Re: HCl-KCl buffers

Shankar Bhattacharyya wrote:
Marvin <physchem@xxxxxxxxxxx> wrote in
news:daX3j.395$AX6.69@trnddc07:

David Stone wrote:
In article <m1C3j.24061$rg1.16846@trndny04>,
Marvin <physchem@xxxxxxxxxxx> wrote:

johngo88@xxxxxxxxx wrote:
[...] Can someone please explain to me how an HCl-KCl buffer
works?
What kind of buffer is it? It doesn't make sense as a pH buffer.
According to the web site, it's a pH buffer for ~ pH 1-2. There's
also a NaOH / KCl buffer listed for pH 12-13. I didn't see a link
to an actual recipe, though, just links to the MSDS information
for each component:

http://ptcl.chem.ox.ac.uk/MSDS/buffers.html
That would be a very unusual buffer. pH buffers are normally made with an acid and a base, not an acid and a salt. All I can think of is that the KCl is there to control the activity coefficient of the solution, which in turn controls the ionization of the HCl.

That, if you look at it carefully, is exactly what a buffer made of
an acid and a base is, if there is more acid than base.

While common "acidic" buffers rely upon the control of a weak acid's
ionization by manipulating the relative concentration of a
substantially ionized salt of that acid, you get the same effect
whether you get to the relative concentration by mixing an acid and a
suitablebase or an acid and it's corresponding salt.

As others have pointed out, of course, the buffer capacity of an
HCl/KCl buffer is going to be fairly small since you can manipulate
the ionization of HCl over only a relatively narrow range, over
towards the "completely" ionized end of the range. However, the
principle is the same.

Sorry for the mostly untrimmed quoted text. It seemed sensible to leave it in place.

- Shankar
What you can control, to some extent, with this kind of buffer is the activity coefficient. It is a very limited pH buffer, but pH electrodes respond to H+ activity, not to the concentration of the ion.

Ion activity is a difficult concept for those who haven't taken a course in physical chemistry, but you can't really understand pH measurement otherwise.

.

Relevant Pages

  • Re: WHAT is the pH of a weak-weak salt?
    ... acid and the base from which the salt was made ie HCN and NH3 (rather ... Salt a of weak-acid-weak-base is not a buffer. ... They include whole water molecules, so I don't see how they ...
    (sci.chem)
  • Re: Stuck on a Biochemistry question
    ... acid to be 1.45M. ... and it does come in handy. ... when pH meters were cranky devices that never seemed to be ... little bit of a quickly made buffer in a constant temp. ...
    (sci.chem)
  • Re: HCl-KCl buffers
    ... normally made with an acid and a base, not an acid and a salt. ... All I can think of is that the KCl is there to control the activity coefficient of the solution, which in turn controls the ionization of the HCl. ... That, if you look at it carefully, is exactly what a buffer made ...
    (sci.chem.analytical)
  • TRIS buffer- how does it work?
    ... >HCl in order to make it a buffer? ... What is TRIS hydrochloride? ... weak base-tris + salt of tris with a strong acid. ...
    (sci.chem)
  • Re: HCl-KCl buffers
    ... normally made with an acid and a base, ... That, if you look at it carefully, is exactly what a buffer made ... While common "acidic" buffers rely upon the control of a weak ... substantially ionized salt of that acid, ...
    (sci.chem.analytical)