Coulometric micro-titrator with a ruthenium dioxide pH-electrode

Analytica Chimica Acta 412 (2000) 69–75

Coulometric micro-titrator with a ruthenium dioxide pH-electrode Carlo Colombo1 , Thomas Kappes, Peter C. Hauser∗ The University of Basel, Department of Chemistry, Spitalstrasse 51 CH-4056 Basel, Switzerland Received 22 October 1999; received in revised form 17 January 2000; accepted 26 January 2000

Abstract A coulometric diffusional titrator with a ruthenium dioxide pH-sensing electrode for the end-point detection in acid–base titrations is presented. The device was prepared by galvanic deposition of noble metals onto electrodes etched on a copper clad printed circuit board. Five electrodes were employed: three gold electrodes (two actuator and one counter) for the coulometric production of the titrating protons or hydroxide ions, and a RuO2 -electrode and Ag/AgCl pseudo-reference electrode pair for the end-point detection. The RuO2 -electrode for pH-measurements showed a linear response to the variation of the pH-value in the range from 2 to 12 pH-units and was useable for a period of 3 months. The device was found to yield linear relationships between the analyte concentration and the equivalence time for concentration ranges of acetic acid and ammonia between about 0.1 and 100 mM. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Coulometry; Micro-titrator; Ruthenium dioxide pH-electrode

1. Introduction Titrations are frequently carried out in many laboratories and automated systems employing piston operated burettes and pH- or ion-selective electrodes for end-point detection are widely used. The miniaturization of this common method is attractive because of the reduced reagent consumption and the ability to analyse small sample volumes. Several different approaches to the scale down of titration systems have indeed been reported in recent years. Sagara et al. have described a conventional burette driven system with spectrophotometric end-point detection for small sample volumes which was constructed from fused silica capillaries with narrow internal diameters and me∗ Corresponding author. Tel.: +41-61-267-1003; fax: +41-61-267-1013. E-mail address: [email protected] (P.C. Hauser) 1 Present address: Addiment Italia S.r.l., Direzione Generale, Via Roma 65, 24030 Medolago (BG), Italy.

chanically driven microsyringes [1]. Similar systems with automatic sample injection and potentiometric end-point detection have been described by Alerm and Bartrol´ı [2]. The use of piston operated microburettes to meter the volume of a reagent solution, however, does only allow a limited degree of miniaturization and simplification. Gratzl et al. have reported reagent delivery to drop sized samples by diffusion from a plug of a gelled material serving as reservoir [3]. A different possible route to eliminate conventional burettes is the use of flow-injection analysis (FIA)-titration [4]. A further attractive approach in the simplification is coulometric reagent generation as the control of electrical charge is much more readily achieved then that of reagent volume. This has indeed been exploited for FIA-titrations [5]. Similarly, electrochemical end-point detection methods lead to less complex systems than optical detection. Bustin et al. [6] described a device for redox titrations which employed two interdigitated microelectrode arrays

0003-2670/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 0 0 ) 0 0 7 6 5 - 0

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as actuator and detector and conventional reference and counter electrodes. The array electrodes were constructed by sputtering of platinum onto a glass substrate. The reagent was generated galvanostatically from a precursor which was added to the solution and detection was carried out amperometrically. Bergveld et al. have reported micro-scale acid-base titrators employing gold actuator electrodes for the coulometric generation of protons or hydroxide ions by water hydrolysis and pH-ISFETs (Ion-Selective Field Effect Transistor) for end-point detection [7]. The working, detector and auxiliary electrodes were constructed on a silicon substrate employing silicon microfabrication methods. These devices rely on the dynamic process of diffusion of the titrant from the actuator electrode to the sensing electrode and do not employ bulk mixing and complete reaction of the sample. The cells in their compactness resemble chemical sensors. Recently Guenat et al. reported a similar titrator manufactured in silicon with micromachining methods which was demonstrated for all four types of titration, namely gravimetric, complexometric, redox and acid-base titrations, employing potentiometric end-point detection [8]. Silver and platinum electrodes were used as actuators and detectors. In the acid-base titration a platinum electrode was used for the generation of protons or hydroxide ions. End-point detection was also achieved with a platinum electrode by monitoring the redox potential of the hydroquinon/quinone couple which was added as indicator substance. An ISFET was not employed in order to limit the complexity of the system [8]. An alternative route to solid-state pH-sensors recently introduced is the use of ruthenium dioxide electrodes [9–11]. The device reported herein for acid-base titrations relies on such a pH-sensor which does not necessitate the addition of an auxiliary reagent for end-point detection. Construction is carried out by employing readily accessible methods used in the manufacture of printed circuit boards and galvanic deposition of metals.

Fig. 1. Cross-sectional view of the cell assembly, not to scale.

printed circuit board material (consisting of a 1.5 mm thick epoxy board clad with a copper layer of 35 ␮m thickness). This base was covered with a Teflon sheet (250 ␮m thickness) which had a slot of 30 mm length and 0.5 mm width cut to form a flow channel on top of the electrodes. The electrode support and the spacer were placed between two perspex blocks (40 mm×60 mm) and the whole assembly tightened together with eight screws. The top perspex cover had threads for standard 1/4×280 -UNF-fittings machined to allow the passing of solutions through the channel. The total volume of the channel is 3.75 ␮l, however, since the titration is localized to the region of the electrodes the effective sample volume is only approximately one tenth of this. A schematic diagram of the electrode arrangement is shown in Fig. 2. Two coulometric actuator electrodes and one counter electrode made of gold were employed. The pH-sensitive electrode consists of ruthenium dioxide and is referenced against a silver/silver chloride electrode. The width of the generating electrodes is approximately 350 ␮m and they are separated from the 300 ␮m wide pH-electrode by about 400 ␮m. The

2. Experimental 2.1. Cell design A sandwich design as illustrated in Fig. 1 was employed. The electrodes were constructed on standard

Fig. 2. Top–down view of the active section of the titrator cell, approximately to scale. Actuator and counter electrodes: Au: pH-Electrode: Ru/RuO2: Reference-electrode: Ag/AgCl.

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Fig. 3. Circuit diagram.

reference (550 ␮m wide) and counter (700 ␮m wide) electrodes were positioned at a distance of 700 and 900 ␮m from the nearest other electrode, respectively, in order to avoid interferences due to diffusion of generated ions. Note that the length of the electrodes is greater than the width of the flow channel, but that only the parts exposed to the solution are active. Connections to these electrodes were made at the edge of the printed circuit board, extending slightly beyond the perspex covers, by soldering wires to the copper tracks. A purpose made galvanostat was used to generate the current for the coulometric titration and the indicator and reference electrodes were fitted with impedance convertors. The electronic circuitry is detailed in Fig. 3.

2.2. Fabrication of the electrodes The layout of the electrodes was prepared using a drawing program on an computer and reproduced with a laser printer on a special transparency film used for manufacturing printed circuit boards (Part No. 51 95 70-44, Conrad Electronics, Solothurn, Switzerland). Printed circuit boards (PCBs) pre-coated with a photoresist were employed (Part No. 45 00 27, Distrelec,

Nänikon, Switzerland). The PCBs were exposed using a standard UV-unit (Model UV2M, 16W, 370 nm, Farnell Instrument, Bognor Regis, UK) and the photoresist developed with a solution of approximately 1% of sodium hydroxide in deionized water. Etching was carried out in a solution of 350 g l−1 Na2 S2 O8 at a temperature of ca. 40–45◦ C in a tank employing bubble agitation (Art. No. 141080 1000, Isel-Automation, Eiterfeld, Germany). When the process was terminated, the PCB was washed thoroughly with water, dried and protected with a lacquer spray (Plastik 70, Distrelec). The protective cover for the individual tracks to be further treated was then removed with the help of acetone and the ends of the copper lines coated with either Au, Ag or Ru as required by employing standard galvanic deposition methods as detailed below. Note that in the electrodeposition processes also lateral growth of the tracks occurs, and the width of the narrower electrodes was found to be increased by up to 50%. The dimensions given in Section 2.1 refer to the final product. Gold was electroplated from a solution containing 4.5 g l−1 of AuCN, 1.5 g l−1 KCN, 20 g l−1 K2 HPO4 and 15 g l−1 KH2 PO4 with a current density of 10 ␮A/mm2 for 8 h [12]. The colour of the deposit varied from a pale yellow to a reddish brown. To obtain a bright and smooth surface the electrodes were

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then subjected to a bright dipping bath (30 g l−1 KCN and 40 ml l−1 H2 O2 (30%)) and electropolishing at a constant potential of +2 V against a saturated calomel electrode (SCE) (Model REF401, Radiometer Analytical, Villeurbanne, France) for 120 s in a solution of 60 g l−1 of KCN and 15 g l−1 Na2 HPO4 [13]. Silver was electroplated from a solution of 35 g l−1 AgCN, 50 g l−1 KCN and 7.5 g l−1 K2 CO3 with a current density of 55 ␮A/mm2 for 6 h. The colour of the Ag deposit was white. To obtain a bright and smooth Ag surface, the electrode was electropolished at +0.2 V (against SCE) for 60 s in a solution of 5 g l−1 AgCN and 75 g l−1 KCN. Silver/silver chloride reference elements were prepared by anodizing the silver electrodes prepared as described above in a solution of 50 g l−1 KCl at a current density of 1 ␮A/mm2 for 2 h [14]. Ruthenium was electrochemically deposited for 2 h on a smooth gold electrode at a current density of 1 ␮A/mm2 from a solution containing 10 g l−1 of ruthenium as the potassium diaquooctachloronitridoruthenate salt, 10 g l−1 of ammonium formate and concentrated HCl to adjust the pH-value to 1.3 [12]. The ruthenium surface was subsequently oxidised in a 1 M HClO4 solution by applying a constant potential of +1.2 V against a mercury/mercurous sulphate (MSE) (Model XR200, Radiometer) reference electrode for 1 h [15]. 2.3. Procedures Water was purified by a Milli-Q (Millipore, Bedford, MA, USA) purifying system. Reagents were purchased from various suppliers (Fluka, Buchs, Switzerland; Merck, Darmstadt, Germany; Siegfried AG, Zofingen, Switzerland; and J.T. Baker, Deventer, Holland) and were all of analytical reagent grade. All data was recorded with a MacLab/4e data acquisition system (ADInstruments PTY, Castle Hill, Australia) and a Macintosh 7300/166 personal computer (Apple, Cupertino, CA) using the program Chart 3.4.3 (ADInstrument) which also allowed the simultaneous recording of the first derivative of the titration. A MacLab Potentiostat (ADInstruments) connected to this system was used for electroplating and the characterisation of the electrodes. In order to obtain fixed currents for plating, the potentiostat was converted to a galvanostat by inserting an appropriate resistor

between the working and reference inputs of the instrument and then connecting the working electrode (WE) to the reference electrode (RE) input and the counter electrode (CE) in the normal configuration [16]. A peristaltic pump (Minipuls 3, Gilson, France) was used to fill the titrator with the standard solutions.

3. Result and discussion 3.1. Characterisation of the electrodes The Ag and Au electrodes were characterised by acquiring cyclic voltammograms in a 1 M H2 SO4 solution and comparison to those of Au and Ag wires. The voltammograms showed the expected features with the formation of the gold and silver oxide layers [14,17,18]. The absence of detectable oxidation currents in the region of the copper oxide formation indicated a complete homogeneous coverage of the copper surface. This was confirmed by scanning electron microscopy (SEM) of a cross-section of the electrodes which showed a clear distinction between the copper and the gold or silver layers. The Ag/AgCl electrode was characterised by measuring the potentiometric response against a mercury/mercurous sulphate (MSE) reference electrode to increased concentrations (10−4 M–10−1 M) of potassium chloride ion and calculating the regression line from the activities for chloride. The slope was −60.1 mV in close agreement with the theoretical value (59 mV) of the Nernst equation. The intercept at aCl− gives the value of Eo for the system |Ag|AgCl|K+ Cl+ |Hg2 SO4 |Hg. The value was 406 mV versus MSE corresponding to a value of 234 mV versus SHE, which is in reasonable agreement with the E0 -value for the Ag/AgCl half-cell (222 mV versus SHE). The response of the ruthenium dioxide pH-electrode was tested against a conventional reference electrode and compared with a glass-pH-electrode by stepwise additions of acid or base to a stirred universal buffer solution. The responses of a newly prepared RuO2 -pH-electrode and an electrode which had been used for a period of 3 months are shown in Fig. 4. For the newly prepared electrode a steady state after each addition of base was reached in a maximum of about 30 s. This response time is comparable to that

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Fig. 4. Response to the variation of the pH-value for a newly prepared RuO2 electrode and a 3-month-old electrode. The numbers on the steps indicate pH-values measured with a glass pH-electrode. Starting solution: 4 mM H3 PO4 /CH3 COOH/H3 BO3 in 0.1 M KNO3 ; added solutions: 0.1 M NaOH and 0.1 M HCl in 0.1 M KNO3 .

of a glass pH electrode. For pH-values outside the range between about 2 and 12 pH-units a significant drift was encountered and the electrode could not be used at the extreme ends of the pH-scale. It can also be seen in Fig. 4 that the aged electrode displayed an increase in the response time. The steady-state potentials for both electrodes showed a slope of approximately 40 mV per pH-unit. Repetitions of this experiment showed that the ruthenium electrode was behaving consistently, with values for the slope within 5% of the reported value. This slope is significantly sub-Nernstian, but this is not a problem for end-point detection in a titration. An electrode prepared using a different published method, in which RuO2 is directly plated without an intermediate metallic Ruthenium-layer on the gold surface by repeated potential cycling in a solution of a Ruthenium-salt [9], was found to show a slope close to the theoretical value. However, at least in our hands, this electrode was found to be significantly less stable and the ruthenium oxide film had disappeared completely after 7 days. Presumably thinner oxide layers are obtained in this latter procedure, leading to the shorter lifetime. 3.2. Titration of acetic acid The titration of a solution of 10 mM acetic acid was carried out by generating hydroxide ions at the actu-

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Fig. 5. Titration curve and first derivative for a 10 mM acetic acid solution containing 0.1 M KNO3 . Current applied: 6.8 ␮A. Current switching is indicated by ‘start’ and ‘stop’. Units for the first derivative are arbitrary.

ator electrodes by hydrolysis. The resulting titration curve is shown in Fig. 5 and it can be seen that it resembles that of a conventional titration. It should be borne in mind, however, that here the solution is not stirred, but the transport of hydroxide ions relies on the dynamic diffusion from the actuator electrodes to the pH-sensing electrode. Complete titration of the cell volume is not carried out. The shifts in potential on turning the actuator current on and off are thought to be due to the creation of a small iR-drop between actuator and counter electrodes. The equivalence time, 48 s in this case, is readily obtained from the first derivative of the titration curve and could be reproduced to within a standard deviation of 1.5% (10 titrations) at this concentration. It can also be seen that the pH-sensing electrode was not affected by memory effects, as a potential close to the initial one was reached in about 30 s when the pump to renew the solution was started. Fresh solution was pumped through the cell for approximately 30 s at a flow rate of about 1 ml min−1 before each titration. Please also note that the Ag/AgCl electrode served as pseudo-reference only as chloride was not added. This is not a limitation, however, as the distance of the reference from the actuator electrode assures a constant composition of the surrounding solution for the duration of the titration. Calibration curves obtained for acetic acid using currents from 0.75 to 32 ␮A are given in Fig. 6. It can be noted from the figure that the calibration curves do not go through the origin and all show an intercept

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Fig. 6. Calibration curves obtained for different applied currents for acetic acid solutions containing 0.1 M KNO3 .

of approximately 15 s, a fact attributable to the finite diffusion time required for the hydroxide ions generated to cover the distance from the actuator electrode to the sensor electrode. The time required for diffusion over a given distance may be estimated according to t=x2 (2D)−1 , where t is the time, x the distance and D the diffusion coefficient [19]. For the diffusion of hydroxide ions across 400 ␮m, the approximate distance between the actuator electrodes and the pH-electrode, a time of 15.4 s is calculated (D for OH− =5.2×10−3 mm2 s−1 [20]). Therefore, this experimental observation agrees closely with the theoretically expected. Linear relationships were obtained for equivalence times up to 300 s for all the currents used. The deviations encountered for longer times can again be rationalized with the help of the above equation by considering the time required for the diffusion of protons from the counter electrode to the actuator or pH-sensing electrodes. In practice it is, therefore, necessary to adjust the current to bring the equivalence time for the sample into the range between about 15 and 300 s and it may generally be desired to use a current level that yields short analysis times. However, by making this simple adjustment, which may furthermore be automated, titrations of acetic acid concentrations ranging from about 0.1 mM up to 100 mM were found possible. The effect of the total ionic strength of the sample was also tested using different background electrolyte concentrations of KNO3 . It was found that for total salt concentrations of less than approximately 1 mM it was not possible to detect the end point due to significant noise on the detector signal.

Fig. 7. Titration curves, first derivative and calibration plot for different ammonia concentrations in 0.1 M KNO3 . Current applied: 6.8 ␮A. Units for the first derivative are arbitrary.

3.3. Titration of ammonia Ammonia solutions ranging from 0.1 to 20 mM were titrated by reversing the polarity of the applied current in order to generate protons at the actuator electrodes. The titration curves obtained for 1, 5, 10 and 20 mM ammonia shown in Fig. 7, together with the calibration graph, indicate that the titration of base is also feasible. The response to ammonia concentrations was found to be linear over the concentration range considered using a current of 6.8 ␮A as a compromise. An intercept at approximately 7 s was found. By considering the diffusion coefficient, D of 9.3×10−3 mm2 s−1 for H+ [20] a value of 8.6 s is calculated as lag time for diffusion of protons from the actuator electrode to the pH-electrode, indicating again close agreement with the expected.

4. Conclusions The data presented demonstrates that a miniature coulometric acid–base titrator can be prepared using conventional manufacturing techniques. Its behaviour is similar to the previously reported devices constructed with silicon micromachining methods [7]. However, the dimensions are somewhat larger for the titrator described here, which results in longer diffu-

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sion times from actuator to sensor electrode and correspondingly the analyses cannot be carried out quite as fast. Titrations of acid and base over the large concentration range from 10−4 –10−1 M were found to be readily possible by the simple adjustment of the actuator current. The key to the simplification was the use of the ruthenium dioxide electrode as pH-sensor. This electrode was found to be stable for an extended period of time. To our knowledge, the two-step galvanic preparation procedure used for the fabrication of the sensing electrode has not been reported previously.

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