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Thermometric titration of acids in pyridine
SHORT COMMUNICATIONS
THERMOMETRIC
TITRATION OF ACIDS IN PYRIDINE
(Received 25 July 1973. Accepted 9 October 1973)
Titration of simpie monobasic acids in pyridine with an uncharged base, has been shown”’ to involve the followmg equilibria: HX(H+ X-) ZZZ=H+ KHx B+ H’ m?% BH+ +X-A c_
+X-,
(1)
BH+,
(2)
BH+ X- (BHX).
(3)
Although the evidence from potentiometric titrations’ is not convincing in our conductometric titration’ of nitric acid us. 1,3-diphenylguanidine (DPG) consideration of the equilibria above has yielded satisfactory agreement between the observed and calculated plots. The conductance of the solution decreased linearly up to the end-point and then rose gradually (also in a linear fashion) and the detection of the end-point was relatively easy. It was of interest to investigate the thermometric titration of such acids in pyridine. The acids considered for the study were: perchlortc, hydriodic, nitric, hydrobromic, acetic, benzoic, o-nitrobenzoic and picric acids and 2.4 and 2,5-dini~ophenol. With the exception of o-mtrobenzoic acid for which no pii, estimate is available the pll, vaiues of all these acids and phenols are known.3*4 An approximate estimate of pK, is available for picric acid.’ It was expected that a correlation between these values and the trends observed in the thermometric titrations would be exhibited. It was further expected that the results would be of value in providing some estimate of the heats of reaction fneutralization) in favourable cases.
EXPERIMENTAL The picric. benzoic and o-nitrobenzoic acids were of reagent quality; the slightly damp picric acid sample was squeezed between folds of filter paper and then air-dried at room temperature. Reagent-quality acetic acid was dried by the usual method. Preparation and purification of all other chemicals have been reported earlier.’ Procedure
The titrant was delivered at a constant rate mto a calorimeter, while the change in temperature of the solution was detected by a thermistor (Sargent Model S-81620) and recorded. In each run, 40 ml of titrand were placed m a vacuum-jacketed (glass) calorimeter. A constant-drive syringe pump (Sage Instrument Model 249:2) burette was used; a length of Teflon tubing was attached to the end of the syringe for delivering the titrant into the calortmeter. Before starting a run, the solution ofacid in the calorimeter and the titrant, ~.e.,the solution of DPG contamed m the syringe burette, were allowed to attain temperature equilibrium in an air-conditioned room maintamed at 21 & 2’. The contents of the calorimeter were kept agitated with a Teflon-coated magnetic stirrer for about a minute before a tstration was actually started, and sttrring.was continued until well beyond the theoretical end-point. A Wheatstone bridge (Sargent Model S-8 1601) was used, with the thermistor as one arm. The bridge imbalance was amplified through a Keithley Model 150 AR microvolt ammeter and then recorded on a Sargent strip-chart recorder. An R-C hook-up with a 250-mF capacitor and a 100: 1 voltage divider consisting of a l-MR resistor and a I O-k52 resistor, was attached across the recorder terminals to minimize the noise. A chart speed of 1 in./min and a rate of titrant delivery of 0.843 ml/min were used in all runs. ~ere~~i~j~lur~o~~ ~~wuf~r-eq~jvuie~f of&e ca~ori~efer assembly. The water~qui~lent of the calorimeter system was ascertained by determining the temperature change caused by the addition of a known amount of ice-cold water to a given quantity of water contained in the calorimeter assembly. An average value of 16-Ocal/deg was obtained from three separate determinations. 303 TAL Vol 21 No 4-D
304
SHORT
COMMUNICATIONS
Time
-
Fig. 1. Time-temperature plots. A-HClO* (O~lOOOkf);B-HI (0~0247M); C-HNO, (01105M): D-HBr (@0479&f);E-2&%nitropheno1(0~1013M); F-picric acid (0.1013M); G-o-nitrobenzoic acid (@1013&f). Full-scale recorder sensitivity: plots A, C, D, E, G, 3mV: plot g. 1mV; plot F, 10 mV. Temperature calibration ofthe chart paper. The temperature change as recorded in mm of chart paper was calibrated by running several titrations of aqueous solutions of sodium hydroxide against hydrochloric acid under identical experimental conditions. The total heat liberated per mole of reactant in each of these titrations was obtained on the basis of the averagc number of chart divisions (mm) representing the temperature change at the titration end-point, the volume of solution at the end-point and the water-equivalent of the calorimeter
I I”. -
Time
-
Fig. 2. Time-temperature plots. A-acetic acid (CO112M); E-benzoic acid (MI 16M); C-benzoic acid (Q1164M); D-_ZSdinitrophenol(@O990M); E-2,5-dinitrophenol (OW99M). Full-scale recorder sensitivity: plots A, B, E, 1 mV; plots C, D. 3 mV.
SHORT
305
COMMUNlCAnONS
determined as above. The average of the heats (657 i @30 x JO’ mm.cal/deg) was then equated with the heat of neutralization of sodmm hydroxide. 13.5 kcal/mole. Thus. at a sensitivity of I mV full-male deflection. I mm of chart paper on the temperature axis would correspond to 2.06 + QO9 x 10m3 deg. This value was used m ascertaining the temperature change associated with a particular titration in pyridine. RESULTS AND DISCUSSION Typical titration curves for perchloric, hydriodic, nitric, hydrobromic. picric and o-nitrobenzoic acids and 2.4 dinitrophenol are given in Fig. 1. The plots are linear with little or no curvature and are marked by fairly sharp breaks in the vtcinity of the theoretical end-points. Comparison of the results summarized in Table 1, indicates that at higher concentrations the correspondence of the thermometric end-points with those ~iculated on the basis of a 1: 1 reaction between the acid and DPG is good. The lack of agreement at lower concentrations of the acids may be due to a number of factors. The amount of heat evolved at lower concentrations is small, and dilution effects may become serious. This suggests that higher titrant concentrations might improve the situation, but when a higher concentration of the titrant is used. the small time-interval required in delivering the titrant mtroduces additional problems. Table I. Results of thermometric titrations of acids in pyridme HX*
HCIOI (3.26) ;.I39, HNO, 14.06)
cnx. iW*
cope, M
Number of chart divisions& tF?h?
01000 0~0100
1.023 0.486
107.5 (3) 175 (1)
3.890 0.762
3.890 0.823
574 -
DO247 0.0025
1.057 0.486 1.023 1.023 0.486
135.0 (1) 48.5 (0.3) 1665 (3) 208.0 (0.3) 86.5 (0.3)
@940 0.227 4.290 O-480 0.472
0940 0.210
9.35 -
4.000 0408 0.455
8.12 -
1Q43
1035 (3) 675 (1) 325 (0.3)
I.910 0446 0.160 3.840 0.420 0.084
1.830 0.397 0115
11.27 -
3.870 0.387 0.082
7.74 -
3.850 0.432 @i185
3.880 0.388 0.0825
8.85 -
3-880 0440 0,084
3-910 0*39I 0.083
11.19 -
0.1 105 00111 0.0055
Volume of DPG. ml Obsd. C&d.
Heat of reactionf ~CUl/~~~
HBr (436)
QO479 0.0104 O+JOJ4
1.043 0.486
Picric acid (35)
0.1013 0.0101 0~0010
l@J3 0.486
o-Nttrobenzoic acid 2.4-Dmitrophenol (4.38) Acetic acid (JO 1) Benzoic acid 19.8) 2+Dimtrophenol (5.76)
0.1013 0~0101 0‘0010 0.1013 O*OlOJ 0@010
1.043 1.039 0.486 1.039 1,039 0.486
0.0112
I.057
0.425
-
01164 0.0116 @ooJJ
1.039 1.039 0.486
4.500 0446 0.091
-
0@990 0.0099 O%JlO
J ,039 1.039 0.486
3.820 0.380 0*080
-
1.043
44.0 (10) 198.0 (0.3) 16-O(@3) 167.0 (3) gZ lif 2105 (3) 83.5 (1) 10.0 (0.3)
* Numbers m brackets indicate the pK, values of the actds. t 40.0 ml of solution used. # Represents the distance travelled by the pen, on the temperature axis. to reach the end-point at the full-scale sensitivity (mVJ given in parentheses. il’The specific heats and the densities of the solutions were taken to be the same as those of pyridine (specific heat = 0.431 cal:deg. density = 0.9781 g/ml at the experimental temperature), and heats of mixing and dilution were not taken into account. Each mm of chart paper was equated to 2% x 10e3 deg, at 1-OmV sensitivity, for calculating the heats. assuming a J : 1 reaction between the acid and the base.
306
SHORTCOMMUNlCATlONS
The plots for acetic and benzoic acid and 2.5-dinitrophenol show considerable curvature (Fig. 2). and the curve-shape is not improved at increased acid concentration. Consequently, location of the end-point for these acids is not feasible. In view of the large difference in pK, values (acetic acid? 10.1: benzoic acid:’ 9.8; 2.5-dinitrophenol:” 5.8) it seems possible that the titrations in these cases are far less complete than for the first group of acids discussed above (Fig. 1). Incidentally. the earlier report’ on potentiometric titration of the acids appears to be vague and incongruous. We have not notlced any homoconJugation for any of the acids. and we are inclined to believe that they all conform to the simple titration scheme suggested m equations (I t(3). It is interestmg to note in this connection that although acetic acid proved too weak for end-point estimation, thermometric titration using DPG as the base5 yielded a distinct end-point for benzoic acid in acetonitrile which is an aprotic but protophobic solvent of moderate dielectric constant. The molar heats of reaction (in kcal) as calculated in the present work from titration data at the highest acid concentration used are given in Table 1. Excluding the case of o-nitrobenzoic acid for which no pK, estimate is available, and of hydriodic acid which might involve a large experimental error due to the low concentration (@0247M) used, the heats seem to follow the same sequence as the pK, values of the acids. No explanation of the trend can be offered at this stage. RAUL VIDAL
Chemistry Department Fnculty of Science
L. M.
MUKHERJEE*
Banaras Hindu University Vianasi 221005, India
REFERENCES 1. 2. 3. 4. 5.
M. Bos and E. A. M. F. Dahmen, Anal. Chim. Acta, 1971,s. 39; 1971,55,285. L. M. Mukherjee, D. W. Suwala and R. S. Schultz, ibid., 1972, 60, 247. L. M. Mukherjee, J. J. Kelly, W. Baranetzky and J. Sica, J. Phys. Chem., 1968.72, 3410. L. M. Mukherjee and R. S. Schultz, Talanta. 197219, 707. E. J. Forman and D. N. Hume, ibid., 1964.11, 129.
* Address all correspondence
to this author.
Summary-Thermometric titration of HClO,. HI, HNOI, HBr, picric acid. o-nitrobenzoic acid, 2,4- and 2,5_dinitrophenol, acetic acid and benzoic acid have been attempted in pyridine as solvent, using 1.3-diphenylguanidine as the base. Except in the case of 25dinitropheno1, acetic acid and benzoic acid, the results are, in general, reasonably satisfactory. The approximate molar heats of neutralization have been calculated. Zusammenfaasung-Die thermometrische Titration von HCIOI, HJ, HNOs, HBr, Pikrinslure, oNitrobenzoes;iure, 2,4- und 2,5-Dinitrophenol, Essigsiiure und Benzoetiure wurde in Pyridin als LGsungsmittel mit 1,3-Diphenylguanidin als Base versucht. Au&r bei 25-Dinitrophenol, Essigs%ure und Benzoedure sind die Ergebnisse im allgemeinen recht zufriedenstellend. Die ungelhren molaren Neutralisationswlrmen wurden berechnet. Rhtm&-On a essay6 le titrage thermomktrique de HClO,, HI. HNOs, HBr, acide picrlque. acide o-nitrobenzo’ique. 2,4- et 2,5-dinitrophinols, acide acetique et acide benzo’ique en pyridine comme solvant, utilisant la 1,3-diphknylguanidine comme base. En exceptant les cas du 2,5-dinitrophlnol, de l’acide acdtique et de l’acide benzo’ique, les rtsultats sent, en gin&al, raisonnablement satisfaisants. On a calculd les chaleurs molaires de neutralisation approximatives.
THERMOMETRIC
TITRATION OF ACIDS IN PYRIDINE
(Received 25 July 1973. Accepted 9 October 1973)
Titration of simpie monobasic acids in pyridine with an uncharged base, has been shown”’ to involve the followmg equilibria: HX(H+ X-) ZZZ=H+ KHx B+ H’ m?% BH+ +X-A c_
+X-,
(1)
BH+,
(2)
BH+ X- (BHX).
(3)
Although the evidence from potentiometric titrations’ is not convincing in our conductometric titration’ of nitric acid us. 1,3-diphenylguanidine (DPG) consideration of the equilibria above has yielded satisfactory agreement between the observed and calculated plots. The conductance of the solution decreased linearly up to the end-point and then rose gradually (also in a linear fashion) and the detection of the end-point was relatively easy. It was of interest to investigate the thermometric titration of such acids in pyridine. The acids considered for the study were: perchlortc, hydriodic, nitric, hydrobromic, acetic, benzoic, o-nitrobenzoic and picric acids and 2.4 and 2,5-dini~ophenol. With the exception of o-mtrobenzoic acid for which no pii, estimate is available the pll, vaiues of all these acids and phenols are known.3*4 An approximate estimate of pK, is available for picric acid.’ It was expected that a correlation between these values and the trends observed in the thermometric titrations would be exhibited. It was further expected that the results would be of value in providing some estimate of the heats of reaction fneutralization) in favourable cases.
EXPERIMENTAL The picric. benzoic and o-nitrobenzoic acids were of reagent quality; the slightly damp picric acid sample was squeezed between folds of filter paper and then air-dried at room temperature. Reagent-quality acetic acid was dried by the usual method. Preparation and purification of all other chemicals have been reported earlier.’ Procedure
The titrant was delivered at a constant rate mto a calorimeter, while the change in temperature of the solution was detected by a thermistor (Sargent Model S-81620) and recorded. In each run, 40 ml of titrand were placed m a vacuum-jacketed (glass) calorimeter. A constant-drive syringe pump (Sage Instrument Model 249:2) burette was used; a length of Teflon tubing was attached to the end of the syringe for delivering the titrant into the calortmeter. Before starting a run, the solution ofacid in the calorimeter and the titrant, ~.e.,the solution of DPG contamed m the syringe burette, were allowed to attain temperature equilibrium in an air-conditioned room maintamed at 21 & 2’. The contents of the calorimeter were kept agitated with a Teflon-coated magnetic stirrer for about a minute before a tstration was actually started, and sttrring.was continued until well beyond the theoretical end-point. A Wheatstone bridge (Sargent Model S-8 1601) was used, with the thermistor as one arm. The bridge imbalance was amplified through a Keithley Model 150 AR microvolt ammeter and then recorded on a Sargent strip-chart recorder. An R-C hook-up with a 250-mF capacitor and a 100: 1 voltage divider consisting of a l-MR resistor and a I O-k52 resistor, was attached across the recorder terminals to minimize the noise. A chart speed of 1 in./min and a rate of titrant delivery of 0.843 ml/min were used in all runs. ~ere~~i~j~lur~o~~ ~~wuf~r-eq~jvuie~f of&e ca~ori~efer assembly. The water~qui~lent of the calorimeter system was ascertained by determining the temperature change caused by the addition of a known amount of ice-cold water to a given quantity of water contained in the calorimeter assembly. An average value of 16-Ocal/deg was obtained from three separate determinations. 303 TAL Vol 21 No 4-D
304
SHORT
COMMUNICATIONS
Time
-
Fig. 1. Time-temperature plots. A-HClO* (O~lOOOkf);B-HI (0~0247M); C-HNO, (01105M): D-HBr (@0479&f);E-2&%nitropheno1(0~1013M); F-picric acid (0.1013M); G-o-nitrobenzoic acid (@1013&f). Full-scale recorder sensitivity: plots A, C, D, E, G, 3mV: plot g. 1mV; plot F, 10 mV. Temperature calibration ofthe chart paper. The temperature change as recorded in mm of chart paper was calibrated by running several titrations of aqueous solutions of sodium hydroxide against hydrochloric acid under identical experimental conditions. The total heat liberated per mole of reactant in each of these titrations was obtained on the basis of the averagc number of chart divisions (mm) representing the temperature change at the titration end-point, the volume of solution at the end-point and the water-equivalent of the calorimeter
I I”. -
Time
-
Fig. 2. Time-temperature plots. A-acetic acid (CO112M); E-benzoic acid (MI 16M); C-benzoic acid (Q1164M); D-_ZSdinitrophenol(@O990M); E-2,5-dinitrophenol (OW99M). Full-scale recorder sensitivity: plots A, B, E, 1 mV; plots C, D. 3 mV.
SHORT
305
COMMUNlCAnONS
determined as above. The average of the heats (657 i @30 x JO’ mm.cal/deg) was then equated with the heat of neutralization of sodmm hydroxide. 13.5 kcal/mole. Thus. at a sensitivity of I mV full-male deflection. I mm of chart paper on the temperature axis would correspond to 2.06 + QO9 x 10m3 deg. This value was used m ascertaining the temperature change associated with a particular titration in pyridine. RESULTS AND DISCUSSION Typical titration curves for perchloric, hydriodic, nitric, hydrobromic. picric and o-nitrobenzoic acids and 2.4 dinitrophenol are given in Fig. 1. The plots are linear with little or no curvature and are marked by fairly sharp breaks in the vtcinity of the theoretical end-points. Comparison of the results summarized in Table 1, indicates that at higher concentrations the correspondence of the thermometric end-points with those ~iculated on the basis of a 1: 1 reaction between the acid and DPG is good. The lack of agreement at lower concentrations of the acids may be due to a number of factors. The amount of heat evolved at lower concentrations is small, and dilution effects may become serious. This suggests that higher titrant concentrations might improve the situation, but when a higher concentration of the titrant is used. the small time-interval required in delivering the titrant mtroduces additional problems. Table I. Results of thermometric titrations of acids in pyridme HX*
HCIOI (3.26) ;.I39, HNO, 14.06)
cnx. iW*
cope, M
Number of chart divisions& tF?h?
01000 0~0100
1.023 0.486
107.5 (3) 175 (1)
3.890 0.762
3.890 0.823
574 -
DO247 0.0025
1.057 0.486 1.023 1.023 0.486
135.0 (1) 48.5 (0.3) 1665 (3) 208.0 (0.3) 86.5 (0.3)
@940 0.227 4.290 O-480 0.472
0940 0.210
9.35 -
4.000 0408 0.455
8.12 -
1Q43
1035 (3) 675 (1) 325 (0.3)
I.910 0446 0.160 3.840 0.420 0.084
1.830 0.397 0115
11.27 -
3.870 0.387 0.082
7.74 -
3.850 0.432 @i185
3.880 0.388 0.0825
8.85 -
3-880 0440 0,084
3-910 0*39I 0.083
11.19 -
0.1 105 00111 0.0055
Volume of DPG. ml Obsd. C&d.
Heat of reactionf ~CUl/~~~
HBr (436)
QO479 0.0104 O+JOJ4
1.043 0.486
Picric acid (35)
0.1013 0.0101 0~0010
l@J3 0.486
o-Nttrobenzoic acid 2.4-Dmitrophenol (4.38) Acetic acid (JO 1) Benzoic acid 19.8) 2+Dimtrophenol (5.76)
0.1013 0~0101 0‘0010 0.1013 O*OlOJ 0@010
1.043 1.039 0.486 1.039 1,039 0.486
0.0112
I.057
0.425
-
01164 0.0116 @ooJJ
1.039 1.039 0.486
4.500 0446 0.091
-
0@990 0.0099 O%JlO
J ,039 1.039 0.486
3.820 0.380 0*080
-
1.043
44.0 (10) 198.0 (0.3) 16-O(@3) 167.0 (3) gZ lif 2105 (3) 83.5 (1) 10.0 (0.3)
* Numbers m brackets indicate the pK, values of the actds. t 40.0 ml of solution used. # Represents the distance travelled by the pen, on the temperature axis. to reach the end-point at the full-scale sensitivity (mVJ given in parentheses. il’The specific heats and the densities of the solutions were taken to be the same as those of pyridine (specific heat = 0.431 cal:deg. density = 0.9781 g/ml at the experimental temperature), and heats of mixing and dilution were not taken into account. Each mm of chart paper was equated to 2% x 10e3 deg, at 1-OmV sensitivity, for calculating the heats. assuming a J : 1 reaction between the acid and the base.
306
SHORTCOMMUNlCATlONS
The plots for acetic and benzoic acid and 2.5-dinitrophenol show considerable curvature (Fig. 2). and the curve-shape is not improved at increased acid concentration. Consequently, location of the end-point for these acids is not feasible. In view of the large difference in pK, values (acetic acid? 10.1: benzoic acid:’ 9.8; 2.5-dinitrophenol:” 5.8) it seems possible that the titrations in these cases are far less complete than for the first group of acids discussed above (Fig. 1). Incidentally. the earlier report’ on potentiometric titration of the acids appears to be vague and incongruous. We have not notlced any homoconJugation for any of the acids. and we are inclined to believe that they all conform to the simple titration scheme suggested m equations (I t(3). It is interestmg to note in this connection that although acetic acid proved too weak for end-point estimation, thermometric titration using DPG as the base5 yielded a distinct end-point for benzoic acid in acetonitrile which is an aprotic but protophobic solvent of moderate dielectric constant. The molar heats of reaction (in kcal) as calculated in the present work from titration data at the highest acid concentration used are given in Table 1. Excluding the case of o-nitrobenzoic acid for which no pK, estimate is available, and of hydriodic acid which might involve a large experimental error due to the low concentration (@0247M) used, the heats seem to follow the same sequence as the pK, values of the acids. No explanation of the trend can be offered at this stage. RAUL VIDAL
Chemistry Department Fnculty of Science
L. M.
MUKHERJEE*
Banaras Hindu University Vianasi 221005, India
REFERENCES 1. 2. 3. 4. 5.
M. Bos and E. A. M. F. Dahmen, Anal. Chim. Acta, 1971,s. 39; 1971,55,285. L. M. Mukherjee, D. W. Suwala and R. S. Schultz, ibid., 1972, 60, 247. L. M. Mukherjee, J. J. Kelly, W. Baranetzky and J. Sica, J. Phys. Chem., 1968.72, 3410. L. M. Mukherjee and R. S. Schultz, Talanta. 197219, 707. E. J. Forman and D. N. Hume, ibid., 1964.11, 129.
* Address all correspondence
to this author.
Summary-Thermometric titration of HClO,. HI, HNOI, HBr, picric acid. o-nitrobenzoic acid, 2,4- and 2,5_dinitrophenol, acetic acid and benzoic acid have been attempted in pyridine as solvent, using 1.3-diphenylguanidine as the base. Except in the case of 25dinitropheno1, acetic acid and benzoic acid, the results are, in general, reasonably satisfactory. The approximate molar heats of neutralization have been calculated. Zusammenfaasung-Die thermometrische Titration von HCIOI, HJ, HNOs, HBr, Pikrinslure, oNitrobenzoes;iure, 2,4- und 2,5-Dinitrophenol, Essigsiiure und Benzoetiure wurde in Pyridin als LGsungsmittel mit 1,3-Diphenylguanidin als Base versucht. Au&r bei 25-Dinitrophenol, Essigs%ure und Benzoedure sind die Ergebnisse im allgemeinen recht zufriedenstellend. Die ungelhren molaren Neutralisationswlrmen wurden berechnet. Rhtm&-On a essay6 le titrage thermomktrique de HClO,, HI. HNOs, HBr, acide picrlque. acide o-nitrobenzo’ique. 2,4- et 2,5-dinitrophinols, acide acetique et acide benzo’ique en pyridine comme solvant, utilisant la 1,3-diphknylguanidine comme base. En exceptant les cas du 2,5-dinitrophlnol, de l’acide acdtique et de l’acide benzo’ique, les rtsultats sent, en gin&al, raisonnablement satisfaisants. On a calculd les chaleurs molaires de neutralisation approximatives.