2,695
edits
Lidstrom:Buffers: Difference between revisions
From OpenWetWare
Jump to navigationJump to search
→Q&A
(→Q&A) |
(→Q&A) |
||
| Line 39: | Line 39: | ||
== 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? === | ||
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 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) | |||
=== How does temperature affect the pH of a buffer? === | === How does temperature affect the pH of a buffer? === | ||
Amine-containing buffers are most sensitive to changes in temperature. Example: Tris-HCl adjusted to pH=8.0 at 25°C will have a pH of 8.78 at 0°C, whereas carboxylic acid buffers are least sensitive to changes in temperature. Example: acetate buffer adjusted to pH 4.5 at 25°C will have a pH of 4.495 at 0°C. '''These differences are due to the differences in ΔH for ionization of the acids.''' | Amine-containing buffers are most sensitive to changes in temperature. Example: Tris-HCl adjusted to pH=8.0 at 25°C will have a pH of 8.78 at 0°C, whereas carboxylic acid buffers are least sensitive to changes in temperature. Example: acetate buffer adjusted to pH 4.5 at 25°C will have a pH of 4.495 at 0°C. '''These differences are due to the differences in ΔH for ionization of the acids.''' | ||
(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 do method developer chose between sodium and potassium phospahte buffers? === | === How do method developer chose between sodium and potassium phospahte buffers? === | ||
