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By Anonymous on Saturday, June 12, 2004 - 04:47 pm:
I saw the lengthy thread on buffers. Hope you all not buffered out. This ones easy.
In my new job I see some methods where people prep 0.05 M Phosphate buffer by adding 6.8 g of potassium phosphate monobasic to 800 mL of water. but wouldn't there need to be both acid and salt added together to make buffer. My new colleagues dont have agood answer for me.
Thanks
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By syx_gf on Sunday, June 13, 2004 - 06:45 pm:
You didn't give specified pH you want to get but it is possible to prepare the buffer with weights and volumes. You can find it in USP Buffer Prep and NIST.
Read this literature: GW Tindall, LCGC North America 21(1) Jan 2003: Mobile-Phase Buffers, Part III - Preparation of Buffers.
http://www.lcgcmag.com/lcgc/data/articl ... rticle.pdf
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By Bill tindall on Tuesday, June 15, 2004 - 02:35 pm:
Anon:
Remember, a buffer is defined by what it does-buffers- not by how it is prepared. Monobasic phosphate is both an acid(it can donate protons) and a base(it can accept protons. So by your definition, which is not correct, this salt is a buffer. In fact the solution you quoted will provide some buffering action. But, buffers prepared from acids and conjugate salts buffer best at a ph equal to the acid pKa. There is still good buffering action 1 pH unit from the pKa and sometimes useful buffering action 2 units from the pKa. As I recall (and I hope someone will confirm my somewhat potenitally unreliable recollection) the pH of the solution in the example you quoted is in between the 1st and second pKa of phosphoric acid and hence the solution in the example does not have a very great buffering capacity. It will buffer better than just water and maybe for some applications this is enough. But there are better buffers for this pH. (I hope I am remembering that the pH of solutions of this salt is around 4??)
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By HW Mueller on Tuesday, June 15, 2004 - 11:52 pm:
Bill, your recollection is quite good. Values which I measured are, for NaH2PO4 (should be roughly the same as for K...):
0.025M has pH~4.6
0.1M has pH~4.4
The buffer index indicates lousy buffering at those pH.
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By Anonymous on Friday, June 18, 2004 - 07:31 pm:
most things useful as bufferes require two components
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By Anonymous on Saturday, June 19, 2004 - 07:23 am:
Good responses. I just have one followup. This is something I have always wondered about. All the textbooks say that buffering is not very good if you are far from the pka of the buffer system. But wouldn't it be the case that, in these cases, it could buffer in one direction.
Example: The pka of the acetic acid/acetate system is 4.8 (I looked it up). Suppose we have an acetic acid/acetate solution at a pH of 3. This would mean that the molecules are mostly in the acid form. OK, so then wouldn't that solution have the ability to buffer quite well against the addition base but very poorly against the addition of acid. In sum, it can buffer in one direction.
I always thought it was limiting - and somewhat inaccurate - to say that such systems have no buffering ability.
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By Anonymous on Saturday, June 19, 2004 - 11:15 am:
No, it works the same in both directions. The pH will change the same way, if you add a small amount of acid H+ or base OH-, and it will change more at pH 3 than at pH 4.8.
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By HW Mueller on Monday, June 21, 2004 - 12:16 am:
See the discussion "Necessity of buffers in HPLC".
In short:
Last Anon,
it´s strictly the same only if the addition approaches infinatly small amounts.
Anon 11:15 am, if an addition is not nearly infinatly small, lets say you add enough acid to change the pH, you will go toward the enormous buffering of low pH solutions. In other words, in a practical sense the buffering may be indeed better in one direction, but opposite to your suggestion. If you add enough base to change the pH you will quickly be in a no buffering range. Again: Look at buffering index formulars.
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By HW Mueller on Monday, June 21, 2004 - 06:54 am:
Looking at the buffering index of acetate again, myself, it became clear that a bit above pH 3 the buffering capacity increases again due to the acetate-acetic acid equilibrium. Still, the buffering capacity toward lower pH (below pH 2) rises faster.
I saw the lengthy thread on buffers. Hope you all not buffered out. This ones easy.
In my new job I see some methods where people prep 0.05 M Phosphate buffer by adding 6.8 g of potassium phosphate monobasic to 800 mL of water. but wouldn't there need to be both acid and salt added together to make buffer. My new colleagues dont have agood answer for me.
Thanks
-------------------------------------------------------------------------------------------------------
By syx_gf on Sunday, June 13, 2004 - 06:45 pm:
You didn't give specified pH you want to get but it is possible to prepare the buffer with weights and volumes. You can find it in USP Buffer Prep and NIST.
Read this literature: GW Tindall, LCGC North America 21(1) Jan 2003: Mobile-Phase Buffers, Part III - Preparation of Buffers.
http://www.lcgcmag.com/lcgc/data/articl ... rticle.pdf
-------------------------------------------------------------------------------------------------------
By Bill tindall on Tuesday, June 15, 2004 - 02:35 pm:
Anon:
Remember, a buffer is defined by what it does-buffers- not by how it is prepared. Monobasic phosphate is both an acid(it can donate protons) and a base(it can accept protons. So by your definition, which is not correct, this salt is a buffer. In fact the solution you quoted will provide some buffering action. But, buffers prepared from acids and conjugate salts buffer best at a ph equal to the acid pKa. There is still good buffering action 1 pH unit from the pKa and sometimes useful buffering action 2 units from the pKa. As I recall (and I hope someone will confirm my somewhat potenitally unreliable recollection) the pH of the solution in the example you quoted is in between the 1st and second pKa of phosphoric acid and hence the solution in the example does not have a very great buffering capacity. It will buffer better than just water and maybe for some applications this is enough. But there are better buffers for this pH. (I hope I am remembering that the pH of solutions of this salt is around 4??)
-------------------------------------------------------------------------------------------------------
By HW Mueller on Tuesday, June 15, 2004 - 11:52 pm:
Bill, your recollection is quite good. Values which I measured are, for NaH2PO4 (should be roughly the same as for K...):
0.025M has pH~4.6
0.1M has pH~4.4
The buffer index indicates lousy buffering at those pH.
-------------------------------------------------------------------------------------------------------
By Anonymous on Friday, June 18, 2004 - 07:31 pm:
most things useful as bufferes require two components
-------------------------------------------------------------------------------------------------------
By Anonymous on Saturday, June 19, 2004 - 07:23 am:
Good responses. I just have one followup. This is something I have always wondered about. All the textbooks say that buffering is not very good if you are far from the pka of the buffer system. But wouldn't it be the case that, in these cases, it could buffer in one direction.
Example: The pka of the acetic acid/acetate system is 4.8 (I looked it up). Suppose we have an acetic acid/acetate solution at a pH of 3. This would mean that the molecules are mostly in the acid form. OK, so then wouldn't that solution have the ability to buffer quite well against the addition base but very poorly against the addition of acid. In sum, it can buffer in one direction.
I always thought it was limiting - and somewhat inaccurate - to say that such systems have no buffering ability.
-------------------------------------------------------------------------------------------------------
By Anonymous on Saturday, June 19, 2004 - 11:15 am:
No, it works the same in both directions. The pH will change the same way, if you add a small amount of acid H+ or base OH-, and it will change more at pH 3 than at pH 4.8.
-------------------------------------------------------------------------------------------------------
By HW Mueller on Monday, June 21, 2004 - 12:16 am:
See the discussion "Necessity of buffers in HPLC".
In short:
Last Anon,
it´s strictly the same only if the addition approaches infinatly small amounts.
Anon 11:15 am, if an addition is not nearly infinatly small, lets say you add enough acid to change the pH, you will go toward the enormous buffering of low pH solutions. In other words, in a practical sense the buffering may be indeed better in one direction, but opposite to your suggestion. If you add enough base to change the pH you will quickly be in a no buffering range. Again: Look at buffering index formulars.
-------------------------------------------------------------------------------------------------------
By HW Mueller on Monday, June 21, 2004 - 06:54 am:
Looking at the buffering index of acetate again, myself, it became clear that a bit above pH 3 the buffering capacity increases again due to the acetate-acetic acid equilibrium. Still, the buffering capacity toward lower pH (below pH 2) rises faster.
