- Email: [email protected]
A fast response membrane-based pH indicator optode
0039-9140/93 s6.00+ 0.00 FergamonRed8Ltd
Talanla.Vol. 40, No. 5, pp. 765-168,1993 Printedin Great Britain. All rights reserved
A FAST RESPONSE MEMBRANE-BASED OPTODE
pH INDICATOR
THENCE J. CARDWELL,ROBERTW.
CAITRALL, LESLIEW. DEADY,* MARIA DORKOSand GREOORYR. O’CONNELL
Centre for Scientific Instrumentation, La Trobe University, Bundoora, Victoria u)83, Australia (Received 15 June 1992. Revised 11 August 1992. Accepted 12 August 1992) Sunmary-A cellulose acetate based optode membrane containing 4-dimethylamino-4’-octylazobenzene as an acid-base indicator is described. Other essential components of the membrane for a fast-responding and durable sensor are diethyl phthalate, triethyleneglycol and potassium tetrakis@chlorophenyl)borate. Factors affecting the sensor behaviour are discussed and an application in a flow-cell is demonstrated.
There is much current interest in the development of optical sensors (optodes) for a variety of analytical purposes. For example, a number of indicators have been immobilized in polymer films and used as pH sensors.’ A considerable amount of work has dealt with poly(viny1 chloride) based membranes and Morf and coworkers have described the mechanisms which lead to successful optical sensors2 However, PVC based membranes tend to suffer from a slow response time, in the order of seconds to minutes, which is not too different from their potentiometric counterparts. A rapid colour change is essential if a fIhn is to be used in an acid/base titration and, for repeated use, the indicator must not leach out of the membrane. Among the most successful attempts at achieving a fast response for optode membranes has been the incorporation of an unmodified “direct dye” such as Congo Red into a modified cellulose acetate lYm.3 Our work in this field is related in that cellulose acetate was found to be the best polymer from which to construct a water-permeable membrane, but we have modified a standard indicator to make it more polymer-compatible and less prone to leaching in aqueous solutions. We have also developed a simple formulation for a membrane sensor which has a fast response to pH change and which does not require an ultra-thin film. This paper reports studies on factors critical to the response time and the use of the optode in a flow system.
*Author for correspondence.
EXPERIMENTAL
4-Dimethylamino-I’-octylazobenzene A solution of sodium nitrite (0.12 g) in water (1 ml) was added, dropwise with stirring, to a solution of 4octylaniline (0.25 g) in water (3 ml) and concentrated hydrochloric acid (1.2 ml) at < 5”. After 15 min at 5”, this was added, slowly with stirring, to a solution containing an excess of k,N,N-dimethylaniline and sodium acetate in aqueous ethanol. The yellow solid which separated was filtered, washed with water, and recrystallized from acetonitrile to give orange-yellow flakes, m.p. 80-81” (found: C, 78.6; H, 9.4; N, 12.7. CZZH3,N3 requires C, 78.3; H, 9.2; N, 12.5%). Construction of membrane A typical composition was (weight %): cellulose acetate (Aldrich-acetate content 39.8%), 50; diethyl phthalate, 10; triethylene glycol, 34.7; dye, 0.3; sodium tetraphenylborate, 5. This mixture (100 mg) was dissolved in ca. 2 ml of tetrahydrofuran (which was purified by passage through a short alumina column), a coating applied to the appropriate surface and the solvent allowed to completely evaporate. In some circumstances, the membrane was heated briefly to improve the transparency. Coated in this way were a glass microscope slide and plastic coated stirrer bar (for qualitative testing), the inside of a normal l-cm quartz cuvette (for visible spectroscopy), and the walls of a 14-mm x 2.3~mm diameter tunnel (roughened with a diamond drill to improve adhesion of the plastic) in a 7.5~mm diameter glass cylinder (for use in the flow-cell). The membrane when separated from 765
TERENCE J. CARDWELL
166
a glass surface was quite robust. The thickness of the membrane in both the glass cylinder and on a flat glass surface was typically ca. 50 pm, measured by micrometer, which is considerably thicker than the fast response examples of Porter.3 Spectroscopy The membrane coated cuvette was used and gave I, values of 420 nm (free base) and 545 nm (conjugate acid). A standard spectroscopic technique4 was applied to determine the pK of the dye in this membrane. In spite of a nonuniform coating, the membrane gave useable values within + 1 pH unit of the pK, determined as 3.8. This compares with the value of 2.96 reported by Yeh and Jaffe,’ obtained in 25% ethanol-sulphuric acid for the parent dimethylaminoazobenzene. The values reported by Yeh and Jaffe are not true p& values, since tautomeric forms are possible for the protonated species.
et al.
3. During early fabrication attempts it was noted that, if the membrane retained any of the water miscible tetrahydrofuran used to initially dissolve the components, the response time was much reduced. This essential feature has been incorporated in the final formulation by including the non-volatile triethylene glycol as “wetting-agent”. About 10% by weight seemed to be necessary and up to 50% was used. 4. This first study has only considered a single indicator (I) which is a simple azo-dye to which is attached the long-chain octyl group. This was prepared by standard chemistry and the compound at 0.3% was miscible with the polymer and did not leach from the membrane on repeated use.
Flow -cell measurements The Discontinuous Flow Analyser used has been described previously.6 In this work, the indicator application was demonstrated by the titration of O.lM sodium hydroxide or potassium hydroxide with 0.M hydrochloric acid, the membrane coated glass cylinder (i.d., 2.3 mm) providing the flow channel and the optical window for a yellow LED (emission A,,,, = 580 nm). RESULTS AND DISCUSSION
Composition of the sensor There are five necessary components of the successful sensor: 1. Cellulose acetate was the polymer support of choice for making a water-permeable membrane. Cellulose triacetate, polyvinyl acetate, polyvinyl pyrrolidone, and polyvinyl chloride were much less satisfactory. A noteworthy feature was that the optode worked with much thicker membranes than are normally specified for sensors, so that casting the membrane required no special technique. 2. Diethyl phthalate (S-20%) was included as plasticizer and this improved the transparency of the membrane. Slow drying, as from inside a cuvette, gave a more transparent membrane; one cast on a glass slide was opaque initially but gentle heat for a short time gave a clear membrane.
5. For this dye, which goes from a yellow neutral form in base to a red cationic form in acid, it was essential to have an ion-balance reagent (IBR) present and sodium tetraphenylborate (NaTPB) and potassium tetrakis(4chlorophenyl)borate (KTKB) were each used. Without the IBR, no reaction occurred in aqueous hydrochloric acid. To maintain ionic balance in these conditions, it is necessary for an anion to be taken into the membrane with the proton and it was evident that the hydrophilic chloride anion would not transport along with the proton into this membrane. The IBR provided mobile metal cations in the membrane to exchange with the incoming protons to overcome this problem. A molar excess (with respect to the dye) was necessary for rapid response. However, too much gave an opaque/crystalline membrane, and 5% by weight (ca. IO-fold molar excess) was a good working amount. It emerged that the response time was affected by the nature of the IBR reagent, as described below. The membranes with NaTPB as the IBR, though thicker than those reported previously,3 were characterized by a rapid response to pH change in either direction. For a coated slide immersed in the appropriate solution, the colour change to red was “instant” and to yellow, marginally slower, but too fast to measure quantitatively in a batch type experiment. A more quantitative answer is available
A fast response membrane-based pH indicator optode
from flow-cell experiments (below). The colour change could be quantitatively related to pH by visible spectroscopy only over cu. a 2-unit range about the pK value of 3.8. Thus, this dye is more appropriate for an indicator of pH change rather than for a measure of pH value. Flow-cell application The application of this pH-optode to use in a discontinuous flow analysis system6 was demonstrated. The optode membrane (cu. 50 pm thick) was incorporated into the flow-cell of the instrument as described in the experimental section. Figure l(a) shows a typical DFA acid-base titration curve for a membrane with NaTPB as the IBR. At A, the liquid in the flow-cell is O.lM hydrochloric acid. At B, the titration cycle of the instrument provides a rapid flush of O.lM sodium hydroxide and the indicator immediately turns yellow. The actual titration begins at
Relative
Cam Position
Fig. 1. DFA titration of 0.1M HCl with base. (a) Base, 0. 1M NaOH; IBR, NaTPB. (b) Base, O.lM NaOH; IBR, KTKB. (c) Base, O.lM KOH; IBR, KTKB. The time-bars relate to the instrument cam speed.
761
C as acid is added and the observable colour change back to red commences at about D. The end-point E is indicated by the rapid change in the detector signal and is defined in the DFA instrumental method by taking the first derivative. The times for the colour changes can be estimated from the time-bar in the figure, which refers to the speed of rotation of the instrument cam. These times are virtually identical to the case where a water-soluble indicator is introduced separately into the flowing stream.6 The behaviour is ideal for the DFA application, which requires a rapid colour change at the end-point. Unfortunately, NaTPB as IBR slowly leached from the membrane and the optode ceased to function after about ten titration cycles (though the coated glass slide lasted many more batch changes of acid and base, probably because there is much less agitation of the membrane surface than in the DFA cell). This problem was overcome by the use of the less hydrophilic KTKB as IBR and there was no perceptible deterioration in the membrane performance after more than forty cycles. However, with sodium hydroxide as the base, there was now an unacceptably slow response for the acid to base change (but no effect on the reverse). This is seen in Fig. l(b) as a very slow rise, A, in the detector signal to the maximum, B, which is reached just before the change back to the acid form. Clearly, the movement of the various ions involved in the indicator colour change, across the membrane/solution interface, is a complex process. The tetrakis(4chlorophenyl)borate anion apparently has a lower affinity for a sodium ion in the membrane than does the unsubstituted tetraphenylborate anion and the replacement of a proton by a sodium ion is slower. The situation, however, is also cation dependent and, when potassium hydroxide was used as the base, the response times in both directions were quite acceptable. The membrane was stable indefinitely and Figure l(c) shows the titration sequence in these conditions after seventy cycles. This DFA application represents an advance on the previous example using photometric end-point detection, where an indicator had to be added in reasonably high concentrations to the flow system.6 Compound I, while being a quite satisfactory indicator for a strong acidstrong base titration, has limited potential because of its low pK, value. We are currently investigating compounds applicable to other pH
'l-msm J. CARDWELL eta/.
168
regions and which will be compatible with the polymer system. Acknowledgements-We thank the Australian Council for fhlancial support.
Research
REFERENCES 1. S. M. Stole, T. P. Jones, L.-K. Chau and M. D. Porter, in Chemical Sensors and Microinstrumentation, R. W. Murray, R. E. Dessy, W. R. Heineman, J. Janata, W. R. Seitz (eds), Chap. 19, ACS, Washington DC, 1989.
2. W. E. Morf, K. Se&x, B. L&mann, Ch. B&ringer. S. Tan, K. Hartman. P. R. !bensen and W. Simon, in Zon-selective Electrodes, 5, E. Pungor (cd.), p. 115. Pergamon Press, oxford, 1989. 3. T. P. Jones and M. D. Porter, Anal. C/tern.. 1988,60, 404. 4. A. Albert and E. P. Sejeant, Zonization Constants 01 Acti and Bases, Methuen, London. 1962. 5. S. J. Yeh and H. H. Jaff&, J. Am. Gem. Sot., 1959.81, 3283. 6. T. J. Cardwell, R. W. Cattrall, G. J. Cross, G. R. O’Connell, J. D. Petty and G. R. Scollary, Analyst, 1991, 116, 1051.
Talanla.Vol. 40, No. 5, pp. 765-168,1993 Printedin Great Britain. All rights reserved
A FAST RESPONSE MEMBRANE-BASED OPTODE
pH INDICATOR
THENCE J. CARDWELL,ROBERTW.
CAITRALL, LESLIEW. DEADY,* MARIA DORKOSand GREOORYR. O’CONNELL
Centre for Scientific Instrumentation, La Trobe University, Bundoora, Victoria u)83, Australia (Received 15 June 1992. Revised 11 August 1992. Accepted 12 August 1992) Sunmary-A cellulose acetate based optode membrane containing 4-dimethylamino-4’-octylazobenzene as an acid-base indicator is described. Other essential components of the membrane for a fast-responding and durable sensor are diethyl phthalate, triethyleneglycol and potassium tetrakis@chlorophenyl)borate. Factors affecting the sensor behaviour are discussed and an application in a flow-cell is demonstrated.
There is much current interest in the development of optical sensors (optodes) for a variety of analytical purposes. For example, a number of indicators have been immobilized in polymer films and used as pH sensors.’ A considerable amount of work has dealt with poly(viny1 chloride) based membranes and Morf and coworkers have described the mechanisms which lead to successful optical sensors2 However, PVC based membranes tend to suffer from a slow response time, in the order of seconds to minutes, which is not too different from their potentiometric counterparts. A rapid colour change is essential if a fIhn is to be used in an acid/base titration and, for repeated use, the indicator must not leach out of the membrane. Among the most successful attempts at achieving a fast response for optode membranes has been the incorporation of an unmodified “direct dye” such as Congo Red into a modified cellulose acetate lYm.3 Our work in this field is related in that cellulose acetate was found to be the best polymer from which to construct a water-permeable membrane, but we have modified a standard indicator to make it more polymer-compatible and less prone to leaching in aqueous solutions. We have also developed a simple formulation for a membrane sensor which has a fast response to pH change and which does not require an ultra-thin film. This paper reports studies on factors critical to the response time and the use of the optode in a flow system.
*Author for correspondence.
EXPERIMENTAL
4-Dimethylamino-I’-octylazobenzene A solution of sodium nitrite (0.12 g) in water (1 ml) was added, dropwise with stirring, to a solution of 4octylaniline (0.25 g) in water (3 ml) and concentrated hydrochloric acid (1.2 ml) at < 5”. After 15 min at 5”, this was added, slowly with stirring, to a solution containing an excess of k,N,N-dimethylaniline and sodium acetate in aqueous ethanol. The yellow solid which separated was filtered, washed with water, and recrystallized from acetonitrile to give orange-yellow flakes, m.p. 80-81” (found: C, 78.6; H, 9.4; N, 12.7. CZZH3,N3 requires C, 78.3; H, 9.2; N, 12.5%). Construction of membrane A typical composition was (weight %): cellulose acetate (Aldrich-acetate content 39.8%), 50; diethyl phthalate, 10; triethylene glycol, 34.7; dye, 0.3; sodium tetraphenylborate, 5. This mixture (100 mg) was dissolved in ca. 2 ml of tetrahydrofuran (which was purified by passage through a short alumina column), a coating applied to the appropriate surface and the solvent allowed to completely evaporate. In some circumstances, the membrane was heated briefly to improve the transparency. Coated in this way were a glass microscope slide and plastic coated stirrer bar (for qualitative testing), the inside of a normal l-cm quartz cuvette (for visible spectroscopy), and the walls of a 14-mm x 2.3~mm diameter tunnel (roughened with a diamond drill to improve adhesion of the plastic) in a 7.5~mm diameter glass cylinder (for use in the flow-cell). The membrane when separated from 765
TERENCE J. CARDWELL
166
a glass surface was quite robust. The thickness of the membrane in both the glass cylinder and on a flat glass surface was typically ca. 50 pm, measured by micrometer, which is considerably thicker than the fast response examples of Porter.3 Spectroscopy The membrane coated cuvette was used and gave I, values of 420 nm (free base) and 545 nm (conjugate acid). A standard spectroscopic technique4 was applied to determine the pK of the dye in this membrane. In spite of a nonuniform coating, the membrane gave useable values within + 1 pH unit of the pK, determined as 3.8. This compares with the value of 2.96 reported by Yeh and Jaffe,’ obtained in 25% ethanol-sulphuric acid for the parent dimethylaminoazobenzene. The values reported by Yeh and Jaffe are not true p& values, since tautomeric forms are possible for the protonated species.
et al.
3. During early fabrication attempts it was noted that, if the membrane retained any of the water miscible tetrahydrofuran used to initially dissolve the components, the response time was much reduced. This essential feature has been incorporated in the final formulation by including the non-volatile triethylene glycol as “wetting-agent”. About 10% by weight seemed to be necessary and up to 50% was used. 4. This first study has only considered a single indicator (I) which is a simple azo-dye to which is attached the long-chain octyl group. This was prepared by standard chemistry and the compound at 0.3% was miscible with the polymer and did not leach from the membrane on repeated use.
Flow -cell measurements The Discontinuous Flow Analyser used has been described previously.6 In this work, the indicator application was demonstrated by the titration of O.lM sodium hydroxide or potassium hydroxide with 0.M hydrochloric acid, the membrane coated glass cylinder (i.d., 2.3 mm) providing the flow channel and the optical window for a yellow LED (emission A,,,, = 580 nm). RESULTS AND DISCUSSION
Composition of the sensor There are five necessary components of the successful sensor: 1. Cellulose acetate was the polymer support of choice for making a water-permeable membrane. Cellulose triacetate, polyvinyl acetate, polyvinyl pyrrolidone, and polyvinyl chloride were much less satisfactory. A noteworthy feature was that the optode worked with much thicker membranes than are normally specified for sensors, so that casting the membrane required no special technique. 2. Diethyl phthalate (S-20%) was included as plasticizer and this improved the transparency of the membrane. Slow drying, as from inside a cuvette, gave a more transparent membrane; one cast on a glass slide was opaque initially but gentle heat for a short time gave a clear membrane.
5. For this dye, which goes from a yellow neutral form in base to a red cationic form in acid, it was essential to have an ion-balance reagent (IBR) present and sodium tetraphenylborate (NaTPB) and potassium tetrakis(4chlorophenyl)borate (KTKB) were each used. Without the IBR, no reaction occurred in aqueous hydrochloric acid. To maintain ionic balance in these conditions, it is necessary for an anion to be taken into the membrane with the proton and it was evident that the hydrophilic chloride anion would not transport along with the proton into this membrane. The IBR provided mobile metal cations in the membrane to exchange with the incoming protons to overcome this problem. A molar excess (with respect to the dye) was necessary for rapid response. However, too much gave an opaque/crystalline membrane, and 5% by weight (ca. IO-fold molar excess) was a good working amount. It emerged that the response time was affected by the nature of the IBR reagent, as described below. The membranes with NaTPB as the IBR, though thicker than those reported previously,3 were characterized by a rapid response to pH change in either direction. For a coated slide immersed in the appropriate solution, the colour change to red was “instant” and to yellow, marginally slower, but too fast to measure quantitatively in a batch type experiment. A more quantitative answer is available
A fast response membrane-based pH indicator optode
from flow-cell experiments (below). The colour change could be quantitatively related to pH by visible spectroscopy only over cu. a 2-unit range about the pK value of 3.8. Thus, this dye is more appropriate for an indicator of pH change rather than for a measure of pH value. Flow-cell application The application of this pH-optode to use in a discontinuous flow analysis system6 was demonstrated. The optode membrane (cu. 50 pm thick) was incorporated into the flow-cell of the instrument as described in the experimental section. Figure l(a) shows a typical DFA acid-base titration curve for a membrane with NaTPB as the IBR. At A, the liquid in the flow-cell is O.lM hydrochloric acid. At B, the titration cycle of the instrument provides a rapid flush of O.lM sodium hydroxide and the indicator immediately turns yellow. The actual titration begins at
Relative
Cam Position
Fig. 1. DFA titration of 0.1M HCl with base. (a) Base, 0. 1M NaOH; IBR, NaTPB. (b) Base, O.lM NaOH; IBR, KTKB. (c) Base, O.lM KOH; IBR, KTKB. The time-bars relate to the instrument cam speed.
761
C as acid is added and the observable colour change back to red commences at about D. The end-point E is indicated by the rapid change in the detector signal and is defined in the DFA instrumental method by taking the first derivative. The times for the colour changes can be estimated from the time-bar in the figure, which refers to the speed of rotation of the instrument cam. These times are virtually identical to the case where a water-soluble indicator is introduced separately into the flowing stream.6 The behaviour is ideal for the DFA application, which requires a rapid colour change at the end-point. Unfortunately, NaTPB as IBR slowly leached from the membrane and the optode ceased to function after about ten titration cycles (though the coated glass slide lasted many more batch changes of acid and base, probably because there is much less agitation of the membrane surface than in the DFA cell). This problem was overcome by the use of the less hydrophilic KTKB as IBR and there was no perceptible deterioration in the membrane performance after more than forty cycles. However, with sodium hydroxide as the base, there was now an unacceptably slow response for the acid to base change (but no effect on the reverse). This is seen in Fig. l(b) as a very slow rise, A, in the detector signal to the maximum, B, which is reached just before the change back to the acid form. Clearly, the movement of the various ions involved in the indicator colour change, across the membrane/solution interface, is a complex process. The tetrakis(4chlorophenyl)borate anion apparently has a lower affinity for a sodium ion in the membrane than does the unsubstituted tetraphenylborate anion and the replacement of a proton by a sodium ion is slower. The situation, however, is also cation dependent and, when potassium hydroxide was used as the base, the response times in both directions were quite acceptable. The membrane was stable indefinitely and Figure l(c) shows the titration sequence in these conditions after seventy cycles. This DFA application represents an advance on the previous example using photometric end-point detection, where an indicator had to be added in reasonably high concentrations to the flow system.6 Compound I, while being a quite satisfactory indicator for a strong acidstrong base titration, has limited potential because of its low pK, value. We are currently investigating compounds applicable to other pH
'l-msm J. CARDWELL eta/.
168
regions and which will be compatible with the polymer system. Acknowledgements-We thank the Australian Council for fhlancial support.
Research
REFERENCES 1. S. M. Stole, T. P. Jones, L.-K. Chau and M. D. Porter, in Chemical Sensors and Microinstrumentation, R. W. Murray, R. E. Dessy, W. R. Heineman, J. Janata, W. R. Seitz (eds), Chap. 19, ACS, Washington DC, 1989.
2. W. E. Morf, K. Se&x, B. L&mann, Ch. B&ringer. S. Tan, K. Hartman. P. R. !bensen and W. Simon, in Zon-selective Electrodes, 5, E. Pungor (cd.), p. 115. Pergamon Press, oxford, 1989. 3. T. P. Jones and M. D. Porter, Anal. C/tern.. 1988,60, 404. 4. A. Albert and E. P. Sejeant, Zonization Constants 01 Acti and Bases, Methuen, London. 1962. 5. S. J. Yeh and H. H. Jaff&, J. Am. Gem. Sot., 1959.81, 3283. 6. T. J. Cardwell, R. W. Cattrall, G. J. Cross, G. R. O’Connell, J. D. Petty and G. R. Scollary, Analyst, 1991, 116, 1051.