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The control of acidity influences the structural assembly of biopolymers that are essential for a wide range of applications. Its miniaturization can increase the speed and the possibilities of combinatorial throughput for their manipulation, similarly to the way that the miniaturization of transistors allows the high throughput of logical operations in microelectronics. Here we present a device containing multiplexed micro-reactors, each one enabling independent electrochemical control of the acidity in ~ 2.5 nL volumes, with a large acidity range in aqueous solutions from pH 3 to 7 and an accuracy of at least 0.4 pH units. The attained pH within each microreactor (with footprints of ~ 0.3 mm2 for each spot) was kept constant for long retention times (~10 minutes) and over repeated cycles >100. The acidity is driven by redox proton exchange reactions, which can be driven at different rates that influence the efficiency of the device in order to achieve more charge exchange (larger acidity range) or better reversibility. By the performance in the acidity control the miniaturisation and the possibility to multiplex paves the way for the control of combinatorial chemistry through pH and acidity controlled reactions.