We know, it is possible to interpret pH for aqueous solutions.but what about non aqueous solutions (e.g. ionic liquids). Should the acid/base be an Arrhenius acid/base to correctly interpret pH?
The concept of pH is technically limited to aqueous solutions as it is based on the logarithm of the equilibrium process of the autoionization of water. However, the same ideas do apply to other mediums. Meaning acidity and basicity measures are still used. Another fairly common solvent for those types of measurements is dimethyl sulfoxide (Bordwell pKa table is a common and useful example of this).
Most pH measurement processes are not extensible to organic solvents however. This is because common pH meters rely on electric current/charge transduction through the solution. Most organic solvents are not conductive to any appreciable extent (large resistivity).
pH measurement in organic solvents is possible like in water - by spectroscopy. Well known method is application of color indicators (visible light area - eyes as spectroscope :-). Proton is elementary particle (and electron also) and usually can not exist in free form in liquids - it is solvated. Solvation of protons cause color (spectra in common case) change. Solution of potassium metal (not KOH) in water and ammonia is blue in color - it is indication of solvated electrons visible by human eyes.
https://www.iupac.org/publications/pac/1985/pdf/5703x0531.pdf
https://books.google.ru/books?id=k0YksYIQ1j0C&lpg=PA43&ots=ztRy1e9IF2&dq=ph%20definition%20in%20ionic%20liquids&hl=ru&pg=PA43#v=onepage&q=ph%20definition%20in%20ionic%20liquids&f=false
There are already publications about acidity and pH scales for ionic liquids. Check out the article of Himmel et al., ChemPhysChem, 16, 1428-1439, 2015
http://onlinelibrary.wiley.com/doi/10.1002/cphc.201402906
Hello Elsayed,
Attached is the reference for an ACS article that describes the pH measurement of ab ionic liquid (EMIM-Ac) that you may find useful. Also copied is relevant text that provides the necessary experimental details. Hope it helps.
pH Control of Ionic Liquids with Carbon Dioxide and Water: 1-Ethyl-3-methylimidazolium Acetate
Courtney A. Ober and Ram B. Gupta*
Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849, United States
Ind. Eng. Chem. Res., 2012, 51 (6), pp 2524–2530
DOI: 10.1021/ie201529d
Publication Date (Web): January 03, 2012
Here are the details from abovementioned paper:
pH was measured using an extra-long-neck pH electrode from Cole-Parmer (Vernon Hills, IL, USA) connected to an Accumet BASIC microprocessor meter from Fisher Scientific (Pittsburgh, PA, USA).
pH of ionic liquid is reported in many papers. but what is the anion part in ionic liquid?
pH is mostly depend on acidic or basic ion in ionic liquid?
Dear Elsayed,
Please, find in attach, an article related on your question on: Polarity study in two classes of imidazolium based protic ionic liquids (PILs) incorporated with [HSO4]-, [HCOO]-, [CH3COO]-
and [CH3CH2COO]- anions were carried out using solvatochromic method from 298.15 to 353.15 K. For 1-methylimidazolium
class of PILs, ET(30) was found independent over the entire range of temperature, but for 1-butylimidazolium class of PILs
containing [CH3COO]- and [CH3CH2COO]- anions, ET(30) was noted to decrease with a rise in temperature. ET(30) decreases in
both the classes on varying the anions ([HSO4]-, [HCOO]-, [CH3COO]- and [CH3CH2COO]-). ET(30) value is controlled by
hydrogen bond accepter basicity, β and dipolarity/polarizability, π*. ET(30) value for PIL varies inversely to the strength of the
Coulombic interaction between ions in PILs. Strong interaction between ions leads to the lower ET(30) value. Unlike, ET(30)
thermal effect on the Kamlet-Taft parameters α, β and π* is more pronounced in imidazolium based PILs. Variation in Kamlet -
Taft parameters is solely controlled by the degree of proton transfer from Bronsted acid to Bronsted base.
Actual measurement with a simple system is actually quite tricky. In order to get a pH in the true sense of the word, you have to have a diluted aqueous solution, which is of course not possible for many ILs, what we usually do is to have a 10% IL solution in water. It is of course questionable if that is the real pH, but if always measured under the same conditions it gives an idea about the acidity of the IL.
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