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Direct titration of glycerol by constant current potentiometry
Talanta.1967,Vol. 14.pp. 1309to 1314.P~atnon Pmr L1d. Printedin Northernkeland
DIRECT TITRATION OF GLYCEROL BY CONSTANT CURRENT POTENTIOMETRY J. R. SAG and C. 0. HUBER Department of Chemistry, University of Wisconsin-Milwaukee, Wisconsin 53201, U.S.A. (Received 13 ApriI 1967. Accepted 8 June 1967)
Summary-A direct, constant-current potentiometric titration method allows quantitative determination of 0~50-5~00mg samples of glycerol in aqueous solution with errors ranging from O-2gfor t& large &&plos to 2.0% for the smaller samnles. Perchloratocerate is used as the standard oxidant and pyrolyt& graphite as the electrode material. A direct determina tion is made possible by elevated temperatures, high acidity, and a relatively large constant current. Sources of error and possible interferences are discussed.
BECAUSEof its wide use and natural occurrence many methods have been devised for the determination of glycerol in aqueous solution. The convenience and simplicity of the “wet” volumetric methods account for their continued importance. The most common of these are esterification to the triacetate followed by hydrolysis and an acid-base titration, and oxidation by excess of chromate, periodate or cerate followed by back-titration of the excess oxidant. The indirect cerate titration was fist described by Smith and Duke,* who showed that the reaction rate in perchloric acid was definitely higher than in sulphuric acid solution. The equation basting the stoi~~ome~ is as follows: 8Ce4+ + &H&(OH), j- 3HaO + 3HCOOH -j- SCest + 8H*
The work presented here describes the direct titration of glycerol in perchloric acid solution by constant-current potentiomet@ with pyrolytic graphite electrodes (PGE). Miller and Zittela’have shown the apparent reversibility of the cerate-cerous system at a pyrolytic graphite cathode. EXPERIIMENTAL electrodes used were twoi x f- x & in. rods of pyrolytic graphite with the higher conductivity along the rod axis. The sides of the cathode were covered with a mixture of 80% Epolene C-10@ (Pastman-TeMessee) and 20% .high-melting pa&in wax, leaving only the end of the cathode exposed. The constant current in the system was maintained by a 90-V battery in series with a 900,ooO ohm resistor. A commercially available pH meter with O-1400 mV scale, or a digital vacuum tube voltmeter, was used to measure th& potential difference between the two pyrolytic graphite electrodes. Titrations were carried out in 15Oml beakers. A combination heater-magnetic stirrer was used to heat and stir the titration solutions. A heat reservoir in the form of a water-bath or an aluminium block maintained the temperature sutbcientty constant during the titration. The
Reagents All chemicals were reagent grade and used as supplied. The perchloratoccrate was prepared by
mixing ammonium hexanitratocerate with enough concentratedperchloric acid to give a solution 2F in perchloric acid when diluted. Any hydrolysis precipitates were removed before the solution was standardized and used. Procedure
Two pyrolytic graphite electrodes polarized by a IOO-FA constant current, were immersed in a stirred solution of 2F per&lo& acid which was heated to 80”. The glycerol sample was then added 1309 8
1310
J. R.
SAND
and C. 0. HIJEER
:
rC
I-
1.
A____
~
yvs.scF
I
+2.0
-1.0
f IOpCt
-
!i!otL_o -
-B
----
44 Cerate reduction FIG. l.-Current-voltage curves in 2F HClO, at 25°C. 1. Residual current. 2. Cerate reduction.
---
1311
Titration of glycerol
to the heated solution. The resulting solution was titrated with the perchloratocerate untif the voltage between the electrodes changed to 0.3 V from initially high values of about 1.6 V. The end-point was defined as the vohune at which a one-drop increment of titmnt caused the potential to remain at or below O-3V for 30 sec. A bhmk was determined. RESULTS
AND
DISCUSSION
The accuracy and precision of the proposed method were established by comparing the volume of titrant used for several direct titrations of 5-ml aliquots of 0*01114M glycerol solution with the volume used in a well-established indirect cerate oxidation methodS The results of this comparison are summarized in Table I. This table TABLE.I Direct determination of 5.13 mg of glycerol Found, mg 5.12 5.12 5.05 5.20 5.19 5.17 5.10 5.15 Mean 5.14 Std. devn. 0.05
Error, mg -0.01 -0.01 -0.08 -to*07 +OW +0*04 -0.03 $0.02
Indirect determination of 5.13 mg of glycerol Found, w 5,28 5.14 5,32 488 5.12 504 4.89 5.19 Mean 5.14 Std. devn. @20
Error, rng i-O.15 +0*28 SO.19 -0.25 -0.01 -o*OP -0.24 -l-W6 _
_
~.__
ilfustrates the agreement between the indirect and direct methods in the 5*OOmg range. Greater precision was obtained for the direct method. This was presumably a result of the fewer ma~puIations needed in it. Observation of the current-voltage curves for the perchloratocerate ion in acid solution at a pyrolytic graphite electrode (Fig. 1) indicates nearly reversible behaviour of the cerate-cerous system in the vicinity of 0.90 V (US.S.C.E.). This is in distinct contrast to the highly irreversible reduction of cerate which occurs at other common electrodes. The titration curve of glycerol (Fig. 2) can be interpreted in terms of the current-voltage curves shown in Fig. 1. The potential at the beginning of the titration is determined by the intersection of the 100~,uAconstant current lines with current-voltage curve 1. The potential difference is indicated by the two points A and B. With the exception of a slight increase at the beginning of the titration, the voltage remains constant across the electrodes until an excess of cerate in the solution causes a shift in the cathode reaction from curve 1 to curve 2, and a corresponding potential drop from AB to CB which indicates the end-point of the titration. The faster reaction rate needed for a direct determination was obtained by using elevated temperatures, high acid concentration, and a relatively large constant current (100 ,uA). At temperatures below 70” the reaction is slow enough to make direct titration impractical. At the recommended temperatures delays due to slow reaction rate are still perceptible, but a titration can easily be performed in less than 10 min. A 2F mineral acid solution was used as the solvent for the titration because the reaction rate is favoured by high acidity. l Perchloric acid was selected as the most suitable of the strong mineral acids. Nitric acid can be used, but the reaction is
J. R. SANDand C. 0. HUBER
1312
I.
I-
End
--_--_-------
Point
I
1.0
Potential
,
2.0 ML,
FIG. Z.-Titration
of
i
I
3.0
4.0
I
r
5.0
Ce’+
curve for glycerol with perchloratocerate.
slightly slower. Guilbault and McCurdyl have shown that suiphate and bisulphate ions affect the reaction rate adversely. Chloride is oxidized under these conditions. Titration at the relatively large constant current of 100 ,uA provides for a definite excess of titrant at the end-point, aud thus a significant decrease in reaction time. Positive error is avoided by use of a blank titration at the same current. Figure 2 indicates also that the voltage change at the end-point is quite large and ~~~~ble
1313
Titration of glycerol
when this current is used. When a thermostatically controlled combination heaterstirrer is used, current drain by the heater coils may cause fluctuations in the stirring rate. Turning the heater coils off prior to reading the end-point potentials prevents such fluctuations. The stirring rate at the blank end-point and the end-point of the actual glycerol determinations must be the same in order to maintain the accuracy and the precision of the proposed method. Sample loss due to evaporation was estimated, using Raoult’s law. Such computations indicated that less than O-1% of a 5-mg sample volatilized during a IO-min titration. As an experimental check, several titrations were carried out in an Erlenmeyer flask fitted with a water-cooled condenser. These titrations showed no signiticant deviation from those performed in open beakers. The large shift toward more positive cathode potentials at the end-point was quite definite, but somewhat slow. This is evidence to support the formation of an electrode film as suggested by Miller and Zitte1.a Slow potential increases at the beginning of the titration were found to be anodic and are presumably due to electrochemical oxidation of the anode, or to film formation at the surface of the anode by temporary local concentrations of cerate. Neither of these effects significantly affects titration accuracy or convenience. Reference to Fig. 2 indicates that in principle any potential along the relatively large potential shift could be selected for end-point indication. Titrating to O-3 V allows the major portion of the potential shift to provide a convenient anticipation of the end-point. In addition, this potential is positive relative to the region where Ghn formation occur~,~ thereby eliminating spurious potentials resulting from alternate formation and reduction of the film. Several titrations were carried out with more dilute solutions of glycerol and/or cerate to test the sensitivity limits of the method. Table II shows that less than 1 mg TABLEII.-TITRATION Sample number+ 1 2 3 4 : 7
Cerate concentration, 0.09629 O-09629 0.09629 0.04814 0.00963 O-00963 0.00963
M
SENSTMTY
Glycerol present, mg -~5.29 2.64 0.53 5.29
Glycerol found, mg
;:z 0.53
26Ot 0*52+
5.30 261 0.54 5.27 5.27t
Relative error, % +0*2 -0.8 +1*9 -0.4 -0.4 -1.5 -1.9
l Each individually listed result is an average of several results. t These values were corrected for dilution caused by the addition of titrant.
of glycerol could be determined accurately by this method. When dilute perchloratocerate solutions (approx O*OlM) were used, corrections were made for the relatively large increases in the volume of titration solution. The negative errors found for the most dilute solutions may be due to anodic reoxidation of cerium(II1). Titrations at greater dilutions than those presented here would be inconveniently slow. Cerate solubility limits the titrant concentration to about 0.1M. Thus titration of samples larger than about 5-Omg is not feasible unless rather large dilution corrections are applied.
J. R. SANG and C. 0. HUBER
1314
Since no electrode which was sensitive to glycerol condensation was known, the major requirement for a suitable anode was that it should keep at constant potential in the vicinity of the end-point. Copper, platinum, gold, pyrolytic graphite, and a saturated calomei electrode were tested as possible anodes. The last was rejected because of the ditkulties with the salt bridge at the temperatures used. Pyrolytic graphite was selected as the most suitable of the solid anode materials because it provided the most stable anode potentials under these conditions, A study was conducted to see what interferences could be tolerated in the determination. As expected, easily oxidized or hydrolysed organic compounds interfere. Such interferences are less severe in the direct method than in the indirect method, because direct titration minimizes the concentrations of excess titrant. Neither saturated acids nor alcohols interfere sig~fican~y at concentrations up to Ttbthat of the glycerol. Acknowledgement-?&r.
Gerald Nelesen obtained the current-voltage
piots.
Zusammen&aunR-Ein direktes potentiometrisches Titrationsverfahren bei konstanter Stromst&rke erlaubt die quantitative Bestimmung von 0,50-5,OO mg-Proben Glycerin in wtiriger Liisung mit Fehlern von 0,2% bei den DoDen und 2.0% bei den kleineren Probementren. Perchloratocerat &d als eingesteihes Oxidationsmittel verwandt &d pyroDie direkte Bestimmuna wird lvtischer Granhit als Electrodenmaterial. &m$@lrt d&h erhtihte Temperatur, hohe Aciditiit und eine &ativ groQe konstante Stromst&ke. Fehlerquellen und mogliche St&ungen werden diikutiert. R6smn6-Une methode dire& de titrage par potentiom~t~e a courant constant permet le dosage quantitatif ~~~ti~ons de glyc&ol de 0,SO 215,OOmg en solution aqueuse aver: des erreurs comprises entre 0.2% pour lea gros echantillons et 2,0% pour les &hantilions les plus petits. On utilise le perchloratoc&ate comme oxydant standard et le graphite pyrolytique comme mat&e d%lectrode. Une determination diiecte est rendue possible aux temperatures tlevees, forte aciditb et courant constant relativement fort. On discute des sources d’erreur et des interferences possibles, REFERENCES 1. 2. 3. 4. 5.
G. G. Guilbault and W. H. McCurdy Jr., J. Phys. Chem., 1963,67,283. F. J. Miller and H. E. Zittel, J. Electroamd. Chem., 1964, 7, 116. Idem, ibid., 1965,9, 305. C, N. Reilly, W. D. Cooke and N. H. Furman, Anal. Gem., 1951,X$1223. G. F. Smith, &r&e Ox~~~try, pp. 97-98. G. Frederick Smith Chemical Ohio, 1942. 6. G. F. Smith and F. R. Duke, Ind. Eng. Chem., Anal. ES., 1941,67,283.
Co.,
DIRECT TITRATION OF GLYCEROL BY CONSTANT CURRENT POTENTIOMETRY J. R. SAG and C. 0. HUBER Department of Chemistry, University of Wisconsin-Milwaukee, Wisconsin 53201, U.S.A. (Received 13 ApriI 1967. Accepted 8 June 1967)
Summary-A direct, constant-current potentiometric titration method allows quantitative determination of 0~50-5~00mg samples of glycerol in aqueous solution with errors ranging from O-2gfor t& large &&plos to 2.0% for the smaller samnles. Perchloratocerate is used as the standard oxidant and pyrolyt& graphite as the electrode material. A direct determina tion is made possible by elevated temperatures, high acidity, and a relatively large constant current. Sources of error and possible interferences are discussed.
BECAUSEof its wide use and natural occurrence many methods have been devised for the determination of glycerol in aqueous solution. The convenience and simplicity of the “wet” volumetric methods account for their continued importance. The most common of these are esterification to the triacetate followed by hydrolysis and an acid-base titration, and oxidation by excess of chromate, periodate or cerate followed by back-titration of the excess oxidant. The indirect cerate titration was fist described by Smith and Duke,* who showed that the reaction rate in perchloric acid was definitely higher than in sulphuric acid solution. The equation basting the stoi~~ome~ is as follows: 8Ce4+ + &H&(OH), j- 3HaO + 3HCOOH -j- SCest + 8H*
The work presented here describes the direct titration of glycerol in perchloric acid solution by constant-current potentiomet@ with pyrolytic graphite electrodes (PGE). Miller and Zittela’have shown the apparent reversibility of the cerate-cerous system at a pyrolytic graphite cathode. EXPERIIMENTAL electrodes used were twoi x f- x & in. rods of pyrolytic graphite with the higher conductivity along the rod axis. The sides of the cathode were covered with a mixture of 80% Epolene C-10@ (Pastman-TeMessee) and 20% .high-melting pa&in wax, leaving only the end of the cathode exposed. The constant current in the system was maintained by a 90-V battery in series with a 900,ooO ohm resistor. A commercially available pH meter with O-1400 mV scale, or a digital vacuum tube voltmeter, was used to measure th& potential difference between the two pyrolytic graphite electrodes. Titrations were carried out in 15Oml beakers. A combination heater-magnetic stirrer was used to heat and stir the titration solutions. A heat reservoir in the form of a water-bath or an aluminium block maintained the temperature sutbcientty constant during the titration. The
Reagents All chemicals were reagent grade and used as supplied. The perchloratoccrate was prepared by
mixing ammonium hexanitratocerate with enough concentratedperchloric acid to give a solution 2F in perchloric acid when diluted. Any hydrolysis precipitates were removed before the solution was standardized and used. Procedure
Two pyrolytic graphite electrodes polarized by a IOO-FA constant current, were immersed in a stirred solution of 2F per&lo& acid which was heated to 80”. The glycerol sample was then added 1309 8
1310
J. R.
SAND
and C. 0. HIJEER
:
rC
I-
1.
A____
~
yvs.scF
I
+2.0
-1.0
f IOpCt
-
!i!otL_o -
-B
----
44 Cerate reduction FIG. l.-Current-voltage curves in 2F HClO, at 25°C. 1. Residual current. 2. Cerate reduction.
---
1311
Titration of glycerol
to the heated solution. The resulting solution was titrated with the perchloratocerate untif the voltage between the electrodes changed to 0.3 V from initially high values of about 1.6 V. The end-point was defined as the vohune at which a one-drop increment of titmnt caused the potential to remain at or below O-3V for 30 sec. A bhmk was determined. RESULTS
AND
DISCUSSION
The accuracy and precision of the proposed method were established by comparing the volume of titrant used for several direct titrations of 5-ml aliquots of 0*01114M glycerol solution with the volume used in a well-established indirect cerate oxidation methodS The results of this comparison are summarized in Table I. This table TABLE.I Direct determination of 5.13 mg of glycerol Found, mg 5.12 5.12 5.05 5.20 5.19 5.17 5.10 5.15 Mean 5.14 Std. devn. 0.05
Error, mg -0.01 -0.01 -0.08 -to*07 +OW +0*04 -0.03 $0.02
Indirect determination of 5.13 mg of glycerol Found, w 5,28 5.14 5,32 488 5.12 504 4.89 5.19 Mean 5.14 Std. devn. @20
Error, rng i-O.15 +0*28 SO.19 -0.25 -0.01 -o*OP -0.24 -l-W6 _
_
~.__
ilfustrates the agreement between the indirect and direct methods in the 5*OOmg range. Greater precision was obtained for the direct method. This was presumably a result of the fewer ma~puIations needed in it. Observation of the current-voltage curves for the perchloratocerate ion in acid solution at a pyrolytic graphite electrode (Fig. 1) indicates nearly reversible behaviour of the cerate-cerous system in the vicinity of 0.90 V (US.S.C.E.). This is in distinct contrast to the highly irreversible reduction of cerate which occurs at other common electrodes. The titration curve of glycerol (Fig. 2) can be interpreted in terms of the current-voltage curves shown in Fig. 1. The potential at the beginning of the titration is determined by the intersection of the 100~,uAconstant current lines with current-voltage curve 1. The potential difference is indicated by the two points A and B. With the exception of a slight increase at the beginning of the titration, the voltage remains constant across the electrodes until an excess of cerate in the solution causes a shift in the cathode reaction from curve 1 to curve 2, and a corresponding potential drop from AB to CB which indicates the end-point of the titration. The faster reaction rate needed for a direct determination was obtained by using elevated temperatures, high acid concentration, and a relatively large constant current (100 ,uA). At temperatures below 70” the reaction is slow enough to make direct titration impractical. At the recommended temperatures delays due to slow reaction rate are still perceptible, but a titration can easily be performed in less than 10 min. A 2F mineral acid solution was used as the solvent for the titration because the reaction rate is favoured by high acidity. l Perchloric acid was selected as the most suitable of the strong mineral acids. Nitric acid can be used, but the reaction is
J. R. SANDand C. 0. HUBER
1312
I.
I-
End
--_--_-------
Point
I
1.0
Potential
,
2.0 ML,
FIG. Z.-Titration
of
i
I
3.0
4.0
I
r
5.0
Ce’+
curve for glycerol with perchloratocerate.
slightly slower. Guilbault and McCurdyl have shown that suiphate and bisulphate ions affect the reaction rate adversely. Chloride is oxidized under these conditions. Titration at the relatively large constant current of 100 ,uA provides for a definite excess of titrant at the end-point, aud thus a significant decrease in reaction time. Positive error is avoided by use of a blank titration at the same current. Figure 2 indicates also that the voltage change at the end-point is quite large and ~~~~ble
1313
Titration of glycerol
when this current is used. When a thermostatically controlled combination heaterstirrer is used, current drain by the heater coils may cause fluctuations in the stirring rate. Turning the heater coils off prior to reading the end-point potentials prevents such fluctuations. The stirring rate at the blank end-point and the end-point of the actual glycerol determinations must be the same in order to maintain the accuracy and the precision of the proposed method. Sample loss due to evaporation was estimated, using Raoult’s law. Such computations indicated that less than O-1% of a 5-mg sample volatilized during a IO-min titration. As an experimental check, several titrations were carried out in an Erlenmeyer flask fitted with a water-cooled condenser. These titrations showed no signiticant deviation from those performed in open beakers. The large shift toward more positive cathode potentials at the end-point was quite definite, but somewhat slow. This is evidence to support the formation of an electrode film as suggested by Miller and Zitte1.a Slow potential increases at the beginning of the titration were found to be anodic and are presumably due to electrochemical oxidation of the anode, or to film formation at the surface of the anode by temporary local concentrations of cerate. Neither of these effects significantly affects titration accuracy or convenience. Reference to Fig. 2 indicates that in principle any potential along the relatively large potential shift could be selected for end-point indication. Titrating to O-3 V allows the major portion of the potential shift to provide a convenient anticipation of the end-point. In addition, this potential is positive relative to the region where Ghn formation occur~,~ thereby eliminating spurious potentials resulting from alternate formation and reduction of the film. Several titrations were carried out with more dilute solutions of glycerol and/or cerate to test the sensitivity limits of the method. Table II shows that less than 1 mg TABLEII.-TITRATION Sample number+ 1 2 3 4 : 7
Cerate concentration, 0.09629 O-09629 0.09629 0.04814 0.00963 O-00963 0.00963
M
SENSTMTY
Glycerol present, mg -~5.29 2.64 0.53 5.29
Glycerol found, mg
;:z 0.53
26Ot 0*52+
5.30 261 0.54 5.27 5.27t
Relative error, % +0*2 -0.8 +1*9 -0.4 -0.4 -1.5 -1.9
l Each individually listed result is an average of several results. t These values were corrected for dilution caused by the addition of titrant.
of glycerol could be determined accurately by this method. When dilute perchloratocerate solutions (approx O*OlM) were used, corrections were made for the relatively large increases in the volume of titration solution. The negative errors found for the most dilute solutions may be due to anodic reoxidation of cerium(II1). Titrations at greater dilutions than those presented here would be inconveniently slow. Cerate solubility limits the titrant concentration to about 0.1M. Thus titration of samples larger than about 5-Omg is not feasible unless rather large dilution corrections are applied.
J. R. SANG and C. 0. HUBER
1314
Since no electrode which was sensitive to glycerol condensation was known, the major requirement for a suitable anode was that it should keep at constant potential in the vicinity of the end-point. Copper, platinum, gold, pyrolytic graphite, and a saturated calomei electrode were tested as possible anodes. The last was rejected because of the ditkulties with the salt bridge at the temperatures used. Pyrolytic graphite was selected as the most suitable of the solid anode materials because it provided the most stable anode potentials under these conditions, A study was conducted to see what interferences could be tolerated in the determination. As expected, easily oxidized or hydrolysed organic compounds interfere. Such interferences are less severe in the direct method than in the indirect method, because direct titration minimizes the concentrations of excess titrant. Neither saturated acids nor alcohols interfere sig~fican~y at concentrations up to Ttbthat of the glycerol. Acknowledgement-?&r.
Gerald Nelesen obtained the current-voltage
piots.
Zusammen&aunR-Ein direktes potentiometrisches Titrationsverfahren bei konstanter Stromst&rke erlaubt die quantitative Bestimmung von 0,50-5,OO mg-Proben Glycerin in wtiriger Liisung mit Fehlern von 0,2% bei den DoDen und 2.0% bei den kleineren Probementren. Perchloratocerat &d als eingesteihes Oxidationsmittel verwandt &d pyroDie direkte Bestimmuna wird lvtischer Granhit als Electrodenmaterial. &m$@lrt d&h erhtihte Temperatur, hohe Aciditiit und eine &ativ groQe konstante Stromst&ke. Fehlerquellen und mogliche St&ungen werden diikutiert. R6smn6-Une methode dire& de titrage par potentiom~t~e a courant constant permet le dosage quantitatif ~~~ti~ons de glyc&ol de 0,SO 215,OOmg en solution aqueuse aver: des erreurs comprises entre 0.2% pour lea gros echantillons et 2,0% pour les &hantilions les plus petits. On utilise le perchloratoc&ate comme oxydant standard et le graphite pyrolytique comme mat&e d%lectrode. Une determination diiecte est rendue possible aux temperatures tlevees, forte aciditb et courant constant relativement fort. On discute des sources d’erreur et des interferences possibles, REFERENCES 1. 2. 3. 4. 5.
G. G. Guilbault and W. H. McCurdy Jr., J. Phys. Chem., 1963,67,283. F. J. Miller and H. E. Zittel, J. Electroamd. Chem., 1964, 7, 116. Idem, ibid., 1965,9, 305. C, N. Reilly, W. D. Cooke and N. H. Furman, Anal. Gem., 1951,X$1223. G. F. Smith, &r&e Ox~~~try, pp. 97-98. G. Frederick Smith Chemical Ohio, 1942. 6. G. F. Smith and F. R. Duke, Ind. Eng. Chem., Anal. ES., 1941,67,283.
Co.,