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H+-selective electrodes based on neutral carriers: Specific features in behaviour and quantitative description of the electrode response
Tahta, Vol. 37, No. 5, pp. 461469, 1990 Printed in Great Britain. All rights reserved
0039-9140/90 $3.00+ 0.00
Copyright 0 1990Perpmon Press plc
H+-SELECTIVE ELECTRODES BASED ON NEUTRAL CARRIERS: SPECIFIC FEATURES IN BEHAVIOUR AND QUANTITATIVE DESCRIPTION OF THE ELECTRODE RESPONSE V. V.
EG~ROV
and YA. F.
LUSHCHIK
Institute for Physico-Chemical Problems, Byelorussian State University, Minsk, USSR (Received 18 January 1989. Revised 24 August 1989. Accepted 29 September
1989)
Summary-The influence has been studied of the membrane and solution composition on the response of H+-ISEs with plasticized polymer and liquid membranes based on the neutral carriers NJV-dioctylaniline and tridecylamine in association with trioctyloxybenxene sulphonic acid. It is shown that the extraction processes at the membrane/solution interface exert the main effect on the response limits by inducing essential changes in the activity of potential-determining ions in the membrane. At low pH, the amine extraction of acids followed by neutralization (free amines binding in ion-pairs) is the relevant process, while at high pH it is the extraction of metal cations with amine salts of a lipophilic acid, with the consequent displacement of amine from the salts. Equations are suggested to represent the interphase potential of the H+-ISE membranes with allowance for these extraction processes, ‘Ihe experimental electrode responses of both liquid and polymer membranes are shown to be well described by the equations for the interphase potential, thus indicating its dominant contribution to the membrane potential.
The first H+-ion selective electrodes based on ionophoreP have already exhibited a number of advantages over the traditional glass H+electrodes, including low resistance (with consequent advantages for miniaturization), safe handling, and promising applications in some aggressive media. Therefore, although H+-ISEs based on ionophores are characterized by much lower selectivity and narrower response ranges than glass electrodes, their applications in medical and biological studies, where they have most advantages over glass H+-ion selective electrodes, are still increasing. Various compounds have been studied as carriers of hydrogen ions, mainly aliphatic and heterocyclic amines and their derivatives,24 but also other sufficiently strong organic bases that can form a complex cation with a proton, e.g., hexabutylphosphotriamide.9 Simon6 has shown for such electrodes that deviations from the ideal proton-response in the acidic region are caused by protonation of the membrane ionophore, while in the alkaline region metal cations interfere. The ISE response range (dynamic interval) depends mainly on the basicity of the carrier. Since Simon did not study in detail the effect of membrane/solution interface extraction on the form of the electrode response, his conclusions were basically qualita-
tive. Moreover, the mechanisms of the interfering effect of metal cations in alkaline media remains unclear. The present work is aimed at developing a quantitative description of the potential of amine carrier-based H+-ISEs, allowing for membrane/solution extraction processes. The validity of this approach is supported by experimental measurements showing the electrode responses of the membranes to be well approximated by the boundary potential equations with regard to the extraction processes at the membrane/solution interface. Neglecting the contribution of the diffusion potential, which is difficult to allow for because of the mobility differences and activity gradients of charged components in the membrane, is, therefore, justifiable in these cases, although this may not always be so.
EXPERIMENTAL
Reagents N,N-Dioctylaniline (DOA) and tridecylamine (TDA) of analytical grade were further purified” and trioctyloxybenzene sulphonic acid (TOBS) was synthesized with a purity 297% for use as the electroactive components in the 461
V. V. EGCJROV and YA. F. LUSHCHIK
462
Table 1. Compositions of H+-ISE membranes used Membrane composition (the content of electroactive material is given in mole/kg or mole/l., other components in % w/w or v/v) Membrane I II III IV V VI VII VIII IX X XI XII
DOA 0.050 0.050
TDA 0.050
0.029 0.050 0.050 0.015 0.010 0.012 0.029 0.015 0.001
TOBS 0.026 0.026 0.010 0.026 0.026 0.010 0.010 0.010 0.010 0.010 0.0005
PVC 24.5 24.2 24.1 24.7 24.2 24.1 24.8 24.8 24.8 24.6 24.8
membranes. Poly(viny1 chloride) (PVC) PZh-S70 was used as the membrane matrix and was dissolved in commercial grade cyclohexanone that had previously been distilled. Dioctyl phthalate (DOP), trihexylphosphate (THP) and I-bromonaphthalene (l-BN) of commercial grade, nitrobenzene of analytical grade and carbon tetrachloride of reagent grade were used as membrane plasticizers or solvents. DOP and THP were purified by acid treatment” and I-BN was distilled. The membranes prepared from these components are summarized in Table 1. Distilled water solutions of electrolytes were made from boric, acetic, phosphoric, hydrofluoric, hydrochloric and perchloric acids as well as sodium, potassium, rubidium and caesium chlorides, sodium thiocyanate and bromide, sodium hydroxide and silver nitrate. These electrolytes were of at least analytical reagent grade. Preparation of samples The ISE membranes were produced conventionally.12 The buffer solutions for pH between 1.8 and 12.0 were made by mixing acetic, phosphoric and boric acids and sodium hydroxide.13 The same mixtures were employed to prepare buffered solutions of sodium chloride, sodium thiocyanate and other salts. Hydrochloric acid was used to adjust solutions to pH < 1.8. Estimation of extraction equilibrium constants The extraction equilibrium constants, equations (7) and (18) below, were measured at 293 f 1 K. In determining the extraction constants of hydrobromic and thiocyanic acids, the initial TDA concentration in the organic phase was O.OlM, the sodium thiocyanate concentration was O.OlM and the sodium bromide con-
DOP 73.6 72.6 72.3 73.9
I-BN -
THP -
Carbon tetrachloride -
Nitrobenzene -
72.6 72.3 74.2 74.4 74.3 73.6 74.2 90
10
centration ranged from 0.01 to O.lM. The aqueous phase pH was adjusted with universal buffer mixture solutions in the range 1.8-7.0. The equilibrium concentrations of either bromide and thiocyanate in the aqueous and organic phases or of free amine in the organic phase were measured, depending on the extraction range. The anions were measured by potentiometric titration with silver nitrate in 1-propanol. Free TDA concentrations were measured by potentiometric titration with hydrochloric acid solution in acetic acid. The equilibrium concentrations of the other components were determined by difference. The extraction constants were calculated from the formula: ch
Lx-
aH+
(1)
where C.&,,,u+x-and c, are the equilibrium concentrations of the ion-associate and free amine in the organic phase and aH+and C,- are the hydrogen-ion activity and the acid anion concentration, respectively. In assessing the extraction constants of sodium ions, the initial concentrations of TDA and DOA in the organic phase ranged from 0.011 to O.O15M, that of TOBS was O.OlOM and the sodium bromide concentration in the aqueous phase ranged between 0.1 and 0.065M. When equilibrium had been achieved, the organic phase was separated and the sodium ions were re-extracted with an equal volume of 0. 1M hydrochloric acid. The sodium concentration in this aqueous phase was measured with a PFMU4.2 flame photometer. The extraction constants were estimated from the formula K:, =
ifhQ+R- CAm
C AmH+R-
(2) CM:aOH-
463
Electrodes based on neutral carriers
where CMzR- is the concentration of the ionassociate of the metal cation with the TOBS of the anion, chH+R- is the concentration protonated amine ion-associate with the TOBS anion and aoH- and C,: are the hydroxide ion activity and metal ion concentration, respectively. Potentiometric measurements Potentiometric measurements were performed with an EV-74 universal ionometer operated as a millivoltmeter. The pH-values of aqueous solutions were measured with an ESP-43-07 glass electrode and an EVL-IMZ silver-silver chloride reference electrode. Measurements were made at 293 + 1 K with the cell KC1 (sat.)
Ag, AgCl
I RESULTS
! I
Sample solution
Membrane
I
I
I I
AND DISCUSSION
For the membranes under consideration, the potential is due to selective transfer of hydrogen ions from the sample solution to the membrane phase by means of an amine or other organic base: -H+ + AmsAmH+ (3) Here and elsewhere, a bar indicates the membrane phase. The complex cation AmH+ is in equilibrium with free amine and hydrogen ions: AmH +$liKpr+H+
(4)
where K,, is the amine protonation the membrane phase. According to the expression
constant in
E=EO+ylnE
Reference solution; buffer (pH 7.0 or 2.7), 0. 1M NaCl
AgCl, Ag I
The data in Figs 2 and 3 show that the upper response limit of the H+-ISE is strongly dependent on the nature of the anions in the test solution, on the neutral carrier and on the anion concentration. This parameter is also greatly affected by the solvent (plasticizer) and the amine : TOBS concentration ratio. As noted above, protonation of the ionophore in the membrane is mainly responsible for the deviation of the proton-response of these H+-ISEs in acidic media.6 In this case, the extraction of acids from the test solution into the membrane phase is described by the equation H++X-+zA
-AmH+X-
(7)
(5)
H
the proton-response will be ideal provided the activity of hydrogen ions in the membrane is constant. The latter condition is achieved by stabilizing the concentration of alkylammonium cations ChH+ by adding a lipophilic acid RH in less than the amount equivalent to the amine in the membrane. An amine salt is thereby produced which partly dissociates into AmH+ and R- in the membrane: --
RH + Am+AmH+R-kdl_AmH+
where &, is the dissociation constant of the ion-pair AmH+R-. It is seen from Fig. 1 that inclusion of a subequivalent quantity of TOBS in the DOAbased membrane results in a Nernst slope for the proton-response and extends the electrode response range to a lower pH. Moreover, the TOBS-modified membrane has more stable potentials. Therefore, despite some reduction in sensitivity to H+ at high pH, the advantages of the TOBS-modified membrane are greater than its shortcomings. Similar results were obtained for H+-selective membranes based on other neutral carriers and plasticizers as well as for liquid membranes.
+ R-
(6)
Fig. I. TOBS effect on the DOA-based H+-ISE responsein universal buffer solutions: (1) membrane I, without TOBS; (2) membrane II, with TOBS (compositions of the membranes are in Table I).
V. V. EGQROVand Ya. F. LUSHCHIK
It is worth noting here that the formalism suggested by Morfr4v” for a quantitative description of the extraction effect on the membrane potential of cation-selective ISEs based on neutral carriers, which assumed complete ionic dissociation in the membrane phase, cannot directly be applied to H+-ISEs based on aminetype neutral carriers, because amine salts in the low dielectric constant media of ordinary ISE membranes mainly exist as non-associates. If the concentration of ions in such membranes can be neglected in comparison with the concentration of the ion-associates, the mass balance for the amine is cz PH
Fig. 2. Effect of nature and concentration of the anion on the TDA-based H+-ISE response (membrane III) in universal buffer solutions: (1) no background electrolyte, (2) l.OM NaCl, (3) l.OM NaBr, (4) O.lM NaSCN and (5) l.OM NaSCN.
where K& is the extraction constant of the ion-pair. The salt generated is partly dissociated into ions: AmH+X&AmH+
+X-
(g)
As a result, the concentration of the complex cation AmH+ in the membrane layer adjacent to the test solution increases, while that of the free amine decreases, resulting, in accordance with equation (4), in the increasing activity of hydrogen ions in the membrane and, eventually, in distortion of the proton-response.
= C,&,,,.,+a-+ &,,,,,+x- + CA,,,
(9)
where c!$,, is the total concentration for all forms of the amine in the membrane, and c AmH+R- is the concentration of the ion-pair of the complex alkylammonium cation with the lipophilic anion R- introduced into the membrane. In the case considered, virtually all of the lipophilic acid HR introduced into the membrane is bound with amine as an ion-pair and so to a first approximation, chH+R- = CF. Subject to the condition of ideal behaviour of membrane components, from equations (7) and (9) we obtain the following expression for the concentration of the ion-pair AmH+X- extracted into the membrane:
cAmH+X-=
Kix(C!$ - CT)‘+,+ Cl,1+
KixaH+ax-
(10)
The concentrations of the free anions R- and X- in the membrane are CR_ _ F”’
(11)
AmH+ ,x_=cA;+X-
Ka (12) AmHf
Since the concentration of free hydrogen ions in the membrane is much less than the concentration of alkylammonium cations AmH+, the electroneutrality condition is c
AmH+ = CR-
+cX-
(13)
Combining (1 l), (12) and (13), we obtain
cAmH+ Fig. 3. Effect of plasticizer nature and Am:TOBS ratio on the upper response limit of H+-ISEs based on DOA in buffer solutions with O.lM NaSCN: (1) DOP, Am: TOBS = 1.9: 1 (membrane II); (2). DOP, Am:TOBS = 2.9: 1 (membrane IV); (3) I-BN, Am:TOBS = 1.9: I (membrane V).
= Jcp
Kd, + Ch,.,+X- Kdz
(14)
Taking into account that dissociation constants of ion-pairs depend only slightly on the nature of the anion, being mainly a function of the dielectric constant of the solvent,16 the following
465
Electrodes based on neutral carriers approximation
can
be made:
&, F~S lydl = 4.
Then equation (14) becomes:
c
AmH+
(14’)
=
In accordance with equation (4), the concentration of free hydrogen ions in the membrane is: - ‘hH+ H+ L&r
z;
(15)
Substituting (IO) and (14’) into (15) yields the following expression for the hydrogen-ion concentration:
L
0
L
’
1
I
I
I
2
3
4
PH
Fig. 4. H+-ISE response in hydtofluoric acid solutions (membrane II).
Equations (6) and (15) can be used to show that the first factor in equation (16) is the initial concentration of hydrogen ions in the membrane, CL+. Substituting if,+ from equation (16) into equation (5) and combining In P”+ and the constants with E” (to give EO’), we arrive at the membrane potential equation allowing for acid extraction from the sample solution
(K~x)2af.‘+a:--I- l]
+ g
(17)
R
Analysis of equation (17) implies that when extraction is insignificant and the first two terms inside the square brackets can be neglected, the second logarithmic term will be zero and an ideal proton-response is obtained. As the degree of extraction increases, the slope of the function E vs. log aH+decreases until sensitivity to the hydrogen-ion is completely lost (when the second term inside the square brackets is predominant and measurements are performed at constant extracted anion concentration, ax-). For measurements in solutions of varied acid concentration when the acid itself can be extracted into the membrane (uH+ = ax-), the curve E vs. log ++ can pass through a maximum, and a negative slope appears at high concentrations (Fig. 4). Equation (17) implies that the amine-extraction constant of the acid,
which is strongly dependent on the amine extracting power and the nature of the anions in the test solution, is the main factor responsible for the upper limit of the proton-response. To compare the extractability of different anions, their hydration energies can be used.” The extracting power of amines is, to a first approximation, proportional to their protonation constants or those of their lower homologues, in water. For instance, the p& values of trie~yla~ne and ~,~-diethylaniline are 10.87 and 6.56, respectiveiy” and the upper response limits of the ISEs based on their corresponding higher homologues are some 5 orders of magnitude different, other conditions being equal (Figs. 2 and 3). According to equation (17), interference by extraction should decrease as the lipophilic acid concentration in the membrane increases, which is confirmed by the experimental results (Fig. 3). However, the relationship CEt c cz restricts the use of this factor to shift the upper response limit of the H”-ISE to a lower pH, because violation of it results in the loss of membrane selectivity for hydrogen ions. Because measuring extraction constants for polymer membranes involves great difficulties, the constants of acid extraction with amine solutions in pure plasticizers have been used to evaluate the adequacy of equation (17). Results of extraction experiments are given in Table 2. It has been assumed that a comparatively inert polymer matrix, constituting 25% of the total membrane mass does not sibilantly affect the extraction constant. The data in Fig. 5 show quite satisfactory agreement between the experimental and theoretical E vs. log a,.#+functions, thus supporting the assumptions above.
466
V. V. EGC~ROV and YA. F. LUSHCHX Table 2. Extraction equilibrium constants Solvent I-BN 1-BN ccl, + nitrobenzene (9: 1 v/v) I-BN 1-BN
Electroactive substance
Extracted acid
TDA + TOBS TDA + TOBS TDA + TOBS
HSCN HBr HBr
TDA+TOBS DOA+TOBS
-
H+-ISE function at high pH
In contrast to the upper limit of the H+-ISE function, the lower limit is independent of the
1
5: E .P z
I
2 a”
1OOmV
2.3 1
I
0
PH Fig. 5. Theoretical and experimental response of TDAbased H+-ISE (membrane VI): (l)-(3) experimental curves in buffer solutions containing backgrounds of 1.0 x lo-‘M NaSCN, l.OM NaBr and l.OM NaSCN, respectively; (l/)-(3’), theoretical curves in the same solutions, calculated by equation (17) with &a,,, = 2.0 x 10’ and K&a,, = 2.0 x 106.
&X 2.0 x lo* 2.0 x 106 4.2 x lo5
-
Extracted alkali -
K:, -
NaOH NaOH
12 1.5 x lo5
form of the anions in the test solution and is determined by the nature and concentration of interfering cations (Fig. 6). The non-ideality of the electrode response is apparently the same as for other ISEs based on neutral carriers. In fact, however, the deviation of the H+-ISE response obeys a specific mechanism, which is peculiar to these electrodes and requires a special consideration. Indeed, displacement of the analyte ion from the carrier complex by interfering ions, which depends on the relationship between the corresponding extraction and complexing constants, cannot be responsible for the loss of the proton-response, because the amine protonation constants (which are formally similar to complex stability constants) are rather high and amines do not typically form complexes with alkali-metal cations. The experimental results can be accounted for by the extraction process: AmH+R-
+ M: + OHK,x -_‘Am
+ MZR- + H,O
(18)
At fairly high pH, amines, being comparatively
Fig. 6. Effect of the O.lM electrolyte (A) and dilution of buffer solution (B) on the lower response limit of DOA-based H+-ISEs (membrane IV). A: (1) CsCl, (2) RbCl, (3) KC1 and (4) NaCl; B: (1) initial buffer mixture, (2) diluted 1: 10, (3) diluted 1: 100.
Electrodes based on neutral carriers
461
0
0
10
5 PH
Fig. 7. Effect of the plasticizer nature, neutral carrier and Am:TOBS ratio on the H+-ISE lower response limit: (1) THP, DOA (membrane VII); (2)-(4) DOP with DOA:TOBS = 1: 1 (membrane VIII), 1.2: 1(membrane IX) and 2.9: 1 (membrane IV), respectively, (5) DOP, TDA (membrane X).
weak bases, are replaced in the lipophilic acid salts by metal ions. The concentration of molecular amine in the membrane is thereby increased, while the concentrations of the protonated amine AmH+ and the hydrogen ion in equilibrium with it decrease, thus causing nonNernstian electrode response at high pH. Naturally, the extent of the extraction process depends on the extractability of the metal cations and the basicity of the amines. As shown above, the basicity of TDA is more than 4 orders of magnitude larger than that of DOA, so the lower response limit of the TDA-based ISE is shifted to the alkaline region by about 4 pH units (Fig. 7). The strong dependence of the lower limit of the H+-ISE function on the solvating power of plasticizers (THP and DOP) relative to metal cations further supports the suggested mechanism of deviation from ideal behaviour, implying the absence of appreciable interaction of metal cations with the neutral carrier in the membrane phase. As the amine: lipophilic acid ratio in the membrane increases, the lower H+-ISE response limit naturally shifts to higher pH owing to the decreased degree of extraction, described by equation (18). Thus, the suggested model logically accounts for the dependence of the change in the lower H+-ISE response limit on the membrane and test solution compositions. The equation for evaluating the effect of foreign cations on the H +-ISE potential can be obtained from the approximations above, assuming also equality of the
Fig. 8. Responses of DOA-based H+-ISE (membrane XI) in solutions of dilute (1: 100)buffer mixture, containing 0. IM NaCl background: (1) experimental; (2) calculated from equation (25) with K&,c,, = 1.5 x IO’.
dissociation constants of the ion-pairs M:Rand AmH+R-. With these assumptions, the following relationships hold: (19)
(20) CAmH+
=
cAmH+R-JKd
(21)
fi
where all the lipophilic acid in the membrane (Ct,o’) is bound in the ion-pairs M:Rand AmH+ R- and Kd is the dissociation constant of these ion-pairs. Equation (18) implies that
CAmH+R_ =
cMe+R-
K:, a,:
CAm
(22)
aOH -
where K:, = &,/X55 (with allowance for C,,, in the aqueous phase). Substituting equations (19) and (20) into (22) and solving for CAmH+R-gives
(CT+ (?ilk+ Ex)’ _ c,.,t 4
R
clot
Am
(23)
where Ex (= K:, a,: aOH-) characterizes the extraction effect on the membrane composition. Substituting equation (23) into (21) and (15) and taking ch from equation (20), we arrive at the equation which describes the concentration of free hydrogen ions in the membrane, with
448 allowance
V. V. EGOROV and YA. F.
for extraction equation
LIJSHCHIK
(IS)]:
where e”x+ is the initial concentration of hydrogen ions in the membrane (without extraction) as given by the first factor in equation (16). Substituting equation (24) into equatjon (5) and again ineluding In &+ and the constants with E’, gives the following equation for the lower branch of the E us. log a,+ curve:
based on neutral carriers, 3ecause the efkets of the two types of extraction are manifest at opposite ends of the pH range, where their interdependence is impossible, equations (17) and (25) may be ~ombiued into a gen~a~i~ equation for the proton-response of an H’-ISE over the entire pH range [equation (26)f.
t
Analysis of equation (25) shows that when .83x is ~egligib~e~ the second and the third Iogarithmie terms cancel, resulting in Nernstian response. Figure 8 compares the theoreti~ai and experimental E z?.s.pH curves for a PVC-membrane plasticized with I-BN and containing DOA as neutral carrier. The value of KG, was capsulated for pure I-BN. The satisfactory agreement between the experimental and thearetical curves proves the predominant contribution of the extraction process described by equation (18) to the Ioss (at high pH) of the proton-response of H+-ISEs
The good a~~ment in Fig. 9 between the ex~rimental E US.pH curve and that calculated by equation (26) for a liquid memb~ne containing TDA as the neutral carrier support the considerations presented.
Extraction processes at the m~mbran~test solution interface, by causing changes in the ~on~ntration of hydrogen ions in the membrane, are mainly responsible for the restrictions
Electrodes based on neutral carriers
amine basicity described by Simon6 and will stimulate further purposeful search for membrane compositions giving a required response range.
2 1
5
E
‘0 .Y 5
REFERENCES
1OOmV
:
I I 8 , I I I 0
469
5
:
10
PH
Fig. 9. Responses of TDA-based H+-ISE (membrane XII) in solutions of buffer mixture containing 0. IM NaBr background: (I) experimental; (2) calculated from equation (26) with K&a,, = 4.2 x 10’ and K&NP,,r)= 12.
on the response range of H+-ISEs based on neutral amine-type carriers. In a.cidic media the extraction of ion-associates of anions with protonated amines is the operative factor, whereas in alkaline media, it is the extraction of metal cations by amine salts of lipophilic acids. The degree of extraction is strongly dependent on the composition of both the membrane and the test solution. Among the membrane components, the greatest effect on the extraction processes and, hence, on the response range of H+-ISEs is exerted by the nature (primarily the basicity) of the amine used as the neutral carrier. The solvating power of the plasticizer and the amine : lipophilic acid concentration ratio in the membrane have less pronounced, but still significant, influence. It should be noted that the proposed approach yields good results only when the potential-determining reaction is solely specified by the electroactive substance introduced into the membrane. In a number of cases, however, the presence of electroactive impurities in the membrane polymer or plasticizer can lead to alternative mechanisms of charge transfer through the membrane. In addition, the interphase processes can sometimes be controlled by kinetic as well as equilibrium factors. In this connection, approaches that take into account the role of all the substances capable of charge transfer’9.20 and those including kinetic factors*‘-” seem to be very promising. We hope that these results add to the understanding of the mechanism of the relationship between the dynamic range of H+-ISEs and
1. R. L. Coon, N. C. J. Lai and J. P. Kampine, J. Appl. Physiol., 1976, 40, 625. 2. D. Eme, D. Arnmann and W. Simon, Chimiu, 1979,33, 88. 3. D. Ammann, F. Lanter, R. A. Steiner, P. Schulthess, Y. Shijo and W. Simon, Anal. Chem., 1981, 53, 2267. 4. D. Eme, K. V. Schenker, D. Ammann, E. Pretsch and W. Simon, Chimiu, 1981, 35, 178. 5. C. Hongbo, E. H. Hansen and J. RbiiEka, Anal. Chim. Acta, 1985, 169, 209. 6. U. Qesch, 2. Brzbzka, A. Xu, B. Rusterholz, G. Suter, H. V. Pham, D. H. Weiti, D. Ammann, E. Pretsch and W. Simon, Anal. Chem., 1986, 58, 2285. 7 G. L. Starobinets, V. V. Egorov and Ya. F. Lushchik, Zh. Analit. Khim., 1986, 41, 1030. 8. H.-L. Wu and R.-Q. Yu, Talanta, 1987, 34, 577. 9. K. N. Mikhel’son, 0. V. Mukbacheva, V. M. Lutov, V. Ya. Semenii and A. N. Khutsishvili, USSR Author Cert., N 1326977, IB, 1987, N 28. 10. S. M. Leshchev, E. M. Rakhmanko, E. V. Vorobieva and G. L. Starobinets, USSR Author Cert., N 1027151, IB, 1983, N 25. 11. T. M. Alkhazishvili, N. V. Astakhova, V. V. Egorov, Z. F. Kirpichnikova, L. V. Koleshko, E. M. Rakhmanko, G. L. Starobinets, Yu. M. Tarasova and A. N. Khutsishvili, USSR Author Cert., N 1310401, IB, 1987, N 18. 12. K. Cammann, Das Arbeiten mit ionenselektiven Elektroden, Springer, Berlin, 1979. 13. Yu. Yu. Lurie, Handbook of Analytical Chemislry, Khimiya, Moscow, 1979. 14. W. E. Morf, G. Kahr and W. Simon, Anal. L&t., 1974, 7, 9. 15. W. E. Morf and W. Simon, in Ion-Selective Electrodes in Analytical Chemistry, Vol. 1, H. Freiser (ed.), p. 211. Plenum Press, New York, 1978. 16. K. S. Krasnov, Radiochemistry, 1963, 5, 222. 17. K. P. Mishchenko and A. A. Ravdel (eds.) A Handbook of Physico-Chemical Quantities, Khimiya, Moscow, 1967. 18. A. Albert and E. Serjeant, Ionization Constanfs of Acids and Bases, Chapman & Hall, London. 1964. 19. S. Kihara and Z. Yoshida, Talanta, 1984, 31, 789. 20. T. Kakiuchi and M. Senda, Bull. Chem. Sot. Japan, 1984, 57, 1801. 21. K. Cammann, Anal. Chem., 1978, SO, 936. 22. J. Koryta, Anal. Chim. Acta, 1979, 111, 1. 23. T. Fujinaga, Phil. Trans. R. Sot., 1982, 305A, 631. 24. J. Koryta, Ion-Seleclive Electrode Rev., 1983, 5, 131. 25. K. Cammann and G. A. Rechnitz, in Ion-Selective Electrodes, 4, p. 35. Akademiai Kiado, Budapest, 1985. 26. R. D. Armstrong. J. Electroanal. Chem., 1988,245, 113. 27. Sheng-Luo Xie and K. Cammann, ibid., 1988,245, 117. I.
0039-9140/90 $3.00+ 0.00
Copyright 0 1990Perpmon Press plc
H+-SELECTIVE ELECTRODES BASED ON NEUTRAL CARRIERS: SPECIFIC FEATURES IN BEHAVIOUR AND QUANTITATIVE DESCRIPTION OF THE ELECTRODE RESPONSE V. V.
EG~ROV
and YA. F.
LUSHCHIK
Institute for Physico-Chemical Problems, Byelorussian State University, Minsk, USSR (Received 18 January 1989. Revised 24 August 1989. Accepted 29 September
1989)
Summary-The influence has been studied of the membrane and solution composition on the response of H+-ISEs with plasticized polymer and liquid membranes based on the neutral carriers NJV-dioctylaniline and tridecylamine in association with trioctyloxybenxene sulphonic acid. It is shown that the extraction processes at the membrane/solution interface exert the main effect on the response limits by inducing essential changes in the activity of potential-determining ions in the membrane. At low pH, the amine extraction of acids followed by neutralization (free amines binding in ion-pairs) is the relevant process, while at high pH it is the extraction of metal cations with amine salts of a lipophilic acid, with the consequent displacement of amine from the salts. Equations are suggested to represent the interphase potential of the H+-ISE membranes with allowance for these extraction processes, ‘Ihe experimental electrode responses of both liquid and polymer membranes are shown to be well described by the equations for the interphase potential, thus indicating its dominant contribution to the membrane potential.
The first H+-ion selective electrodes based on ionophoreP have already exhibited a number of advantages over the traditional glass H+electrodes, including low resistance (with consequent advantages for miniaturization), safe handling, and promising applications in some aggressive media. Therefore, although H+-ISEs based on ionophores are characterized by much lower selectivity and narrower response ranges than glass electrodes, their applications in medical and biological studies, where they have most advantages over glass H+-ion selective electrodes, are still increasing. Various compounds have been studied as carriers of hydrogen ions, mainly aliphatic and heterocyclic amines and their derivatives,24 but also other sufficiently strong organic bases that can form a complex cation with a proton, e.g., hexabutylphosphotriamide.9 Simon6 has shown for such electrodes that deviations from the ideal proton-response in the acidic region are caused by protonation of the membrane ionophore, while in the alkaline region metal cations interfere. The ISE response range (dynamic interval) depends mainly on the basicity of the carrier. Since Simon did not study in detail the effect of membrane/solution interface extraction on the form of the electrode response, his conclusions were basically qualita-
tive. Moreover, the mechanisms of the interfering effect of metal cations in alkaline media remains unclear. The present work is aimed at developing a quantitative description of the potential of amine carrier-based H+-ISEs, allowing for membrane/solution extraction processes. The validity of this approach is supported by experimental measurements showing the electrode responses of the membranes to be well approximated by the boundary potential equations with regard to the extraction processes at the membrane/solution interface. Neglecting the contribution of the diffusion potential, which is difficult to allow for because of the mobility differences and activity gradients of charged components in the membrane, is, therefore, justifiable in these cases, although this may not always be so.
EXPERIMENTAL
Reagents N,N-Dioctylaniline (DOA) and tridecylamine (TDA) of analytical grade were further purified” and trioctyloxybenzene sulphonic acid (TOBS) was synthesized with a purity 297% for use as the electroactive components in the 461
V. V. EGCJROV and YA. F. LUSHCHIK
462
Table 1. Compositions of H+-ISE membranes used Membrane composition (the content of electroactive material is given in mole/kg or mole/l., other components in % w/w or v/v) Membrane I II III IV V VI VII VIII IX X XI XII
DOA 0.050 0.050
TDA 0.050
0.029 0.050 0.050 0.015 0.010 0.012 0.029 0.015 0.001
TOBS 0.026 0.026 0.010 0.026 0.026 0.010 0.010 0.010 0.010 0.010 0.0005
PVC 24.5 24.2 24.1 24.7 24.2 24.1 24.8 24.8 24.8 24.6 24.8
membranes. Poly(viny1 chloride) (PVC) PZh-S70 was used as the membrane matrix and was dissolved in commercial grade cyclohexanone that had previously been distilled. Dioctyl phthalate (DOP), trihexylphosphate (THP) and I-bromonaphthalene (l-BN) of commercial grade, nitrobenzene of analytical grade and carbon tetrachloride of reagent grade were used as membrane plasticizers or solvents. DOP and THP were purified by acid treatment” and I-BN was distilled. The membranes prepared from these components are summarized in Table 1. Distilled water solutions of electrolytes were made from boric, acetic, phosphoric, hydrofluoric, hydrochloric and perchloric acids as well as sodium, potassium, rubidium and caesium chlorides, sodium thiocyanate and bromide, sodium hydroxide and silver nitrate. These electrolytes were of at least analytical reagent grade. Preparation of samples The ISE membranes were produced conventionally.12 The buffer solutions for pH between 1.8 and 12.0 were made by mixing acetic, phosphoric and boric acids and sodium hydroxide.13 The same mixtures were employed to prepare buffered solutions of sodium chloride, sodium thiocyanate and other salts. Hydrochloric acid was used to adjust solutions to pH < 1.8. Estimation of extraction equilibrium constants The extraction equilibrium constants, equations (7) and (18) below, were measured at 293 f 1 K. In determining the extraction constants of hydrobromic and thiocyanic acids, the initial TDA concentration in the organic phase was O.OlM, the sodium thiocyanate concentration was O.OlM and the sodium bromide con-
DOP 73.6 72.6 72.3 73.9
I-BN -
THP -
Carbon tetrachloride -
Nitrobenzene -
72.6 72.3 74.2 74.4 74.3 73.6 74.2 90
10
centration ranged from 0.01 to O.lM. The aqueous phase pH was adjusted with universal buffer mixture solutions in the range 1.8-7.0. The equilibrium concentrations of either bromide and thiocyanate in the aqueous and organic phases or of free amine in the organic phase were measured, depending on the extraction range. The anions were measured by potentiometric titration with silver nitrate in 1-propanol. Free TDA concentrations were measured by potentiometric titration with hydrochloric acid solution in acetic acid. The equilibrium concentrations of the other components were determined by difference. The extraction constants were calculated from the formula: ch
Lx-
aH+
(1)
where C.&,,,u+x-and c, are the equilibrium concentrations of the ion-associate and free amine in the organic phase and aH+and C,- are the hydrogen-ion activity and the acid anion concentration, respectively. In assessing the extraction constants of sodium ions, the initial concentrations of TDA and DOA in the organic phase ranged from 0.011 to O.O15M, that of TOBS was O.OlOM and the sodium bromide concentration in the aqueous phase ranged between 0.1 and 0.065M. When equilibrium had been achieved, the organic phase was separated and the sodium ions were re-extracted with an equal volume of 0. 1M hydrochloric acid. The sodium concentration in this aqueous phase was measured with a PFMU4.2 flame photometer. The extraction constants were estimated from the formula K:, =
ifhQ+R- CAm
C AmH+R-
(2) CM:aOH-
463
Electrodes based on neutral carriers
where CMzR- is the concentration of the ionassociate of the metal cation with the TOBS of the anion, chH+R- is the concentration protonated amine ion-associate with the TOBS anion and aoH- and C,: are the hydroxide ion activity and metal ion concentration, respectively. Potentiometric measurements Potentiometric measurements were performed with an EV-74 universal ionometer operated as a millivoltmeter. The pH-values of aqueous solutions were measured with an ESP-43-07 glass electrode and an EVL-IMZ silver-silver chloride reference electrode. Measurements were made at 293 + 1 K with the cell KC1 (sat.)
Ag, AgCl
I RESULTS
! I
Sample solution
Membrane
I
I
I I
AND DISCUSSION
For the membranes under consideration, the potential is due to selective transfer of hydrogen ions from the sample solution to the membrane phase by means of an amine or other organic base: -H+ + AmsAmH+ (3) Here and elsewhere, a bar indicates the membrane phase. The complex cation AmH+ is in equilibrium with free amine and hydrogen ions: AmH +$liKpr+H+
(4)
where K,, is the amine protonation the membrane phase. According to the expression
constant in
E=EO+ylnE
Reference solution; buffer (pH 7.0 or 2.7), 0. 1M NaCl
AgCl, Ag I
The data in Figs 2 and 3 show that the upper response limit of the H+-ISE is strongly dependent on the nature of the anions in the test solution, on the neutral carrier and on the anion concentration. This parameter is also greatly affected by the solvent (plasticizer) and the amine : TOBS concentration ratio. As noted above, protonation of the ionophore in the membrane is mainly responsible for the deviation of the proton-response of these H+-ISEs in acidic media.6 In this case, the extraction of acids from the test solution into the membrane phase is described by the equation H++X-+zA
-AmH+X-
(7)
(5)
H
the proton-response will be ideal provided the activity of hydrogen ions in the membrane is constant. The latter condition is achieved by stabilizing the concentration of alkylammonium cations ChH+ by adding a lipophilic acid RH in less than the amount equivalent to the amine in the membrane. An amine salt is thereby produced which partly dissociates into AmH+ and R- in the membrane: --
RH + Am+AmH+R-kdl_AmH+
where &, is the dissociation constant of the ion-pair AmH+R-. It is seen from Fig. 1 that inclusion of a subequivalent quantity of TOBS in the DOAbased membrane results in a Nernst slope for the proton-response and extends the electrode response range to a lower pH. Moreover, the TOBS-modified membrane has more stable potentials. Therefore, despite some reduction in sensitivity to H+ at high pH, the advantages of the TOBS-modified membrane are greater than its shortcomings. Similar results were obtained for H+-selective membranes based on other neutral carriers and plasticizers as well as for liquid membranes.
+ R-
(6)
Fig. I. TOBS effect on the DOA-based H+-ISE responsein universal buffer solutions: (1) membrane I, without TOBS; (2) membrane II, with TOBS (compositions of the membranes are in Table I).
V. V. EGQROVand Ya. F. LUSHCHIK
It is worth noting here that the formalism suggested by Morfr4v” for a quantitative description of the extraction effect on the membrane potential of cation-selective ISEs based on neutral carriers, which assumed complete ionic dissociation in the membrane phase, cannot directly be applied to H+-ISEs based on aminetype neutral carriers, because amine salts in the low dielectric constant media of ordinary ISE membranes mainly exist as non-associates. If the concentration of ions in such membranes can be neglected in comparison with the concentration of the ion-associates, the mass balance for the amine is cz PH
Fig. 2. Effect of nature and concentration of the anion on the TDA-based H+-ISE response (membrane III) in universal buffer solutions: (1) no background electrolyte, (2) l.OM NaCl, (3) l.OM NaBr, (4) O.lM NaSCN and (5) l.OM NaSCN.
where K& is the extraction constant of the ion-pair. The salt generated is partly dissociated into ions: AmH+X&AmH+
+X-
(g)
As a result, the concentration of the complex cation AmH+ in the membrane layer adjacent to the test solution increases, while that of the free amine decreases, resulting, in accordance with equation (4), in the increasing activity of hydrogen ions in the membrane and, eventually, in distortion of the proton-response.
= C,&,,,.,+a-+ &,,,,,+x- + CA,,,
(9)
where c!$,, is the total concentration for all forms of the amine in the membrane, and c AmH+R- is the concentration of the ion-pair of the complex alkylammonium cation with the lipophilic anion R- introduced into the membrane. In the case considered, virtually all of the lipophilic acid HR introduced into the membrane is bound with amine as an ion-pair and so to a first approximation, chH+R- = CF. Subject to the condition of ideal behaviour of membrane components, from equations (7) and (9) we obtain the following expression for the concentration of the ion-pair AmH+X- extracted into the membrane:
cAmH+X-=
Kix(C!$ - CT)‘+,+ Cl,1+
KixaH+ax-
(10)
The concentrations of the free anions R- and X- in the membrane are CR_ _ F”’
(11)
AmH+ ,x_=cA;+X-
Ka (12) AmHf
Since the concentration of free hydrogen ions in the membrane is much less than the concentration of alkylammonium cations AmH+, the electroneutrality condition is c
AmH+ = CR-
+cX-
(13)
Combining (1 l), (12) and (13), we obtain
cAmH+ Fig. 3. Effect of plasticizer nature and Am:TOBS ratio on the upper response limit of H+-ISEs based on DOA in buffer solutions with O.lM NaSCN: (1) DOP, Am: TOBS = 1.9: 1 (membrane II); (2). DOP, Am:TOBS = 2.9: 1 (membrane IV); (3) I-BN, Am:TOBS = 1.9: I (membrane V).
= Jcp
Kd, + Ch,.,+X- Kdz
(14)
Taking into account that dissociation constants of ion-pairs depend only slightly on the nature of the anion, being mainly a function of the dielectric constant of the solvent,16 the following
465
Electrodes based on neutral carriers approximation
can
be made:
&, F~S lydl = 4.
Then equation (14) becomes:
c
AmH+
(14’)
=
In accordance with equation (4), the concentration of free hydrogen ions in the membrane is: - ‘hH+ H+ L&r
z;
(15)
Substituting (IO) and (14’) into (15) yields the following expression for the hydrogen-ion concentration:
L
0
L
’
1
I
I
I
2
3
4
PH
Fig. 4. H+-ISE response in hydtofluoric acid solutions (membrane II).
Equations (6) and (15) can be used to show that the first factor in equation (16) is the initial concentration of hydrogen ions in the membrane, CL+. Substituting if,+ from equation (16) into equation (5) and combining In P”+ and the constants with E” (to give EO’), we arrive at the membrane potential equation allowing for acid extraction from the sample solution
(K~x)2af.‘+a:--I- l]
+ g
(17)
R
Analysis of equation (17) implies that when extraction is insignificant and the first two terms inside the square brackets can be neglected, the second logarithmic term will be zero and an ideal proton-response is obtained. As the degree of extraction increases, the slope of the function E vs. log aH+decreases until sensitivity to the hydrogen-ion is completely lost (when the second term inside the square brackets is predominant and measurements are performed at constant extracted anion concentration, ax-). For measurements in solutions of varied acid concentration when the acid itself can be extracted into the membrane (uH+ = ax-), the curve E vs. log ++ can pass through a maximum, and a negative slope appears at high concentrations (Fig. 4). Equation (17) implies that the amine-extraction constant of the acid,
which is strongly dependent on the amine extracting power and the nature of the anions in the test solution, is the main factor responsible for the upper limit of the proton-response. To compare the extractability of different anions, their hydration energies can be used.” The extracting power of amines is, to a first approximation, proportional to their protonation constants or those of their lower homologues, in water. For instance, the p& values of trie~yla~ne and ~,~-diethylaniline are 10.87 and 6.56, respectiveiy” and the upper response limits of the ISEs based on their corresponding higher homologues are some 5 orders of magnitude different, other conditions being equal (Figs. 2 and 3). According to equation (17), interference by extraction should decrease as the lipophilic acid concentration in the membrane increases, which is confirmed by the experimental results (Fig. 3). However, the relationship CEt c cz restricts the use of this factor to shift the upper response limit of the H”-ISE to a lower pH, because violation of it results in the loss of membrane selectivity for hydrogen ions. Because measuring extraction constants for polymer membranes involves great difficulties, the constants of acid extraction with amine solutions in pure plasticizers have been used to evaluate the adequacy of equation (17). Results of extraction experiments are given in Table 2. It has been assumed that a comparatively inert polymer matrix, constituting 25% of the total membrane mass does not sibilantly affect the extraction constant. The data in Fig. 5 show quite satisfactory agreement between the experimental and theoretical E vs. log a,.#+functions, thus supporting the assumptions above.
466
V. V. EGC~ROV and YA. F. LUSHCHX Table 2. Extraction equilibrium constants Solvent I-BN 1-BN ccl, + nitrobenzene (9: 1 v/v) I-BN 1-BN
Electroactive substance
Extracted acid
TDA + TOBS TDA + TOBS TDA + TOBS
HSCN HBr HBr
TDA+TOBS DOA+TOBS
-
H+-ISE function at high pH
In contrast to the upper limit of the H+-ISE function, the lower limit is independent of the
1
5: E .P z
I
2 a”
1OOmV
2.3 1
I
0
PH Fig. 5. Theoretical and experimental response of TDAbased H+-ISE (membrane VI): (l)-(3) experimental curves in buffer solutions containing backgrounds of 1.0 x lo-‘M NaSCN, l.OM NaBr and l.OM NaSCN, respectively; (l/)-(3’), theoretical curves in the same solutions, calculated by equation (17) with &a,,, = 2.0 x 10’ and K&a,, = 2.0 x 106.
&X 2.0 x lo* 2.0 x 106 4.2 x lo5
-
Extracted alkali -
K:, -
NaOH NaOH
12 1.5 x lo5
form of the anions in the test solution and is determined by the nature and concentration of interfering cations (Fig. 6). The non-ideality of the electrode response is apparently the same as for other ISEs based on neutral carriers. In fact, however, the deviation of the H+-ISE response obeys a specific mechanism, which is peculiar to these electrodes and requires a special consideration. Indeed, displacement of the analyte ion from the carrier complex by interfering ions, which depends on the relationship between the corresponding extraction and complexing constants, cannot be responsible for the loss of the proton-response, because the amine protonation constants (which are formally similar to complex stability constants) are rather high and amines do not typically form complexes with alkali-metal cations. The experimental results can be accounted for by the extraction process: AmH+R-
+ M: + OHK,x -_‘Am
+ MZR- + H,O
(18)
At fairly high pH, amines, being comparatively
Fig. 6. Effect of the O.lM electrolyte (A) and dilution of buffer solution (B) on the lower response limit of DOA-based H+-ISEs (membrane IV). A: (1) CsCl, (2) RbCl, (3) KC1 and (4) NaCl; B: (1) initial buffer mixture, (2) diluted 1: 10, (3) diluted 1: 100.
Electrodes based on neutral carriers
461
0
0
10
5 PH
Fig. 7. Effect of the plasticizer nature, neutral carrier and Am:TOBS ratio on the H+-ISE lower response limit: (1) THP, DOA (membrane VII); (2)-(4) DOP with DOA:TOBS = 1: 1 (membrane VIII), 1.2: 1(membrane IX) and 2.9: 1 (membrane IV), respectively, (5) DOP, TDA (membrane X).
weak bases, are replaced in the lipophilic acid salts by metal ions. The concentration of molecular amine in the membrane is thereby increased, while the concentrations of the protonated amine AmH+ and the hydrogen ion in equilibrium with it decrease, thus causing nonNernstian electrode response at high pH. Naturally, the extent of the extraction process depends on the extractability of the metal cations and the basicity of the amines. As shown above, the basicity of TDA is more than 4 orders of magnitude larger than that of DOA, so the lower response limit of the TDA-based ISE is shifted to the alkaline region by about 4 pH units (Fig. 7). The strong dependence of the lower limit of the H+-ISE function on the solvating power of plasticizers (THP and DOP) relative to metal cations further supports the suggested mechanism of deviation from ideal behaviour, implying the absence of appreciable interaction of metal cations with the neutral carrier in the membrane phase. As the amine: lipophilic acid ratio in the membrane increases, the lower H+-ISE response limit naturally shifts to higher pH owing to the decreased degree of extraction, described by equation (18). Thus, the suggested model logically accounts for the dependence of the change in the lower H+-ISE response limit on the membrane and test solution compositions. The equation for evaluating the effect of foreign cations on the H +-ISE potential can be obtained from the approximations above, assuming also equality of the
Fig. 8. Responses of DOA-based H+-ISE (membrane XI) in solutions of dilute (1: 100)buffer mixture, containing 0. IM NaCl background: (1) experimental; (2) calculated from equation (25) with K&,c,, = 1.5 x IO’.
dissociation constants of the ion-pairs M:Rand AmH+R-. With these assumptions, the following relationships hold: (19)
(20) CAmH+
=
cAmH+R-JKd
(21)
fi
where all the lipophilic acid in the membrane (Ct,o’) is bound in the ion-pairs M:Rand AmH+ R- and Kd is the dissociation constant of these ion-pairs. Equation (18) implies that
CAmH+R_ =
cMe+R-
K:, a,:
CAm
(22)
aOH -
where K:, = &,/X55 (with allowance for C,,, in the aqueous phase). Substituting equations (19) and (20) into (22) and solving for CAmH+R-gives
(CT+ (?ilk+ Ex)’ _ c,.,t 4
R
clot
Am
(23)
where Ex (= K:, a,: aOH-) characterizes the extraction effect on the membrane composition. Substituting equation (23) into (21) and (15) and taking ch from equation (20), we arrive at the equation which describes the concentration of free hydrogen ions in the membrane, with
448 allowance
V. V. EGOROV and YA. F.
for extraction equation
LIJSHCHIK
(IS)]:
where e”x+ is the initial concentration of hydrogen ions in the membrane (without extraction) as given by the first factor in equation (16). Substituting equation (24) into equatjon (5) and again ineluding In &+ and the constants with E’, gives the following equation for the lower branch of the E us. log a,+ curve:
based on neutral carriers, 3ecause the efkets of the two types of extraction are manifest at opposite ends of the pH range, where their interdependence is impossible, equations (17) and (25) may be ~ombiued into a gen~a~i~ equation for the proton-response of an H’-ISE over the entire pH range [equation (26)f.
t
Analysis of equation (25) shows that when .83x is ~egligib~e~ the second and the third Iogarithmie terms cancel, resulting in Nernstian response. Figure 8 compares the theoreti~ai and experimental E z?.s.pH curves for a PVC-membrane plasticized with I-BN and containing DOA as neutral carrier. The value of KG, was capsulated for pure I-BN. The satisfactory agreement between the experimental and thearetical curves proves the predominant contribution of the extraction process described by equation (18) to the Ioss (at high pH) of the proton-response of H+-ISEs
The good a~~ment in Fig. 9 between the ex~rimental E US.pH curve and that calculated by equation (26) for a liquid memb~ne containing TDA as the neutral carrier support the considerations presented.
Extraction processes at the m~mbran~test solution interface, by causing changes in the ~on~ntration of hydrogen ions in the membrane, are mainly responsible for the restrictions
Electrodes based on neutral carriers
amine basicity described by Simon6 and will stimulate further purposeful search for membrane compositions giving a required response range.
2 1
5
E
‘0 .Y 5
REFERENCES
1OOmV
:
I I 8 , I I I 0
469
5
:
10
PH
Fig. 9. Responses of TDA-based H+-ISE (membrane XII) in solutions of buffer mixture containing 0. IM NaBr background: (I) experimental; (2) calculated from equation (26) with K&a,, = 4.2 x 10’ and K&NP,,r)= 12.
on the response range of H+-ISEs based on neutral amine-type carriers. In a.cidic media the extraction of ion-associates of anions with protonated amines is the operative factor, whereas in alkaline media, it is the extraction of metal cations by amine salts of lipophilic acids. The degree of extraction is strongly dependent on the composition of both the membrane and the test solution. Among the membrane components, the greatest effect on the extraction processes and, hence, on the response range of H+-ISEs is exerted by the nature (primarily the basicity) of the amine used as the neutral carrier. The solvating power of the plasticizer and the amine : lipophilic acid concentration ratio in the membrane have less pronounced, but still significant, influence. It should be noted that the proposed approach yields good results only when the potential-determining reaction is solely specified by the electroactive substance introduced into the membrane. In a number of cases, however, the presence of electroactive impurities in the membrane polymer or plasticizer can lead to alternative mechanisms of charge transfer through the membrane. In addition, the interphase processes can sometimes be controlled by kinetic as well as equilibrium factors. In this connection, approaches that take into account the role of all the substances capable of charge transfer’9.20 and those including kinetic factors*‘-” seem to be very promising. We hope that these results add to the understanding of the mechanism of the relationship between the dynamic range of H+-ISEs and
1. R. L. Coon, N. C. J. Lai and J. P. Kampine, J. Appl. Physiol., 1976, 40, 625. 2. D. Eme, D. Arnmann and W. Simon, Chimiu, 1979,33, 88. 3. D. Ammann, F. Lanter, R. A. Steiner, P. Schulthess, Y. Shijo and W. Simon, Anal. Chem., 1981, 53, 2267. 4. D. Eme, K. V. Schenker, D. Ammann, E. Pretsch and W. Simon, Chimiu, 1981, 35, 178. 5. C. Hongbo, E. H. Hansen and J. RbiiEka, Anal. Chim. Acta, 1985, 169, 209. 6. U. Qesch, 2. Brzbzka, A. Xu, B. Rusterholz, G. Suter, H. V. Pham, D. H. Weiti, D. Ammann, E. Pretsch and W. Simon, Anal. Chem., 1986, 58, 2285. 7 G. L. Starobinets, V. V. Egorov and Ya. F. Lushchik, Zh. Analit. Khim., 1986, 41, 1030. 8. H.-L. Wu and R.-Q. Yu, Talanta, 1987, 34, 577. 9. K. N. Mikhel’son, 0. V. Mukbacheva, V. M. Lutov, V. Ya. Semenii and A. N. Khutsishvili, USSR Author Cert., N 1326977, IB, 1987, N 28. 10. S. M. Leshchev, E. M. Rakhmanko, E. V. Vorobieva and G. L. Starobinets, USSR Author Cert., N 1027151, IB, 1983, N 25. 11. T. M. Alkhazishvili, N. V. Astakhova, V. V. Egorov, Z. F. Kirpichnikova, L. V. Koleshko, E. M. Rakhmanko, G. L. Starobinets, Yu. M. Tarasova and A. N. Khutsishvili, USSR Author Cert., N 1310401, IB, 1987, N 18. 12. K. Cammann, Das Arbeiten mit ionenselektiven Elektroden, Springer, Berlin, 1979. 13. Yu. Yu. Lurie, Handbook of Analytical Chemislry, Khimiya, Moscow, 1979. 14. W. E. Morf, G. Kahr and W. Simon, Anal. L&t., 1974, 7, 9. 15. W. E. Morf and W. Simon, in Ion-Selective Electrodes in Analytical Chemistry, Vol. 1, H. Freiser (ed.), p. 211. Plenum Press, New York, 1978. 16. K. S. Krasnov, Radiochemistry, 1963, 5, 222. 17. K. P. Mishchenko and A. A. Ravdel (eds.) A Handbook of Physico-Chemical Quantities, Khimiya, Moscow, 1967. 18. A. Albert and E. Serjeant, Ionization Constanfs of Acids and Bases, Chapman & Hall, London. 1964. 19. S. Kihara and Z. Yoshida, Talanta, 1984, 31, 789. 20. T. Kakiuchi and M. Senda, Bull. Chem. Sot. Japan, 1984, 57, 1801. 21. K. Cammann, Anal. Chem., 1978, SO, 936. 22. J. Koryta, Anal. Chim. Acta, 1979, 111, 1. 23. T. Fujinaga, Phil. Trans. R. Sot., 1982, 305A, 631. 24. J. Koryta, Ion-Seleclive Electrode Rev., 1983, 5, 131. 25. K. Cammann and G. A. Rechnitz, in Ion-Selective Electrodes, 4, p. 35. Akademiai Kiado, Budapest, 1985. 26. R. D. Armstrong. J. Electroanal. Chem., 1988,245, 113. 27. Sheng-Luo Xie and K. Cammann, ibid., 1988,245, 117. I.