[Biotech78]

I have a bit of confusion. I need to make 0.2M NaOAc buffer (as per the papers I am following). Now there are 2 different recipes of "sodium acetate buffer" available and that has got me confused. One is straight forward, following CSHL protocols

Sodium acetate buffer (0.2 M, pH 5.0)
Reagent Quantity (for 2 L) Sodium acetate trihydrate 54.43 g Glacial acetic acid 12 mL H2O
1988 mL

Now that is perfectly fine for me (except that I need pH 3.5-4.0 so all I need to do is to add a bit of extra acetic acid)

 

But here's another recipe

"Acetate buffer pH 3.6–5.6 Stock solutions A: 0.2 M solution of acetic acid (11.55 mL in 1 L distilled water) B: 0.2 M solution of sodium acetate (16.4 g of C2H2Na or 27.2 g of C2H3O2Na.3H2O in 1 L distilled water) x mL of A plus y mL of B, dilute to a total of 100 mL with distilled water."

Ref: https://link.springer.com/content/pdf/bbm%3A978-1-59745-425-4%2F1.pdf

(have found the same recipe in one of the old book on buffers available in print in our lab)

 

Apparently, people in my lab who have worked previously on similar experiments have used this later recipe while calling it 0.2M NaOAc buffer. In my opinion this (later) recipe cannot make a 0.2M buffer solution, it's something else. Papers other than our own group have usually used the specific term "0.2M NaOAc buffer (pH xyz)". Wondering what these other people are calling as NaOAc buffer?

 

Can someone explain what am I missing here?

[Borek]

Let's start with the most basic things first: what is a 0.2 M buffer?

[Biotech78]

Well I understand where you are leading this but let's just answer what you have asked.
So, a 0.2M buffer of substance x is the buffer which contain 0.2 mol/L of substance x in the solution

I completely understands that. But what's worrying me is the people from my lab have published papers in Anal. Chem. using the same "0.2M buffer" term and using the later formula (which actually makes up the final concentration of NaAoC as 0.07M). While other papers use the same term, my concern is whether I am using the correct calculations (I am following CSHL protocol) or the other papers who have reported 0.2M buffer.... have actually used the formula my former colleagues have used

[Borek]

So, a 0.2M buffer of substance x is the buffer which contain 0.2 mol/L of substance x in the solution

What is "substance" here?

It is a bit confusing, I know, but acetate buffer is not 0.2 M in acetate.

[Biotech78]

But then how do we define 0.2M NaAoC buffer?

[Borek]

[CH₃COO^-] + [CH₃COOH] = 0.2 M

[Biotech78]

But CSHL's recipe differs with this definition of yours

[Babcock_Hall]

Borek is simply pointing out that when one writes 0.200 M acetate buffer, one means that the sum of conjugate acid and conjugate base is 0.200 M.  The ratios of acetic acid and acetate will be different in a pH 5 buffer versus a pH 4 buffer.  If you mix a 0.200 M solution of acetate with a 0.200 M solution of acetic acid, you will alway remain at 0.200 M in all forms.  Your second recipe takes advantage of that property.   By the way, the Henderson-Hasselbalch equation is useful in finding the ratio of conjugate acid to conjugate base, subject to certain caveats.

[Biotech78]

I completely understand that. But then why is the CSHL's recipe different?
And when people report in a paper 0.2M sodium acetate buffer, what do they mean? the 0.2M:0.2M ratio of sodium acetate:acetic acid, or a buffer made from some weighed amount of sodium acetate and acetic acid used to adjust pH?

[Borek]

I completely understand that. But then why is the CSHL's recipe different?

IMHO CSHL's recipe produces 0.3 M acetate buffer, not 0.2 M acetate buffer. I am not sure if the buffer concentration is precisely defined by IUPAC or any other similar body, so there is a bit of ambiguity here. Still, I don't think I have ever seen any chemist meaning 0.2 M acetate buffers means 0.2 M in just conjugate base.

And when people report in a paper 0.2M sodium acetate buffer, what do they mean? the 0.2M:0.2M ratio of sodium acetate:acetic acid, or a buffer made from some weighed amount of sodium acetate and acetic acid used to adjust pH?

We told you twice: they mean buffer in which sum of conjugate acid and base is 0.2 M. Doesn't matter how it was prepared.

[Biotech78]

Thanks for explaining. I think it has resolved my confusion

[AWK]

http://cshprotocols.cshlp.org/content/2017/8/pdb.err101634.short

"a final concentration of 0.2 M sodium acetate in the solution" !!!

[Biotech78]

all was good until I consulted CSHL website

[Babcock_Hall]

I think that the way that CSHL described this buffer is, shall we say, uncommon.  Yet if I had been using the CSHL recipe for years, and it worked, I would be hesitant to change what I was doing.

[Borek]

Definitely, if it ain't broke, don't fix it.

Just remember "0.2 M acetate buffer" is a misnomer in this case.