Electrokinetic control of a chemical reaction in a lab-on-a-chip micro-reactor: measurement and quantitative modelling

We have investigated the complex formation/dissociation reaction between Ni(2+) ions and the ligand pyridine-2-azo-p-dimethylaniline (PADA) in a glass micro-reactor operating under electrokinetic control. An in situ, microscope-imaging technique was used to determine the spatial and temporal evolution of the reaction within the channel network of the micro-reactor. Using appropriately controlled voltage sequences, a 'slug' of PADA was injected into a stream of Ni(2+) solution. Under the experimental reaction conditions used, Ni(2+) ions are mixed with the PADA as a consequence of the species' different electrokinetic mobilities allowing the complex formation to occur at the trailing edge of the PADA slug. Following complex formation, reversal of the flow results in the partial re-formation of free PADA by dissociation of the complex, demonstrating that voltage control can be used to drive the reaction either forwards or backwards. We discuss the methods whereby all the parameters required to predict the spatial and temporal evolution of the reaction in the micro-reactor can be either measured or estimated. Based on the estimated parameters, model calculations of the concentration profiles as a function of time show good agreement with the measured data.