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Acid-Base Behavior in 50-Percent Aqueous Methanol: Thermodynamics of the Dissociation of Protonated Tris(Hydroxymethyl)Aminomethane and Nature of the Solvent Effect*

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Acid-Base Behavior in 50-Percent Aqueous Methanol: Thermodynamics of the Dissociation of Protonated Tris(Hydroxymethyl)Aminomethane and Nature of the Solvent Effect* JOURNAL OF RESEARCH of the National Bureau of Standards- A. Physics and Chemistry Va!. 69A, No. 3, May- June 1965 Acid-Base Behavior In 50-Percent Aqueous Methanol: Thermodynamics of the Dissociation of Protonated Tris(hydroxymethyl)aminomethane and Nature of the Solvent Effect* Maurice Woodhead,! Maya Paabo, R. A. Robinson, and Roger G. Bates (February 16, 1965) Electro mutive-force methods a nd cell s without liquid junc ti on have been uti lized to dete rmine the acid ic dissociati on constant of protonated tris(hydroxymethyl)a minome th a ne 12- amino-2-( hy­ droxymethyl)- 1, 3- propanedi ulJ in 50 wt percent methanol solv e nt at seve n te mperatures fro m ]0 to 40 °C. The c hange of the di ssociat ion consta nt with te mpera ture has been used to calcul ate the c ha nges of enthalpy, entrop y, and heat capacity when the dissocia ti on takes pl ace in the sta ndard state. Compari sons with earli er measureme nts in the aqueous medium reveal no great difl'ere nces in the entha llJ Y and entropy, s uggestin g that wate r participates in prefere nce to metha nol in the proto­ lyti c react io n even in 50-percent mcthanol. It is shown th at electrostatic considerations alone a re unable to explain the solve nt efl'ect on the dissociation ene rgy, a nd a subs ta nt ia l " basic it y e fl'ect" is indicated . The activity-coeffi c ie nt te rm for the a mine hydrochl oride in eq uimolal a mine-salt bufl'e rs has been evaluated and compa red with s imila r data in the water solve nt. T he di ssociation cons tant of an acid of charge type 1. Introduction A+Bo, namely the acid conjugate to the wea k un­ charged base 2-amino-2-{hydroxymethyl)-1, 3-p ro­ Acid-base studies of organic compounds only sli ghtly panediol or tri s{hydroxyme thyl)aminome thane ,3 soluble in water are some times conveni e ntly made in has been studied in 50-percent methanol from 10 to a solvent consisting of equal parts by weight of water 40 °C. The associated thermodynamic quantities and methanol. In order to facilitate the de termination have bee n derived. Buffe r solutions composed of of pH and pK in 50-percent me thanol, an operational this primary amine and its salt have proved extraordi­ pH scale has recently been established for this solvent narily useful for pH control in biological syste ms [2] _ mixture [1)2. The standard scale, defin ed in a manner consistent with thermodynamic di ssociation con­ 2. Experime'Iltal Methods stants and activity coeffi cients in 50-percent me thanol, is fixed by three suitable reference solutions, namely 2 .1. Materials an acetate buffer, a phosphate buffer, and a solution of sodium hydrogen succinate. An aqueous solution of twice-di stilled hydrochloric As a part of the earlier work, the di ssociation con­ acid was used as a primary standard. Its molality stants of acetic acid. and dihydrogen phosphate ion was determined by gravimetric chloride determination; in 50-percent methanol were determined from 10 to the standard deviation derived from three determina­ 40°C These two acids are of charge types AOB ­ tions was 0.01 percent. The purity of four lots of and A- B=, respectively. Here A and B refer respec­ crystalline tris{ hydroxyme thyl)aminome thane (ob­ tively to the acid and its conjugate base. The results tained from commercial sources) was found to be for the enthalpy and entropy of ionization in 50- 99.92, 99.94, 99.94, and 100.22 percent b y titration percent methanol were of interest for their bearing with the standard solution of hydrochloric acid _ on the relati on between charge type and solvent Weight burets were used, and each sample was ad­ effec t. justed to the theoretical equivalence point (PH 4.54 in a 0.1 M solution of the ne utralized base) with the aid of glass-electrode measurements. If: Presented before the Di vision of Analytical Chemi stry at the 149th National meeting of the Am e ri can Chemical Society, Detroit , Mic h., on April 7, 1965. I Guest wo rk er, on leave from l\'1a kerere Unive rsit y College, Kampala, Ug anda. .3 For brevity, this base will sometimes be refe rred 10 as " tri s" and the corresponding 2 Fi gures in brackets in dicate the lit erature references at the end of th is paper. hydrochloride as " tris hydrochloride." 263 For convenience, many of the cell solutions were , 2.2. Procedures prepared from a commercial grade of tris hydrochloride together with the free base. The emf of cells contain­ In general, the experimental procedures followed ing buffer solutions prepared in this way was compared those used in determining the dissociation constant with solutions of identical nominal molalities prepared of tris in water [5]. Electromotive force measure­ from tris and the standard solution of hydrochloric ments of the cell acid. Three buffer concentrations spanning the range covered in the study were compared in this way. The Pt;H2(g, 1 atm), tris' HCl(ml), tris(m2) in 50 wt percent difference in emf was found to be 0.39 m V with a methanol, AgCl; Ag standard deviation of 0.05 m V. The cells prepared with the tris hydrochloride gave the higher emf. where ml and m2 are molalities, were used. Thirty­ The recorded emf for cells prepared in this way was four solutions, all with buffer ratio close to unity, were adjusted to correspond to that obtainable with pure studied. When tris hydrochloride was used, a stock tris (assay 100%) and the standard solution of hydro­ buffer solution was prepared by weighing the free chloric acid. base, the salt, and water. The remainder of the solu­ On examination, the tris hydrochloride obtained tions were prepared by dissolving a known weight of commercially was found to be very close to stoichio­ tris in a known weight of standard hydrochloric acid. metric neutrality; measurements of pH and buffer The necessary weight of methanol was then added, capacity showed that the product contained no ap­ together with more water, to achieve the desired preciable excess of either the base or of hydrochloric solvent composition. acid. Analysis for chloride by gravimetry showed, Each cell contained one hydrogen electrode and one however, that the acidic and basic components, while silver-silver chloride electrode. The emf measure­ present in equivalent amounts, were both present in ments at the seven temperatures were made in a lower quantity than expected (about 99.1 percent of variety of sequences. A complete series required theoretical). Furthermore, samples of the salt pre­ from two to three days. The solubility of silver pared by the authors by neutralizing the pure tris chloride in a methanol-water solution of tris was not with hydrochloric acid were also found to assay from determined, as it had been found insufficient to require 99.1 to 99.6 percent. corrections in aqueous solutions [5]. Methanol IS The inert impurity in the commercial product, presumed to lower the solubility still further. thought to be water by the manufacturer, could not be identified.4 Drying at temperatures low enough 2.3. Results to preclude decomposition of the salt did not mater­ ially increase the assay value.5 Although drying The emf data are summarized in table 1. Cor­ brought about some improvement in the results of the rections have been made to the reference hydrogen carbon and hydrogen analysis, the figures were partial pressure of 1 atm with the aid of the recorded inconclusive: barometric pressure and the known total vapor pres­ sure of the methanol-water solvent at the temperature in question [4]. The emf recorded for those solutions Carbon, Hydrogen, prepared by weighing tris and the solid hydrochloride percent percent was further corrected for the known deficiency of ionized chloride in these solutions. Commercial sample", ...... ".""""" "" .. " ,,. 30.81 7.83 30.56 7.69 3. Calculation of the Dissociation Constant Commercial sample, after drying""" .. """" 30.60 7.69 30.60 7.84 The calculation of pK, where K is the acidic dis­ sociation constant of 2·ammonium-2-(hydroxymethyl)- Theoretical ..... .. ........... .. " .................... " 30.49 7.67 1,3-propanediol (that is, tris' H+) in water, has been described in detail in earlier papers [3, 5]. The cor­ The supposition that the impurity was electrochem­ responding dissociation constant in 50 wt percent ically inert was confirmed by a comparison of the methanol, referred not to the standard state in water correction to the emf observed (0.39 m V) with that to but to that in this solvent, is termed p(sK). The pro­ be expected from the known assay (0.46 m V). The cedure for obtaining p(sK) is entirely analogous to difference corresponds to only 0.001 unit in pK. that for the computation of pK (that is, g(wK)) in water. The methanol with which the solvents were prepared "Apparent" values of p(sK)' were computed from the was "Spectro Grade," of the same quality as that used emf E by the equation in the earlier studies in 50 wt percent methanol solvents [1, 4]. p(sK) I == p(sK) - f3! = (R; ~ sf;)/F + 2 log ml -log m2 4 Datta, Grzybowski. and Wilson [3] reported a similar low chloride assay for the samples of tris hydrochloride prepared for their study of the di ssociation of tri s in aqueous solution.
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