Progress in Buffer Choice for the Villermaux–Dushman Reaction

TRIS [tris­(hydroxymethyl)­aminomethane] is proposed as a new buffer with the advantages of being nontoxic and having only one conjugated acid. Moreover, it is well-established in biochemical applications, has good solubility in water (550 g/L), and is reasonably priced. Comparable micromixing experiments in a TRIS buffer and a borate buffer led to similar segregation indices with respective micromixing times. Furthermore, TRIS was successfully transferred to experiments in viscous glycerin–water solution. Comparable micromixing experiments with sulfuric acid in TRIS highlight the importance of including the ionic strength dependency of the second dissociation constant of sulfuric acid to obtain similar micromixing times. Micromixing experiments become thereby independent of the acid choice. The application and limitation of the phosphate buffer were investigated in more detail where comparable experiments have shown that HPO4 2– is the only species to be considered in micromixing time calculations when the HPO4 2–/H2PO4 – ratio is over 20. It is shown whether and how the effect of the ionic strength on the pK a2 value of phosphoric acid should be considered when micromixing experiments are conducted in a phosphate buffer as it is close to the dismutation pH* of iodine.