 M), and tetrahydrofuran (THF) were purchased from Sigma-Aldrich (analytical grade). Electrochemical Equipment. A double-junction Ag/ AgCl/3 M KCl/1 M LiOAc reference electrode was used in potentiometric and chronopotentiometric measurements (Mettler-Toledo AG, Schwerzenbach, Switzerland). Note that alkalinity measurements in very dilute solutions might suffer from leakage of acetate into the sample. Electrode bodies (Oesch Sensor Technology, Sargans, Switzerland) were used to mount the polymeric membranes. A platinum working rod (3.2 cm2 surface area) served as a counter electrode. Selectivity coefficients were determined by zero current potentiometry employing high impedance input 16-channel EMF monitor (Lawson Laboratories, Inc., Malvern, PA). Potentiometric and chronopotentiometric measurements were performed with an Autolab PGSTAT302N (MULTI 16 module, Metrohm Autolab, Utrecht, The Netherlands) that allows one to read up to 16 working electrodes placed in the same electrochemical cell. A Faraday cage was used to protect the system from undesired noise. Membrane Preparation. Porous polypropylene (PP) membranes (Celgard, 0.237 cm2 surface area, 25 μm thickness, and kindly provided by Membrana Wuppertal, Germany) were used as supporting material. The membranes were washed with THF for 10 min to remove any possible contaminants. When the membrane was found to be completely dry (in a matter of seconds), an excess volume of 3 μL of the cocktail solution was deposited on it (see cocktail preparation below). The impregnation of the cocktail was found to be instantaneous; however, the membrane was let in the Petri Dish for ∼10 min to ensure a homogeneous and reproducible impregnation of the pores. The pore filling solution composition is assumed to remain identical to the initial THF-free cocktail. Afterward, the membrane was conditioned in the buffer solution for 40 min. Finally, the membrane was mounted in the electrode body. The inner compartment was filled with 10 mM acetic acid/10 mM sodium acetate buffer in 10 mM of NaCl for all experiments. All the solutions were prepared in 10 mM of NaCl as background electrolyte avoiding migration process (higher current densities might require higher background electrolyte concentration). Membrane H1 for alkalinity measurements contained 120 mmol kg−1 of ionophore I, 60 mmol kg−1 of KTFPB, 90 mmol kg−1 of ETH 500, 190 mg of o-NPOE, and 1 mL of THF. Membrane H2 for acidity measurements contained 120 mmol kg−1 of ionophore I, 5 mmol kg−1 of KTFPB, 90 mmol kg−1 of ETH500, 190 mg of o-NPOE, and 1 mL of THF. Note that THF was only used to enhance the solubility of the solid compounds into the plasticizer and was removed by evaporation before casting the membranes. Membrane H1 was also employed to titrimetric analysis to determine alkalinity, acidity, and pH.

Figure 1. Schematic illustration of pH and total alkalinity determination with a permselective membrane that contains a hydrogen ion-selective ionophore (L), a cation exchanger (R−), and a lipophilic salt (ETH 500). (a) pH determination with an unperturbed membrane at zero current. (b) A membrane perturbed by an anodic current pulse allows one to determine total alkalinity in a sample that contains a weak base B, owing to the controlled release of hydrogen ions from the membrane. At the transition time, the diffusion of B can no longer sustain the imposed hydrogen ion flux, resulting in a change of the observed potential as (c) raw (E vs time) and processed (dE/dt vs time) chronopotentiometric data.

membrane is composed of the ionophore (L), cation exchanger (R−), and ETH 500 and is sandwiched between an inner solution and the sample. The cation exchanger R− in the membrane dictates the concentration of its counterion, LH+, the protonated form of the H+-ionophore. This is a typical formulation for a liquid membrane electrode used in potentiometry and the expected permselective membrane behavior should allow one to measure pH by zero current potentiometry (Figure 1a). It is here postulated that an anodic current imposes a hydrogen ion flux in the direction of the sample solution. The hydrogen ion is expelled from the membrane to satisfy the imposed flux, resulting in a net transport of protons from the inner solution across the membrane into the sample (see Figure 1b). Recent work on the coulometric delivery of ions from an ion-selective membrane into a bulk sample confirmed that permselectivity can be maintained at moderately low current densities.12 When a base is present in the sample, it will become protonated near the electrode surface. While the proton flux remains constant, the flux of this base from the solution bulk to the electrode membrane continuously diminishes owing to flattening diffusion profiles. At a transition time (τ), the proton flux exceeds the flux of base near the membrane electrode, and the local pH is drastically reduced. This transition can be

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RESULTS AND DISCUSSION The schematic membrane composition for the chronopotentiometric determination of alkalinity is shown in Figure 1. The 10166

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Figure 2. (a) Time derivative of the chronopotentiometric response in contact with a sample containing 0.2−1.1 mM of Tris base. The transition time increases with increasing Tris concentration, as expected from the Sand equation, eq 1. Inset: structure of the ionophore (pKa 14.8,38 linear range of ∼pH 2−12 in potentiometry). (b) Square root of the transition time (τ1/2, left axis) as a function of Tris concentration follows a linear relationship, giving information on total alkalinity. Also shown (right axis): sample pH from the same membrane electrode at zero current (red line shows theory).

Figure 3. Determination of pH, total alkalinity, and total acidity with the same permselective membrane H1 in a volumetric titration of 1 mM acetic acid with NaOH and 1 mM Tris with HCl. (a) Total acidity measurement by chronopotentiometry during the titration (left axis), where the blue line shows the ideal linear decrease of acid with added NaOH. Also shown (right axis): Experimental pH values from the same membrane at zero current and comparison to theory (solid red line). (b) Total alkalinity measurement during the titration of 1 mM Tris with HCl by chronopotentiometry, showing the expected linear decrease of Tris concentration in the course of the titration as the blue line (left axis). Again, pH measurements from the same membrane are also shown along with theoretical expectations (red line).

recorded in chronopotentiometry (see Figure 1c) since the membrane is intrinsically responsive to the local pH in the sample solution. The relationship between the transition time (τ), the concentration of base cB, the diffusion coefficient Daq of the base in the aqueous phase, the Faraday constant F, and the current density i/A is given by the Sand equation:35 i 1 τ = πDaq Fc B (1) A 2 The observed potential change is a direct function of the pH change at the transition time and the membrane selectivity. A stronger base (higher pKa value) will give rise to larger potential changes for this reason. Figure 2 shows an example of total alkalinity determination by chronopotentiometry with a membrane that contains 120 mM of hydrogen ion-selective ionophore and 60 mM of cation exchanger. Specifically, Figure 2a shows the chronopotentiometric data for total alkalinity determination of Tris from 0.3 to 1.1 mM at an anodic current of 100 μA cm−2 (see Figure 2S in the Supporting Information for raw chronopotentiometric data). The transition time is visualized as the maximum of the

time derivative of the potential in Figure 2a. The higher the concentration of Tris in the solution, the longer the observed transition time, as expected from eq 1. After each chronopotentiometric determination, the membrane is relaxed at the open circuit for a period of 30 s, in which time the open circuit potential is confirmed to return to within 2 mV of the value measured before current excitation. As with established ion-selective electrodes, this potential is a function of the sample pH, which is confirmed in Figure 2b. The relationship between the square root of transition time (τ1/2) and Tris concentration with the potentiometric results are shown in Figure 2b. A linear behavior is observed in accordance with the Sand equation, eq 1. From the obtained slope (2.02 ± 0.01 s1/2 mM−1), the diffusion coefficient of Tris is estimated as (5.79 ± 0.02) 10−6 cm2 s−1.36 The linear range can be tuned by the applied current magnitude. Figure 3S in the Supporting Information shows the variation of the square root 10167

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used to validate the measurements. Figure 3a shows a 1 mM acetic acid titration by successive additions of NaOH. Total acidity determined by chronopotentiometry agrees with the end point of the titration curve. A slight deviation from ideal behavior (shown as a blue line) is observed for the initial points of the chronopotentiometric detection. This is explained by the degree of dissociation of acetic acid, which is not negligible for the first three points of the titration (αHA = 0.85, 0.88, 0.92). Total acidity is overestimated because of the much larger hydrogen ion mobility relative to that of undissociated acid. On the other hand, Figure 3b shows the titration of 1 mM Tris base with HCl. Total alkalinity determined by chronopotentiometry agrees quantitatively with the titration end point, and the base concentration can be directly traced in the course of the titration. While more work needs to be done to fully validate this methodology, it may become a powerful tool for the direct detection alkalinity and acidity, along with pH, in situ without bulk sample modification.

of the transition time as a function of the total alkalinity at different applied currents. Increasing the current from 5 to 50 μA, the linear range is increased from 0.1−0.3 to 0.2−1 mM, respectively. A linear response is also observed when the inverse of the square root of transition time is plotted as a function of the applied current at several concentration levels, as expected from eq 1. The linear range and the lower and upper detection limits are modulated by two key operational parameters, the applied current density and the membrane composition. A decrease of the concentration of cation exchanger in the membrane (from 60 to 5 mM) was found to result in a decrease of the upper limit of detection from 2 to 0.7 mM at the same current density (Figure 4S in the Supporting Information). A lower cation exchanger concentration translates into a smaller attainable outward flux of hydrogen ions. The lower detection limit was here not yet optimized, but a recent report on similarly formulated membranes for the uptake and detection of total calcium34 suggested a detection limit on the order of 10 μM. On the other hand, when a cathodic pulse is applied to the membrane for determining total acidity (Figure 5S in the Supporting Information), the observed inflection point corresponds to the localized proton depletion at the phase boundary, in analogy to previous work.34 At this transition time, the undissociated acid (HA) can no longer sustain the imposed flux and a background cation must be extracted along with the analyte ion to maintain the imposed diffusional ion flux. In acidity determinations, HA diffuses to the phase boundary across the diffusion layer to the membrane surface, where it dissociates to give rise to hydrogen ion transfer into the membrane phase. Consequently, the same membrane used above for alkalinity is explored for direct acidity measurements by using a cathodic rather than anodic current perturbation. For this purpose, the concentration of an acetic acid/acetate buffer is increased stepwise and the resulting solution observed by chronopotentiometry and potentiometry at the same membrane (Figure 5S in the Supporting Information). The observed pH is confirmed to remain constant in this buffer system, while chronopotentiometry is clearly sensitive to the concentration of the acid species in solution. The diffusion coefficient of acetic acid is obtained from the calibration slope and the Sand equation as (1.25 ± 0.02) 10−5 cm2 s−1, which is in agreement with reported values.37 The observed linear range is found as 0.1−1.2 mM at −180 μA cm−2. At higher current amplitudes (−70 μA), the upper detection limit could be extended to up to 2 mM (see Figure 6S in the Supporting Information). Again, diffusion is confirmed to be the predominant mode of transport with experiments with increasing applied current (−10 to −70 μA). While migration may certainly be expected at high current amplitudes, its relevance can be identified by performing measurements at varying current density. There is no evidence for this effect in the current range and with the ionic strength used here (see Figure 6S in the Supporting Information). One may note that strong bases and acids such as NaOH (for alkalinity) and HCl (acidity) were also successfully measured by the proposed methodology (data not shown), confirming that the technique is not limited to the systems reported here. Finally, we present an additional example that compares the determination of total acidity and alkalinity by the proposed method to conventional titrimetric analysis. The titration end point detector is the very same membrane used above and used in the potentiometric mode. An additional pH glass electrode is

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ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. † On leave from Imam Khomeini International University, Qazvin, Iran.

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ACKNOWLEDGMENTS This work was supported by the Swiss National Science Foundation (SNF). M.G.A. is grateful to an international fellowship from Khomeini International University.

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REFERENCES

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