2,695
edits
Lidstrom:Buffers: Difference between revisions
From OpenWetWare
Jump to navigationJump to search
→Why does the pH of a buffer change with the addition of salts (changes in ionic strength)?
| Line 97: | Line 97: | ||
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]: | 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. | 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. | ||
