persat - basic principles of electrolyte chemistry for microfluidic electrokinteics part 1 - acid...

Basic principles of electrolyte chemistry for microfluidic electrokinetics. Part I:† Acid–base equilibria and pH buffers ‡x Alexandre Persat, Robert D. Chambers and Juan G. Santiago * Recei ved 1st April 2009, Accepte d 4th June 2009 First published as an Advance Article on the web 7th July 2009 DOI: 10.1039/b906465f We review fundamental and applied acid–base equilibrium chemistry useful to microfluidic electrokinetics. We present elements of acid–base equilibrium reactions and derive rules for pH calculation for simple buffers. We also present a general formulation to calculate pH of more complex, arbitrary mixtures of electrolytes, and discuss the effects of ionic strength and temperature on pH calculation. More practically, we offer advice on buffer preparation and on buffer reporting. We also discuss ‘‘real world’’ buffers and likely contamination sources. In particular, we discuss the effects of atmospheric carbon dioxide on buffer systems, namely, the increase in ionic strength and acidification of typical electrokinetic device buffers. In Part II of this two-paper series, we discuss the coupling of acid–base equilibria with electrolyte dynamics and electrochemistry in typical microfluidic electrokinetic systems. Introduction Microfluidics is a rapidly expanding group of enabling technol- ogies in the fields of chemistry, biology, and biomedical engi- neering. 1–3 System designs are growing increasingly complex and more specialized, and a wide array of electrolyte chemistry is being employed. Electrolyte chemistry is a key consideration in nea rly all mic rofl uid ic dev ice s and tec hni que s, and thi s is parti cular ly true of elec troki netic systems. On-c hip mixin g, react ion, separ atio n, puri ficati on, elec troph oretic separa tion , and elec troos moti c flow are each inhe rentl y physi coch emica l proce sses and conse quent ly coup led to ionic concentration s, reac ti ons, and equi li br ia. Furt he r, Faraday current and associ ated electrod e react ions are coup led to and can signi fi- cantl y affec t elec troly te chemi stry and primary func tions of a microfluidic device. Well-controlled and well-specified chem- ist ry in ele ctr oki net ics is cru cia l to syst ematic experi men tal design, robust operation, and reproducibility. Despi te its importan ce, the ratio nale behind the choi ce of electrolyte chemistries and pH buffers used for specific applica- tions are rarely directly addressed in the microfluidics literature. In par tic ula r, the var iou s pri nci ple s ass oci ate d wit h the se phenomena have not, to our knowledge, been summarized in a paper easily addressable to the microfluidics community. We here present a general introduction to the fundamental principles of electrolytes and acid–base chemistr y. We also summarize some misconceptions and inconsistencies we have observed in the microfluidics literature. These include errors in ionic concentration estimates; erroneous or incomplete reports of buffer chemistry; and inconsistent or incomplete reports of electrophoretic mobilities. Ou r intent is not to merely poi nt these out, b ut to offer a basic description of the problem, suggest formulations, and provide a practical summary of how to choose electrolytes and buffers for microfluidics. Depar tment of Mechanical Enginee ring, Stanford University, Stanford, CA, 94305, USA. E-mail: juan.santiago@stanfor d.edu; Fax: +1 650- 723-5689; Tel: +1 650-723-7657 † For Part II see ref. 4. ‡ Part of a Lab on a Chip themed issue on fundamental principles and tech niqu es in micro fluid ics, edited by Juan Santiago and Chuan-Hua Chen. x Elec tron ic supp lemen tary infor matio n (ESI ) avai lable : Summary of buffer survey. See DOI: 10.1039/b906465f Al exa nd re P er sa t is a PhD candid ate at Stanf ord Unive r- sit y, under the supe rvisi on of Prof . Juan G. Santiago. He received his MS in Chemical En gineerin g at Stanford Univer sit y, and hi s Di pl ome d’Ing eni eur from l’Ec ole Poly - technique, France. Robert D. Chambe rs receiv ed h is BS fro m Ha rve y Mud d College in 2006. He is currently pursuing his PhD at Stanford Universit y, where he is sup- ported by a fellowship from Kodak corporation. This journal is ª The Royal Society of Chemistry 2009 Lab Chip, 2009, 9, 2437–2453 | 2437 TUTORIAL REVIEW www.rsc.org/loc | Lab on a Chip

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