Determination of sulphate in water by flow-injection analysis with electrode-separated piezoelectric quartz crystal sensor

Available online at www.sciencedirect.com

Sensors and Actuators B 130 (2008) 551–560

Determination of sulphate in water by flow-injection analysis with electrode-separated piezoelectric quartz crystal sensor Y.S. Fung ∗ , C.C.W. Wong, J.T.S. Choy, K.L. Sze Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China Available online 29 September 2007

Abstract A new flow-injection analysis (FIA) system with detection by an electrode-separated piezoelectric quartz crystal (ESPQC) was developed for sulphate determination in water. The method was based on the decrease in conductivity upon precipitation of barium sulphate due to the mixing between reagent and sample with change indicated by the frequency shift in the ESPQC sensor. The best performance was achieved using a three-channel FI system consisted of a carrier stream of water, a buffer stream containing 0.05% KCl, 10 mM acetic acid and l mM sodium acetate, and a reagent stream of 0.1% BaCl2 . Under the optimized conditions, the linear range for sulphate determination was 0.5–50 ppm with a correlation coefficient of 0.9991, detection limit 0.3 ppm (S/N = 2) and sample throughput 50 samples/h. The effect of co-existing cations and anions were investigated and found not to cause interference for most samples except those with very high salt content. The reliability of the method was established by parallel determination using standard turbidimetric method for sulphate in drinking and pond water samples with results within statistical variation. The method developed was shown to provide a simple, sensitive, rapid and reproducible FIA detection method for ionic compounds in the liquid medium. © 2007 Elsevier B.V. All rights reserved. Keywords: Flow-injection analysis; Electrode-separated piezoelectric quartz crystal; Sulphate analysis

1. Introduction Sulphate is one of the dominant sulphur species in aquatic environment and its level is greatly elevated in polluted environment as the result of acidic deposition due to the use of coal, oil and other sulphur-containing fuel [1,2]. Although the presence of sulphate in drinking water affects its aesthetic quality and does not pose immediate threat to public health as recommended by the World Health Organization [3,4], increasing sulphate level in water provides a strong indicator for the acidification of water bodies due to water pollution that causes health concern on quality of water [5–7]. Thus, it is an important parameter for monitoring environmental water quality with analysis carried out on routine basis worldwide by governmental regulatory bodies. The common methods for the determination of SO4 2− in water are the gravimetric method [8], turbidimetric method [9,10], ion-selective electrode [11], ion chromatography

∗

Corresponding author. Tel.: +852 28592162; fax: +852 2548 2132. E-mail address: [email protected] (Y.S. Fung).

0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.09.070

[12–15], capillary electrophoresis [16–19] and flow-based automatic analyzers [20–25]. The gravimetric method is the classical method for analysis at a relatively high concentration range (>10 mg/L) and can be interfered by the presence of silica, nitrates and heavy metals. The turbidimetric method is applicable from 1 to 40 mg SO4 2− /L and dilution is required for higher concentrations. Both these two manual methods provide the backup for instrumental methods in routine analysis. Ion chromatography (IC) is suitable for low concentrations of SO4 2− (∼1 mg/L) with detection limit at 0.1 mg/L. However, it requires long elution time for SO4 2− and high capital and operational cost. Capillary electrophoresis provides a faster and more efficient separation technique. However, the high capital cost, slow sample throughput, and requirements of skilled staffs have limited its application for handling a large number of samples on daily basis. When a large number of sample has to be analyzed for SO4 2− on routine basis, procedure based on an automatic continuous flow is normally used. In early work, a dedicated analyzer based on automatic continuous flow with spectrophotometric detection (automated methylthymol blue method) has been developed [26], applicable to sulphate determination in potable,

552

Y.S. Fung et al. / Sensors and Actuators B 130 (2008) 551–560

surface and waste waters over a range of 10–300 mg/L, and 30 samples can be analyzed per hour. However, the technique is restricted to specialized laboratory due to its high cost, complicated flow control and procedures. With the development of flow-injection analysis (FIA) by Ruzicka and Hansen [27], automatic analysis for large numbers of samples can be achieved in ordinary laboratories due to its simple experimental set-up, excellent repeatability and high sample throughput. FIA determination of sulphate in water has been achieved by turbidimetric or spectrophotometric methods. The turbidimetric methods use the precipitation reaction of barium ion with sulphate in the sample to form barium sulphate suspension for detection at 480 nm [21,25]. Different versions of the method have been developed with an aim to improve the sensitivity, repeatability and sample throughput. Most of the turbidimetric methods are applicable in the range from 10 to 200 mg SO4 2− /L. To handle samples with lower levels of sulphate, different spectrophotometric methods have been developed, using different complexing agents [20,23,24]. Upon the injection of samples, barium will be displaced from the complex, resulting in a decrease in the absorbance which is proportional to sulphate concentration. The method is sensitive (0.1–0.6 mg/L) although the precision is lowered (R.S.D., ca. 4%). Another version of the method is based on the displacement of barium from solid reagents packed in a small reaction column [22,28,29]. Upon displacement, deeply coloured ions such as chloranilate or dimethylsulphonazo III were released and subsequently determined by a spectrophotometer. With these techniques, determination of SO4 2− in water samples in the range from 0.5 to 5 mg/L can be achieved with an average precision of about 2% in R.S.D. [28]. Although the sensitivity of the method has been improved using the spectrophotometric method, its working range is narrowed, more complicated flow control is used and suffers additional interference due to the use of a secondary reaction with highly absorptive dyes to enhance spectrophotometric detection. The recent advance of the piezoelectric quartz crystal (PQC) sensor offers an alternative method for sulphate detection, in particular the recently developed electrode-separated piezoelectric quartz crystal sensor (ESPQC) [30–35] which can oscillate stably in solution. It is developed by Nomura et al. [30] for detection in liquid based on the change in the physiochemical property under flow condition. Two types of ESPQC have been developed with one or two separated electrodes, depending on the conductivity change in the solution to be detected. The production of barium sulphate upon mixing between the barium reagent and the sulphate ions could lead to a significant change in conductivity. Thus, the one electrode ESPQC system is adopted in the present work, as it can oscillate stably in solution of very high specific conductivity (ca. 10 S/m), offering sensitive detection by measuring the frequency shift of the ESPQC sensor during the precipitation reaction. As no additional reagent is added, it offers a direct detection method with simple FIA procedure. The present work will report on the application of ESPQC for direct FIA detection of the barium sulphate reaction with an aim to provide an automatic procedure with a wider working

range, simplified flow and more sensitive detection for sulphate in water. 2. Experimental 2.1. Standards, reagents and sample preparation All chemicals used (Aldrich, BDH, Merck) were of analytical-reagent grade and doubly quartz-distilled water was used to prepare stock and standard solutions. The stock sulphate solution (1000 mg SO4 2− /L) was prepared by dissolving 0.1479 g of anhydrous sodium sulphate in 100 mL water. It was used to prepare working standard solutions after appropriate dilution. Barium chloride solution (0.1%) was prepared by dissolving 0.25 g of BaCl2 ·2H2 O in 250 mL water. Stock acetic acid buffer (l00 mM acetic acid–10 mM sodium acetate) was prepared by dissolving a given amount of sodium acetate in 100 mL of 0.1 M acetic acid. Solutions for interference study (NaCl, NaNO3 , NaHCO3 , Na2 CO3 , K2 CO3 , CaCl2 and MgCl2 ·6H2 O, each of 1000 ppm) were prepared by dissolving respective amount of the salts in l00 mL water. Commercially available natural mineral waters containing specified amount of sulphate and other ions were used as “mixed standard” solutions for the study of interference. Fresh samples of drinking water and pond water were collected in the University using 250-ml polyethylene bottles. All standard solutions and drinking water samples were filtered through a 0.45 ␮mmembrane filter (Rainin, USA) before injection into the FIA system. For pond water samples containing a large amount of suspended particles, the samples were pre-filtered through a Advantec no. 2 cellulose filter paper (Tokyo, Japan) prior to membrane filtration. The samples were then passed through a cation-exchange resin column (Dowex 50WX8, 20–50 mesh, sodium form) of length 15 cm and i.d. 2 cm. The first 20 mL of eluent was discarded and the next 10 mL portion was used for subsequent analysis. 2.2. Piezoelectric crystal detection system The electrode-separated piezoelectric quartz crystal (ESPQC) supplied from C & Y Trading Co. Ltd., Hong Kong was consisted of a 10 MHz, AT-cut piezoelectric quartz crystal with silver electrodes on both sides (diameter: 5 mm) mounted on HC-51 U holder. One silver electrode was dissolved in aqua regia and washed thoroughly with deionized water and acetone prior to drying in air. The flow-through detector cell (Fig. 1) was constructed by fixing one ESPQC with silicone resin in a block made of PTFE (50 mm × 20 mm × 2 mm). The separate electrode was a Pt disc (1 mm thick and 3 mm diameter) placed opposite to the quartz surface of ESPQC with a separation of 0.5 mm. PTFE tubings (i.d. 0.8 mm) were used in the solution inlet and outlet. The sample solution was passing through the channel, contacting the separated electrode and one side of the crystal. The temperature of the detector cell was controlled at 22 ± 0.2 ◦ C by circulating water supplied from a thermostated water bath. A laboratory-made IC oscillator driven by a 5 V dc power supply was used as the crystal oscillator with frequency

Y.S. Fung et al. / Sensors and Actuators B 130 (2008) 551–560

Fig. 1. The configuration of the electrode-separated piezoelectric quartz crystal (ESPQC) flow-through detector.

monitored by a digital universal frequency counter (CHY 8220R) with a resolution of ±1 Hz. The frequency signal was input to a microcomputer via a RS-232 interface cable for recording of the FIA signal. 2.3. FIA system and measurement The FIA system was consisted of a computerized pumping system with three peristaltic pump heads (Model 7550-92, Masterflex, Cole-Parmer Instrument Co., USA), an electrically actuated six-port rotary valve (Model E60-220, Valco Ltd., Australia) loaded with a 200 ␮L sample loop, and an autosampler with microprocessor control (LS-3200, SGE, Australia). The sample was displaced from the sample vial (0.8 mL) under pneumatic control through a transfer line connected directly to the sample loop loaded on the rotary valve. Two PTFE mixing coils, 20 cm × 0.8 mm i.d. and 100 cm × 0.8 mm, each with 1.5 cm coil diameter were used as the reactors. Schematic diagram showing the FIA manifold is shown in Fig. 2. For measurement of the FIA signal, standard and sample solutions were injected into the FIA system using an auto-sampling system with a 200-␮L sample loop filled for 5 s by applying a vial pressure of 60 psi. Flow rates of the carrier, buffer and reagent streams were 0.25, 0.25 and 0.5 ml/min, respectively. Calibration was done based on the peak height measurement of the FIA signals corresponding to the maximum frequency shift of the detector. For method comparison, the water samples were

553

Fig. 2. Experimental set-up of the flow-injection/electrode-separated piezoelectric crystal for the determination of sulphate in water. (A) A two-channel FIA manifold; R, reagent: 0.1% BaCl2, 0.05% KCl and 10 mM acetic acid–1 mM acetate; C, carrier: H2 O; P, peristaltic pump; V, sample injection valve; AS, autosampler; L, mixing loop; D, ESPQC detector; W, waste; 1, oscillator; 2, voltage regulator; 3, frequency counter; 4, microcomputer. (B) A three-channel FIA manifold; R, reagent: 0.1% BaCl2; B, buffer: 0.05% KCl and 10 mM acetic acid–1 mM acetate; C, carrier: HP; P, peristaltic pump; V, sample injection valve; AS, autosampler; L, L, mixing loop; D, ESPQC detector; W, waste; 1, oscillator; 2, voltage regulator; 3, frequency counter; 4, microcomputer.

analyzed for sulphate using the manual turbidimetric method with photometric detection at 480 nm [9]. 3. Results and discussion 3.1. Preliminary study of ESPQC for FIA application To explain the response of PQC detector in liquid phase, Nomura and Minemura [36] had derived an empirical formula to correlate the frequency shift of the PQC sensor in aqueous solution with the specific conductivity and specific gravity and showed that the crystal with only one electrode in contact with the aqueous solution oscillates stably. The change in resonant frequency depends on density and conductivity of the solution. Simple physical model had been developed by Kanazawa and Gordon [37] to predict the change in resonance frequency for one side of a quartz crystal resonator in contact with the liquid. Bruckenstein and Shay [38] derived a similar relationship between frequency shift and physical properties of the liquid. While most of the theories proves with experimental support that the resonance frequency were determined mainly by density and viscosity of the liquid, the crystal was found oscillating in electrolyte solution with frequency shift dependent on the spe-

554

Y.S. Fung et al. / Sensors and Actuators B 130 (2008) 551–560

Fig. 3. Dependence of frequency shift of the ESPQC detector on the specific conductivity of KCl. Flow rate of solution, 0.5 mL/min.

cific conductivity of the electrolyte [36,39]. In the present work, the frequency shift due to the change in conductivity of the solution is monitored to follow the change in BaCl2 concentration. As proposed by Nomura et al. [40,41], a reagent containing barium ion, acetic acid and potassium chloride was suitable for the precipitation of sulphate while giving constant properties of the solution such as density, viscosity and specific conductance. Therefore, such reagent was investigated further in the present FIA system. The composition of the reagent was evaluated in terms of sensitivity, stability and the detector’s noise level. The frequency characteristics of the flow-through ESPQC sensor used in the present method were studied by passing KCl solution of different concentrations through the cell at a flow rate of 0.5 mL/min using doubly quartz-distilled water as the reference. The dependence of the frequency shift (Hz) on the specific conductivity of the electrolyte is shown in Fig. 3. The frequency shift was found to be proportional to the specific conductivity in the range from about 2 × 10−4 to 5 × l0−2 S/m (0.002–0.5 mS cm−1 ), and the slope of the linear region was found to be 14,843 Hz/mS cm−1 with a correlation coefficient of 0.9995. It can also be seen that the detector is extremely sensitive to the specific conductivity of the solution in the range of interest. Moreover, the linear range obtained provides valuable information for the selection of a suitable reference solution passing through the detector. The reagent for the mixing of sample should be properly selected in such a way that the background conductivity is in the frequency range where the detector is the most sensitive to the change in the specific conductivity. The reagent proposed by Nomura et al. [40,41] containing barium ion, acetic acid and potassium chloride with constant properties of the solution such as density, viscosity and specific conductance was investigated in the present FIA system. The composition of the reagent was evaluated in terms of sensitivity, stability and the detector’s noise level. In the preliminary studies, a two-channel FIA manifold shown in Fig. 2A with water using as a carrier and a reagent containing 0.1% BaCl2 , 0.05% KCl and 10 mM acetic acid −l mM acetate buffer was employed. The sensitivity to sulphate (10 ppm) was found to be rather poor. The range of detection was relatively narrower and the detector was unstable with reference to this reagent. The situation was improved by

Fig. 4. The difference of frequency response of ESPQC detector for the two configurations of FIA manifold. A, Three-channel manifold; B, two-channel manifold.

developing a three-channel FIA manifold as shown in Fig. 2B. The effect of sulphate concentration on the peak height (maximum frequency shift) for the two configurations is shown in Fig. 4. The lower sensitivity and narrow detection range obtained using the two-channel configuration was attributed mainly due to the improper background conductivity of the reagent which was not in a suitable range detected by the ESPQC detector. Using the three-channel FIA configuration in which the sample was first mixed with the KCl/acetate buffer prior to mixing with the barium reagent stream in a mixing ratio of 1:1, the sensitivity was greatly improved. Moreover, the baseline noise was observed to be smaller. As the result, the three-channel configuration with a carrier stream of water, a buffer stream containing 0.05% KCl, 10 mM acetic acid and l mM sodium acetate, and a reagent stream of 0.1% BaCl2 was employed for further study. The effect of the concentration of the barium reagent on the FIA signal was investigated by varying the concentration of BaCl2 from 0.025 to 0.15% while keeping the others constant. The results showing the frequency response (peak height) and precision after injection of 10 ppm sulphate are given in Table 1. It is obvious that increased sensitivity is obtained using higher BaCl2 concentrations. However, the noise level was increased correspondingly, especially at 0.15% BaCl2 . With consideration on both sensitivity and stability, the optimum concentration of the barium reagent was chosen at 0.1%. The purposes of the buffer stream were to provide an acidic condition for the precipitation of barium sulphate and a constant solution property due to the presence of other ions but it can be altered sufficiently by the reaction between barium ions and sulphate ions. The effect of the concentration of acetate buffer was studied by injecting alternatively a solution of 10 ppm SO4 2− and a solution containing 10 ppm SO4 2− + 50 ppm CO3 2− . The results are shown in Table 1. For higher concentration of the buffer, the oscillation of the crystal was found very unstable. Accordingly, a concentration of the acetate buffer containing 10 mM acetic acid and 1 mM sodium acetate which gave the highest response without the effect of other ions was thus selected. The pH was controlled at 5.0. The three-channel FIA system employing a buffer stream also allowed mixing of the sample with the buffer to achieve a constant solution property before reacting with the reagent.

Y.S. Fung et al. / Sensors and Actuators B 130 (2008) 551–560

555

Table 1 The effect of barium chloride, potassium chloride and acetate buffer on peak height and precision of the FIA signal Salt concentration/buffer composition

Maximum frequency shift (Hz) due to flow injectiona of 10 ppm SO4 2− only

Precision (% R.S.D.)b

10 ppm SO4 2− + 50 ppm CO3 2−

BaCl2 (%, w/w)c

0.025 0.05 0.1 0.15

64 89 197 325

3.1 2.4 1.9 3.1

KCl (%, w/w)d

0.01 0.05 0.1 0.15

148 210 182

2 1.8 2.2 –

Acetate bufferf

a b c d e f

e

2.5 mM acetic acid + 0.25 mM acetate 5 mM acetic acid + 0.5 mM acetate 10 mM acetic acid + 1 mM acetate 20 mM acetic acid + 2 mM acetate

182 197 212

134 195 210

e

e

1.5 1.8 1.8 –

Flow rates of carrier, buffer and reagent are 0.25, 0.25 and 0.5 mL/min, respectively. R.S.D. = relative standard deviation from five measurements of 10 ppm SO4 2− for BaCl2 and KCl and 10 ppm SO4 2− + 50 ppm CO3 2− for acetate buffer. Carrier: water; buffer: 0.05% KCl, 5 mM acetic acid–0.5 mM acetate. Carrier: water; buffer: 10 mM acetic acid–1 mM acetate; reagent: 0.1% BaCl2 . Oscillation of crystal was very unstable. Carrier: water; buffer: 0.05% KCl; reagent: 0.1% BaCl2 .

For the detection based on the change in conductivity of the solution using ESPQC sensor, the addition of foreign electrolyte such as KCl has been shown to improve the response and stability of the detector [27,42]. The effect of KCl on the FIA signal and its precision was investigated with results shown in Table 1. The optimum concentration of KCl was found to be 0.05%. From the above investigation, a FIA manifold with optimized composition of the reagents given in Fig. 2B was found to be suitable for the determination of sulphate in water using the ESPQC sensor based on the non-mass effect. Under this condition, a stable oscillation of the quartz crystal was observed, producing a noise level within ±5 Hz over a period of 30 min. 3.2. Optimization of the operational parameters The flow rates of all the streams (carrier/reagent flow ratio) in a FIA system are known to affect the sensitivity, baseline stability and sample throughput. The effects of the flow rates of carrier (C), buffer (B) and reagent (R) on the response of the detector and sample throughput were studied under five different conditions with results shown in Table 2. Increase in the barium reagent flow rate by a factor of two and four (Conditions I, II and III)

corresponding to a decrease in the (C + B)/R ratio from 2 to 0.5 resulted in a decrease in the peak height, whereas increase in both the carrier and buffer flow rates by a factor of two (Conditions III and IV) corresponding to an increase in the (C + B)/R ratio from 0.5 to 2 led to an increase in the sensitivity. Moreover, a better crystal stability was observed when the flow rate of solution passing through the detector was reduced. As expected and shown in Table 2, a higher sampling frequency was obtained at higher flow rates for all the reagents at the expense of the signal sensitivity (Conditions II and V). As a compromise between sensitivity, stability and sample throughput, the carrier, buffer and reagent flow rates used were 0.25, 0.25 and 0.5 mL/min, respectively, to give a (C + B)/R ratio of 1 and a throughput of 50 samples/h. The length of the mixing coil is important in the control of the reaction time between reagent and sample, hence the rate of precipitation of barium sulphate, and the sample throughput of the present FIA system. Under the optimized flow rate condition (Condition II in Table 2), the effect of the length of the mixing coil for mixing between samples in the carrier/buffer stream and the barium reagent stream was studied by varying the length of the PTFE tubing (i.d., 0.8 mm) from 20 to 200 cm. The results

Table 2 The effect of relative flow rates of carrier, buffer and reagent streams on the FIA signal and sample throughput Conditions

I II III IV V a b

Flow rate (ml/min) Carrier (C)

Buffer (B)

Reagent (R)

(C + B)/R

0.25 0.25 0.25 0.5 0.5

0.25 0.25 0.25 0.5 0.5

0.25 0.5 1 0.5 1

2 1 0.5 2 1

Peak heighta (Hz)

Samples per hourb

235 208 120 167 104

35 50 65 65 73

Sulphate concentration: 10 ppm; carrier: water; buffer: 0.05% KCl, 10 mM acetic acid–1 mM acetate; reagent: 0.1% BaCl2 . Mean of five measurements.

556

Y.S. Fung et al. / Sensors and Actuators B 130 (2008) 551–560

Table 3 The effect of the length of mixing coil on the FIA signal for 10 ppm sulphate Length of mixing coil (cm)

Peak heighta (Hz)

Repeatability (% R.S.D., n = 5)

20 50 100 200b

55 114 211 165

3.4 2 1.8 5.7

Carrier: water; buffer: 0.05% KCl, 10 mM acetic acid–1 mM acetate; reagent: 0.1% BaCl2 ; flow rates of carrier, buffer and reagent were 0.25, 0.25 and 0.5 ml/min, respectively. a Mean of five measurements. b At reagent flow rate of 0.25 mL/min.

are shown in Table 3. The peak height was found to be elevated when the length of mixing coil was increased from 20 to 100 cm, showing that a suitable reaction time between the reagent and sample was required to produce sufficient amount of barium sulphate needed to cause a significant change in the physiochemical property of the reagent. Instead of using reaction coil of length greater than 200 cm normally used in turbidimetric determination of sulphate, a mixing coil with length of 100 cm is sufficient without the need of going to complete precipitation of all barium sulphate as particles. This fact was supported by observing the formation of barium sulphate particles adhering on the tubing wall when the length of the mixing coil used was greater than 100 cm. In addition, when a mixing coil of length 200 cm was used and the flow rate of the reagent passing through the detector was reduced to 0.25 mL/min, the frequency of the detector was found to be decreased after injection of 200 ␮L of 10 ppm sulphate solution. This phenomenon was most probably due to the increase in the residence time of the resulting solution which permits the adsorption of barium sulphate on the surface of the ESPQC detector. Under this condition, a poorer precision of the response (R.S.D., ca. 6%) was also obtained. From the above investigation, it is worth noting that the mixing of reagent and sample can be controlled by manipulating the conditions in the FI system to achieve a particular analytical purpose. Furthermore, the mixing is highly reproducible to achieve good precision of the signal. As the result, a mixing coil of length 100 cm was employed in the present system. No cleaning of the tubing for the removal of barium sulphate after each determination was needed. The effect of varying the length of mixing loop from 20 to 50 cm for mixing between sample in carrier stream and the buffer stream was found not significant on the FIA signal. To reduce the effect of dispersion, a length of 20 cm was selected. Moreover, the coil diameter used in the two mixing coils was 15 mm, which was sufficient large to reduce the effect of dispersion by radial diffusion (secondary flow in coiled tubing) [42]. When the sample injection volume was increased from 50 to 500 ␮L, the intensity of the signal was found to increase with higher injection volume from 50 to 200 ␮L. No significant increase in the peak height was observed when the volume was greater than 200 ␮L. At a higher sample volume (500 ␮L), the peak shapes were also found to be broadened. This may due

Fig. 5. The effect of cell constant of the ESPQC detector on the FIA signals. Flow rates of carrier, buffer and reagent: 0.25, 0.25 and 0.5 mL/min, respectively; length of mixing loop, 100 cm; sample volume, 200 ␮L; A, 20 ppm, B, 10 ppm and C, 5 ppm.

to the fact that more time is needed to clear the sample from the system. Moreover, the sample throughput was decreased from 50 sample/h to about 30 sample/h when the sample size was increased from 200 to 500 ␮L due to the increase in the residence time. Thus a sample volume of 200 ␮L was injected into the system to achieve a higher sensitivity without sacrificing the sampling frequency. According to Yao and co-workers [43], the effect of temperature of the electrolyte solution on the frequency shift of the ESPQC detector (frequency–temperature coefficient) is highly dependent on the specific conductivity of the solution. Within a given range of the solution conductivity (10−4 to 10−3 S/m), the effect of temperature was significant. Thus, the temperature of the detector cell was maintained at 22 ± 0.2 ◦ C to ensure high oscillation stability of the crystal and good repeatability of the signal. As the reaction rate between barium and sulphate in the mixing coil was sufficiently high under the present conditions at room temperature, the effect of temperature on the FIA signal was not investigated in the present study. Therefore, the mixing was allowed to proceed under room temperature condition without the need of temperature control. As the sensitivity of the ESPQC detector is affected by its cell constant, i.e. the electrode area (A) and the distance between the separated electrode and the quartz plate (1). The effect of varying the cell constant, by adjusting l, on the frequency response was investigated under constant FIA conditions and the results are shown in Fig. 5. The sensitivity was found to increase when Table 4 The optimized operational conditions for FIA determination of sulphate using ESPQC detector Carrier stream (C) Buffer stream (B) Reagent stream (R) Flow rates of C, B and R Length of mixing coil Sample volume Temperature of detector cell Cell constant of detectora a

Water 0.05% KCl, 10 mM acetic acid–1 mM sodium acetate 0.1% BaCl2 0.25, 0.25 and 0.5 mL/min, respectively 20 and 100 cm 200 ␮L 22 ± 0.2 ◦ C 1.41 cm

Ratio of electrode area to distance between electrodes.

Y.S. Fung et al. / Sensors and Actuators B 130 (2008) 551–560

557

the cell constant varied from 0.35 to 2.83 cm. The baseline noise, however, was also increased which affected the stability of the signals when the cell constant was large (2.83 cm). In addition, a poor repeatability of the signals was obtained due to the disturbance of the liquid flowing through the cell when the electrode separation was small. With consideration on both sensitivity and stability, a cell constant of 1.41 cm corresponding to the electrode separation of ca. 0.5 rnm was thus employed throughout the study. The optimized analytical conditions of the FIA system with detection by ESPQC sensor for the determination of sulphate in water are summarized in Table 4. 3.3. Analytical performance and application study Under the optimized conditions, an output of the FIA signals for the standard sulphate solutions is shown in Fig. 6. The peak height is linearly proportional to the concentration of SO4 2− in the range from 0.5 to 50 ppm with a correlation coefficient of 0.9991. The linear range was described by the following equation: F = 19.93 [SO4 2− ] + 1.12 The detection limit, based on a signal-to-ratio (S/N) of 2, was found to be 0.3 ppm, the lowest reported for automated FIA determination of sulphate based on direct barium sulphate reaction. The relative standard deviation (R.S.D.) obtained for five determinations of 10 ppm SO4 2− was 1.8%. The autosampling system was carefully adjusted so as to achieve the highest sampling frequency of the FIA system by controlling the vial pressure for withdrawing sample, the sample loop fill time and the run time. The effect of the loop fill time on the percentage carry-over of the analyte was studied by injecting 20 and 2 ppm SO4 2− alternatively with result shown in Fig. 7. Using a loop fill time of 5 s for a 200 ␮L sample under a vial pressure of 60 psi, a throughput of 50 samples/h was achieved. The analytical application of the system for real samples was evaluated for its selectivity and reliability by analyzing real water samples and water samples with interferents added. Using the present method for the determination of sulphate in water samples, the major potential interferents are co-existing cations and anions which may increase the specific conductivity of the

Fig. 7. The effect of loop fill time on the percentage carry-over of the analyte. Concentration of SO4 2− injected, 20 and 2 ppm alternatively; sample volume, 200 ␮L.

reagent after injection into the flowing streams, giving rise to negative errors. The interfering effect of cations commonly found in environmental water bodies was removed by passing the solution containing cations through a cation-exchange resin column (Naform) before injection into the system. The results, shown in Table 5, indicate that the percentage recovery for the injection of 200 ␮L of 20 ppm SO4 2− in the presence of divalent cations such as Ca2+ and Mg2+ was greatly improved after passing through the column and close to 100%. The small deviation from 100% recovery is probably due to the effect of the exchanged Na+ on the ionic property of the reagents, disturbing the effect of the buffer stream (0.05% KCl, 10 mM acetic acid and l mM sodium acetate) to maintain a constant ionic environment in the FIA system. The effect of anions was also investigated with results shown in Table 6. Interferents were identified by noting those with a change in the frequency signal of more than 3%. Thus, the tolerance limit of the anions studied was found to be less than 100 ppm. The reduction of the effect of anions was again attributed to the use of the buffer stream in the system intended to keep a constant value of the solution properties such as density, viscosity and conductivity. In the experiment, the influence of mass loading is negligible due to the use of a flow system. Table 5 The effect of cations on the recovery of FIA signal before and after passing through a cation-exchange resin column Cation

Fig. 6. The FIA peak profiles for the sulphate standards (concentrations in ppm are shown over the peaks).

Added as

Concentration (ppm)

Recovery (%) Before exchanger

After exchanger

Na+

NaCl

5 10

99.2 98.8

99 98.5

K+

K2 CO3

5 10

98.5 99.3

99 99.1

Mg2+

MgCl2 ·6H2 O

10 20

93.5 89.1

98.7 98.2

Ca2+

CaCl2

50 100

76 61.5

99.2 97.5

Note: Sample solution: 200 ␮L of 20 ppm SO4 2− + cations.

558

Y.S. Fung et al. / Sensors and Actuators B 130 (2008) 551–560

Table 6 The effect of anions on the percentage error of FIA signals Aniona

Concentration (ppm)

Frequency shiftb (Hz)

Errorc (%)

Cl−

50 100 200

412 407 302

−1.9 −3 −28.1

5 10

413 415

−1.7 −1.2

HCO3 −

50 100 200

415 410 307

−1.2 −2.3 −26.9

CO3 2−

50 100 200

422 408 285

0.48 −2.9 −32.1

NO3 −

Note: Sample solution: 200 ␮L 20 ppm SO4 2− + anions. a All anions were added as the sodium salts. b Mean of five measurements. c Percentage error was calculated relative to the mean frequency shift of 420 Hz due to the injection of 20 ppm SO4 2− alone.

Once the precipitation of BaSO4 has occurred due to the reaction between analytes and the reagent stream, the precipitates are simultaneously washed away from the detection cell by the incoming flow. Thus, the effect caused by mass loading on the electrodeless quartz surface does not affect the results. It should be note that the effect of co-existing ions are additive and does not dependent on the types of ions present because the major effect are due to the overall conductance of the sample, a sum of conductance of individual ions. Therefore, error may arise if the sample contains high concentrations of other dissolved ions in comparison with the SO4 2− concentration. To illustrate this, three natural mineral water samples with ions of known concentrations were obtained commercially and analyzed for their sulphate contents using the present method. One sample shows a significant difference of results from the labeled values as the sample contains very high concentrations of Ca2+ (78 ppm) and HCO3 − (357 ppm) ions as compared to the labeled sulphate content (10 ppm). The other two samples show agreeable results within statistical variation with the labeled values (the first sample labeled as 6.9 ppm versus measured value at 6.4 ± 0.2 ppm (S.D. for n = 5) and the second as 18 ppm versus 17.2 ± 0.3 ppm (S.D. for n = 5)) in the presence of Ca2+ (9.9 and 17.6 ppm) and HCO3 − (65.3 and 104 ppm), respecTable 7 Paraellel determination of sulphate in water samples using the FIA–ESPQC method and the standard turbidimetric method Water samplea

Concentration of SO4 2− (ppm)b found by FIA–ESPQC

DW1 DW2 DW3 PW1 PW2

13.92 13.85 14.20 19.77 19.94

± ± ± ± ±

0.21 0.20 0.28 0.30 0.31

Turbidimetry 14.01 14.02 14.27 20.02 20.04

± ± ± ± ±

0.25 0.27 0.28 0.32 0.34

a DW1–DW3, drinking water; PW1–PW2, pond water; samples collected on different days. b Values are given as mean ± standard deviation for n = 5.

tively. Thus, there are interference problems for samples with high ionic contents. In addition to the analysis of commercial mineral water samples with known concentration of sulphate, the method developed was applied to the determination of sulphate in drinking water and pond water. The results for the analysis of five samples are shown in Table 7. The reliability of the present method was established by comparing the results with those obtained by the standard manual turbidimetric method [5] as given in Table 8. By using the matched pair t-test method [44], the computed t value (2.50) was within the statistical value (2.78) for 4 d.f. It can be concluded that there is no statistically significant difference between the two methods for the determination of sulphate in natural water samples at the 95% confidence level. In summary, by applying the non-mass effect of ESPQC sensor for detection in FIA, a simple, rapid, sensitive and reliable method was developed for the determination of sulphate in water. 4. Conclusions A new automatic FIA system with detection by an electrodeseparated piezoelectric quartz crystal (ESPQC) was developed for the determination of sulphate in water. The three-channel FI system was consisted of a carrier stream of water, a buffer stream containing 0.05% KCl, 10 mM acetic acid and l mM sodium acetate, and a reagent stream of 0.1% BaCl2 . The method was based on the decrease in the specific conductivity of the reagent stream upon production of barium sulphate due to the mixing between the reagent and the sample. The change in the liquid property was indicated by the shift in the resonating frequency of the ESPQC sensor when the resulting solution had passed through the detector. The frequency shift of the flow-through ESPQC detector developed was found proportional to the change in specific conductivity of solution, with a sensitivity of about 15,000 Hz/mS cm−l . A small change in the conductivity can be detected by the crystal against a highly conducting background solution. The detector is characterized by its simple construction, high sensitivity, good stability, long lifetime and low capital and operational cost. Using the FIA system, the reaction between reagent and sample was controlled and optimized to allow proper mixing without causing precipitation of the barium sulphate particles. Under the optimized conditions, the linear range for the determination of sulphate was found to range from 0.5 to 50 ppm with a correlation coefficient of 0.9991. The detection limit was found to be 0.3 ppm based on a S/N ratio of 2, the lowest reported for automated FIA determination of sulphate based on direct barium sulphate reaction. The sample throughput was 50 samples/h. As viscosity, density and mass loading can influence the frequency shift of the PQC crystal, a reagent containing barium ion, acetic acid and potassium chloride was found to be suitable for the reaction between sulphate and barium as the physical properties of the solution such as density, viscosity and specific conductance could be kept constant during the reaction within the working range of the FIA procedure under which a significant reduction in solution conductivity had occurred at the ESPQC cell junction before the precipitation of BaCl2 .

Y.S. Fung et al. / Sensors and Actuators B 130 (2008) 551–560

Thus, the major interferents are due to samples with very high ionic content. This limits the application of the present method to sulphate determination in natural water with low ionic content. Interference due to co-existing ions could be reduced by using buffer stream which kept the properties of the solution containing other ions constant. The effect of excessive cations could be reduced by passing the sample solution through a cation-exchange resin column (Na-form) before injection. The reliability of the method was established statistically as no significant difference was observed at the 95% confidence level for results obtained using the present method and the standard turbidimetric method for the analysis of sulphate in drinking and pond water samples. In summary, the salient features of combining automatic FIA with piezoelectric detection based on the non-mass effect was shown to provide a simple, sensitive, rapid and reproducible detection method for determining ionic compounds in liquid medium with low ionic content.

[13]

[14]

[15]

[16]

[17]

[18]

[19]

Acknowledgements We would like to acknowledge the financial supports from the Seed Fund for Basic Research of the Hong Kong University Research and Conference Grants Committee, the Competitive Research Grants (HKU 7043/03P) from the Hong Kong Research Grants Council, and the Innovation and Technology Fund from the Hong Kong SAR Government.

[20]

[21]

[22]

References [1] J.W. Moore, Inorganic Contaminants of Surface Water: Research and Monitoring Priorities, Springer-Verlag, NY, 1992, pp. 266– 272. [2] J.O. Nriagu, J.D. Hem, Chemistry of pollutant sulfur in natural waters, in: J.O. Nriagu (Ed.), Sulfur in the Environment. Part II. Ecological impacts, Wiley, NY, USA, 1978, pp. 211–270. [3] Guidelines for Drinking-water Quality, 3rd ed. vol. 1. Recommendations, World Health Organization, Geneva, 2004. [4] M. Van Der Aa, Classification of mineral water types and comparison with drinking water standards, Environ. Geol. 44 (2003) 554–563. [5] N. Stambuk-Giljanovic, The quality of water resources in Dalmatia, Environ. Monit. Assess. 104 (2005) 235–268. [6] M. Shakirullah, I. Ahmad, K. Mehmood, A. Khan, H. Rehman, S. Alam, A.A. Shah, Physicochemical study of drinking water from Dir districts, J. Chem. Soc. Pak. 27 (2005) 374–387. [7] M. Rafi, F. Karim, Drinking water in the hilly region of Bangladesh: how is the quality? Int. J. Water Resour. Dev. 18 (2002) 439–452. [8] I.M. Kolthoff, E.J. Meehan, E.B. Sandell, S. Bruckenstein, Quantitative Chemical Analysis, 4th ed., Macmillan, NY, USA, 1969. [9] J.R. Rossum, P. Villarruz, Suggested methods for determination of sulfate in water, J. Am. Water Works Assoc. 53 (1961) 873. [10] J.A. Vieira, I.M. Raimundo, B.F. Reis, Turbidimentric determination of sulfate employing gravity flow-based systems, Anal. Chim. Acta 438 (2001) 75–81. [11] Y.G. Mourzina, Y.E. Ermolenko, T. Yoshinobu, Y. Vlasov, H. Iwasaki, M.J. Schoning, Anion-selective light-addressable potentiometer sensors (LAPS) for the determination of nitrate and sulfate ions, Sens. Actuators B 91 (2003) 32–38. [12] Q. Xu, C. Xu, Q.J. Wang, K. Tanaka, H. Toada, W. Zhang, L.T. Jin, Application of a single electrode, modified with polydiphenylamine and dodecyl sulfate, for the simultaneous amperometric determination of

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

559

electro-inactive anions and cations in ion chromatography, J. Chromatogr. A 997 (2003) 65–71. J.W. O’Reilly, G.W. Dicinoski, M.J. Shaw, P.R. Haddad, Chromatographic and electrophoretic separation of inorganic sulfur and sulfur–oxygen species, Anal. Chim. Acta 432 (2001) 165–192. Q. Xu, C. Xu, Y.P. Wang, W. Zhang, L.T. Jin, K. Tanaka, H. Haraguchi, A. Itoh, Simultaneous amperometric detection of electroinactive anions and cations in ion chromatography, Analyst 125 (2000) 1799– 1804. P. Bruno, M. Caselli, G. de Gennaro, B. De Tommaso, G. Lastella, S. Mastrolitti, Determination of nutrients in the presence of high chloride concentrations by column-switching ion chromatography, J. Chromatogr. A 1003 (2003) 133–141. P. Kosobucki, B. Buszewski, Application of isotachophoresis for quality control of drinking and mineral waters, J. Liq. Chromatogr. Relat. Technol. 29 (2006) 1951–1960. S. Kulka, G. Quintas, B. Lendl, Automated sample preparation and analysis using a sequential-injection-capillary electrophoresis (SI-CE) interface, Analyst 131 (2006) 729–744. W.P. Yang, B. O’Flaherty, A.L. Cholli, Fast analysis of water samples for detection of anions by capillary zone electrophoresis, J. Environ. Sci. Health Part A-Tox./Hazard. Subst. Environ. Eng. 36 (2001) 1271– 1285. P. Kuban, P. Kuban, V. Kuban, Flow injection-capillary electrophoresis system with contactless conductivity detection and hydrostatic pressure generated flow-application to the quantitative analysis of inorganic anions in water samples, Electrophoresis 24 (2003) 1935–1943. P.M. Crnkovic, A.O. Jacintho, Use of reagent in suspension in flow injection analysis spectrophotometric determination of sulphate in natural water, Quimica Nova 25 (2002) 254–258. I.P.A. Morais, M.R.S. Souto, T.I.M.S. Lopes, A.O.S.S. Rangel, Use of a single air segments to minimize dispersion and improve mixing in sequential injection: turbidimetric determination of sulphate in waters, Water Res. 37 (2003) 4243–4249. R. Buraham, K. Higuchi, M. Oshima, K. Grudpan, S. Motomizu, Flow injection spectrophotometry coupled with a crushed barium sulfate reactor column for the determination of sulfate ion in water samples, Talanta 64 (2004) 1147–1150. R.A.S. Lapa, J.L.F.C. Lima, I.V.O.S. Pinto, Simultaneous determination of nitrite, nitrate, sulphate and phenolic compounds, by sequential injection analysis, in wastewaters, Analysis 28 (2000) 295–301. R.A.S. Lapa, J.L.F.C. Lima, I.V.O.S. Pinto, Sequential injection analysis determination of sulphate in wastewaters by ultraviolet-spectrophotometry, J. Braz. Chem. Soc. 11 (2000) 170–174. I.P.A. Morais, A.O.S.S. Rangel, M.R.S. Souto, Determination of sulfate in natural and residual waters by turbidimetric flow-injection analysis, JAOAC Int. 84 (2001) 59–64. A.L. Lazrus, K.C. Hill, J.P. Lodge, Automated determination of sulfate in natural waters in Automation in Anal. Chem. Technicon Symposium, White Plains, NV, 1965, 1966, pp. 291–293. J. Ruˇziˇcka, E.H. Hansen, Flow injection analyses. Part I. A new concept of fast continuous flow analysis, Anal. Chim. Acta 78 (1975) 145– 157. U. Keihei, S. Fumio, H. Katsuya, Y. Ken’shi, Y. Isao, I. Daido, Flowinjection spectrophotometric determination of sulphate ion with a barium chloranilate–cellulose reaction column, Anal. Chim. Acta 261 (1992) 241–245. A. Sakuragawa, S. Nakayama, T. Okutani, Flow-injection spectrophotometric determination of micro amounts of sulfate ion in surface and sea water samples with a barium chromate reaction column, Anal. Sci. 10 (1994) 77–81. T. Nomura, Y. Ohno, Y. Takaji, Single drop method for determination of ions using an electrodeless piezoelectric quartz crystal, Anal. Chim. Acta 272 (1993) 187–192. W.H. Zhu, W.Z. Wei, L.H. Nie, S.Z. Yao, Piezoelectric quartz crystal with separated electrode for simultaneous determination of atropine sulfate and sodium chloride, Anal. Chim. Acta 282 (1993) 535– 541.

560

Y.S. Fung et al. / Sensors and Actuators B 130 (2008) 551–560

[32] D.Z. Shen, Q. Kang, J.T. Hu, Influence of oscillator phase on osciallation frequency and sensitivity of an electrode-separated piezoelectric sensor, Chinese J. Chem. 14 (1996) 235–243. [33] D.P. de Jesus, C.A. Neves, C.L. do Lago, Improving the sensitivity of electrode-separated piezoelectric quartz crystal sensor for copper (II) ions by immobilization of the N-2-aminoethyl-3-aminopropylsilane group, J. Braz. Chem. Soc. 12 (2001) 123–128. [34] Y. Mao, W.Z. Wei, J.Z. Zhang, S.F. Zhang, Interaction process between ionic surfactant and protein probed by series piezoelectric quartz crystal technique, J. Biochem. Biophys. Methods 52 (2002) 19–29. [35] M.H. Huang, D.Z. Shen, L.M. Chow, M.S. Yang, Impedance analysis of an electrode-separated piezoelectric sensor as a surface monitoring technique for surfactant adsorption on quartz surface, Analyst 127 (2002) 940– 946. [36] T. Nomura, A. Minemura, Behavior of a piezoelectric quartz crystal in an aqueous solution and the application to the determination of minute amounts of cyanide, Nippon Kagaku Kaishi 10 (1980) 1621– 1625. [37] K.K. Kanazawa, J.G. Gordan, The oscillation frequency of a quartz resonator in contact with liquid, Anal. Chim. Acta 175 (1985) 99– 105. [38] S. Bruckenstein, M. Shay, Experimental aspects of use of the quartz crystal microbalance in solution, Electrochim. Acta 30 (1985) 1295– 1300. [39] S.Z. Yao, T.A. Zhou, Dependence of the oscillation frequency of a piezoelectric crystal on the physical parameters of liquids, Anal. Chim. Acta 30 (1985) 1295–1300. [40] T. Nomura, Adsorption determination of sulfate in water with a quartz mass sensor, Bunseki Kagaku 36 (1987) 93–97. [41] T. Nomura, F. Tanaka, T. Yamada, Electrodeless piezoelectric quartz crystal and its behaviour in solutions, Anal. Chim. Acta 243 (1991) 273–278. [42] M. Valcarcel, M.D. Luque De Castro, Flow-Injection Analysis: Principles and Applications, Ellis Horwood Ltd., UK, 1987. [43] Y.J. Xu, D.Z. Shen, Q.J. Xie, L.H. Nie, S.Z. Yao, Frequency–temperature coefficient of an electrode-separated piezoelectric sensor in the liquid phase, J. Electroanal. Chem. 387 (1995) 23–28. [44] L. Bauer, A Statistical Manual for Chemist, 2nd ed., Academic Press, NY, USA, 1971, p. 65.

Biographies Y.S. Fung received his PhD degree from the Imperial College of Science, Technology and Medicine, University of London, United Kingdom in 1980. He has served as Chairman of the Hong Kong Chemical Society and the Hong Kong Air and Waste Management Association, and Vice President of the Hong Kong Association for the Advancement of Science and Technology. He had appointed as a guest professor for Changchun Institute of Applied Chemistry, Jilin University and Dongguan Institute of Technology, as well as academic advisor for various industrial and environmental groups and associations in Hong Kong. He is currently an associate professor in the Department of Chemistry, the University of Hong Kong. His research interests include: (1) the application of capillary electrophoresis (CE) and microchip CE for environmental, food and clinical investigation; (2) the development of chemical and biosensor in the analysis of environmental pollutants and bacteria using the quartz piezoelectric crystal detector, (3) the integration of chemometric approach and PQC array sensors for electronic tongue in quality control and taste assessment of beverage drinks. C.C.W. Wong received his PhD degree from the University of Hong Kong in 1999. He is currently working as a chemist in the Analytical Laboratory in Bureau Veritas (Hong Kong) Ltd. His research includes developing methods for analysis of environmental pollutants using piezoelectric quartz crystal (PQC) detectors operating in both gas and liquid phases, flow-injection analysis in couple with various PQC sensors for automation in chemical analysis and the application of carbon fibre ultramicroelectrode for studying the impact of heavy metals in biological systems. J.T.S. Choy received her BSc degree from the University of Hong Kong in 2002. She is currently a MPhil student in the University of Hong Kong under the supervision of Prof. Y.S. Fung. Her research interests include: (1) the application of piezoelectric crystal sensor array for electronic tongue application; and (2) the development of chemical and biosensor in the determination of environmental pollutants and bacteria using the quartz piezoelectric crystal detector. K.L. Sze received her BSc from the University of Hong Kong in 2001. She is currently a PhD student in the University of Hong Kong under the supervision of Prof. Y.S. Fung. Her research interests include: (1) application of particles sizer coupled with ICPAES for particle-bounded metal analysis and (2) trace element analysis in milk using capillary electrophoresis.