Anal. Chem. 1988, 60, 295-301 Sandblom. J.; Eisenman, G.; Walker, J. L.. Jr. J . phvs. Chem. 1967,
71.3871-3tm. Oesch, U.; Simon, W. Anal. Chem. 1960. 52, 692-700. Luch, J. R.; Higuchi, T.; Sternson, L. A. Anal. Chem. 1982, 5 4 , 1583-1 588. Ybshida, 2.; Freiser, H. J . Electroanal. Chem. 1984, 179, 31-41.
~ h o m aA. , P.; VivlanCNauer. A.; Anfanhis, s.; M O ~ ,w. E.; Simon, w . Anal. Chem. 1977, 49, 1567-1572. Salthouse, E. C.; McIihagger, D. S. Roc. Inst. Electr. Eng. 1963,
110.9a3. Caselli, M.; Papoff, P. J . Necfrmnal. Chem. 1969, 23, 23-40.
295
(15) Thoma, A. P. Dissertation, Swiss Federal Institute of Technology, Zurich, 1977. (16) Armstrong, R. D.; Todd, M. Electrochim. Acts 1987, 32, 155-157. (17) Sandifer, J. R., private communication.
RECEIVED for review June
12,1987. Accepted September 24, 1987. Support from NSF (under Grants CHE8406976 and INST-8403331) and the Hungarian Academy of Sciences is gratefully acknowledged.
Responses of Site-Controlled, Plasticized Membrane Electrodes Ern8 Lindner, Etelka Grlf, Zsuzsa Niegreisz, Klhra Tbth, and Ern0 Pungor Department of General and Analytical Chemistry, The Technical University of Budapest, Gellert Ter 4, H-1111 Budapest XI,Hungary
Richard P. Buck* Department of Chemistry, Uniuersity of North Carolina, Chapel Hill, North Carolina 27514
Ion exchange wlth atomk absorptkm methods has been uued to determine the anlonlc M e concentrations of membranes and membrane-support materlak poly(Viny1 chloride) (PVC) and carboxylated PVC. Total &le concentratlons can be determined. Impedance measurements of &lacontro#ed mem branes prove that not an Mes are available to K+ In PVC-COOH based neutral carrler membranes. The theory of selectlrlty, based on the assurnptlon that the carrler/avallable slte concentratlon ratio Is the most Important factor, Is thoroughly tested. New results on she concentratlon control by tetraphenylborate (TPB-) and tetrakls(p-chtorophenyl)borate emphasize the systematic varlatlon of selectlvlty with carrler/site ratlo. At low cartler levels, TPB- can be spontaneously lost from membranes until the unlty carrler/slte ratlo Is achleved. This paper in part V in a series on the general topic of polymer and plasticizer properties as they affect electrical and electrochemical characteristics of membrane electrodes. In particular, we have previously inferred the prevalence of intrinsic, negative sites in various poly(viny1 chloride) (PVC) samples from different sources (1-3) and in polysiloxane (silicone rubber) membranes (4).Although this observation is not original with us ( 5 , 6 ) ,certain response characteristics (potentiometric slopes and selectivities) and electrical properties (current-voltage curves, resistance vs time, etc.) can be conveniently interpreted. The effects of bathing electrolyte concentrations and neutral, lipophilic complexing agent (valinomycin) loadings are best understood by assuming polymer membranes contain fEed, negative sites and possibly some mobile negative sites. The origin of these sites seems to be the manufacturing process for the polymers and impurities spontaneously formed from ester plasticizer decomposition. This paper focuses on remaining problems: the quantitative determination of actual ion exchangeable sites available for different countercations. It is probable that actual sites used may be equal to, or less than, the total available. This distinction is important in choosing between mechanisms and potential distributions. The intention of this work is to make clear the distinction between total and active ion exchangeable, 0003-2700/88/0360-0295$01.50/0
anionic site concentrations. A general method is applied for the determination of ion exchanging sites in PVC and in compounded membranes Containing known concentrations of carboxylated PVC (PVC-COOH). Potentiometric responses of the characterized membranes are then measured as a function of neutral carrier loadings: valinomycin for potassium sensors and ETH 1001 for calcium sensors. The latter experiments verify the prior observation (7) that slopes, selectivities, and membrane resistances become optimal or ideal at high carrier loadings relative to active, accessible fixed (or mobile) sites. However, at low carrier loadings (less than the accessible active fiied site concentration), membranes lose their ideal responses. This series of effects has been interpreted by the assumption that accessible negative site concentrations determine, electrostatically, the btal concentration of extracted counterions. The ionophore valinomycin (val) behaves as a selective reagent to form the complex Kval+. For ideal responses of sensors to potassium activities, all of the sites should be covered by Kval+ as counterion. In fact, Kval+ concentration may be equal to the fixed site concentration or less, depending on ionophore (carrier) loadings. Excess valinomycin, typically 1w t % or 9 mM, is sufficient to ensure Kval+ at 0.1 mM, the typical site concentration. However, at low carrier loadings, less than 0.1 mM, response degradation occurs because other cations can serve as counterions for the negative sites. This competition for negative sites by many cations is believed to be a principal factor in the mechanism of cation interference. A further conclusion from the model is that addition of foreign (extrinsic) sites, such as lipophilic anion salts (sodium tetraphenylborate, NaTPB, or potassium tetra(p-ch1oro)phenylborate, KTpClPB) is strictly limited to concentrations less than the neutral carrier concentration. The reasoning is that potassium selectivity remains dominant only when Kval+ counterions balance the total negative sites. These agents for adding anionic sites, and thus reducing membrane resistances and anion interferences, should be limited to concentration less than the carrier loading. We will show that this reasoning still applies to membranes which seem to contain very high total added COOH, because only a small portion of the sites are ionized (“active”)and available for potassium exchange. The availability of functionally modified forms of PVC containing 1wt % COOH groups has made possible prepa0 1988 American Chemical Society
296
ANALYTICAL CHEMISTRY, VOL. 60, NO. 4, FEBRUARY 15, 1988
Table I. Composition of the Dummy Membranes" as 1
2
3
PVC HMW PVC-COOH
33.4
33.4
DOS
66.6
32.8 0.6 66.6
0-NPOE
% (w/w) and
4
Bulk Properties
membrane 5
32.2
6
31.7 1.7 66.6
1.2
66.6
30.6 2.8 66.6
7
8
9
27.8 5.6 66.6
33.4 66.6
33.4
66.6
RbulLtb kQ Cbulkt pF ?bulk,
66.6
15000-30000 1500-1800 15000 14000 12000 40-60 170 60-70 60-70 60-70 60-70 60-70 1 x 10-3 3 x 10-4 1 x 10-3 1 x 10-3 1 x 10-3 1 x 10-3 8X 10-14 38-42 15-17 15-17 15-17 15-17 15-17
€
Dummy membranes do not have ionophore. Data determined after 5 min of soaking in capacitance; 7, RC; e , dielectric constant.
5500 60-70 4 X 10" 15-17
500 60-70 9 X 10" 44-47
M KC1 solution. Key: R, resistance; C,
Table 11. Composition of the Valinomycin-Based Membranes as % (w/w)and Selected Membrane Properties N- 1 valinomycin NaTPB
N-2
1.0
1.0
33.0
33.0
N-3
N-4
membrane N-5 N-6
N-7
1.0
1.0
1.0
1.0
31.5 29.9 1.5 3.1 (70) (150) 66.0 66.0
29.9 3.1
33.0
33.0
1.0
a
DOS 0-NPOE
66.0
Cbulk, ?bulk*
kQ pF
66.0 66.0
6400
1770 66
77
18 56.6
N-10
(150)
66.0
3700 f 250 300-600 61-75 130-170 3X 6X e 16-20 37-45 Sl-,(KCl),cmV/dec 56.4 R-*
N-9
1.0 1.0 1.0 0.2 0.5 0.2 (70) (150) (70) 32.9 32.8 32.9
a
PVC HMW PVC-COOH
N-8
49-52
a Mole percent sites relative to valinomycin are given in parentheses. footnote, Table 111.
65.7
1.0 0.2 (70) 29.9 3.1 (150)
65.9
66.0
137 220
15-16 55.8
65.9
N-11
1060 60 61-66 60-74 7 x 10-5 2 x 10-5 14-16 60 54.5
350 70
65.8
33
56.3
52.1
56.6
Data determined after 5 min of soaking in 10-3M KC1.
See
Table 111. Composition of the ETH 1001 Based Membranes as % (w/w) Ca-1 ETH 1001 NaTPB
Ca-2
1.0
1.0
1.0
1.0 (200) 32.7 65.3
PVC HMW PVC-COOH O-NPOE
33.0
0.3 (70) 32.9
66.0
65.8
S1+in CaCl,,b mV/dec
25.1
27.5 28.2
a
Si-5
membrane Ca-4
Ca-3
in CaCl,
Mole percent of sites relative to ETH 1001 are given in parentheses. l0-l-lO4 M Si-5 = slope over 10-1-10-5 M. ration of PVC membranes with a range of known functional group concentrations by compounding with low-site, ordinary high molecular weight (HMW) PVC. Although these potentially high site density modifications were made primarily for better adhesion to solid surfaces (8), their availability is advantageous for testing as possible low resistance supports in ion selective electrodes (ISEs) and as supports that might reduce anion interferences that occur in low site density polymers (9-12). Also, it is interesting to study responses of PVC-COOH based sensors because anion interferences and uni- and divalent ion selectivities may depend on the dielectric constant. The latter values increase with increasing COOH content according to measurements reported here.
EXPERIMENTAL SECTION Chemicals. Valinomycin and high molecular weight and carboxylated poly(viny1 chloride) (designated HMW and PVCCOOH) were purchased from Aldrich. Bis(2-ethylhexyl) sebacate,
Ca-5
Ca-6
Ca-7
1.0
1.0
1.0
1.0 0.3
33.0 66.0
30.5 2.5 66.0
25.8 7.2 66.0
28.3 28.9
27.1
(70)
25.6
32.9 65.8 27.3 28.6
Potentiometric slopes over subscripted range: SI+ = slope over 2-nitrophenyl octyl ether, potassium tetrakis(4-chloropheny1)borate, and sodium tetraphenylborate (designated DOS,o-NPOE, KTpClPB, and NaTPB) were made by Fluka. Tetrahydrofuran (THF) was Aldrich Gold Label quality. All other chemicals were analytical grade reagents: KSCN, CaC12-2H20,and MgClz.6H20 (Fisher Scientific Corp.), NH&l (Mallinkrodt), KC104and LiCl (J.T. Baker Chemical Co.), LiOAc (Fluka AG), CsCl (GallardSchlesinger Chem. Mfg. Co.), Ca(SCN)z in 62% solution (Pfaltz and Bauer, Inc.), Ca(C104)2-XH20(Alfa Products). For solution preparation, deionized water, distilled in Pyrex glass, was used. Membranes. Membranes were prepared by the method suggested by Craggs et al. (13). The compositions of the membranes studied are summarized in Tables 1-111. For potentiometric measurements these membranes were mounted into Phillips IS-560 liquid membrane electrode bodies. Apparatus. Resistances were obtained from an impedance plot for each membrane with active area of 0.64 cm2 in contact with aqueous solutions. Resistances of THF solutions of membrane components, in a conventional conductance cell, also used
ANALYTICAL CHEMISTRY, VOL. 60, NO. 4, FEBRUARY 15, 1988
297
R [Mohl
250
.
-
Aldrich HMW Carboxylated PVC
0 rng PVC in l O r n l THF
Flgure 1. Conducttvity of a THF solution as a function of added PVC powders (see Experimental Section).
50
100
150 200 ul sdtener/lO ml THF
Flgure 2. Conductivity of a THF solution as a function of teners (plasticizers) (see Experimental Section).
Table IV. Ion-Exchange Capacity of PVC Powders and PVC-DOS Membranes
the Solartron impedance measuring system as described in ref 1.
emf Measurements. The emf values were measured at room temperature with an Orion Research Model 701 pH meter coupled to an Orion Research Model 605 electrode switch. The external reference electrode was a double junction Ag/AgCl electrode, Orion Model 90-02, with 1M lithium acetate outer filling solution. The activity coefficients of the aqueous ions were calculated according to the Debye-Htickel approximation with the parameters published in the literature (14). The measured emf data were corrected for the diffusion potential at the reference electrode/test solution interface according to the Henderaon equation. The selectivity factors were determined by the separate solution method using 0.1 M solutions. Materials Testing. Determination of “Impurities” by Spectrometry. Determination of cationic impurities of different PVC powders (Aldrich HMW PVC and PVC-COOH) in our work used emission spectrography, atomic absorption spectroscopy (AAS),and inductively coupled plasma (ICP). Approximately 1g of the PVC powder was weighed and ashed in a quartz beaker. The cations detected are listed in order of decreasing concentrations: Na, Ca, K, Fe, Si, Mg, Al, Mn, Pb, Cd, and Zn. However, the s u m of all impurities but sodium and calcium was negligible. The values determined for these two cations were 0.15 mmol of Na/kg and 0.1 mmol of Ca/kg for Aldrich HMW PVC and 18 mmol of Na/kg and 2 mmol of Ca/kg for PVC-COOH. A report on the chemical identification of anionic sites using X-ray photoelectron spectroscopy (Xes), secondary ion mass spectrometry (SIMS), and X-ray fluorescence was published recently by van den Berg et al. (15). Determination of “Impurities”by Conductivity Measurements. To determine the contribution of individual membrane components to the total conductance of the membranes, and later to control the concentrations of impurities and/or sites in the membranes, all of the components used in the membrane preparation were examined. A conductivity cell with two parallel platinized platinum electrodes of 1cm2 area at a distance of 1 cm (cell constant 0.318 cm-’) and containing 10 mL of THF solution was used. First the conductivity of THF was checked and then small amounts of the tested materials were added to the THF solution in subsequent steps. The solution was mixed until complete dissolution of the compound and the conductivity of the cell was measured after each addition (Figures 1 and 2). Determination of Ion Exchange Capacity of PVC Powders. The procedure was a variation of one used commonly for ion exchange resins (16,17).First 1g of the PVC powder was treated with acidic and alkaline solution several times successively,since an ion exchange resin reaches its optimal ion exchange capacity after two to three cycles of “formation”. Between and after the treatments, the powder was washed with distilled water until neutrality. Following the above ”formation”procedure, the PVC powder was loaded with 1 M HC1 and washed to acid-free. Then 5 mL 0.1 M LiOH was filtered through the PVC powder for 1 h. The LiOH eluted from the PVC was titrated with 1 M
added sof-
form
type of PVC
dummy membr dummy membr powder powder normal membr normal membr
HMW PVC-COOH HMW PVC-COOH HMW PVC-COOH
ion-exchange capacity, mmol/kg 0.09 f 0.03 0.27 f 0.01 67
276 0.32 (0.05)’ 1.48 (0.21)‘
‘The data in parentheses are determined with the procedure applied for dummy membranes. HCl using methyl red indicator. The ion exchange capacity was calculated on the basis of the amount of LiOH bound to the PVC powders. The results are summarized in Table IV. “Dummy”Membrane: Plasticized PVC without Carrier. For this study dummy membranes of 2 cm diameter, 0.022 cm thickness, and 70 mg weight were soaked in 1mL of 0.1 M LiCl solution for 1week. The sodium content of the LiCl solution was determined. After soaking, the membranes were rinsed three times with distilled water and transferred to 1mL of 0.1 M HC1 solution for another 2 days to displace the bound Li+. The Li+ concentration leached out was determined by AAS (Varian Techtron AA6). The ion exchange capacity values obtained for PVC-DOS membranes are summarized in Tables IV and V. “Normal”Membranes: Plasticized PVC Containing Valinomycin. The exchangeable potassium in valinomycin-loaded membranes was determined in two successive steps. First, the procedure applied to determine the ion exchange capacity of dummy membranes was adapted to these membranes. Next, the membranes were loaded with potassium by soaking them in lo4 M KC1 for 1 day. Then they were rinsed with and soaked in distilled water for another day. Finally, 1 mL of M CsCl solution was applied to displace the extracted potassium, since the selectivity coefficient of valinomycin-based electrodes for cesium is large, approximately 0.4. The potassium concentration of M CsCl solution was determined by flame AAS in a propane-butane-air flame (Table V).
RESULTS AND DISCUSSION I t is known that the presence of strictly limited amounts of salts consisting of hydrophilic cations and lipophilic anions, e.g. NaTPB or KTpCLPB in neutral carrier based membranes, are beneficial in many respects (10, 11). These additives reduce interferences by lipophilic anions in the sample, may give a rise in selectivity, and are able to boost cation sensitivity in the case of carriers with poor extraction capabilities and lower the electrical membrane resistance considerably (2419). The amount of salt added should not exceed the critical molar ratio of ionophore/lipophilic salt (depending on the ionophorecation complex stoichiometry). However, an excess salt situation is self-correcting. It is important to mention that
298
ANALYTICAL CHEMISTRY, VOL. 60, NO. 4, FEBRUARY 15, 1988
a
kn
Z[Iml
1200
c
n
1 LRel k
Z0 \
n
Z[hl kn Re(1) Re(2)"0°4
12.6
--
800
I
4 [Re1 k
40k-".,'
n
40t /
3
R
:::[ 30
0
300
+
600
900
c
ZIRel
Flgure 3. Compiex impedance plane plots of HMW PVC, PVC-COOH compounded HMW, and pure PVC-COOH based normal membranes. Re(l)/Re(2)is the ratio of the diameters of the first and second (bulk and surface) semicircles: (a) DOS based membranes; (b) o-NPOE based membranes. The numbers refer to membrane compositions in Table 11.
membranes which have KTPB in excess of valinomycin levels spontaneously lose KTPB into the contacting aqueous KCl solutions until the concentration of KTPB becomes exactly the same as that of the valinomycin (18). This effect leads to recovery of lost selectivity discussed in a later section. Recently it has been found that excess salt can be tolerated without loss of selectivity when both the cation and the anion incorporated into the membrane are lipophilic (20,21). It seemed likely that increasing the low site concentration of ordinary HMW PVC, using PVC-COOH, would give further or similar benefits found for added mobile sites, e.g. TPB-. However, PVC-COOH used to enhance adhesion on SiOz surfaces (8) is difficult to remove from glass plates normally used for PVC membrane casting. Teflon plates solve this problem. Impedance Measurements. By compounding ordinary HMW PVC (of low site concentration) with PVC-COOH, beeides the decrease in bulk membrane resistance (Tables I and 11), a significant decrease in the surface film resistance could be observed with both DOS and o-NPOE based mem-
branes (Figure 3). In contrast to observations on HMW membranes, the second or surface film semicircle could not be recorded at all when the membranes were conditioned in M KCl for longer periods of time (2)to allow plasticizer exudation. Accordingly, the good adhesion of this type of PVC is understandable since exudation seems not to be important for PVC-COOH. There is a further advantage of this membrane property; it makes possible the study of ionic electrokinetics at pure ionic interfaces which will be described in a subsequent paper (4). By preparation of PVC-COOH compounded HMW membranes it was expected that both the advantages of incorporated sites (e.g. NaTPB or KTpClPB) and PVC-COOH features could be achieved without disadvantages (e.g. dissolution of s a l b and plasticizer from the membranes cannot be entirely prevented). With an increase of the PVC-COOH content in compounded HMW membranes, the resistances decrease (Tables I and 11). However the effect is far less than that expected on the basis of the calculated site concentration and on the experimental results with TPB- containing membranes
ANALYTICAL CHEMISTRY, VOL. 60, NO. 4, FEBRUARY 15, 1988
299
Table V. Ion Exchange Capacity of PVC-DOS Membranes Incorporating NaTPB or KTpClPB in Different Amounts cation Nat or Kt leached out (mmol/kg) into 2 mL of conditioning
incorporated sites, mmol/ke NaTPB KTpClPB I
solution dist water' dist wateP
-
0.1 M LiCl
residual boron in me. ! . . L1.
0.001 M CsC1'
membraneb
Dummy Membranes 7.2 5.4 3.6 1.8
6.4 5.2 3.4 1.9
2.4
5.4
1.2
2.0
2.1
0.4
0.05
5.1
Normal Membranes 5.4
1.6
1.8
5.4
0.2
4.3
'Experiments carried out on freshly prepared membranes in subsequent steps. bData correspond to compositions on the left; boron determined by atomic absorption and is presumed to be residual anions. a
lop K',":
Loa K E ' . C..,
7
CI.5
c.-4
C..2
C..7
C.-3
C.-Sll
- ca
- Ca
2
0
1
1
0
-2
1
-3
-2
- ca - cs
-
Ca
- ca - cs -
K
-
Mo
- K
-4
-3
-5
-4
-6
-5
-7
-6
b N-2
@ K r M
N-5
N-7
N-11
N-10
-K
- K
'I -K
-K
C.
-1
-2
i
-
NH4
-
C.
-
CI
- c.
NH4
-
NH,
-
NI
NH4
-
NH4
-
Na
-
H.
-
-
-
Figure 4. Selectivity coefficients of valinomycin-based potassiumselective membranes of different compositions (see Table 11): (a) membrane plasticizer, bis(24hylhexyl) sebacate (DOS) N-911, the measurement of N-9 membrane aiter 2 days of soaking in 200 mL of lo3 M KCI; (b) membrane plastlcizer, 2-nitrophenyl octyl ether (oNPOE).
(20).The addition of the allowed amount of NaTPB (70 mol % in respect to the valinomycin) induced a far greater resistance decrease compared to that expected for a membrane completely made from PVC-COOH in which the site concentration is more than one order of magnitude larger than that of the carrier. Effect on the Selectivity of Uni- and Divalent Cations. Comparison of the potentiometric selectivities of HMW, PVC-COOH, and compounded membrane electrodes are shown in Figures 4 and 5. These data include potassium- and calcium-selective membranes based on valinomycin and ETH
Figure 5. Selectivity coefficients of ETH 1001 based calciumselectie membranes of different composition (see Table 111) Ca-311: the measurement of Ca-3 membrane after 2 days of soaking in 200 mL of M CaCI,.
1001 ionophores. For NaTPB containing membranes, excess of NaTPB over the critical ratio of lipophilic salt/cmier yields a severe degradation of selectivity (compare N-3 or N-4 with N-9). The membrane N-9 has the poor selectivity properties of a classical ion exchanger (22). Practically no selectivity degradation can be seen for the HMW PVC, PVC-COOH, and low-loaded TPB- potassium selective membranes for a range of site concentration/ionophore ratios (compare Figure 4a N-1 with N-4, N-6, and N-8). Similar observations were made on polar o-NPOE based membranes (Figure 4b); i.e. the beneficial effect of increased PVC-COOH site concentrations could be obtained without the risk of a drastic drop in selectivity as an effect of overdose of PVC-COOH. The data for membrane N-11 compounded with 150 mol % PVC-COOH and 70 mol % NaTPB (in respect to valinomycin) showed the same selectivity as N-7 (cast completely from PVCCOOH) and the same as N-8 prepared from HMW and compounded with 70 mol % NaTPB. Recovery of lost selectivity from an overdose of TPB- is illustrated by comparing N-9 with N-9/1, and Ca-3 with Ca-3/1. The same statements can be made concerning the experiments on calcium-selective membranes (Figure 5). A membrane cast completely from PVC-COOH (calculated site concentration in the membrane 140 mmol/kg) and compounded additionally with 70 mol 70NaTPB (Ca-7) had the same selectivity as Ca-2 (HMW + 70 mol % NaTPB) or Ca-5 (HMW 70 mol % PVC-COOH) or Ca-4 (PVC-COOH), etc. All these must be considered with reference to the normal ionophore concentration which is about 9 mmol/kg.
+
300
ANALYTICAL CHEMISTRY, VOL. 60, NO. 4, FEBRUARY 15, 1988 EMF lrnV1 3 0 0 ~ -
EMF
*,
a
[&I
KO KCI
200 2001
/ 150
100
5c
0-
h
-L
C
-2
log
EMF h V 1
300 J
/*
/
KC'
7' 200
100
0
I _ 8 -
log OK
Potential responses of valinomycin-based PVC-COOH compounded potassium-selectivemembranes In solutions containing different anions: (a) DOS plasticized membranes: (b) o-NPOE plasticked membranes. Since there was practica#y no difference between various DOS membranes (N-1,N-3, N-4, N-6, and N-8) or between various o-NPOE based membranes (N-2, N-5, N-7, N-10, N-1 1). only one of each plasticizer is selected and shown on the figure. Flgure 6.
The effect of the plasticizer dielectric constant on PVCCOOH compounded membranes was the same aa for ordinary HMW based membranes (Tables I and 11). However, higher e values were observed for PVC-COOH membranes. This result was thought to be an effect of the greater water uptake by PVC-COOH (23). Effect on the Anion Interference. The anion interference of sensors responsive to monovalent cations may be nearly eliminated by using membrane plasticizers of low dielectric constant (18). In agreement with this observation it was found that DOS based membranes have, by far, less anion interference compared with o-NPOE based membranes (Figure 6), but practically no differences upon varying the PVC-COOH content were found. Unfortunately with Ca2+ sensitive membranes, where the polar solvent cannot be replaced (%), incorporation of various amounts of PVC-COOH
Flgure 7. Potential response of ETH 1001 based PVC-COOH compounded calciumselective membranes in solution of different anions. Since there was no difference between membranes (Ca-1, Ca-2, Ca-4,5,6,7),as an example Ca-4 was selected and is shown in the figure.
into the membranes did not lead to improvement in the anion interferences (Figure 7). These results are in contrast to those reported for TPB-containing membranes (IO,11). The observed impedances and selectivity data suggest that only a negligible part of the total site concentration of PVCCOOH is active, e.g. in dissociated and available form. Furthermore, the data suggest that the absolute amount of active ion exchangeable sites remains small in comparison with dissolved ionophore concentration,regardless of the membrane solvent polarity, e.g. from DOS to o-NPOE. In order to justify this conclusion the ion exchange capacities of PVC powders and those of dummy and normal membranes were determined and compared (Table IV). The ion exchange capacities of the PVC powders can be taken as total site concentrations, while those of the membrane are active site concentrations. Table IV clearly indicates that the measured ion exchange capacity (active site concentration) of normal, valinomycincontaining membranes is about 5 times larger than that of plasticized, carrier-free dummy membranes. However, the values determined are still far less than the valinomycin concentration in the normal membranes and almost negligible with respect to the total site concentrations (Table IV). These results correlate well with the bulk resistance ratios found for dummy and normal membranes (see Tables I and 11). The apparent higher site concentrationsof normal membranes may be explained by the greater dissociation of Kval+/sites compared to the dissociation of any simple cations attached to the Tied sites in the solvent polymeric dummy membranes. To demonstrate the capability of the method for site concentration determination, ion exchange capacities of dummy and normal membranes containing known additional sites, e.g. NaTPB or KTpClPB in varying amounts, were also determined (Table V). The results clearly show that the "site concentration" found by Li+ replacement approaches that of the known addition. We have also found that the spontaneous dissolution of added lipophilic salts into distilled water from normal membranes containing KTpClPB is negligible. But, spontaneous loss of NaTPB is not negligible, although dissolution rates and the dissolved amount of lipophilic salts into the contacting aqueous electrolyte are considerably smaller from normal membranes compared to that from dummy
Anal. Chem. lQ66, 60,301-305
membranes. This result may mean that complexed cation Kval'TPB- is more oil soluble than the simple K+TPB-. But there may also be some association between cations and anions. Nevertheless, the advantageous effect of added NaTPB on the potentiometric behavior of ionophore based electrodes is still useful, although limited in time. Registry No. DOS,122-62-3;o-NPOE, 37682-29-4; KTpCPB, 14680-77-4; NaTPB, 143-66-8; ETH 1001, 58801-34-6; PVC, 9002-86-2; K, 1440-09-1;Ca, 7440-70-2; valinomycin, 2001-95-8.
LITERATURE CITED Horvai, G.; Grif, E.; Tbth, K.; Pungor, E.; Buck, R. P. Anal. Chem. 1988, 58, 2735-2741. Tdth, K.; Grif, E.; Horvai, 0.; Pungor, E.; Buck, R. P. Anal. Chem. 1988, 58, 2741-2744. Buck. R. P.; Tbth, K.; Grif, E.; Pungor. E. J. Elechoanal. Chem. 1987, 223, 51-66. Lindner, E.: Niegreisz, Zs.; Pungor, E.; Buck, R. P., in preparation. Perry, M.; Lobei, E.; Bloch, R . J . Membr. Scl. 1976, 7 , 223-235. Kumlns, C. A.; London, A. J. folym. Sci. 1980, 46, 395-408. Oesch, U.; Simon, W. Anal. Chem. 1980, 52,692-700. Satchwill, T.; Harrison, D. J. J . Electroanal. Chem. 1988, 202, 75-81. Boles, J. H.; Buck, R. P. Anal. Chem. 1973, 4 5 , 2075. Morf, W. E.; Kahr, G.; Simon, W. Anal. Lett. 1974. 7 , 9. Morf, W. E.; Ammann, D.; Simon, W. Chlmk 1974, 28, 65. Kedem, 0.; Perry, M.; Bloch, R. IUPAC Internatlonal Symposium on Selective Ion-Sensitive Electrodes, Cardiff, 1973 paper 44. Craggs, A.; Moody, G. J.; Thomas, J. D. R. J . Chem. Educ. 1974, 51, 54 1-544.
30 1
(14) Meier, P. C.; Ammann, D.; Morf, W. E.; Simon, W. I n Medical and Biologlcel Appllcatlons of Electrochemical Devlces; Koryta. J., Ed.; Wliey: New York, 1980. (15) van den Berg, A.; van der Wai, P.; Ptasinski, D.; Sudhoiter, E. J. R.; Bergveld, P.; Reinhoudt, D. N. Anal. Chem. 1987, 59, 2827-2829. (16) Heifferich, F. Ionenaustauscher; Verlag Chemie: Weinheim, 1959; Band I. (17) Incz&Iy, J. Analytical Application of Ion-Exchangers ; Akaddmiai Kiadb: Budapest, 1966. (18) Ammann, D.; Morf. W. E.; Anker, P.; Meier, P. C.; Pretsch, E.; Simon, W. Ion-Sel. Electrode Rev. 1983, 5 , 3. (19) Armstrong. R. D., submitted for publication in Electrochim. Acta. (20) Ammann, D.;Pretsch, E.; Simon, W.; Lindner, E.; Bezegh, A,; Pungor, E. Anal. Chlm. Acta 1985, 171, 119-129. (21) Pretsch, E.; Wegmann, D.; Ammann, D.; Bezegh. A.; Dinten, 0.; Uubil, M. W.; Morf, W. E.; Oesch, U.; Sugahara, K.; Weiss, H.; Simon, W. In Recent Advances in the Theory and Application of Ion-Selectlve Electrodes h Physiology and Medlclne; Kessler, M., Harrison, D. K., Hoper, J., Eds.; Springer-Verlag, Berlin, 1985. (22) Meier, P. C.; Morf, W. E.; Laubii, M.; Simon, W. Anal. Chim. Acta 1984, 156, 1. (23) Tdth, K., unpublished resuits. (24) Lindner, E.; Tbth, K.; Pungor, E.; Behm, F.: Oggenfuss, P.; Welti, D. H.; Ammann, D.; Morf, W. E.; Pretsch, E.; Simon, W. Anal. Chem. 1984, 56, 1127.
RECEIVED for review June 24, 1987. Accepted September 24, 1987. Support from the Hungarian Academy of Sciences and NSF (under Grants CHE8406976 and INST-8403331) is gratefully acknowledged.
Liquid Chromatographic Determination of Nitrilotriacetic Acid, Ethylenediaminetetraacetic Acid, and Related Aminopolycarboxylic Acids Using an Amperometric Detector Jihong Dai' and George R. Helz*
Chemistry and Biochemistry Department, University of Maryland, College Park, Maryland 20742
An amperometric detector empioylng a carbon-paste electrode is used to determine aminopoiycarboxylic aclds, Inciudlng nltrliotriacetic acld (NTA) and ethyienediamlnetetraacetic acld (EDTA), after liquld chromatographlc separation on a reversed-phase column with an aqueous trlchioroacetk acld mobile phase at pH lower than 2. The aminopoiycarboxylic acids are dlrectly oxidlzed at the detector electrode wlthout Involving an lntermedlate species. Glycine, hninodlacetic acld, common amino aclds, cltrk acld, and fulvlc acids do not interfere with the determination of NTA and EDTA. The low moblie-phase pH limits Interference from metal lons In natural waters. Where such interference occurs, a stronger chelatlng reagent [e.g. dlethylenetriamlnepentaacetic acid (DTPA)] can be used to suppress it. NTA and EDTA In aqueous samples, including wastewater treatment plant influent and effluent, can be determlned without prlor sample preparation. The minimum detectable amounts are 0.1 ppm for NTA and 0.15 ppm for EDTA with a precision of less than 7% relative standard deviation.
Aminopolycarboxylic acids, such as ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid ("FA), are widely 'Present address: B d Polar Research Center, Ohio State University, Columbus, O g 43210.
used in industry and agriculture. Their use in commercial detergents, specialized cleaning reagents, and a variety of other products, including foods, results in their ultimate release to the environment ( 1 , 2 ) . Their strong chelating capacity may play an important role in the distribution and transportation of metals in the aquatic environment. These chelating agents can enhance the levels of dissolved heavy metals by both releasing them from sediments (I,3) and inhibiting removal through precipitation ( 4 ) . They are synthetic products with undetermined health and toxicological effects (5, 6 ) . It has been suggested that they may constitute a source of nutrient nitrogen for aquatic algae (7), although this has been questioned (8). A number of analytical methods exist for NTA and EDTA. Among them, the most widely used for environmental purposes are the highly sensitive gas chromatographic methods, originally developed by Aue et al. (9) and subsequently improved and modified by others (10-15). However, the GC methods are limited by long analysis time and various interferences (16). Recently, in efforts by researchers to develop analytical methods adequate for environmental applications, liquid chromatography (LC) techniques have been exploited, especially for EDTA and NTA (17-24). So far, these techniques have been based on detection by ultraviolet (UV) absorption of complexes formed between the analyte ligand and a metal ion in the mobile phase. The reported detection limits by the L C - W methods are from 1to 10 FM. However, UV detection
0003-2700/88/0360-0301$01.50/00 1988 American Chemical Society
