Unified pH Measurements of Ethanol, Methanol, and Acetonitrile, and Their Mixtures with Water

Abstract

Measurement of pH in aqueous-organic mixtures with different compositions is of high importance in science and technology, but it is, at the same time, challenging both from a conceptual and practical standpoint. A big part of the difficulty comes from the fundamental incomparability of conventional pH values between solvents (spH, solvent-specific scales). The recent introduction of the unified pH (pHabs) concept opens up the possibility of measuring pH, expressed as pHabsH2O, in a way that is comparable between solvent, and, thereby, removing the conceptual problem. However, practical issues remain. This work presents the experience of the authors with measuring pHabsH2O values in mixtures of methanol, ethanol, and acetonitrile, with water, but without the presence of buffers or other additives. The aim was to assigned pHabsH2O values to solvent–water mixtures using differential potentiometry and the ‘pHabs-ladder’ method. Measurements were made of the potential difference between glass electrodes immersed in different solutions, separated by an ionic liquid salt bridge. Data were acquired for a series of solutions of varying solvent content. This work includes experiences related to: a selection of commercial electrodes, purity of starting material, and comparability between laboratories. Ranges of pHabsH2O values for selected compositions of solvent–water mixtures are presented.

Polyamide composite membranes for enhanced organic solvent nanofiltration performance by metal ions assisted interfacial polymerization method

“…The surface charge of the PMIA/PA(Co 2+ ) membrane under different pH conditions was tested by zeta (ζ)‐potential. The zeta potential of the membrane in the solute/ethanol solution is expected to be similar to the value in the neutral water (pH = 7) since ethanol could weakly release the same amount of proton and CH 3 CH 2 O − 55 . As can be seen in Figure 3B, the membranes are negatively charged at pH 7.…”

“…pH conditions was tested by zeta (ζ)-potential. The zeta F I G U R E 1 Representative SEM top-view images of (A) PMIA substrate, (B and C) PMIA/PA(Co 2+ ) membrane, the SEM cross-section view images of (D) PMIA/PA membrane, and (E) PMIA/PA(Co 2+ ) membrane, (F) relationship between the thickness of the polyamide layer and concentration of Co 2+ , (G and H) representative AFM images and mean roughness (Ra) surfaces (5 Â 5 μm 2 ) of PMIA/PA(Co 2+ ) membrane prepared with Co 2+ concentration of 0.04 wt%, and (I) relationship between Ra of the polyamide layer and the concentration of Co 2+ potential of the membrane in the solute/ethanol solution is expected to be similar to the value in the neutral water (pH = 7) since ethanol could weakly release the same amount of proton and CH 3 CH 2 O À 55. …”

Kinetics of oligomer-forming reactions involving the major functional groups present in atmospheric secondary organic aerosol particles

“…34 Combining this value with the total concentration of sulfuric acid in both phases gives [HSO 4 − ] org = [H + ] org = 0.37 M. This concentration can be compared to the value obtained by submerging a pH probe in the organic phase for a range of sulfuric acid concentrations and assuming pH org = −log[H + ] org . Although measurements of pH in organic solvents can be difficult to interpret, 45 the presence of 2.2 M water in the 1-octanol (the solubility of water in 1-octanol 46 ) improves the validity of this measurement. As shown in Fig.…”

Does Urea Preferentially Interact with Amide Moieties or Nonpolar Sidechains? A Question Answered Through a Judicious Selection of Model Systems

“…where K a is the dissociation constant of the acid measured in pure water, $${{a}_{H}}$$ is the activity of the hydronium ion, while $${{\gamma }_{HA}}$$ and $${{\gamma }_{{A}^{-}}}$$ are the activity coefficients of the acidic and basic forms of HA . Eq‐7 is of limited use in mixed solvent systems because of the challenges associated with directly measuring pH at high concentrations of organic cosolvent using glass pH electrodes [82,83] . To circumvent this problem for o ‐HBA and p ‐HBA, we have modified the method of Frohlinger [84] for our system: i. e., added a small amount of fluoride ion to our buffer system which will act as an NMR pH probe; [85] Eq‐7 can then be re‐written as: $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr log{{\left[{A}^{-}\right]}\over{\left[HA\right]}}=log{K}_{a}+log{{{\gamma }_{HA}}\over{{\gamma }_{{A}^{-}}}}-{\rm l}{\rm o}{\rm g}{K}_{HF}-{\rm l}{\rm o}{\rm g}{{{\gamma }_{HF}}\over{{\gamma }_{{F}^{-}}}}+{\rm l}{\rm o}{\rm g}{{\left[{F}^{-}\right]}\over{\left[HF\right]}}={\rm \ }\hfill\cr ({\rm p}{K}_{HF}^{apparent}-{\rm p}{K}_{HA}^{apparent})+{\rm l}{\rm o}{\rm g}{{\left[{F}^{-}\right]}\over{\left[HF\right]}}\hfill\cr}}$$$ …”

“…Eq-7 is of limited use in mixed solvent systems because of the challenges associated with directly measuring pH at high concentrations of organic cosolvent using glass pH electrodes. [82,83] To circumvent this problem for o-HBA and p-HBA, we have modified the method of Frohlinger [84] for our system: i. e., added a small amount of fluoride ion to our buffer system which will act as an NMR pH probe; [85] Eq-7 can then be re-written as:…”