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Lloyd Parker 02-20-2004 01:31 PM

buffer preparation
 
In article <[Only registered and activated users can see links. Click Here To Register...]> ,
[Only registered and activated users can see links. Click Here To Register...] (Biochemist) wrote:
You can. For example, you can prepare a pH 4.74 acetate buffer by mixing,
say, 0.5 mol of HOAc and 0.5 mol of NaOAc. Or you could start with say, 1.0
mol of HOAc and add 0.5 mol of NaOH, thereby converting 0.5 mol of the HOAc
to NaOAc.

This is why you go through a buffer region when titrating a weak acid with a
strong base (or vice-versa).

Biochemist 02-20-2004 04:43 PM

buffer preparation
 
I was taught to prepare carbonate, phosphate, etc. buffers by
calculation that uses a conjugate acid/base pair. For example: to
prepare a 0.1M phosphate buffer at a given pH, I use predetermined
amounts of monosodium phosphate and disodium phosphate. Someone asked
me why I couldnt make the exact same 0.1M phosphate buffer by using
only one reagent (ie. only monophosphate form) and titrate with
HCl/NaOH to the same pH. He said he makes his Tris buffers this way. I
admit I had no good answer, so I am curious if anyone here knows if it
makes a difference.
Thanks

GeneralChemTutor 02-20-2004 06:29 PM

re:buffer preparation
 
I suppose it could work if one added large amounts of the buffer
initially than added the HCl and NaOH. Relatively wasteful though.


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raconte 02-21-2004 05:46 AM

buffer preparation
 
[Only registered and activated users can see links. Click Here To Register...] (Biochemist) wrote in message news:<bd2258a5.0402200843.6542770@posting.google.c om>...
I'd heard biologists say that they perfer to use two components to
make their buffers because they're worried about the effect of adding
another counterion.

Example, you prepare a 0.1 M potassium phosphate buffer using the acid
and base salts using Henderson-Hasselbach.-assuming you don't want
other thermodynamic corrections - if you do, try this:
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Someone else prepares a 0.1 M buffer from the acid potassium salt that
has been titrated up with sodium hydroxide. Same pH, same phosphate
molarity, but the mix of sodium and potassium ions could affect a
living system differently.

Now try this: Someone else does the same, but overshoots with the
hydroxide and has to go back with ... oh, what acid is around
here...dang out of phosphoric acid...oh well, a few drops of conc.
sulfuric acid is ok...it will be the right molarity at the end anyway
....

In the end, all you want is a consistant preparation method to avoid
surprises. Innovate as little as possible.

Bruce Sinclair 02-22-2004 10:42 PM

buffer preparation
 
In article <[Only registered and activated users can see links. Click Here To Register...] >, [Only registered and activated users can see links. Click Here To Register...] (raconte) wrote:
(snip)

VERY good advice here. Once you have a method that works, don't change
it :) Luckily, you've writen down exactly what you have done so others
can do it too :) :)

Bruce

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