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The iodide interference with silver chloride electrodes
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@Envier
Chimka Actq 88 (1977) 41-46 Scientific PuhiishingCompany, Amsterdam - Printedin The Netherlvlds
THE iODIDE INTERFERENCE
I-LAkASENSand
WITH SILVER CHLORIDE
ELECTRODES
J.GOOSSEN
Philips, Scientific and Industrial Equipment Division, and Philips Research Lubomtories, Eindhoven (!iXe Netherlands)
(Received 24t.hMay1976)
SUMMARY
‘Ibe interference of iodide in the mesurement of chloride in water with a silver chloride electrode is smaller than is generally assumed. Selectivity coefficients (Kgt1) varying from 2 to 360 have been reported; values determined with a Philips eiectrohe range from l-4 to 14. The results are explained in terms of how AgI is depositid on the membrane, the E VS. pCi plots for different ccnditions being correlated with electron microscope photographs.
The behaviour of a solid-state silver halide (AgX) electrode in solutions containing not only the ion X- but also another halide Y- which forms a sparingly soluble salt with Ag’, can be described by a Nicolsky equation [l] :
7
E=E,+
KF$.
ln(a,+
av)
(1)
where ax and ay are the activities of the halides X and Y-, respectively, and HE% is the selectivity coefficient. This behaviour i:as been explained by assuming that, after the exchange reaction: xLkl
+
y- liquid
=
Xipuid
+
ykid
(2)
the AgX cry$als become covered by a surface layer of a mixed phase Ag(X, Y) which is in theknodynamic equilibrium with both halides in the solution [Z-4]. The selectivity coefficient is calculated to be equal to the ratio of the of AgX and AgY, respectively, i.e. solubiity products 2&,X and S,,, K$.
= %2d%6Y
(3)
If a mised phase did not occur, the electrode potential would be given by the conventional equation, E = E, + RT In ax/F in solutions containing and would change abruptly to E = Eo bothXandY_atlowvaluesof+, + RT K*,“+ a,[F when ay was increased so that ay 2 a, S,,,/S,,, ami the AgX crystals became covered completely by the single phase AgY. Tbii Qpe of behaviour has been reported for the interference of tbioCYanat’2 on .AgBr electrodes [ 51.
Experimentally obtained sefectivity coefficients show reasonable agreement with those calculated from the solubility products [3,6, 71. Many suppliers of ion-selective electrodes also quote these values in their brochures [S] _ Since S,,, /S,,, is of the order of 106 at room temperature, this would mesn that it would not be possible, for example, to measure the concentration of chloride soluti5ns m5re dilute than 1 Ni in the pxesence of iodirie concentrations of 1C6 M or more, with Age! electrodes, Lower values for K$$, ranging from 2 to 360 are, however, quoted by Philips, Pungor and Padiometer. The value of 360 is in accordance with the measurements of Rechnitz and Kresz [93. The work reported here was carried out to see if such Iarge discrepancies between calculated and experimental values could be confirmed and if so, whether such a discrepancy could be explained. EX?ER.lMENTAL
Two methods are commonly used to determine selectivity coefficients. In the separate solution method [lO], potentid measurements are carried cut i-ntwo series of solutions, one series containing only the primary ion and the other containing only the interfering ion, If the slopes of the curves stowing the electrode potential as a function of the logarithm of the ion activities are the same for both ions, the selectivity coefficient can be calcubatedfrom the difference in potential at equal act+ities: log Kx”y . = (Ev-
E&/Q
(4)
*where Q is the slope of the curves, X is the primary ion and Y the interfering ion. In the mixed sofution method ET], eIectrode potentidts an mezuzzzd in soluti513s with a constant activity of the interfering or the primary ion and ,3fferent activities of the primary or the interfering ion, respectively. The theoretical variation of E with czx, -hen czy is constant, is shown schematically in Fig. 1. It follows fr5m eqn. (1) that the intercept of the tw5 asymptotes is at ai = K~:
uY, Or HX~*
=T~tlo,
(5)
Measurements were made prith Philips IS 55~CI ebsctrodes; an X 44!2SD/l was used as the reference electrode. The output was measured with a Philips PW 9408 digital pH meter. 1t was of paramount importance, when miYed sclutions of chloride and iodide were measured, to earth alI equipment, including the electric motor for stirring, at one and the same point in order to obtain accurate and repr
43
RESULTS
El~trode potentials obtained with sepamtc solutions of chloride and iodide are shown in Fig. 2; the slopes are virtually the same for both curves. From the potential difference at a concentration of 1(X3 M, the value of Kg1 is calculated from eqn. (4) to be 14. Figure 3 gives the result for mixed sol&ions with a constant iodide concentration of IO4 I&I.From measurements in stationary solutions the value of Kg”r was found to be 1.4. When the solutions are stirred the interference is somewhat great=, and Kayi is 5. When an electrode is kept for some time in a solution containing iodide, the membrane turns yellow because of deposition of silver iodide. It was therefore expected that the response of the electrode might change with time under such conditions. To. check this, one electrode was kept for 90 h in a 1G4 M KI solution. The Nemstian behaviour of this electrode in chloride solutions hardly changed (Fig. 4); the same was true for an electrode kept for 24 h in a lo-’ M NaCI-IO4 M KI solution, but when the iodide concentration exceeded lG_’ M, the behaviour of the electrode towards chloride was affected considerably (Fig. 4). Figure 5 shows scanning electron microscope photographs of the precipitated layers with a Philips PSEM 500 microscope. REtI was also determined before and after an electrode had been kept for 135 h ‘in a solution of 10e7 M Nail and, initially, 10e4 M KI. The electrode potential was recorded over the whole period, and did not change by more than 3 mV. The iodide concentration dropped during 135 h to 10m6M. From
L
-06
Fig. 1. Schewtic behaviourof an eleCxroaeIn a wluttch WIUIa cor!&Ultactivity0~ as a functionof .jz, Rg, 2. Electrode&.&tialpersus log [I-] (0) and [Ck-] (A).
Fig. 3. Electrode potential as a function of [Cl-] in mixed solutions witi [I-: 0: Stationary solutions. A: @?ntly stiTIed solutions.
= lo4 M.
IQ: 4. Potential of AgCl electrodes in NaSl solutions. 0: SVhennew. CJ : After keeping for 2~% h in a solution c~ntabin(i 107 TV% chloride and 10-O M iodide. 7 : After 90 h in a 10% M ICI solution. a: After 30 min in a lo* lV¶IU solution t : After 45 min in a 10-l M KI solution. t&a total vobxne
of
the solution in which the electrode was kept, the axnount
of silver iodide deposited on the ZO-mm2membrane area was calculated to be 3 mg. ROyz changed only from 2 to 3 during the storage time {Pig. 6). DISCUSSION The experimental results prove that the interference from iodide ions in the measurement of chloride concentrations nrith AgCl electrodes can in practice be 5 orders of magnitude smaller than expected from eqn. (3), Rayz being only about 3 instead of X6. This may be explained as follows. TZIorder to observe a selectivity coefficient as given by eqn. (3), two conditions must be fulfilled: (a) the A&l crystals must be covered comptetely by the depos%ed layer; and (b) the solution must be in equ%brium with the deposited layer. Neither condition is fulfiUed in normal usage of the chlotide electrode. Figure HA, B) shows clearly that no compact layer is deposited on the membrane but that a layer of loosely packed crys*~s is formed when the membrane is held in diluted iodid.e solutions, The crystals that grow are much larger than f&e A@ crystals of-the membrane; the latter rem&s in free col=tact with the sampfe solution througfi the farge holes between the deposited crystals. This is further confirmed by Figs. 4 and 6 which show that an electrode covered with a layer of crys*& grown from ICY4 %I ‘XI
46
solutions can still be used to measure chloride contents in sodium chloride solutions; the Nemstian response of such an electrode is only slightly different from that of a fresh AgCl electrode. Larger deviations occur (Fig. 4) when an electrode is held for some time in solutions with a high iodide content. Figure 5 shows that layers grown from a 10m2M KI solution consist of small crystals with small holes between them, whereas layers grown from a 10-l M KI solution contain large flat crystals which shut off the membrane more completely from the test solution. When A&l is in contact with water, some of it will dissoive until the solubility product of AgCl is reached. When it is in contact with iodide solutions, AgI will be precipitated if aI > aaSAgf/SASCI, so that iodide is removed from the solution adjacent to the AgCl membrane. The iodide is continuously resupplied from the hulk solution by diffusion and reacts with Ag’ ions originating from the membrane. The chain reaction involving dissolution of silver chloride, and reaction of iodide with the Ag’ ions released to form silver iodide continues until either the AgCl of the membrane or the iodide of the solution becomes exhausted. In the tests described above with the electrode kept in a finite volume of a 10e4 M KI solution, the latter was the case. The solution in the immediate vicinity of the membrane remains exhausted of iodide when the supply of Ag+ ions from the membrane is sufficiently high, or when the supply of iodide from the bulk solution to the exhausted layer adjacent to the membrane is sufficiently low. Under those conditions the electrode potential of AgCl electrodes in chloride solutions is little affected by the presence of iodide ions. Figure ‘2 shows that deviations do occur when the I- : Cl- ratio, and therefore the supply of iodide ions from the bulk, becomes too high. ‘Ibis deviation sets in earlier when the supply of iodide ions from the bulk is increased by stirring_ We are grateful to Dr. L. Heyne for stimulating discussion and to J. L. C. Daams for the scanni.ng electron microscope photographs. REFERENCES 1 B. P. Nicolsky, Zh. Fiz. Khim.. 10 (1937) 495. 2 R. P. Buck, Anal. Chem.. 40 (1983) 1432. 3 IV. E. Morf. G. K&r and W. Simon, Am& Chem.. 46 (1974) 1538. 4H.E.Vfuhrmarm. W. FL Morf znd W. Simon, Helv. Chim. Acta, 56 (1974) 1011. 5 J. \V_ Ross in R. -4 Dust (Ed.), Ion Selective Electrodes. Nat. BureauStand. Spec. Publ. No. 314 (1969) p. 83. 6 E. Pungor. Anal. Chem., 39 (1967) 28A. 7 E. Pungorand K. Toth, Anal. Chim. Acta, 47 (1969) 291. 8 G. J. Moody and J. D. R Thomas,SelectiveIon SensitiveElectrodes,Merrow, England, 1971, p. 26. 9 G. E. R.echnitz and M. R. Kresz, Anal. Chem., 38 (1966) 1786. 10 G. Eiienman, D. 0. Rudin znd J. U. C&by, Science. 126 (1957) 831. 11 R P. Buck Anal. Chim A&a. 73 (1974) 321. 12 k Ic Coving-ton, CRC Crit Rev. Anal. Chem., 4 (1974) 355.
@Envier
Chimka Actq 88 (1977) 41-46 Scientific PuhiishingCompany, Amsterdam - Printedin The Netherlvlds
THE iODIDE INTERFERENCE
I-LAkASENSand
WITH SILVER CHLORIDE
ELECTRODES
J.GOOSSEN
Philips, Scientific and Industrial Equipment Division, and Philips Research Lubomtories, Eindhoven (!iXe Netherlands)
(Received 24t.hMay1976)
SUMMARY
‘Ibe interference of iodide in the mesurement of chloride in water with a silver chloride electrode is smaller than is generally assumed. Selectivity coefficients (Kgt1) varying from 2 to 360 have been reported; values determined with a Philips eiectrohe range from l-4 to 14. The results are explained in terms of how AgI is depositid on the membrane, the E VS. pCi plots for different ccnditions being correlated with electron microscope photographs.
The behaviour of a solid-state silver halide (AgX) electrode in solutions containing not only the ion X- but also another halide Y- which forms a sparingly soluble salt with Ag’, can be described by a Nicolsky equation [l] :
7
E=E,+
KF$.
ln(a,+
av)
(1)
where ax and ay are the activities of the halides X and Y-, respectively, and HE% is the selectivity coefficient. This behaviour i:as been explained by assuming that, after the exchange reaction: xLkl
+
y- liquid
=
Xipuid
+
ykid
(2)
the AgX cry$als become covered by a surface layer of a mixed phase Ag(X, Y) which is in theknodynamic equilibrium with both halides in the solution [Z-4]. The selectivity coefficient is calculated to be equal to the ratio of the of AgX and AgY, respectively, i.e. solubiity products 2&,X and S,,, K$.
= %2d%6Y
(3)
If a mised phase did not occur, the electrode potential would be given by the conventional equation, E = E, + RT In ax/F in solutions containing and would change abruptly to E = Eo bothXandY_atlowvaluesof+, + RT K*,“+ a,[F when ay was increased so that ay 2 a, S,,,/S,,, ami the AgX crystals became covered completely by the single phase AgY. Tbii Qpe of behaviour has been reported for the interference of tbioCYanat’2 on .AgBr electrodes [ 51.
Experimentally obtained sefectivity coefficients show reasonable agreement with those calculated from the solubility products [3,6, 71. Many suppliers of ion-selective electrodes also quote these values in their brochures [S] _ Since S,,, /S,,, is of the order of 106 at room temperature, this would mesn that it would not be possible, for example, to measure the concentration of chloride soluti5ns m5re dilute than 1 Ni in the pxesence of iodirie concentrations of 1C6 M or more, with Age! electrodes, Lower values for K$$, ranging from 2 to 360 are, however, quoted by Philips, Pungor and Padiometer. The value of 360 is in accordance with the measurements of Rechnitz and Kresz [93. The work reported here was carried out to see if such Iarge discrepancies between calculated and experimental values could be confirmed and if so, whether such a discrepancy could be explained. EX?ER.lMENTAL
Two methods are commonly used to determine selectivity coefficients. In the separate solution method [lO], potentid measurements are carried cut i-ntwo series of solutions, one series containing only the primary ion and the other containing only the interfering ion, If the slopes of the curves stowing the electrode potential as a function of the logarithm of the ion activities are the same for both ions, the selectivity coefficient can be calcubatedfrom the difference in potential at equal act+ities: log Kx”y . = (Ev-
E&/Q
(4)
*where Q is the slope of the curves, X is the primary ion and Y the interfering ion. In the mixed sofution method ET], eIectrode potentidts an mezuzzzd in soluti513s with a constant activity of the interfering or the primary ion and ,3fferent activities of the primary or the interfering ion, respectively. The theoretical variation of E with czx, -hen czy is constant, is shown schematically in Fig. 1. It follows fr5m eqn. (1) that the intercept of the tw5 asymptotes is at ai = K~:
uY, Or HX~*
=T~tlo,
(5)
Measurements were made prith Philips IS 55~CI ebsctrodes; an X 44!2SD/l was used as the reference electrode. The output was measured with a Philips PW 9408 digital pH meter. 1t was of paramount importance, when miYed sclutions of chloride and iodide were measured, to earth alI equipment, including the electric motor for stirring, at one and the same point in order to obtain accurate and repr
43
RESULTS
El~trode potentials obtained with sepamtc solutions of chloride and iodide are shown in Fig. 2; the slopes are virtually the same for both curves. From the potential difference at a concentration of 1(X3 M, the value of Kg1 is calculated from eqn. (4) to be 14. Figure 3 gives the result for mixed sol&ions with a constant iodide concentration of IO4 I&I.From measurements in stationary solutions the value of Kg”r was found to be 1.4. When the solutions are stirred the interference is somewhat great=, and Kayi is 5. When an electrode is kept for some time in a solution containing iodide, the membrane turns yellow because of deposition of silver iodide. It was therefore expected that the response of the electrode might change with time under such conditions. To. check this, one electrode was kept for 90 h in a 1G4 M KI solution. The Nemstian behaviour of this electrode in chloride solutions hardly changed (Fig. 4); the same was true for an electrode kept for 24 h in a lo-’ M NaCI-IO4 M KI solution, but when the iodide concentration exceeded lG_’ M, the behaviour of the electrode towards chloride was affected considerably (Fig. 4). Figure 5 shows scanning electron microscope photographs of the precipitated layers with a Philips PSEM 500 microscope. REtI was also determined before and after an electrode had been kept for 135 h ‘in a solution of 10e7 M Nail and, initially, 10e4 M KI. The electrode potential was recorded over the whole period, and did not change by more than 3 mV. The iodide concentration dropped during 135 h to 10m6M. From
L
-06
Fig. 1. Schewtic behaviourof an eleCxroaeIn a wluttch WIUIa cor!&Ultactivity0~ as a functionof .jz, Rg, 2. Electrode&.&tialpersus log [I-] (0) and [Ck-] (A).
Fig. 3. Electrode potential as a function of [Cl-] in mixed solutions witi [I-: 0: Stationary solutions. A: @?ntly stiTIed solutions.
= lo4 M.
IQ: 4. Potential of AgCl electrodes in NaSl solutions. 0: SVhennew. CJ : After keeping for 2~% h in a solution c~ntabin(i 107 TV% chloride and 10-O M iodide. 7 : After 90 h in a 10% M ICI solution. a: After 30 min in a lo* lV¶IU solution t : After 45 min in a 10-l M KI solution. t&a total vobxne
of
the solution in which the electrode was kept, the axnount
of silver iodide deposited on the ZO-mm2membrane area was calculated to be 3 mg. ROyz changed only from 2 to 3 during the storage time {Pig. 6). DISCUSSION The experimental results prove that the interference from iodide ions in the measurement of chloride concentrations nrith AgCl electrodes can in practice be 5 orders of magnitude smaller than expected from eqn. (3), Rayz being only about 3 instead of X6. This may be explained as follows. TZIorder to observe a selectivity coefficient as given by eqn. (3), two conditions must be fulfilled: (a) the A&l crystals must be covered comptetely by the depos%ed layer; and (b) the solution must be in equ%brium with the deposited layer. Neither condition is fulfiUed in normal usage of the chlotide electrode. Figure HA, B) shows clearly that no compact layer is deposited on the membrane but that a layer of loosely packed crys*~s is formed when the membrane is held in diluted iodid.e solutions, The crystals that grow are much larger than f&e A@ crystals of-the membrane; the latter rem&s in free col=tact with the sampfe solution througfi the farge holes between the deposited crystals. This is further confirmed by Figs. 4 and 6 which show that an electrode covered with a layer of crys*& grown from ICY4 %I ‘XI
46
solutions can still be used to measure chloride contents in sodium chloride solutions; the Nemstian response of such an electrode is only slightly different from that of a fresh AgCl electrode. Larger deviations occur (Fig. 4) when an electrode is held for some time in solutions with a high iodide content. Figure 5 shows that layers grown from a 10m2M KI solution consist of small crystals with small holes between them, whereas layers grown from a 10-l M KI solution contain large flat crystals which shut off the membrane more completely from the test solution. When A&l is in contact with water, some of it will dissoive until the solubility product of AgCl is reached. When it is in contact with iodide solutions, AgI will be precipitated if aI > aaSAgf/SASCI, so that iodide is removed from the solution adjacent to the AgCl membrane. The iodide is continuously resupplied from the hulk solution by diffusion and reacts with Ag’ ions originating from the membrane. The chain reaction involving dissolution of silver chloride, and reaction of iodide with the Ag’ ions released to form silver iodide continues until either the AgCl of the membrane or the iodide of the solution becomes exhausted. In the tests described above with the electrode kept in a finite volume of a 10e4 M KI solution, the latter was the case. The solution in the immediate vicinity of the membrane remains exhausted of iodide when the supply of Ag+ ions from the membrane is sufficiently high, or when the supply of iodide from the bulk solution to the exhausted layer adjacent to the membrane is sufficiently low. Under those conditions the electrode potential of AgCl electrodes in chloride solutions is little affected by the presence of iodide ions. Figure ‘2 shows that deviations do occur when the I- : Cl- ratio, and therefore the supply of iodide ions from the bulk, becomes too high. ‘Ibis deviation sets in earlier when the supply of iodide ions from the bulk is increased by stirring_ We are grateful to Dr. L. Heyne for stimulating discussion and to J. L. C. Daams for the scanni.ng electron microscope photographs. REFERENCES 1 B. P. Nicolsky, Zh. Fiz. Khim.. 10 (1937) 495. 2 R. P. Buck, Anal. Chem.. 40 (1983) 1432. 3 IV. E. Morf. G. K&r and W. Simon, Am& Chem.. 46 (1974) 1538. 4H.E.Vfuhrmarm. W. FL Morf znd W. Simon, Helv. Chim. Acta, 56 (1974) 1011. 5 J. \V_ Ross in R. -4 Dust (Ed.), Ion Selective Electrodes. Nat. BureauStand. Spec. Publ. No. 314 (1969) p. 83. 6 E. Pungor. Anal. Chem., 39 (1967) 28A. 7 E. Pungorand K. Toth, Anal. Chim. Acta, 47 (1969) 291. 8 G. J. Moody and J. D. R Thomas,SelectiveIon SensitiveElectrodes,Merrow, England, 1971, p. 26. 9 G. E. R.echnitz and M. R. Kresz, Anal. Chem., 38 (1966) 1786. 10 G. Eiienman, D. 0. Rudin znd J. U. C&by, Science. 126 (1957) 831. 11 R P. Buck Anal. Chim A&a. 73 (1974) 321. 12 k Ic Coving-ton, CRC Crit Rev. Anal. Chem., 4 (1974) 355.