Technicolour pKa indicator

Scientists in Japan have shown that a dye can present more than five different colours according to the acidity of the solution it is in and can be used to visualise acid–base equilibria in non-polar solvents. It is extremely unusual for a single dye to demonstrate so many different colours.

Hydrogen bonding, deprotonation and different degrees of protonation all turn the dye, oxoporphyrinogen, a different colour. Hydrogen bonding of an anion to oxoporphyrinogen gives an increasingly blue colour depending on the strength of the interaction. Strongly basic substances cause oxoporphyrinogen to deprotonate and turn a pale brown hue. In the opposing direction, acids of certain strength can doubly protonate the dye. This induces tautomerisation of the dye’s structure, which varies its conjugated electronic form, leading to another significant colour change, in this case to red. Stronger acids quadruply protonate the dye, so all available electronegative atoms are protonated, turning it bright green.

Jonathan Hill, from the National Institute for Materials Science, Ibaraki, who led the work, says there are two main reasons to want to see acid–base equilibria in non-polar solvents. ‘Firstly, from an analytical viewpoint, we want a simple protocol based on changes in indicator colour for standardisation or estimation of solutions of acids or bases which are only soluble in non-polar solvents. Secondly, we want an indicator to directly observe changes in solution acidity to monitor the course or completion of a reaction.’

‘This is a well-designed system to visualise acid–base equilibria thanks to the clear color changes induced by the protonation and deprotonation of the unique nonplanar and flexible oxoporphyrinogen molecule,’ comments Yongshu Xie, an expert in colorimetric sensors at East China University of Science and Technology.

Hill adds that it is easy to incorporate the dye into solid state polymer films to make reusable or disposable indicator strips for situations where acidic vapours might be released and represent a hazard. These strips could also be integrated into optoelectronic sensing devices for environmental monitoring.