Acrylic acid grafted PVC membrane based ion selective electrode for calcium and hardness measurement of water

Transcription

1 Indian Journal of Chemistry Vol. 46A, February 2007, pp Acrylic acid grafted PVC membrane based ion selective electrode for calcium and hardness measurement of water D Nanda, M S Oak* & M Pravin Kumar Department of Atomic Energy, Raja Ramanna Centre for Advanced Technology (RRCAT), Indore , India mso@cat.ernet.in Received 27 January 2005; rerevised 2 January 2006 A poly(vinyl chloride) based membrane ion selective electrode has been developed in which acrylic acid has been grafted on poly(vinyl chloride) powder. The ion selective electrode system, prepared without an internal solution, contains DBP (plasticizer) and NaTPB (anion excluder), and shows near Nernstian response to Mg 2+, Cu 2+ and super-nernstian response to Ca 2+ in the concentration range mol L -1. EDTA has been used to mask the interference of transition metal ions. The best performance of the electrode was obtained when it was stored in 10-2 mol/l Ca 2+ solution when the electrode was not in use. The electrode, with response time 20 s, has been found useful in the determination of concentration of Ca 2+ and hardness in river water. IPC Code: Int. Cl. 8 G01N27/333 Many environmentally important cations are now routinely monitored using ion-selective electrodes based on impregnated polymeric matrices like PVC or Polysulfone. Among these cations, determination of calcium content is of great importance in water analysis. Treated water from water softening plant, water used in cooling tower and boiler water should not have calcium concentration beyond the permissible limit of 20 ppm, because of it s scale forming tendency over time. Among the several analytical methods available for calcium analysis, most frequently used methods are titration, flame photometry, gravimetry, potentiometry, etc. depending upon the sample matrix and the concentration of calcium in solution. Though the ion selective electrodes (ISE) for different mono- and divalent cations are available, polyvinyl chloride (PVC) membrane based potentiometric sensors 1 9 are now being used routinely by analytical chemists because of their ease of fabrication, storage, stability, better performance and wide application for different metal ions. The basic composition is generally the same for all PVC sensors but the choice of ionophore is a very important factor that determines selectivity for a particular ion among the other ions present in the solution. The nature of binding of ionophore in the PVC membrane determines the duration with which the sensor can be used with Nernstian response having reasonable accuracy in the determination of ionic species being analyzed. Possibility of leaching out of the ionophore from membrane matrix is always there as the ionophore is adsorbed on the polymer matrix and not bonded with it covalently. Studies on development of Ca 2+ - selective electrode have been reported by several researchers using calcium selective organophosphate H + DEHP -, PANI containing the calcium selective organophosphate H + DEHP -10 and Ca 2+ -ionophore ETH ,12 as the neutral carrier. Lakshminarayanaiah 13, has made mixed ester ion selective electrodes for calcium that show nernstian response in the range 10-1 to 10-7 M. A novel calcium PVC based membrane sensor based on 2-[(2-hydroxyph enyl)imino]-1,2-diphenylethanone (HD),as a new ionophore, has been developed by Ganjali 14 et al., which is independent of ph in the range 4.0 to In the present study, acrylic acid has been grafted on PVC with the help of γ-radiation to prevent loss of ionophore by leaching and a sensor has been developed for the determination of calcium-cumhardness in water. The electrode shows super- Nernstian response to the calcium solutions of mol L -1. Materials and Methods All the reagents used in the studies were of AR grade. De-ionized water was used throughout the study. Solutions of alkali, alkaline earth and transition

2 NANDA et al.: ACRYLIC ACID GRAFTED PVC MEMBRANE ION SELECTIVE ELECTRODE FOR CALCIUM 259 metal ions were prepared by dissolving weighed quantities of metal chlorides/nitrates (S. D. Fine Chemicals, India) in dilute acids. The solutions of transition metal ions were standardized by complexometric titration with EDTA using suitable metallochromic indicator. Polyvinyl chloride (PVC) of high molecular weight, dibutyl phthalate (DBP) and sodium tetra phenyl borate (NaTPB) were obtained from E. Merck, Germany. Lancaster make acrylic acid was used for grafting without further purification. A combination electrode was used for measurement of ph of all the solutions required in the studies. All potential measurements were carried out at room temperature using a microprocessor based ph meter (model TMP 85,Toshniwal Brothers, India) having provision of operating in the mv mode. A saturated calomel electrode was used as reference electrode for the potential measurements with the membrane electrode as indicator electrode. A Co 60 gamma irradiator (model Gc-1200) with gamma dose rate of 2.82 kgy/hour was used for irradiation of PVC powder. PVC powder samples in glass ampoules were irradiated in the dose range kgy. Aqueous acrylic acid solution 20%, v/v, bubbled with nitrogen gas,(to remove dissolved oxygen), was taken in a stoppered glass tube. The irradiated PVC powder was carefully transferred, with minimum exposure to atmosphere, to the glass tube containing acrylic acid. The glass tube along with its contents was placed in a constant temperature water bath maintained at 40 C for 4 hours. The acrylic acid grafted PVC powder (AA-g-PVC) obtained was washed several times with methanol and water to Fig. 1 Superimposed FTIR spectra of un-grafted PVC (A) powder and Acrylic acid grafted PVC powder (B). make it free from any traces of acrylic acid and dried in vacuum oven before further use. The grafting of acrylic acid on the PVC powder was confirmed by FT-IR (Jasco model FTIR-660 PLUS) analysis. AA-g-PVC powder (Fig.1) and ungrafted PVC were characterized separately by FTIR spectroscopy. The two spectra have been superimposed in Fig. 1 confirming the grafting of acrylic acid on PVC. In the spectra of the AA-g-PVC powder a strong peak appears at 1725 cm -1, which indicates the presence of carboxylic carbonyl group whereas in ungrafted PVC, no such peak is observed. The presence of above peak at 1725 cm -1, for >C=O group in COOH, confirms the grafting of AA on PVC. Preparation of the membrane electrode A mixture of grafted PVC, DBP and NaTPB in a predetermined ratio was dissolved in 3 ml of THF and the clear solution was allowed to stand and evaporate slowly till an oily concentrate was formed. A piece of graphite (approx. 10 mm long and 5 mm dia) was cut from spectroscopic grade graphite rod and fixed with epoxy resin to one end of a 50 mm glass tube of appropriate diameter. A shielded copper wire was fixed to the enclosed end of the graphite rod using soldering element. The free end of the copper wire was fitted with an adapter suitable for connecting it to ph meter. The graphite surface marked for holding membrane was polished with fine emery paper. The polished surface was washed thoroughly with deionized water, methanol and tetrahydrofuran (THF) and dried in air. The polished and cleaned graphite end was dipped in the concentrated solution for about a minute so that a nontransparent coating was formed. The electrode was removed from the mixture and allowed to dry overnight at room temperature. (The standard procedure of preparing membrane electrode is to cast the membrane solution containing all the essential ingredients on a polished graphite surface and connecting this to a potentiometer. The electrode used in the present study was made by casting the membrane solution directly on the graphite electrode surface as described by Amini et al. 3. The main advantage with this type of electrode is that no internal solution is required and the chance of the internal solution leaking out and mixing with the analyte solution is negligible. The external contact to the measuring instrument was made through a shielded copper wire directly connected to the graphite rod. This type of electrode is simple and easy

3 260 INDIAN J CHEM, SEC A, FEBRUARY 2007 therefore be assumed that the ability to form covalent bonds with hydrogen ion may be extended to other ions capable of forming covalent bonds. Fig. 2 Experimental setup for potentiometric measurement of calcium/hardness. to construct. The lifetime is also sufficiently long and the results are reproducible. The electrode after completely drying was conditioned by equilibrating with 0.01 M solution of respective metal ion for two days to obtain a stable and reproducible potential values against calibration standards. Three electrodes were developed and were numbered as electrode 1, electrode 2 and electrode 3. The electrode 1 was used as calcium sensing electrode whereas electrodes 2 and 3 were used as magnesium and copper sensing electrodes respectively. The experimental setup is shown in the Fig. 2. The ion exchange capacity of the membrane was determined by an acid-base titration. The membrane was immersed overnight in a solution of NaOH (0.02N) prepared in 5% NaCl. The excess NaOH was back-titrated against standard H 2 SO 4 (0.02N) solution using phenolphthalein as indicator. The ion-exchange capacity was determined to be 0.5 meq/g dry. Results and Discussion Acrylic acid was chosen for grafting to PVC as it has a vinyl group CH=CH 2 required for the grafting reaction to take place and unusual affinity of the carboxylic acid group for divalent metal ions such as Ca 2+ and Mg 2+ extending it to other divalent ion like Cu 2+. The unusual affinity for divalent ions can be possibly related to the acidity of the carboxylic acid. The low acidity of the carboxylic acid may be attributed to the ability of the carboxylic acid group to form covalent bonds with hydrogen ions and it might Electrode performance The performances of the electrodes were tested at various membrane compositions and under different experimental conditions. Grafting of acrylic acid with different acid concentrations like 5, 10, 15, and 20% was studied and it was observed that grafting was maximum when 20% acrylic acid solution was used with irradiated PVC. The COOH group is the ionophore in grafted acrylic acid and is responsible for the ion exchange reaction with divalent ions and the electrode response is due to the difference in the concentration of divalent ion between the analyte solution and the membrane electrode. The electrode response was very poor with respect to Ca 2+ and Mg 2+ when 10% acrylic acid solution was used for grafting. Only a potential difference of 6.0 mv/decade was recorded for Ca 2+ solutions ranging from 10-2 to 10-5 mol/l, whereas approximately 15.0 mv/decade was noticed with fluctuation in reading for Mg 2+ solutions in the same concentrations range. DBP was used as a plasticizer and sodium tetraphenyl borate as an anion excluder. The effect of the individual constituent with variation in its amounts in membrane solution on the electrode performance has been studied keeping the amounts of other constituents constant. The actual amount of grafted PVC required for the electrode was decided on the basis of the performance of the membrane electrode. The amount of grafted PVC was varied to change the thickness of the membrane coating. This variation did not significantly affect the electrode performance. If the amount of grafted PVC was too low, the membrane became soft and brittle and at higher grafted PVC concentration the electrode response was slow. The optimum concentration of PVC was found to be mg in 2.0 ml solution of THF. The optimum amount of DBP for the same volume was 2 drops (about 96 mg). At lower DBP content, proper membrane coating could not be obtained and at higher values, the electrode response was poor. The anion excluder, NaTPB had significant influence on the measured potential values. In the absence of NaTPB, the variation of the measured potential was non-linear and there was pronounced deviation from linearity particularly at the extreme ends of the concentration axis (i.e. at low and high concentrations). The optimum membrane composition

4 NANDA et al.: ACRYLIC ACID GRAFTED PVC MEMBRANE ION SELECTIVE ELECTRODE FOR CALCIUM 261 Table 1 Optimum membrane (electrode) composition and working conditions. PVC mg Acrylic acid 20% v/v DBP 96 mg. (approx.) NaTPB 14.5 mg Equilibrating solution 10-2 mol. L -1 of concerned ionic species Equilibration time > 2 days ph range 6.0 (Ca 2+ & Mg 2+ ) & 4.0 (Cu 2+ ) Storing In 10-2 mol / L (M 2+ ), (where M = Ca, Mg and Cu) for best calibration coupled with mechanical characteristics is given in Table 1. Effect of ph The effect of ph on the potential measurement was studied for each of the divalent metal ions. In all the cases, interference from hydrogen ion was noticed at lower ph. There was no significant change in potential for Ca 2+ and Mg 2+ measurement at higher ph though change in electrical potential was noticed in the case of Cu 2+ due to the formation of copper hydroxide precipitate. The potential value was found to be stable for Ca 2+ and Mg 2+ in the ph range of whereas narrow ph range of was maintained at the time of Cu 2+ ion measurement in the solution. A ph of 6.0 was chosen and adjusted with hexamine buffer for all potential measurements for Ca 2+ and Mg 2+ solutions whereas ph 4.5 was chosen as the ph of potential measurement in case of Cu 2+. Electrode 1 showed super-nernstian response to the Ca 2+ ion solution in the concentration range of 10-2 to 10-5 mol/l with an average slope of 34.0 mv/decade even when the ph was not adjusted for all the solutions used for calibration. Upon adjustment of ph 6.0 with hexamine, the potential value was raised to 37 mv/decade. The reason for a super-nernstian response may be attributed to the unusual affinity of the carboxylic acid group for the divalent ions like Ca 2+ and Mg 2+. The response time which is the time taken for the potential to stabilize to ± 2 mv was 15 to 20s. There was no significant variation in the calibration and response of the electrode over a period of 30 days. The potential values were reproducible within ± mv of the initial values and there was no noticeable change in the slope of the calibration curve. This certainly indicates a longer electrode life and a stable electrode performance. The best performance of the electrode could be obtained if it was stored by dipping in 10-2 mol/l Ca 2+ ion solution when the electrode was not in use. Electrode 2 was equilibrated with 10-2 mol./l Mg 2+ ion solution maintained at a ph of 6.0. An average slope of 28.0 mv/decade was observed in the calibration of solutions of 10-2 to 10-5 mol./l of Mg 2+ ion. As in the case of electrode 1, there was no change in potential, with or without the adjustment of ph but fluctuation in response was noticed leading to unstable reading. The reason of fluctuation may be attributed to the lower stability constant (log K) as compared with calcium and other common divalent ions. Electrode 3 was equilibrated with 10-2 mol./l Cu 2+ ion solution maintained at a ph of 4.5. Different potential values were recorded with and without adjustment of ph of the calibration standards. An average slope of 27.0 V/decade was observed without the adjustment of ph and with ph adjustment to 4.5, the potential value came down to 23.0 mv. The ph value was restricted to 4.5 to prevent formation of copper hydroxide at higher ph. Further experiments towards Mg 2+ ions were discontinued as the response of electrode 2 fluctuated for each observation. Similarly, during Cu 2+ ions measurement with electrode 3, the experiments had to be terminated as the emf values obtained were lower than those expected. Selectivity Ion exchange resin having carboxylic acid functionality have been used for producing pure soft water because of its unusual affinity for Ca 2+ and Mg 2+ ion present in feed water. Interferences from various cations are generally eliminated either by masking or separating the interfering ions. The presence of anion excluder NaTPB in the membrane, however, helps to minimize the interferences from anions like chloride, sulfate, EDTA, etc. The reduced form of Eisenman s equation 12 given below is often used to calculate the selectivity pot coefficient, K Ca, B ( E E n K pot 1 2 ) log Ca, B = 1 * log[] a S z where E 1 and E 2 are the potentials measured in 10-3 mol/l solutions of interfering ion and Ca 2+ ; S is the calibration slope, n and z are the charges of Ca 2+ and interfering ions respectively and [a] is the concentration of the ions used (10-3 mol/l). A simplified form of the equation 15 as given below was used to calculate the selectivity coefficients of mono, di and trivalent cations.

5 262 INDIAN J CHEM, SEC A, FEBRUARY 2007 Table 2 Analysis of water in Narmada river Parameter Value ph 8.29 Conductivity at 25 C, μs/cm 435 Total residue (TS), ppm 216 Total dissolved solid (TDS), ppm 201 P. Alkalinity as CaCO 3, ppm 19.6 M. Alkalinity as CaCO 3, ppm 147 Total hardness as CaCO 3, ppm 140* # Ca hardness as CaCO 3, ppm 102* # Mg hardness as CaCO 3, ppm 38 Silica as SiO 2, ppm 29 Nitrate + Nitrite, ppm < 0.2 Sodium, ppm 26 Potassium, ppm 0.93 Total iron, ppm 0.21 Chloride, ppm 34 Sulphate, ppm 07 *This work. # EDTA titration values K = [Ca 2+ ]/[M n+ ] 2/n The potential measured by the mixed solution method with constant concentration of Ca 2+ ion (10-3 mol/l) but varying concentrations of interfering ions from 10-4 to 10-1 mol/l was used to evaluate the selectivity coefficients from the above equation. Alkali metal ions interfere only at their higher concentrations but divalent and trivalent ions interfere in the measurement of Ca 2+ ion concentrations. The interference of divalent and trivalent metal ions was masked by the addition of EDTA and adjustment of ph. The stable anionic complexes, formed by the interfering ions with EDTA, are rejected by the membrane. The stability constants for the EDTA complexes of the transition metal ions are very high. The log K values of Cu 2+, Al 3+ and Fe 3+ are 18.8, 16.1 and 25.1 respectively 16. Thus the presence of EDTA in the experimental solution enables the determination of calcium even in the presence of transition metal ions. Calcium ion forms a weak or no complex with EDTA at ph = 6 (ref. 17). Consequently, the addition of EDTA does not affect the measurement of the calcium ion concentration. To eliminate interference from the cations and fix the Ca 2+ ion in the membrane matrix, Ca 2+ loaded in the membrane was converted to calcium oxalate with ammoniacal sodium oxalate. The performance of electrode was not satisfactory. Electrode 1 was found suitable for measuring Ca 2+ concentration of any pure Ca 2+ ion solution or hardness of water, which is the combination of Ca 2+ and Mg 2+ ion concentration. The electrode was used in determining the hardness of Narmada river water, which contains mainly Na +, Ca 2+ and Mg 2+ ions with concentration of sodium only about ppm. (Though fluctuation was noticed in the case of electrode 2 in the measurement of magnesium but electrode 1 was quite stable for the measurement of the hardness of river water). The hardness values obtained by this method have been found to be in good agreement with those obtained by complexometric titration with EDTA. The relative error has been calculated to be within 2 3% of the real value. This study indicates that electrode developed can be used in the determination of hardness of almost all river waters similar to the water quality found in Narmada water (Table 2). In conclusion, an ion selective electrode has been fabricated for the determination of Ca 2+ and total hardness of raw water samples. It has a super- Nernstian in the concentration range 10-2 to 10-5 mol/l response with a slope of 37mV/decade when the ph is adjusted to 6.0 with hexamine,. This electrode is different from the other PVC based ion selective membrane electrodes in that the ion exchange group is covalently bonded to the PVC matrix by radiation grafting, instead of being physically adsorbed onto the PVC powder. References 1 Jain A K, Gupta V K, Singh L P & Khurana U, Talanta, 46 (1998) Rouhollahi A, Ganjali M R.& Shamsipur M, Talanta, 46 (1998) Amini Mohammad K, Shahrokhian Said & Tangestaninejad S, Anal Chem 71 (1999) Gupta VK, Jain A K, Singh L P, Khurana U, & Kumar Pankaj, Anal Chim Acta, 379 (1999) Florido A, Casas Ignasi, Raurich-Garcia Josep, Yellin-Arad Rina & Warshawsky Abraham, Anal. Chem, 72 (2000) Nanda D, Chouhan H P S & Maiti B, Indian J Chem, 43A (2004) Singh A K, Bhattacharjee G, Baniwal S & Singh M J, Indian J Chem Soc, 76 (1999) Gupta V K & Kumar Pankaj, Anal Chim Acta, 389 (1999) Hassan S S M, Saleh M B, Abdel Gaber A A, Mekheimer R A H & Kream N A A, Talanta, 53 (2000) Lindfors Tom & Ivaska Ari, Anal. Chim. Acta., 404 (2000) Lindfors Tom & Ivaska Ari, Anal. Chim. Acta., 404 (2000) Hassan S S M, Ali M M& Attawiya Amr M Y, Talanta,54 (2001) Lakshminarayaniah, N, J Memb Sci,8,(1981), Mohammed Reza Ganjali et. al, Acta Chim Slov, 52, (2005), Awasthi S P, Shenoy N S & Radhakrishnan T P, Analyst, 119 (1994) Ringbom A Complexation in analytical chemistry, Interscience, New York, Martel A.E, Smith R.M, Critical stability constants, Vol.1-Amino acids, Plenum Press, New York, 1974.