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Lidstrom:Buffers: Difference between revisions
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→Does pH of a buffer depend on the concentration of buffer?
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== Q&A== | == Q&A== | ||
=== Does pH of a buffer depend on the concentration of buffer? === | === Does pH of a buffer depend on the concentration of buffer? === | ||
According to the Henderson-Hasselbalch equation, the pH of a buffer depends only on the ratio of the conjugate base activity to conjugate acid activity. Explain then why the pH of a buffer changes when it is diluted. | |||
Answer in 3 pieces: (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] | |||
# The activity coeffecients of different ions are not the same at any given concentration and do not change in an identical manner with a given change in concentration. | |||
##There are tables that have activity coefficients of different ions (e.g. HPO<sub>4</sub><sup>2-</sup>) at different molarities. | |||
##You can use a modified form of the Henderson-Hasselbalch equation that accounts for activity coefficients. | |||
##In general, the log(A<sup>-</sup>/HA) term of "acidic" buffers increases upon dilution resulting in an increase in pH/ In "basic" buffers, the log (R-NH<sub>2</sub>/R-NH<sub>3</sub><sup>+</sup>) term decreases upon dilution, resulting in a decrease in pH. | |||
#The degree of dissociation of HA increases as the solution is diluted. | |||
## | |||
A buffer would be expected to maintain its pH upon dilution, if both [A<sup>-</sup>] and [HA] are reduced in equivalent proportions. This is not strictly the case, although it is a useful approximation provided the dilution is not large. A discussion of ionic strength follows, informing you that K<sub>a</sub> depends on the ionic strength and hence to some degree on dilution. They provide an equation for calculating the effect of dilution or change in ionic strength of a buffer on its pH arising from changes in activity coefficients. | A buffer would be expected to maintain its pH upon dilution, if both [A<sup>-</sup>] and [HA] are reduced in equivalent proportions. This is not strictly the case, although it is a useful approximation provided the dilution is not large. A discussion of ionic strength follows, informing you that K<sub>a</sub> depends on the ionic strength and hence to some degree on dilution. They provide an equation for calculating the effect of dilution or change in ionic strength of a buffer on its pH arising from changes in activity coefficients. | ||
The changes in pH arising from the dilution of a buffer are generally small where the buffering ion is monovalent. Example: dilution of a 0.1M buffer comprising equal amounts of HA and [A<sup>-</sup>] to 0.05M causes a change of 0.024 pH units. However, if the buffer ions are polyvalent, e.g. phosphate or citrate, the change may be appreciable and large dilutions should be avoided. | The changes in pH arising from the dilution of a buffer are generally small where the buffering ion is monovalent. Example: dilution of a 0.1M buffer comprising equal amounts of HA and [A<sup>-</sup>] to 0.05M causes a change of 0.024 pH units. However, if the buffer ions are polyvalent, e.g. phosphate or citrate, the change may be appreciable and large dilutions should be avoided. | ||
(source: [http://www.amazon.com/Enzyme-Assays-Practical-Approach-Series/dp/0199631425/ref=sr_1_1?ie=UTF8&qid=1383921457&sr=8-1&keywords=0199631425 ISBN 0-19-963142-5] pg 318) | (source: [http://www.amazon.com/Enzyme-Assays-Practical-Approach-Series/dp/0199631425/ref=sr_1_1?ie=UTF8&qid=1383921457&sr=8-1&keywords=0199631425 ISBN 0-19-963142-5] pg 318) | ||
=== How does temperature affect the pH of a buffer? === | === How does temperature affect the pH of a buffer? === | ||
