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Spectrophotometric investigations in aqueous solution at elevated temperatures. The effect of temperature on the ionisation constant of the 2,2′ bipyridyl cation

 

作者: David H. Buisson,  

 

期刊: Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases  (RSC Available online 1977)
卷期: Volume 73, issue 1  

页码: 157-163

 

ISSN:0300-9599

 

年代: 1977

 

DOI:10.1039/F19777300157

 

出版商: RSC

 

数据来源: RSC

 

摘要:

Spectrophotometric Investigations in Aqueous Solutionat Elevated TemperaturesThe Effect of Temperature on the Ionisation Constant of the 2,2' Bipyridyl CationBY DAVID €3. BUISSON-~ AND ROGER J. IRVISG'~Department of Chemistry, University of Surrey, Guild ford, Surrey GU2 5XHReceived 30th April, 1976The ionisation constant, Ka, of the 2,2' bipyridyl cation has been determined at various tempera-tures between 298 and 473 K from spectrophotometric data. The results, expressed as a functionof absolute temperature, are given by the equationpKa = (968.58/T)+0.735 53+0.001 131 1 T.The increase in ionisation with increasing temperature is attributed to increasing solvation of thehydronium ion relative to the 2,2' bipyridyl cation.The changes of enthalpy, entropy and heat capacity for the ionisation reaction have been calculatedfrom the temperature coefficients of the ionisation constant at each of the experimental temperatures.'There have been a number of determinations of the acid dissociation constant(K,) of thc 2,2' bipyridyl cation (Bipy H+) under a variety of experimental csndi-tions 2 v but only up to a temperature of 323 K4 A knowledge of the behaviourof compounds such as bipyridyl in water at high temperatures is desirable becauseof their potential use as ligands to chelate metal ions which could otherwise causeprobleins in high pressure steam generating plants.This paper reports the determination of K, of Bipy H+ over the temperaturerange 298-473 K.EXPERIMENTALAnalytical grade 2,2' bipyridyl (Koch Light) and AnalaR perchloric acid were usedwithout further purification, water was twice aistilled and deionised.For determinations in the range 298-333 K a Unicam SP3000 spectrophotometer fittedwith a 1 cm flow-through cell and thermostatted cell block was used.In the temperaturerange 353-373 K a Unicam SP1800 U.V. spectrophotometer fitted for digital readout inabsorbance was used. The temperature was maintained to kO.5"C using a thermostattedelectrically heated aluminium block. Matched 1 cm Spectrosil cells were used in each ofthese series of experiments.At temperatures above 373 K spectroscopic measurements were carried out using a hightemperature, Teflon lined cell between quartz windows, heated in an aluminium furnace asdescribed pre~iously.~ The temperature in the cell, which was controlled to kl"C, wasmeasured by means of an iron-constantan thermocouple which extended to the surface ofihe inner cell.In a typical series of experiments at temperatures below 373 K one cell contained waterand the other a series of solutions of varying hydrogen ion cencentration (adjusted usingAnalaR perchloric acid) suitable for the determination of the ionisation constant.Afteran equilibration period the absorbance of each solution was measured and corrected forthe appropriate cell blank. The extinction coefficient of the Bipy was determined for usepresent address : Chemistry Division, DSIR, Private Bag, Pstone, New Zealand.15158 SPECTROPHOTOMETRY AT ELEVATED TEMPERATURESin calculations rather than HBipy+ as experimental determinations showed that withincreasing acid concentrations there was a progressive shift in the spectral peaks of a solutionof 2,2’ bipyridyl.This has been attributed by McBrycie to a second stage protonation of2,2’ bipyridyl. Measurements in the high temperature cell entailed measurements of theseries of solutions only : it was not necessary to know the background absorbance of thecell and optical path length provided these were constant within very small limits from oneexperiment to the next. A level of reproducibility of 0.5-1 % was maintained over the wholetemperature range. Variations in pressure of as much as 100 bar had a negligible effect onabsorbance readings, so for convenience the pressure was maintained at just above thesaturation vapour pressure of water at the appropriate temperature.RESULTS AND DISCUSSIONThe equilibrium for the dissociation of HBipy+ may be writtenH Bipy+ + H+ + Bipy.HenceIn order to avoid the difficulty of estimating activity coefficients, the present workhas been carried out in solutions having ionic strengths of the order of where itis reasonable to assume, within experimental error, that all activity coefficients areunity.Thus K, was considered to be equal to K,.The calculation of K, was based on the equations, due to Maroni and Calmanand revised by Albert and Serjeant as follows :where D is the absorbance of a solution of Bipy of hydrogen ion concentration H+.Letting EO and 8, represent the extinction coefficients of Bipy and BipyH+ respectivelyat wavelength A thenDo = c o d and D, = E,CZwhere c is the total concentration of 2,2’ bipyridyl and Zis the optical path length in cm.Usingthis value of D, and eqn (2), pKa is calculated for each hydrogen ion concentrationand an average value of pK, determined.In view of the discussion of Quist andMarshall the molar concentration scale was used throughout. Concentrations of2,2’ bipyridyl and perchloric acid were, therefore, corrected for expansion of thesolutions on heating assuming that the solutions were sufficiently dilute SO as toexpand as pure water.l0. l1 The determinations of pK, were carried out at a numberof different wavelengths and at least two parallel determinations were carried out ateach temperature.The experimental data were incorporated in a least squares computer programand processed on an ICL 1905F computer.Absorbance values and the results ofcalculations to determine Ka at 298 K and 423 K are given in table 1. These aretypical calculations at widely different temperatures. Table 2 contains values of K8calculated for the full temperature range 298 K (pKa = 4.323) to 473 K (PKa = 3.32).Eqn (3) l2 in the form proposed by Harned and Robinson,13 expresses the pK, valuesas a function of absolute temperature :From eqn (1) a plot of D against (D-D,)/[H+] has an intercept D,.pKa = A/T-B+CT (3D. H. BUISSON AND R. J . IRVING 159where A = 968.58, B = -0.735 53 and C = 0.001 131 1. Thermodynamic quan-tities for the dissociation were computed from the coefficients of eqn (3) using thefollowing :AGO = 2.3026 R ( A - BT+ CT2)AHo = 2.3026 R (A-CT2)(4)(5)AS" = 2.3026 R (B-2CT)AC; = 2.3026 R (-ZCT).TABLE ACIDITY CONSTANT OF 2,2' BIPYRIDYL CATION AT 298 AND 423 K2 = 310.0 nm ; c = 6 .0 ~ mol dmA3 ; Do = 0.0058T = 298 K D , = 0.750104[HC104]/mol dm-3 D 104[H+]/mol dm-3 M a 1 0 5 ~ ~1.960 0.572 1.502 4.323 4.7482.450 0.605 1.965 4.321 4.7723.430 0.646 2.91 1 4.326 4.7264.900 0.678 4.354 4.329 4.6829.800 0.71 3 9.219 4.313 4.861average PKa = 4.322 apKa = 0.005average Ka = 4.785 x aKa = 0.066~ lo5A, = 310.0 nm ; c = 6.0 x mol dm-3 ; Do = 0.0203T = 423K D , = 0.7652.082 0.280 1.720 3.492 32.203.644 0.394 3.077 3.514 30.625.205 0.456 4.461 3.499 31.706.767 0.501 5.861 3.492 32.2210.41 0.573 9.158 3.497 31.84average PKa = 3.499 up& = 0.009average Ka = 31.72 x UKa = 0.65 xThese are also included in table 2.It has been assumed that the pressure remainsconstant over the whole temperature range as the effect it would have on the thermo-dynamic constants would be expected to be small.TABLE 2.-THERMODYNAMIC CONSTANTS OF THE DISSOCIATION OF THE 2,2' BIPYRIDYL CATIONPKS PKU AG" AH" AS" A&temp/K lO4Ka (expt) OpKa (calc) /kJ mol-1 /kJ mol-1 /J K-1 mol-1 /J K-1 mol-12983083183383593803984234484730.47580.59420.72731.0391.4451.8912.373.174.054.784.3234.2264.1383.983.843.723.623.503.393.320.0050.0050.0050.010.010.010.010.010.010.014.3214.2274.1403.983.853.713.623.503.403.3224.7 _+ 0.0125.0 f 0.0125.2 f 0.0125.8 -t 0.0126.4 -t 0.0127.1 k 0.0127.6 -t 0.0128.4 k 0.0129.2 k 0.0130.1 & 0.0216.6-tO.0416.5 k 0.0416.4 k 0.0316.1 & 0.1015.8 f 0.0715.4 f 0.0515.1 k0.0714.7k 0.1014.2 f 0.2013.7k0.2- 27.0 & 0.1- 27.4 & 0.1- 27.9 f 0.1- 28.7 f 0.3- 29.7 -t 0.2-30.6k0.1-31.3k0.2- 32.4 f 0.3- 33.5 & 0.4- 34.6 & 0.5- 12.9 & 0.4-13.4f0.4-13.8k0.4-14.7t-2-15.652-16.5+2-17.3k4- 18.3 f 5- 19.4k 5-20.5f160 SPECTROPHOTOMETRY AT ELEVATED TEMPERATURESComparison of the pK,'s in table 2 with other data is not possible as measurementshave not been attcmpted before on 2,2' bipyridyl at elevated temperatures.Potentio-metric neasurements, however, at 298 K and at ionic strengths ( I ) of 0.025 and" extrapolated to zero " gave pK,'s of 4.33 l4 and 4.35 respectively and a spectro-photomeirk determination at an I of 0.01 gave a pK, OF 4.34 ; I s all in agreement withthe value obtained in this study. Calorimetric studies at 293 K ( I = 0.1) and 303 K( I = 1.0) gave values of AH" of 15.3 l 5 and 16.8k2.1 l7 kJ mol-1 respectivelycompared with values of 16.7 and 16.6 kJ m01-~ calculated using eqn (5) and AS'of - 34.3 l6 J K-' mol-1 compared with -25.1 J 3C-l mob1 calculated using eqn (6).The plot of AHagainst TAS" shown in fig. 1 is a straight line and is an excelleiitexample for a single compound of tbe so called " Compensation Law ".Theimportance of the concept of this linear enthalpy-entropy effect was emphasized byHaanmett.' * It has been proposed that the major source of this effect of compensationin solutions is attributable to solvation changes.lg The fact that the plot in fig. 1is a straight line shows that the same principles apply to a single compound over arange of temperatures, and lends support to the explanation of the " compensationlaw * ' in ternis o f solvation, which in this case is a function of temperature.1.71.63 - I0 E2dt 1.5 2L,\I.?\I I I I I t I 1 0- TAS/J mol-'FIG. 1.-Variation of AH" with TAS" for the ionisation of the 2,2' bipyridyl cation.The process of ionisation is usually divided into two parts, one, the reaction part,a function of the solute and the other, the hydration part, a function o f the solvent.20In ionisation reactions Ives and Marsden 2o suggested that the enthalpy and entropyof hydration make the major contribution to the total AH" and AS" of the reactionand hence the coinpensation law is believed to have its main application to reactionswhich differ from each other principally in the extent of solvation changes whichaccompany them.For an isoelectric process a plot of -log K, against 1/T will be a straight lineassuming the "reaction part " work done in the proton transfer is independent oftemperature, and the hydration part work is zero or close to zero.21 Such a plotshown in fig.2 is a straight line indicating that these assumptions are justifiedD .H . BUISSON AND R. J . IRVINGI _ _-2 I I2.5 3.0 3.5d2 .clo3 KITFIG. 2.-Variation of log Ka with 1/T for the ionisation of the 2,2' bipyridyl cation.000I I I I2.2 2.L 2.6 2.0161lo2 DklFIG, 3.-Variation of TpKa with as derived from the Born equation AG,l = B+A/&(T) forthe cations of the following: aminopyrine 2 2 x , 2,2' bipyridyl (this work) 0, aniline (in aqueous 220, and methanol + water 24 0 mixtures), and 4 aminoantipyrine 22 + .1-162 SPECTROPHOTOMETRY AT ELEVATED TEMPERATURESFor many reactions the electrostatic or hydration contribution to the free energy(AG,,) of dissociation can be expressed in terms of the Born or Bjerrum equationswhere Dk is the dielectric constant at temperature T and A and B are constants.Hence, over a variable temperature range there should be a linear relationshipbetween TpKa and l / D k .Fig.3 shows plots of TpKa against l / D k for a number of isoelectric ionisations.Over a relatively small temperature range 288-323 K TpK, against l / D k is very closeto a straight line for anilinium, 4-aminoantipyrine, aminopyrine 22 and 2,2' bipyridylbut over the full temperature range (298-473 K) the plot for the latter substance showsa marked curvature. There are no studies over a comparable temperature rangefor similar compounds but, through the use of mixed solvents, the dielectric constant(Dk) can be varied over a similar range to that covered by the temperature change.Thus for water+methanol mixtures at 25"C, Dk changes from 78 in pure water to36.8 in 10 % water+%) % methanol.At 473 K water has a D, of 3 4 . C ~ ~ ~ Acomparison can be made of the pKa of the anilinium ion of 4.45 in 20 % methanol/80 % water 24 with a Dk of 71 and the pK, of 4.25 of the anilinium ion in water at319 K ( D , = 71) interpolated using eqn (3).The difference in slope of the plots of TpKa against l/Dk for the anilinium ion,where the change in one case is due to variation of Dk with temperature and in theother with variation of solvent composition, is largely due to selective hydration inthe latter case so that although comparisons can be made, a detailed analysis of thedifference would be fruitless.We thank the Central Electricity Generating Board for financial support of thiswork.Previous paper in this series.A. W. L. Dudeney and R. J. Irving, J.C.S. Faraday I, 1975,71, 1215.Stability Constants, Special Publication No. 17 (The Chemical Society, London, 1964).Stability Constants, Supplement No. 1, Special Publication 25 (The Chemical Society, London,1971).R. Nasanen, Suomen Kemi., 1955, 28, 161.R. D. Alexander, A. W. L. Dudeney and R. J. Irving, J. Phys. E, 1974,7,522.W. A. E. McBryde, Canad. J .Chem., 1965, 43,3472.A. Albert and E. P. Serjeant, The Determination ofIonisation Constants (Chapman and Hall,London, 2nd edn., 1971).A. S. Quist and W. L. Marshall, J, Phys. Chem., 1968, 72, 684.lo R. E. Mesmer, F. H. Sweeton, B. F. Hitch and C. F. Baes, Proc. Int. Conf. High Temp. HighPress. Electrochem. in Aq. Soh. (University of Surrey, Guildford, 1973).l1 G. S. Kell, in Handbook of Chemistry and Physics (Chemical Rubber Co., Cleveland, 49thedn., 1968).l 2 The constants were calculated by fitting a set of data points as a series of Chebyshev poly-nomials : E. Stiefel, Numerical Methods of Chebyshev Approximation in On NumericalApproximation ed. R. E. Langer (U. of Wisconsin Press, Madison, 1959).' P. Maroni and J. P. Calman, Bull. SOC. Chim. France, 1964, 519.l3 H. S. Harned and R. A. Robinson, Trans Faraday Soc., 1940, 36,973.l4 J. H. Baxendale and P. George, Trans. Faraday SOC., 1950, 46, 55.l5 M. T. Falqui, Gazetta, 1958, 88, 57.l6 G. Anderegg, Helv. Chim. Acta, 1963, 46, 2813.l7 R. L. Davies and K. W. Dunning, J. Chem. Sac., 1965, 4168.l8 L. P. Hammett, Trans. Faraday SOC., 1938, 34, 156.l9 K. J. Laidler, Trans. Faraday SOC., 1959, 55, 1725.2o D. J. G. Ives and P. D. Marsden, J. Chem. Suc., 1965, 649D. H. BUISSON AND R . J . IRVING 16321 R. W. Gurney, Ionic Processes in Solution (Dover Publications, New York, 1962).2 2 Data taken from F. Kopecky, M. Pesak and J. Celechovsky, COIL Czech. Chem. Camm.,23 G. C . Akeslof and H. I. Oshry, J. Arner. Chern. SOC., 1950,72,2844.1970, 35, 576.R. A. Robinson and R. H. Stokes, EZectroZyte Solution (Butterworth, London, 1959).(PAPER 6/833

 

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