Alkaline Solutions for pH Control

The dissociation constants of protonated 2-amino-2-ethyl-1,3-propanediol, 2-amino-2-methyl-1-propanol, diethylmonoethanolamine, diisopropanolamine, dimethylmonoethanolamine, monoethanolamine, 1-amino-2-propanol, methylmonoethanolamine, triethanolamine, and the first and the second dissociation constants of piperazine and hydroxyethylpiperazine have been determined by electromotive force measurements from (293 to 353) K. The dissociation constants of protonated triethylamine have been determined with the same technique from (293 to 333) K. The experimental results and derived values of the standard state thermodynamic properties are reported and compared to available literature values.

Dissociation Constants and Thermodynamic Properties of Amines and Alkanolamines from (293 to 353) K

The dissociation constants of the conjugate acids of 14 amines (diethylethanolamine, monoethanolamine, n-butyldiethanolamine, t-butyldiethanolamine, n,n-dimethylpropanolamine, methyl-diethanolamine, ethyldiethanolamine, monoethylethanolamine, n,n-dimethylisopropanolamine, triethanolamine, 4-methylpiperazine-1-amine, 3-morpholino propylamine, 4,2-hydroxylethylmorpholine, and triethylamine) were measured over a temperature range between 293.15 and 333.15 K using the potentiometric titration method. The change in standard state thermodynamic properties was derived from the van’t Hoff equation. The influence of the steric hindrance, number of −OH groups, and length of alkyl chain on the dissociation constants was identified. Of the studied amines, few sterically hindered derivatives of piperazine, a secondary amine monoethylethanolamine, and a tertiary amine n,n-dimethylpropanolamine have high pKa values but lower standard enthalpy than those of the benchmark amine, monoethanolamine (MEA), and thus were deeme...

Dissociation Constants (pKa) of Tertiary and Cyclic Amines: Structural and Temperature Dependences

Switching the basicity of ionic liquids by CO2

1. INTRODUCTION A. Background "Plus ca change, plus c'est la meme chose." In a review article published more than 30 years ago, the present author stated " … several scales, all masquerading under the name pH, are in common use. Many investigators are thinking and computing in terms of one definition and measuring a different quantity."1 An improvement toward an understanding of the nature of the practical pH value has been seen in the last three decades, but the precise manner of interpreting these values is still largely a mystery. To emphasize this situation, one has only to survey a dozen modern texts used in the teaching of analytical chemistry. The results of such an examination are summarized in Table 1. As observed earlier … "this state of confusion results in part from the impossibility of determining exactly … the molality and the activity of hydrogen (hydronium) ion for a buffer solution of moderate concentration."1

The Modern Meaning of PH

In three series of solutions: HCl, an acetate buffer and a triethanolamine buffer, the solvent water has been replaced successively by ethanol. Hammett's acidity functions H0 and H− have been determined with an indicator of aniline type and an indicator of phenol type respectively. The indicators show a strong individual behaviour and the acidity function is by no means simply dependent on the charge type of the indicator used. The product of the proton activity and the activity coefficient of the chloride ion also has been determined in these solutions as well as the pH value, measured in a conventional manner using a cell with a liquid junction.

Über pH-Werte nichtwässeriger Lösungen

The determination of thermodynamic acidity constants (dissociation constants) requires the elimination of all salt effects by an extrapolation to zero ionic strength. Laborious curvefitting may be necessary, and the results may be of low accuracy unless an extensive series of data is available. It is shown that the extrapolation can be reduced to a simple extension of a straight line of small slope when suitable experimental data are combined in the proper way. Measurements of spectral absorption and of e.m.f., taken together, will yield thermodynamic acidity constants not obtainable by either method alone. The procedures are illustrated by the determination of the acidity constants for three uncharged acids (p-nitrophenol, 2,4-dinitrophenol and 4-chloro-2,6-dinitrophenol) and for three cationic acids (m-nitroanilinium-ion, p-chloranilinium-ion, and N,N-diethyl-p-toluidinium-ion).

Die Bestimmung thermodynamischer Aciditätskonstanten

The thermodynamic acidity constant of triethanolammonium ion has been determined at 20, 25 and 30°. Triethanolamine is recommended as a buffer substance in the pH-region 7–8,5.

Roger G. Bates

Papers

Über pH-Werte nichtwässeriger Lösungen

Die Bestimmung thermodynamischer Aciditätskonstanten

Triäthanolamin als Puffersubstanz