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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== | ||
=== | === Why does the pH of a buffer change with dilution? === | ||
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. | 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] | 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. | # 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. | ##There are tables that have activity coefficients of different ions (e.g. HPO<sub>4</sub><sup>2-</sup>) at different molarities. | ||
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##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. | ##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. | #The degree of dissociation of HA increases as the solution is diluted. | ||
## | ## The log(A<sup>-</sup>/HA) term increases as the solution is diluted. In a solution of a weak acid alone (no added conjugate base(, the acid is 10% dissociated when [HA]<sub>orig</sub>=100K<sub>a</sub> and 50% dissociated when [HA]<sub>orig</sub>=2Ka. Thus the log(A<sup>-</sup>/HA) term increases as the solution is diluted. In a buffer solution (weak acid plus added conjugate base), the A<sup>-</sup> tends to suppress the dissociation of HA. Consequently, in a buffer solution containing [HA]=2K<sub>a</sub>, the HA is somewhat less than 50% dissociated. {There are mathematical examples in the book} | ||
# If you dilute a buffer enough, the pH becomes 7 because the contribution toward the H<sup>+</sup> or OH<sup>-</sup> ion concentration of the solution approaches that of water and the pH approaches 7. | |||
'''As a general rule, you can assume that the concentration of conjugate acid and conjugate base present in solution are the same as the concentrations of each originally added to the solution (or produced by partial titration of one or the other). This general rule does not hold when the buffer is extremely dilute (when the concentrations of the buffer components are in the region of the K<sub>a</sub> value). In such dilute buffers, the changes (y) are large compared to the original concentrations. (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 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) | |||
=== Why does the pH of a buffer change with the addition of salts (changes in ionic strength)? === | |||
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. | |||
=== How does temperature affect the pH of a buffer? === | === How does temperature affect the pH of a buffer? === | ||
