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isoelectric point of exotic oxides: Tl2O3
Journal of Colloid and Interface Science 280 (2004) 544–545 www.elsevier.com/locate/jcis
Letter to the Editor
Point of zero charge/isoelectric point of exotic oxides: Tl2 O3 Marek Kosmulski ∗ , Czesław Saneluta Department of Electrochemistry, Lublin University of Technology, Nadbystrzycka 38A, 20618 Lublin, Poland Received 10 May 2004; accepted 3 August 2004 Available online 2 October 2004
Abstract The pristine point of zero charge of Tl2 O3 determined in the presence of NaNO3 using the inert electrolyte titration method and confirmed by microelectrophoresis is at pH 7.9. 2004 Elsevier Inc. All rights reserved. Keywords: Zeta potential; Isoelectric point; Point of zero charge; Thallium (III) oxide
1. Introduction
2.2. Methods
The pristine point of zero charge (PZC), defined as the pH value at which the net proton charge equals zero, is an important parameter characterizing the adsorption properties of metal oxides and related materials. It is usually determined as the crossover point of three or more surfacecharging curves obtained at various concentrations of inert electrolytes, usually alkali nitrates or halides. With very pure materials, the PZC obtained by means of titration matches the isoelectric point (IEP) obtained by means of electrokinetic methods. The PZCs/IEPs of various materials were reviewed by Parks [1], Kosmulski [2–4], and many others (cf. [2] for references). A survey of the literature reveals that the PZCs/IEPs of certain materials are very well documented, while for other relatively common materials, no single literature entry could be found. In the present study the PZC/IEP of thallium oxide is reported for the first time ever.
In order to avoid experimental problems which may be brought about by adsorption of silicates (when the experiments are carried out in glass), all operations in the present work were carried out in Nalgene. Malvern IIc was used to determine the electrophoretic mobility at a solid-to-liquid ratio of 1:5000 w/w at 25 ◦ C, and the ζ potential was calculated by means of the Smoluchowski equation. The 0.01 mol dm−3 NaNO3 was always present as a supporting electrolyte, and the pH was adjusted by means of dilute HNO3 or NaOH solutions. The electrokinetic mobility was measured in fresh dispersions (no more than 20 min contact between the powder and the solution). Each data point in the figure is an average of 10 measurements (without refilling the measurement cell). The course of the titrations was very sensitive to the titration rate and aging; thus the potentiometric titrations failed to produce reversible charging curves, and the attempt to obtain the PZC as the crossover point of the titration curves at various ionic strengths failed. Under such circumstances, the PZC was determined using the inert electrolyte titration method [5].
2. Experimental 2.1. Materials Thallium (III) oxide (avicennite) 99.99% was from Aldrich. The other reagent grade chemicals were used without further purification, and the water was MilliQ. * Corresponding author. Fax: +48-81-5254601.
E-mail address: [email protected] (M. Kosmulski). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.08.079
3. Results and discussion Fig. 1 shows the ζ potential of thallium (III) oxide as a function of pH. The IEP is at pH 7.9. The inert electrolyte
M. Kosmulski, C. Saneluta / Journal of Colloid and Interface Science 280 (2004) 544–545
Fig. 1. The ζ potential of thallium (III) oxide in the presence of 0.01 mol dm−3 NaNO3 . Various symbols denote two series of measurements.
titration produced the following results. At pH 7.79, addition of saturated NaNO3 solution to a dispersion of thallium (III) oxide in NaNO3 solution always produced an increase in the pH of the dispersion. At pH 7.90–7.96 addition of saturated NaNO3 solution to a dispersion of thallium (III) oxide in NaNO3 solution did not affect the pH. At pH 8.00, addition of saturated NaNO3 solution to a dispersion of thallium (III) oxide in NaNO3 solution always produced a decrease
545
in the pH of the dispersion; with the exception of one data point (pH 8.03) where there was no significant effect. We then conclude that the PZC is at pH about 7.9 (±0.1) and it matches the IEP. Thus 7.9 is very likely the pristine PZC of thallium (III) oxide. This result is not surprising; most Me2 O3 oxides, including chemical analogs of Tl2 O3 (oxides of 13-group metals), have their pristine PZCs in this range [2]. The above value can be useful to test correlations between the pristine PZCs of metal oxides and various physical constants (ionic radii, electronegativities, etc.; cf. [2] for detail) and to test the validity of theories attempting to derive the pristine PZCs of metal oxides from their crystallographic data.
References [1] G.A. Parks, Chem. Rev. 65 (1965) 177. [2] M. Kosmulski, Chemical Properties of Material Surfaces, Dekker, New York, 2001. [3] M. Kosmulski, J. Colloid Interface Sci. 253 (2002) 77. [4] M. Kosmulski, J. Colloid Interface Sci. 275 (2004) 214. [5] M. Kosmulski, J. Matysiak, J. Szczypa, J. Colloid Interface Sci. 164 (1994) 280.
Letter to the Editor
Point of zero charge/isoelectric point of exotic oxides: Tl2 O3 Marek Kosmulski ∗ , Czesław Saneluta Department of Electrochemistry, Lublin University of Technology, Nadbystrzycka 38A, 20618 Lublin, Poland Received 10 May 2004; accepted 3 August 2004 Available online 2 October 2004
Abstract The pristine point of zero charge of Tl2 O3 determined in the presence of NaNO3 using the inert electrolyte titration method and confirmed by microelectrophoresis is at pH 7.9. 2004 Elsevier Inc. All rights reserved. Keywords: Zeta potential; Isoelectric point; Point of zero charge; Thallium (III) oxide
1. Introduction
2.2. Methods
The pristine point of zero charge (PZC), defined as the pH value at which the net proton charge equals zero, is an important parameter characterizing the adsorption properties of metal oxides and related materials. It is usually determined as the crossover point of three or more surfacecharging curves obtained at various concentrations of inert electrolytes, usually alkali nitrates or halides. With very pure materials, the PZC obtained by means of titration matches the isoelectric point (IEP) obtained by means of electrokinetic methods. The PZCs/IEPs of various materials were reviewed by Parks [1], Kosmulski [2–4], and many others (cf. [2] for references). A survey of the literature reveals that the PZCs/IEPs of certain materials are very well documented, while for other relatively common materials, no single literature entry could be found. In the present study the PZC/IEP of thallium oxide is reported for the first time ever.
In order to avoid experimental problems which may be brought about by adsorption of silicates (when the experiments are carried out in glass), all operations in the present work were carried out in Nalgene. Malvern IIc was used to determine the electrophoretic mobility at a solid-to-liquid ratio of 1:5000 w/w at 25 ◦ C, and the ζ potential was calculated by means of the Smoluchowski equation. The 0.01 mol dm−3 NaNO3 was always present as a supporting electrolyte, and the pH was adjusted by means of dilute HNO3 or NaOH solutions. The electrokinetic mobility was measured in fresh dispersions (no more than 20 min contact between the powder and the solution). Each data point in the figure is an average of 10 measurements (without refilling the measurement cell). The course of the titrations was very sensitive to the titration rate and aging; thus the potentiometric titrations failed to produce reversible charging curves, and the attempt to obtain the PZC as the crossover point of the titration curves at various ionic strengths failed. Under such circumstances, the PZC was determined using the inert electrolyte titration method [5].
2. Experimental 2.1. Materials Thallium (III) oxide (avicennite) 99.99% was from Aldrich. The other reagent grade chemicals were used without further purification, and the water was MilliQ. * Corresponding author. Fax: +48-81-5254601.
E-mail address: [email protected] (M. Kosmulski). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.08.079
3. Results and discussion Fig. 1 shows the ζ potential of thallium (III) oxide as a function of pH. The IEP is at pH 7.9. The inert electrolyte
M. Kosmulski, C. Saneluta / Journal of Colloid and Interface Science 280 (2004) 544–545
Fig. 1. The ζ potential of thallium (III) oxide in the presence of 0.01 mol dm−3 NaNO3 . Various symbols denote two series of measurements.
titration produced the following results. At pH 7.79, addition of saturated NaNO3 solution to a dispersion of thallium (III) oxide in NaNO3 solution always produced an increase in the pH of the dispersion. At pH 7.90–7.96 addition of saturated NaNO3 solution to a dispersion of thallium (III) oxide in NaNO3 solution did not affect the pH. At pH 8.00, addition of saturated NaNO3 solution to a dispersion of thallium (III) oxide in NaNO3 solution always produced a decrease
545
in the pH of the dispersion; with the exception of one data point (pH 8.03) where there was no significant effect. We then conclude that the PZC is at pH about 7.9 (±0.1) and it matches the IEP. Thus 7.9 is very likely the pristine PZC of thallium (III) oxide. This result is not surprising; most Me2 O3 oxides, including chemical analogs of Tl2 O3 (oxides of 13-group metals), have their pristine PZCs in this range [2]. The above value can be useful to test correlations between the pristine PZCs of metal oxides and various physical constants (ionic radii, electronegativities, etc.; cf. [2] for detail) and to test the validity of theories attempting to derive the pristine PZCs of metal oxides from their crystallographic data.
References [1] G.A. Parks, Chem. Rev. 65 (1965) 177. [2] M. Kosmulski, Chemical Properties of Material Surfaces, Dekker, New York, 2001. [3] M. Kosmulski, J. Colloid Interface Sci. 253 (2002) 77. [4] M. Kosmulski, J. Colloid Interface Sci. 275 (2004) 214. [5] M. Kosmulski, J. Matysiak, J. Szczypa, J. Colloid Interface Sci. 164 (1994) 280.