Appendix B
Properties of electrolyte solutions

Our interest in flows of electrolyte solutions in microdevices requires that we keep track of electrolyte solutions themselves, as well as acid-base chemistry at surfaces and in buffers. The coupling of electric fields and fluid mechanics that is common in microfluidics leads also to coupling between fluid mechanics and chemistry, since acid-base chemistry describes most interfacial charge and the charge state of many common analytes. Acid-base reactions at interfaces dictate the interfacial charge and, in turn, the electroosmotic mobility of the interface. Acid-base reactions, for example, between water and DNA or between water and proteins dictate the electrophoretic mobility of proteins and DNA in solution.

Because of this, this appendix provides a description of the properties of water, electrolyte solutions, and the associated acid-base chemistry. We define solution terminology, derive the Henderson-Hasselbach equation for dissociation equilibrium, show how the Henderson-Hasselbach equation relates reaction pK_a to acid dissociation, and show how the water dissociation equation leads to a simple relation between pH and pOH for water at room temperature. This basic understanding explains, for example, the pH dependence of the electroosmotic mobility.

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