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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? === | ||
A buffer would be expected to maintain its pH upon dilution, if both [A<sup>-</sup>] and [HA] are reduced in equivalent proportions | 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. (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) 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. | ||
(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) | |||
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 do method developer chose between sodium and potassium phospahte buffers? === | === How do method developer chose between sodium and potassium phospahte buffers? === | ||
