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== Notes specific to buffers == | == Notes specific to buffers == | ||
=== phosphate buffer === | === phosphate buffer === | ||
==== phosphate buffer basics ==== | |||
*pH rane: 5.8-8 | *pH rane: 5.8-8 | ||
* Phosphate buffering is based on the fact that phosphate can be in four states based on the pH. | * Phosphate buffering is based on the fact that phosphate can be in four states based on the pH. | ||
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** Sodium and potassium phosphates are interchangeable. You can use either based on whether you would rather have Na or K in soltuion. | ** Sodium and potassium phosphates are interchangeable. You can use either based on whether you would rather have Na or K in soltuion. | ||
** You can also start with one buffer and use HCl or NaOH to force the pH in one direction or another. For example, if you add enough HCl to a PO<sub>4</sub><sup>3-</sup> solution, you will force the phosphate into a pH7.4 solution with comparable amounts of HPO<sub>4</sub><sup>2−</sup>) and H2PO<sub>4</sub><sup>−</sup>. | ** You can also start with one buffer and use HCl or NaOH to force the pH in one direction or another. For example, if you add enough HCl to a PO<sub>4</sub><sup>3-</sup> solution, you will force the phosphate into a pH7.4 solution with comparable amounts of HPO<sub>4</sub><sup>2−</sup>) and H2PO<sub>4</sub><sup>−</sup>. | ||
==== pH depends on presence of other salts for phosphate buffers ==== | |||
This section is written based on [http://www.public.asu.edu/~laserweb/woodbury/classes/chm467/samplelab/SAMPLAB.html this educational lab] | |||
''' Why: ''' | |||
*When salts are present (ionic strength increased), the activity coefficient of a doubly charged ion (the base form of phosphate in this case) is always smaller than that of a singly charged ion (the acid form of phosphate in this case). Thus, the presence of salts shifts the pH down as the Henderson–Hasselbalch equation shows: [[image:2014_09_10 henderson hasselbalch for phosphates]] | |||
''' Examples: ''' | |||
Examples of how this affects buffer preparations: | |||
* Say you have a phosphate buffer with pH 6.87 made by mixing NaH<sub>2</sub>PO<sub>4</sub> and Na<sub>2</sub>HPO<sub>4</sub> . If you dilute this buffer 10x with water, and separately dilute it 10x into 1M NaCl, the pH of the two diluted buffers will be different. In [http://www.public.asu.edu/~laserweb/woodbury/classes/chm467/samplelab/SAMPLAB.html this example], the pH of the buffer diluted into water will be 7.01 and the pH of the buffer diluted into NaCl will be 6.38. | |||
* Why are both pHs different from the starting pH? | |||
** The pH of buffers is pretty strongly dependent on the ionic strength of the solution. '''Higher ionic strengths → decrease in phosphate buffer pH''' because the the activity coefficient of a doubly charged ion (base form in this case) is always smaller than that of a singly charged ion (acid form in this case) making the last term in [[image:2014_09_10 henderson hasselbalch for phosphates]]. | |||
** The lower concentration of ions in the buffer resulting from a 10x dilution into water → lower ionic strength → increased pH relative to the starting buffer. | |||
** The higher concentration of ions in the buffer resulting from dilution into NaCl → higher ionic strength → decreased pH relative to the starting buffer. | |||
==== advantages ==== | ==== advantages ==== | ||
* Most physiological of common buffers. Mimics certain components of extracellular fluids. | * Most physiological of common buffers. Mimics certain components of extracellular fluids. |
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