# What pH indicators are commonly used for non-polar medium?

Compounds, commonly used as pH-indicators, are soluble in aqueous media and polar solvents (DMSO, DMF, NMP etc.) and are functional pretty much across all these solvents. However, practically none these indicators are soluble in non-polar solvents (hexane, chloroform, dioxane etc.).

I briefly searched for the suitable available systems and so far there is only an article about oxoporphyrinogen derivatives used as pH-indicators in $\ce{CH2Cl2}$ [1]. Are there any other common substances that are typically used for the visualization of acid-base equilibrium in solvents with low polarity?

### Reference

1. Shundo, A.; Ishihara, S.; Labuta, J.; Onuma, Y.; Sakai, H.; Abe, M.; Ariga, K.; Hill, J. P. Chem. Commun. 2013, 49 (61), 6870–6872. DOI: 10.1039/C3CC42859A.
• pH does not really make sense outside of aquaeous solutions, so~ – Jan Dec 17 '17 at 17:35
• I disagree with @Jan's comment a bit. The notational definition of pH isn't specific to water. Relative activity of a proton is equally well/poorly defined in non-aqueous solvents. Comparing them is difficult because of different reference points. There are proposed solutions (e.g., a unified acidity scale), and this has been realised experimentally for HPLC liquid phases. That said, I don't see a straight-forward way of comparing pH across solvents via a colour indicator. If this is not a problem, five titration indicators are apparently listed here. (not a sci paper) – Linear Christmas Dec 17 '17 at 19:15
• (ref-s for previous comment: doi: 10.1002/anie.201000252, doi: 10.1021/ac504692m) – Linear Christmas Dec 17 '17 at 19:25
• @Jan, pH+ is quite sensible for Lewis and for Bronsted-Lowery acids-bases in non-aqueous media. – DrMoishe Pippik Dec 17 '17 at 22:49
• True, pH+ is associated with aqueous solutions in most people's minds. BTW, 7 is neutral only at ~300 K. At boiling, 373 K, neutral's ~6.14. – DrMoishe Pippik Dec 18 '17 at 23:30