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AN ALTERNATIVE METHOD FOR RELATING MACROSCOPIC TO MICROSCOPIC ACIDITY CONSTANTS WITH ZWITTERIONIC SPECIES
Citation:
Loux, N T., T W. Collette, AND J W. Washington. AN ALTERNATIVE METHOD FOR RELATING MACROSCOPIC TO MICROSCOPIC ACIDITY CONSTANTS WITH ZWITTERIONIC SPECIES. Presented at 223rd American Chemical Society National Meeting, Orlando, FL, April 7-11, 2002.
Impact/Purpose:
Elucidate and model the underlying processes (physical, chemical, enzymatic, biological, and geochemical) that describe the species-specific transformation and transport of organic contaminants and nutrients in environmental and biological systems. Develop and integrate chemical behavior parameterization models (e.g., SPARC), chemical-process models, and ecosystem-characterization models into reactive-transport models.
Description:
Using the notation of Adams (1916. JACS, 38:1503), zwitterionic microscopic acidity constants defined by: ka = [H+] [+H3NRCOO-]/ [+H3NRCOOH]; kb = [H+] [H2NRCOOH]/ [+H3NRCOOH]; kc = [H+] [H2NRCOO-]/ [+H3NRCOO-]; and kd = [H+] [H2NRCOO-]/ [H2NRCOOH] are historically related to the experimentally measurable macroscopic acidity constants by: K1 = ka + kb and 1/K2 = 1/kc + 1/kd (ignoring activity coefficients). These expressions can be viewed as procedures for approximating biprotic systems with a monoprotic formulation. The reader can observe that under conditions where ka >> kb and kc >> kd, then K1 ka and K2 kd; and when ka kb and kc kd, then K1 2ka (or 2kb) and K2 kc/2 (or kd/2). We will present evidence that while a monoprotic approximation of a 2 pK system is appropriate under some circumstances, a geometric mean (rather than an arithmetic sum) of the microscopic acidity constants can provide superior accuracy in some applications.