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New potentiometric sensor for the determination of iodine species
Materials Science and Engineering C 32 (2012) 2286–2291
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New potentiometric sensor for the determination of iodine species Zholt Kormosh ⁎, Tanya Savchuk Department of Analytical Chemistry and Eco-technology, Volyn National University, 13 Voli Ave, 43025 Lutsk, Ukraine
a r t i c l e
i n f o
Article history: Received 25 October 2011 Received in revised form 10 June 2012 Accepted 19 June 2012 Available online 27 June 2012 Keywords: Triiodide potentiometric sensor Ion-pair Iodine species Rhodamine B Indicator electrode
a b s t r a c t The characteristics, performance and application of novel triiodide potentiometric sensor based on ion-pair of Rhodamine B triiodide as a membrane carrier are described. The electrode has a linear dynamic range between 1×10−6 and 1×10−1 M, with a Nernstian slope of 68±1mV pC−1 and detection limit of 3.9×10−7 M. Fast and stable response, good reproducibility, long-term stability, very good selectivity over a large number of common organic and inorganic anions, applicability over a pH range of 2–10 are demonstrated. The proposed sensor has been applied for potentiometric determination of some iodine species. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Food and pharmaceutical industry is considered the most important in the world since it has sustained continuous research to improve nutritional quality of foodstuffs. Iodine is an essential micronutrient in human growth and is very important for iodine supplementation metabolism. Deficiency of iodine causes serious delay in neurological development, goiter and hypothyroidism. Table salt is supplemented with iodate or iodide to serve as a source of iodine. The iodinated salt is recognized as the method of choice and the most successful strategy for the prevention of iodine deficiency disorders. The recommended concentration of iodate in salts is 40ppm. Iodine is an effective germicide for a wide range of microorganisms. In addition, iodine finds applications as dietary supplements, catalysts, pharmaceutical preparations, stabilizers and in photography. The varied applications of iodine have made the determination of iodine very important [1,2]. Several types of analytical procedures have been reported on the determination of iodine species, such as spectrophotometry [3–8], high performance liquid chromatography [9,10], ion-chromatography [11], spectrofluorimetry [12], differential pulse polarography [13] etc. Ion-selective electrodes (ISEs) remain an important and promising field of analytical chemistry. This is related to the advantages of potentiometry (simplicity, fast response, sensitivity, selectivity, analysis in muddy and colored media etc.) over other analysis methods [14]. Despite the need of direct or indirect determination of iodine as triiodide in many reactions, there have been few reports on triiodide electrodes [15–29]. A research of new electrode-active substances is important. Due to the functional properties of ion-pair (IP) to donate ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Kormosh). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.06.016
both a cation and an anion as potential-determining ions, they are the most universal ionophores [30–33]. It is likely that solubility is the main restriction for their use as ionophores. The solubility of IPs should be sufficiently low in aqueous phase; otherwise they will be washed out from the membranes, and at the same time, high in the membrane phase, otherwise it will be difficult to obtain a homogeneous membrane. Ion-pairs formed by various quaternary ammonium or phosphonium bases most often exhibit such properties [32–38]. Previously, we analyzed the applicability of ISEs based on ion-pair of basic dyes as electrode-active substances in analytical chemistry [39–45]. The effectiveness of Rhodamine B (RB)-based IPs for making potentiometric sensors was demonstrated [41,43]. The purpose of the present work was to develop a simple and low-cost potentiometric PVC-based triiodide ion sensor, and evaluate the applicability of this new sensor to the determination of iodine species in iodinated drinking water, salt and pharmaceuticals. 2. Experimental 2.1. Reagents and solutions All chemicals were of analytical-reagent grade. Twice-distilled water was used to prepare all solution and in all experiments. Dibutylphtalate (DBP), dioctylphtalate (DOP), dinonylphtalate (DNP), dibutylsebacate (DBS), tricresylphosphate (TCP), cyclohexanone (CHN), and high molecular weight polyvinylchloride (PVC) were obtained from Sigma-Aldrich. Rhodamine B (RB) was purchased from Merck. Standard iodate solution, (1/6)×10 −2 M, was prepared by accurately weighed KIO3 dissolved in 1000 ml twice-distilled water. Standard thiosulfate solution, 1×10 −1 M, was prepared from Na2S2O3·5H2O and standardized with potassium iodate solution.
Z. Kormosh, T. Savchuk / Materials Science and Engineering C 32 (2012) 2286–2291
Standard iodine (triiodide) solution, 1×10 −1 M, was prepared from of 40 g KI and 12.7 g of I2, dissolved in 500 ml of twice-distilled water and standardized with thiosulfate solution. Working solutions were prepared by dilution of the corresponding standard solutions. Triiodide solutions used for the sensor characterization and analytical determinations (1×10 −8–1×10 −1 M) were prepared daily from the stock solution, diluting the required volume with 0.2 M KI and using 50 ml volumetric flasks. The stock solution of the basic dye Rhodamine B with a concentration of 1×10−2 M was prepared by dissolving an accurately weighed portion of dye, which was recrystallized from methanol, in twice-distilled water. The ionic strength of the solutions was adjusted with 0.2 M KI solution. The pH value of solutions was maintained with the use of a buffer mixture (0.04 M CH3COOH, H3BO3, H3PO4, and a 0.2 M NaOH solution) and controlled potentiometry with a glass electrode. 2.2. Preparation of ion-pair and membrane for sensor The ion-pair was preparing by mixing 1×10 −2 M solutions of Rhodamine B and triiodide in a ratio of 1:1. The resulting solution was settled for 2 h, and the sediment of ion-pair was filtered out (quantitative rapid filter paper). This residue was treated with 50 ml of cold distilled water. The filter paper containing the precipitate was then dried for 24 h at room temperature. These IPs (low solubility in water and good solubility in membrane plasticizers, along with ion-exchange properties) were used as an electrode-active material for preparing membranes of triiodide sensor. The generally accepted technique of plasticized membrane production by Moody, Oke, Thomas [46,47] consists of thorough mixing of the electrode-active substance with PVC dissolved in cyclohexanone or tetrahydrofuran followed by the evaporation of the solvent in a glass ring. We weighed 0.1 g PVC and the respective amount of isolated IP (such that its concentration in the membrane was 5–25 mass %), then mixed the materials thoroughly for homogenizing. The degree of materials homogeneity was estimated from photomicrographs obtained with a LEICA VMHT Auto microhardness tester. Afterwards, 0.1 ml of a plasticizer (DBP, DBS, DOP, DNP or TCP) and 0.5 ml of solvent (cyclohexanone or tetrahydrofuran) were added. The resulting solution was transferred into a previously polished glass form 1.7 cm in diameter, glued to the glass substrate, and dried in air for 1–2 days. The films obtained after evaporation of the solvent were cut using a rubber cork cutter into disks of 0.5–1.0 cm diameter. These were then glued to the end of the PVC tube with 10% PVC solution in cyclohexanone. The tube was filled with the concentrated standard triiodide solution (1×10 −2 M), and a copper wire was immersed into it. The sensor was then used for the investigation.
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3. Results and discussion 3.1. Association constant and IR spectra Considerable changes in the light absorption of the system were observed upon mixing the solutions of the basic dye Rhodamine B. As the concentration of triiodide was increased at a constant concentration of the dye, a bathochromic shift of the dye absorption band was observed (Fig. 1), which can be indicative of the formation of compounds like ion-pair. This was also supported by the experimental IR spectra. Тhe IR spectra of Rhodamine B and Rhodamine B triiodide have little difference. This is typical for compounds such as ion-pair. The vibrational bands of the groups responsible for the association are the most informative ones. As seen from Fig. 2, the spectra of Rhodamine B feature an intense νС N band at 1620 cm −1. In the formation of the Rhodamine B triiodide ion-pair this band shifts to 1570 cm −1. If sufficiently dilute reagent solutions are used and acidity is maintained at a level at which singly charged reagent species are predominant, the formation of ion-pair can be represented by the following reaction scheme: −
−
I2 þ I ↔I3 þ
ð1Þ þ
−
RB þ I3 ⇔RB ⋅I3
−
ð2Þ
Then, the concentration constant of the IP formation (association constant): þ − RB ⋅I3 K as ¼ ; C I3 − −½RBþ ⋅I3 − ⋅ðC RB −½RBþ ⋅I 3 − Þ
ð3Þ
þ − where RB ⋅I3 is the equilibrium concentration of the ion associate. The equilibrium concentration of the IP can be easily calculated from spectroscopic data: h
þ
RB ⋅I3
−
i
¼
εRB ⋅C RB ⋅l−A ; ðε RB −εas Þ⋅l
ð4Þ
where A is the absorbance of solutions at an absorption band maximum of the RB cation, and εRB and εas are the molar absorption coefficients of the dye cation and the ion-pair, respectively. The association constant thus found is (1.6±0.2)×10 4. Because of the restricted water solubility of this compound, the formation of a suspension (precipitate) of the ion-pair was observed as the concentrations of reacting components were increased (more as 1.2×10 −4 M of triiodide-ions); this allowed us
2.3. Apparatus All EMF measurements were carried out with the following cell assembly. An I-160M model pH–mV meter with an EVL-1MZ silver–silver chloride reference electrode was used for the measurements of potential difference at 25.0±1.0°C. The electronic absorption spectra of solution were measured on an SF-2000 spectrophotometer (LOMO, RUSSIA) over a range of 450–750 nm. The IR spectra were measured on an AVATAR 330 FT-IR instrument (Thermo Nicolet). The thermal studies (TG, DTG and DTA) were carried out on an apparatus for complex dynamic thermal analysis—MOM derivatograph (Hungary) under the following conditions: temperature range 20– 900 °C, heating rate 5 K/min, sample weight 36 mg, static air medium, channel sensitivities: DTA—150 mV, TG—50 mV.
Fig. 1. Absorption spectra of the triiodide-ion – RB system, 1×10−5 M RB. Triiodide-ion concentrations, M: 1–0, 2–4×10−5, 3–8×10−5, 4–1.2×10−4, 5–1.6×10−4, 6–2×10−4.
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Z. Kormosh, T. Savchuk / Materials Science and Engineering C 32 (2012) 2286–2291 Table 1 Data of DTA curve of the ion-pair (C28H31O3N2+·I3−·1.5H2O) and decomposition products. Temperature range, °С
tmax, °С
20–100 50 endo 240–280 265 exo 280–325 300 exo 325–388 388–680 485 exo Total loss of mass
Loss of mass, %
Decomposition products
Experimental
Theoretical
3.32 20.28 5.56 2.78 62.50 94.44
3.17 19.03
1.5Н2О 2NH3 +Н
9.88 62.27 94.35
3C2H4 (18С+12Н+3О+2 ) 4С (solid product)
these stages, the gaseous thermolysis products contain NH3, C2H4, C4H8, НI, and I2. An intense exothermal effect is observed above 380 °С with the maximum at 485 °С which is accompanied by a significant loss of mass. This is identified as the destructive combustion of the major part of IP. The total loss of mass is 94.4% indicating that some carbon does not combust completely and remains as a black film on the crucible wall. The process of the thermal destruction of IP can be described in the following scheme: þ
C28 H31 O3 N2 ⋅I3 ⋅1:5H2 O→1:5H2 O↑ þ 2NH3 ↑ þ НI↑ þ 3C2 H4 ↑þ
Fig. 2. IR spectra of RB (1) and ion-pair (2).
to isolate this ion-pair: þ
−
þ
−
RB þ I3 ⇔RB ⋅I3 ↓
þðC22 H12 O3 Þ⋅I2 þ 4Cresidue ð6Þ
3.2. Thermal behavior of the ion-pair Thermal decomposition of the triiodide-Rhodamine B ion-pair in air undergoes five stages accompanied by mass loss and endo- or exothermic effects (Fig. 3, Table 1). At the first stage of the IP thermo destruction water molecules are separated in the range of 20–100 °С (tmax =50 °С). This indicates the presence of hygroscopic 1.5 molecules of water in the complex. This is easily seen by heating the sample in a hard-glass test tube where water drops or mist condensate at the cold walls. No decomposition of the complex is observed in the 100–240 °С range. We predict that the sensors based on this IP can operate in hot solutions. The second stage (240–280 °С) is accompanied by large mass loss and blends smoothly into the third and fourth stages which end at t=388 °С. At
Fig. 3. Simultaneous TG, DTA and DTG curves of the IP (C28H31O3N2+·I3−·1.5H2O) in air.
C22 H12 O3 ⋅I2 þ 13:5O2 →11CO2 þ 6H2 O þ I2 Experimental and calculated mass loss agrees satisfactorily which confirms the correctness of the proposed scheme of the IP thermolysis process.
3.3. Electrode response It is well known that the sensitivity and selectivity of ion-selective electrodes depend not only on the nature of ionophore used, but also significantly on the membrane composition and the properties of the plasticizer employed. To evaluate the potentiometric characteristics of sensor based on IP triiodide of Rhodamine B, five different plasticizers (DBP, DBS, DOP, DNP, TCP) were considered in the selective membrane preparation. The plasticizer content was 65% of the total mass of the membrane components, and the ionic strength of the solution was adjusted with 0.2 M KI solution. The results of the study of the influence of solvent mediators on the potentiometric response characteristics of the sensors are summarized in Table 2.
Fig. 4. Calibration curve for triiodide-ion sensor.
Z. Kormosh, T. Savchuk / Materials Science and Engineering C 32 (2012) 2286–2291
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Table 2 The effect of plasticizer on the response of triiodide sensor (10 mass. % IP). Plasticizer (65%)
Slope, mV pC−1
Linearity range, M
Low detection limit, M
DOP DBP DBS TCP DNP
63±1 68±1 63±1 68±1 56±1
1×10−4–1×10−1 1×10−5–1×10−1 1×10−5–1×10−1 1×10−6–1×10−1 1×10−5–1×10−1
3.2×10−5 4.5×10−6 3.2×10−6 3.9×10−7 3.0×10−6
It was found that the best characteristics on the electrode function slope are exhibited by the membranes plasticized with slope values of 56–68 mV рС −1. The low detection limits were n∙(10 −7–10 −6) M, regardless of the plasticizer employed. Therefore the nature of the plasticizer (its polarity ε) does not affect the sensitivity of the detection of the investigated ions but contributes remarkably to the value of the electrode function slope. It was established that an increase in the permittivity of the solvent in the series DOPbDBPbTCP leads to an increase in the slope of the electrode function. This is important for the selection of the optimal conditions for sensor performance. The effect of the plasticizer itself on the electrode function slope, its linearity, and the detection limit were investigated. The ratio of plasticizer to added PVC in the membrane was varied from 1:1 to a 2-fold excess. The optimal component ratio was found to be 1.5 plasticizer to 1.0 PVC. A membrane of sufficient elasticity and homogeneity could not be produced at other ratios of the components. No dependence of a cation or a mixed electrode function on the ratio membrane polymer:plasticizer was found; therefore the matrix does not affect the selectivity of the film membranes. This assumption is indirectly corroborated by the closeness of the electrode parameters of the sensors (linearity of the electrode function and detection limits) as the PVC:plasticizer ratio increases. The plasticizer content is also related to the lifetime of the electrode, which is determined by the frequency of its use. It was established that electrodes with lower plasticizer content last somewhat less time than those with high plasticizer content. The solvent content in the membrane decreases with time, which leads to structure failure and loss of elasticity and limits the electrode lifetime. It was determined that sensors with 60–70% plasticizer content have a shelf life of 18–20 weeks from the preparation date, unlike sensors with a lower (40–50%) plasticizer content. After this period of slope of electrode function can be reduced to 45–50 mV pC −1 and that equilibrium is reached in a long time (~15–30 s). We prepared membranes with 5–15% content of the electrode-active substance. The study of the electroanalytical properties of the produced sensors with various ion-pair concentrations shows the response to the concentration of triiodide-ions over a wide range of n×10−6–1×10−
Fig. 6. The effect of the solution pH on the electrode potential of triiodide: pC=4 (1), pC=3 (2).
1
M (Fig. 4). The best results were obtained for membranes containing 10% EAS content (low detection limit (LDL) 3.9×10−7 M). The response time of the electrodes was obtained by measuring the time required to achieve a steady state potential (within ± 1 mV) after successive immersion of the electrodes in a series of triiodide-ions solutions, each having a 10-fold increase in concentration from 1.0×10 −7 to 1.0×10 −1 mol L −1. As shown in Fig. 5 the electrodes yielded steady potentials within 1–8 s (in solutions at 1.0×10 −4–1.0×10 −1 mol L −1 of triiodide-ions). The potential reading stays constant, to within ±1 mV, for at least 5 min. It is much better than known in the literature triiodide-ions sensors. The developed sensors work well for 18–20 weeks. After this period of slope of electrode function can be reduced to 45–50 mV pC −1 and that equilibrium is reached in a wary long time (~15–30 s).
3.4. Effect of the acid medium The factor that most affects the performance of practically all sensors is the acidity of the solution. One should also take into account that the acidity affects the state of the ion-pair and other membrane components, i.e. the membrane–solution interface can host a series of parallel protolytic processes, a description of which may become quite complex. The working range of an IP-based sensor is determined by the protolytic properties of the large ions that compose the ion-pair. We have investigated the effect of the acidity on the electrode characteristics of sensors. The results show a stable potential response over pH ranges of 2–10 (Fig. 6). At higher pH values, irreversible Table 3 Comparative characteristics of the selectivity (−log Kpot) of various triiodide potentiometric sensors. Anion
F− Cl− Br− I− NO3− SCN− ClO4− SO42− HPO42− Sal− Benz− Pikr− CH3COO− Fig. 5. Dynamic responses of the triiodide-ion sensor.
a
This worka
Published data [15]
[16]
[17]
[22]
[25]
[26]
>4 >4 >4 >4 >4 – >4 – – 3.3 3.4 – –
– 3.19 2.76 1.69 3.19 2.76 2.90 3.31 3.14 2.97 – – 3.86
– 3.2 3.3 2.6 3.1 2.7 3.5 4.5 3.4 – – – 2.7
– >5 >5 (3.35) 2.79 (3.29) >5 (4.72) 2.19 (2.72) 3.41 (4.44) 3.78 (3.22) 3.10 (3.61) 3.04 (2.62) 2.80 (4.16) – 4.43 (3.96)
>5 3.82 4.60 3.52 – 3.82 – 4.60 – – – – >5
– 5.20 4.80 3.15 4.30 3.90 5.00 4,37 4.68 – – – 4.71
65% TCP, 10% IP, 25% PVC.
>5 >5 >5 >5 >5 >5 >5 >5 >5 >5 4.5 5.0 >5
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Table 4 Result of the determination of iodates in iodinated drinking water (n=5; P=0.95). Notes: F⁎ and Ftabl =5.192, calculated and table value of F-test, respectively; t⁎ and ttabl =2.78, calculated and table value of Student's test, respectively. Product, manufacturer
Composition
Iodate found, mg Potentiometric titration
“Etalon” still iodinated drinking water, Subos Company Iodanka Pavlivska, Iodanka LLC
3
3
Chlorides ≤100.0 mg/dm , sulfates≤50.0 mg/dm , 0.03–0.06 mg/dm3 iodate Hydrocarbonate, chloride-hydrocarbonate, sodium-calcium, 100±10 μg/dm3 iodate
disproportionation of triiodide ions to hypoiodate and iodide ions occurs both of which are insensitive to the membrane sensor. 3.5. Selectivity Selectivity behavior is obviously one of the most important characteristics of sensor, determining whether a reliable measurement in the target sample is possible. In this work, selectivity coefficients over interfering anions were evaluated by the separate solution method. The resulting values are listed in Table 3. It is immediately obvious that the proposed triiodide membrane sensor is highly selective with respect to most common organic and inorganic anions such as fluoride, chloride, bromide, iodide, nitrate, sulfate, and thiocyanate. The logarithms of selectivity coefficients for all anions tested are −5 or smaller which seems to indicate that these anions have negligible disturbance on the functioning of the triiodide membrane sensor. The surprisingly high selectivity of the sensor for triiodide ion over other anions used arises most probably from the strong tendency of the carrier molecule RB for triiodide ion in the form of the very stable ion-pair. The terms of selectivity coefficients the proposed triiodide sensor are superior to those reported for other triiodide-selective electrodes. 4. Analytical applications The proposed membrane sensor was successfully employed for the assay of different iodine species in iodinated drinking water, salt and pharmaceuticals.
4.1. Determination of iodates in drinking water Iodine as iodate ions was determined in bottled still drinking water Etalon (Subos Company, Ukraine) and Iodanka Pavlivska (Iodanka LLC, Ukraine). An aliquot of 5–25 ml of the investigated water was diluted with 5 ml of 1 М К and 2 ml of 4 М H2SO4. The triiodide sensor and the silver chloride reference electrode were immersed in the beaker, and obtained solution was titrated with sodium thiosulfate.
X
ΔX
S
40.10
0.11
99.71
0.13
Comparison technique [48] t⁎
F⁎
0.0064
2.06
1.031
0.0062
2.43
1.256
2
X
ΔX
S2
38.81
0.42
0.1024
101.82
0.63
0.2304
4.2. Determination of iodates in salt Iodine as iodate ions was determined in iodinated and iodinefluorinated salt Golden Bison (Belarus) and iodinated salts Artemsil and Drogobych (both Ukraine). An exactly weighed batch of salt was dissolved in 20 ml water, and 5 ml of 1 М К and 2 ml of 4 М H2SO4 were added. The triiodide sensor and the silver chloride reference electrode were immersed in the beaker. The iodate content was determined by the standard addition techniques and by the titration with sodium thiosulfate. 4.3. Determination of iodates in pharmaceuticals Iodine as iodide ions was determined in the pharmaceuticals Iod-active (Farmakom, Ukraine), Antistrumin (Darnytsia, Ukraine) and Iodomarin (Вerlin-Chemie AG, Germany). Ten tablets were powdered in an agate mortar, an exactly weighed batch was dissolved in 25 ml twice-distilled water, the solution was quantitatively transferred into a 50 ml volumetric flask, and twice-distilled water was added to the mark. An aliquot of the prepared solution was poured into a beaker, and 4 ml of 0.1 M NaOH, 5 ml of 0.1 М KMnO4 and 1.5 ml of 2 М H2SO4 were added and heated to 80 °C. The excess was potassium permanganate and was removed by stirring in 5 ml H2C2O4 solution. After cooling to room temperature, 5 ml of 1 М К and 2 ml of 4 М H2SO4 were added. The triiodide sensor and the silver chloride reference electrode were immersed in the beaker, and obtained solution was titrated with sodium thiosulfate. The results of determination agree well with data declared in the specifications, and are comparable with the official methods. Obtained data are shown in Tables 4–6 [48]. Data on testing the homogeneity of averages and on estimating uniform precision and correctness of proposed techniques of direct potentiometric titration of iodates in iodinated drinking water and of the comparison technique are summarized in Table 4. Comparing the dispersions of the comparison technique and the proposed techniques of the determination of iodates using F-test, one can see that the techniques are uniformly precise and uniformly correct. The comparison of the set of averages by Student's test shows that the uniform correctness of obtained data is observed for both proposed sensors. Calculated values of F-test (F*) and Student's test (t*) from S2 and
Table 5 Result of the determination of iodates in iodinated salt (n=5; P=0.95). Notes: F⁎ and Ftabl =5.192, calculated and table value of F-test, respectively; t⁎ and ttabl =2.78, calculated and table value of Student's test, respectively. G* and Gtabl =0.5440, calculated and table value of Cochren test, respectively. Manufacturer, country
Techniques Potentiometric titration X
ΔX
S2
Direct potentiometry, additions technique t⁎
F⁎
10−4% **Golden Bison, Belarus *Golden Bison, Belarus *Artemsil, Ukraine *Drogobych, Ukraine
42 45 43 68
X
ΔX
S2
t⁎
F⁎
Comparison technique [48] G*
10−4% 0.9 1.0 0.8 2.0
0.5041 0.6561 0.4096 0.1156
Composition: *—iodinated; **—iodinated and fluorinated
0.41 0.36 0.35 1.20
1.467 1.347 2.250 1.752
44 41 42 66
X
ΔX
S2
1.1 1.2 1.2 2.3
0.7396 0.8836 0.9216 0.2025
10−4% 1.3 1.3 1.5 2.2
1.2100 1.1236 1.4884 0.1521
0.26 0.16 0.57 0.38
1.636 1.272 1.615 1.331
0.4074 0.4256 0.5007 0.4453
43 43 40 65
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Table 6 Result of the determination of iodides in pharmaceuticals (n=5; P=0.95). Notes: F⁎ and Ftabl =5.192, calculated and table value of F-test, respectively; t⁎ and ttabl =2.78, calculated and table value of Student's test, respectively. Product, manufacturer
Composition
Found, mg Potentiometric titration X
Iod-active, Farmakom Antistrumin, Darnytsia Iodomarin, Вerlin-Chemie AG
Potassium iodide, dried fat-free milk, lactose monohydrate, iodine casein, calcium stearate Potassium iodide, sodium thiosulfate, cellulose, microcrystalline lactose monohydrate, magnesium stearate Potassium iodide, lactose monohydrate, basic magnesium carbonate, gelatin, sodium carboxymethylstarch, silicon dioxide colloid, magnesium stearate
X of the comparison technique are significantly lower than the respective table data (F*bFtabl =5.192, t*bttabl =2.78) for n=5 and confidence probability P=0.95; therefore they are uniform for the developed sensor. Developed sensor was successfully approved for the determination of iodates in iodinated salt by potentiometric titration and by direct potentiometry (additions technique). The results of the comparative estimate of the precision and correctness of the determination of iodates in iodinated salt applying developed techniques are presented in Table 5. The distribution parameters of the salt analysis results were estimated. The results given in Table 5 show that those techniques are uniformly precise for the accepted confidence probability. Calculated from S2 and X of the comparison technique values of F-test (F*) and Student's test are significantly lower than the table data (F*bFtabl =5.192, t*bttabl =2.78). Obtained values of Cochren test (G*) were compared to Gtabl for the number of groups k and degrees of freedom in each group f=n−1. As the calculated value of Cochren test is lower than the table value (G*bGtabl = 0.5440) for k=4, f=5, zero hypothesis of the insignificance of the dispersions is confirmed. Summarizing, we note that the potentiometric titration and by direct potentiometry (additions technique) are worthwhile. The techniques are mutually complementary. The results of the comparative estimate of the precision and correctness of the determination of iodides in pharmaceuticals applying developed techniques are presented in Table 6. Lower reproducibility of the results is achieved with application of a triiodide sensor. The developed methods permit the determination of iodides in the presence of common excipients used in tablet formulations, such as lactose, magnesium and calcium stearate, colloid silica, cellulose etc. 5. Conclusions The results obtained in present work demonstrate that membrane potentiometric sensors based on the ion-pair of Rhodamine B and triiodide with an internal comparison solution were developed. The proposed sensors exhibit long lifetime, good stability, sensitivity, precision, accuracy and selectivity. They are low cost and easy to both prepare and use. Techniques with high metrological characteristics for the quantitative determination of iodine species in iodinated drinking water, salt and pharmaceuticals were developed. According to the results of the statistical analysis, the proposed techniques are uniformly precise and uniformly correct. The results of this study show that a potentiometric method based on triiodide sensor may provide an attractive alternative for the iodine species determination. References [1] B.S. Hetzel, M.T. Mano, J. Nutr. 119 (1989) 145–151. [2] J.S. Edmonds, M. Morita, Pure Appl. Chem. 70 (1998) 1567–1584. [3] G. Mary, N. Balasubramanian, K.S. Nagaraja, Chem. Pharm. Bull. 56 (2008) 888–893.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
[37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48]
2
Comparison technique F⁎
t⁎
X
ΔX
S2
52.75
0.82
0.2916
2.13
101.79
0.47
0.1024
1.05
201.67
0.48
0.1764
ΔX
S
49.14
0.53
0.2025
5.192
0.64
99.68
0.12
0.0980
1.045
198.80
0.25
0.0784
2.240
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
New potentiometric sensor for the determination of iodine species Zholt Kormosh ⁎, Tanya Savchuk Department of Analytical Chemistry and Eco-technology, Volyn National University, 13 Voli Ave, 43025 Lutsk, Ukraine
a r t i c l e
i n f o
Article history: Received 25 October 2011 Received in revised form 10 June 2012 Accepted 19 June 2012 Available online 27 June 2012 Keywords: Triiodide potentiometric sensor Ion-pair Iodine species Rhodamine B Indicator electrode
a b s t r a c t The characteristics, performance and application of novel triiodide potentiometric sensor based on ion-pair of Rhodamine B triiodide as a membrane carrier are described. The electrode has a linear dynamic range between 1×10−6 and 1×10−1 M, with a Nernstian slope of 68±1mV pC−1 and detection limit of 3.9×10−7 M. Fast and stable response, good reproducibility, long-term stability, very good selectivity over a large number of common organic and inorganic anions, applicability over a pH range of 2–10 are demonstrated. The proposed sensor has been applied for potentiometric determination of some iodine species. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Food and pharmaceutical industry is considered the most important in the world since it has sustained continuous research to improve nutritional quality of foodstuffs. Iodine is an essential micronutrient in human growth and is very important for iodine supplementation metabolism. Deficiency of iodine causes serious delay in neurological development, goiter and hypothyroidism. Table salt is supplemented with iodate or iodide to serve as a source of iodine. The iodinated salt is recognized as the method of choice and the most successful strategy for the prevention of iodine deficiency disorders. The recommended concentration of iodate in salts is 40ppm. Iodine is an effective germicide for a wide range of microorganisms. In addition, iodine finds applications as dietary supplements, catalysts, pharmaceutical preparations, stabilizers and in photography. The varied applications of iodine have made the determination of iodine very important [1,2]. Several types of analytical procedures have been reported on the determination of iodine species, such as spectrophotometry [3–8], high performance liquid chromatography [9,10], ion-chromatography [11], spectrofluorimetry [12], differential pulse polarography [13] etc. Ion-selective electrodes (ISEs) remain an important and promising field of analytical chemistry. This is related to the advantages of potentiometry (simplicity, fast response, sensitivity, selectivity, analysis in muddy and colored media etc.) over other analysis methods [14]. Despite the need of direct or indirect determination of iodine as triiodide in many reactions, there have been few reports on triiodide electrodes [15–29]. A research of new electrode-active substances is important. Due to the functional properties of ion-pair (IP) to donate ⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Kormosh). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.06.016
both a cation and an anion as potential-determining ions, they are the most universal ionophores [30–33]. It is likely that solubility is the main restriction for their use as ionophores. The solubility of IPs should be sufficiently low in aqueous phase; otherwise they will be washed out from the membranes, and at the same time, high in the membrane phase, otherwise it will be difficult to obtain a homogeneous membrane. Ion-pairs formed by various quaternary ammonium or phosphonium bases most often exhibit such properties [32–38]. Previously, we analyzed the applicability of ISEs based on ion-pair of basic dyes as electrode-active substances in analytical chemistry [39–45]. The effectiveness of Rhodamine B (RB)-based IPs for making potentiometric sensors was demonstrated [41,43]. The purpose of the present work was to develop a simple and low-cost potentiometric PVC-based triiodide ion sensor, and evaluate the applicability of this new sensor to the determination of iodine species in iodinated drinking water, salt and pharmaceuticals. 2. Experimental 2.1. Reagents and solutions All chemicals were of analytical-reagent grade. Twice-distilled water was used to prepare all solution and in all experiments. Dibutylphtalate (DBP), dioctylphtalate (DOP), dinonylphtalate (DNP), dibutylsebacate (DBS), tricresylphosphate (TCP), cyclohexanone (CHN), and high molecular weight polyvinylchloride (PVC) were obtained from Sigma-Aldrich. Rhodamine B (RB) was purchased from Merck. Standard iodate solution, (1/6)×10 −2 M, was prepared by accurately weighed KIO3 dissolved in 1000 ml twice-distilled water. Standard thiosulfate solution, 1×10 −1 M, was prepared from Na2S2O3·5H2O and standardized with potassium iodate solution.
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Standard iodine (triiodide) solution, 1×10 −1 M, was prepared from of 40 g KI and 12.7 g of I2, dissolved in 500 ml of twice-distilled water and standardized with thiosulfate solution. Working solutions were prepared by dilution of the corresponding standard solutions. Triiodide solutions used for the sensor characterization and analytical determinations (1×10 −8–1×10 −1 M) were prepared daily from the stock solution, diluting the required volume with 0.2 M KI and using 50 ml volumetric flasks. The stock solution of the basic dye Rhodamine B with a concentration of 1×10−2 M was prepared by dissolving an accurately weighed portion of dye, which was recrystallized from methanol, in twice-distilled water. The ionic strength of the solutions was adjusted with 0.2 M KI solution. The pH value of solutions was maintained with the use of a buffer mixture (0.04 M CH3COOH, H3BO3, H3PO4, and a 0.2 M NaOH solution) and controlled potentiometry with a glass electrode. 2.2. Preparation of ion-pair and membrane for sensor The ion-pair was preparing by mixing 1×10 −2 M solutions of Rhodamine B and triiodide in a ratio of 1:1. The resulting solution was settled for 2 h, and the sediment of ion-pair was filtered out (quantitative rapid filter paper). This residue was treated with 50 ml of cold distilled water. The filter paper containing the precipitate was then dried for 24 h at room temperature. These IPs (low solubility in water and good solubility in membrane plasticizers, along with ion-exchange properties) were used as an electrode-active material for preparing membranes of triiodide sensor. The generally accepted technique of plasticized membrane production by Moody, Oke, Thomas [46,47] consists of thorough mixing of the electrode-active substance with PVC dissolved in cyclohexanone or tetrahydrofuran followed by the evaporation of the solvent in a glass ring. We weighed 0.1 g PVC and the respective amount of isolated IP (such that its concentration in the membrane was 5–25 mass %), then mixed the materials thoroughly for homogenizing. The degree of materials homogeneity was estimated from photomicrographs obtained with a LEICA VMHT Auto microhardness tester. Afterwards, 0.1 ml of a plasticizer (DBP, DBS, DOP, DNP or TCP) and 0.5 ml of solvent (cyclohexanone or tetrahydrofuran) were added. The resulting solution was transferred into a previously polished glass form 1.7 cm in diameter, glued to the glass substrate, and dried in air for 1–2 days. The films obtained after evaporation of the solvent were cut using a rubber cork cutter into disks of 0.5–1.0 cm diameter. These were then glued to the end of the PVC tube with 10% PVC solution in cyclohexanone. The tube was filled with the concentrated standard triiodide solution (1×10 −2 M), and a copper wire was immersed into it. The sensor was then used for the investigation.
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3. Results and discussion 3.1. Association constant and IR spectra Considerable changes in the light absorption of the system were observed upon mixing the solutions of the basic dye Rhodamine B. As the concentration of triiodide was increased at a constant concentration of the dye, a bathochromic shift of the dye absorption band was observed (Fig. 1), which can be indicative of the formation of compounds like ion-pair. This was also supported by the experimental IR spectra. Тhe IR spectra of Rhodamine B and Rhodamine B triiodide have little difference. This is typical for compounds such as ion-pair. The vibrational bands of the groups responsible for the association are the most informative ones. As seen from Fig. 2, the spectra of Rhodamine B feature an intense νС N band at 1620 cm −1. In the formation of the Rhodamine B triiodide ion-pair this band shifts to 1570 cm −1. If sufficiently dilute reagent solutions are used and acidity is maintained at a level at which singly charged reagent species are predominant, the formation of ion-pair can be represented by the following reaction scheme: −
−
I2 þ I ↔I3 þ
ð1Þ þ
−
RB þ I3 ⇔RB ⋅I3
−
ð2Þ
Then, the concentration constant of the IP formation (association constant): þ − RB ⋅I3 K as ¼ ; C I3 − −½RBþ ⋅I3 − ⋅ðC RB −½RBþ ⋅I 3 − Þ
ð3Þ
þ − where RB ⋅I3 is the equilibrium concentration of the ion associate. The equilibrium concentration of the IP can be easily calculated from spectroscopic data: h
þ
RB ⋅I3
−
i
¼
εRB ⋅C RB ⋅l−A ; ðε RB −εas Þ⋅l
ð4Þ
where A is the absorbance of solutions at an absorption band maximum of the RB cation, and εRB and εas are the molar absorption coefficients of the dye cation and the ion-pair, respectively. The association constant thus found is (1.6±0.2)×10 4. Because of the restricted water solubility of this compound, the formation of a suspension (precipitate) of the ion-pair was observed as the concentrations of reacting components were increased (more as 1.2×10 −4 M of triiodide-ions); this allowed us
2.3. Apparatus All EMF measurements were carried out with the following cell assembly. An I-160M model pH–mV meter with an EVL-1MZ silver–silver chloride reference electrode was used for the measurements of potential difference at 25.0±1.0°C. The electronic absorption spectra of solution were measured on an SF-2000 spectrophotometer (LOMO, RUSSIA) over a range of 450–750 nm. The IR spectra were measured on an AVATAR 330 FT-IR instrument (Thermo Nicolet). The thermal studies (TG, DTG and DTA) were carried out on an apparatus for complex dynamic thermal analysis—MOM derivatograph (Hungary) under the following conditions: temperature range 20– 900 °C, heating rate 5 K/min, sample weight 36 mg, static air medium, channel sensitivities: DTA—150 mV, TG—50 mV.
Fig. 1. Absorption spectra of the triiodide-ion – RB system, 1×10−5 M RB. Triiodide-ion concentrations, M: 1–0, 2–4×10−5, 3–8×10−5, 4–1.2×10−4, 5–1.6×10−4, 6–2×10−4.
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Z. Kormosh, T. Savchuk / Materials Science and Engineering C 32 (2012) 2286–2291 Table 1 Data of DTA curve of the ion-pair (C28H31O3N2+·I3−·1.5H2O) and decomposition products. Temperature range, °С
tmax, °С
20–100 50 endo 240–280 265 exo 280–325 300 exo 325–388 388–680 485 exo Total loss of mass
Loss of mass, %
Decomposition products
Experimental
Theoretical
3.32 20.28 5.56 2.78 62.50 94.44
3.17 19.03
1.5Н2О 2NH3 +Н
9.88 62.27 94.35
3C2H4 (18С+12Н+3О+2 ) 4С (solid product)
these stages, the gaseous thermolysis products contain NH3, C2H4, C4H8, НI, and I2. An intense exothermal effect is observed above 380 °С with the maximum at 485 °С which is accompanied by a significant loss of mass. This is identified as the destructive combustion of the major part of IP. The total loss of mass is 94.4% indicating that some carbon does not combust completely and remains as a black film on the crucible wall. The process of the thermal destruction of IP can be described in the following scheme: þ
C28 H31 O3 N2 ⋅I3 ⋅1:5H2 O→1:5H2 O↑ þ 2NH3 ↑ þ НI↑ þ 3C2 H4 ↑þ
Fig. 2. IR spectra of RB (1) and ion-pair (2).
to isolate this ion-pair: þ
−
þ
−
RB þ I3 ⇔RB ⋅I3 ↓
þðC22 H12 O3 Þ⋅I2 þ 4Cresidue ð6Þ
3.2. Thermal behavior of the ion-pair Thermal decomposition of the triiodide-Rhodamine B ion-pair in air undergoes five stages accompanied by mass loss and endo- or exothermic effects (Fig. 3, Table 1). At the first stage of the IP thermo destruction water molecules are separated in the range of 20–100 °С (tmax =50 °С). This indicates the presence of hygroscopic 1.5 molecules of water in the complex. This is easily seen by heating the sample in a hard-glass test tube where water drops or mist condensate at the cold walls. No decomposition of the complex is observed in the 100–240 °С range. We predict that the sensors based on this IP can operate in hot solutions. The second stage (240–280 °С) is accompanied by large mass loss and blends smoothly into the third and fourth stages which end at t=388 °С. At
Fig. 3. Simultaneous TG, DTA and DTG curves of the IP (C28H31O3N2+·I3−·1.5H2O) in air.
C22 H12 O3 ⋅I2 þ 13:5O2 →11CO2 þ 6H2 O þ I2 Experimental and calculated mass loss agrees satisfactorily which confirms the correctness of the proposed scheme of the IP thermolysis process.
3.3. Electrode response It is well known that the sensitivity and selectivity of ion-selective electrodes depend not only on the nature of ionophore used, but also significantly on the membrane composition and the properties of the plasticizer employed. To evaluate the potentiometric characteristics of sensor based on IP triiodide of Rhodamine B, five different plasticizers (DBP, DBS, DOP, DNP, TCP) were considered in the selective membrane preparation. The plasticizer content was 65% of the total mass of the membrane components, and the ionic strength of the solution was adjusted with 0.2 M KI solution. The results of the study of the influence of solvent mediators on the potentiometric response characteristics of the sensors are summarized in Table 2.
Fig. 4. Calibration curve for triiodide-ion sensor.
Z. Kormosh, T. Savchuk / Materials Science and Engineering C 32 (2012) 2286–2291
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Table 2 The effect of plasticizer on the response of triiodide sensor (10 mass. % IP). Plasticizer (65%)
Slope, mV pC−1
Linearity range, M
Low detection limit, M
DOP DBP DBS TCP DNP
63±1 68±1 63±1 68±1 56±1
1×10−4–1×10−1 1×10−5–1×10−1 1×10−5–1×10−1 1×10−6–1×10−1 1×10−5–1×10−1
3.2×10−5 4.5×10−6 3.2×10−6 3.9×10−7 3.0×10−6
It was found that the best characteristics on the electrode function slope are exhibited by the membranes plasticized with slope values of 56–68 mV рС −1. The low detection limits were n∙(10 −7–10 −6) M, regardless of the plasticizer employed. Therefore the nature of the plasticizer (its polarity ε) does not affect the sensitivity of the detection of the investigated ions but contributes remarkably to the value of the electrode function slope. It was established that an increase in the permittivity of the solvent in the series DOPbDBPbTCP leads to an increase in the slope of the electrode function. This is important for the selection of the optimal conditions for sensor performance. The effect of the plasticizer itself on the electrode function slope, its linearity, and the detection limit were investigated. The ratio of plasticizer to added PVC in the membrane was varied from 1:1 to a 2-fold excess. The optimal component ratio was found to be 1.5 plasticizer to 1.0 PVC. A membrane of sufficient elasticity and homogeneity could not be produced at other ratios of the components. No dependence of a cation or a mixed electrode function on the ratio membrane polymer:plasticizer was found; therefore the matrix does not affect the selectivity of the film membranes. This assumption is indirectly corroborated by the closeness of the electrode parameters of the sensors (linearity of the electrode function and detection limits) as the PVC:plasticizer ratio increases. The plasticizer content is also related to the lifetime of the electrode, which is determined by the frequency of its use. It was established that electrodes with lower plasticizer content last somewhat less time than those with high plasticizer content. The solvent content in the membrane decreases with time, which leads to structure failure and loss of elasticity and limits the electrode lifetime. It was determined that sensors with 60–70% plasticizer content have a shelf life of 18–20 weeks from the preparation date, unlike sensors with a lower (40–50%) plasticizer content. After this period of slope of electrode function can be reduced to 45–50 mV pC −1 and that equilibrium is reached in a long time (~15–30 s). We prepared membranes with 5–15% content of the electrode-active substance. The study of the electroanalytical properties of the produced sensors with various ion-pair concentrations shows the response to the concentration of triiodide-ions over a wide range of n×10−6–1×10−
Fig. 6. The effect of the solution pH on the electrode potential of triiodide: pC=4 (1), pC=3 (2).
1
M (Fig. 4). The best results were obtained for membranes containing 10% EAS content (low detection limit (LDL) 3.9×10−7 M). The response time of the electrodes was obtained by measuring the time required to achieve a steady state potential (within ± 1 mV) after successive immersion of the electrodes in a series of triiodide-ions solutions, each having a 10-fold increase in concentration from 1.0×10 −7 to 1.0×10 −1 mol L −1. As shown in Fig. 5 the electrodes yielded steady potentials within 1–8 s (in solutions at 1.0×10 −4–1.0×10 −1 mol L −1 of triiodide-ions). The potential reading stays constant, to within ±1 mV, for at least 5 min. It is much better than known in the literature triiodide-ions sensors. The developed sensors work well for 18–20 weeks. After this period of slope of electrode function can be reduced to 45–50 mV pC −1 and that equilibrium is reached in a wary long time (~15–30 s).
3.4. Effect of the acid medium The factor that most affects the performance of practically all sensors is the acidity of the solution. One should also take into account that the acidity affects the state of the ion-pair and other membrane components, i.e. the membrane–solution interface can host a series of parallel protolytic processes, a description of which may become quite complex. The working range of an IP-based sensor is determined by the protolytic properties of the large ions that compose the ion-pair. We have investigated the effect of the acidity on the electrode characteristics of sensors. The results show a stable potential response over pH ranges of 2–10 (Fig. 6). At higher pH values, irreversible Table 3 Comparative characteristics of the selectivity (−log Kpot) of various triiodide potentiometric sensors. Anion
F− Cl− Br− I− NO3− SCN− ClO4− SO42− HPO42− Sal− Benz− Pikr− CH3COO− Fig. 5. Dynamic responses of the triiodide-ion sensor.
a
This worka
Published data [15]
[16]
[17]
[22]
[25]
[26]
>4 >4 >4 >4 >4 – >4 – – 3.3 3.4 – –
– 3.19 2.76 1.69 3.19 2.76 2.90 3.31 3.14 2.97 – – 3.86
– 3.2 3.3 2.6 3.1 2.7 3.5 4.5 3.4 – – – 2.7
– >5 >5 (3.35) 2.79 (3.29) >5 (4.72) 2.19 (2.72) 3.41 (4.44) 3.78 (3.22) 3.10 (3.61) 3.04 (2.62) 2.80 (4.16) – 4.43 (3.96)
>5 3.82 4.60 3.52 – 3.82 – 4.60 – – – – >5
– 5.20 4.80 3.15 4.30 3.90 5.00 4,37 4.68 – – – 4.71
65% TCP, 10% IP, 25% PVC.
>5 >5 >5 >5 >5 >5 >5 >5 >5 >5 4.5 5.0 >5
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Table 4 Result of the determination of iodates in iodinated drinking water (n=5; P=0.95). Notes: F⁎ and Ftabl =5.192, calculated and table value of F-test, respectively; t⁎ and ttabl =2.78, calculated and table value of Student's test, respectively. Product, manufacturer
Composition
Iodate found, mg Potentiometric titration
“Etalon” still iodinated drinking water, Subos Company Iodanka Pavlivska, Iodanka LLC
3
3
Chlorides ≤100.0 mg/dm , sulfates≤50.0 mg/dm , 0.03–0.06 mg/dm3 iodate Hydrocarbonate, chloride-hydrocarbonate, sodium-calcium, 100±10 μg/dm3 iodate
disproportionation of triiodide ions to hypoiodate and iodide ions occurs both of which are insensitive to the membrane sensor. 3.5. Selectivity Selectivity behavior is obviously one of the most important characteristics of sensor, determining whether a reliable measurement in the target sample is possible. In this work, selectivity coefficients over interfering anions were evaluated by the separate solution method. The resulting values are listed in Table 3. It is immediately obvious that the proposed triiodide membrane sensor is highly selective with respect to most common organic and inorganic anions such as fluoride, chloride, bromide, iodide, nitrate, sulfate, and thiocyanate. The logarithms of selectivity coefficients for all anions tested are −5 or smaller which seems to indicate that these anions have negligible disturbance on the functioning of the triiodide membrane sensor. The surprisingly high selectivity of the sensor for triiodide ion over other anions used arises most probably from the strong tendency of the carrier molecule RB for triiodide ion in the form of the very stable ion-pair. The terms of selectivity coefficients the proposed triiodide sensor are superior to those reported for other triiodide-selective electrodes. 4. Analytical applications The proposed membrane sensor was successfully employed for the assay of different iodine species in iodinated drinking water, salt and pharmaceuticals.
4.1. Determination of iodates in drinking water Iodine as iodate ions was determined in bottled still drinking water Etalon (Subos Company, Ukraine) and Iodanka Pavlivska (Iodanka LLC, Ukraine). An aliquot of 5–25 ml of the investigated water was diluted with 5 ml of 1 М К and 2 ml of 4 М H2SO4. The triiodide sensor and the silver chloride reference electrode were immersed in the beaker, and obtained solution was titrated with sodium thiosulfate.
X
ΔX
S
40.10
0.11
99.71
0.13
Comparison technique [48] t⁎
F⁎
0.0064
2.06
1.031
0.0062
2.43
1.256
2
X
ΔX
S2
38.81
0.42
0.1024
101.82
0.63
0.2304
4.2. Determination of iodates in salt Iodine as iodate ions was determined in iodinated and iodinefluorinated salt Golden Bison (Belarus) and iodinated salts Artemsil and Drogobych (both Ukraine). An exactly weighed batch of salt was dissolved in 20 ml water, and 5 ml of 1 М К and 2 ml of 4 М H2SO4 were added. The triiodide sensor and the silver chloride reference electrode were immersed in the beaker. The iodate content was determined by the standard addition techniques and by the titration with sodium thiosulfate. 4.3. Determination of iodates in pharmaceuticals Iodine as iodide ions was determined in the pharmaceuticals Iod-active (Farmakom, Ukraine), Antistrumin (Darnytsia, Ukraine) and Iodomarin (Вerlin-Chemie AG, Germany). Ten tablets were powdered in an agate mortar, an exactly weighed batch was dissolved in 25 ml twice-distilled water, the solution was quantitatively transferred into a 50 ml volumetric flask, and twice-distilled water was added to the mark. An aliquot of the prepared solution was poured into a beaker, and 4 ml of 0.1 M NaOH, 5 ml of 0.1 М KMnO4 and 1.5 ml of 2 М H2SO4 were added and heated to 80 °C. The excess was potassium permanganate and was removed by stirring in 5 ml H2C2O4 solution. After cooling to room temperature, 5 ml of 1 М К and 2 ml of 4 М H2SO4 were added. The triiodide sensor and the silver chloride reference electrode were immersed in the beaker, and obtained solution was titrated with sodium thiosulfate. The results of determination agree well with data declared in the specifications, and are comparable with the official methods. Obtained data are shown in Tables 4–6 [48]. Data on testing the homogeneity of averages and on estimating uniform precision and correctness of proposed techniques of direct potentiometric titration of iodates in iodinated drinking water and of the comparison technique are summarized in Table 4. Comparing the dispersions of the comparison technique and the proposed techniques of the determination of iodates using F-test, one can see that the techniques are uniformly precise and uniformly correct. The comparison of the set of averages by Student's test shows that the uniform correctness of obtained data is observed for both proposed sensors. Calculated values of F-test (F*) and Student's test (t*) from S2 and
Table 5 Result of the determination of iodates in iodinated salt (n=5; P=0.95). Notes: F⁎ and Ftabl =5.192, calculated and table value of F-test, respectively; t⁎ and ttabl =2.78, calculated and table value of Student's test, respectively. G* and Gtabl =0.5440, calculated and table value of Cochren test, respectively. Manufacturer, country
Techniques Potentiometric titration X
ΔX
S2
Direct potentiometry, additions technique t⁎
F⁎
10−4% **Golden Bison, Belarus *Golden Bison, Belarus *Artemsil, Ukraine *Drogobych, Ukraine
42 45 43 68
X
ΔX
S2
t⁎
F⁎
Comparison technique [48] G*
10−4% 0.9 1.0 0.8 2.0
0.5041 0.6561 0.4096 0.1156
Composition: *—iodinated; **—iodinated and fluorinated
0.41 0.36 0.35 1.20
1.467 1.347 2.250 1.752
44 41 42 66
X
ΔX
S2
1.1 1.2 1.2 2.3
0.7396 0.8836 0.9216 0.2025
10−4% 1.3 1.3 1.5 2.2
1.2100 1.1236 1.4884 0.1521
0.26 0.16 0.57 0.38
1.636 1.272 1.615 1.331
0.4074 0.4256 0.5007 0.4453
43 43 40 65
Z. Kormosh, T. Savchuk / Materials Science and Engineering C 32 (2012) 2286–2291
2291
Table 6 Result of the determination of iodides in pharmaceuticals (n=5; P=0.95). Notes: F⁎ and Ftabl =5.192, calculated and table value of F-test, respectively; t⁎ and ttabl =2.78, calculated and table value of Student's test, respectively. Product, manufacturer
Composition
Found, mg Potentiometric titration X
Iod-active, Farmakom Antistrumin, Darnytsia Iodomarin, Вerlin-Chemie AG
Potassium iodide, dried fat-free milk, lactose monohydrate, iodine casein, calcium stearate Potassium iodide, sodium thiosulfate, cellulose, microcrystalline lactose monohydrate, magnesium stearate Potassium iodide, lactose monohydrate, basic magnesium carbonate, gelatin, sodium carboxymethylstarch, silicon dioxide colloid, magnesium stearate
X of the comparison technique are significantly lower than the respective table data (F*bFtabl =5.192, t*bttabl =2.78) for n=5 and confidence probability P=0.95; therefore they are uniform for the developed sensor. Developed sensor was successfully approved for the determination of iodates in iodinated salt by potentiometric titration and by direct potentiometry (additions technique). The results of the comparative estimate of the precision and correctness of the determination of iodates in iodinated salt applying developed techniques are presented in Table 5. The distribution parameters of the salt analysis results were estimated. The results given in Table 5 show that those techniques are uniformly precise for the accepted confidence probability. Calculated from S2 and X of the comparison technique values of F-test (F*) and Student's test are significantly lower than the table data (F*bFtabl =5.192, t*bttabl =2.78). Obtained values of Cochren test (G*) were compared to Gtabl for the number of groups k and degrees of freedom in each group f=n−1. As the calculated value of Cochren test is lower than the table value (G*bGtabl = 0.5440) for k=4, f=5, zero hypothesis of the insignificance of the dispersions is confirmed. Summarizing, we note that the potentiometric titration and by direct potentiometry (additions technique) are worthwhile. The techniques are mutually complementary. The results of the comparative estimate of the precision and correctness of the determination of iodides in pharmaceuticals applying developed techniques are presented in Table 6. Lower reproducibility of the results is achieved with application of a triiodide sensor. The developed methods permit the determination of iodides in the presence of common excipients used in tablet formulations, such as lactose, magnesium and calcium stearate, colloid silica, cellulose etc. 5. Conclusions The results obtained in present work demonstrate that membrane potentiometric sensors based on the ion-pair of Rhodamine B and triiodide with an internal comparison solution were developed. The proposed sensors exhibit long lifetime, good stability, sensitivity, precision, accuracy and selectivity. They are low cost and easy to both prepare and use. Techniques with high metrological characteristics for the quantitative determination of iodine species in iodinated drinking water, salt and pharmaceuticals were developed. According to the results of the statistical analysis, the proposed techniques are uniformly precise and uniformly correct. The results of this study show that a potentiometric method based on triiodide sensor may provide an attractive alternative for the iodine species determination. References [1] B.S. Hetzel, M.T. Mano, J. Nutr. 119 (1989) 145–151. [2] J.S. Edmonds, M. Morita, Pure Appl. Chem. 70 (1998) 1567–1584. [3] G. Mary, N. Balasubramanian, K.S. Nagaraja, Chem. Pharm. Bull. 56 (2008) 888–893.
[4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
[37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48]
2
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0.0980
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198.80
0.25
0.0784
2.240
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