The pH of methanol in water is a complex topic due to the solvent-specific nature of pH measurements in non-aqueous media. Traditional pH measurements are based on the activity of hydrogen ions (H+) in aqueous solutions, but in non-aqueous solvents like methanol, the concept of pH is not as straightforward.
Understanding the Challenges of pH Measurement in Methanol-Water Mixtures
In the context of DIY users, it is essential to understand that pH measurements in methanol-water mixtures are not directly comparable to pH measurements in pure water or aqueous solutions. The pH scale in methanol and methanol-water mixtures is defined in terms of the pHst values of standard buffer solutions in these solvents, and the unified pH (pH abs) concept has been introduced to allow for comparable pH measurements between solvents.
For methanol-water mixtures, the pH can be measured using differential potentiometry and the ‘pH abs-ladder’ method, which involves assigning pH values to solvent-water mixtures using potential differences between glass electrodes immersed in different solutions, separated by an ionic liquid salt bridge. However, this method has practical issues, such as the selection of suitable commercial electrodes, purity of starting materials, and comparability between laboratories.
Factors to Consider when Measuring pH in Methanol-Water Mixtures
When dealing with methanol-water mixtures, it is crucial to consider the potential contaminants and chemicals present. For instance, the pH of acidified methanol can be determined by adding one mole of H+ per liter (including methanol in units of liter), resulting in a pH of 0.0. However, in dilute aqueous solutions of methanol (greater than 40 mol %), a pH probe can be used, but it is more likely to damage the probe than to provide accurate results.
Recommended Methods for Accurate pH Determination in Methanol-Water Mixtures
To accurately determine the pH of methanol in water, DIY users should consider using methods like differential potentiometry and the ‘pH abs-ladder’ method. These methods take into account the solvent-specific nature of pH measurements and provide a more reliable way to measure the pH in methanol-water mixtures.
Conclusion
In summary, the pH of methanol in water is a complex topic that requires a thorough understanding of non-aqueous pH measurements. DIY users should be aware of the challenges associated with pH measurements in methanol-water mixtures and consider using specialized methods for accurate pH determination. Additionally, understanding the potential contaminants and chemicals present in methanol-water mixtures is crucial for safe and effective use.
References:
- The universal pH‐scale in methanol and methanol‐water mixtures – 1967. https://onlinelibrary.wiley.com/doi/abs/10.1002/recl.19670861104
- Unified pH Measurements of Ethanol, Methanol, and Acetonitrile … – NCBI. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8201227/
- How can the pH of acidified methanol be determined? – ResearchGate. https://www.researchgate.net/post/How_can_the_pH_of_acidified_methanol_be_determined
- Abrash 2001.pdf – A paper discussing the pKa of methanol and its implications for pH measurements in methanol-water mixtures. https://www.sciencemadness.org/talk/viewthread.php?tid=14877
- pH in Methanol | Yokogawa Electric Corporation. https://www.yokogawa.com/library/resources/application-notes/ph-in-methanol/
- Discussion on Sciencemadness Discussion Board regarding pH measurements in methanol. http://www.sciencemadness.org/talk/viewthread.php?tid=14877