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New reference electrodes based on tungsten-substituted molybdenum bronzes
Solid State Ionics 169 (2004) 75 – 80 www.elsevier.com/locate/ssi
New reference electrodes based on tungsten-substituted molybdenum bronzes J. Gabel a,*, W. Vonau a, P. Shuk b, U. Guth a,c a
Kurt-Schwabe-Institute for Measuring and Sensor Technology Meinsberg, D-04720, Ziegra-Knobelsdorf, Germany b Emerson Process Management, Rosemount Analytical Inc., 1201 North Main, St. Orrville, OH 44667, USA c Institute of Physical Chemistry and Electrochemistry, Dresden University of Technology, D-01062 Dresden, Germany Received 20 January 2003; received in revised form 6 August 2003; accepted 6 August 2003
Abstract New reference electrodes have been developed and tested by a solid state contact method. The performance was demonstrated at ambient temperature with polycrystalline powders from tungsten-substituted alkali molybdenum bronzes with the general formula AxMo1yWyO3. These electrodes show no significant response to the changing of pH, cation concentration and redox potential of the measuring solution. Therefore, it is possible to use them as reference electrode for electrochemical measurements. D 2003 Elsevier B.V. All rights reserved. Keywords: Oxide bronzes; pH sensitivity; Solid state reference electrode
1. Introduction Using transition metal oxide bronzes for ion selective electrodes in aqueous solutions is known. It has been reported in detail by Dobson [1], Dobson and Comer [2], Kohler and Go¨pel [3], Shuk et al. [4,5,7] and Greenblatt et al. [6]. Electrodes made of bronzes have been found suitable as sensors for measurements of pH, sodium concentration, redox potential, dissolved oxygen and EDTA titration. However, there are restrictions because of cross sensitivities to univalent cations. Some bronzes with bivalent cations, especially the barium tungsten bronzes BaxWO3, have been tested as material for reference electrodes [8] without success because of potential instability. Conventional reference electrodes are mostly electrodes of the second kind: a metallic phase in equilibrium with its sparingly soluble metal salt in an aqueous electrolyte. Therefore, the measurement range of these electrodes as regards temperature and pressure is strongly restricted. Electrochemical solid state electrodes might help to overcome some of the disadvantages of conventional glass electrodes or liquid reference electrodes such as fragility, * Corresponding author. Tel.: +49-343276080; fax: +49-34327608131. E-mail address: [email protected] (J. Gabel). 0167-2738/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.08.001
temperature and pressure limitation. Most important is the absence of any liquid system component like the inner electrolytic solution. So it is easy to make planar or miniaturised sensors. Also, the measurement at higher temperature and in wide pressure range as well as the measurement in non aqueous solutions is possible with low maintenance. The problem of a solid state reference electrode is not resolved satisfactory up to the present [9].
2. Experimental The bronzes have been synthesised by solid state reaction under inert conditions. The stoichiometric quantities of the reactants were weighed out according to the following equations: nA2 MoO4 þ 2ð1 n xÞMoO3 þ nMoO2 þ 2xWO3 ! 2An Mo1x Wx O3
ð1Þ
3nA2 WO4 þ 3ð3 2n 3xÞWO3 þ nW þ 6xMoO3 ! 6An W1x Mox O3 ;
ð2Þ
with A=Na, Li, K and n, xV1. The charge size was maintained at 10 g. The reactants have been mixed carefully and filled into a quartz tube. The
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J. Gabel et al. / Solid State Ionics 169 (2004) 75–80
tube was evacuated and sealed. A special heat treatment was carried out: to begin an initial heating on 500 jC for several days was followed by the heating up to the reaction temperature between 600 and 1000 jC depending on the composition. The runs were terminated by a controlled cooling. In this way, single crystals as well as polycrystalline materials were obtained. After synthesis, the products were alternate washed with hot diluted hydrochloric acid, carbonate solution and water. The phases were characterised and identified by X-ray powder diffraction using a Bruker axs D8 advance. Fig. 1a shows the powder XRD pattern of the LixMo0.95W0.05O3 compared to Li0.3WO3 (cubic) and Li0.3MoO3 (monoclinic). There is no congruence between the patterns. In Fig. 1b, the powder XRD pattern of different charges of LixMo0.95W0.05O3 can be seen. The synthesis of the compound is reproducible. The differences in intensity of the peaks are based on the needled structure of the bronze. The exact content of Li could not be detected but x is assumed between 0.3 and 0.4 caused by the colour. The value of W was determined by EDX. A precise chemical
analytic of the compound was not possible until now because of the high chemical resistance of the substance. The polycrystalline samples were preferred for electrode fabrication because it was not possible every time to get single crystals in the requested size and quality. The electrodes were produced by mixing carefully ground powders with unsaturated polyester resin. The ratio powder/resin was chosen in that way that the mixture could be casted into silicon moulds. The moulds have been centrifuged to get a high particle density on one end. From that end, discs have been cut of. These discs were contacted with a copper disc or a platinum wire by a conductive adhesive. For insulation and stability the purpose entire electrode assembly was coated by an epoxy resin. For better handling, it was stuck into a glass tube. In some cases, it was necessary to abrade the surface of the disc. Unfortunately, all the electrodes showed a relative high electrical resistance (about 1– 500 MV). But this was the only way to get stable electrodes from the polycrystalline samples. These electrodes show results in good agreement with single crystal electrodes. We
Fig. 1. Powder XRD pattern of LixMo0.95W0.05O3: (a) compared to Li0.3WO3 and Li0.3MoO3; (b) of different charges of LixMo0.95W0.05O3.
J. Gabel et al. / Solid State Ionics 169 (2004) 75–80
77
Fig. 2. Crystal structure of NaxWO3 (generated by PowderCell 2.2; atomic coordinates from the ICSD, No. 1949).
were not able to get reproducible results with electrodes using thick film technology so far.
3. Results and discussion
phases are wide range nonstoichiometric compounds all form with perovskite-like structure (Fig. 2) [4]. The molybdenum oxide bronzes are stoichiometric or nearly so. The structure can be described as a channel system of sheets of MoO6 octahedrons. The sheets are held together by the inserted cations [10]. Fig. 3 shows the crystal structure of a
The potentiometric characteristics of the bronze electrodes were evaluated by measuring their emf against standard saturated silver/silver chloride electrode. All measurements were carried out on 25 jC. Table 1 gives the electrode abbreviations and the summary about the electrode response. The reasons for the behaviour of the bronzes as electrode material have been studied intensively in the recent years. It is assumed that the pH and other ion sensing properties of the transition metal oxide bronze electrodes depend on the crystal structure. In general, bronzes may be regarded as a solid solution of the cations in the MO3 lattice. The AxWO3 Table 1 List of the electrodes Formula
Abbreviation
pH
Redox potential
LixMo0.95W0.05O3 LixMo0.95W0.05O3 Na0.9Mo6O17
S1 S2 S3
no response no response f 76 mV/pH
Li0.3MoO3
S4
f 46 mV/pH
no response no response response like Pt electrode response like Pt electrode
Fig. 3. Crystal structure of K0.3MoO3 [3].
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J. Gabel et al. / Solid State Ionics 169 (2004) 75–80
Fig. 4. Schematic view on ion exchange [3].
potassium molybdenum oxide bronze as an example. These structures permit the exchange and intercalation of cations on the surface. The pH sensing characteristics of the bronze electrodes are based on the ion exchange reactions. In Fig. 4, the simplified reaction mechanism can be seen. The reaction on the surface is believed to the following equations [7]: Ax My Oz þ yH þ þ ye ! Ax Hd My Oz
ð3Þ
Ax My Oz þ yH þ ! Axd Hd My Oz þ yAþ ;
ð4Þ
with A=Li, Na, K and M=W, Mo.
To adjust stoichiometric imbalances caused by the cations in the MO3 lattice, the transition metals take different valence states (usually V and VI). So an electron exchange and transportation by electron hopping is possible too. However, it is assumed that the sensing properties will be significantly different from the bulk structure. Fig. 5 shows the results of a measurement in buffer solutions with different pH values. The electrodes made from the well known bronzes S 3 (line 4) and S 4 (line 5) show a good response (S 3: 71 mV/pH, S 4: 52 mV/ pH) to the changing pH. But the electrodes S 1 and S 2 show pH-independent potential in wide pH range like a reference electrode. The same results could be observed at other pH measurements. Therefore, an acidic solution (0.04 M on boric, acetic and phosphoric acid) was titrated with a 0.2 M sodium hydroxide solution (Fig. 6). The slopes of the electrodes S 3 (line 4) and S 4 (line 5) are lower than in Fig. 5 (S 3: 59 mV/pH, S 4: 31 mV/pH) because of the influence of the increasing Na+ concentration. It was intensively reported about the cross sensitivities and the influence of univalent cations on bronze electrodes elsewhere in the past [4,5,7]. The new electrodes (S 1 and S 2) show no significant change of the potential. Only a small drift was observed (about 5– 10 mV). Consequence of this, there is no response to the changes of pH and Na+ concentration. That suggest the possibility of a new solid state reference electrode. This behaviour differs from the theory of cation exchange and intercalation. Presumed, this mechanism is prevented. The exact mechanism is not clear at the present. The response to the changing redox potential of the transition metal oxide bronze electrodes depends on electron
Fig. 5. Measurement in different pH solutions (system Na2HPO4/C6H8O7 [11]) vs. Ag/AgCl, Cl.
J. Gabel et al. / Solid State Ionics 169 (2004) 75–80
79
Fig. 6. Titration of a 0.04 M acetic solution by 0.2 M NaOH [12] vs. Ag/AgCl, Cl.
exchange and transportation. The redox potential measurements in solutions with different rations of Fe2+ and Fe3+ have been carried out. The results can be seen in Fig. 7. The electrodes from the known bronzes S 3 and S 4 (lines 4 and 5) show a very good response to the changing redox potential. Surprisingly, it is nearly the same voltage like this obtained on the platinum electrode (line 1). Only a little difference (about 2– 5 mV) could be observed. But the electrodes from the new bronzes S 1 and S 2 (lines 2 and 3) show no significant reaction to the changing measure-
ment solution. That encourages the application of these materials in solid state reference electrodes. Cyclic voltammograms confirm the results. Fig. 8 represents the measuring results from the known bronze S 3 and the new bronze S 1 in comparison to the platinum electrode. The oxidation and reduction peaks of the electrode S 3 are recognisable analogue to platinum but with lower amplitude. However, the curve of the new bronze electrode S 1 possess no observable peaks. This result approve the findings from Fig. 7. Because of this, it would appear that the electron
Fig. 7. Measurement of the redox potential in solutions of various ratios of Fe(II)/Fe(III) vs. Ag/AgCl, Cl.
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J. Gabel et al. / Solid State Ionics 169 (2004) 75–80
Fig. 8. Cyclic voltammograms of a Na0.9Mo6O17 (S 3) and LixMo0.95W0.05O3 (S 1) in comparison to Pt in 0.1 M FeCl2 (50 mV/s).
exchange reaction on the new bronzes is disabled too. However, this mechanism is not clear too.
4. Conclusion/outlook It was demonstrated that polycrystalline samples of tungsten substituted lithium molybdenum oxide bronzes can be used as material for solid state reference electrodes. These electrodes show no response to changing pH, Na+ concentration and redox potential. The exact mechanisms on this electrodes are not clear at the present. The next technological step will be the advancement of the electrode fabrication getting low resistance electrodes. Acknowledgements Financial support of this work from Humboldt Foundation is gratefully acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
J.V. Dobson, T. Dickinson, GB-Pat. 1,597,493 (1977). J.V. Dobson, J. Comer, J. Electroanal. Chem. 220 (1987) 225. H. Kohler, W. Go¨pel, Sens. Actuators B 4 (1991) 345. P. Shuk, U. Guth, M. Greenblatt, J. Solid State Electrochem. 6 (2002) 374. P. Shuk, M. Greenblatt, K.V. Ramanujachary, Solid State Ionics 91 (1996) 233. M. Greenblatt, P. Shuk, K.V. Ramanujachary, US-Pat. 6,015,481 (2000). P. Shuk, K.V. Ramanujachary, M. Greenblatt, Electrochim. Acta 41 (1996) 2055. R. Lukowski, U. Guth, O. Scha¨f, DE-Pat. 19,823,056 A1 (1999). H. Kaden, W. Vonau, J. Prakt. Chem. 340 (1998) 710. M. Greenblatt, Chem. Rev. 88 (1988) 31. K. Schwabe, Fortschritte der pH-Messtechnik, Berlin, (1958) p. 287. D.D. Perrin, B. Dempsey, Buffers for pH and Metal Ion Control, Chapman and Hall, London, 1974, p. 17.
New reference electrodes based on tungsten-substituted molybdenum bronzes J. Gabel a,*, W. Vonau a, P. Shuk b, U. Guth a,c a
Kurt-Schwabe-Institute for Measuring and Sensor Technology Meinsberg, D-04720, Ziegra-Knobelsdorf, Germany b Emerson Process Management, Rosemount Analytical Inc., 1201 North Main, St. Orrville, OH 44667, USA c Institute of Physical Chemistry and Electrochemistry, Dresden University of Technology, D-01062 Dresden, Germany Received 20 January 2003; received in revised form 6 August 2003; accepted 6 August 2003
Abstract New reference electrodes have been developed and tested by a solid state contact method. The performance was demonstrated at ambient temperature with polycrystalline powders from tungsten-substituted alkali molybdenum bronzes with the general formula AxMo1yWyO3. These electrodes show no significant response to the changing of pH, cation concentration and redox potential of the measuring solution. Therefore, it is possible to use them as reference electrode for electrochemical measurements. D 2003 Elsevier B.V. All rights reserved. Keywords: Oxide bronzes; pH sensitivity; Solid state reference electrode
1. Introduction Using transition metal oxide bronzes for ion selective electrodes in aqueous solutions is known. It has been reported in detail by Dobson [1], Dobson and Comer [2], Kohler and Go¨pel [3], Shuk et al. [4,5,7] and Greenblatt et al. [6]. Electrodes made of bronzes have been found suitable as sensors for measurements of pH, sodium concentration, redox potential, dissolved oxygen and EDTA titration. However, there are restrictions because of cross sensitivities to univalent cations. Some bronzes with bivalent cations, especially the barium tungsten bronzes BaxWO3, have been tested as material for reference electrodes [8] without success because of potential instability. Conventional reference electrodes are mostly electrodes of the second kind: a metallic phase in equilibrium with its sparingly soluble metal salt in an aqueous electrolyte. Therefore, the measurement range of these electrodes as regards temperature and pressure is strongly restricted. Electrochemical solid state electrodes might help to overcome some of the disadvantages of conventional glass electrodes or liquid reference electrodes such as fragility, * Corresponding author. Tel.: +49-343276080; fax: +49-34327608131. E-mail address: [email protected] (J. Gabel). 0167-2738/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2003.08.001
temperature and pressure limitation. Most important is the absence of any liquid system component like the inner electrolytic solution. So it is easy to make planar or miniaturised sensors. Also, the measurement at higher temperature and in wide pressure range as well as the measurement in non aqueous solutions is possible with low maintenance. The problem of a solid state reference electrode is not resolved satisfactory up to the present [9].
2. Experimental The bronzes have been synthesised by solid state reaction under inert conditions. The stoichiometric quantities of the reactants were weighed out according to the following equations: nA2 MoO4 þ 2ð1 n xÞMoO3 þ nMoO2 þ 2xWO3 ! 2An Mo1x Wx O3
ð1Þ
3nA2 WO4 þ 3ð3 2n 3xÞWO3 þ nW þ 6xMoO3 ! 6An W1x Mox O3 ;
ð2Þ
with A=Na, Li, K and n, xV1. The charge size was maintained at 10 g. The reactants have been mixed carefully and filled into a quartz tube. The
76
J. Gabel et al. / Solid State Ionics 169 (2004) 75–80
tube was evacuated and sealed. A special heat treatment was carried out: to begin an initial heating on 500 jC for several days was followed by the heating up to the reaction temperature between 600 and 1000 jC depending on the composition. The runs were terminated by a controlled cooling. In this way, single crystals as well as polycrystalline materials were obtained. After synthesis, the products were alternate washed with hot diluted hydrochloric acid, carbonate solution and water. The phases were characterised and identified by X-ray powder diffraction using a Bruker axs D8 advance. Fig. 1a shows the powder XRD pattern of the LixMo0.95W0.05O3 compared to Li0.3WO3 (cubic) and Li0.3MoO3 (monoclinic). There is no congruence between the patterns. In Fig. 1b, the powder XRD pattern of different charges of LixMo0.95W0.05O3 can be seen. The synthesis of the compound is reproducible. The differences in intensity of the peaks are based on the needled structure of the bronze. The exact content of Li could not be detected but x is assumed between 0.3 and 0.4 caused by the colour. The value of W was determined by EDX. A precise chemical
analytic of the compound was not possible until now because of the high chemical resistance of the substance. The polycrystalline samples were preferred for electrode fabrication because it was not possible every time to get single crystals in the requested size and quality. The electrodes were produced by mixing carefully ground powders with unsaturated polyester resin. The ratio powder/resin was chosen in that way that the mixture could be casted into silicon moulds. The moulds have been centrifuged to get a high particle density on one end. From that end, discs have been cut of. These discs were contacted with a copper disc or a platinum wire by a conductive adhesive. For insulation and stability the purpose entire electrode assembly was coated by an epoxy resin. For better handling, it was stuck into a glass tube. In some cases, it was necessary to abrade the surface of the disc. Unfortunately, all the electrodes showed a relative high electrical resistance (about 1– 500 MV). But this was the only way to get stable electrodes from the polycrystalline samples. These electrodes show results in good agreement with single crystal electrodes. We
Fig. 1. Powder XRD pattern of LixMo0.95W0.05O3: (a) compared to Li0.3WO3 and Li0.3MoO3; (b) of different charges of LixMo0.95W0.05O3.
J. Gabel et al. / Solid State Ionics 169 (2004) 75–80
77
Fig. 2. Crystal structure of NaxWO3 (generated by PowderCell 2.2; atomic coordinates from the ICSD, No. 1949).
were not able to get reproducible results with electrodes using thick film technology so far.
3. Results and discussion
phases are wide range nonstoichiometric compounds all form with perovskite-like structure (Fig. 2) [4]. The molybdenum oxide bronzes are stoichiometric or nearly so. The structure can be described as a channel system of sheets of MoO6 octahedrons. The sheets are held together by the inserted cations [10]. Fig. 3 shows the crystal structure of a
The potentiometric characteristics of the bronze electrodes were evaluated by measuring their emf against standard saturated silver/silver chloride electrode. All measurements were carried out on 25 jC. Table 1 gives the electrode abbreviations and the summary about the electrode response. The reasons for the behaviour of the bronzes as electrode material have been studied intensively in the recent years. It is assumed that the pH and other ion sensing properties of the transition metal oxide bronze electrodes depend on the crystal structure. In general, bronzes may be regarded as a solid solution of the cations in the MO3 lattice. The AxWO3 Table 1 List of the electrodes Formula
Abbreviation
pH
Redox potential
LixMo0.95W0.05O3 LixMo0.95W0.05O3 Na0.9Mo6O17
S1 S2 S3
no response no response f 76 mV/pH
Li0.3MoO3
S4
f 46 mV/pH
no response no response response like Pt electrode response like Pt electrode
Fig. 3. Crystal structure of K0.3MoO3 [3].
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J. Gabel et al. / Solid State Ionics 169 (2004) 75–80
Fig. 4. Schematic view on ion exchange [3].
potassium molybdenum oxide bronze as an example. These structures permit the exchange and intercalation of cations on the surface. The pH sensing characteristics of the bronze electrodes are based on the ion exchange reactions. In Fig. 4, the simplified reaction mechanism can be seen. The reaction on the surface is believed to the following equations [7]: Ax My Oz þ yH þ þ ye ! Ax Hd My Oz
ð3Þ
Ax My Oz þ yH þ ! Axd Hd My Oz þ yAþ ;
ð4Þ
with A=Li, Na, K and M=W, Mo.
To adjust stoichiometric imbalances caused by the cations in the MO3 lattice, the transition metals take different valence states (usually V and VI). So an electron exchange and transportation by electron hopping is possible too. However, it is assumed that the sensing properties will be significantly different from the bulk structure. Fig. 5 shows the results of a measurement in buffer solutions with different pH values. The electrodes made from the well known bronzes S 3 (line 4) and S 4 (line 5) show a good response (S 3: 71 mV/pH, S 4: 52 mV/ pH) to the changing pH. But the electrodes S 1 and S 2 show pH-independent potential in wide pH range like a reference electrode. The same results could be observed at other pH measurements. Therefore, an acidic solution (0.04 M on boric, acetic and phosphoric acid) was titrated with a 0.2 M sodium hydroxide solution (Fig. 6). The slopes of the electrodes S 3 (line 4) and S 4 (line 5) are lower than in Fig. 5 (S 3: 59 mV/pH, S 4: 31 mV/pH) because of the influence of the increasing Na+ concentration. It was intensively reported about the cross sensitivities and the influence of univalent cations on bronze electrodes elsewhere in the past [4,5,7]. The new electrodes (S 1 and S 2) show no significant change of the potential. Only a small drift was observed (about 5– 10 mV). Consequence of this, there is no response to the changes of pH and Na+ concentration. That suggest the possibility of a new solid state reference electrode. This behaviour differs from the theory of cation exchange and intercalation. Presumed, this mechanism is prevented. The exact mechanism is not clear at the present. The response to the changing redox potential of the transition metal oxide bronze electrodes depends on electron
Fig. 5. Measurement in different pH solutions (system Na2HPO4/C6H8O7 [11]) vs. Ag/AgCl, Cl.
J. Gabel et al. / Solid State Ionics 169 (2004) 75–80
79
Fig. 6. Titration of a 0.04 M acetic solution by 0.2 M NaOH [12] vs. Ag/AgCl, Cl.
exchange and transportation. The redox potential measurements in solutions with different rations of Fe2+ and Fe3+ have been carried out. The results can be seen in Fig. 7. The electrodes from the known bronzes S 3 and S 4 (lines 4 and 5) show a very good response to the changing redox potential. Surprisingly, it is nearly the same voltage like this obtained on the platinum electrode (line 1). Only a little difference (about 2– 5 mV) could be observed. But the electrodes from the new bronzes S 1 and S 2 (lines 2 and 3) show no significant reaction to the changing measure-
ment solution. That encourages the application of these materials in solid state reference electrodes. Cyclic voltammograms confirm the results. Fig. 8 represents the measuring results from the known bronze S 3 and the new bronze S 1 in comparison to the platinum electrode. The oxidation and reduction peaks of the electrode S 3 are recognisable analogue to platinum but with lower amplitude. However, the curve of the new bronze electrode S 1 possess no observable peaks. This result approve the findings from Fig. 7. Because of this, it would appear that the electron
Fig. 7. Measurement of the redox potential in solutions of various ratios of Fe(II)/Fe(III) vs. Ag/AgCl, Cl.
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J. Gabel et al. / Solid State Ionics 169 (2004) 75–80
Fig. 8. Cyclic voltammograms of a Na0.9Mo6O17 (S 3) and LixMo0.95W0.05O3 (S 1) in comparison to Pt in 0.1 M FeCl2 (50 mV/s).
exchange reaction on the new bronzes is disabled too. However, this mechanism is not clear too.
4. Conclusion/outlook It was demonstrated that polycrystalline samples of tungsten substituted lithium molybdenum oxide bronzes can be used as material for solid state reference electrodes. These electrodes show no response to changing pH, Na+ concentration and redox potential. The exact mechanisms on this electrodes are not clear at the present. The next technological step will be the advancement of the electrode fabrication getting low resistance electrodes. Acknowledgements Financial support of this work from Humboldt Foundation is gratefully acknowledged.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
J.V. Dobson, T. Dickinson, GB-Pat. 1,597,493 (1977). J.V. Dobson, J. Comer, J. Electroanal. Chem. 220 (1987) 225. H. Kohler, W. Go¨pel, Sens. Actuators B 4 (1991) 345. P. Shuk, U. Guth, M. Greenblatt, J. Solid State Electrochem. 6 (2002) 374. P. Shuk, M. Greenblatt, K.V. Ramanujachary, Solid State Ionics 91 (1996) 233. M. Greenblatt, P. Shuk, K.V. Ramanujachary, US-Pat. 6,015,481 (2000). P. Shuk, K.V. Ramanujachary, M. Greenblatt, Electrochim. Acta 41 (1996) 2055. R. Lukowski, U. Guth, O. Scha¨f, DE-Pat. 19,823,056 A1 (1999). H. Kaden, W. Vonau, J. Prakt. Chem. 340 (1998) 710. M. Greenblatt, Chem. Rev. 88 (1988) 31. K. Schwabe, Fortschritte der pH-Messtechnik, Berlin, (1958) p. 287. D.D. Perrin, B. Dempsey, Buffers for pH and Metal Ion Control, Chapman and Hall, London, 1974, p. 17.