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Making a buffer


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#1 Axolotl9250

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Posted 04 September 2012 - 06:54 AM

Hi,

I need to make 0.05M sodium tartrate buffer at pH 4.5.
Are the two components I need sodium tartrate dibasic dihydrate and tartric acid?

Or is it Sodium tartrate (Na2C4H4O6) and Monosodium tartrate or sodium bitartrate.

I've made buffers from the two sodium phosphates before and with the HH equation. The sodium tartrate and sodium bitartrate kind of sounds similar. But I don't know if this is the same. If someone can also explain as to how this works or to where I can find out. I note that when these kinds of things, one has an extra H in it where the other has an extra Na (It makes me wonder if most chemicals have two forms like this). But I'm definitely not a chemist - with the phosphate buffer I used a worked example of the equation and double checked with my lab partner to make sure it's right.

Thanks,
Ben.

#2 Axolotl9250

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Posted 04 September 2012 - 07:06 AM

I see a answer on a web page - someone asked a similar question and got the answer to simply dissolve the sodium tartrate into water at the molar concentration desired and adjust the pH with HCl or NaOH and pH meter. So now my question is why is it for some buffers - the preparation is the precise combination of two things, like the sodium phosphate buffer, and for some buffers people simply weight it out, dissolve it and adjust the pH? Can you in reality do either method for any buffer or is it only suitable for some?

Edited by Axolotl9250, 04 September 2012 - 07:12 AM.


#3 mdfenko

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Posted 04 September 2012 - 11:33 AM

you can do it either way for most buffers except where you have to limit additional ions (like chlorine, sulfate, etc).

some buffer salts are very strong and hard to adjust. you have to allow for a relatively large volume of acid or base prior to bringing to final volume.

the nice thing about using equimolar acid and base components of the buffer is that final volume is not important, concentration remains the same.
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#4 Axolotl9250

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Posted 05 September 2012 - 02:18 AM

I see, if I need the pKa of tartaric acid, I've got the problem that 4 different values are given:

L(+) 25 °C :


pKa1= 2.95

pKa2= 4.25


meso 25 °C:

pKa1= 3.22

pKa2= 4.85



The natural form is L-(+)-tartaric acid or dextrotartaric acid. So is that the pKa to use? With the phosphate buffer I used a pKa close to the desired pH for the calculation, so if this applies then I should use pKa2 of 4.25 rather than pKa1 2.95?

Edited by Axolotl9250, 05 September 2012 - 02:19 AM.


#5 mdfenko

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Posted 06 September 2012 - 06:44 AM

if you insist on calculating then use the pK closest to the desired pH (all pKs should be taken into consideration but one is okay for an approximation, make sure you confirm the pH, don't assume).

i use the pKs to determine buffering range and prepare buffers to the desired pH by titration (either with acid-base components of the buffering compound or by adjusting with acid or base).

Edited by mdfenko, 06 September 2012 - 06:47 AM.

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