pH

[HW Mueller]

That´s the idea behind selling these for the purpose of preparing buffers for use in HPLC (see Fisher, as an example).

[HW Mueller]

That´s the idea behind selling these for the purpose of preparing buffers for use in LC (see Fisher, as an example).

[Bill Tindall]

Victor, so that it does not get lost in this discussion I have attempted to answer your question in a separate post

[sudhis]

As of now i really didnt got the exact answer to my question which indeed its diverted for other topics.

my question again is PH will change with the increase in temperature and even with the reduce in temperature lets say at 5°c.

[JesseG]

As of now i really didnt got the exact answer to my question which indeed its diverted for other topics.

my question again is PH will change with the increase in temperature and even with the reduce in temperature lets say at 5°c.

The answer was there if you read all the info.
If you make your solution by weighing out the components in the correct concentrations, your soultion will be properly buffered regardless of temp.

[Bill Tindall]

I am not sure that I understood your question. I think you are confused by the definition of a buffer. A buffer "resists changes in pH upon adding a small increment of acid or base". Temperature is not part of the buffer difinition. There are buffers that are more or less sensitive to changes in temperature. But, just because a buffer changes pH with temperature does not mean that it has some how lost its ability to resist changes in pH caused by adding small increments of acid or base.

Indeed, the pH of a solution may change with a change in temperature. Furthermore, because of the way pH is defined, even the pH scale chages with temperature. The pH values of calibration buffers are provided at various temperatures because of these scale changes. Therefore, rigorous comparisons of pH can not be made at different temperatures.

But, all these limitations of the way pH is defined and measured have NO practical effect on using buffers in LC separations.

If a solution is buffered at some temperature, it will still be be buffered at a different temperature. However, it will be buffered at a different pH at this different temperature. If the separation is adequate at this pH and temperature then there is no reason to be concerned.

[rc_12321]

PH can indeed vary with temperature. The reasons why
depend on the context, but even a simple solution of a weak
acid (HA) will exhibit a (weak) temperature dependence. The pH is
given by the Henderson-Hasselbalch equation:

pH = pKa + log { [A-]/[HA] }

where Ka is the equilibrium constant for the reaction

HA ---> H+ + A-

( ka = [H+][A-] / [HA] )

and pKa = - log Ka .

Ka is itself a function of temperature, since it is related to
the Gibbs free energy of reaction (delta G) by the equation

delta G = - RT ln Ka = -2.303 RT log Ka = 2.303 RT * pKa

so we have

pkA = delta G / (2.303 RT)

delta G is itself given by

delta G = delta H - T * delta S

where delta H is the enthalpy of reaction and delta S is the entropy
of reaction. Combining these, we get

pKa = (delta H / (2.303 RT)) - (delta S / (2.303 R))

If we assume for the sake of simplicity that delta H and delta S
are approximately independent of temperature T (constant), then
the variation with temperature is determined by the sign of delta H.
For example, if delta H is positive (endothermic dissociation),
pKa gets smaller as the temperature gets larger. A
decrease in pKa amounts to an increase in Ka, which means that the
reaction favors dissociation more as temperature increases (in
agreement with LeChatelier's principle). This increases [H+] and
decreases the pH. If the reaction is exothermic the opposite effect will
be observed. Either way, we expect the pH to depend on temperature.

These arguments can be extended to strong acids too. Things get
complicated when there are multiple chemical reactions. Biological
systems can use enzyme-catalyzed reactions to keep the pH constant even
when T varies (within limits, of course).

[HW Mueller]

rc, not having been comfortable with your temperature explanation I checked into my old thermodynamics books on this. I am as confused as ever, it seems, though, that one has to look at work of van der Waals and van´t Hoff to get at this ..... too complicated for me, any thermodynamicists out there?
(One thing is clear, the free energy relation to mass law action holds at a constant T and one has to be careful not to confuse G with Go, the mass action law with Kequ, and not to use a Go of Tx at Ty, etc.)

Luckily, chromatography is still largely empirical.