phosphate buffer pH7

[Provetech]

Hi,
could you confirm me that a 5 times dilution of 50mM phosphate buffer pH7 gives a 10mM phosphate with still pH7...
Thanks

[zokitano]

Yes, the theory says that 5 times dilution of one solution gives a new solution with 5 times lower concentration. And by definition, BUFFERS should not change the pH value due to dilution.

But sometimes people know to say that solution of for example K2HPO4 is a buffer solution, and they make mistake.
Because simple dilution of that solution will change the pH value of the solution (dilution of a salt solution, K2HPO4 is salt of a strong base and weak acid)
Buffers are solutions containing conjugate weak acid/weak base pair that are resistant to a change in pH when strong acid or strong base is added.

For example: solution containing H2PO4(1-) and HPO4(2-) (an acid and its conjugate base) is an example of buffer solution

Best regards
Zoran

[Noser222]

Why not just measure it?

[HW Mueller]

zokitano, where did you get this definition of a buffer regarding dilution?

Besides what Noser222 suggested one can run a program like

http://www.liv.ac.uk/buffers/buffercalc.html

to get an idea on how component amaunts look for different buffer concentrations. You will be in for surprises.

[zokitano]

Dear Mr.Mueller,

Buffers are solutions containing conjugate weak acid/weak base pair that are resistant to a change in pH when strong acid or strong base is added.

According to Henderson-Hasselbalch equation, pH value of a buffer solution can be calculated (at constant temperature, of course):

pH = pKa + log [salt]/[acid] or

pH = pKa + log [conj.base]/[acid] or

pOH = pKb + log[salt]/[base] or

pOH = pKb + log [conj.acid]/[base]

where [] indicates for equilibrium concentration of the conjugate base/acid and its acid/base.

So, according to Henderson-Hasselbalch equation (at constant temperature) if one buffer solution is diluted to greater volume, the ratio between concentrations of conjugate base/acid and acid/base won't change. That's why dilution (by theory and practice - at constant temperature) won't change the pH value of the buffer.

Viele Grussen,
Zoran

[zokitano]

Or more accurately, pH value can be calculated using the activities (a) of the buffer components:

pH = pKa + log a(salt)/a(acid)

PH = pKa + log a(conj.base)/a(acid) e.t.c

By this correction, you consider other factors that can influence activity (and also pH): ionic strength; ion charge and ion radius of the ionic species in the buffer.

Best regards

[HW Mueller]

zokitano,
In theory: The H.-H. equation is essentially just a rearrangement of the equation for the equilibrium constant which holds only over certain concentrations, because it ignores the Kw = [H+][OH-]. The activities don´t help here either.

In practice: Try what Noser and I suggested above.

[zokitano]

Mr Mueller,

I am not saying that your and Noser222's suggestion isn't correct or OK.
In practice when there is a lack of time during buffer preparation, the suggested link for immediate calculation of the buffer's pH gives a relief for analysts, I am sure (by the way, thanks for the link).

But that link, that web application, have to use some equations for computing the pH. And I guess those equations include Henderson-Hasselbalch, Debay-Huckel or other equation that normally can be used for pH calculation when different additional factors like temperature and ionic strength are taken in to consideration.

Best regards

[Rande]

I am not sure about a pure phosphate buffer going from 50 mM to 10 mM , but I do know that 10X PBS reads pH 7.4 and then is pH 7.2 when diluted.

I also have another Buffer (that I make often) that is pH 6.7 when diluted to 1X but I need to titrate the 10X concentrate to pH 6.3 .

I thought that when concentrated, the ionic concentration "pushed the equilibration constant" of H+ a little. This could be due to other salts in the mix. It also could be effected by the "sodium error" of the pH probe (but I understand this is minor at neutral pH).

To be practical .... I don't care! I am happy to let an inorganic chemist worry about the theory, I just want to make sure that I can make a 10X buffer that will always dilute to the expected pH.

[HW Mueller]

Again, not even considering inaccuracies due to using concentrations instead of activities, the H.-H. ignores the Kw and assumes that the buffer components added to the H2O are the concentrations at equilibrium. Therefore, the H.-H. must give, and does give, misleading impressions regarding pH stability in relation to dilution.

[jitender]

I agree with Zokitano. Theoretically, as long as activity ratio of conjugate acid-base is same , pH of system shall remain same with 'lowered ionic strength'.

1. pH is a measure of activity coefficient of 'hydrogen ions' not concentration

2. pH of buffer is dependent on 'ratio of activity coefficients' of acid and base component, rather than individual concentration.

3. In some buffers dilution can cause slight change in the degree of ionization of acid and base (in most cases ionization increases slightly), and therefore change in activity ratio (not concentration ratio) and subsequently small change in pH. [concept of degree of hydrolysis of electrolyte].

Since ionization increases mostly on dilution therefore 'ionic strength' of solution increases slightly.

In the buffer calculator link provided: look for the effect of dilution by altering volume and concentration in opposite directions. Look for 'ionic strength change due to the buffer'. If this term changes, amount of acid and base component will change accordingly else not. Therefore, small effect of dilution on degree of ionization is compensated by calculator.
So how much small change will occur that depends on buffer species and extent of dilution.

[zokitano]

Thank you Jitender, for your input. I totally agree with you

Regards

[HW Mueller]

Ok, so some have gone in a circle such that all of us now agree that there are changes to be expected when diluting buffers.
Except: some of you are forgetting that H.-H. is ignoring the Kw, etc. so when you are at [H+] close to [buffer components] H.-H. fails abismally and so will your theoretical predictions on pH change with dilution.
And, why are we arguing when experiments, etc., clearly show that pH can vary unacceptably with dilution.