Liquid state membrane electrode sensitive to Ln(III)

Vol. 41, No. 8, pp. 139S1396, 1994 Copyright 0 1994 Elsevier ScienceLtd Printed in Great Britain. All rights reserved

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LIQUID STATE MEMBRANE ELECTRODE SENSITIVE TO Ln(II1) WALENTY SZCZEPANIAK and MARIA

F&N

Department of Instrumental Analysis, Faculty of Chemistry, A. Mickiewicz University, Poznati, Poland (Received

11 May 1993. Revised

12 July 1993. Accepted

7 October

1993)

Summary-A liquid ion-exchange electrode containing a chloroform solution of the complex of Ln(II1) (Gd, La) with tetraphenyl ester of imidodiphosphoric acid is described. The slope of the calibration graph (electrode potential us concentration) is 18.5 mV/pLn in the pLn range 4.7-2 @H = 5). Fe(III), AI(III), Co(II), Ni(I1) and Ca(I1) ions do not interfere, unlike ions of other lanthanides. It was found that the electrode might be applied to detect the end point of the titration of Ln3+ ions.

The purpose of constructing ion selective electrodes for multivalent ions is often questioned, as such electrodes are characterized by low sensitivity. For example, for trivalent cations, a change in potential under a tenfold change in ion activity is 19 mV. Thus a big error is likely to be committed if a method of direct potentiometry is applied for measurements. The advocates of these electrodes argue, however, that in the case of potentiometric titration no such danger is involved and these electrodes may be applied as an indicator to determine the titration end-point. A number of electrodes sensitive, among others, to Fe3+,’ Bi3+,2*3La3+ 4*5cations have been developed and then employed as indicator electrodes in potentiometric titration. The commonly known electrodes sensitive to lanthanide ions feature low selectivity, and the slope of calibration curves is different from the Nerstine slope. In the present paper, a new electrode sensitive to lanthanide ions (Ln3+) has been proposed, where the liquid ion exchanger is a complex of Ln3+ with imidodiphosphate acid tetraphenyl ester (I).

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This compound is a monoprotonic acid (HL), which with lanthanides forms LnL, complex which is well extracted to an organic phase, and stable. The values of extraction coefficients for complexes with lanthanides for various lanthanides are different.6 For example, the DEu/DSmratio is 1.13, while the DYb/DT,,, ratio is 2.90. These properties of the LnL, complex made us use it as a liquid ion exchanger in electrodes sensitive to lanthanides. EXPERIMENTAL

Liquid ion -exchanger

Imidodiphosphate acid tetraphenyl ester (I) was prepared as described earlier.’ A solution of ion-exchanger (of a concentration 3 x 10m3M Ln (L), in chloroform) was obtained by shaking of a liquid solution of lanthanide salt with (I) in chloroform (in ratio 1: 3). Measuring cell

A Teflon ion-selective electrode’ in which the liquid membrane was stabilized in a porous circular plate, was used. A silanized filter disc (Sartorius type SM-11306) saturated with ionexchanger solution was used as the liquid membrane. The composition of the inner solution of the electrode was 10m3A4Ln3+ and 10m2M KC1 (Ln3+ = La3+, Gd3+, Lu3+). A silver/silver chloride electrode with salt bridge was used as the reference electrode. The measuring cell was:

1 Inner / Liquid 1 Test 1 1M 1 1~ / Ag,AgC1 solution membrane solution KNO, KC1 1393

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WALENTY SZCZEPAN~AK and MARIAREN

The emf was measured with accuracy _+0.1 mV. Reagent

All reagents were of analytical grade. Doublydistilled water from a quartz still was used. RESULTS

Figure 1 shows a calibration curve of a lanthanum electrode in which the liquid membrane was made of a solution of La(L), in chloroform, while the inner solution was made of a solution of La3+ in KCl. The calibration graph was determined for aqueous solution of lanthanum ions at a constant ionic strength (0. 1M NaNO,). Analogous studies were carried out for a gadolinium and lutetium electrode with the liquid membrane made of the solutions of Gd(L), and Lu(L),, respectively. Very good curves were obtained for lanthanum and gadolinium electrodes. Within the linear range, the slopes for these electrodes correspond to 18.5 mV/decade change in concentration, which is close to the theoretical value for an electrode sensitive to tervalent cations (19.3 mV/dec.). The detection limit is the same for both electrodes and equals to 4 x lo-6M. The lutetium electrode has a lower value of the slope (S = 18 mV), and a significantly longer period of potential establishment (a few minutes, whereas for the lanthanum and gadolinium electrodes it takes only a few seconds). Infr’ence of pH on the electrode potential

The influence of the potential on the concentration of hydrogen ions was studied for the electrode sensitive to La3+. pH influence on

120

-

i

-

Fig. 2. pH effect on the potential of La-electrode solutions of La(II1) with concentration: (1) lo-‘M, 10-411!f.

electrode potential is illustrated in Fig. 2. It was found that the analytically favorable pH range of the electrode operation spans from 4.3 to 7. Eflect of interfering ions

In order to characterize the selectivity of electrode, a number of selectivity coefficients against many interfering ions for lanthanum and gadolinium electrodes were established. The coefficients were determined by measuring the potential of electrodes in pure solutions of Gd3+ and La3+ and in solutions which beside Gd3+ and La3+ also contain interfering ions.’ The values of the selectivity coefficients obtained are presented in Tables 1 and 2. Electrodes sensitive to La3+ and to Gd3+ are not selective against ions of other lanthanides, which is indicated by high values of selectivity coefficients against, e.g. Ce(III), Nd(III),

Table 1. Selectivity coefficients of the lanthanum electrode

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ra

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loo-

e

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B 80

./’

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Fig. 1. Calibration graph for the La-electrode in La(NO,), solution in O.lM NaNO,.

for (2)

Ni co Ca Al SC Y Ce Nd Sm Tb Gd LU

10-4, lo-2 10-4, IO-’ 10-5, lo-’ lo-5,10-2

lo-4.10-r 10-r, 10-r 10-4*IO-~ lo-4,10-d 10-d. 10-d 10-d. 10-4 10-d. to-4 10-4, 1o-4 lo-4,10-d

2.8 x 2.5 x <6.4 x <6.4 x 0.47 >I 0.52 0.76 0.86 0.89 0.75 >I 0.96

IO-’ lo-’ 10m4 IO-’

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Liquid state membrane electrode sensitive to Ln(II1) Table 2. Selectivity coetTicients of the gadolinium electrode M Ni co La Ce Lu

cod, C,(M) lo-‘, 10-4, IO-“, 10-4, 10-4,

IO-2 IO-2 lo-4 lo-4 lo-4

Kk%W 2.5 x 10-Z 4.6 x lO-2 0.5 0.14 0.93

120 -

IO 5

100 -

1

: :

Sm(III), and Tb(II1). They exhibit however high selectivity against Ni(II), Co(II), Al(II1) and Ca(I1).

20 -: : 1

Analytical application of the electrode The method of direct potentiometry. The influence of the presence of a number of cations in La(II1) and Gd(II1) solutions on the run of calibration curves was studied. emf measurements were performed in standard solutions La3+ (Gd3+ for a gadolinium electrode) containing interfering ions of a given concentration. Figure 3 shows calibration graphs for the La-electrode in the presence of various interfering ions at constant concentration. The shape of the curve is not affected by lo-*A4 Fe(III), Al(III), Ca(I1). It was also possible to determine 10-3-10-ZM La in the presence of lo-*MNi(II), Co(I1). Analogously, Fig. 4 presents calibration graphs for the Gd-electrode in the presence of various interfering ions at constant concentration. The calibration graph remains unchanged in the presence of Fe(II1) of a concentration lo-*M. It was also possible to determine 10-3-10-ZM Gd in the presence of lo-*M Ni(II), Co(I1). The influence of other lanthanides on calibration curves of both electrodes can be clearly observed already for the concentration of 10e4 M of interfering ions. It is interesting from the analytic point of view that La and Gd may be determined against Fe(II1). A high pH (=5) value of La(II1) and Gd(II1) solutions, i.e. required for correct operation of the electrode, is responsible for hydrolysis of Fe3+ ions and precipitation of Fe(OH),. Due to a unique form of Fe(II1) ions masking (soft precipitation of Fe(II1) in the form of hydroxide) obtained by increasing pH to 5, the course of calibration graph for electrodes within the whole concentration range of La and Gd is not interferred. Gd(II1) concentration in solutions of Ni(II), Co(II), Ca(II), and Fe(II1) with concentration of lo-*M (pH = 5) was determined by a direct

I

I

I

I

6

4

2

2

pLa(II1) Fig. 3. Calibration graph for La-electrode in lanthanum solutions containing interfering ions: 1-La + Fe(II1) [10-2M]; La + Al(II1) [IO-*Ml; La + Ca(I1) [IO-‘Ml; 2La + Ni(I1) [IO-*A4]; 3-La + Co(I1) [10-2M]; 4-La + Nd(II1) [10-4M]; La + Sm(II1) [10w4M]; 5-La + Tb(II1) [10-4&f]; 6-La + ce(III) [W4M], La + Lu(III) [lo-%]; 7-La + Gd(II1) [10-4M]; 8-La + Sc(II1) [10-3M]; 9La + Y(II1) [10e4M]; IO-La + Ce(II1) [10-3M].

potentiometry method using a gadolinium electrode. Determinations were performed by the standard addition method, in a solution (50 ml) containing Gd(II1) with concentration of 1.6 x 10e4M in the presence of Fe(II1) and Ca(I1) with concentration of lo-*M and 1.6 x 10m3M Gd in the presence of Ni(I1) or Co(H) with concentration of lo-*M. The results of statistical analysis of 10 Gd-determinations in the presence of Ni(II), Co(II), Fe(II1) or Ca(I1) ions are given in Table 3. The

I

I

I

I

I

6

4

2

2

pGd(II1) Fig. 4. Calibration graph for gadolinium electrode in gadolinium solutions containing interfering ions: I-Fe(II1) [lO-2M]; 2-Ni(I1) [10V2M]; 3X0(11) [10-2M]; 4-Ce(II1) [IO-‘Ml; 5-La(II1) [10e4M]; 6-Lu(II1) [IO-‘Ml.

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WALENTY SZCZEPANIAK and MARIAREN Table 3. Result of statistical analysis of 10 Gd-determinations Co(H), Fe(II1) and Ca(II) ions

Gd (w)lM

kg)

in 50 cm’ Average Standard Relative Absolute Relative

result (mg) deviation standard deviation (%) error (mg) error (%)

in the presence of Ni(II),

Gd, Fe 1.258/28

Gd, Ni 12.58129

Gd, Co 12.58/30

Gd, Ca 1.258120

1.241 0.0556 4.48 0.017 1.35

12.55 0.1262 1.00 0.03 0.24

12.63 0.18 1.44 0.05 0.40

1.252 0.0439 3.5 0.006 0.48

accuracy and precision of the determinations presented in Table 3 are satisfactory. Slightly worse results obtained in determination of Gd in the presence of Fe(II1) (iron concentration almost by two orders of magnitude higher than Gd concentration) are a consequence of the presence of iron hydroxide precipitate in the solution (pH = 5). The precipitate depositing on the membrane surface can block the reproducibility of the electrode potential. The method of potentiometric titration. Potentiometric titration may prove unreliable in the presence of interfering cations. The titration curve is affected by selectivity of the electrode as well as selectivity of the applied titrant. Curves 1 and 2 in Fig. 5 illustrate titration of pure La-solutions of concentrations 10p4M and 10m3M, respectively, with EDTA solutions. The course of the curves is satisfactory from the point of view of analytical purposes, however, the observed jumpwise potential changes differ from the theoretical predictions based on analysis of stability ofthe formed complex of La with EDTA (log KLa-EDTA = 15.5). The titration curves end at the potentials corresponding to La concentrations by a few orders of magnitude higher than the theoretically predicted value. The lower

potential jump observed is caused by the presence of different interfering cations (e.g. H+, K+ from the salt bridge, Na+ from EDTA), which despite low values of their selectivity coefficients exert decisive influence on the electrode potential at very low La concentrations. A nonselective titrant will lead to titration of a sum of the cation under determination and interfering cations. Curve 4 in Fig. 5 (titration of 10m3MLa against 5 x 10e4M Gd) illustrates the case of the nonselective titrant (log &.&nTA = 15.5, log Kod_rnTA= 17.37) and nonselective electrode (KE),, > 1). The result of titration corresponds to the sum of La and Gd ion content. Curve 3 in Fig. 5 presents titration of La in the presence of Ca. In this case both the electrode (Q‘& < 6.4 x 10-4) and the titrant 10.96) are selective. The curve (log &a-EDTA = reveals two potential jumps corresponding to the titration of La and Ca which are suitable for analytical interpretation. The potential jump corresponding to Ca titration is lower than that predicted by the theory because of the presence of interfering cations mentioned earlier. Because of insufficient selectivity of the electrode and a lack of a selective titrant, the method of titration is of less suitable for analytical purposes than the method of direct potentiometry (the method of standard addition). REFERENCES 1. P. Gabor, K. Toth and E. Pungor, Symposium on Ion Selective Electrodes, MitrafGred, Hungary, 23-25 October 1972. 2. M. Ren and W. Szczepaniak, Talanfa, 1983,30(12), 945. 3. M. Ren and W. Szczepaniak, Talanfa, 1984,31(3), 212. 4. J. B. Harrel, A. D. Jones and G. R. Choppin, Anal. Chem., 1969,41,

i

Fig. 5. Titration curves: 1-10-4M La; 2-lo-‘M La; 3La(lO-)M) + Ca(10e3M); 4-La(lO-‘M) + Gd(5 x 10w4M) with EDTA using a lanthanum electrode. For the sake of clarity of illustration the curves have been separated.

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5. K. Sykut, R. Dumkiewicz and J. Dumkiewicz, Zeszyry Naukowe Politechniki slpskiej, Chemia 108,78 1. 6. E. Herrmann, 0. Navratil, Hoang ba Nang, J. Smola, J. Friedrich, J. Prihoda, R. Dreyer, V. A. Chalkin and S. Kulpe, Collection Czechoslovak Chem. Commun., 1984,49,201. 7. M. L. Nilsen, Inorg. Chemistry, 1964,3(12), 1760. 8. K. Ren and W. Szczepaniak, Chem. Anal., 1976,21,1365. 9. G. J. Moody and J. D. R. Thomas, Selective Ion Sensitive Electrodes. Merrow, Watford, 1971.