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Flow injection potentiometry of primary and interfering ion with a gold complex ion-exchange membrane
Talanta 55 (2001) 201– 207 www.elsevier.com/locate/talanta
Flow injection potentiometry of primary and interfering ion with a gold complex ion-exchange membrane Concepcio´n Sa´nchez-Pedren˜o, Joaquı´n A. Ortun˜o, Jorge Herna´ndez Department of Analytical Chemistry, Faculty of Chemistry, Uni6ersity of Murcia, 30071 Murcia, Spain Received 4 January 2001; received in revised form 20 April 2001; accepted 20 April 2001
Abstract A gold(I) organic complex in the perchlorate form was incorporated in plasticized poly(vinyl chloride) (PVC) membranes and the potentiometric response towards perchlorate (primary ion) and permanganate (interfering ion) was studied. Membranes with 2:1 and 1:2 (w/w) plasticizer to PVC ratio were selected for the determination of primary and interfering ions, respectively. In the selected flow injection conditions, a linear relationship between peak height and log c were obtained between 1 × 10 − 2 and 2× 10 − 5 mol l − 1 perchlorate and 1 × 10 − 3 –1× 10 − 5 mol l − 1 permanganate. Good reproducibilities and excellent selectivity coefficients towards many common ions were obtained. The methods proposed were applied satisfactorily to the determination of perchlorate in water and of permanganate in pharmaceutical preparations. Permanganate can be directly determined even in the presence of a high amount of manganese dioxide. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ion-selective electrodes; Flow-injection analysis; Perchlorate; Permanganate
1. Introduction The use of flow injection methodology to extend the capabilities of ion-selective electrodes has been reviewed [1]. Compared to manual potentiometry, flow injection potentiometry (FIP) has several advantages including the reproducibility of the analytical signal and the reproducible use of small sample volumes. Several perchlorate-selective electrodes using liquid [2,3] or plasticized PVC [4 – 6] membranes, * Corresponding author. Tel.: + 34-968-367400; fax: +34968-364148. E-mail address: [email protected] (C. Sa´nchez-Pedren˜o).
or based on carbon paste electrodes [7] have been reported. An FIP device using a membranecoated rod ion-selective electrode has been proposed for the determination of perchlorate [8]. With regard to permanganate, the perchlorate-selective electrode described in Ref. [2] was also used for the determination of interfering permanganate ion. An interpretation of the enhanced potential response of liquid ion-exchange membrane electrodes towards interfering ions has been reported [9]. FIP offers a convenient way to exploit this phenomenon to determine the interfering ion. In the present paper, FIP methods for the determination of perchlorate (primary ion) and
0039-9140/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 1 ) 0 0 4 1 8 - 0
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C. Sa´nchez-Pedren˜o et al. / Talanta 55 (2001) 201–207
permanganate (interfering ion) using gold complex ion-exchange membranes are proposed and applied to the analysis of water and pharmaceuticals.
model was refined by constructing the cell from a sole Perspex block incorporating a circular 2-mm thick silicone gasket, in the centre of which a 7-mm diameter hole had been made.
2.2. Reagents 2. Experimental
2.1. Apparatus Potentials were measured with an Orion 960 Autochemistry System (Boston, MA), whose recorder output was connected to a personal computer via a DGH Corporation 1121 module analogue-to-digital converter (Manchester, UK). A Fluka (Buchs, Switzerland) glass ring of 28-mm inner diameter and 30-mm height, a glass plate, vial, punch and electrode body ISE, were used for the membrane construction and mounting. An Orion 90-02 double junction silver– silver chloride reference electrode containing 10% ammonium nitrate solution in the outer compartment was used. A Gilson (Villiers le Bel, France) Minipuls 3 peristaltic pump, Omnifit (Cambridge, UK) injection valve, connecting tubing of 0.5 mm bore, PTFE tubing and various end fittings and connectors were used to construct the flow injection system. A flow-cell (Fig. 1) similar to that described previously [10] was used although the new
The Bis[tri-(p-metoxyphenyl) phosphine] gold(I) perchlorate was obtained as described before [6]. Poly(vinyl chloride) (PVC) high molecular mass, 2-nitrophenyl octyl ether (NPOE) and tetrahydrofuran (THF) were Selectophore products from Fluka. All other reagents used were of analytical reagent grade and doubly distilled water was used throughout. A concentration of 0.1 mol l − 1 sodium chloride, sodium sulphate or hydrochloric acid were used as carrier solutions. A 1 mol l − 1 standard perchlorate solution was prepared from sodium perchlorate monohydrate (Fluka) and standardised gravimetrically following the procedure described before [11] with 1,2,4,6-tetraphenylpyridinium acetate prepared accordingly. A 1 mol l − 1 standard permanganate solution was prepared from potassium permanganate (Merck) and standardised with sodium oxalate (Merck). Working standard solutions were prepared by dilution with the corresponding carrier solution.
2.3. Preparation of the membranes The membranes were prepared by dissolving 3 mg gold(I) complex, 200, 150 or 100 mg NPOE and 100, 150 or 200 mg PVC for membranes A, B and C, respectively, in 3 ml of THF. This solution was poured into the glass ring resting on the glass plate and was left overnight to allow the solvent to evaporate slowly. A 7-mm diameter piece was cut out with the punch and incorporated into the electrode body containing 0.1 mol l − 1 sodium chloride and 0.01 mol l − 1 sodium perchlorate saturated with excess of AgCl as internal filling solution.
2.4. Flow injection procedures Fig. 1. Flow-through cell design: (1) ISE; (2) Perspex block; (3) Gasket; and (4) PTFE tubes.
A one-channel flow injection assembly, similar to that described before [10], was used. The corre-
C. Sa´ nchez-Pedren˜ o et al. / Talanta 55 (2001) 201–207
sponding ion-selective electrode was incorporated in a flow injection system by means of the flowcell mentioned above. The distance between the injection valve and the cell was 20 cm. The carrier solution was pumped at a flow rate of 1 ml min − 1 and 500 ml aliquots of sample solutions were injected. Potentials were monitored and the peak heights were determined. Working standard solutions (10 − 6 – 10 − 2 mol l − 1) were used to obtain the corresponding calibration graphs. For the determination of perchlorate in waters, 2 ml of 1.25 mol l − 1 hydrochloric acid solution was added to 20 ml of sample and the solution was diluted to 25 ml with distilled water. Aliquots of this solution were injected into a 0.1 mol l − 1 hydrochloric acid carrier solution and the perchlorate concentration was determined using the ISE containing membrane A, as described above. For the determination of permanganate in pharmaceuticals, 2 ml of 1.25 mol l − 1 sodium sulphate solution was added to a suitable volume (0.25–2.0 ml) of sample and the solution was diluted to 25 ml with distilled water. Aliquots of this solution were injected into a 0.1 mol l − 1 sodium sulphate carrier solution and the permanganate concentration was determined, using the ISE containing membrane C, as described above.
3. Results and discussion
3.1. Influence of membrane composition One of the most important parameters affecting sensitivity in the combination of FIA with ISEs, is the dynamic response of the membrane, which is dependent on the membrane composition. Three polymeric membranes containing different NPOE/PVC ratios, (2/1, 1/1 and 1/2; membranes A, B and C, respectively), were tested. The corresponding calibration graphs of the three membranes towards perchlorate and permanganate, obtained by injecting 500 ml of sample at a flow rate 1.0 ml min − 1, are shown in Fig. 2. The transient signals obtained for 10 − 4 mol l − 1 of the corresponding analyte are also included in each graph. With regard to perchlorate (primary ion; Fig. 2(a)), the potential response decreased as the
203
Fig. 2. Calibration graphs and transient signals for (a) perchlorate and (b) permanganate with ( ) membrane A; () membrane B; and () membrane C. The transient signals correspond to a concentration of 10 − 4 mol l − 1.
plasticizer to PVC ratio of the membrane composition decreased, perhaps due to the different speed of response of the membranes. Their response rates were obtained from the corresponding derivative transient signals. The maximum response rates obtained for 10 − 4 mol l − 1 perchlorate were 5.4, 4.2 and 1.7 mV s − 1 for membranes A, B and C, respectively. Membrane A was selected for the FIP determination of perchlorate. With regard to permanganate (interfering ion), the potential responses obtained with the three membranes (Fig. 2(b)) were higher than the corresponding response obtained for perchlorate, for concentrations 10 − 4 mol l − 1 or higher. This is in accordance with the permanganate selectivity coefficient KClO−,MnO− values of higher than 1, 4 4 which have been reported for different perchlorate-selective electrodes. In addition, the three calibration graphs obtained showed a super-Nernstian potential response, as predicted from theory [9]. As regards the shape of the transient signals, they showed non-monotonic behaviour as they returned to the base line, as a result of the permanganate entering the membrane through ionexchange during exposure to the sample. The extent of the non-monotonic part of the signals decreased with decreasing plasticizer-to-PVC ratios in the membrane composition. This can be explained by the drastic reduction observed in the
204
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ion mobilities, which hindered penetration of the ions into the bulk of the membrane. This represents a practical advantage since the time needed for the electrode to be restored is considerably decreased, thus increasing sample throughput. The magnitude of the analytical signal measured (peak height) was only very slightly influenced by membrane composition. Membrane C was selected for permanganate determination.
3.2. Influence of flow injection-6ariables and pH The influence of the injected sample volume on the determination of perchlorate and permanganate with ion-selective electrodes based on membranes A and C respectively was studied at a flow rate of 1 ml min − 1. An increase in sample volume produced an increase in peak height, which reached an almost constant value at 300 ml for perchlorate and 400 ml for permanganate. A sample volume of 500 ml was used for both ions in further studies. The influence of the flow rate on the determination of perchlorate and permanganate with the respective membranes was studied in the range 0.25–1.5 ml min − 1. The results are shown in Fig. 3. As can be seen, a slight increase in peak height was obtained by raising the flow rate in the case
of both anions. On the other hand, the peak width decreased when the flow rate was increased, thus improving sample throughput. However, the reproducibility of the signal diminished at the highest flow rates assayed. Taking all this into account, a flow rate of 1 ml min − 1 was selected. The influence of pH on the determination of perchlorate and permanganate was studied by adjusting the pH of the carrier solutions and samples with HCl or NaOH. The peak height remained constant in the pH range 2–10 during the determination of perchlorate and 2–9 during the determination of permanganate, above which the peak height decreased sharply.
3.3. Features of the methods The calibration graphs (potential versus log concentration) showed a linear relationship in the range 10 − 2 –2× 10 − 5 mol l − 1 perchlorate and 10 − 3 –10 − 5 mol l − 1 permanganate, with slopes of − 48.3 and − 89.5 mV per decade, respectively. The detection limit, considered as the analyte concentration at which the potential deviates by 18 mV from the extrapolation of the linear portion, was 5.6× 10 − 6 mol l − 1 for perchlorate and 4.3× 10 − 6 mol l − 1 for permanganate. The repeatability of the proposed methods was studied by carrying out consecutive injections of the sample. The relative standard deviations (S.D.) for the peak height for six injections were: 0.36 and 0.87% for 10 − 4 and 5×10 − 5 mol l − 1 perchlorate, respectively, and 0.33 and 1.50% for 10 − 4 and 10 − 5 mol l − 1 permanganate, respectively. The between-day reproducibility was studied by carrying out the same procedure on 3 days during a period of 9 days. The peak height relative S.D. obtained from the corresponding mean values were 0.70 and 0.68 for 10 − 4 and 5× 10 − 5 mol l − 1 perchlorate, respectively, and 0.22 and 1.05% 10 − 4 and 10 − 5 mol l − 1 permanganate, respectively.
3.4. Selecti6ity Fig. 3. Influence of the flow rate (; left) on the peak height and ( ; right) on the peak width for (1) perchlorate, 10 − 5 mol l − 1, and (2) permanganate, 10 − 4 mol l − 1.
The selectivity coefficients KClO−,j and KMnO−,j 4 4 of the ISEs based on membrane A and C, respectively, were obtained by the fixed interference
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205
Table 1 Potentiometric selectivity coefficients 4
a
Phosphate Acetate SO2− 3 Cl− F− HCO− 3 SO2− 4 − OH NO− 2 Br− NO− 3 ClO− 3 I− SCN− IO− 4 ClO− 4 MnO− 4 a b
Log KMnO−,j (present)b
Log KClO−,j
Ion
4
Present
Batch 2
Batch 3
Batch 4
Batch 5
Batch 6
Batch 7
−5.7 −5.4 −5.4 −5.4 −5.3 −5.3 −5.3 −4.9 −4.8 −4.5 −2.5 −2.4 −1.7 −0.9 −0.4
−2.3 −2.4 −3.5 −3.1 −2.4 – −3.4 – −2.5 −2.6 −3.0 −2.4 −1.1 0.26 0.56
– −1.4 – −1.4 −1.4 – −2.5 – −1.3 −1.3 −1.5 −1.4 −1.1 −0.74 −0.36
−5.1 – – −5.0 – −4.2 −4.9 – −4.0 −4.7 −2.7 – −1.8 −1.0 –
– – – −2.5 −3.2 – −4.4 – −2.6 −2.2 −2.1 −1.9 −1.4 – 0.04
−5.1 −5.2 −5.0 −5.0 – −4.7 −5.0 −4.6 −4.4 −4.0 −3.2 −3.9 −2.0 −1.4 0.11
– – – −2.2 −2.5 −1.5 – – – −2.2 −2.7 – 0.45 – –
−0.3
0.41
0.74
–
–
0.75
−4.2 – −4.4 – −4.7 −4.2 −4.7 −4.8 – – −2.8 −2.8 – −1.0 −0.8 −0.1
–
Flow injection, membrane A. Flow injection, membrane C.
method [12]. The concentration of interfering anion was fixed at 0.1 mol l − 1 except for iodide and thiocyanate (1× 10 − 3 mol l − 1), periodate and perchlorate (1× 10 − 4 mol l − 1) and permanganate (1×10 − 5 mol l − 1). The values obtained are shown in Table 1. A very good degree of selectivity with respect to most ions was found. The KClO−,j values reported for some previous perchlo4 rate-selective electrodes are also included.
3.5. Applications The flow injection methods proposed were applied to the determination of perchlorate in waters and of permanganate in pharmaceuticals. In the absence of samples containing perchlorate, known amounts of perchlorate were added to water samples from different sources. The determinations were carried out in 0.1 mol l − 1 hydrochloric acid medium to remove the bicarbonate, which is found at high concentrations in some waters. The results are shown in Table 2. Good recoveries
were obtained. It was found that the lowest perchlorate concentration that could be determined without error was dependent on the type of water. To clarify this, the concentration of other anions present in water (chloride, nitrate and sulphate) was determined (also included in Table 2). From these results and from the selectivity coefficients shown above, it was concluded that the nitrate concentration present in the sample was the limiting factor, except in the case of the reservoir water, where the limiting factor was attributed to organic matter. Due to the presence in pharmaceutical solutions of amounts of manganese dioxide which increase with storage time, the effect of this compound on the method applied to the determination of permanganate in pharmaceuticals was first studied. Different amounts of manganese dioxide were added to permanganate solutions and the permanganate concentration was determined by the proposed method without previous filtration. The results obtained are shown in Table 2, where they
C. Sa´ nchez-Pedren˜ o et al. / Talanta 55 (2001) 201–207
206
Table 2 Determination of perchlorate and permanganate in different samples Sample
Concentration Cl− (mg l−1)
−1 NO− ) 3 (mg l
SO2− (mg l−1) 4
−1 ClO− ) 4 (mg l
Added
Founda
Tap water
104.4
3.9
177.4
1.00 5.00 10.00
1.04 9 0.01 5.03 9 0.02 9.83 9 0.26
Reser6oir water
149.4
1.2
46.7
1.00 5.00 10.00
0.97 90.02 4.92 90.14 9.50 90.25
Spring water 1
0.8
4.8
Not detected
2
7.9
2.1
17.7
3
6.2
7.9
17.9
1.00 5.00 0.50 1.00 5.00 1.00 5.00
0.98 90.01 4.90 90.05 0.50 90.01 0.95 90.01 5.14 90.07 1.13 90.03 4.95 90.03
Sample
MnO2 added (g l−1)
−1 MnO− ) 4 (g l
Pharmaceutical preparations 1 0.4785 2 – 3 0.2301 4 – 5 –
Proposed methoda
Comparative methoda
0.0717 9 0.0007 0.0860 90.0009 0.2964 90.0021 0.1416 90.0010 0.9137 90.0048
0.0735 9 0.0011b 0.0885 9 0.0008b 0.3024 90.017c 0.1431 9 0.0007c 0.9028 9 0.0032c
Mean 9S.D. (n= 3). Molecular absorption spectroscopy [13]. c Volumetric method [14]. a
b
are compared with those obtained by the reference methods, which needed previous filtration. As can be seen manganese dioxide does not interfere in the proposed method, at least up to an 8/1 mole to mole ratio of manganese dioxide to permanganate. The proposed method was then applied to the determination of permanganate in pharmaceutical preparations. The results obtained were compared with those obtained by the corresponding reference method (Table 2) by applying the paired t-test and F-test at the 95% confidence level. No significant difference in accuracy or precision was found between the methods.
References [1] L.K. Shpigun, Flow injection analysis with ion-selective electrodes: recent developments and applications, in: D. Littlejohn, D. Thorburn Burns (Eds.), Reviews on Analytical Chemistry, Royal Society of Chemistry, London, 1994, pp. 246 – 272. [2] S.S. Hassan, M.M. Elsaied, Talanta 33 (1986) 679. [3] A.K. Jain, M.J. Jahan, V. Tyagi, Anal. Chim. Acta 231 (1990) 69. [4] J. Casabo´ , L. Escriche, C. Pe´ rez-Jime´ nez, J.A. Mun˜ oz, F. Teixidor, J. Bausells, A. Errachid, Anal. Chim. Acta 320 (1996) 63. [5] R. Pe´ rez-Olmos, A. Rı´os, M.P. Martı´n, R.A.S. Lapa, J.L.F.C. Lima, Analyst 124 (1999) 97.
C. Sa´ nchez-Pedren˜ o et al. / Talanta 55 (2001) 201–207 [6] C. Sa´ nchez-Pedren˜ o, J.A. Ortun˜ o, J. Herna´ ndez, Anal. Chim. Acta 415 (2000) 159. [7] J. Jezkova, J. Musilova, K. Vytras, Electroanalysis 9 (1997) 1433. [8] E. Wang, S. Kamata, Anal. Chim. Acta 261 (1992) 399. [9] M. Maj-Zurawska, T. Sokalski, A. Hulanicki, Talanta 35 (1988) 281. [10] J.A. Ortun˜ o, C. Sa´ nchez-Pedren˜ o, R. Ferna´ ndez de
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Bobadilla, Talanta 41 (1994) 627. [11] T.C. Chadwick, Anal. Chem. 48 (1976) 1201. [12] P. Kane, D. Diamond, Talanta 44 (1997) 1847. [13] F.D. Snell, Photometric and Fluorometric Method of Analysis – Metals. Part 2, Wiley, New York, 1978, p. 1019. [14] I.M. Kolthoff, R. Belcher, Volumetric Analysis, vol. 3, Interscience, New York, 1957, p. 52.
Flow injection potentiometry of primary and interfering ion with a gold complex ion-exchange membrane Concepcio´n Sa´nchez-Pedren˜o, Joaquı´n A. Ortun˜o, Jorge Herna´ndez Department of Analytical Chemistry, Faculty of Chemistry, Uni6ersity of Murcia, 30071 Murcia, Spain Received 4 January 2001; received in revised form 20 April 2001; accepted 20 April 2001
Abstract A gold(I) organic complex in the perchlorate form was incorporated in plasticized poly(vinyl chloride) (PVC) membranes and the potentiometric response towards perchlorate (primary ion) and permanganate (interfering ion) was studied. Membranes with 2:1 and 1:2 (w/w) plasticizer to PVC ratio were selected for the determination of primary and interfering ions, respectively. In the selected flow injection conditions, a linear relationship between peak height and log c were obtained between 1 × 10 − 2 and 2× 10 − 5 mol l − 1 perchlorate and 1 × 10 − 3 –1× 10 − 5 mol l − 1 permanganate. Good reproducibilities and excellent selectivity coefficients towards many common ions were obtained. The methods proposed were applied satisfactorily to the determination of perchlorate in water and of permanganate in pharmaceutical preparations. Permanganate can be directly determined even in the presence of a high amount of manganese dioxide. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ion-selective electrodes; Flow-injection analysis; Perchlorate; Permanganate
1. Introduction The use of flow injection methodology to extend the capabilities of ion-selective electrodes has been reviewed [1]. Compared to manual potentiometry, flow injection potentiometry (FIP) has several advantages including the reproducibility of the analytical signal and the reproducible use of small sample volumes. Several perchlorate-selective electrodes using liquid [2,3] or plasticized PVC [4 – 6] membranes, * Corresponding author. Tel.: + 34-968-367400; fax: +34968-364148. E-mail address: [email protected] (C. Sa´nchez-Pedren˜o).
or based on carbon paste electrodes [7] have been reported. An FIP device using a membranecoated rod ion-selective electrode has been proposed for the determination of perchlorate [8]. With regard to permanganate, the perchlorate-selective electrode described in Ref. [2] was also used for the determination of interfering permanganate ion. An interpretation of the enhanced potential response of liquid ion-exchange membrane electrodes towards interfering ions has been reported [9]. FIP offers a convenient way to exploit this phenomenon to determine the interfering ion. In the present paper, FIP methods for the determination of perchlorate (primary ion) and
0039-9140/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 1 ) 0 0 4 1 8 - 0
202
C. Sa´nchez-Pedren˜o et al. / Talanta 55 (2001) 201–207
permanganate (interfering ion) using gold complex ion-exchange membranes are proposed and applied to the analysis of water and pharmaceuticals.
model was refined by constructing the cell from a sole Perspex block incorporating a circular 2-mm thick silicone gasket, in the centre of which a 7-mm diameter hole had been made.
2.2. Reagents 2. Experimental
2.1. Apparatus Potentials were measured with an Orion 960 Autochemistry System (Boston, MA), whose recorder output was connected to a personal computer via a DGH Corporation 1121 module analogue-to-digital converter (Manchester, UK). A Fluka (Buchs, Switzerland) glass ring of 28-mm inner diameter and 30-mm height, a glass plate, vial, punch and electrode body ISE, were used for the membrane construction and mounting. An Orion 90-02 double junction silver– silver chloride reference electrode containing 10% ammonium nitrate solution in the outer compartment was used. A Gilson (Villiers le Bel, France) Minipuls 3 peristaltic pump, Omnifit (Cambridge, UK) injection valve, connecting tubing of 0.5 mm bore, PTFE tubing and various end fittings and connectors were used to construct the flow injection system. A flow-cell (Fig. 1) similar to that described previously [10] was used although the new
The Bis[tri-(p-metoxyphenyl) phosphine] gold(I) perchlorate was obtained as described before [6]. Poly(vinyl chloride) (PVC) high molecular mass, 2-nitrophenyl octyl ether (NPOE) and tetrahydrofuran (THF) were Selectophore products from Fluka. All other reagents used were of analytical reagent grade and doubly distilled water was used throughout. A concentration of 0.1 mol l − 1 sodium chloride, sodium sulphate or hydrochloric acid were used as carrier solutions. A 1 mol l − 1 standard perchlorate solution was prepared from sodium perchlorate monohydrate (Fluka) and standardised gravimetrically following the procedure described before [11] with 1,2,4,6-tetraphenylpyridinium acetate prepared accordingly. A 1 mol l − 1 standard permanganate solution was prepared from potassium permanganate (Merck) and standardised with sodium oxalate (Merck). Working standard solutions were prepared by dilution with the corresponding carrier solution.
2.3. Preparation of the membranes The membranes were prepared by dissolving 3 mg gold(I) complex, 200, 150 or 100 mg NPOE and 100, 150 or 200 mg PVC for membranes A, B and C, respectively, in 3 ml of THF. This solution was poured into the glass ring resting on the glass plate and was left overnight to allow the solvent to evaporate slowly. A 7-mm diameter piece was cut out with the punch and incorporated into the electrode body containing 0.1 mol l − 1 sodium chloride and 0.01 mol l − 1 sodium perchlorate saturated with excess of AgCl as internal filling solution.
2.4. Flow injection procedures Fig. 1. Flow-through cell design: (1) ISE; (2) Perspex block; (3) Gasket; and (4) PTFE tubes.
A one-channel flow injection assembly, similar to that described before [10], was used. The corre-
C. Sa´ nchez-Pedren˜ o et al. / Talanta 55 (2001) 201–207
sponding ion-selective electrode was incorporated in a flow injection system by means of the flowcell mentioned above. The distance between the injection valve and the cell was 20 cm. The carrier solution was pumped at a flow rate of 1 ml min − 1 and 500 ml aliquots of sample solutions were injected. Potentials were monitored and the peak heights were determined. Working standard solutions (10 − 6 – 10 − 2 mol l − 1) were used to obtain the corresponding calibration graphs. For the determination of perchlorate in waters, 2 ml of 1.25 mol l − 1 hydrochloric acid solution was added to 20 ml of sample and the solution was diluted to 25 ml with distilled water. Aliquots of this solution were injected into a 0.1 mol l − 1 hydrochloric acid carrier solution and the perchlorate concentration was determined using the ISE containing membrane A, as described above. For the determination of permanganate in pharmaceuticals, 2 ml of 1.25 mol l − 1 sodium sulphate solution was added to a suitable volume (0.25–2.0 ml) of sample and the solution was diluted to 25 ml with distilled water. Aliquots of this solution were injected into a 0.1 mol l − 1 sodium sulphate carrier solution and the permanganate concentration was determined, using the ISE containing membrane C, as described above.
3. Results and discussion
3.1. Influence of membrane composition One of the most important parameters affecting sensitivity in the combination of FIA with ISEs, is the dynamic response of the membrane, which is dependent on the membrane composition. Three polymeric membranes containing different NPOE/PVC ratios, (2/1, 1/1 and 1/2; membranes A, B and C, respectively), were tested. The corresponding calibration graphs of the three membranes towards perchlorate and permanganate, obtained by injecting 500 ml of sample at a flow rate 1.0 ml min − 1, are shown in Fig. 2. The transient signals obtained for 10 − 4 mol l − 1 of the corresponding analyte are also included in each graph. With regard to perchlorate (primary ion; Fig. 2(a)), the potential response decreased as the
203
Fig. 2. Calibration graphs and transient signals for (a) perchlorate and (b) permanganate with ( ) membrane A; () membrane B; and () membrane C. The transient signals correspond to a concentration of 10 − 4 mol l − 1.
plasticizer to PVC ratio of the membrane composition decreased, perhaps due to the different speed of response of the membranes. Their response rates were obtained from the corresponding derivative transient signals. The maximum response rates obtained for 10 − 4 mol l − 1 perchlorate were 5.4, 4.2 and 1.7 mV s − 1 for membranes A, B and C, respectively. Membrane A was selected for the FIP determination of perchlorate. With regard to permanganate (interfering ion), the potential responses obtained with the three membranes (Fig. 2(b)) were higher than the corresponding response obtained for perchlorate, for concentrations 10 − 4 mol l − 1 or higher. This is in accordance with the permanganate selectivity coefficient KClO−,MnO− values of higher than 1, 4 4 which have been reported for different perchlorate-selective electrodes. In addition, the three calibration graphs obtained showed a super-Nernstian potential response, as predicted from theory [9]. As regards the shape of the transient signals, they showed non-monotonic behaviour as they returned to the base line, as a result of the permanganate entering the membrane through ionexchange during exposure to the sample. The extent of the non-monotonic part of the signals decreased with decreasing plasticizer-to-PVC ratios in the membrane composition. This can be explained by the drastic reduction observed in the
204
C. Sa´ nchez-Pedren˜ o et al. / Talanta 55 (2001) 201–207
ion mobilities, which hindered penetration of the ions into the bulk of the membrane. This represents a practical advantage since the time needed for the electrode to be restored is considerably decreased, thus increasing sample throughput. The magnitude of the analytical signal measured (peak height) was only very slightly influenced by membrane composition. Membrane C was selected for permanganate determination.
3.2. Influence of flow injection-6ariables and pH The influence of the injected sample volume on the determination of perchlorate and permanganate with ion-selective electrodes based on membranes A and C respectively was studied at a flow rate of 1 ml min − 1. An increase in sample volume produced an increase in peak height, which reached an almost constant value at 300 ml for perchlorate and 400 ml for permanganate. A sample volume of 500 ml was used for both ions in further studies. The influence of the flow rate on the determination of perchlorate and permanganate with the respective membranes was studied in the range 0.25–1.5 ml min − 1. The results are shown in Fig. 3. As can be seen, a slight increase in peak height was obtained by raising the flow rate in the case
of both anions. On the other hand, the peak width decreased when the flow rate was increased, thus improving sample throughput. However, the reproducibility of the signal diminished at the highest flow rates assayed. Taking all this into account, a flow rate of 1 ml min − 1 was selected. The influence of pH on the determination of perchlorate and permanganate was studied by adjusting the pH of the carrier solutions and samples with HCl or NaOH. The peak height remained constant in the pH range 2–10 during the determination of perchlorate and 2–9 during the determination of permanganate, above which the peak height decreased sharply.
3.3. Features of the methods The calibration graphs (potential versus log concentration) showed a linear relationship in the range 10 − 2 –2× 10 − 5 mol l − 1 perchlorate and 10 − 3 –10 − 5 mol l − 1 permanganate, with slopes of − 48.3 and − 89.5 mV per decade, respectively. The detection limit, considered as the analyte concentration at which the potential deviates by 18 mV from the extrapolation of the linear portion, was 5.6× 10 − 6 mol l − 1 for perchlorate and 4.3× 10 − 6 mol l − 1 for permanganate. The repeatability of the proposed methods was studied by carrying out consecutive injections of the sample. The relative standard deviations (S.D.) for the peak height for six injections were: 0.36 and 0.87% for 10 − 4 and 5×10 − 5 mol l − 1 perchlorate, respectively, and 0.33 and 1.50% for 10 − 4 and 10 − 5 mol l − 1 permanganate, respectively. The between-day reproducibility was studied by carrying out the same procedure on 3 days during a period of 9 days. The peak height relative S.D. obtained from the corresponding mean values were 0.70 and 0.68 for 10 − 4 and 5× 10 − 5 mol l − 1 perchlorate, respectively, and 0.22 and 1.05% 10 − 4 and 10 − 5 mol l − 1 permanganate, respectively.
3.4. Selecti6ity Fig. 3. Influence of the flow rate (; left) on the peak height and ( ; right) on the peak width for (1) perchlorate, 10 − 5 mol l − 1, and (2) permanganate, 10 − 4 mol l − 1.
The selectivity coefficients KClO−,j and KMnO−,j 4 4 of the ISEs based on membrane A and C, respectively, were obtained by the fixed interference
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Table 1 Potentiometric selectivity coefficients 4
a
Phosphate Acetate SO2− 3 Cl− F− HCO− 3 SO2− 4 − OH NO− 2 Br− NO− 3 ClO− 3 I− SCN− IO− 4 ClO− 4 MnO− 4 a b
Log KMnO−,j (present)b
Log KClO−,j
Ion
4
Present
Batch 2
Batch 3
Batch 4
Batch 5
Batch 6
Batch 7
−5.7 −5.4 −5.4 −5.4 −5.3 −5.3 −5.3 −4.9 −4.8 −4.5 −2.5 −2.4 −1.7 −0.9 −0.4
−2.3 −2.4 −3.5 −3.1 −2.4 – −3.4 – −2.5 −2.6 −3.0 −2.4 −1.1 0.26 0.56
– −1.4 – −1.4 −1.4 – −2.5 – −1.3 −1.3 −1.5 −1.4 −1.1 −0.74 −0.36
−5.1 – – −5.0 – −4.2 −4.9 – −4.0 −4.7 −2.7 – −1.8 −1.0 –
– – – −2.5 −3.2 – −4.4 – −2.6 −2.2 −2.1 −1.9 −1.4 – 0.04
−5.1 −5.2 −5.0 −5.0 – −4.7 −5.0 −4.6 −4.4 −4.0 −3.2 −3.9 −2.0 −1.4 0.11
– – – −2.2 −2.5 −1.5 – – – −2.2 −2.7 – 0.45 – –
−0.3
0.41
0.74
–
–
0.75
−4.2 – −4.4 – −4.7 −4.2 −4.7 −4.8 – – −2.8 −2.8 – −1.0 −0.8 −0.1
–
Flow injection, membrane A. Flow injection, membrane C.
method [12]. The concentration of interfering anion was fixed at 0.1 mol l − 1 except for iodide and thiocyanate (1× 10 − 3 mol l − 1), periodate and perchlorate (1× 10 − 4 mol l − 1) and permanganate (1×10 − 5 mol l − 1). The values obtained are shown in Table 1. A very good degree of selectivity with respect to most ions was found. The KClO−,j values reported for some previous perchlo4 rate-selective electrodes are also included.
3.5. Applications The flow injection methods proposed were applied to the determination of perchlorate in waters and of permanganate in pharmaceuticals. In the absence of samples containing perchlorate, known amounts of perchlorate were added to water samples from different sources. The determinations were carried out in 0.1 mol l − 1 hydrochloric acid medium to remove the bicarbonate, which is found at high concentrations in some waters. The results are shown in Table 2. Good recoveries
were obtained. It was found that the lowest perchlorate concentration that could be determined without error was dependent on the type of water. To clarify this, the concentration of other anions present in water (chloride, nitrate and sulphate) was determined (also included in Table 2). From these results and from the selectivity coefficients shown above, it was concluded that the nitrate concentration present in the sample was the limiting factor, except in the case of the reservoir water, where the limiting factor was attributed to organic matter. Due to the presence in pharmaceutical solutions of amounts of manganese dioxide which increase with storage time, the effect of this compound on the method applied to the determination of permanganate in pharmaceuticals was first studied. Different amounts of manganese dioxide were added to permanganate solutions and the permanganate concentration was determined by the proposed method without previous filtration. The results obtained are shown in Table 2, where they
C. Sa´ nchez-Pedren˜ o et al. / Talanta 55 (2001) 201–207
206
Table 2 Determination of perchlorate and permanganate in different samples Sample
Concentration Cl− (mg l−1)
−1 NO− ) 3 (mg l
SO2− (mg l−1) 4
−1 ClO− ) 4 (mg l
Added
Founda
Tap water
104.4
3.9
177.4
1.00 5.00 10.00
1.04 9 0.01 5.03 9 0.02 9.83 9 0.26
Reser6oir water
149.4
1.2
46.7
1.00 5.00 10.00
0.97 90.02 4.92 90.14 9.50 90.25
Spring water 1
0.8
4.8
Not detected
2
7.9
2.1
17.7
3
6.2
7.9
17.9
1.00 5.00 0.50 1.00 5.00 1.00 5.00
0.98 90.01 4.90 90.05 0.50 90.01 0.95 90.01 5.14 90.07 1.13 90.03 4.95 90.03
Sample
MnO2 added (g l−1)
−1 MnO− ) 4 (g l
Pharmaceutical preparations 1 0.4785 2 – 3 0.2301 4 – 5 –
Proposed methoda
Comparative methoda
0.0717 9 0.0007 0.0860 90.0009 0.2964 90.0021 0.1416 90.0010 0.9137 90.0048
0.0735 9 0.0011b 0.0885 9 0.0008b 0.3024 90.017c 0.1431 9 0.0007c 0.9028 9 0.0032c
Mean 9S.D. (n= 3). Molecular absorption spectroscopy [13]. c Volumetric method [14]. a
b
are compared with those obtained by the reference methods, which needed previous filtration. As can be seen manganese dioxide does not interfere in the proposed method, at least up to an 8/1 mole to mole ratio of manganese dioxide to permanganate. The proposed method was then applied to the determination of permanganate in pharmaceutical preparations. The results obtained were compared with those obtained by the corresponding reference method (Table 2) by applying the paired t-test and F-test at the 95% confidence level. No significant difference in accuracy or precision was found between the methods.
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