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Lidstrom:Buffers: Difference between revisions
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→pH depends on presence of other salts for phosphate buffers
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''' Why: ''' | ''' 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]] | *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.png]] | ||
''' Examples: ''' | ''' Examples: ''' | ||
Examples of how this affects buffer preparations: | 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. | * 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? | * 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 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.png]]. | ||
** 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 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. | ** 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 ==== | ||
