HCl-KCl buffers

Post by j***@gmail.com
This is with reference to Oxford University's Physical and Chemical
Laboratory's preparation tables for preparation of HCl-KCl buffers.
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.

David Stone

2007-11-30 13:04:08 UTC

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

Marvin

2007-11-30 16:49:13 UTC

What kind of buffer is it? It doesn't make sense as a pH
buffer.

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.

Daddy Tadpole

2007-12-01 20:12:50 UTC

What kind of buffer is it? It doesn't make sense as a pH buffer.

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.

I tried to look this up some time ago, but didn't get very far.

The pKa of HCl is -6.2 according to Lange's handbook, and I didn't find any
reference to a value closer to 0.

It's well known that HCl is a weak acid in solvents of low dielectric
constant such as acetic acid, but this information is not relevant to
aqueous systems.

Regards

Shankar Bhattacharyya

2007-12-25 00:43:33 UTC

[...] 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.

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

Marvin

2007-12-25 16:51:47 UTC

[...] 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.

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.

Shankar Bhattacharyya

2007-12-26 11:15:16 UTC

Post by Marvin
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.

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.

You are right, of course. I should have been more precise. However,
"ionization" is also, in the end, an activity-based concept, not a
concentration-based concept. I did not intend to imply otherwise but
I can see where my post could be read that way.

My point, however, was that your post appeared to be distinguishing
between the situation of adding a base to an acid as against adding a
salt to an acid. You can do either to get to a specified state,
within defined context. There is no fundamental difference between
the two in terms of final conditions.

The more extreme condition of adding a stoichiometric excess of base
to an acid is, correspondingly, equivalent to adding a salt to a
base.

So one can achieve the same final effect by adding a base to an acid
or by adding a salt to an acid or to a base, depending on which side
of "neutralization" you want to be.

There is obviously a certain amount of attention to detail in getting
the stoichiometry of the buffers right. Operationally it is probably
easier to figure out and control the process by adding a base to an
acid than by adding a salt to an acid. Adding the salt changes the
stoichiometry in more ways.

- Shankar

Marvin

2007-12-26 17:10:28 UTC

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.

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.

You are right, of course. I should have been more precise. However,
"ionization" is also, in the end, an activity-based concept, not a
concentration-based concept. I did not intend to imply otherwise but
I can see where my post could be read that way.
My point, however, was that your post appeared to be distinguishing
between the situation of adding a base to an acid as against adding a
salt to an acid. You can do either to get to a specified state,
within defined context. There is no fundamental difference between
the two in terms of final conditions.
The more extreme condition of adding a stoichiometric excess of base
to an acid is, correspondingly, equivalent to adding a salt to a
base.
So one can achieve the same final effect by adding a base to an acid
or by adding a salt to an acid or to a base, depending on which side
of "neutralization" you want to be.

But the essential component of a buffer is either a weak
acid or a weak base. A mixture of a strong acid and a salt
cannot stabilize pH in the same way. It can have a weaker
stabilizing effect by regulating the activity coefficient.
I wouldn't call it equivalent.

Post by Shankar Bhattacharyya
There is obviously a certain amount of attention to detail in getting
the stoichiometry of the buffers right. Operationally it is probably
easier to figure out and control the process by adding a base to an
acid than by adding a salt to an acid. Adding the salt changes the
stoichiometry in more ways.
- Shankar

Unknown

2007-12-01 23:44:52 UTC