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Simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional ion-selective electrodes in series
Talanta 49 (1999) 1017 – 1022 www.elsevier.com/locate/talanta
Simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional ion-selective electrodes in series J.F. van Staden *, R.I. Stefan Department of Chemistry, Uni6ersity of Pretoria, Pretoria 0002, South Africa Received 26 October 1998; accepted 4 January 1999
Abstract An on-line automated system for the simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional calcium-selective and fluoride-selective membrane electrodes as sensors in series is described. Samples (30 ml) are injected into a TISAB II (pH = 5.50) carrier solution as an ionic strength adjustment buffer. The sample-buffer zone formed is first channeled to a fluoride-selective membrane electrode and then via the calcium-selective membrane electrode to the reference electrodes. The system is suitable for the simultaneous on-site monitoring of calcium (linear range 10 − 5 –10 − 2 mol l − 1 detection limit 1.94 × 10 − 6 mol l − 1 recovery 99.22%, RSD B 0.5%) and fluoride (linear range 10 − 5 –10 − 2 mol l − 1 detection limit 4.83 × 10 − 6 mol l − 1 recovery 98.63%, RSD = 0.3%) at a sampling rate of 60 samples h − 1. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Simultaneous detection; Calcium-selective membrane electrode; Fluoride-selective membrane electrode; Flow injection analysis
1. Introduction Since its conception in 1974 – 1975 flow injection analysis (FIA) has developed into a well established analytical technique [1 – 3]. According to the numerous publications to date, spectrophotometric determination still seems to be the most Presented at ICFIA’98, August 23–27, 1998, Seattle, WA, USA. * Corresponding author. Tel.: +27-12-4202515; fax: +2712-3625297. E-mail address: [email protected] (J.F. van Staden)
popular choice in FIA systems, but there has been an increase in the use of electrochemical detection in flowing streams. Various types of automatic or semi-automatic electrochemical sensors have been employed in continuous-flow measurements [1,2,4]. However, in most of the methods reported, the emphasis was placed on the determination of a single species in a sample. A survey of literature indicated a lack of procedures describing the simultaneous determination of more than one species in the same sample [5,6]. Although ICP or chromatographic methods offer the possibility for simultaneous determinations, the expen-
0039-9140/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 9 9 ) 0 0 0 6 0 - 0
1018
J.F. 6an Staden, R.I. Stefan / Talanta 49 (1999) 1017–1022
sive instrumentation (ICP) or laborious and time consuming (chromatography) procedures make these systems not very suitable for small routine laboratories and on-site field analysers. The use of ion-selective electrodes (ISE) in FIA on the other hand seems very attractive because of a number of advances [6], such as simple low cost instrumentation, little or no pretreatment of the sample [7], no interference from sample colour and turbidity [8,9], reliability, compatibility, capability, etc. [10]. Ru˚z' ic' ka et al. [11] used a cascade electrode arrangement to determine sodium and potassium simultaneously in blood serum and Mascini and Palleschi [12] glucose and urea in serum samples. Virtanen [13] developed a method for the simultaneous determination of potassium, sodium, calcium and chloride in serum by placing four cascade ion-selective electrodes sequentially in an FIA system with the reference electrode downstream. The same idea of electrodes in series was implemented by Hansen et al. [14] for the simultaneous determination of pH and calcium in serum. Van Staden [15] exploited the concept of flow-through tubular arrangements for the simultaneous determination of chloride and pH in a single sample. It has been recognised that one of the fundamental requirements for the successful performance of ion-selective electrode arrays in flow injection analysis is high selectivity coefficients with respect to the primary ions [16–18]. One of the advances associated with ion-selective electrode arrays in FIA is an enhanced selectivity [17,18] for the primary ion over interferents due to the hydrodynamic conditions of the sample plug at the detector, where the response is under kinetic rather than equilibrium control. These simultaneous detection systems are necessary in clinical analysis (e.g. for the determination of Na + , K + and Ca2 + in blood) [17,18] as well as in environmental analysis [19]. The miniaturization of these sensor arrays also made possible the simultaneous in vivo assay of inorganic cations and pH [20,21]. In South Africa, water is a very scarce commodity, and in some rural areas, where the people depend on borehole water, water quality should be controlled very carefully. There is therefore a need for a robust low-cost field analyser for the
on-site simultaneous monitoring of calcium and fluoride in natural and borehole water. Our laboratory was approach by the water authorities to adapt and combine the two single conventional automated segmented systems currently in use for the determination of calcium and fluoride with conventional electrodes into one robust low-cost ISE-FIA on-site field analyser capable of simultaneous determination of both components on a single sample. This present paper therefore describes a simultaneous detection system for calcium and fluoride ions in natural and borehole water, using a conventional calcium-selective membrane electrode in series with a conventional fluoride-selective membrane electrode in a flow injection system.
2. Experimental
2.1. Reagents and solutions All reagents were prepared from analytical reagent grade chemicals unless specified otherwise. Deionised water from a Modulab system (Continental Water Systems, San Antonio, TX) was used for dilution. The solutions were prepared as follows: TISAB II was used as carrier stream; 57 ml glacial acetic acid and 74 g reagent grade potassium chloride were carefully dissolved in 500 ml distilled water in a 1-l beaker. The pH was adjusted to 5.50 by adding 5 mol l − 1 KOH solution. The solution was quantitatively diluted to mark in a 1-l volumetric flask with distilled water. Standard working calibration solutions containing both calcium and fluoride ions were prepared by serial dilutions from standard 0.10 mol l − 1 CaCl2 and NaF stock solutions.
2.2. Apparatus Two ion-selective membrane electrodes: calcium-selective membrane electrode (Orion Research) Model 93-20 and fluoride-selective membrane electrode (Orion Research) Model 9409 were used for all measurements in combination with two double junction reference electrodes
J.F. 6an Staden, R.I. Stefan / Talanta 49 (1999) 1017–1022
(Orion Research) Model 90-02. The potentials were measured at room temperature using two Orion Research (Model 901) microprocessor Ionalyzers.
2.3. Flow injection system The electrodes were incorporated into the conduits of a flow injection system with basic design similar to that previously described [15,22]. A Carle microvolume two-position sampling valve (Carle No. 2014) containing two identical sample loops was used. Each loop has a volume of 30 ml. A Cenco peristaltic pump operating at 10 rev. min − 1 supplied the carrier streams to the manifold system. Tygon tubing (0.51 mm i.d.) was used to construct the manifold; coils were wound round suitable lengths of glass tubing (15 mm o.d.). The carrier solution containing the sample is first channeled to a fluoride-selective membrane electrode and then via the calcium-selective membrane electrode to the reference electrodes. A 60-s cycle sampling time was used, giving the system a capacity of about 60 samples h − 1. Data acquisition and device control were achieved using a PC30-B interface board (Eagle Electric, Cape Town, South Africa). The FlowTEK [23] software package (obtainable from MINTEK) for computer-aided flow analysis was used throughout for device control and data acquisition.
3. Results and discussion The main purpose of this investigation was to combine the two single automated segmented systems using two conventional calcium and fluoride electrodes currently in use by the water authorities into a single robust low-cost FIA-ISE on-site field analyser for the simultaneous determination of both components on a single sample as requested. As a result the incorporation and performance of the electrodes in array in an FIA system was investigated. The response characteristics and selectivity coefficients of both the fluoride- and calcium-selective electrodes incorporated in series into the
1019
conduits of the flow injection system were evaluated under optimum running conditions. Preliminary experiments indicated that the position of the fluoride- and calcium-selective electrodes in the series of arrays in the flow injection system is important and this was investigated. It was found that the best results were obtained when the fluoride-selective membrane electrode was place before the calcium-selective membrane electrode in the series. The analytical applicability of the proposed system was further evaluated on natural and borehole water.
3.1. Electrodes response The electrode characteristics of both electrodes incorporated into the FIA system are summarised in Table 1. The equations for the calibration graphs are: H= H o − 0.06 pCa and H= H o − 0.25 pF where H are the relative peak heights; pCa = − log[cCa2 + ] and pF= − log[cF − ]; with correlation coefficients of 0.9670 and 0.9787, respectively. It is clear from the results in Table 1 that with the linear response ranges between 10 − 5 and 10 − 2 Table 1 Response characteristics of the ion-selective membrane electrodes incorporated into the conduits of the flow injection analysis systema Parameter
Response Ca2+-selective mem- F−-selective membrane electrode brane electrode
Slope of calibration graph Intercept, H° Linear range
0.060 90.002
0.932 90.020 10−5–10−2 mol l−1 (approximately 0.4– 400 mg l−1) Detection limit 1.94×10−6 mol l−1 (about 0.078 mg l−1) a
0.250 90.002 1.323 9 0.070 10−5–10−2 mol l−1 (approximately 0.19– 190 mg l−1) 4.83×10−6 mol l−1 (about 0.092 mg l−1)
All measurements are the average of 10 determinations.
J.F. 6an Staden, R.I. Stefan / Talanta 49 (1999) 1017–1022
1020
Table 2 Selectivity coefficients of the calcium- and fluoride-selective membrane electrodes incorporated into the FIA systema Electrode
Ca2+−selective F−-selective a
Interfering species Na+
Mg2+
Ca2+
F−
Cl−
Br−
CO2− 3
1.2×10−3 –
6.0×10−3 –
– BB10−5
BB10−4 –
– 1.4×10−5
– 4.0×10−5
– 1.0×10−5
All measurements are the average of seven determinations.
mol l − 1 (approximately 0.4 – 400 mg l − 1 Ca2 + ) and 10 − 5 – 10 − 2 mol l − 1 (approximately 0.19– 190 mg l − 1 F − ), the very low detection limits of 1.94× 10 − 6 mol l − 1 (about 0.078 mg l − 1 Ca2 + ) and 4.83 × 10 − 6 mol l − 1 (about 0.092 mg l − 1 F − ), the very high precision obtained and stability of the ISE-FIA system with time for both electrodes, the proposed on-line ISE-FIA system seems to be ideally suitable as a reliable and effective low-cost robust field analyser.
3.2. Selecti6ity of the electrodes One of the main requirements for electrodes in series is that for ideal conditions no mutual interferences should occur and that the electrodes should have high selectivity coefficients with respect to the primary ions [16 – 18]. The effect of some possible mutual interferences on the response of both electrodes as well as possible interferences from other anions and cations were studied with the proposed ISE-FIA system. The selectivity coefficients presented in Table 2 shows that sodium, magnesium and fluoride do not interfere in the response of the calcium-selective membrane electrode even when the ratio between the calcium and interfering ions (mol l − 1) was 1:100 (1:102 for fluoride) and that the response of the fluoride-selective membrane electrode was not affected by the presence of Br − , Cl − and CO23 − ions even when the ratio between the concentration of fluoride and interfering ions (mol l − 1) was 1:1000 (1:103 for calcium). This fact demonstrates the specificity of the two electrodes for the primary ions and that the electrodes could be used in series.
3.3. Analytical applications The response characteristics as well as the selectivity of both ion-selective membrane electrodes incorporated in series into the conduits of the FIA system ensure a reliable simultaneous detection of calcium and fluoride ions in natural and borehole water without any sample preparation. The accuracy of the proposed ISE-FIA system was tested by comparing the results of a number of natural and borehole water samples with those obtained by a standard automated segmented method. Results compare favourably as can be seen from Table 3. The RSD (%) for water samples having different concentrations of calcium and fluoride was less than 0.5 and 0.3% for calcium and fluoride respectively on 10 tests of each sample. Analyses were also performed in a random order to test carry-over effects. Carry-over from one sample to another is negligible at a sample throughput of 60 samples h − 1. The average recoveries of calcium and fluoride in natural and borehole water samples are 99.22 and 98.63%, respectively.
4. Conclusion The proposed ISE-FIA system using a conventional calcium-selective membrane electrode and a conventional fluoride-selective membrane electrode as detectors in series is suitable for the simultaneous on-line monitoring of calcium and fluoride at a rate of approximately 60 actual water samples h − 1. The very high selectivity coefficients obtained for both ion-selective electrodes with respect to their primary ions coupled with their
Sample
Automated segmented method [Ca2+] (mg)
Proposed ISE-FIA system [Ca2+] (mg)
RSD (%)
Automated segmented method [F−] (mg)
Proposed ISE-FIA system [F−] (mg)
RSD (%)
1 2 3 4 5
8.4 60.1 61.9 51.5 5.0
8.0 59.6 55.2 49.2 6.1
0.2 0.1 0.1 0.3 0.5
0.123 0.133 0.338 0.311 0.136
0.112 0.130 0.315 0.294 0.136
0.3 0.2 0.2 0.1 0.1
a
Mean result of 10 tests in each case, with relative standard deviation for the proposed ISE-FIA system.
J.F. 6an Staden, R.I. Stefan / Talanta 49 (1999) 1017–1022
Table 3 Performance and reproducibility of the proposed ISE-FIA system for the simultaneous determination of calcium and fluoride ions in natural and borehole watera
1021
1022
J.F. 6an Staden, R.I. Stefan / Talanta 49 (1999) 1017–1022
response characteristics contributed to the successful performance of both sensors in series in the FIA system. As a low-cost, low maintenance (especially with the low cost calcium- and fluoride-selective electrodes as detectors) reliable analyser, the system satisfies the requirements given to us by the water authorities and can be particularly attractive for on-site water laboratories where natural and borehole water streams could be monitored continuously.
References [1] M. Valca´rcel, M.D.L. de Castro, Flow-injection Analysis. Principles and Applications, Ellis Horwood, Chichester, UK, 1987. [2] J. Ru˚z' ic' ka, E.H. Hansen, Flow Injection Analysis, 2nd edn, Wiley, New York, 1988. [3] J.L. Burguera, Flow Injection Atomic Spectroscopy, Marcel Dekker, New York, 1989. [4] K. S& tulik, V. Paca´kova´, Electroanalytical Measurements in Flowing Liquids, Ellis Horwood, Chichester, UK, 1987. [5] M.D. Luque de Castro, M. Valca´rcel, Analyst 109 (1984) 413. [6] A. Izquierdo, M.D. Luque de Castro, Electroanalysis 7 (1995) 505.
.
[7] M.E. Meyerhoff, P.M. Kovach, J. Chem. Educ. 60 (1983) 766. [8] M.E. Meyerhoff, Y.M. Fraticelli, Anal. Lett. 14 (1981) 415. [9] E. Pungor, Z. Feher, G. Nagy, K. Toth, G. Horvai, M. Gratzl, Anal. Chim. Acta 109 (1979) 1. [10] J.F. van Staden, Anal. Chim. Acta 179 (1986) 407. [11] J. Ru˚z' ic' ka, E.H. Hansen, E.A. Zagatto, Anal. Chim. Acta 88 (1977) 1. [12] M. Mascini, G. Palleschi, Anal. Chim. Acta 145 (1983) 213. [13] R. Virtanen, in: E. Pungor (Ed.), Ion-selective Electrodes 3. Proceedings of the Third Symposium, Matrafu¨red, Hungary, October 1980; Analytical Symposium Series, Volume 8, Elsevier, Amsterdam, 1981, p. 375. [14] E.H. Hansen, J. Ru˚z' ic' ka, A.K. Ghose, Anal. Chim. Acta 100 (1980) 151. [15] J.F. van Staden, Analyst 111 (1986) 1231. [16] Y.G. Vlasov, Ann. Chim. 87 (1997) 261. [17] D. Diamond, R.J. Forster, Anal. Chim. Acta 276 (1993) 75. [18] R.J. Forster, F. Regan, D. Diamond, Anal. Chem. 63 (1991) 876. [19] Y.G. Vlasov, A.V. Legin, A.M. Rudnitskaya, A. Damico, C. Dinatale, J. Anal. Chem. 52 (1997) 1087. [20] L. Ratton, T. Kunt, T. Mcavoy, T. Fuja, R. Cavicchi, S. Semancik, Sensors Actuators B 41 (1997) 105. [21] V.V. Cosofret, M. Erdo¨sy, A.J. Timothy, R.P. Buck, R. Bruce Ash, M.R. Neuman, Anal. Chem. 67 (1995) 1647. [22] J.F. van Staden, Analyst 112 (1987) 595. [23] G.D. Marshall, J.F. van Staden, Anal. Instrum. 20 (1992) 79.
Simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional ion-selective electrodes in series J.F. van Staden *, R.I. Stefan Department of Chemistry, Uni6ersity of Pretoria, Pretoria 0002, South Africa Received 26 October 1998; accepted 4 January 1999
Abstract An on-line automated system for the simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional calcium-selective and fluoride-selective membrane electrodes as sensors in series is described. Samples (30 ml) are injected into a TISAB II (pH = 5.50) carrier solution as an ionic strength adjustment buffer. The sample-buffer zone formed is first channeled to a fluoride-selective membrane electrode and then via the calcium-selective membrane electrode to the reference electrodes. The system is suitable for the simultaneous on-site monitoring of calcium (linear range 10 − 5 –10 − 2 mol l − 1 detection limit 1.94 × 10 − 6 mol l − 1 recovery 99.22%, RSD B 0.5%) and fluoride (linear range 10 − 5 –10 − 2 mol l − 1 detection limit 4.83 × 10 − 6 mol l − 1 recovery 98.63%, RSD = 0.3%) at a sampling rate of 60 samples h − 1. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Simultaneous detection; Calcium-selective membrane electrode; Fluoride-selective membrane electrode; Flow injection analysis
1. Introduction Since its conception in 1974 – 1975 flow injection analysis (FIA) has developed into a well established analytical technique [1 – 3]. According to the numerous publications to date, spectrophotometric determination still seems to be the most Presented at ICFIA’98, August 23–27, 1998, Seattle, WA, USA. * Corresponding author. Tel.: +27-12-4202515; fax: +2712-3625297. E-mail address: [email protected] (J.F. van Staden)
popular choice in FIA systems, but there has been an increase in the use of electrochemical detection in flowing streams. Various types of automatic or semi-automatic electrochemical sensors have been employed in continuous-flow measurements [1,2,4]. However, in most of the methods reported, the emphasis was placed on the determination of a single species in a sample. A survey of literature indicated a lack of procedures describing the simultaneous determination of more than one species in the same sample [5,6]. Although ICP or chromatographic methods offer the possibility for simultaneous determinations, the expen-
0039-9140/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 9 9 ) 0 0 0 6 0 - 0
1018
J.F. 6an Staden, R.I. Stefan / Talanta 49 (1999) 1017–1022
sive instrumentation (ICP) or laborious and time consuming (chromatography) procedures make these systems not very suitable for small routine laboratories and on-site field analysers. The use of ion-selective electrodes (ISE) in FIA on the other hand seems very attractive because of a number of advances [6], such as simple low cost instrumentation, little or no pretreatment of the sample [7], no interference from sample colour and turbidity [8,9], reliability, compatibility, capability, etc. [10]. Ru˚z' ic' ka et al. [11] used a cascade electrode arrangement to determine sodium and potassium simultaneously in blood serum and Mascini and Palleschi [12] glucose and urea in serum samples. Virtanen [13] developed a method for the simultaneous determination of potassium, sodium, calcium and chloride in serum by placing four cascade ion-selective electrodes sequentially in an FIA system with the reference electrode downstream. The same idea of electrodes in series was implemented by Hansen et al. [14] for the simultaneous determination of pH and calcium in serum. Van Staden [15] exploited the concept of flow-through tubular arrangements for the simultaneous determination of chloride and pH in a single sample. It has been recognised that one of the fundamental requirements for the successful performance of ion-selective electrode arrays in flow injection analysis is high selectivity coefficients with respect to the primary ions [16–18]. One of the advances associated with ion-selective electrode arrays in FIA is an enhanced selectivity [17,18] for the primary ion over interferents due to the hydrodynamic conditions of the sample plug at the detector, where the response is under kinetic rather than equilibrium control. These simultaneous detection systems are necessary in clinical analysis (e.g. for the determination of Na + , K + and Ca2 + in blood) [17,18] as well as in environmental analysis [19]. The miniaturization of these sensor arrays also made possible the simultaneous in vivo assay of inorganic cations and pH [20,21]. In South Africa, water is a very scarce commodity, and in some rural areas, where the people depend on borehole water, water quality should be controlled very carefully. There is therefore a need for a robust low-cost field analyser for the
on-site simultaneous monitoring of calcium and fluoride in natural and borehole water. Our laboratory was approach by the water authorities to adapt and combine the two single conventional automated segmented systems currently in use for the determination of calcium and fluoride with conventional electrodes into one robust low-cost ISE-FIA on-site field analyser capable of simultaneous determination of both components on a single sample. This present paper therefore describes a simultaneous detection system for calcium and fluoride ions in natural and borehole water, using a conventional calcium-selective membrane electrode in series with a conventional fluoride-selective membrane electrode in a flow injection system.
2. Experimental
2.1. Reagents and solutions All reagents were prepared from analytical reagent grade chemicals unless specified otherwise. Deionised water from a Modulab system (Continental Water Systems, San Antonio, TX) was used for dilution. The solutions were prepared as follows: TISAB II was used as carrier stream; 57 ml glacial acetic acid and 74 g reagent grade potassium chloride were carefully dissolved in 500 ml distilled water in a 1-l beaker. The pH was adjusted to 5.50 by adding 5 mol l − 1 KOH solution. The solution was quantitatively diluted to mark in a 1-l volumetric flask with distilled water. Standard working calibration solutions containing both calcium and fluoride ions were prepared by serial dilutions from standard 0.10 mol l − 1 CaCl2 and NaF stock solutions.
2.2. Apparatus Two ion-selective membrane electrodes: calcium-selective membrane electrode (Orion Research) Model 93-20 and fluoride-selective membrane electrode (Orion Research) Model 9409 were used for all measurements in combination with two double junction reference electrodes
J.F. 6an Staden, R.I. Stefan / Talanta 49 (1999) 1017–1022
(Orion Research) Model 90-02. The potentials were measured at room temperature using two Orion Research (Model 901) microprocessor Ionalyzers.
2.3. Flow injection system The electrodes were incorporated into the conduits of a flow injection system with basic design similar to that previously described [15,22]. A Carle microvolume two-position sampling valve (Carle No. 2014) containing two identical sample loops was used. Each loop has a volume of 30 ml. A Cenco peristaltic pump operating at 10 rev. min − 1 supplied the carrier streams to the manifold system. Tygon tubing (0.51 mm i.d.) was used to construct the manifold; coils were wound round suitable lengths of glass tubing (15 mm o.d.). The carrier solution containing the sample is first channeled to a fluoride-selective membrane electrode and then via the calcium-selective membrane electrode to the reference electrodes. A 60-s cycle sampling time was used, giving the system a capacity of about 60 samples h − 1. Data acquisition and device control were achieved using a PC30-B interface board (Eagle Electric, Cape Town, South Africa). The FlowTEK [23] software package (obtainable from MINTEK) for computer-aided flow analysis was used throughout for device control and data acquisition.
3. Results and discussion The main purpose of this investigation was to combine the two single automated segmented systems using two conventional calcium and fluoride electrodes currently in use by the water authorities into a single robust low-cost FIA-ISE on-site field analyser for the simultaneous determination of both components on a single sample as requested. As a result the incorporation and performance of the electrodes in array in an FIA system was investigated. The response characteristics and selectivity coefficients of both the fluoride- and calcium-selective electrodes incorporated in series into the
1019
conduits of the flow injection system were evaluated under optimum running conditions. Preliminary experiments indicated that the position of the fluoride- and calcium-selective electrodes in the series of arrays in the flow injection system is important and this was investigated. It was found that the best results were obtained when the fluoride-selective membrane electrode was place before the calcium-selective membrane electrode in the series. The analytical applicability of the proposed system was further evaluated on natural and borehole water.
3.1. Electrodes response The electrode characteristics of both electrodes incorporated into the FIA system are summarised in Table 1. The equations for the calibration graphs are: H= H o − 0.06 pCa and H= H o − 0.25 pF where H are the relative peak heights; pCa = − log[cCa2 + ] and pF= − log[cF − ]; with correlation coefficients of 0.9670 and 0.9787, respectively. It is clear from the results in Table 1 that with the linear response ranges between 10 − 5 and 10 − 2 Table 1 Response characteristics of the ion-selective membrane electrodes incorporated into the conduits of the flow injection analysis systema Parameter
Response Ca2+-selective mem- F−-selective membrane electrode brane electrode
Slope of calibration graph Intercept, H° Linear range
0.060 90.002
0.932 90.020 10−5–10−2 mol l−1 (approximately 0.4– 400 mg l−1) Detection limit 1.94×10−6 mol l−1 (about 0.078 mg l−1) a
0.250 90.002 1.323 9 0.070 10−5–10−2 mol l−1 (approximately 0.19– 190 mg l−1) 4.83×10−6 mol l−1 (about 0.092 mg l−1)
All measurements are the average of 10 determinations.
J.F. 6an Staden, R.I. Stefan / Talanta 49 (1999) 1017–1022
1020
Table 2 Selectivity coefficients of the calcium- and fluoride-selective membrane electrodes incorporated into the FIA systema Electrode
Ca2+−selective F−-selective a
Interfering species Na+
Mg2+
Ca2+
F−
Cl−
Br−
CO2− 3
1.2×10−3 –
6.0×10−3 –
– BB10−5
BB10−4 –
– 1.4×10−5
– 4.0×10−5
– 1.0×10−5
All measurements are the average of seven determinations.
mol l − 1 (approximately 0.4 – 400 mg l − 1 Ca2 + ) and 10 − 5 – 10 − 2 mol l − 1 (approximately 0.19– 190 mg l − 1 F − ), the very low detection limits of 1.94× 10 − 6 mol l − 1 (about 0.078 mg l − 1 Ca2 + ) and 4.83 × 10 − 6 mol l − 1 (about 0.092 mg l − 1 F − ), the very high precision obtained and stability of the ISE-FIA system with time for both electrodes, the proposed on-line ISE-FIA system seems to be ideally suitable as a reliable and effective low-cost robust field analyser.
3.2. Selecti6ity of the electrodes One of the main requirements for electrodes in series is that for ideal conditions no mutual interferences should occur and that the electrodes should have high selectivity coefficients with respect to the primary ions [16 – 18]. The effect of some possible mutual interferences on the response of both electrodes as well as possible interferences from other anions and cations were studied with the proposed ISE-FIA system. The selectivity coefficients presented in Table 2 shows that sodium, magnesium and fluoride do not interfere in the response of the calcium-selective membrane electrode even when the ratio between the calcium and interfering ions (mol l − 1) was 1:100 (1:102 for fluoride) and that the response of the fluoride-selective membrane electrode was not affected by the presence of Br − , Cl − and CO23 − ions even when the ratio between the concentration of fluoride and interfering ions (mol l − 1) was 1:1000 (1:103 for calcium). This fact demonstrates the specificity of the two electrodes for the primary ions and that the electrodes could be used in series.
3.3. Analytical applications The response characteristics as well as the selectivity of both ion-selective membrane electrodes incorporated in series into the conduits of the FIA system ensure a reliable simultaneous detection of calcium and fluoride ions in natural and borehole water without any sample preparation. The accuracy of the proposed ISE-FIA system was tested by comparing the results of a number of natural and borehole water samples with those obtained by a standard automated segmented method. Results compare favourably as can be seen from Table 3. The RSD (%) for water samples having different concentrations of calcium and fluoride was less than 0.5 and 0.3% for calcium and fluoride respectively on 10 tests of each sample. Analyses were also performed in a random order to test carry-over effects. Carry-over from one sample to another is negligible at a sample throughput of 60 samples h − 1. The average recoveries of calcium and fluoride in natural and borehole water samples are 99.22 and 98.63%, respectively.
4. Conclusion The proposed ISE-FIA system using a conventional calcium-selective membrane electrode and a conventional fluoride-selective membrane electrode as detectors in series is suitable for the simultaneous on-line monitoring of calcium and fluoride at a rate of approximately 60 actual water samples h − 1. The very high selectivity coefficients obtained for both ion-selective electrodes with respect to their primary ions coupled with their
Sample
Automated segmented method [Ca2+] (mg)
Proposed ISE-FIA system [Ca2+] (mg)
RSD (%)
Automated segmented method [F−] (mg)
Proposed ISE-FIA system [F−] (mg)
RSD (%)
1 2 3 4 5
8.4 60.1 61.9 51.5 5.0
8.0 59.6 55.2 49.2 6.1
0.2 0.1 0.1 0.3 0.5
0.123 0.133 0.338 0.311 0.136
0.112 0.130 0.315 0.294 0.136
0.3 0.2 0.2 0.1 0.1
a
Mean result of 10 tests in each case, with relative standard deviation for the proposed ISE-FIA system.
J.F. 6an Staden, R.I. Stefan / Talanta 49 (1999) 1017–1022
Table 3 Performance and reproducibility of the proposed ISE-FIA system for the simultaneous determination of calcium and fluoride ions in natural and borehole watera
1021
1022
J.F. 6an Staden, R.I. Stefan / Talanta 49 (1999) 1017–1022
response characteristics contributed to the successful performance of both sensors in series in the FIA system. As a low-cost, low maintenance (especially with the low cost calcium- and fluoride-selective electrodes as detectors) reliable analyser, the system satisfies the requirements given to us by the water authorities and can be particularly attractive for on-site water laboratories where natural and borehole water streams could be monitored continuously.
References [1] M. Valca´rcel, M.D.L. de Castro, Flow-injection Analysis. Principles and Applications, Ellis Horwood, Chichester, UK, 1987. [2] J. Ru˚z' ic' ka, E.H. Hansen, Flow Injection Analysis, 2nd edn, Wiley, New York, 1988. [3] J.L. Burguera, Flow Injection Atomic Spectroscopy, Marcel Dekker, New York, 1989. [4] K. S& tulik, V. Paca´kova´, Electroanalytical Measurements in Flowing Liquids, Ellis Horwood, Chichester, UK, 1987. [5] M.D. Luque de Castro, M. Valca´rcel, Analyst 109 (1984) 413. [6] A. Izquierdo, M.D. Luque de Castro, Electroanalysis 7 (1995) 505.
.
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