Mikrochim. Acta 135, 97±104 (2000)
Construction and Analytical Applications of Plastic Membrane Electrodes for Oxytetracycline Hydrochloride Yousry M. Issa , Aida L. El-Ansary, and Ahmed S. Tag-Eldin Chemistry Department, Faculty of Science, Cairo University, Giza Egypt
Abstract. Oxytetracycline hydrochloride-selective electrodes of both the coated wire and the conventional polymer membrane types based on oxytetracyclinium phosphotungstate and phosphomolybdate have been prepared. A Nernstian response is shown by these electrodes within 1:0 10ÿ6 ÿ 1:0 10ÿ2 M concentration ranges depending on the type of electrode. The response is unaffected by the change of pH over the range 4±11. The standard electrode potentials, E , were determined at different temperatures and used to calculate the isothermal temperature coef®cients of the electrodes. The electrodes show good selectivity to oxytetracycline hydrochloride with respect to many inorganic cations, sugars and amino-acids. Oxytetracycline hydrochloride is determined successfully in pure solutions and in pharmaceutical preparations using calibration by standard addition and potentiometric titration. A regeneration process for the exhausted electrodes has been developed. Key words: Oxytetracycline hydrochloride; ion-selective electrodes; potentiometry; coated wire; phosphotungstate; phosphomolybdate, tetraphenylborate.
Oxytetracycline hydrochloride (OTC.Cl) [CAS 205846-0] is one of the broad-spectrum antibiotics which possesses a wide range of antibacterial activity. Several methods have been reported for the determination of oxytetracycline hydrochloride, including spectrophotometry [1, 2], high performance liquid chromatography [3, 4], ¯uorimetry [5], voltammetry [6], polarography [7], capillary electrophoresis [8], titrimetry [9], ¯ow-injection chemiluminescence [10].
Selective electrodes for oxytetracycline of the conventional membrane type based on OTC-tetraphenylborate ion-pairs have been reported [11], however, this electrode has a short life-span and a narrow working pH range (7±11). This electrode is conventional and the coated wire electrodes were not constructed. In the present work, plastic-membrane oxytetracycline-selective electrodes of the conventional and coated wire types are constructed and their performance characteristics are investigated. The electrodes are based on incorporation of the oxytetracyclinephosphotungstate [(OTC)3 PT] or oxytetracyclinephosphomolybdate [(OTC)3 PM] ion-associates in a poly(vinyl chloride) (PVC) membrane plasticized with dibutylphthalate (DBP). The electrodes are used successfully as sensors to determine oxytetracycline hydrochloride in pure solutions and in pharmaceutical preparations. The work also includes the development of methods for the regeneration of the exhausted electrodes. Experimental Apparatus Potentiometric and pH measurements were carried out using a SEIBOLD G-103 digital pH/mV meter (Vienna, Austria). A Techne circulator thermostat Model C-100, was used to control the temperature of the test solution. A saturated calomel electrode (SCE) was used as the external reference and a Ag/AgCl electrode as the internal reference. Reagents and Materials
To whom correspondence should be addressed
All reagents used were chemically pure. Double distilled water was used throughout all experiments. Oxytetracycline hydrochlor-
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Table 1. Elemental analysis of the ion-associates OTC-PT and OTC-PM OTC-PT (3:1)
OTC-PM (3:1}
Element
Calculated%
Found%
Calculated%
Found%
C H N
18.59 1.76 1.97
18.8 1.8 2.1
24.72 2.34 2.62
25.0 2.2 2.6
ide (OTC.Cl) (Aldrich, Wisconsin, U.S.A), Phosphotungstic acid (PTA) [H3 PO4 .12WO3 .xH2 O] (Fluka, Buchs, Switzerland), phosphomolybdic acid (PMA) [H 3 PO 4 .12MoO 3 .xH2 O] (Fluka), sodium tetraphenylborate (NaTPB) (Fluka), dibutylphthalate (DBP) (Fluka), PVC of relatively high molecular weight (Aldrich) and tetrahydrofuran (THF) (Aldrich) were used. The effect of the ionic strength was studied using sodium chloride. The following buffer solutions: 0.1 M acetic acid 0.1 M sodium acetate (pH 4.60), 0.01 M KH2 PO4 0.01 M NaH2 PO4 (pH 6.85) and 0.05 M borax (pH 9.00) were prepared. The pharmaceutical preparations (Oxytetracid) capsules (250 mg) (Chemical Industries Development Co., Giza, Egypt) were obtained from local drug stores. A stock oxytetracycline hydrochloride solution, 0.1 M was prepared daily by dissolving the appropriate amount of the drug in double distilled water. More dilute solutions were prepared by appropriate dilution. All (OTC.Cl) solutions were kept in dark brown bottles. Preparation of (OTC)3 PT and (OTC)3 PM Ion-Associates The ion-associates were prepared by mixing 200 ml 10ÿ2 M (OTC.Cl) and 50 ml 10ÿ2 M (PTA) or (PMA) solutions. The resulting yellow precipitates were ®ltered, washed with deionized water till chloride free and dried at room temperature. The compositions of the ion-associates were found to be 3:1 (OTC:PT) or (OTC:PM), as con®rmed by elemental analysis data (Table 1), available from the Microanalytical Centre, Cairo University. Membrane Composition Trials were made to attain the optimum membrane composition. Five membrane compositions were prepared by varying the percentage of (OTC)3 PT and (OTC)3 PM ion-associate. The percentages of (OTC)3 PT ion-associate are 5, 10, 15, 20 and 25 and the slopes obtained are 55.0, 59.5, 57.5, 54.0 and 51.0 mV/ decade, respectively. In the case of the (OTC)3 PM ion-associate, the percentages are 5, 8, 10, 15 and 20; the slopes obtained are 53.0, 59.0, 55.0, 46.5 and 38.0 mV/decade, respectively. So, the optimum membrane compositions (w/w) for OTC-PT and OTCPM electrodes were 10.0% (OTC)3 PT, 45.0% DBP and 45.0% PVC and: 8.0% (OTC)3 PM, 46.0% DBP and 46.0% PVC, respectively. The components (0.35 g) were dissolved in tetrahydrofuran (THF) and the resulting solution was used either to coat wires or to cast membrane discs.
about 1.0 mm thickness was obtained. A disc of 14 mm diameter was cut out and glued to the polished end of a PVC tube by means of a PVC-THF solution. The electrodes were then ®lled with 0.1 M NaCl 10ÿ3 M OTC.Cl solution (The ®lling solution must be changed after changing its colour) and a Ag/AgCl wire was immersed in this solution. The resulting electrodes were preconditioned by soaking them for 1/4 h in 10ÿ3 M OTC.Cl solution. The electrochemical system is composed as follows: Ag/ AgCl/inner solution/membrane/test solution//KCl salt bridge// SCE. (b) Coated wire electrodes (CWEs) [13]: Spectroscopically pure copper, silver, graphite and platinum wires of 2 mm diameter and 12 cm in length were insulated by tight polyethylene tubes leaving 2 cm at one end for coating and 1 cm at the other end for connection. Seven types of electrodes, Cu, Cu/CuS, Ag, Ag/AgCl, Ag/Ag2 S, Pt and graphite were prepared. The polished electrode surface was coated with the active membrane by quickly dipping the exposed end into the coating solution (described previously under membrane composition) twenty times and allowing the film left on the wire to dry in air for about 1 min. The process was repeated until a plastic film of approximately 1.0 mm thickness was formed. The prepared electrodes were preconditioned by soaking them for 1/4 h in 10ÿ3 M OTC.Cl solution. The electrochemical system in this case of the coated wire type was: wire/ membrane/test solution//KCl salt bridge//SCE. Construction of the Calibration Graphs Suitable increments of standard (OTC.Cl) solution were added to 50 ml of 10ÿ6 M OTC.Cl solution to cover the concentration range 1.010ÿ6 ÿ1.010ÿ2 M. In this solution, the sensor and the reference electrodes were immersed and after each addition the emf was recorded, at 250.1 C. The cell potentials, Ecell , were plotted versus pOTC. The process was repeated at 25, 35, 45, 50, 55 and 60 C. The calibration graphs were also constructed in presence of some buffers and in the presence of different concentrations from NaCl solutions. Selectivity of the Electrodes Selectivity coef®cients were determined with separate solutions, [14] where the following equation was applied: Z 1=Z pot logKOTC;J Z
E2 ÿ E1 =S logOTC ÿ log J E1 is the electrode potential in 10ÿ3 M OTC.Cl solution, E2 is the potential of the electrode in 10ÿ3 M solution of the interferent J Z and S is the slope of the calibration graph. The tolerance was considered as the concentration imparting a 2 mV drift in potential reading. Sample Preparation The contents of 20 capsules were weighed, ®nely powdered and intimately mixed. An accurate amount of the powder containing an appropriate amount of drug was dissolved in distilled water.
Preparation of the Electrodes
Potentiometric Determination of OTC.Cl
(a) Conventional membrane type electrodes [12]: The solution containing the ion-associate, PVC and DBP dissolved in THF as given above was poured into a 7.5-cm Petri dish. The solvent was allowed to evaporate at room temperature whereupon a ®lm of
The standard additions method [14] was used, in which small increments of 0.1 M OTC.Cl solution were added into the 100-ml double wall titration cell containing 50 ml samples of various concentrations. The change in emf was recorded after each
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Construction and Analytical Applications of Plastic Membrane Electrodes addition and used to calculate the concentration of the OTC.Cl sample solutions. Potentiometric Titration of OTC.Cl An aliquot of OTC.Cl solution containing 4.97±49.69 mg was transferred into the titration cell, and the solution was diluted to 50 ml with distilled water. The resulting solution was titrated with 10ÿ2 M TPB solution at 250.1 C.
Results and Discussion Electrode Response The response characteristics of the OTC-PT and OTCPM electrodes under investigation are summarized in Table 2. The results indicate that the electrodes show a Nernstian response to OTC.Cl over a relatively wide range (1.010ÿ6 ÿ1.010ÿ2 M) of concentrations depending on the type of electrode. The linear range depends on the nature of the electrode (Table 2). Effect of Soaking The performance characteristics of OTC-PT and OTCPM electrodes were studied as a function of the soaking time. For this purpose, the electrodes were soaked in 10ÿ3 M solution of OTC.Cl and the performance of the electrodes was investigated after 1/4, 1/2, 1, 2, 3, 5, 6 and 24 h. The results indicated that soaking in OTC.Cl solution for intervals extending from 1/2 to 24 h led to very good Nernstian behaviour (59±62 mV/decade) for the conventional
type, Cu-, Ag-, and Pt-coated wire electrodes with (OTC)3 PT ion-associate. For the (OTC)3 PM ionassociate, a Nernstian behaviour was obtained with conventional type Cu-, Pt- and graphite-coated wire electrodes. The calibration graph slopes decreased slightly to 55.0 mV/decade after 10 and 25 days and continued to decrease reaching 44.0 and 48.0 mV/ decade after 40 days for the conventional and the Ptcoated wire OTC-PT electrodes, respectively. In case of OTC-PM electrodes (conventional type and graphite-coated wire), the calibration graph slopes decreased slightly to 57.0 mV/decade after 20 and 2 days and continued to decrease reaching 45.0 mV/ decade after 60 and 10 days for conventional and graphite-coated wire electrodes, respectively. Figure 1 includes representative curves illustrating the effect of soaking on OTC-PT electrode performance. In case of Cu/CuS, Ag/AgCl, Ag/Ag2 S, graphite and Cu/CuS, Ag, Ag/AgCl, Ag/Ag2 S coated wire electrodes for (OTC)3 PT and (OTC)3 PM ion-associates, respectively, the electrodes were exhausted after 24 h. The decrease in the performance of the electrodes is due to a diminished (OTC ) ion-exchange rate on the membrane gel layer-test solution interface, which is responsible for the membrane potential. There are two possible reasons for this decrease in the exchange rate: (i) leaching of the ion-associate from the gel layer of the membrane into the bathing solution and (ii) poisoning of the electrode surface. A trial to regenerate the exhausted conventional electrodes, was
Table 2. Response characteristics of oxytetracycline drug electrodes (soaking in 10ÿ3 M OTC.Cl) Electrode
Slope (mV/decade)
Usable concentration range (M)
Soaking time (h)
Response time (s)
OTC-PT electrode Conventional Cu-CWE Cu/CuS-CWE Ag-CWE Ag/AgCl-CWE Ag/Ag2 S-CWE Pt-CWE Graphite-CE
59.5 62.0 56.0 60.0 55.0 50.5 58.5 59.0
2:5 10ÿ6 ±1:0 10ÿ2 1:0 10ÿ6 ±1:0 10ÿ2 1:0 10ÿ6 ±1:0 10ÿ4 5:0 10ÿ6 ±3:2 10ÿ3 1:0 10ÿ5 ±1:0 10ÿ2 5:0 10ÿ6 ±5:0 10ÿ4 1:0 10ÿ6 ±1:0 10ÿ3 5:0 10ÿ6 ±5:0 10ÿ4
1/4 1/2 1/4 1 1/4 1/4 1/2 1/4
5 5 5 5 5 5 5 5
OTC-PM electrode Conventional Cu-CWE Cu/CuS-CWE Ag-CWE Ag/AgCl-CWE Ag/Ag2 S-CWE Pt-CWE Graphite-CE
58.0 60.0 60.0 60.0 62.0 60.0 60.0 60.0
1:0 10ÿ6 ±1:0 10ÿ2 5:0 10ÿ6 ±1:0 10ÿ3 5:0 10ÿ6 ±1:0 10ÿ2 1:0 10ÿ5 ±1:0 10ÿ3 2:5 10ÿ6 ±1:0 10ÿ3 2:5 10ÿ6 ±1:0 10ÿ3 5:0 10ÿ6 ±1:0 10ÿ3 5:0 10ÿ6 ±1:0 10ÿ3
2 1/2 1 1/4 1/2 1/4 1/4 1
5 5 5 5 5 5 5 5
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Fig. 1. Calibration curves of OTC-PT electrode (conventional type) obtained after soaking for 1/4(a), 1(b), 2(c), 6(d), 24 hrs(e), 2( f ), 5(g), 10(h), 20(i), 30( j), and 35 days(k)
made by dipping three times alternately in 10ÿ2 M OTC.Cl and PTA or PMA solutions for 1 h [15]. In the case of coated wire electrodes, the electrodes can be regenerated by quickly dipping the exposed end into the coating solution (described previously under membrane compositions). The process succeeded to reactivate the electrodes (Table 3). Effect of pH The effect of the pH of the test solutions (10ÿ5 , 10ÿ4 and 10ÿ3 M OTC.Cl) on the electrodes (OTC-PT and OTC-PM) potential was investigated by following the variation of potential readings with the change in pH when adding very small volumes of HCl and/or NaOH (0.1 or 1 M from each). For each pH value, the potential was recorded and thus the potential-pH
curves for three OTC.Cl concentrations were constructed. They exhibit nearly the same pattern in each case, and representative plots are shown in Fig. 2. Within the pH range 4±11, the potentials of the electrodes are practically independent of the pH. The increase in mV readings at a pH less than 4, may be due to penetration of H into the membrane surface, while the increase in the potential readings after a pH 11 is most probably attributed either to the penetration of hydroxyl ions into the gel layer of the membrane or changes in the liquid-junction potential. Selectivity of the Electrodes The selectivity of the ion-associates based membrane electrodes depends on the selectivity of the ionexchange process at the membrane-test solution
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Construction and Analytical Applications of Plastic Membrane Electrodes Table 3. Performance characteristics of OTC-PT and OTC-PM electrodes before and after regeneration Before regeneration Electrode
After regeneration
Slope mV/decade
Usable concentration range (M)
Slope mV/decade
Usable concentration range (M)
OTC-PT electrode Conventional Cu-CWE Cu/CuS-CWE Ag-CWE Ag/AgCl-CWE Ag/Ag2 S-CWE Pt-CWE Graphite-CE
44.0 47.0 46.0 45.0 46.0 43.0 48.0 43.0
1:0 10ÿ5 ±5:0 10ÿ4 1:0 10ÿ6 ±4:0 10ÿ5 5:0 10ÿ6 ±5:0 10ÿ5 1:0 10ÿ5 ±5:0 10ÿ4 1:0 10ÿ5 ±5:0 10ÿ4 1:0 10ÿ5 ±2:5 10ÿ4 1:0 10ÿ5 ±1:0 10ÿ4 1:0 10ÿ5 ±1:0 10ÿ4
56.0 60.0 53.0 58.0 55.0 50.0 63.0 60.0
5:0 10ÿ6 ±3:2 10ÿ3 1:0 10ÿ6 ±2:5 10ÿ4 2:5 10ÿ6 ±1:0 10ÿ4 5:0 10ÿ6 ±5:0 10ÿ4 1:0 10ÿ5 ±1:0 10ÿ2 1:0 10ÿ5 ±5:0 10ÿ4 2:5 10ÿ6 ±5:0 10ÿ4 5:0 10ÿ6 ±5:0 10ÿ4
OTC-PM electrode Conventional Cu-CWE Cu/CuS-CWE Ag-CWE Ag/AgCl-CWE Ag/Ag2 S-CWE Pt-CWE Graphite-CE
45.0 38.0 46.0 48.0 44.0 48.0 42.0 45.0
1:0 10ÿ5 ±1:0 10ÿ3 1:0 10ÿ5 ±1:0 10ÿ3 1:0 10ÿ5 ±1:0 10ÿ3 1:0 10ÿ5 ±1:0 10ÿ3 1:0 10ÿ5 ±1:0 10ÿ3 1:0 10ÿ5 ±1:0 10ÿ3 1:0 10ÿ5 ±1:0 10ÿ3 1:0 10ÿ5 ±1:0 10ÿ3
58.0 58.0 61.0 60.0 60.0 60.0 60.0 60.0
1:0 10ÿ6 ±1:0 10ÿ3 2:5 10ÿ6 ±1:0 10ÿ3 5:0 10ÿ6 ±3:2 10ÿ3 5:0 10ÿ6 ±1:0 10ÿ3 2:5 10ÿ6 ±1:0 10ÿ3 2:5 10ÿ6 ±1:0 10ÿ3 5:0 10ÿ6 ±1:0 10ÿ3 5:0 10ÿ6 ±1:0 10ÿ3
interface and the mobilities of the respective ions in the membrane. The data presented showed that (OTC)3 PT and (OTC)3 PM electrodes are highly selective for OTC.Cl (Table 4). The inorganic cations
did not interfere due to the differences in their mobilities and permeabilities as compared to OTC.Cl. For natural species, the tolerated ratios of their concentration to that of OTC.Cl were as follows:
Fig. 2. In¯uence of the pH on the potential of OTC-PM membrane selective electrodes
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Table 4. Selectivity coef®cients for OTC-PT and OTC-PM conventional electrodes pot log KOTC;J Z
Interferant
K Na NH 4 Ba2 2 Sr Pb2 Mn2 Ca2 Ni2
OTC-PT
pot log KOTC;J Z
OTC-PM ÿ2
5:88 10 4:75 10ÿ2 2:89 10ÿ2 1:45 10ÿ4 3:78 10ÿ3 8:23 10ÿ4 8:23 10ÿ4 8:53 10ÿ5 7:40 10ÿ5
Interferant ÿ2
3:98 10 3:47 10ÿ2 1:91 10ÿ2 1:66 10ÿ4 4:37 10ÿ3 1:58 10ÿ3 1:00 10ÿ4 1:38 10ÿ4 1:75 10ÿ4
2
Zn Fe2 Cu2 Mg2 Co2 Fe3 Al3 TC.Cl CTC.Cl
OTC-PT
OTC-PM ÿ4
1:00 10ÿ4 1:85 10ÿ4 4:37 10ÿ3 2:75 10ÿ4 1:05 10ÿ4 2:63 10ÿ3 2:40 10ÿ5 8:46 10ÿ1 9:35 10ÿ1
1:20 10 8:40 10ÿ5 3:52 10ÿ3 2:56 10ÿ4 1:22 10ÿ4 3:98 10ÿ4 1:64 10ÿ5 6:10 10ÿ1 6:75 10ÿ1
Tetracycline hydrochloride. Chlortetracycline hydrochloride.
Table 5. Performance characteristics of OTC-PT and OTC-PM conventional electrodes at different temperatures Temperature ( C)
Slope mV/decade
Usable concentration range (M)
E (mV)
Response time (s)
OTC-PT electrode 25 35 45 50 55 60
59.0 60.0 62.0 63.0 65.0 71.0
2:5 10ÿ6 ±1:0 10ÿ2 2:5 10ÿ6 ±1:0 10ÿ2 5:0 10ÿ6 ±6:3 10ÿ3 1:0 10ÿ5 ±3:2 10ÿ3 2:0 10ÿ5 ±1:3 10ÿ3 2:0 10ÿ5 ±1:0 10ÿ3
456 468 481 494 498 515
5 5 10 10 10 10
OTC-PM electrode 25 35 45 50 55 60
59.0 61.0 63.0 65.0 68.0 72.0
2:5 10ÿ6 ±1:0 10ÿ2 2:5 10ÿ5 ±1:0 10ÿ2 5:0 10ÿ5 ±6:3 10ÿ3 5:0 10ÿ5 ±5:6 10ÿ3 5:0 10ÿ5 ±4:0 10ÿ3 2:0 10ÿ5 ±3:2 10ÿ3
508 515 520 524 530 553
5 5 10 10 10 10
1900:1 and 1700:1 (glucose), 1800:1 and 1800:1 (glycine) for OTC-PT and OTC-PM electrodes, respectively. In the case of sugars and amino-acids, the high selectivity is mainly attributed to the difference in polarity and the lipophilic nature of their molecules relative to OTC.Cl. Though tetracycline hydrochloride and chlortetracycline hydrochloride interfere, yet they don't exist together in one pharmaceutical preparation. Effect of Temperature of the Test Solution Calibration graphs (cell potential, Ecell , versus pOTC) were constructed at test solution temperatures (25, 35, 45, 55, 60 and 65 C) for OTC-PT and OTC-PM electrodes. The slope, usable concentration range, the standard electrode potentials (E ) and the response time of the electrodes corresponding to each tem-
perature are reported in Table 5. It is clear that the electrodes have a good Nernstian response in the temperature range 25±55 C. For the determination of the isothermal coef®cients (dE /dt) of the electrodes, the standard electrode potentials (E ) at different temperatures were determined from the calibration graphs as the intercepts at pOTC 0 and plotted versus (tÿ25) (Fig. 3), where t is the temperature of the test solution. Straight line plots are obtained using the following equation [16]: E E
25
dE =dt
t ÿ 25 The slopes of the straight lines obtained represent the isothermal coef®cients of the electrodes, amounting to 1.4 10ÿ3 and 7.50 10ÿ4 V/ C for OTC-PT and OTC-PM electrodes, respectively. This reveals a fairly good thermal stability of the electrodes within the investigated temperatures range.
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Construction and Analytical Applications of Plastic Membrane Electrodes
Fig. 3. Variation of E of OTC-PT (a) and OTC-PM (b) electrodes with temperature
Effect of Buffers The slopes of the calibration graphs, obtained in different buffering media [acetate (pH 4.60), phosphate (pH 6.85) and borax buffer (pH 9.00)], decreased from the normal Nernstian value down to a non-Nernstian value. The value of the slopes for OTC-PT and OTC-PM electrodes decreased from
59 mV/decade (when no buffers is used) down to 37 mV/decade in the presence of buffers. Effect of Ionic Strength The performance characteristics for OTC-PT and OTC-PM electrodes were studied in different sodium
Table 6. Standard additions and potentiometric titration methods for the determination of OTC.Cl using different electrodes Standard additions Sample
Taken (mg)
Potentiometric titration Mean recover (%)
a) Conventional electrode (OTC-PT) 0.25±24.85 101.5 0.50±24.85 102.5 b) Coated electrodes (OTC-PT) i) Cu-coated electrode 0.25±12.42 100.5 0.25±6.21 101.2 ii) Ag-coated electrode 0.25±12.42 100.2 0.25±12.42 103.6 iii) Pt-coated electrode 0.25±12.42 99.8 0.25±12.42 101.8 a) Conventional electrode (OTC-PM) 0.25±12.42 100.1 0.25±12.42 98.8 b) Coated electrodes (OTC-PM) i) Cu-coated electrode 0.25±4.96 99.9 0.25±4.96 102.2 ii) Pt-coated electrode 0.25±2.48 99.9 0.25±2.48 102.3 iii) Graphite-coated electrode 0.25±2.48 98.8 0.25±2.48 101.6
1 2 1 2 1 2 1 2 1 2
1 2 1 2 1 2
Standard deviation (®ve determinations). Pure solutions. 2 Pharmaceutical preparation, Oxytetracid capsules (250 mg). 1
S (%)
Taken (mg)
Mean recovery (%)
S (%)
0.20 0.12
9.94±49.69 9.94±49.69
101.0 103.2
0.13 0.26
0.07 0.04
4.97±49.69 9.94±49.69
102.3 103.2
0.14 0.28
0.18 0.27
9.94±49.69 9.94±49.69
101.6 102.3
0.18 0.28
0.02 0.17
9.94±49.69 9.94±49.69
99.4 101.8
0.02 0.16
0.15 0.29
9.94±49.69 9.94±49.69
99.8 101.8
0.05 0.12
0.05 0.04
9.94±49.69 9.94±49.69
102.8 101.2
0.04 0.03
0.12 0.28
9.94±49.69 9.94±49.69
98.8 99.2
0.18 0.26
0.24 0.26
9.94±49.69 9.94±49.69
99.4 98.7
0.24 0.32
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Construction and Analytical Applications of Plastic Membrane Electrodes
chloride concentrations. The slopes for OTC-PT and OTC-PM electrodes show their best Nernstian value (slope 59 mV/decade) at 25 C in sodium chloride concentrations of 3.0 10ÿ3 and 2.0 10ÿ3 M, respectively. Analytical Applications Determination of OTC.Cl. The electrodes proved to be useful for the determination of OTC.Cl by the standard addition method and by potentiometric titration for the case of pure solutions and in pharmaceutical preparations. Collective results, given in Table 6, indicate the high accuracy and precision of the present work as compared to those previously reported [1, 8, 10]. The latter depend on more complicated instrumentations or time-consuming pretreatment steps, while the combination of sensitivity, selectivity and simplicity of ion-selective electrode potentiometry makes it to an excellent and versatile technique. The performance of the method was assessed by calculation of the t- and F-values in comparison to the of®cial method [11]. Mean values were obtained in a Student's t- and F-test at 95% con®dence limits for ®ve degrees of freedom [17], and the results showed that the calculated t- and F-values did not exceed the theoretical values. Conclusions New OTC-selective PVC membrane electrodes of both coated wire and conventional types based on (OTC)3 PT or (OTC)3 PM ion-associates are constructed and applied in potentiometric determination of OTC.Cl. The electrodes proved to be successful and provide a rapid, simple and of low cost potentiometric methods for the determination of OTC.Cl in pure solutions and in pharmaceutical preparations. The use of OTC-phosphotungstate (OTC)3 PT or OTCphosphomolybdate (OTC)3 PM ion-associates, instead of OTC-TPB ion-pair [11], leads to better performance characteristics, e.g. the life time of the electrodes is 40 and 60 days for OTC-PT and OTC-PM electrodes,
respectively, in comparison to 3 days for the OTCTPB electrode. The working pH range of OTC-PT and OTC-PM electrodes is 4±11, which is 7±11 in case of OTC-TPB electrode. The effect of temperature indicated that the proposed electrodes have good Nernstian response within the temperature range 25± 55 C, which was not studied for the OTC-TPB electrode. Also, simple ion-selective electrodes of the coated wire type were prepared using Cu, Ag, Pt and graphite conductors coated with the membranes. These simple and inexpensive electrodes can be used advantageously as sensors and may easily be constructed even from waste materials [18] and in a miniaturized form. The present electrodes are easily and simply regenerated. References [1] B. Morelli, P. Peluso, Anal. Lett. 1985, 18, 1865. [2] M. Basanti Rao, P. S. Ramamurthy, V. Suryanarayana Rao, Indian Drugs 1996, 33, 350. [3] R. Maruyama, K. Uno, Shokuhin Eiseigaku Zasshi (Japanese) 1997, 38, 425. (Analyt. Abst. 1998, 60, 7H240). [4] T. Agasoster, K. E. Rasmussen, J. Pharm. Biomed. Anal. 1992, 10, 349. [5] A. Regosz, Z. Anal. Chem. 1976, 280, 383. [6] J. Wang, T. Peng, M. S. Lin, Bioelectrochem, Bioenerg. 1986, 15, 147. (Analyt. Abst. 1986, 48, 10E30). [7] I. Ganescu, M. Braun, E. Gavrila, Arch. Pharm. 1977, 310, 544. [8] J. Tjornelund, S. H. Hansen, J. Pharm. Biomed. Anal. 1997, 15, 1077. [9] M. Novak-Pekli, M. El-Hadi Mesbah, G. Petho, J. Pharm. Biomed. Anal. 1996, 14, 1025. [10] Z. Li, M. Feng, J. Lu, Z. Gong, H. Jiang, Anal. Lett. 1997, 30, 797. [11] A. F. Shoukry, S. S. Badawy, Microchem. J. 1987, 36, 107. [12] G. J. Moody, J. D. R. Thomas, H. Freiser (Ed.) Ion-Selective Electrodes in Analytical Chemistry. Plenum Press, New York, 1978, p. 287. [13] C. R. Martin, H. Freiser, Anal. Chem. 1980, 52, 562. [14] S. S. Badawy, A. F. Shoukry, Y. M. Issa, Analyst 1986, 111, 1363. [15] A. F. Shoukry, S. S. Badawy, Y. M. Issa, Anal. Chem. 1987, 59, 1078. [16] L. I. Antropov, Theoretical Electrochem. Mir, Moscow, 1972. [17] J. C. Miller, J. N. Miller, Statistics in Analytical Chemistry, 2nd Edn. Ellis Horwood, Chichester, 1988. [18] K. Vytras, V. Dlabka, Prir. Vedy Sk. 1985, 37, 182. Received February 2, 2000. Revision April 7, 2000.
