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Lidstrom:Buffers: Difference between revisions
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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. | ||
=== 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.''' | |||
=== How do method developer chose between sodium and potassium phospahte buffers? === | === How do method developer chose between sodium and potassium phospahte buffers? === | ||
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*** Sometimes Na precipitates when K wouldn't & vice versa | *** Sometimes Na precipitates when K wouldn't & vice versa | ||
* Michael Konopka's words: "What assays are you trying to measure? As for the buffering capacity, that really shouldn't matter since the pertinent components for the buffer (mono- and di-basic phosphate) are the same. The issue is if the potassium or sodium ion will form a precipitate which you don't want around if mixing with other solutions (or do want). A classic example is potassium will precipitate SDS while sodium is soluble. That's why in minipreps one adds potassium acetate/acetic acid after using SDS/NaOH to lyse the cells/dissolve lipids & proteins. The proteins, lipids, and chromosomal DNA is then trapped in the precipitate (plasmid DNA still in solution)." | * Michael Konopka's words: "What assays are you trying to measure? As for the buffering capacity, that really shouldn't matter since the pertinent components for the buffer (mono- and di-basic phosphate) are the same. The issue is if the potassium or sodium ion will form a precipitate which you don't want around if mixing with other solutions (or do want). A classic example is potassium will precipitate SDS while sodium is soluble. That's why in minipreps one adds potassium acetate/acetic acid after using SDS/NaOH to lyse the cells/dissolve lipids & proteins. The proteins, lipids, and chromosomal DNA is then trapped in the precipitate (plasmid DNA still in solution)." | ||
== Notes == | == Notes == | ||
{{Reflist}} | {{Reflist}} | ||
