[galaxydefender]

Hi everyone,

I'm struggling to correctly make a 20 mM phosphate buffer using Na2HPO4 (MW 141,96 g/mole) and NaH2PO4 (MW 137,99 g/mole)+ 1M ammonium sulphate pH 6.2

I understand how to correctly make just a phosphate buffer pH 6.2 using HH equation. I know the theoretical result =/= actual measured result in terms of pH but by slightly tweaking the obtained grams of the components I obtained a nice pH 6.2.

Now, I need to make the same buffer with the addition of 1 M (NH4)₂SO4. I actually attempted to make the buffer by weighing everything and dissolving, but the pH had dropped to 5.1!
I have no idea how to account for the pH shift caused by NH4+ ions in the henderson-hasselbach equation. In my old theory book I can only find info about adding a strong acid/base to a buffer, not a weak acid (ammonium).

I would really appreciate any possible input you might have on the subject. Maybe to clarify: I really would like to get a concrete amount of each component to weigh, as this buffer will be upscaled and during the preparation there will be minimal time to check/adjust pH. Thanks in advance!

[Borek]

The simplest approach I can think of is to use pH calculator built into my Buffer Maker and to play with parameters till you will get correct pH and concentrations. This is not a simple application of the HH equation.

[AWK]

The simplest approach I can think of is to use pH calculator built into my Buffer Maker and to play with parameters till you will get correct pH and concentrations. This is not a simple application of the HH equation.

Does you Buffer Maker works correctly with the ionic over 3 M?
In the mixture of 20 mM phosphate buffer and 1 M ammonium sulphate rather hydrolysis of the later is important.

[Borek]

Does you Buffer Maker works correctly with the ionic over 3 M?

No. But I don't know any tool that will work in such a soup.

In the mixture of 20 mM phosphate buffer and 1 M ammonium sulphate rather hydrolysis of the later is important.

Even if so, it would become apparent after trying to calculate the result. But seems like it shouldn't be a problem. 1M mixture of ammonium sulfate neutralized with 0.02M disodium hydrogenphosphate has pH over 7, at least ignoring the ionic strength of the solution. That most likely means there is a room for adjustments.

[AWK]

But final pH=5.1 means at least [H₂PO₄^-]/[HPO₄^2-] > 100

[galaxydefender]

Argh! Sorry to give you the wrong information, but I need 10 mM phosphate, not 20 mM as described in my previous post.

I have been given the advice to make higher molarity stock solutions of each phosphate component, and tweak pH by keeping track of the added volume of the stock solutions. I can then backtrack and get the amounts of grams needed. I will give it a try.

I'm not familiar with Buffer Maker but it might be useful. Thanks for all the input so far!

[Borek]

Target pH is 6.2.

5.1 was the effect of acidyfying buffer that was already 1 pH unit down from the pKa2, so its capacity was 3 times lower than the max possible value.

Actually it is about preparing buffer from disodium hydrogenphosphate (base) and ammonium sulfate (acid).

[Borek]

Argh! Sorry to give you the wrong information, but I need 10 mM phosphate

By my trials it should be still doable.

[AWK]

http://www.chemicalforums.com/index.php?topic=29954.0

10 % (close to 1M) ammonium sulphate shows pH=5.3.

[Borek]

http://www.chemicalforums.com/index.php?topic=29954.0

10 % (close to 1M) ammonium sulphate shows pH=5.3.

Please elaborate, this is cryptic. No idea what you are aiming at.

Starting pH of ammonium sulfate is a little bit over 5, no doubt about it. When we add disodium hydrogenphosphate we add a base, which neutralizes ammonium sulfate solution, so pH goes up. From my calculations when we add 0.01M disodium hydrogenphosphate pH goes up far enough to allow preparation of the pH 6.2 solution. Because of the high ionic strength calculations are not exact, but there is enough margin for an error.

Experimentally the best approach would be most likely to start preparing solution that is 1M in ammonium sulfate and 0.01M in phosphoric acid, then to titrate the solution with a relatively concentrated NaOH (to avoid substantial dilution) till the pH is right.

[AWK]

Galaxydefender

I'm struggling to correctly make a 20 mM phosphate buffer using Na2HPO4 (MW 141,96 g/mole) and NaH2PO4 (MW 137,99 g/mole)+ 1M ammonium sulphate pH 6.2

After correction by Galaxydefender  0.01 M phosphate buffer (with omitting ammonium sulfate) shows pH=6.2.  OK(AWK)

Galaxydefender

Now, I need to make the same buffer with the addition of 1 M (NH4)2SO4. I actually attempted to make the buffer by weighing everything and dissolving, but the pH had dropped to 5.1!

In my opinion we can completely neglect buffer for calculation of pH at this point - 0.01 M buffer cannot buffering 1 M ammonium sulfate. We can calculate an approximate pH just for hydrolysis of this salt instead. (AWK)

Borek

Experimentally the best approach would be most likely to start preparing solution that is 1M in ammonium sulfate and 0.01M in phosphoric acid, then to titrate the solution with a relatively concentrated NaOH (to avoid substantial dilution) till the pH is right.

I agree. This is an usual procedure done by biochemists for adjusting pH in experiments but I just tried to explain the value of pH after addition of ammonium sulfate. (AWK)

[Babcock_Hall]

What about making two stock solutions? A.  1 M ammonium sulfate/10 mM NaH2PO4  B.  1 M ammonium sulfate/10 mM Na2HPO4.  Mix A and B to whatever proportion gives the desired pH.