2,695
edits
Lidstrom:Buffers: Difference between revisions
From OpenWetWare
Jump to navigationJump to search
→phosphate buffer
| Line 110: | Line 110: | ||
* 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. | ||
** [[image:2014_02_09 phosphate states.png]] | ** [[image:2014_02_09 phosphate states.png]] | ||
** [[image:2014_02_09 phosphate state at various pHs.png]] | |||
** It will shift between the different levels of protonation based on how many protons are available in solution. Thus it can be a source/sink for protons. Like all (good) buffers, if more H+ ions show up in solution, it can help absorb these and maintain the initial pH. Converseley, it can release H+ ions when they disappear from solution. | ** It will shift between the different levels of protonation based on how many protons are available in solution. Thus it can be a source/sink for protons. Like all (good) buffers, if more H+ ions show up in solution, it can help absorb these and maintain the initial pH. Converseley, it can release H+ ions when they disappear from solution. | ||
*Aqueous phosphate exists in four forms. In strongly basic conditions, the phosphate ion (PO<sub>4</sub><sup>3−</sup>) predominates, whereas in weakly basic conditions, the hydrogen phosphate ion (HPO<sub>4</sub><sup>2−</sup>) is prevalent. In weakly acid conditions, the dihydrogen phosphate ion (H2PO<sub>4</sub><sup>−</sup>) is most common. In strongly acidic conditions, trihydrogen phosphate (H<sup>3</sup>PO<sub>4</sub>) is the main form. | ** The details: | ||
***Aqueous phosphate exists in four forms. In strongly basic conditions, the phosphate ion (PO<sub>4</sub><sup>3−</sup>) predominates, whereas in weakly basic conditions, the hydrogen phosphate ion (HPO<sub>4</sub><sup>2−</sup>) is prevalent. In weakly acid conditions, the dihydrogen phosphate ion (H2PO<sub>4</sub><sup>−</sup>) is most common. In strongly acidic conditions, trihydrogen phosphate (H<sup>3</sup>PO<sub>4</sub>) is the main form. | |||
* You can make phosphate buffers several ways. The difference between them is how much salt and what type of salt is also present in solution. | |||
** If you want a desired pH, you can use a ratio of hydrogen phosphate and dihydrogen phosphate salts to get the pH you desire. There are tables that correlate desired pH values and concentrations to volumes you should use. | |||
*** | |||
** 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>. | |||
==== advantages ==== | ==== advantages ==== | ||
* Most physiological of common buffers. Mimics certain components of extracellular fluids. | * Most physiological of common buffers. Mimics certain components of extracellular fluids. | ||
