Inexpensive Instrumental Analysis Part 1. lon-Selective Electrodes Louis ama ale^,' Paula J. Wedge, and Sheila M. Crain Dalhousie University, Halifax, N.S. CANADA B3H 4J3 In an effort to make chemistry more interesting and relevant to the undergraduate, curriculum revision has become a continuous Drocess a t manv universities. In terms of the first class in analytical rhemistry curriculum revision often takes thc form of thc ~ntrodunionof more mode m instrumental techniques at the expense of the classical volumetric and gravimetric methods of analysis. This gives rise to problems in the laboratory, especially if student numbers a t this level are substantial. The classical techniques have the advantages of simplicity, use of inexpensive equipment, and provision of accurate and precise data. Supplying enough burets for a class of 30 to perform individual titrations is usually no problem; to perform the same feat with atomic absorption ipectromet&s is an impossibility. One solution to this problem is to spread experiments that require expensive equipment over the entire term. A class of 30 might use only three or fonr instruments of a kind in this case. This has the disadvantage that some of these experiments must be done before the particular technique is covered in the lecture, requiring a more thorough presentation in the laboratory manual. However, it can be argued that this actually boosts understanding when the topic is finally encountered in lecture. Even so, obtaining three or fonr modern, commercial instruments for each experiment would still be prohibitively expensive. Furthermore, an additional drawback to providing sophisticated eauioment for students a t the introductorv level is that students are often overawed by such instruments and view them as "maeic black boxes". Thev do the experiment bv rote and gain-little understandingof the equipment or thk principles underlying the technique. Another solution might be to have the students do the experiments in groups. We consider such a n approach pedagogically unsound, because often only one student in the group actually learns the technique well. Furthermore, it is difficult to assign marks for accuracy in group work. Our approach to these problems is to design and build in house our own simole instruments and accessories to illustrate the principles involved in modcrn analytical chcmistrv. The availat~ilitvof inexoensive intemated circuits, simple optical components, and a widevariety ofconstruction materials allows the cost to be keot low so that enough instruments can be provided for each student to work independently. For this approach to succeed, the in~ t r u m e n t ~ m ubd s t simple but rugged, work well without being "fiddly", and provide accurate and reliable data. Such equipment does not have to provide state-of-the-art sensitivity or versatility but must illustrate the analytical technique well. Construction is made as open as possible to demonstrate readily the principles of operation. The measurement of p H , d m o s t u n i v e r s a l l y done potentiometrically with a cell using a glass membrane electrode, is one of the most wmmon and important measurements made hv chemists and other scientists. Normally students are exposed early in their academic careers to such measurements, usually in experiments demonstrating simple, direct pH measurements, buffer behavior, A
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or acid-base titration curves. Unfortunatelv. snch exneriments do little to shed light on the basics ion-selective electrode measurements or behavior. The reasons for this are as follows:
of
(1) the experimentsthemselves are not designed to demonsh.. ate the properties of membrane electrodes; (2) the elass electrode is somewhat mvsteriaus because its anpearance prowdcs little h m t us to its operauun, espermlly i n the conibinatwn furm onm uwd, and smre its mrchanism is not easily understood; and (3) the use of a pH meter effectively disguises the fact that cell potential is the measured quantity. ~
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A membrane electrode cell is made up of the membrane or ion-selective electrode and an external reference electrode. The membrane electrode consists of the membrane that is sensitive ideally to only one ion, I, a n internal reference electrode, usually a silver/silver chloride electrode, and an internal solution that wntains ion I and chloride ion a t fixed concentrations. The voltage of this cell, when the electrodes are placed in an external solution containing ion I, is given by
ECa = E* + (RTIzFl In [PI where R is the gas constant; X the absolute temperature, F, the Faraday constant; z, the charge of ion I (including sign); and [I",] the concentration in the external solution. E' 1s a constant that lncludes the contrtbut~onof thr exrernal and Internal reference clcctrodcs, the amcmtratlon of ion I in the internal solution. and the liauid iunction ootential between the external reference el&dde and the analyte solution (assuming this to be constant). This treatment assumes an ideal solution; i.e., that activity and concentration are identical. Eel, is taken as the potential of the membrane electrode measured with resoect to the external reference electrode. the normal with pH meters, and often is simply referred to as the potential of the membrane electrode. At 25 "C the above eq;ation predicts that an electrode responsive to a monovalent ion will chance its ootential59.2 mV per order of magnitude change in [I"l.'~uchresponse is termed "Nernstian". Most membranes respond to ions other than the one for which they show maximum selectivity. If snch an interfering ion, J, also is present in the external solution, the response is given by E,," = E'
+ (RTIzF) In ([PI + k l J [ . W i r )
where ku is the selectivity coefficient for ion I with regard to ion J andv is the charge of ion J. More thorough treatments are given in most texts on instrumental analysis , I , 21and in monographs on electrodes and sensors (.?41 Given the simplicity and importance of mcmbrant. electrode measurements, students should be familmized with thc properties ofsuch electrodes in their introductory cluss in analytical chemistry. Thc fluor~dcelectrode orifinally developed hy E'rant and Ross 16, is an cxccllcnt cnndidnte -
'Author to whom correspondence should be addressed.
for such a n exoeriment. resoondine ideallv to fluoride ion with few inteGerences h t h the exception i f hydroxide ion. Determination of fluoride in real samples such as fluoridated tap water and toothpaste is accomplished readily and the interference of hvdroeen and hvdmxide ions is " demonstrated easily. Our experience withuthiselectrode is that it has a useful life of about two vears, and the response becomes rather sluggish, and thus frustrating for the student. durina the second vear. Since these electrodes are quite expensive, the replacement of enough to service an introductory class in chemical analysis every year or two is difficult to justify on a limited budget. Some types of membrane electrodes can be made easily in house, reducing their cost considerably. Polymer-bound are especially well suited liquid membrane electrodes (7,8) in this regard. These employ a membrane that contains a sensing material dissolved in an organic liquid that acts as for the polymer supp& ma&. The sensing a material may he uncharged, in which case it must complex with the analyte ion in some fashion in order to transport it across the membrane, or it must be capable of ion exchange. In the latter case a proton oRen is exchanged for the analyte ion. Crown ethers are examples of neutral carriers, often referred to as ionophores, and diesters of phosphoric acid of ion-exchange materials. Non-volatile hydm~ h o h i cliauids (often esters) such as dioctvl . .ohthalate. tncresyl phosphate, and 2-nltrophmyl octyl ether are used as the solvent mediator. Polvrv~nvl . . chloride, II'VCI I S almost universally employed as the polymer matrix. A certain amount of "witchcraft" has been invoked with regard to reagent source and purity when preparing polymer-bound liquid membrane electrodes. It was our intention to attempt to prepare such electrodes using simple procedures from inexpensive, available materials without any purification steps. These electrodes should have a reasonable lifetime, should have a response a s close to Nernstian as possible, and should respond over a wide range in concentration. We investigated sensor materials with a sodium response because sodium is available in a wide range of natural samples such as tap water and because we wished to compare atomic spectroscopy (9)and ion-selective methods for the same analyte. Commercial pH meters, used in the millivolt mode, were employed in the measurements. pH meters are readily if desired, but are now available constructed in house (101, a t low enough cost that in-house construction may not lead to significant savings.
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Experimental Two sensing materials were investigated, the sodium (Kodak #I87931 a n ion-exsalt of monensin, 1, C36H62011 change material, and hemisodium ionophore, 2, CA,HA,NO. -. -. (Kodak #52499). Two t v ~ e ofs tetrahvdmfuran (THF), 1)stabilized with 0.025% b;;ylated hyd'xytoluene (Aldrich #14,722-2) and 2) stabilized with 0.1% hydroquinone (BDH Chemicals #B30371), were used as solvent in the castine - ste-o. . Several different PVC's were em~lovedas the polymer matrix: Aldrich ff30.628-2v e q low mol~culnr weight (inherent visco4ty (lV10.53,,Aldrich n18,958-8 low HO
Me
Monensin, I
Me
Hemisodium, 2 molecular weight (0.65 N), Aldrich #18,956-1 high molecular weight (1.02 IV), and Fluka #81392, especially designed for membrane electrodes. Several solvent mediators also were studied: dioctyl phthalate (actually bis(2-ethvlhexvl) ohthalate) (Aldrich #D20.115-4). 2nitropheiyl ociyl ether (Fluka #73732), tricksy1 phosphate (Pfaltz and Bauer #T32305),tributyl phosphate (Aldrich #24,049-41, dibutyl sebecate (Pfaltz and Bauer #D12188).and tributvl citrate (Pfaltz and Bauer #T18515). All other chemicals were reagent grade and all materials were used as received. All test, calibration, and internal solutions were buffered at pH 7.8 with a mixture of 0.01 M triethanol amine and 0.01 M triethanolammonium chloride. The membrane casting solution consisted of 0.50 g of PVC, 1.50 g of solvent mediator, and 1 0 3 0 mg of sensing material dissolved in approximately 10 mL of THF. This was placed in a 10-cm glass Petri dish, covered with a filter paper, and the THF allowed to evaporate slowly. The last traces of THF were removed by placing the Petri dish in an oven a t 100 "C or under vacuum for eight hours. Membranes cast in this fashion were 9 cm in diameter and approximately 0.3 mm thick. Circular sections 8 mm dia. were cut from the master membrane with a cork borer and bonded to the solid PVC electrode body (see below) with a d u e consistine of 100 me of PVC dissolved in 2 mL of THF. 6ver 60 electGdes can & prepared from one master membrane and the remnants of the master membrane can be redissolved and recast. New electrodes are easily prepared by removina the old membrane from the PVC body with a rizor blade-and bonding a new membrane in place. CAUTION C a r e should b e exercised in t h e use of THF, because it is volatile, c a n b e irritating t o t h e eyes a n d mucous membranes, a n d is narcotic i n high concentrations (11).When unstablized it c a n form peroxides t h a t a r e explosive if mishandled. Only stabilized THF was used in this work. The electrode body, shown in Figure 1, is made from 11/16 in. dia. PVC rod. The top 1.0 in. of a 5-in. length is machined down to 21/32 in. while the remaining 4 in. are reduced to 0.5 in. dia. A 318-in. bole is then bored to within about 118 in. of the bottom while the upper 718 in. is enlarged to 0.5 in. Finally a 3116-in. hole, over which the memhrane is placed, is bored through the bottom. This body will fit into most pH-electrode stands. The internal reference electrode, also shown in Figure 1, is made by soldering a 0.5-mm silver wire to the inner conductor of an old glass electrode lead. If such leads are not available, BNC leads from any electronic supply company can be substituted provided that the pH meter has a BNC female connector. The lead with its silver electrode is then sealed a t both ends into a 5-mm od borosilicate glass tube with any silicone sealant or bathtub calk. Silver chloride can be plated onto the eleatrode by placing it in a 0.1 M
Volume 71 Number 2 February 1994
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LEAD
+ t SEALANT
MEMBRANE
7'
Figure 1. Details of membrane-electrodewnstruction. NaCl solution, connecting it to second electrode through a 1.5-V dry cell in series with a 10042 resistor, and allowing cnrrent to pass until a gray-purple coating appears. The silver electrode must contact the positive battery terminal. Almost any metal can be used as the other electrode, including silver, copper, stainless steel, or platinum. If the coating is not even, the electrode should be cleaned by rubbing with fine sand paper or by brief immersion in wncentrated nitric acid and recoated. This electrode is held in the PVC body with a #00 rubber stopper that has had a small "V" section cut from its side to allow pressure equalization between the interior and exterior of the assembly. The internal solution used was 0.1 M NaC1. The cell voltages were measured with a Fisher Scientific Model 810 pH meter. The external reference electrode employed was a Fisher Scientific #13-639-52 saturated calomel electrode. Results and Discussion Suzuki et al. (12)have reported an investigation of membrane electrodes based on a series of natural carboxvlic polyether antibiotics including monensin using very careful purification and electrode preparation techniques. They noted that monensin is more selective to Baz+than any other alkali or alkaline earth cation, followed by sodium. The linear range they obtained for BaZ+was from 5 x lo5 to 5 x 10.' M with a slope of 58 mV, an unusual value for a divalent ion. They did not report the range and slope for Na+. They also prepared the methyl ester of monensin and found it to show maximum selectivity for Na+ with a range of 1x lo4 to 0.1 M and a slope of 58 mV. Our monensin-based electrodes showed both a lower linear range, usually from 0.001-0.1 M and a lower slope, from about 25-40 mV. In addition the response degraded after about four weeks of use. In tests using monensin, the best solvent mediator was found to be dioctyl phthalate, which provided the best linear range and longest lifetime. Very little difference in response was noted in going from 10 mg of sensing material to 30 mg or in the origin of the THF casting solvent. The type of PVC polymer matrix employed made little difference in electrode response either, but mechanical membrane strength varied considerably with PVC type. The Fluka and Aldrich #18,958-8 (0.65 N) provided strong membranes that were disengaged easily from the glass Petri dish. The other Aldrich PVC's were sticky or produced easily ruptured membranes. Of the two satisfactory PVC's, the Aldrich is by far the cheaper. We did not attempt to esterify monensin and cannot comment on the behavior of the methyl ester under conditions in our laboratory. 166
Journal of Chemical Education
Electrodes prepared with hemisodium produced results in much hetter accord with theom Thc soluhility limit of hemisodium in the membrane caiting mixture was about 10 mg; therefore, this amount was used in all tests. All hemisodium membranes were prepared with dioctyl phthalate as solvent mediator and Fluka PVC as membrane matrix. Removal of the last remnants of THF from the master membrane by heating was found to degrade the membrane appearance by turning it slightly purple. This, however, did not seem to affect membrane performance. Nevertheless, all membranes were cured under vacuum before use. Hemisodium-based electrodes responded linearly to sodium ion over a range of 0.1 M to 3 x lo5 M with slopes between 56 and 58 mV a t 25 C.The response was rapid. between 10 and 30 s, a t all concentrations and stable in& a n eauilibrium potential was reached. These electrodes are duite se1ecti;e for sodium, although they show some response toward potassium and lithium ions. Typical results are shown in Figure 2, where the data with added lithium are offset by 10 mV and those with added potassium by 20 mV. The response to potassium is slightly greater than to lithium. Values of the selectivity coeffl2 with the cients calculated from the data uoints in Fimre " greatest deviation from the linlar response are 3.0 x 10" for potassium and 1.3 x lo4 for lithium. It has been noted (7)that one way in which liquid membrane electrodes deerade is to leach their active comnonents into solution.- heref fore, three methods of storage were tested for these electrodes: (1)completely dry, no external or internal solution; (2) internal solution always in place, but no external solution; and (3) membranes always stored in both solutions. Over a three-month period small but significant differences were noted. For data like those shown in Figure 2, method two gave results with the most reproducible slopes and with SIODES closest to the Nernsti a i values. variations between klectrodes in a given set were smallest for electrodes stored by method two. In general method one produced the worst results. he inter&ts of the plots for electrodes stored by methods two and three slowlybecame more negative with time, perhaps due to a drift in the internal reference electrodes. Membrane response does degrade slowly with time. Figure 3 shows typical data for a fresh electrode and one that had been used in the undergraduate laboratory for two terms and that was 10 months old, both fabricated from the same master membrane. The linear ranee decreases slightly, the slopes become less Nernstian, ranging from 55-57 mV. the resnonse time increases sliehtlv for low con~
~~
-~
J -5.0 -4.5 -4.0 -3.5 -3.0 -2.5
-2.0 -1.5 -1.0
-0.5
log[~a+]
Figure 2. Membrane-electrode response: circles-no interfering ions; open triangles4.0100M Lit in all solutions; filled triangle&.0100 M K* in ail solutions.
not obvious and usually need not be taken into account. To insure that ionic strength effects do not interfere, the buffer concentration can be raised to 0.1 M and samples can be kept to concentrations of 0.01 M or less, illustrating the principle of the ionic strength adjusting buffer. Craggs et al. (8)have described various methods of examining activity effects in such experiments, if this is desired. Conclusion
Figure 3. Membrane-electroderesponse: circles-fresh electrode with no interferina ions: ooen trianales-10- month-old electrode with no interfering i&s; filied'triangle&lO-month-old electrode with 0.0100 M K+ in all solutions. centration readines. the interceots decrease sliehtlv. and the membranes appear a bit more sensitive to iztellfering ions. In Fieure 3 the data for the old electrode are offset bv 15 mV a n i for the old electrode with added potassium b; 25 mV. None of these effects are meat enough - to make the electrodes unusable in the laboratory. Some membrane electrodes do not work properly from the start and some fail durinp use probably due to imperfections in the membranes or in gl;ing the membranes to thc clcctrode body. Thus, more electrudes are made than are needed and the best are selected. Electrodes are stored dry when not in use and with internal solution only otherwise. When a buffer of ionic strendh 0.01 M is emoloved. the ionic strength for all but the most concentrated sod& solutions is constant. makine activitv effects verv small. With cell potential readings accurate io no better t"han one millivolt, deviations from linearity due to these effects are
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Liquid membrane electrodes prepared a s described above and based on hemisodium provide stable, rapid, and almost Nernstian response for sodium with good selectivity over other ions. These are quite inexpensive and have a reasonable lifetime. They can be disassembled to demonstrate their construction, and, in fact, students can even prepare these electrodes with good expectation of a device that will provide excellent quantitative results in sodium determinations.
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Acknowledament
The authors wish to thank the Trace Analysis Research Centre a t Dalhousie Universitv for eenerous suooort of this project. Literature Cited
3. W a , J . Ion-Seloetiw EloetmdeJ; Cambridge University Ress: Landon, 1975. 4. Laluhminarayanaiah. Membmm Ekcfmdrs: Academic Re-: New Yark, 1976. 5. Birch, J.; Edm0nds.T E. In ChemimlSe"mra; Ed.; Chapman and Hall: NeuYmk, 1988: Chapter 9. 6. Franc, M. S.; Ross. J. W. J r S c i ~ z a1986,154,1563-1555. 7. Mmdy, 0 . J.:Thomas. J. D. R. In C k m i m l SOMOIS;Edmands, T. Ed.; Chapman and Hall: New Ymk. 1988; Chapter 3. 8. Craggs,A.;Moody,G. J.;Thomas, J . D.R. J. Chrm. Edue. 1914,51,541644. 9 . Ramaley, L.: Guy,R D.; Stephens, R. J C h . Educ, in 10. Hallen,R. M.; Sane, K. V. Chemlatq Edumfion 1988,5.454. 11. Sax. I.Dangerous P m p r l l e s of Indust%l Mate%ls: Van Nostrand Rehhold: N e w Y a h 1979. 12. Smuki. K, K;ANga. H.; Matsuzoe, M.; h u e . H.: Shirai, Chem. 1%8,60,171&1721. ~.
N.
B.
Edmonds,T.E.,
E..
press.
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Tohda
T.Aml.
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