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Potentiometric determination of sodium concentrations in aqueous-ethanolic solutions
0039-9140/85 $3.00+ 0.00 Copyright0 1985PergamonPressLtd
Talonto,Vol. 32, No. 8B, pp. 827-829,1985 Printed
in Great Britain. AU rights reserved
POTENTIOMETRIC DETERMINATION OF SODIUM CONCENTRATIONS IN AQUEOUS-ETHANOLIC SOLUTIONS C. J. COETZEE Department of Chemistry, University of the Western Cape, Bellville 7530, South Africa (Received
16 October
1984. Revised 5 March
1985. Accepted
18 March 1985)
potentiometric behaviour of a sodium ion-selective glass electrode in aqueous ethanolic media was studied. The electrode was then used to determine sodium ion concentrations in aqueous Summary-The
ethanolic samples by standard-addition
potentiometry.
The application of ion-selective electrodes in media other than water is still somewhat limited, although there were some applications in the early days of these sensors.‘A Reasons for this slow progress are many, and some are listed by Kakabadse.’ Little difficulty is encountered in the use of the glass pH-electrode in ethanol-water mixtures containing less than 90% w/w ethanol. At higher concentrations of alcohol and in water containing Xi--90% acetone some contraction of the linear portion of the emf-pH curve has been found, as well as variation of the potential with time.6 Since a change in solvent may cause changes in the thermodynamic as well as the kinetic properties of the ions present and of exchange sites in ion-selective membranes, it is to be expected that the sensitivity, selectivity and response time of ion-selective electrodes might be dependent on the nature of the solvent. The reproducibility of potentiometric measurements made in wholly or partially aqueous solvents is generally better than that for those made in pure non-aqueous solvents, because traces of water present in the non-aqueous solvent itself do not pose a problem in the mixed solvent systems, since they will seldom (if ever) be large enough to affect the overall water concentration significantly. The behaviour of the reference electrode in the solvent used must also be known. The reference electrode most commonly used in non-aqueous systems is the aqueous calomel electrode, the use of which is very convenient. Its liquid-junction potential is evidently generally sufficiently constant for electrode potentials in non-aqueous or partly nonaqueous solutions to be measured with high precision.’ If a glass electrode is to be used in non-aqueous or partly non-aqueous media it must be soaked in the appropriate solvent so that the electrode surface layer obtained has a constant vacancy-concentration characteristic of the solvent.’
The use of the standard-addition technique in application of ion-selective electrodes is well known.g,‘0 In this work a sodium ion-selective glass electrode was employed for determination of the sodium ion concentration in aqueous ethanol samples, by the standard-addition method. Because of variations in the non-aqueous contents of the samples the technique was not simple to apply and various precautions had to be taken to standardize the conditions under which determinations could successfully be done. One of the more important prerequisites is that the ionic strength of the solution to be analysed should be the same as that of the added standard. Also, the concentration range covered by the standard additions must fall within the linear part of the calibration graph for the electrode. In addition, if a partially aqueous solvent is used the added standard should have the same solvent composition as the sample in order to cut out any effects from heat of mixing. EXPERIMENTAL Reagents
All reagents used were of analytical-reagent grade. Sodium chloride was dried at 120” overnight. Apparatus
A Radiometer PHM-64 pH-meter was used for all potentiometric measurements. The sodium ion-selective glass electrode used was a Beckman 39278. The manufacturers recommend that the pH of test solutions should be >pNa + 4 if sodium measurements are to be made.” The reference electrode was a Beckman 39170 fibre-junction saturated calomel electrode. Preliminary
experiments
The following experiments were planned and done in order to establish the effect of all the variables, before the standard-addition method for the determination of sodium concentration in ethanolic solutions was attempted. 1. Calibration of the electrode at 25” in sodium solutions containing from 0 to 50% v/v ethanol. 2. Repetition of the calibrations with the ionic strength adjusted to 0.1 M with 0.1 M magnesium sulphate. This was
827
828
SHORT
COMMlJNlCATlONS
necessary as the standard-additions technique calls for working with a constant ionic strength medium to minimize junction-potential effects. 3. Repetition of the calibration with sodium solutions in 50% v/v aqueous dimethylformamide, dioxan, acetone, ethanol, dimethylsulphoxide, methanol and 1-propanol media to obtain an overall picture of the response of the electrode in various aqueous/non-aqueous solvent mixtures. The glass electrode used was preconditioned in the appropriate solvent mixture for 24 hr before use. On the basis of the preliminary experiments, 15% v/v ethanol solution was chosen as the medium for the standard-addition determinations with the glass electrode preconditioned in 15% v/v ethanol solution. Standard-addition
Slope, Solvent
mV/pNa
Water I-Propanol Methanol Dimethylsulphoxide Ethanol Acetone Dioxan Dimethylformamide
58.6 56.5 56.8 58.7 58.7 52.4 56.5 57.2
procedure
The alcohol content of the sample was first determined by density determinationi at 20” and the solution was adjusted to IS% v/v ethanol content and O.lM ionic strength with magnesium sulphate. The sample size was normally 25 ml. The ionic strength of the standard solutions was also adjusted to O.lM with magnesium sulphate when necessary, and the solutions were prepared in 15% v/v ethanol medium. Their sodium concentration ranged between 5.00 x lo-‘M and O.lOOOM.Fixed volumes of sample were mixed with various volumes of standard and diluted to a fixed volume with 15% v/v aqueous ethanol at ionic strength O.lM (MgSO,), the sodium activities were measured with the sodium electrode, and the usual graph was drawn and interpreted. When necessary the pH was adjusted to pNa + 4 by addition of a drop or two of 3M ammonia solution. RESULTS
Table 2. Determmation of the sodium content of aqueous ethanol solutions Original ethanol content, % t’lu
Potentiometry
Flame photometry
12.0 12.0 15.0 15.0 15.0 15.0 15.0 21.0 18.0 16.0 14.0
0.0196, 0.0196, 0.0123, 0.0085, 0.0082, 0.0159, 0.0168, 0.0193, 0.0057, 0.0125, 0.0252,
0.0196 0.0196 0.0124 0.0086 0.0083 0.0159 0.0168 0.0194 0.0058 0.0126 0.0252
[Na+] found, M
AND DISCUSSION
The calibration graphs for various sodium solutions in aqueous ethanol were linear between pNa 1 and 4.5. Stable potential readings were obtained within a minute. Figure 1 shows a selection of these graphs. The slopes of the linear sections were all in the range 58.3-58.7 mV/pNa. The general trend of increase in potential with increasing ratio of organic solvent was also found by Chaudhari and Cheng” for the response of a lead ion-selective electrode.
100
Table 1. Slopes of calibration graphs for sodium in 50’4 v/v water-organic solvent media
1
o-
Fig. 1. Calibration graphs for a sodium ion-selective glass electrode in (1) water, (2) 10% ethanol, (3) 25% ethanol, (4) 50% ethanol.
At the ionic concentrations and solvent compositions used there does not seem to be any problem due to ion-pair formation. The results show, however, that knowledge of the ethanol content of the sample is necessary before a potentiometric determination can be done either by direct measurement or by the standard-additions method. It is also necessary that the ethanol content of both the sample and the standards be the same. The slopes of the calibration graphs for the O.lM ionic strength (MgSO,) solutions also agreed within 0.5 mV, but there is a small parallel shift of the graphs with change in ionic strength, so close matching of the ionic strength of the unknown and standard solution is necessary. Table 1 gives the slopes of the calibration graphs for the sodium electrode in the various 50% v/v aqueous/non-aqueous solvent mixtures tested. The graphs were linear over the sodium concentration range lo-‘-3 x 10m5M. All the slopes except that for the acetone medium are within 3 mV of the value for purely aqueous medium. The cell emf depends on the solvent, and becomes more positive with change of solvent in the descending order given in the table. The shift does not have a direct correlation with the dielectric constant of the organic component of the solvent medium. It may probably be attributed to a combination of increase in activity of the cation, change in junction potentials, and dielectric constant effects.
SHORT
829
COMMUNlCATlONS
Table 2 gives the results of a number of sodium determinations, in which the ethanol content of the sample was adjusted to 15% v/v. The sodium contents by were also determined of the samples flame-emission spectrometry and these results are listed for comparison. A set of 20 determinations of sodium covering the range 0.10-2.0 mg/ml gave an average recovery of 99.9$% with a standard deviation of 0.3%. These results show that with necessary precautions it is possible to use a sodium-selective glass electrode to determine the sodium content of aqueous ethanolic solutions successfully. The selectivity coefficients listed by the manufacturer indicate that only lithium, silver and hydrogen ions could interfere, and in most samples likely to be tested (e.g., wines), the first two species are unlikely to be present in more than traces, and the pH adjustment will take care of the third. Acknowledgemenfs-The author wishes to thank the South African C.S.I.R. for financial assistance, and Dr. R. A. Chalmers for critical comments during preparation of the manuscript.
REFERENCES 1. J. J. Lingane, Anal. Cbem., 1967, 39, 881; 1968,40,935. 2. E. Pungor and K. Toth, Anal. Chim. Acta, 1969, 41, 291. 3. J. W. Ross and M. S. Frant, Anal. Chem., 1969,41,967. 4. G. A. Rechnitz and N. C. Kenny, Anal. Lett., 1969, 2, 395. 5. G. Kakabadse, Ion-Selective Rev.,. 1982, 3, 127. 6. R. G. Bates, Determination of pH, Theory and Practice,
2nd Ed., Wiley, New York, 1984, and references cited therein. 7. G. J. Hills, in Reference Electrodes, Theory and Practice, D. J. G. Ives and G. J. Janz (eds.), Academic Press, New York, 1961. 8. Z. Boksay and B. Csakvari, Acta Chim. Acad. Sri. Hung., 1971, 67, 157. 9. A. Liberti and M. Mascini, Anal. Chem., 1969, 41, 676. 10. C. J. Coetzee, A. J. Basson and S. R. Grobler, Tydskr. Natuurwetenskappe, 1975, 15, 24. 11. Beckman Instructions 1155B, Beckman
Instruments, Fullerton, Cal., August 1964. 12. R. C. Weast (ed.), Handbook of Chemistry and Physics, 47th Ed., The Chemical Rubber Co., Cleveland, Ohio, 1966. 13. S. N. K. Chaudhari and K. L. Cheng, Mikrochim. Acta, 1979 II, 411.
Talonto,Vol. 32, No. 8B, pp. 827-829,1985 Printed
in Great Britain. AU rights reserved
POTENTIOMETRIC DETERMINATION OF SODIUM CONCENTRATIONS IN AQUEOUS-ETHANOLIC SOLUTIONS C. J. COETZEE Department of Chemistry, University of the Western Cape, Bellville 7530, South Africa (Received
16 October
1984. Revised 5 March
1985. Accepted
18 March 1985)
potentiometric behaviour of a sodium ion-selective glass electrode in aqueous ethanolic media was studied. The electrode was then used to determine sodium ion concentrations in aqueous Summary-The
ethanolic samples by standard-addition
potentiometry.
The application of ion-selective electrodes in media other than water is still somewhat limited, although there were some applications in the early days of these sensors.‘A Reasons for this slow progress are many, and some are listed by Kakabadse.’ Little difficulty is encountered in the use of the glass pH-electrode in ethanol-water mixtures containing less than 90% w/w ethanol. At higher concentrations of alcohol and in water containing Xi--90% acetone some contraction of the linear portion of the emf-pH curve has been found, as well as variation of the potential with time.6 Since a change in solvent may cause changes in the thermodynamic as well as the kinetic properties of the ions present and of exchange sites in ion-selective membranes, it is to be expected that the sensitivity, selectivity and response time of ion-selective electrodes might be dependent on the nature of the solvent. The reproducibility of potentiometric measurements made in wholly or partially aqueous solvents is generally better than that for those made in pure non-aqueous solvents, because traces of water present in the non-aqueous solvent itself do not pose a problem in the mixed solvent systems, since they will seldom (if ever) be large enough to affect the overall water concentration significantly. The behaviour of the reference electrode in the solvent used must also be known. The reference electrode most commonly used in non-aqueous systems is the aqueous calomel electrode, the use of which is very convenient. Its liquid-junction potential is evidently generally sufficiently constant for electrode potentials in non-aqueous or partly nonaqueous solutions to be measured with high precision.’ If a glass electrode is to be used in non-aqueous or partly non-aqueous media it must be soaked in the appropriate solvent so that the electrode surface layer obtained has a constant vacancy-concentration characteristic of the solvent.’
The use of the standard-addition technique in application of ion-selective electrodes is well known.g,‘0 In this work a sodium ion-selective glass electrode was employed for determination of the sodium ion concentration in aqueous ethanol samples, by the standard-addition method. Because of variations in the non-aqueous contents of the samples the technique was not simple to apply and various precautions had to be taken to standardize the conditions under which determinations could successfully be done. One of the more important prerequisites is that the ionic strength of the solution to be analysed should be the same as that of the added standard. Also, the concentration range covered by the standard additions must fall within the linear part of the calibration graph for the electrode. In addition, if a partially aqueous solvent is used the added standard should have the same solvent composition as the sample in order to cut out any effects from heat of mixing. EXPERIMENTAL Reagents
All reagents used were of analytical-reagent grade. Sodium chloride was dried at 120” overnight. Apparatus
A Radiometer PHM-64 pH-meter was used for all potentiometric measurements. The sodium ion-selective glass electrode used was a Beckman 39278. The manufacturers recommend that the pH of test solutions should be >pNa + 4 if sodium measurements are to be made.” The reference electrode was a Beckman 39170 fibre-junction saturated calomel electrode. Preliminary
experiments
The following experiments were planned and done in order to establish the effect of all the variables, before the standard-addition method for the determination of sodium concentration in ethanolic solutions was attempted. 1. Calibration of the electrode at 25” in sodium solutions containing from 0 to 50% v/v ethanol. 2. Repetition of the calibrations with the ionic strength adjusted to 0.1 M with 0.1 M magnesium sulphate. This was
827
828
SHORT
COMMlJNlCATlONS
necessary as the standard-additions technique calls for working with a constant ionic strength medium to minimize junction-potential effects. 3. Repetition of the calibration with sodium solutions in 50% v/v aqueous dimethylformamide, dioxan, acetone, ethanol, dimethylsulphoxide, methanol and 1-propanol media to obtain an overall picture of the response of the electrode in various aqueous/non-aqueous solvent mixtures. The glass electrode used was preconditioned in the appropriate solvent mixture for 24 hr before use. On the basis of the preliminary experiments, 15% v/v ethanol solution was chosen as the medium for the standard-addition determinations with the glass electrode preconditioned in 15% v/v ethanol solution. Standard-addition
Slope, Solvent
mV/pNa
Water I-Propanol Methanol Dimethylsulphoxide Ethanol Acetone Dioxan Dimethylformamide
58.6 56.5 56.8 58.7 58.7 52.4 56.5 57.2
procedure
The alcohol content of the sample was first determined by density determinationi at 20” and the solution was adjusted to IS% v/v ethanol content and O.lM ionic strength with magnesium sulphate. The sample size was normally 25 ml. The ionic strength of the standard solutions was also adjusted to O.lM with magnesium sulphate when necessary, and the solutions were prepared in 15% v/v ethanol medium. Their sodium concentration ranged between 5.00 x lo-‘M and O.lOOOM.Fixed volumes of sample were mixed with various volumes of standard and diluted to a fixed volume with 15% v/v aqueous ethanol at ionic strength O.lM (MgSO,), the sodium activities were measured with the sodium electrode, and the usual graph was drawn and interpreted. When necessary the pH was adjusted to pNa + 4 by addition of a drop or two of 3M ammonia solution. RESULTS
Table 2. Determmation of the sodium content of aqueous ethanol solutions Original ethanol content, % t’lu
Potentiometry
Flame photometry
12.0 12.0 15.0 15.0 15.0 15.0 15.0 21.0 18.0 16.0 14.0
0.0196, 0.0196, 0.0123, 0.0085, 0.0082, 0.0159, 0.0168, 0.0193, 0.0057, 0.0125, 0.0252,
0.0196 0.0196 0.0124 0.0086 0.0083 0.0159 0.0168 0.0194 0.0058 0.0126 0.0252
[Na+] found, M
AND DISCUSSION
The calibration graphs for various sodium solutions in aqueous ethanol were linear between pNa 1 and 4.5. Stable potential readings were obtained within a minute. Figure 1 shows a selection of these graphs. The slopes of the linear sections were all in the range 58.3-58.7 mV/pNa. The general trend of increase in potential with increasing ratio of organic solvent was also found by Chaudhari and Cheng” for the response of a lead ion-selective electrode.
100
Table 1. Slopes of calibration graphs for sodium in 50’4 v/v water-organic solvent media
1
o-
Fig. 1. Calibration graphs for a sodium ion-selective glass electrode in (1) water, (2) 10% ethanol, (3) 25% ethanol, (4) 50% ethanol.
At the ionic concentrations and solvent compositions used there does not seem to be any problem due to ion-pair formation. The results show, however, that knowledge of the ethanol content of the sample is necessary before a potentiometric determination can be done either by direct measurement or by the standard-additions method. It is also necessary that the ethanol content of both the sample and the standards be the same. The slopes of the calibration graphs for the O.lM ionic strength (MgSO,) solutions also agreed within 0.5 mV, but there is a small parallel shift of the graphs with change in ionic strength, so close matching of the ionic strength of the unknown and standard solution is necessary. Table 1 gives the slopes of the calibration graphs for the sodium electrode in the various 50% v/v aqueous/non-aqueous solvent mixtures tested. The graphs were linear over the sodium concentration range lo-‘-3 x 10m5M. All the slopes except that for the acetone medium are within 3 mV of the value for purely aqueous medium. The cell emf depends on the solvent, and becomes more positive with change of solvent in the descending order given in the table. The shift does not have a direct correlation with the dielectric constant of the organic component of the solvent medium. It may probably be attributed to a combination of increase in activity of the cation, change in junction potentials, and dielectric constant effects.
SHORT
829
COMMUNlCATlONS
Table 2 gives the results of a number of sodium determinations, in which the ethanol content of the sample was adjusted to 15% v/v. The sodium contents by were also determined of the samples flame-emission spectrometry and these results are listed for comparison. A set of 20 determinations of sodium covering the range 0.10-2.0 mg/ml gave an average recovery of 99.9$% with a standard deviation of 0.3%. These results show that with necessary precautions it is possible to use a sodium-selective glass electrode to determine the sodium content of aqueous ethanolic solutions successfully. The selectivity coefficients listed by the manufacturer indicate that only lithium, silver and hydrogen ions could interfere, and in most samples likely to be tested (e.g., wines), the first two species are unlikely to be present in more than traces, and the pH adjustment will take care of the third. Acknowledgemenfs-The author wishes to thank the South African C.S.I.R. for financial assistance, and Dr. R. A. Chalmers for critical comments during preparation of the manuscript.
REFERENCES 1. J. J. Lingane, Anal. Cbem., 1967, 39, 881; 1968,40,935. 2. E. Pungor and K. Toth, Anal. Chim. Acta, 1969, 41, 291. 3. J. W. Ross and M. S. Frant, Anal. Chem., 1969,41,967. 4. G. A. Rechnitz and N. C. Kenny, Anal. Lett., 1969, 2, 395. 5. G. Kakabadse, Ion-Selective Rev.,. 1982, 3, 127. 6. R. G. Bates, Determination of pH, Theory and Practice,
2nd Ed., Wiley, New York, 1984, and references cited therein. 7. G. J. Hills, in Reference Electrodes, Theory and Practice, D. J. G. Ives and G. J. Janz (eds.), Academic Press, New York, 1961. 8. Z. Boksay and B. Csakvari, Acta Chim. Acad. Sri. Hung., 1971, 67, 157. 9. A. Liberti and M. Mascini, Anal. Chem., 1969, 41, 676. 10. C. J. Coetzee, A. J. Basson and S. R. Grobler, Tydskr. Natuurwetenskappe, 1975, 15, 24. 11. Beckman Instructions 1155B, Beckman
Instruments, Fullerton, Cal., August 1964. 12. R. C. Weast (ed.), Handbook of Chemistry and Physics, 47th Ed., The Chemical Rubber Co., Cleveland, Ohio, 1966. 13. S. N. K. Chaudhari and K. L. Cheng, Mikrochim. Acta, 1979 II, 411.