Local pH gradients and the selectivity of fast reaction—II. Comparisons between model and experiments

Abstract

Fast reaction cause segregation—local, transient concentration gradients—and their product distributions depend upon mixing. For certain reactions protons are important as products, reagents or even catalysts. One of the factors determining the product distribution is then the pH value, and for fast reactions local, transient pH gradients develop. The model explained in Part I was applied to calculate the influence of pH gradients on the selectivity of the diazo coupling between 1-naphthol and diazotised sulphanilic acid. These were induced by limiting the buffer capacity, so that the relative amounts of secondary product varied. The agreement between computed product distributions and those measured in a turbulent tubular reactor at various buffer capacities was good. Final pH values were also well predicted. The minimum quantity of buffer needed to obtain a constant pH at the reaction site was also determined. The rate of turbulent energy dissipation downstream of a feed nozzle (placed on the axis of the pipeline) dependend upon the nozzle design and was greater than in an empty pipe, when the flows were isokinetic.