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Potentiometric titrations in non-conducting solutions with an interfacial antimony electrode
Analytic
Chimica Acta, 132 (1981) 229-233 Ekevier Scientific Publishing Company, Amsterdam -
Printed in The Netherlands
Short Communication POTENTIOMETRIC TITRATIONS IN NON-CONDUCTING WITH AN INTERFACIAL ANTIMONY ELECTRODE
BOLESLAW Department Jagellonian
WALIG6RA*
and MARIA
SOLUTIONS
PALUCH
of Physical Chemistry and Electrochemistry University, 3 Karasia Street, 30-060 Kmcow
of the Institute (Poland)
of Chemistry.
(Received 27th April 1981) The micro interfacial device described is based on an antimony/antimony scratch/ 0.1 M KCl/Hg,CI,/Pt cell. Acid-base reactions occurring in solvents such as dry ligroin, benzene, cyclohexane, dichloroethane, chloroform and pentyl acetate are readily monitored. Summary.
Potentiometric titration in non-aqueous media normally affords positive results only in conducting solvents. In the case of solvents which do not exhibit electric conductance (e.g., hydrocarbons), conventional methods fail because the indicator electrode does not show the characteristic potential changes necessary for quantitative interpretation. However, the interfacial voltaic cell described previously [l] enables acid-base reactions to be monitored in non-conducting media. The inter-facialvoltaic cell discussed here is a development of the microadsorption electrode of Kamieriski et al. [2] which was used initially as a potentiometric detector in chromatography and for atmospheric contaminants [3]. The special construction of that device made it possible to monitor the course of chemical reactions at the phase boundary of two immiscible liquids. The form of the interfacial cell described in the present communication can be immersed in the solutions_ Experimental Measuring system.
The cell used is shown schematically in Fig. 1. It consists of a calomel (CE) and an antimony (AME) electrode with a salt bridge (SB) filled with saturated or 0.1M potassium chloride connecting the two electrodes. After the electrodes have been inserted into ground-glass joints I and II of the bridge, a compact micro cell is obtained (Fig. 1, MC). The groundglass joints facilitate disconnection of the set and cleaning of the electrodes, an operation necessary in serial measurements. The essential part of the interfacial cell is a metallic scratch on the disc of porous corundum (Fig. 1, III) which closes the outlet of the salt bridge. The corundum disc (5-mm diameter and 3-mm thick) is connected to the broadened end of the bridge (SB). The pointed end of the antimony electrode (OS-mm diameter) contacts the external surface of the porous c&undum at point IV and is connected with the reference electrode through the salt bridge. 0003-2670/81/0000-0000/$02_50
0 1981 Elsevier Scientific Publishing Company
230
CE
MC
Fig. 1. The interfacial micro cell (immersible form): CE, calomel electrode; AME. antimony microelectrode; SB, salt bridge; I and II, ground-glass joints for inserting the electrodes; III, corundum disc; MC, assembled cell; IV, working surface of the cell.
Direct contact of +he two electrodes is obviously unnecessary in conducting solutions, but when non-conducting media are used (e.g., gases or pure hydrocarbons), electric conductance must be ensured somehow. The metallic scratch formed on the porous corundum disc wetted by the potassium chloride solution acts as the conductor in nonconducting systems. Before each titration, the cell is disconnected and cleaned, and the surface of the corundum disc is scratched with chemically pure antimony. Construction
and preparation
of the indicator antimony
microelectrode.
The microelectrodes used were constructed by fusing antimony into Supremax glass capillary tubes under vacuum. Figure 2 shows the furnace arrangement (B) and the capillary tube construction (A)_ Powdered antimony metal is poured into a capillary tube (A, ca. 0.5 mm i-d.) fused at its lower end, and then air is evacuated in order to obtain avacuum of lo-*-lo4 mm Hg. The capillary tube is closed at the narrow part (Fig. 2, I). The closed capillary tube is placed in the electric furnace (Fig. 2, B) and heated to the melting point of antimony (630°C); the closed end (Fig. 2, II) of t.he tube is then broken off. Air coming into the tube forces the fused antimony into the elongated part of the capillary tube (Fig. 2, III). The needle-shaped microelectrode thus formed is cooled rapidly after the furnace has been removed. Subsequently the lower end of the capillary (Fig. 2, C-IV) is ground off in order to expose the surface of the antimony and a lead (V) is soldered on. In order to fit the electrode to the salt bridge (Fig. 1, II), the outer wall of the tube (Fig. 2, VI) is ground in a steel sleeve. This procedure is based on the preparation of bismuth, tin, cadmium and lead microelectrodes [ 41.
231 To vacuum
A
pump
C 0
Fig. 2. The furnace and construction explanation.
of the antimony interface electrode. See text for
Procedure. A vibron electrometer or simple valve pH meter, depending on the type of solvent, was used for measuring the changes in the e.m.f. of the interfacial cell. The titrants were added from a microburette in 0.05-0.1-ml portions. After each addition of the acid, the titrated solution was mixed for about 15 s with a magnetic stirrer. The e-m-f. of the cell was measured 30 s after each addition. Samples and reagents_ Strychnine, homatropine, hyoscyamine, aconitine, hydroquinidine, n-dodecylamine, trimethylamine, cY-picoline and o[,a’-lutidine were used as test materials. The solvents tested were dry cyclohexane, benzene, chloroform, ligroin, pentyl acetate and dichloroethane. Trichloracetic acid and picric acid were used as the titrants and were dissolved in the same solvent as the base titrated. Results
and discussion
In all experiments 3-10 ml of dilute solutions of an organic base were titrated. However, the volume of the sample may be reduced by further miniaturization of the immersed interfacial cell. The titration curves obtained are shown in Figs. 3-6. The shapes of these curves in dry organic solvents prove that the interfacial voitaic cell is useful for the determination of organic bases in non-conducting media, for they exhibit sharp changes at the inflection points. During the titration of alkaloid solutions with picric acid in benzene and ligroin, some insoluble salts were formed; they did not interfere, how-
232
Fig. 3. Potentiometric titration curves of strychnine (0.005 M) with picric acid (0.01 M) in: (1) cyclohexane; (2) pentyl acetate; (3) dichloroethane; (4) chloroform; (5) benzene. Fig_ 4. Potentiometric titration curves of: (I) hydrochinidine (0.01 M); (2) trimethylamine (0.005 M); (3) aconitine (0.01 M) and (4) ndodecylamine with trichloroacetic acid (0.01 M) in benzerze.
2 AEm [-!!
z
5
4
-ct.? lrnil
2
2
L
b
I)
Fig. 5. Potentiometric titration curves of: (1) a-picoline (0.001 M); (2) hyoscyamine (0.001 M); (5) homatropine (O_OOlM) M); (3) ndodecylarnine (0.001 M); (4) a,a’-lutidine (0.001 with picric acid (0.002 M); and (6) ndodecylamine(0.001 M); (7) homatropine (0.001 M); (8) hyoscyamine (0.001 M); (9) Q, a’-lutidine (0.001 M); (10) a-picoline with trichloroacetic acid (0.002 LM)in benzene. Fig. 6. Potentiometric titration curves of: (1) homatropine (0.001 M); (2) a,cr’-lutidine (0.001 M); (3) ndodecylamine (0.001 M); (4) a-picoline (0.001 M); (5) hyoscyamine (0.001 M) with pi&c acid (0.002 M); and (6) cL-picoline (0.001 M); (7) u,a,-lutidine (0.001 M); (8) ndodecylamine (0.001 M); (9) homatropine (0.001 M); (10) hyoscyamine (0.001 M) with trichloroacetic acid (0.002 M) in chloroform.
233
ever, with the determination of the bases. Obviously, careful standardization
is necessary in the titration of any compound. The trends of the titration curves in different solvents change only slightly. The distinct potential changes of the interfacial electrode observed during the titration are due to the fact that the chemical reaction is monitored not in the bulk of the solution but at the boundary of two phases with markedly
different dielectric constants. In such a case the reacting substances are con-
centrated at the interface, i.e., at the working surface of the interface cell. REFERENCES 1 2 3 4
B. Walig&a and M. Paluch, Anal. Chim. Acta, 119 (1980) 375. B. Kamietiski, 2. By-l’0 and B. Walig&a, Bull. Acad. Pol. Sci. Lett. B. Kamieliski and J. Kulawik, Bull. Acad. Pol. Sci. Cl. 3,3 (1955) B. Walig&a and M. Paluch, Chem. Anal., 10 (1965) 693.
Ser. A, (1951) 401; 4 (1956)
199. 529.
Chimica Acta, 132 (1981) 229-233 Ekevier Scientific Publishing Company, Amsterdam -
Printed in The Netherlands
Short Communication POTENTIOMETRIC TITRATIONS IN NON-CONDUCTING WITH AN INTERFACIAL ANTIMONY ELECTRODE
BOLESLAW Department Jagellonian
WALIG6RA*
and MARIA
SOLUTIONS
PALUCH
of Physical Chemistry and Electrochemistry University, 3 Karasia Street, 30-060 Kmcow
of the Institute (Poland)
of Chemistry.
(Received 27th April 1981) The micro interfacial device described is based on an antimony/antimony scratch/ 0.1 M KCl/Hg,CI,/Pt cell. Acid-base reactions occurring in solvents such as dry ligroin, benzene, cyclohexane, dichloroethane, chloroform and pentyl acetate are readily monitored. Summary.
Potentiometric titration in non-aqueous media normally affords positive results only in conducting solvents. In the case of solvents which do not exhibit electric conductance (e.g., hydrocarbons), conventional methods fail because the indicator electrode does not show the characteristic potential changes necessary for quantitative interpretation. However, the interfacial voltaic cell described previously [l] enables acid-base reactions to be monitored in non-conducting media. The inter-facialvoltaic cell discussed here is a development of the microadsorption electrode of Kamieriski et al. [2] which was used initially as a potentiometric detector in chromatography and for atmospheric contaminants [3]. The special construction of that device made it possible to monitor the course of chemical reactions at the phase boundary of two immiscible liquids. The form of the interfacial cell described in the present communication can be immersed in the solutions_ Experimental Measuring system.
The cell used is shown schematically in Fig. 1. It consists of a calomel (CE) and an antimony (AME) electrode with a salt bridge (SB) filled with saturated or 0.1M potassium chloride connecting the two electrodes. After the electrodes have been inserted into ground-glass joints I and II of the bridge, a compact micro cell is obtained (Fig. 1, MC). The groundglass joints facilitate disconnection of the set and cleaning of the electrodes, an operation necessary in serial measurements. The essential part of the interfacial cell is a metallic scratch on the disc of porous corundum (Fig. 1, III) which closes the outlet of the salt bridge. The corundum disc (5-mm diameter and 3-mm thick) is connected to the broadened end of the bridge (SB). The pointed end of the antimony electrode (OS-mm diameter) contacts the external surface of the porous c&undum at point IV and is connected with the reference electrode through the salt bridge. 0003-2670/81/0000-0000/$02_50
0 1981 Elsevier Scientific Publishing Company
230
CE
MC
Fig. 1. The interfacial micro cell (immersible form): CE, calomel electrode; AME. antimony microelectrode; SB, salt bridge; I and II, ground-glass joints for inserting the electrodes; III, corundum disc; MC, assembled cell; IV, working surface of the cell.
Direct contact of +he two electrodes is obviously unnecessary in conducting solutions, but when non-conducting media are used (e.g., gases or pure hydrocarbons), electric conductance must be ensured somehow. The metallic scratch formed on the porous corundum disc wetted by the potassium chloride solution acts as the conductor in nonconducting systems. Before each titration, the cell is disconnected and cleaned, and the surface of the corundum disc is scratched with chemically pure antimony. Construction
and preparation
of the indicator antimony
microelectrode.
The microelectrodes used were constructed by fusing antimony into Supremax glass capillary tubes under vacuum. Figure 2 shows the furnace arrangement (B) and the capillary tube construction (A)_ Powdered antimony metal is poured into a capillary tube (A, ca. 0.5 mm i-d.) fused at its lower end, and then air is evacuated in order to obtain avacuum of lo-*-lo4 mm Hg. The capillary tube is closed at the narrow part (Fig. 2, I). The closed capillary tube is placed in the electric furnace (Fig. 2, B) and heated to the melting point of antimony (630°C); the closed end (Fig. 2, II) of t.he tube is then broken off. Air coming into the tube forces the fused antimony into the elongated part of the capillary tube (Fig. 2, III). The needle-shaped microelectrode thus formed is cooled rapidly after the furnace has been removed. Subsequently the lower end of the capillary (Fig. 2, C-IV) is ground off in order to expose the surface of the antimony and a lead (V) is soldered on. In order to fit the electrode to the salt bridge (Fig. 1, II), the outer wall of the tube (Fig. 2, VI) is ground in a steel sleeve. This procedure is based on the preparation of bismuth, tin, cadmium and lead microelectrodes [ 41.
231 To vacuum
A
pump
C 0
Fig. 2. The furnace and construction explanation.
of the antimony interface electrode. See text for
Procedure. A vibron electrometer or simple valve pH meter, depending on the type of solvent, was used for measuring the changes in the e.m.f. of the interfacial cell. The titrants were added from a microburette in 0.05-0.1-ml portions. After each addition of the acid, the titrated solution was mixed for about 15 s with a magnetic stirrer. The e-m-f. of the cell was measured 30 s after each addition. Samples and reagents_ Strychnine, homatropine, hyoscyamine, aconitine, hydroquinidine, n-dodecylamine, trimethylamine, cY-picoline and o[,a’-lutidine were used as test materials. The solvents tested were dry cyclohexane, benzene, chloroform, ligroin, pentyl acetate and dichloroethane. Trichloracetic acid and picric acid were used as the titrants and were dissolved in the same solvent as the base titrated. Results
and discussion
In all experiments 3-10 ml of dilute solutions of an organic base were titrated. However, the volume of the sample may be reduced by further miniaturization of the immersed interfacial cell. The titration curves obtained are shown in Figs. 3-6. The shapes of these curves in dry organic solvents prove that the interfacial voitaic cell is useful for the determination of organic bases in non-conducting media, for they exhibit sharp changes at the inflection points. During the titration of alkaloid solutions with picric acid in benzene and ligroin, some insoluble salts were formed; they did not interfere, how-
232
Fig. 3. Potentiometric titration curves of strychnine (0.005 M) with picric acid (0.01 M) in: (1) cyclohexane; (2) pentyl acetate; (3) dichloroethane; (4) chloroform; (5) benzene. Fig_ 4. Potentiometric titration curves of: (I) hydrochinidine (0.01 M); (2) trimethylamine (0.005 M); (3) aconitine (0.01 M) and (4) ndodecylamine with trichloroacetic acid (0.01 M) in benzerze.
2 AEm [-!!
z
5
4
-ct.? lrnil
2
2
L
b
I)
Fig. 5. Potentiometric titration curves of: (1) a-picoline (0.001 M); (2) hyoscyamine (0.001 M); (5) homatropine (O_OOlM) M); (3) ndodecylarnine (0.001 M); (4) a,a’-lutidine (0.001 with picric acid (0.002 M); and (6) ndodecylamine(0.001 M); (7) homatropine (0.001 M); (8) hyoscyamine (0.001 M); (9) Q, a’-lutidine (0.001 M); (10) a-picoline with trichloroacetic acid (0.002 LM)in benzene. Fig. 6. Potentiometric titration curves of: (1) homatropine (0.001 M); (2) a,cr’-lutidine (0.001 M); (3) ndodecylamine (0.001 M); (4) a-picoline (0.001 M); (5) hyoscyamine (0.001 M) with pi&c acid (0.002 M); and (6) cL-picoline (0.001 M); (7) u,a,-lutidine (0.001 M); (8) ndodecylamine (0.001 M); (9) homatropine (0.001 M); (10) hyoscyamine (0.001 M) with trichloroacetic acid (0.002 M) in chloroform.
233
ever, with the determination of the bases. Obviously, careful standardization
is necessary in the titration of any compound. The trends of the titration curves in different solvents change only slightly. The distinct potential changes of the interfacial electrode observed during the titration are due to the fact that the chemical reaction is monitored not in the bulk of the solution but at the boundary of two phases with markedly
different dielectric constants. In such a case the reacting substances are con-
centrated at the interface, i.e., at the working surface of the interface cell. REFERENCES 1 2 3 4
B. Walig&a and M. Paluch, Anal. Chim. Acta, 119 (1980) 375. B. Kamietiski, 2. By-l’0 and B. Walig&a, Bull. Acad. Pol. Sci. Lett. B. Kamieliski and J. Kulawik, Bull. Acad. Pol. Sci. Cl. 3,3 (1955) B. Walig&a and M. Paluch, Chem. Anal., 10 (1965) 693.
Ser. A, (1951) 401; 4 (1956)
199. 529.