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NaF solutions on the orion Cu(II) ion-selective electrode
Tahta, Vol. 32, No. 3, pp. 235-231, Printed in Great Britain
1985
EFFECT ORION
oo39-9140/85 $3.00 + 0.00 Pergamon Press Ltd
OF EDTA/NaF SOLUTIONS ON THE Cu(I1) ION-SELECTIVE ELECTRODE
HENRY F. STEGER Canada Centre for Mineral and Energy Technology, 555, Booth St., Ottawa, Canada (Received
9 May
1984. Revised 26 June 1984. Accepted
29 September
1984)
Summary-The titration of ethylenediaminetetra-acetic acid in fluoride medium with Cu(II) solution with the Orion Cu(I1) ion-selective electrode as indicator, results (after several titrations) in a broad end-point which is unsatisfactory for exact analytical purposes. This effect has been found to arise from an enhanced rate of response to changes in EDTA concentration when the electrode has been exposed to an EDTA/NaF medium for prolonged periods.
In 1979, the Canadian Certified Reference Materials Project (CCRMP) initiated a programme to confirm the homogeneity of a suite of seven zinc-aluminium alloys (Al 7-30x). For aluminium, a method was developed in which an excess of ethylenediaminetetra-acetic acid (EDTA) was added to the sample and the mixture was boiled to form the Al-EDTA complex; the uncomplexed EDTA was titrated with standard Cu(I1) solution. The Al-EDTA complex was then decomposed by the addition of fluoride and boiling; the EDTA released was also titrated with the Cu(I1) solution. The end-points were detected with an Orion Cu(II)-selective electrode.’ The quality of the response of this electrode was found to deteriorate on continuous use, until a single sharp end-point was not obtained unless the electrode was permitted sufficient recovery time in water or Cu(I1) solution between titrations. This paper reports the results of an investigation on the reduced response of the Orion Cu(II)-selective electrode (Ag,S/CuS homogeneous membrane) under the conditions used in the determination of aluminium in these alloys. EXPERIMENTAL Apparatus
and titration procedure
Titrations of EDTA with standard Cu(I1) solution were performed with a Mettler Automatic Titrator equipped with an Orion 94-29 Cu(I1) ion-selective electrode and a calomel reference electrode. All data were collected and mathematically treated by a home-made dedicated microprocessor. The titrations were of 6.00 ml of O.lOOOMEDTA, 10.00 ml of nH 5.20 acetate buffer (75.0 a of sodium acetate and 8 ml of glacial acetic actd ‘per iitre) and 84.0 ml of water and were performed in a water-jacketed vessel maintained at 25”. Copper titrant (O.lOOOM)was added in 0.5 or 1 ml increments until the total volume added was 5.5 ml or greater than 6.5 ml. Within this range, however, titrant was added either at the minimum continuous rate possible, 0.003 ml/set or in an “equilibrium” titration mode, wherein O.l-ml increments were added at 3-min intervals. Crown copyrights reserved.
The end-points of the titration method of Wolf.*
were calculated by the
General conditions EDTA solutions, 10m2, lo-’ and 10m4M, containing 10.0 ml of acetate buffer per 100 ml were prepared in order to establish electrode calibration graphs. To simulate the effect of continuous titration in a fluoride-medium,’ the Cu(II)-selective electrode was stored for various periods in a mixture of 6 ml of 0. 1M of acetate buffer, 54 ml of water and 25 ml of sodium fluoride solution (42 g/l.).
RESULTS AND
DISCUSSION
Storing the Cu(II)-selective electrode in a solution similar to the titration samples containing fluoride hastened the degradation of the end-point response of the electrode. Indeed a 16-hr or longer storage period makes the electrode ineffective as an end-point indicator in EDTA/Cu(II) titrations. Figure 1 shows the titration curves and corresponding AE/AV vs. V curves for the titration of EDTA with Cu(II), with end-point detection by use of the Cu(II)-selective electrode in (a) polished condition and (b) after treatment for 24 hr with EDTA/NaF solution. The earlier break in the titration curve when the treated electrode is used is typical whenever the AE/A V vs. V curve displays multiple peaks or appreciable broadening. When the titration data are presented as plots of electrode potential vs. either log [EDTA] or log [Cu(II)], linear plots are obtained for the region where the potential increases after the first few additions of Cu(I1) titrant. Figure 2 illustrates this for the titration data in Fig. 1. Table 1 summarizes the slopes of such plots for different treatments of the Cu(II)-selective electrode. It must be pointed out that the electrode potentials, particularly those near the end-point in titrations, do not necessarily attain a steady value before the next addition of titrant. For most applications, this does not have a significant effect on the accuracy of the titration. 235
236
SHORT
COMMIJNICATlONS
Vol (ml)
> Fig. 2. Plot of electrode potential log [EDTA] and log [Cu(II)] for the curves of Fig. 1: O-polished electrode, O-electrode stored in EDTAiNaF solution.
oLJ
I
’
a
’
Vol(ml)
6 ’
b ’
,b
Fig. 1. Titration and AE/AV us. V curves obtained by continuous addition of titrant, with detection by use of (a) the polished electrode and (b) the electrode stored in EDTA/NaF solution.
The major point of interest in Table 1 is that storing the electrode in EDTA/NaF solution appears to increase its sensitivity to changes in [EDTA]. Since the rate of titrant addition was the same in these titrations, the electrode sensitivity is in effect the rate of electrode response to changes in [EDTA] before the end-point and to changes in [Cu(II)] after the end-point. To study the effect of response time, several “equilibrium” titrations of EDTA with Cu(I1) solution were performed in which there were 3-min intervals between additions of titrant, to allow the potential of the electrode (in polished condition) to reach a steady value. Figure 3a shows a typical titration curve for these “equilibrium” titrations. It is evident that the break in the curve occurs earlier than when the continuous-addition procedure is used. The corresponding AEIAV us. V peak is broadened and can be resolved into two peaks. These observations are best explained as due to the rate of response of the polished electrode to changes in [EDTA] being appreciably slower than that to changes in [Cu(II)]. The rate of electrode response to relative changes in [EDTA] is not important until the end-point is closely approached because such changes are small for a given increment of Cu(I1) titrant. Very near the end-point, however, the relative changes in [EDTA] become large for a given increment of titrant, but in a continuous-addition titration the electrode cannot Table
give full response unless complete response is instantaneous. The result is a potential increase that is delayed and smaller than expected. Indeed, the endpoint has been passed and the electrode is responding to free Cu(I1) before it has been able to respond fully to EDTA. The steep potential increase associated with the single end-point suggests that in a continuous-addition titration the polished electrode responds only in a minor fashion to change in [EDTA] and essentially reflects changes in [Cu(II)]. In “equilibrium” titrations, however, the electrode has time to respond fully to changes in [EDTA] before the end-point and to changes in [Cu(II)] after it. That there appear to be two end-points where only one true end-point exists suggests that the electrode cannot respond instantaneously to free Cu(I1) immediately after the end-point. This delay is analogous to the time required for the electrode to reach a stable potential on a change in sample composition. The existence of such a delay is also strongly implied in continuous-addition titrations of EDTA with Cu(II). In 29 titrations of O.lOOOM EDTA with O.lOOOM Cu(II), the concentration of EDTA in the reagent solution was calculated to be 0.1008M (CT= 0.002M) if [Cu(II)] = O.lOOOM. With the same reagents, the reverse titration, where the electrode responds initially to decreasing [Cu(II)] gave the EDTA concentration as O.lOOOM(c = 0.0002M) for 11 runs. The titration of EDTA with Cu(II) has a small bias that is consistent with a delay in the response of the Cu(II)-selective electrode to the initial free Cu(II) immediately after the end-point in titrations of EDTA.
1. Slopes of plots of titration
curves
Slope, m V/decade Electrode
condition
Polished Stored in EDTA/NaF Stored in EDTA/NaF Stored in EDTA/NaF
log [EDTA] for 24 hr for 48 hr for 96 hr
-7.8 - 10.0 -126 - 15.4
log [Cu(II)] 29.1 29.8 30.0 30.4
SHORT
0 0
’
’
2
’
’
4
’ Vol (ml
’
6
J
’
8
’
237
COMh4UNlCATlONS
1
IO
1
Fig. 3. Titration curves for continuous-addition and “equilibrium” procedures with (a) the polished electrode and (b) the electrode stored for 48 hr in EDTAiNaF solution.
The similarity in the curves for the “equilibrium” titrations with the polished electrode and for the continuous-addition titrations with the electrode treated with EDTA/NaF solution, strongly suggests the same cause. Storage or continuous use of the electrode in EDTA/NaF medium results in an enhanced rate of response to changes in [EDTA]. Figure 3b compares the titration curves of EDTA with Cu(II), using the electrode treated for 48 hr with EDTA/NaF solution, by the continuous-addition and “equilibrium” procedures. The strong similarity, especially in the end-point region, substantiates the contention that the EDTA/NaF treatment of the electrode leads to an enhanced rate of response to changes in [EDTA] before the end-point. Neither titration gave a satisfactory single end-point. The treatment of the Cu(II)-selective electrode with EDTA/NaF solution has been postulated to form a sparingly-soluble fluoride compound, because the normal response of the electrode is readily restored
by light polishing of the surface.’ The physical presence of such a compound has not been unequivocally established. There is, however, a visually detectable dulling of the lustre and a corresponding slight decrease in the specular reflectance (in the region 5100-6000 A) of the electrode surface on treatment with EDTA/NaF. The results of this investigation nevertheless do indicate that the surface of the electrode is affected by storage in EDTA/NaF solution. This effect of storage in EDTA/NaF solution was investigated for two Orion 94-29 Cu(I1) electrodes. The first, purchased in 1976 but used very infrequently until 1980,’ displayed an immediate sensitivity to storage or continuous use in EDTA/NaF solutions. This electrode was accidentally broken during the initial stages of the present study. The second electrode, obtained in March 1983, remained unaffected by EDTA/NaF solutions for approximately two weeks but thereafter rapidly developed a sensitivity to EDTA-NaF comparable to that observed for the first electrode. This report summarizes the experimental results for the second electrode. It should be emphasized that the findings of this study should not be taken to mean that all EDTA titrations with Cu(I1) are unsatisfactory. Indeed, the opposite is true for all but very exact analysis. The phenomenon discussed here is observed primarily because of the resolving power of an up-to-date microcomputerized collection and reduction of data. Without it, the end-point in the titrations of EDTA with Cu(I1) and of Cu(I1) with EDTA could not have been determined with sufficient precision to show the slight differences observed. Indeed, the analyst must wonder if, in practical terms, there is any difference. The degeneration of the sharpness of the end-point in titrations of EDTA with Cu(I1) after the electrode has been used or stored in EDTA/fluoride medium may not be noticed at all under less favourable conditions, and only be observed as a reduction in the reproducibility of the end-point. REFERENCES 1. H. F. Steger, Tulanta, 1983, 30, 717. 2. S. Wolf, Z. Anal. Chem., 1970, 250, 13.
1985
EFFECT ORION
oo39-9140/85 $3.00 + 0.00 Pergamon Press Ltd
OF EDTA/NaF SOLUTIONS ON THE Cu(I1) ION-SELECTIVE ELECTRODE
HENRY F. STEGER Canada Centre for Mineral and Energy Technology, 555, Booth St., Ottawa, Canada (Received
9 May
1984. Revised 26 June 1984. Accepted
29 September
1984)
Summary-The titration of ethylenediaminetetra-acetic acid in fluoride medium with Cu(II) solution with the Orion Cu(I1) ion-selective electrode as indicator, results (after several titrations) in a broad end-point which is unsatisfactory for exact analytical purposes. This effect has been found to arise from an enhanced rate of response to changes in EDTA concentration when the electrode has been exposed to an EDTA/NaF medium for prolonged periods.
In 1979, the Canadian Certified Reference Materials Project (CCRMP) initiated a programme to confirm the homogeneity of a suite of seven zinc-aluminium alloys (Al 7-30x). For aluminium, a method was developed in which an excess of ethylenediaminetetra-acetic acid (EDTA) was added to the sample and the mixture was boiled to form the Al-EDTA complex; the uncomplexed EDTA was titrated with standard Cu(I1) solution. The Al-EDTA complex was then decomposed by the addition of fluoride and boiling; the EDTA released was also titrated with the Cu(I1) solution. The end-points were detected with an Orion Cu(II)-selective electrode.’ The quality of the response of this electrode was found to deteriorate on continuous use, until a single sharp end-point was not obtained unless the electrode was permitted sufficient recovery time in water or Cu(I1) solution between titrations. This paper reports the results of an investigation on the reduced response of the Orion Cu(II)-selective electrode (Ag,S/CuS homogeneous membrane) under the conditions used in the determination of aluminium in these alloys. EXPERIMENTAL Apparatus
and titration procedure
Titrations of EDTA with standard Cu(I1) solution were performed with a Mettler Automatic Titrator equipped with an Orion 94-29 Cu(I1) ion-selective electrode and a calomel reference electrode. All data were collected and mathematically treated by a home-made dedicated microprocessor. The titrations were of 6.00 ml of O.lOOOMEDTA, 10.00 ml of nH 5.20 acetate buffer (75.0 a of sodium acetate and 8 ml of glacial acetic actd ‘per iitre) and 84.0 ml of water and were performed in a water-jacketed vessel maintained at 25”. Copper titrant (O.lOOOM)was added in 0.5 or 1 ml increments until the total volume added was 5.5 ml or greater than 6.5 ml. Within this range, however, titrant was added either at the minimum continuous rate possible, 0.003 ml/set or in an “equilibrium” titration mode, wherein O.l-ml increments were added at 3-min intervals. Crown copyrights reserved.
The end-points of the titration method of Wolf.*
were calculated by the
General conditions EDTA solutions, 10m2, lo-’ and 10m4M, containing 10.0 ml of acetate buffer per 100 ml were prepared in order to establish electrode calibration graphs. To simulate the effect of continuous titration in a fluoride-medium,’ the Cu(II)-selective electrode was stored for various periods in a mixture of 6 ml of 0. 1M of acetate buffer, 54 ml of water and 25 ml of sodium fluoride solution (42 g/l.).
RESULTS AND
DISCUSSION
Storing the Cu(II)-selective electrode in a solution similar to the titration samples containing fluoride hastened the degradation of the end-point response of the electrode. Indeed a 16-hr or longer storage period makes the electrode ineffective as an end-point indicator in EDTA/Cu(II) titrations. Figure 1 shows the titration curves and corresponding AE/AV vs. V curves for the titration of EDTA with Cu(II), with end-point detection by use of the Cu(II)-selective electrode in (a) polished condition and (b) after treatment for 24 hr with EDTA/NaF solution. The earlier break in the titration curve when the treated electrode is used is typical whenever the AE/A V vs. V curve displays multiple peaks or appreciable broadening. When the titration data are presented as plots of electrode potential vs. either log [EDTA] or log [Cu(II)], linear plots are obtained for the region where the potential increases after the first few additions of Cu(I1) titrant. Figure 2 illustrates this for the titration data in Fig. 1. Table 1 summarizes the slopes of such plots for different treatments of the Cu(II)-selective electrode. It must be pointed out that the electrode potentials, particularly those near the end-point in titrations, do not necessarily attain a steady value before the next addition of titrant. For most applications, this does not have a significant effect on the accuracy of the titration. 235
236
SHORT
COMMIJNICATlONS
Vol (ml)
> Fig. 2. Plot of electrode potential log [EDTA] and log [Cu(II)] for the curves of Fig. 1: O-polished electrode, O-electrode stored in EDTAiNaF solution.
oLJ
I
’
a
’
Vol(ml)
6 ’
b ’
,b
Fig. 1. Titration and AE/AV us. V curves obtained by continuous addition of titrant, with detection by use of (a) the polished electrode and (b) the electrode stored in EDTA/NaF solution.
The major point of interest in Table 1 is that storing the electrode in EDTA/NaF solution appears to increase its sensitivity to changes in [EDTA]. Since the rate of titrant addition was the same in these titrations, the electrode sensitivity is in effect the rate of electrode response to changes in [EDTA] before the end-point and to changes in [Cu(II)] after the end-point. To study the effect of response time, several “equilibrium” titrations of EDTA with Cu(I1) solution were performed in which there were 3-min intervals between additions of titrant, to allow the potential of the electrode (in polished condition) to reach a steady value. Figure 3a shows a typical titration curve for these “equilibrium” titrations. It is evident that the break in the curve occurs earlier than when the continuous-addition procedure is used. The corresponding AEIAV us. V peak is broadened and can be resolved into two peaks. These observations are best explained as due to the rate of response of the polished electrode to changes in [EDTA] being appreciably slower than that to changes in [Cu(II)]. The rate of electrode response to relative changes in [EDTA] is not important until the end-point is closely approached because such changes are small for a given increment of Cu(I1) titrant. Very near the end-point, however, the relative changes in [EDTA] become large for a given increment of titrant, but in a continuous-addition titration the electrode cannot Table
give full response unless complete response is instantaneous. The result is a potential increase that is delayed and smaller than expected. Indeed, the endpoint has been passed and the electrode is responding to free Cu(I1) before it has been able to respond fully to EDTA. The steep potential increase associated with the single end-point suggests that in a continuous-addition titration the polished electrode responds only in a minor fashion to change in [EDTA] and essentially reflects changes in [Cu(II)]. In “equilibrium” titrations, however, the electrode has time to respond fully to changes in [EDTA] before the end-point and to changes in [Cu(II)] after it. That there appear to be two end-points where only one true end-point exists suggests that the electrode cannot respond instantaneously to free Cu(I1) immediately after the end-point. This delay is analogous to the time required for the electrode to reach a stable potential on a change in sample composition. The existence of such a delay is also strongly implied in continuous-addition titrations of EDTA with Cu(II). In 29 titrations of O.lOOOM EDTA with O.lOOOM Cu(II), the concentration of EDTA in the reagent solution was calculated to be 0.1008M (CT= 0.002M) if [Cu(II)] = O.lOOOM. With the same reagents, the reverse titration, where the electrode responds initially to decreasing [Cu(II)] gave the EDTA concentration as O.lOOOM(c = 0.0002M) for 11 runs. The titration of EDTA with Cu(II) has a small bias that is consistent with a delay in the response of the Cu(II)-selective electrode to the initial free Cu(II) immediately after the end-point in titrations of EDTA.
1. Slopes of plots of titration
curves
Slope, m V/decade Electrode
condition
Polished Stored in EDTA/NaF Stored in EDTA/NaF Stored in EDTA/NaF
log [EDTA] for 24 hr for 48 hr for 96 hr
-7.8 - 10.0 -126 - 15.4
log [Cu(II)] 29.1 29.8 30.0 30.4
SHORT
0 0
’
’
2
’
’
4
’ Vol (ml
’
6
J
’
8
’
237
COMh4UNlCATlONS
1
IO
1
Fig. 3. Titration curves for continuous-addition and “equilibrium” procedures with (a) the polished electrode and (b) the electrode stored for 48 hr in EDTAiNaF solution.
The similarity in the curves for the “equilibrium” titrations with the polished electrode and for the continuous-addition titrations with the electrode treated with EDTA/NaF solution, strongly suggests the same cause. Storage or continuous use of the electrode in EDTA/NaF medium results in an enhanced rate of response to changes in [EDTA]. Figure 3b compares the titration curves of EDTA with Cu(II), using the electrode treated for 48 hr with EDTA/NaF solution, by the continuous-addition and “equilibrium” procedures. The strong similarity, especially in the end-point region, substantiates the contention that the EDTA/NaF treatment of the electrode leads to an enhanced rate of response to changes in [EDTA] before the end-point. Neither titration gave a satisfactory single end-point. The treatment of the Cu(II)-selective electrode with EDTA/NaF solution has been postulated to form a sparingly-soluble fluoride compound, because the normal response of the electrode is readily restored
by light polishing of the surface.’ The physical presence of such a compound has not been unequivocally established. There is, however, a visually detectable dulling of the lustre and a corresponding slight decrease in the specular reflectance (in the region 5100-6000 A) of the electrode surface on treatment with EDTA/NaF. The results of this investigation nevertheless do indicate that the surface of the electrode is affected by storage in EDTA/NaF solution. This effect of storage in EDTA/NaF solution was investigated for two Orion 94-29 Cu(I1) electrodes. The first, purchased in 1976 but used very infrequently until 1980,’ displayed an immediate sensitivity to storage or continuous use in EDTA/NaF solutions. This electrode was accidentally broken during the initial stages of the present study. The second electrode, obtained in March 1983, remained unaffected by EDTA/NaF solutions for approximately two weeks but thereafter rapidly developed a sensitivity to EDTA-NaF comparable to that observed for the first electrode. This report summarizes the experimental results for the second electrode. It should be emphasized that the findings of this study should not be taken to mean that all EDTA titrations with Cu(I1) are unsatisfactory. Indeed, the opposite is true for all but very exact analysis. The phenomenon discussed here is observed primarily because of the resolving power of an up-to-date microcomputerized collection and reduction of data. Without it, the end-point in the titrations of EDTA with Cu(I1) and of Cu(I1) with EDTA could not have been determined with sufficient precision to show the slight differences observed. Indeed, the analyst must wonder if, in practical terms, there is any difference. The degeneration of the sharpness of the end-point in titrations of EDTA with Cu(I1) after the electrode has been used or stored in EDTA/fluoride medium may not be noticed at all under less favourable conditions, and only be observed as a reduction in the reproducibility of the end-point. REFERENCES 1. H. F. Steger, Tulanta, 1983, 30, 717. 2. S. Wolf, Z. Anal. Chem., 1970, 250, 13.