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Lidstrom:Buffers: Difference between revisions
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→Why does the pH of a buffer change with the addition of salts (changes in ionic strength)?
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=== Why does the pH of a buffer change with the addition of salts (changes in ionic strength)? === | === Why does the pH of a buffer change with the addition of salts (changes in ionic strength)? === | ||
* The presence of other salts affects the ionic strength of the buffer, and often change the measured pH value by several tenths of a point. See discussion of phosphates below for a specific example. | |||
from [http://www.amazon.com/Biochemical-Calculations-Mathematical-Problems-Biochemistry/dp/0471774219/ref=sr_1_1?ie=UTF8&qid=1392218929&sr=8-1&keywords=0471774219 ISBN 0-471-77421-9]: | |||
Ions (in addition to those of the buffer components) influence the ionic strength and affect the activity coefficients of the buffer components. Thus even in very dilute solution containing equimolar concentrations of, for example, HPO<sub>4</sub><sup>2-</sup> and H<sub>2</sub>PO<sub>4</sub><sup>2-</sup>, the pH will not equal pKa if a relatively high concentration of NaCl is present. Instead of correcting the activity coefficients of the buffer components for the total ionic strength, it is simpler to define a new, apparent or effective pK<sub>a</sub> that relates the pH to the actual concentrations of a buffer components at a given total ionic strength. | |||
The effect of total ionic strength on K<sub>a</sub> or pK<sub>a</sub> can be quantitatively predicted from the Debye-Huckel equation. Thus, at any total ionic strength the effective pK<sub>a</sub> is given by: '''pK<sub>a</sub><sup>'</sup> = pK<sub>a</sub> + ΔpK<sub>a</sub>'''. The values for ΔpK<sub>a</sub> can be looked up in this book's appendix. | |||
=== How does temperature affect the pH of a buffer? === | === How does temperature affect the pH of a buffer? === | ||
