[MR]

Hi all,

I made a buffer with the pH 6 from a weak acid/conjugated base with a pka of 4.76.
I calculated the amount of the acid and base needed to make a buffer solution with pH 6.

After I made the buffer solution the measured pH was different from 6.

How is this possible? 

[scientific]

Can you show us your calculations? Maybe there is a mistake there.


Otherwise, it could be an issue with the buffers you used or the measuring equipment.

[MR]

The buffer is sorbic acid/potassium sorbate.
My calculations are:

6=4.76+log (potassiumsorbate/sorbic acid)
1.24=log (potassiumsorbate/sorbic acid)
17.378=potassiumsorbate/sorbic acid

potassiumsorbate= 17.378/ (1+17.378)= 0.9456

sorbic acid = 1/(1+17.378)=0.0544

Then i multiplied with the desired molarity and molar masse to get gram.

[Borek]

I made a buffer with the pH 6 from a weak acid/conjugated base with a pka of 4.76.

Using buffer outside of the pKa±1 range is asking for troubles.

[MR]

What kind of troubles?

[Borek]

Pretty small errors in preparation meaning large differences in pH (not to mention the main problem - low buffering capacity, but that's not what we are talking about ATM).

[Babcock_Hall]

One other thing to consider.  The thermodynamic pK_a of a substance pertains to an ionic strength of zero.  Yet the actual ionic strength of a typical buffer is not zero.  This effect for your buffer is not likely to be large unless your buffer was fairly concentrated.  Some tables provide a practical pK_a value, which attempts to correct for this effect.

[MR]

So what i have to make sure of is my preparation and not to make a fairly concwntrated buffer. But how concwntrated are we takling about and where can i find the practical pka values?   

[Borek]

http://www.chembuddy.com/?left=pH-calculation&right=ionic-strength-activity-coefficients

Any serious source should contain the thermodynamic values for different IS values.

Besides, if you need a precise pH control you should always adjust your buffer against a calibrated pH meter, not rely on the calculated recipe.

[MR]

Another question i have is, is it normal for this kind of buffer to have a LOWER buffer capacity for a high concentration compared to the buffer capacity of the same low conc buffer?

[Babcock_Hall]

So what i have to make sure of is my preparation and not to make a fairly concwntrated buffer. But how concwntrated are we takling about and where can i find the practical pka values?   

Obviously I agree with Borek that one should in general make the buffer using a calibrated pH meter.  when I calculate how much acid or base to add, I usually think of that calculation as an estimate, anyway.  Others may have different points of view.

The ability of the buffer to resist changes in pH is greatest at a pH that is identical its the pK_a.  Assuming that I have the freedom to choose any buffer that I want, I typically look for buffers with pK_a values close to the pH of my experiment.  However, when one is trying to duplicate someone else's experiment, this may not be an option.

I may have brought about unnecessary confusion when I mentioned ionic strength.  One usually choose the concentration of the buffer according to the needs of the experiment and then accepts the ionic strength for what it is.  For sorbate buffers the effect will probably be small anyway.