712 Analyst June 1983 "01. 108 pp. 712-716 Synthetic Inorganic lon-exchange Materials Part XXXII." Crystalline Antimonic(V) Acid as a Nitrate Ion-selective Electrode Studies on an Araldite-based Membrane of Sushma Agrawalf and Mitsuo Abe Department of Chemistry Faculty of Science Tokyo Institute of Technology 2-12-1 Ookayama Meguro-ku, Tokyo 152 Japan An Araldite-based membrane of crystalline antimonic(V) acid when acting as a nitrate ion-selective electrode shows a near-Nernstian response for concentra-tions of nitrate ions between 10-5 and 10-1 M and can be used for determining the activity of the ions. Stable potentials are observed within 10-30 s and for about 2min. The useful pH range is 3.5-11 at a higher concentration (5 x M) and 4.5-9 at a lower concentration (5 x 10-4 M) of nitrate ions.This membrane responds to nitrate ions in a solution containing 25% of non-aqueous solvent. Keywords Crystalline antimonic( V ) acid ; nitrate ion-selective electrode ; A raldite-based membrane ; potentiometry Inorganic ion exchangers can be used as ion sensors with an appropriate membrane and suit-able support matrix.l This is implied by the fact that the ion exchangers have fixed ionic groups in their structure ensuring the occurrence of a selective ion-exchange reaction. Liquid membrane nitrate ion-selective electrodes are now acceptable for making analytical and general chemical measurements and specifications of various nitrate ion-selective electrodes were reported by Davies et aZ.2 Various nitrate ion-selective electrodes have been based on an Orion liquid membrane (Model 92-07-2) together with wax-treated carbon powder,3 liquid nitron nitrate electrodes ,* quaternary ammonium compounds in non-porous polymer mem-branes5-7 and a solid-state membrane involving precipitated nitron nitrate.8 Crystalline antimonic(V) acid (C-SbA) behaves as a cation exchanger and shows specific selectivity for cations such as Na+ Cd2+ and Sr2+ with crystal ionic radii of about 0.1 nm.B Many chromatographic applications have been reported by using a relatively small column of this exchanger.1°-13 An attempt to extend the application of C-SbA in Araldite (a polyamide-type epoxy polymer) for use as an ion-selective electrode is described.Experimental Preparation of C- SbA The C-SbA was prepared as previously described.1° A 75-ml volume of antimony(V) chloride was preliminarily hydrolysed with 75 ml of cold water and then hydrolysed with 5 1 of de-mineralised water.The precipitate was kept in the mother solution at 40 "C for over 20 d and washed with cold de-mineralised water using a centrifuge (about 10000 rev. min-l) in order to free it from chloride ions. The precipitate was dried and ground into small grains of size under 325 mesh. Preparation of the Ion-selective Membrane The membrane was prepared by mixing homogeneously 0.7 g of the C-SbA in the hydrogen ion form with 0.3 g of Araldite spreading thinly on a piece of filter-paper and setting aside overnight. The hardened membrane was cut into a circular disc about 2.5 cm in diameter. The filter-paper was then removed with a razor blade and the membrane was equilibrated in a 0.1 M salt solution for about 15 d.The equilibration time can be shortened whenever a * For Part XXXI of this series see M. Abe M. Tsuji and M. Kimura Bull. Cham. SOC. Jpn. 1980 54, Present address Department of Chemistry State University of New York a t Buffalo Buffalo NY 130. 14214 USA AGRAWAL AND ABE 713 concentrated solution is used. The membrane was fixed with Araldite to one end of a Pyrex glass tube 8 cm in length and the tube was immersed in a 50-ml Corning beaker through a plastic lid. The electrode assembly consisted of the following cell: S.C.E. 1 test solution 11 membrane I 0.1 M NaNO 1 S.C.E. E.m.f. Determination Two sleeve saturated calomel electrodes (S.C.E.) were inserted into the solution and the e.m.f.was measured on a digital Toa Model HM-15A pH meter and on a Model R-02 recorder (Rikadenki Kogyo Ltd.). By using a magnetic stirrer all of the measurements could be taken at 25 & 0.1 "C. Between the measurements the electrodes were washed with water and dried with tissue-paper to prevent cross-contamination. The dried electrode was immersed in 0.1 M sodium nitrate solution for 2 h before each use. Supporting Material An epoxy resin first used by Coetzee and Basson14 proved to be the most suitable material. Polyamide Araldite (Araldite standard Ciba-Geigy Switzerland) was used as the supporting material for preparing the membrane. Reagents All of the reagents used were of analytical-reagent grade. Results and Discussion Prepared Membrane Homogeneous distribution of C-SbA in Araldite was confirmed by electron microscope observation.The prepared membrane was stable physically and chemically even in strong mineral acid and alkaline solution. The preparation and electrochemical properties of a number of inorganic ion-exchange membranes have been studied by Alberti and co-workers,l+l7 Coetzee,l Coetzee and Basson'* and Jain and co-workers.18-20 The membranes used were heteropolyacid salts,15J8 chromium hexacyanoferrate(II1) ,l9p2o zirconium phosphate and antimonate(V) ,15-17 with suitable supports. All of these showed a near-Nernstian response for cations. The C-SbA showed very high Kd values for sodium calcium strontium and cadmium ions and was very stable in an aqueous solution with a wide pH range.g For sodium and cadmium nitrate preliminary experiments showed a linear response to the concentration range of 10-5-10-1 M while the direction of potential change was opposite to that of the cations indicat-ing that the membrane was responding to the anion.The e.m.f. values of this membrane were independent of the presence of various cations such as Li+ Na+ K+ Rb+ Cs+ T1+ NH4+ Mg2+ Ca2+ Sr2+ Ba2+ Ni2+ Co2+ and Zn2+ at equivalent concentration. The response of the electrode was relatively fast and the potential reached a constant value within & 0.2 mV (Fig. 1). After 2 min the potential was increased very slowly at a rate of 0.04 mV s-l. When the membrane was prepared without C-SbA using Araldite only the potentials showed almost linear relationships with the concentrations of nitrate ions.Steady potentials were not obtained by immersing the membrane for a long time but were observed when the concentration of C-SbA was increased to more than 50% in the membrane (Fig. 1). Ion-selective electrodes based on ion exchangers involve ion-exchange processes at the electrode interface. The C-SbA is essentially a cation exchanger and has no adsorptive properties towards anions in the pH range ~tudied.~Jl Earlier reports indicate that the C-SbA has a pyrochlore structure and the lattice constant and diffraction intensities of the C-SbA in the H+ form change when H+ is replaced by various cations without any change in the crystal s y ~ t e r n . ~ l - ~ ~ Powdered X-ray analysis revealed that no change was observed on the membrane even when it was immersed in the solution of sodium nitrate or cadmium nitrate for a long time.This result indicates that no apparent cation-exchange reaction occurs on the membrane. It is known that fast conduction of protons occurs within the pyrochlore framework in C-SbA in the H+ form.23 On the other hand the commercial hard 7 14 60 50 40 > E % % . 0 a 50 40 30 AGRAWAL AND ABE SYNTHETIC INORGANIC Analyst Vol. 108 0 20 40 60 80 100 120 140 Response time/s 200 > E E 100 -. - m .-4-0 0 I I I I 6 4 2 0 -Log (activity of NO3) Fig. 1. Response time of the electrode with the membrane containing C-SbA at different concentra-tions A 0% ; B 30%; and C 50-70%. Solution, 0.01 M NaNO,. ener of the Araldite contains polyamide.The functional group of =N= may remain in the polymer after polymerisation and can behave as an anion exchanger. The response on this membrane may be due to the anion-exchange contribution of the polymer and the C-SbA acts as an electro-conductive material. It is known that a small amount of liquid ion exchanger leaks from the commercial liquid membrane and epoxy membrane containing liquid e~changer.~ The prepared membrane gives no leakage of the ion-exchange material. The potentials obtained were plotted against -log(activity of NO3-) (Fig. 2). The activity coefficient for a single nitrate ion was calculated by using the extended Debye-Huckel equation : where p is the ionic strength of the solution. For sodium nitrate the potential response was linear in the range 10-5-10-1 M with a near-Nernstian slope of 56 mV per decade of the activity.However the change in the potential is sufficiently large to permit the determination of the concentration of NO3-. The response of the membrane compares favourably with that of the commercial nitrate ion-selective electrode2 and is much better than any other fabricated nitrate ion-selective electrode. 2-* Fig. 2. Response of the electrode to nitrate ions contained in various solutions. 0 Activity in water; A 10 and 25% ethanol solution; and a 10 and 25% acetone solution. + Logf = -0.15 r-lf/(l + /A+) It deviated from the linear graph in the range lO-'-lO-5 M. Effect of pH The effect of pH on the determination of NO3- was investigated in the pH range 3-13 with a solution of acetic acid and sodium hydroxide.The useful pH ranges were 3.6-1 1 and 4.5-9 for higher (5 x M) and lower (5 x lo-* M) concentrations of nitrate ions respectively (Fig. 3). Selectivity Coefficients The mixed-solution method was employed for studying the response of the membrane to common anions with the solution containing between 10-2 and 10-3 B ions in sodium nitrate solution of different concentrations and the conventional separate-solution r n e t h ~ d ~ ~ ~ * ~ was used for a large variety of the cations and anions at M concentration. Sodium salts were used for anions and chloride salts for cations. The selectivity coefficients (Kg6J-,B) of the separate-solution method were calculated by the following equation June 1983 ION-EXCHANGE MATERIALS.PART XXXII 715 TABLE I SELECTIVITY COEFFICIENTS (kg$; B) FOR THE C-SBA MEMBRANE ELECTRODE f Interfering ion F- . . c1- . . Br- . . I- 10,- . . NO,- . . HSO,-HS0,-H,P04-CH,COO-ClO,- . . * . k d - B I A* B t Interfering ion 0. lob 0.18 SO,e- 0.31a 0.30 0.34a 0.57 C032- . . 0.55a 1.38 W0,2- . . 0.08 MOO,,- . . 0.66 B40,,- . . 0.26 %Fez- . . 0.42b 0.52 S20,2- . . 0.082b 0.15 HASO,&- . . . . 0.058b 0.09 OH- 0.75a kPOt NOj- B - A* Bt 0.051 0.033a 0.047 0.015b 0.012 0.012 0.035 0.017 0.068 0.11 0.042 0.62 * A Mixed-solution method t B Separate-solution method M B ions; b lo- M B ions. M B ions). where E is the potential a is activity z is the charge of ion B and S is the slope of the calibra-tion graph.The k 6 - values are summarised in Table I. The membrane is selective for nitrate ions with respedt to many anions except I- for which the response is more marked than for NO3-. Interferences such as those by I- are known for many nitrate ion-selective elec-trodes.2-8 The interference of I- on this membrane is less than for other electrodes. The value for C1- remains unchanged but that of I- decreased considerably. The selectivity co-efficients for B r and SO,"- decrease to lesser extents and the value for C10 was less than 1. These results indicate that this electrode can determine NO3- concentration in the presence of B r ClO and SO:- which is a great advantage over other electrodes (the &A3- values were higher than 1).Similar selectivity and response are found for ion-selective elecirodes for anions involving an epoxy coated wire matrix membrane.27 E.m.f. Titrations The membrane can be used as an end-point indicator electrode for the potentiometric titration of nitrate ions with nitron. The e.m.f. titration curve is demonstrated in Fig. 4. The curve has almost classical shape which shows that the electrode is more specific to nitrate than to any other anion. The end-point is not very sharp because of the high solubility of the nitron nitrate precipitate which is reported to be 1.3 x M of nitrate ions.28 120 > E % 100 .- ta a to a 80 2 4 6 8 1 0 1 2 PH Fig. 3. Effect of pH on the electrode potential with two differ-ent concentrations of NO3= (A) 5 x 1 0 - a ~ ~ ; and (B) 5 x 1 0 - 4 ~ .> 100 E . -(D al 0 .- to ta 0 80 60 Volume of nitron added/ml 1.0 2.0 3.0 4.0 Fig 4. Potentiometric titration graph of 0.02 M NaNO (20 ml) with 0.2 M nitron 716 AGRAWAL AND ABE Application to the Determination of Nitrate Ion Concentration in Non-aqueous Solution The membrane can be applied to the determination of the concentration of nitrate ions in a medium containing non-aqueous liquid. The potentials are plotted against log (concentration) in Fig. 1. The working curve for the non-aqueous medium was tested for solutions of ethanol and acetone at different concentrations. The stability of the response decreased in 25% non-aqueous solution which contained nitrate ions at concentrations lower than lov4 M.The working curve in the 10% non-aqueous solution is the same as that in the aqueous solution. The membrane responded well to nitrate ions at a concentration of up to low4 M although the drift increased in 50% non-aqueous solution. Life of the Membrane The membrane can be used for a period of 2 months without any significant change in the potentials while the linear range and the slope (-2 mV) decreased slightly after continuous use for about 4 months. The absolute value of the response varies from one preparation to another by a few millivolts. A similar conclusion was found by La1 and Christian,29 but a solid membrane electrode is preferable over a liquid electrode for many reasons as reported by Rechnitz3* Conclusion The Araldite-based membrane of C-SbA can be fruitfully used for the determination of the nitrate ion concentration in aqueous and non-aqueous solution and especially as an indicator electrode in potentiometric titration.The authors express their thanks to Mr. Nobuyuki Hayashi for his help with some of the experiments. 1. 2. 3. 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. References Coetzee C. J. Ion Sel. Electrode Rev. 1981 3 105. Davies J. E. W. Moody G. J. and Thomas J. D. R. Analyst 1972 97 87. Qureshi G. A. and Lindquist J. Anal. Chim. Acta 1973 67 243. Tateda A. and Murakami H. Bull. Chem. SOC. Jpn. 1974 47 2885. Nielsen H. J. and Hansen E.H. Anal. Chim. Acta 1976 85 1. Dobbelstein T. N. and Diehl H. Talanta 1969 16 1341. Hulanicki A. Maj-zurawska M. and Lewandowski R. Anal. Chim. Acta 1978 98 151. Lal U. S. Chattopadhyaya M. C. and Dey A. K. Mikrochim. Acta 1980 11 417. Abe M. Denki Kagaku 1980 48 344. Abe M. and Ito T. Bull. Chem. SOC. Jpn. 1968 41 333. Abe M. and Ito T. Bull. Chem. Soc. Jpn. 1967 40 1013. Abe M. and Uno K. Sep. Sci. Technol 1979 14 355. Abe M. and Kasai K. Sep. Sci. Technol. 1979 14 895. Coetzee C. J. and Basson A. J. Anal. Chim. Acta 1970 56 321. Alberti G. Cont A. and Torracca E. Atti Accad. Nazl. Lincei Rend. Classe Sci. Fis. Mat. Nut., Alberti G. Costantino U. and Zsinka L. J . Inorg. Nucl. Chem. 1972 34 3549. Alberti G. Costantino U. and Gupta J. P. J. Inorg. Nucl.Chem. 1974 36 2103. Srivastava S. K. Jain A. K. Agrawal S. and Singh R. P. J . Electroanal. Chem. 1978 90 291. Jain A. K. Srivastava S. K. Agrawal S. and Singh R. P. Talanta 1978 25 531. Jain A. K. Agrawal S. and Singh R. P. J . Indian Chem. Soc. 1980 57 343. Abe M. J . Inorg. Nucl. Chem. 1979 41 85. Abe M. and Sudoh K. J . Inorg. Nucl. Chem. 1980 42 1051. Abe M. in Clearfield A. Editor “Inorganic Ion Exchange Materials,” CRC Press Cleveland OH, England W. A. Cross M. G. Hamnett A. Wiseman P. J. and Goodennough J. B. Solid State Ammann D. Pretsch E. and Simon W. Anal. Lett. 1972 5 843. Levins J. Anal. Chem. 1972 44 1544. Suzuki K. Ishiwada H. Shirai T. and Yanagisawa S. Bunseki Kagaku 1981 30 751. Hulanicki A. and Maj M. Talanta 1975 22 767. Lal S. and Christian G. D. Anal. Chem. 1971 43 410. Rechnitz G. A. Chem. Eng. News June 12 1967 146. 1963 35 548. 1982 Chapter 6 p. 232. Ionics 1980 1 231. Received July 20th 1982 Accepted January 12th 198