Iodine is an element that has several oxidation states [1, 2]. The determination of various iodine species, especially in pharmaceuticals, is of interest in analytical chemistry. Potentiometry using ion-selective sensors can perform this task [3, 4].

The goals of the present work were to study the possibility of using isolated ionic associates of diiodobromide and rhodamine B as an electrode-active component in plasticized sensor membranes and to fabricate from them a new potentiometric sensor for povidone-iodine determination.

Experimental Part

Stock solutions (10–2 M) of rhodamine B (RhB) were prepared by dissolving an accurately weighed portion of the previously purified compound in doubly distilled H2O with a small amount of added EtOH. A standard solution (10–1 M) of diiodobromide was prepared by dissolving an accurately weighed portion of I2 in KBr solution (0.2 M). Astandard solution (0.1 M) of Na2S2O3 was prepared from fixanal and also standardized by iodometry.

Ionic associates (IA) were prepared by precipitation from stirred solutions (10–2 M) of RhB and KI2Br (1:1 ratio). The mixture was stirred and left at room temperature for 24 h. The resulting precipitate was filtered off, rinsed several times with cold distilled H2O, and dried at room temperature for 4 d.

Plasticized polyvinylchloride (PVC) membranes were prepared according to recommendations [5]. PVC (0.1 g) and a certain amount of IA (1 – 15% of total membrane mass) were mixed, treated with plasticizer (0.1 mL) [dioctylphthalate (DOP), dibutylphthalate (DBP), dinonylphthalate (DNP), dinonylsebacate (DNS), or tricresylphosphate (TCP)] and solvent (0.7 mL) (cyclohexanone or tetrahydrofuran), thoroughly mixed to produce a homogeneous mass, transferred into a form (ring of diameter 1.5 cm), preliminarily polished, glued to a glass substrate, and dried in air for 5 – 7 d. Membranes (0.7 cm in diameter) were cut from the obtained films and glued to the ends of PVC tubes. The degree of homogeneity of the membranes was estimated using a METAM P-1 metallographic microscope.

Potentiometric measurements were taken from an I-160.M ion-meter at room temperature using an EVL-1MZ AgCl standard electrode.

Results and Discussion

The effect of the plasticizer on the sensor electrochemical characteristics was studied. The results in all instances showed that an electrode function was observed in the I2Br–concentration range 10–6 – 10–1 M. The slope of the electrode function for membranes plasticized with DOP, DBP, and TCP was 61 – 73 mV/pC with a minimum determined concentration of n · 10–5 M. Sensors with DNS and DNP plasticizers showed a Nernst function of slope 42 – 43 mV/pC with a minimum determined concentration n · 10–6 M (Table 1). It can be seen that lengthening the plasticizer hydrocarbon chain on going from DOP to DNP gave a more heterogeneous membrane and caused the electrode function slope to decrease.

Table 1 Electrochemical Characteristics of Diiodobromide Sensors
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Acidity also affected the potential of the I2Br– sensors. It was found that the sensor operating range with respect to acidity was pH 2 – 10. The large potential increase of the sensor at pH > 10 was due to hydrolysis of I2Br–.

The response time of the developed sensors showed that the potential was established in 2 – 3 sec for solutions with [I2Br–] = 10–4 – 10–1 M and 5 – 7 sec for those with [I2Br–] = 10–8 – 10–5 M.

The potentiometric selectivity coefficients (−log K pot i,j ) of the I2Br– sensors for Cl– (5.0), Br– (4.6), I– (4.3), NO3 – (4.6), SCN– (4.2), ClO4 – (4.1), SO4 2– (5.0), HPO 4 2– (5.0), etc. were determined by the mixed solutions method and were >4. Model solutions showed that the excipients did not affect the analytical chemical characteristics of the sensors. This indicated that they could be used as analytical indicator electrodes, e.g., to determine povidone-iodine in pharmaceuticals.

Method for determining povidone-iodine

IOX formulation. An aliquot (2 – 5 mL) of the formulation was taken and treated with KBr (0.2 M) and H2SO4 (2 M) solutions. The reference electrode was AgCl; titrant, Na2S2O3 solution (10–3 M).

Betadine formulation (suppositories). Povidone-iodine was determined by dissolving a suppository with heating in distilled H2O (20 mL). The resulting solution was cooled, treated with KBr (0.2 M) and H2SO4 (2 M) solutions, and titrated potentiometrically. Three parallel measurements (p = 0.95) were made. The analytical results were calculated using mathematical statistics (Table 2).

Table 2 Determination of Povidone-Iodine in Pharmaceuticals (n = 3; P = 0.95)
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Povidone-iodine reacts with thiosulfate according to the equation:

$$ {\mathrm{R}}^{+}{\mathrm{I}}_2{\mathrm{Br}}^{-}+2{\mathrm{S}}_2{{\mathrm{O}}_3}^{2-}\overset{{\mathrm{H}}^{+}}{\to }{\mathrm{R}}^{+}+2{\mathrm{I}}^{{}^{\_}}+{\mathrm{Br}}^{-}+{\mathrm{S}}_4{{\mathrm{O}}_6}^{2-}. $$

It was shown that the synthesized ionic associate of I2Br– and RhB could be used as an electrode-active compound for an I2Br–-sensitive potentiometric sensor. The operating conditions of the proposed sensor were investigated (effect of solution pH, plasticizer, I2Br– concentration, response time, etc.). The selectivity of the sensors was studied. Based on the results, a new sensitive and selective method that was simple to use was developed for potentiometric determination of povidone-iodine and was tested for determining it in pharmaceuticals.

The work was supported financially by the Ukraine Ministry of Education and Science.