PVC Matrix Membrane lon-Selective
A. craggs, G. J. Mood.y, and J. D. R. Thomas University of Wales Institute of Science and Technology Cardiff CFI 3NU, Wales
Electrodes
I
Construction a n d l a b o r a t o r y experiments
T h e recent rapid development of ion selective electrodes t o include non-glass membranes has quite properly been the subject of widespread attention and study. Unfortunately, the high cost of commercial ion selective electrodes has precluded their general use i n the teaching laboratory. However, given appropriate emf-measuring equipment, class work is quite feasible, and practical experience with ion selective electrodes need no longer be the prerogative of just the well-endowed laboratory. For example, the recent Summer School a t the University of Manchester Institute of Science and Technology ( 1 ) established t h a t ion selective electrodes based on liquid ion exchanger sensors trapped i n PVC matrix membranes (2-5) can conveniently he studied a t the class level. For the benefit of a wider readership, the instructional material prepared by the authors for t h a t Summer School is described here. The tested experiments are designed t o introduce students to the methodology associated with their construction, evaluation, and usage. The experiments may be performed by students working either alone or i n pairs and are described in terms of both a calcium ion and a nitrate ion selective electrode a s illustrations, respectively, of cation and anion selective electrodes. Details are given of the materials required for each experiment, although in the interest of brevity standard items of laboratory ware are not itemized. Experiment 1: Casting the Master Membrane
Materials Required Glass plate, previously cleaned and dried Glass casting ring (about 30 mm in depth and cut from'glass tubing of 30-35 mm i.d.) with one end ground to give flush contact with the glass plate PVC powder (Breon S 110/10, B.P. Chemicals International Ltd. issuitable) Liquid ion exchanger (Orion1 92-20-02 ion exchanger may be used for a calcium ion selective membrane and the Corning2 477316 ion exchanger for a nitrate ion selective membrane) Tetrahydrofuran Heavy weight with one flat surface Procedure Add 0.40 g liquid ion exchanger to a solution of 0.17 g PVC in 6 ml tetrahydrofuran prepared by sprinkling the PVC powder into the tetrahydrofuran. Stir thoroughly until dissolved. Pour the solution into the glass casting ring resting on the glass plate (Fig. la). Place a pad of filter papers weighted with a heavy weight on top of the glass ring and leave for sufficient time (usually about two days) to allow the solvent to evaporate (Fig. Ih). Carefully remove the glass ring with adhering membrane from the glass plate. If solvent remains on the under surface, place the ring with adhering membrane in a plastic container and store for a day to allow complete solvent evaporation. The membrane may then be stored by placing the lid on the container. Prise the membrane away from the inside edge of the ring. This "master membrane" should be sufficient for about ten electrodes. 1 Address: Orion Research Inc., 11 Blackstone St. Cambridge, Mass. 02139. Address: Corning Glass International, Medfield, Mass. 02052.
0.49 LIOUID ION EXCHANGER 6ml TETRAHYDROFURAN -GLASS
RING
(b) Figure 1. Diagrammatic representation of carting of master PVC matrix membrane.
Experiment 2: Assembly of Ion Selective Electrode
Materials Required Screened cable with suitable terminal for emf-measuring unit connected to a silver-silver chloride electrode as in Figure 3. This fitting may be made by soldering a 25-mm length of 28swg silver wire to the inner conductor of a length of coaxial cable and inserting into a Quiekfit (air leak) cone (MF 1510) so that the silver wire protrudes from the previously constricted lower end. Seal the silver wire to the glass with Araldite epoxy resin (Adhesive AYll1, hardener HY11, Ciha (A.R.L.) Ltd.). Seal the upper end of the Quickfit tube to the cable with a rubber sheath. Coil the silver wire into a spiral, place in coneentrated nitric acid, thoroughly wash with deionized water and deposit silver chloride electrolytically using a solution of 0.1 M sodium chloride titrated to pH 11-12 with 6M sodium hydroxide. The deposition may he carried out at 5-10 mA current for 30 min with the silver wire acting as anode to a platinum wire cathode. Alternatively a silver/silver chloride element from a disused glass electrode may be used. 3 em length of clear PVC tubine (6 mm a d . ) with cork barer of matchLgdiameter Quiekfit socket (SRB 10119)drawn to fit the PVC tubing Master PVC matrix ion selective membrane (for nitrate or calcium) 0.10 M sodium nitrate and 0.10 M sodium chloride for nitrate ion selective electrode 0.10 M calcium chloride for calcium ion selective electrode Tetrahydrofuran and glass plate Procedure Dip one end of the PVC tubing into tetrahydrofuran solvent and hold the tube in a vertical position while rubbing the dipped end on a glass plate in a rotatory fashion. This step may have to be repeated in order to obtain a suitable flat end finish. Cut a disc of the master membrane corresponding to the eaternal diameter of the PVC tubing and carefully mount with a forVolume 51. Number 8, August 1974
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541
enee solution. (0.10 M calcium ehloride for a calcium ion selective electrode and a 1:l mixture of 0.10M sodium chloride and 0.10M sodium nitrate for a nitrate ion selective eleetrode.) To complete the assembly, insert the cone of the internal silver-silver chloride electrode into the socket (Fig. 3). The electrode memhrane should then be conditionedS (for a t least 1 hr) by immersion in a solution containing 10-I M of the ion to be sensed (lo-' M calcium ehloride for a calcium ion selective electrode and 10.' M sodium nitrate for a nitrate ion selective electrode).
Figure 2. Diagrammatic representation of mounting of PVC matrix membrane.
Experiment 3: Calibration and Selectivity Determination of PVC Matrix Membrane Ion Selective Electrodes Materials Required
SCREENED CABLE RUBBER SHEATH QUICKFIT CONE MF 1510
WICKFIT SOCKET SRB 0119
Emf measuring instrument (if possible to +0.1 mV) Assembled PVC matrix memhrane electrode (for calcium or nitrate ions) Reference electrode (saturated calomel, far example, either Orion 90-02 double junction reference electrode or EIL type RJ 23 electrode with remote micro junction) 50-ml beaker with magnetic stirrer lo-', For calcium ion selective electrode: lo-%, and 10.' M solutions of calcium chloride prepared from a stock solution of 10-I Mealcium chloride 1 0 Z M magnesium chloride solution lo-', lo-" For nitrate ion selective eleetrode: lo-', and M solutions of sodium nitrate prepared from a stock solution of 10.' M calcium chloride 1Msodium chloride solution Semi-log paper (6 cycles) Stopclock for static time responses P r o c e d u r e for Calibration
SOLDER JOINT ARALDITE SEAL SILVER-SILVER CHLORIDE REFERENCE ELECTRODE
Figure 3. Longitudinal section of the PVC matrix membrane ion selective electrode. ceps on the polished end of the PVC tubing (Fig. 2). The surface resting on the PVC tubing should he that in contact with the glass plate during casting. Carefully seal the outer edge of the circular memhrane to the end of the PVC tubing with a solution of PVC in tetrahydrofuran as adhesive. This may be achieved by dipping the tip of a drawn-out melting point tube in the PVC adhesive, transferring a drop to the edge of the memhrane and carefully drawing the adhesive round the circumference. Care should be taken to prevent the adhesive coming into contact with the sensing surface of the memhrane. Connect the other end of the PVC tubing to the Quiekfit socket, and two-thirds fill the assembled casing with internal referThis should be done when evaporation of the solvent used in cementing the memhrane to the PVC tubing is complete; up to 3 hr may he needed for this, although the time required could he as low as 15-30 min. The consistent adoption of Ku in the manner described here indicates that when K , < 1, the electrode responds preferentially to the ion of interest, i. Far K ~ > J 1, the electrode responds preferentially to the ion. i. Also. since K i , frequently varies with the relative ;on of intereklinterferent activities, K,, values are more usefullv exmessed with the corresoondine value of the interferent activity, oJ, appearing alongside. The useful limit of the electrode may then be calculated with the aid of eqn. (2).
- .
542
/ Journal of Chemical Education
Take an aliquot of the appropriate lo-' M calibration standard solution in a 50-ml beaker, and assemble a potentiometric cell with a reference electrode and the electrode being calibrated. Ensure that the electrodes are clean and free of extraneous liquid before immersion in the solution. Both the calcium (Orion 92-2002 ion exehanger) and the nitrate (Corning 477316 ion exchanger) PVC matrix membrane electrodes will normally be positive with respect to the saturated calomel reference electrode, but the calcium electmde will become negative as the calibrating salutions become more dilute. Stir the solution to effect equilibrium and note the cell emf and the time taken to reach a stable reading. Repeat the procedure with the remaining calibration standard salutions. Plot the emf (linear ordinate) against concentration on 6 to 8 cycle semi-lag paper. Using the activity values calculated from the activity coefficients (f) derived from the equation
where p = 5 2 ez2, and c, the concentration of each ion of valence r. Plot the emf (linear ordinate) against activity on the same sheet of semi-lag paper. Comment on the plots obtained and also on the static response times. P r o c e d u r e for Determining Selectivity This method is a mixed solution method where the level of interferent is kept constant and illustrates an "at a glance interference nlot" and the calculation of the selectivity coefficient, K,r, where i refers to the ion of interest and j to the interferent ion. Prepare a calibration graph (emf versus log activity of the ion of interest) from the emf readings corresponding to each calibration standard solution in the presence of a fixed concentration (5 X Mmagnesium chloride for the calcium electrode and 5 X 10.' M sodium chloride for the nitrate electrode). Each test solution is prepared by adding 10 ml aliquots of the appropriate calibration standard calcium chloride solution t o 10 ml of M magnesium chloride solution for an assessment of the interference of magnesium on the calcium electrode. For the nitrate eleetrode study, the calibration standard solutions and interferent solution are those of sodium nitrate and 1M sodium chloride, respectively.
Compare the calibration plot obtained with that corresponding t o the absence of interferent obtained in the calibration experiment above. Calculate the selectivity coefficient,' Kij, from the ratio of the ion of interest activity, oj, to the interferent ion activity, n j , appertaining to the intercept of that part of the calibration curve (of near zero slope) corresponding to complete interference by the interferent, j, with that of Nernstian (or near-Nernstian) slope corresoondine. to more or less unfettered response by the ion of interest, i
Table I. PVC Matrix Membrane Calcium Ion Selective Electrode Emf Data at 29E°K for the Multiple Addition of 10-'M Calcium Chloride to 20 ml of 10.' M Calcium Chloride Volume, V. d of 10-1 M Calcium Chloride added Emf. E. mV versus a saturrated calomel electrode
0.0
0.2
0.4
0.6
0.8
1.0
2.0
(Vo + V.1lOtEa'S (ml)
IS =
30 mV
z is the charge of ion i and y the charge of ion j. The powered modification of n j applies only when r and y are different.
Experiment 4: Determinations with the PVC Matrix Membrane Calcium/Nitrate Ion Selective Electrode
This is designed to illustrate direct potentiometry, known addition of standard and Gran's plot (6) procedure. The experimental procedure describes the eolleetion of data while the interpretation of results is considered separately for each determination method. Materials Required Emf measuringinstrument (if possible to +0.1 mV) Reference electrode PVC matrix membrane electrode for calcium or nitrate ions In-' and l W 5 Calibration standard solutions: lo-', M solutions of calcium chloride and sodium nitrate, respectively, for the calcium and nitrate electrodes Test sample containing calcium or nitrate ions Ordinary graph paper. Semi-log (6 cycle) graph paper Experimental P r o c e d u r e Calibrate the PVC matrix membrane ion selective electrode with the appropriate calibration standard solutions as described in Experiment 3. Take 20 ml of the appropriate test sample5 in a 100-ml beaker and record the emf reading. Record the emf reading IE,J of the test solution containing, in addition, 0 3 , 0.4-, 0.6, 0.8.. IS-, and 2.0-ml aliquots (Va), respectively, of 10-1 M calcium chloride or sodium nitrate according to the ion selective electrode in use. Interpretation of Results Direct Potentiometry Read off the aetivity/eoncentration of the test solution from a calibration graph prepared on the semilog graph paper and compare with the expected value. Known Addition Method. An interesting technique for using ion selective electrodes to get total concentration is the known addition method or standard addition to sample method. In this, the total potential, Eo, between the ion selective electrode and reference electrode is measured for the sample solution of volume, VO,and total molar concentration, Co,of the sought species
E,
constant
+ S log xnfnC,
(3)
where S is the calibration slope, fa is the activity coefficient, and xo the fraction of uncomplexed ions. A new patential E, is measured after addition of a small volume, V,. of a standard solution (concentration C,) of ions of the species sought, where C, 100 Co
-
E, = constant
+ S log x. f,
(V,,C" + V"C,) V,) (V,
+
(4)
where fo and x, correspond to the new activity coefficient and fraction of free ions, respectively. An essential assumption is that ra x, and fo fa. Hence the difference between the ahove two equations simplifies to
-
-
E.
- Eo = AE = &
and Cocan he resolved
S log
+ V"CJ + V,,)
(V,C" CdV"
(5)
Figure 4. Gran's plot of data obtained with a calcium ion selective eiectrode by adding lo-' M calcium chloride standard solution lo 10.' M calcium chloride test solution. The ordinate represents, in mi, for A: (VO V.) 10+"a/sand lor B: ( ( V , V,) 101"~l")/la.
+
+
or for no allowance for volume change, that is, V. small in relation to Vo
Treat the results obtained for 0.2-ml and 0.4-ml additions of M solutions by substituting in eqns. (6) and ( I ) , and compare the results for Co with that of the direct potentiometric method ahove. Gmn's Plot Method. A variation of the standard addition method depends on a modification of the method described by Gran (6) for presenting potentiametric titration data in linear form by using a semi-antilag plot. The principle is apparent in eqn. (4) abovefor which the antilog version is
lo-'
(v, + v,)Io*US =
+ V,,C,)
(8)
f,lO+m,"ht""m (V,,Co
where V, is the volume of standard solution added. A plot of the left hand side of eqn. (8) as ordinate versus V, gives a straight line to intercept the abscissa far a V, value where
(9)
C,V, = -C,V, Calculate Coand compare with the values ohtained ahove. Discussion
T h e above scheme is a valuable introduction t o ion selective electrodes w i t h t h e a d d e d advantage of experience i n electrode construction. W h e n tested o n a class of 40 students-all novices a n d working i n pairs-a full complem e n t of functional electrodes w a s obtained for t h e subseo u e n t exneriments. T h e P?C m e m b r a n e is an especially convenient u n i t for it c a n readilv be sealed t o t h e PVC t u b i n g of t h e electrode casing ( ~ i &2 a n d 3). T h e manipulations of t h e construction stages a r e of m i n i m a l t i m e d e m a n d s , a n d t h e ulti-
5 A solution of between 1W3 and 10-2 M of calcium chloride or sodium nitrate would suit the instructions given here.
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543
Table 2.
Activity Coefficient Data from Various Modified Debye-Hiickel Equations with Gran's Plot Data (Eqn. (8)) Corresponding to vo= 20 ml
Value (ml) of (VO V.)lO+Es'"
Activity CoeEeient, f., for various V., [CaC121 and p values, respectively Form of Modified Debye-Hiickel Equation Corresponding to -log f.
V.(ml) [CaCLI(lOSM) c (loa M)
0 1.0 3.0
0.4 2.94 8.81
1.0 5.71 17.1
2.0 10.0 30.0
+
'
18
a t various V, values (ml) 0
0.4
1.0
2.0
0.5115 z 2 K p 0.329 d d E ) (d taken aa 4.73A)
+
(1
mate objectives can readily be achieved as long as experiments 1 and 2 are scheduled a few days before experiments 3 and 4. If desired, the scheme can be extended. For example, the exercise can be preceded by the synthesis of the constituents of the calcium liquid ion exchanger (7,8).A convenient mix for the liquid ion exchanger is to take (9) the di-n-octylphenylphosphonatemediator and calcium didecylphosphate sensor in the ratio 10:l. This avoids the dependence on commercial ion exchangers. Similar independence is also feasible with Aliquat-based sensors (10-13) which have recently been shown to be miscible with PVC. An analysis of the validity of the assumption of the constancy of the activity coefficient term, f , in eqns. (3). (4), and (8) is also very pertinent to the subject of ion selective electrodes together with the study of various modifications of the Debye-Huckel equation. In this respect the assumption concerning constancy of f is unwise for solutions of high ionic strength. This is illustrated from data obtained with a calcium ion selective electrode by adding 10-1 M calcium chloride standard solution to a M calcium chloride test solution (Table 1). The actual Gran's plot (line A in Fig. 4) may be compared with the plot (line B in Fig. 4) obtained by correcting for the change in activity coefficient and using activity coefficients calculated by the equation
the ordinate of Line A in Figure 4. The paper is also volume-corrected for the kind of volumes used in the above discussion, namely, multiple addition of standard in units of 0.01 of the sample volume up to 10% of the Vo value. With this paper, it is only necessary to plot the emf readings on the ordinate for plots like Line A in Figure 4. Since the scale is already calibrated for Nernstiau response at 298°K for electrodes of univalent ions as well as divalent ions. it is onlv necessarv. to . d o t the emf reading" corresponding to the test solution a t an appropriate point on the ordinate in order to obtain a~wrooriateconditions for extrapolation to Ve. The various briinate points can then be labelled accordingly for plotting the remaining data. The paper is not suitable if the ion selective electrode response deviates materially from Nernstian. Acknowledgment
The authors thank Dr. G. J. Kakabadse, Chemistry Department, University of Manchester Institute of Science and Technology, for stimulating the design of these experiments. Literature Cited 11) KakabadPP. G. I . ~ O r g e n i m ) ,A Summer Schml on "The UPP 01 10" Sele~tive Elecfmdes in Chamistw. Biology and Medicine" held at The University of Man-
as quoted in Table 2. It is to be noted that of the figures quoted in Table 2, those calculated from eqn. (10) best fit the experimental data under consideration here and lead to the expected M calcivalue of -0.2 ml for Vo corres~ondineto a urn ion containing testsolution. The other versions of the Debye-Hiickel equations, especially eqn. (I), over-correct for the change in activity coefficient. Orion Gran's Plot Paper The use of Orion Gran's Plot (semi-antilog) Paper (14) avoids the cumbersome antilog calculations required for ~
544
~
1 Journal ot Chemical Education
,Az..>,.
18) Grifiithr. G. H..Mmdy, 0. J., and Thomas. J . D. R.. J. Inor8 and Nucl Ckem.. 31,3042 (1912). (91 GritY7ihr. G.H..MouiyG. J.am1 Tlruaml. J . D.R..Anolyst, 97.120l1972). . . . A ~ ~ Ic. h m . ~ 2 0 7 train). 1 ( l o ) coerzee. C J . . s n d ~ r i e s e r H (11) Coetzee. C.J.. and Frieser. H..Anol. Chem.. d l . 1128 (19691. 112) Matsui,M., sndRieser. H.,AnoI. Left. 3.161 11970). (13) ,lames. H.,Carmsck.G..sndFrieser.H..Anal Chrm, 11.8561197?1. , 11970). (14) OrionResearchInc.. N ~ w r k t l r r 2.49
