Tin ( II ) Selective PVCMembrane Electrode Based on Salicylaldehyde Thiosemicarbazone as an Ionophore

A polymeric membrane-based tin selective electrode was developed by using salicylaldehyde thiosemicarbazone (STSC). e best performance was recorded with amembrane composition of PVC : TBP : ionophore : NaTPB as 28 : 59 : 8 : 5 (w/w%).eNernstian slope calculated from the calibration curve for Sn sensor was 28.8 ± 0.4mV/decade. e detection limit of the sensor was 2.10 × 10M, in the linear concentration range of 1.0 × 10 − 1.1 × 10M. It was relatively fast response time (<8 s for concentration ≥1.0 × 10 and <12 s for concentration of ≥1.0 × 10M) and can be used for 9 months without any considerable divergence in potentials. e proposed sensor exhibit relatively good selectivity and high sensitivity for tin(II) as other mono-, di-, and trivalent cations and can be used in a pH range of 2.0–8.5. e analytical usefulness of the proposed electrode has been evaluated by its application in the determination of stannous in arti�cially made samples.


Introduction
Tin is a natural element in the earth's crust.Tin metal is used to line cans for food, beverages, and aerosols.It can combine with chemicals like chlorine, sulfur, or oxygen to form inorganic tin compounds.ese compounds are used in toothpaste, perfumes, soaps, food additives, and dyes.Tin can also combine with carbon to form organotin compounds which are used to make plastics, food packages, plastic pipes, pesticides, paints, and pest repellents.Inorganic tin compounds are used as pigments in the ceramic and textile industry.In addition, stannous chloride, SnCl 2 , is widely used in daily human life to conserve so drinks, in food manufacturing, processing, and packaging, and in biocidal preparations.e available methods for determination of low concentrations of tin ion in solution include spectrophotometry [1,2], atomic absorption spectrometry [3,4], atomic emission spectrometry [5,6], electrochemical methods [7,8], and spectro�uorimetric methods [9].ese methods are either time consuming, involving multiple sample manipulation, or too expensive for the most analytical laboratories.e potentiometric membrane sensors have shown to be very effective tools for the analysis of a wide variety of metal ions [10,11].ey are very simple, fast, inexpensive, and capable of providing reliable responses in a broad concentration range.
In the present study salicylaldehyde thiosemicarbazone was incorporated as the ionophore.It shows better potentiometric response as compared to the previously reported tin ion selective electrodes [12,13] based on different ionophores.

Potential Measurements
All the membrane electrode potential measurements were performed at constant temperature (25 ± 0.05 ∘ C) using digital pH meter and potentiometer (ELICO L1-10, India) in conjugation with saturated calomel electrodes as reference electrodes.e representation of electrochemical cell for the EMF measurements is shown in Table 1.
3.1.Electrode Preparation.e general procedure was used for the preparation of the PVC membrane.For the preparation of membrane contents� the �xed mixture of PVC : plasticizer : ionophore : excluder in 28 : 59 : 8.0 : 5.0 (w/w%) was taken as �xed.e mixture was thoroughly dissolved in THF (5 mL).e resulting mixture was poured into a glass dish.e solvent evaporated slowly until a sticky and oily mixture was obtained.A 5 mm diameter Pyrex tube was dipped into this mixture for few seconds and then removed.To obtain a 0.3 mm thick nontransparent membrane, �x one end of the Pyrex tube.It was dried for 5 Hrs.Pyrex tube was then �lled with 1.0 × 10 −2 M Sn(NO 3 ) 2 as an internal �lling solution.e electrode was kept as it is and conditioned for 24 hrs by soaking it in Sn(NO 3 ) 2 solution.e blank membrane having only PVC as membrane ingredients was also prepared and studied, while membrane having PVC with plasticizer generates small potential with slope of ∼8.9 mV/decade.

Effect of Membrane Composition and
Response Characteristics.e response of different metal ions and rare earth ions as plotted as the negative log of concentration and the potential values obtained for different metal ions.Best response and slope were seen for the Sn 2+ ion as compared to the other cations as shown Figure 2. e plot between the concentration and EMF in Figure 2 indicates that the best response was obtained for the Sn 2+ , whereas transition metal ion gave a poor response.e calibration curve slope was found to be 28.8 ± 0.4 mV/decade in the linear range of 1 × 10 −2 -1 × 10 −7 M Sn 2+ concentration with the detection limit of 2.10 × 10 −8 M. Further result of changes in the membrane composition on the electrode response was studied.It is well known [14,15] that sensitivity and selectivity obtained for a given ionophore is signi�cantly affected by the membrane composition of an ion sensor membrane.e different membrane ingredients, such as amount of ionophore and nature of the plasticizer and additives, in�uence the potentiometric response behaviour of the sensor [16,17].Best optimized studies membrane for Sn 2+ selective electrode are reported in Table 2 sensor 2. e results given in the Table 2 show that the electrode worked well in plasticizer/PVC ratio of nearly 2, which ensures enough mobility of the membrane constituents.e best sensitivity and selectivity was recorded for TBP as plasticizer.Further the amount of ionophore also affected the Nernstian slope value.It has been observed (Table 2) that on increasing the concentration of the ionophore more than 2% (w/w), the slope deteriorated and a narrow linear concentration range was achieved.Hence, a very small amount of ionophore is required for the study.e presence of lipophilic anion excluder in cation sensor based on neutral carrier not only reduces the ohmic resistance but also enhances the response behaviour and selectivity [14].It also assists in the extraction capability of the carrier in the membrane electrodes study [18,19].According to the results in Table 2, most optimum plasticizer used was TBP and the ionic additive NaTPB, and the optimum ratio for further studies was PVC : TBP : ionophore : NaTPB as 28 : 59 : 8 : 5 (w/w%).

pH and Nonaqueous
Effect.e pH dependence of the electrode potential was tested over the range 0.5-14.0for Sn 2+ in concentration 1.0×10 −3 as shown in Figure 3. e pH of the solutions was adjusted by the addition of dilute hydrochloric acid or sodium hydroxide.It is clear from Figure 3 that the useful pH range is 2.0-8.5, as the potentials remain constant in this range.e sharp change in potentials at higher pH values may be due to the hydrolysis of Sn 2+ , while at lower pH values H + ions start contributing to the charge transport process of the membrane, thereby, causing interference.

Response and Lifetime.
Response time is critically reviewed when it comes to analytical application of the sensor [20].e static response time of the membrane sensor thus obtained was <8 s, for concentration ≥1.0×10 −4 and <12 s for concentration of ≥10 −6 (Figure 4).It should be noted that the equilibrium potential remains constant for more than 8 min.e lifetime of the membrane sensor was about 9 months, during which it could be used without any measurable divergence.e time of contact and concentration of the equilibrium solution was also optimized so that the stable sensor generated and reproducible potentials at relatively short response time obtained.It was found that an equilibrating solution of 1.0 × 10 −2 M and contact time of 24 Hrs was appropriate for smooth functioning of the electrode.Membranes were stored in 1.0 × 10 −2 M Sn(NO 3 ) 2 solution when not in used.

Potentiometric
Selectivity.e selectivity is the most important characteristic, as it determines the extent of utility of a sensor in real sample measurement.e selectivity coefficient values were determined by �xed interference method (FIM) [20,21].e selectivity coefficient values indicate that the electrode is moderately selective to Sn 2+ over a number of other cation (Table 3).e potential of a cell comprising an ion-selective electrode [22] and a reference electrode is measured with solutions of constant level of interference,  B , and varying activity of the primary ion,  A .e potential values obtained are plotted against the activity of the primary ion.e intersection of the extrapolation of the linear portions of this curve will give the value of  A which is to be used to calculate  pot A,B from the equation: where both  A and  B have the same signs, positive or negative.Value of selectivity coefficient equal to 1.0 indicates that the sensor responds equally to primary as well as interfering ions.However, values smaller than 1.0 indicate that membrane sensor is responding more to primary ion than to interfering ions and in such a cases the sensor is said to be selective to primary ion over interfering ion.Further, the smaller the selectivity coefficient is, the higher the selectivity Transfer 50 mL of this solution to a 500 mL conical �ask, and add 5 g of potassium sodium tartrate, and then a cold saturated solution of sodium bicarbonate until the solution is alkaline to litmus paper.Titrate at once with 0.1 N iodine using starch TS as the indicator.e obtained results were comparatively compiled with sensor no. 2 in Table 4.Each mL of 0.1 N iodine consumed is equivalent to 11.28 mg of SnCl 2 ⋅ 2H 2 O.

Conclusions
In this work, the �rst Sn(II) electrochemical sensor was introduced based on of salicylaldehyde thiosemicarbazone (STSC) as a sensing material.e proposed electrochemical sensor with a membrane composition containing 28% poly(vinyl chloride) (PVC), 59% tri-n-butyl phosphate (TBP) as solvent mediator, 5% sodium tetraphenylborate (NaTPB) as cation excluder, and 8% STSC worked as an ionophore.e electrochemical sensor exhibits a nice Nernstian response of 28.8 ± 0.4 mV per decade of tin activity and a wide dynamic working range 1.0 × 10 −2 -1.1 × 10 −7 M. e detection limit of the sensor is 2.10 × 10 −8 M and can be used in the pH range of 2.0-8.5.e electrochemical sensor has fast response time and relatively long lifetime (more than 9 months).

T 2 :F 2 :
Composition of PVC membrane of (STSC) and performance characteristics of Sn 2+ selective sensors based on them.Potential response of various ion-selective electrodes based on STSC.

3 F 3 : 6 F 4 :
e effect of pH of test solution on the response of the Sn 2+ ion-selective electrode.e effect of the time on potentiometric response of Sn 2+ electrode.

T 4 :
Determination of Sn 2+ concentrations in different arti�cially made samples.A perusal of Table4shows that selectivity coefficient values for the present sensor are much smaller than 1.0 over a number of studied mono-, di-, and trivalent cations.Hence, the sensor is sufficiently selective over these ions and can therefore be used to estimate tin in the presence of these ions by direct potentiometry.