[Aiva]

Hi!

My first post Hope this is the right section.

Buffer capacity is the amount of strong acid or strong base needed to change pH of 1L of buffer by 1 pH unit (An. chemistry book says so). Right?

Wouldn't the amount of strong acid and the amount strong base be different?

Is buffer capacity usually determined experimentally by titrating buffer with strong acid/base?

By the way, my buffer has 3 components, calculations look too complicated to do them quickly and Chembuddy buffer calculator does not have a 3 component option.

Edit: found answer - 

http://wiki.answers.com/Q/Is_buffer_capacity_the_same_if_determined_by_titration_with_a_strong_base_instead_of_a_strong_acid

[Borek]

Buffer capacity is the amount of strong acid or strong base needed to change pH of 1L of buffer by 1 pH unit (An. chemistry book says so). Right?

If that's a definition you are required to use - that's OK. But I prefer a different one:

http://www.chembuddy.com/?left=pH-calculation&right=pH-buffer-capacity

Wouldn't the amount of strong acid and the amount strong base be different?

That's one of the reasons I don't like the definition you have mentioned.

Is buffer capacity usually determined experimentally by titrating buffer with strong acid/base?

Again - depends on the definition. You can titrate, you can add a small amount and see what happens.

By the way, my buffer has 3 components, calculations look too complicated to do them quickly and Chembuddy buffer calculator does not have a 3 component option.

That's because 3 component buffer is not defined. See explanation here.

You can use pH calculator built into Buffer Maker to prepare your buffer solution using three components (you will have to select amount of substances by trial and error, but it doesn't take long), then switch to calculation details view - it lists the buffer capacity of the solution (calculated using definition I linked to above).

Edit: found answer -  

http://wiki.answers.com/Q/Is_buffer_capacity_the_same_if_determined_by_titration_with_a_strong_base_instead_of_a_strong_acid

Hardly correct, as it depends on the buffer definition. Different definitions will give different answers.

[Aiva]

What would buffer capacity as defined in Chembuddy page mean? The unit of this buffer capacity would be what? Not mol/L?

If β could be calculated using Chembuddy page formula to be, say, 1.5 and then I would add 1.5 moles of HCl to 1L buffer with pH 7, would the pH of the buffer would rise to 8? What would adding of 1.5 moles of HCl give? pH 6?

Probably not.

Hope late night sentences are not too confusing.

I was trying to deal with determining buffer capacity as painlessly as possible No luck.

[Borek]

What would buffer capacity as defined in Chembuddy page mean?

Why don't you read the page I linked to - it should answer most of your questions.

[Aiva]

Once I helped my cousin with some school trigonometry exercises. Turned out she had rather foggy understanding what a cosine or a function was. My case is similar, probably. Difficulties with basics.

So the amount of strong acid and strong base needed to add would be the same?

Consider eq. 9.10.: β=2.303(Kw/[H]+[H]+(Cbuf·Ka·[H])/(Ka+[H])^2)

Kw is a constant, unitless, yes? [H] is concentration, as in mol/L? How can one do a sum of Kw/[H] + [H]?

Wouldn't it make you count two variables with different units? Like L/mol plus mol/L? I'm missing something important. What? Everything?

Maybe I should take this to highschool chemistry section?

[Borek]

So the amount of strong acid and strong base needed to add would be the same?

Needed to add for what? To change pH by 1 unit? No, as this definition is not about changing pH by 1 unit.

Kw is a constant, unitless, yes?

Constant, yes, unitless, no. [itex]K_w=[H^+][OH^-][/itex], that makes it units concentration squared.

[Aiva]

β=dn/dpH

means that if a very small amount of strong acid/base (dn) is added to buffer solution, the pH would change by dpH or dn/β?

I tried adding strong acid and strong base to buffers until was pH changed by one unit; the quantities weren't equal to each other and to Chembuddy calculated β But about the same order of magnitude.

Which was only to be expected, as adding large amounts of strong acid/base (or larger than a very small amount, anyway) would change the composition of actual buffering species and the buffer will have different β. Right?

The units of β in all cases should be mol/L. Yes.

I am extremely grateful. Anyway, excuse the tortuous reasoning 

[Aiva]

Regarding the calculation of β. I should determine Kw. If the solution is not very dilute - Kw is not 14, anymore. Yes?

Can 0.5 or 1 M solutions be considered dilute if I want to calculate Kw for β?

Hmm. First, I should simply use Kw 14 and see what happens.

[Borek]

Assume pKw to be 14. When solution gets more concentrated this is no longer an exact value but its doesn't differ by more than just a few tenths.

As β=dn/dpH technically there is no problem with writing it as dpH=dn/β, but you have to remember it holds only for infinitesimally small dn, that's what differentials are about.

β units are not mol/L, if anything, they are mols - please remember the derivation assumes V=1L (and this volume is not shown there, even if it IS there all the time). dn/dpH is mol/change in pH, and change in pH is unitless (as all logs are).

[Aiva]

Hmm. β has mol units? Does not seem right. Then it would not matter how large the buffer volume is - I mean - the results would be the same for 1 ml of buffer and a sea of buffer.

Of course, if I consider eq. 19.1 β=dn/dpH,  units are mol.

But what about eq. 19.10 β=2.303(Kw/[H]+[H]+(Cbuf·KaH]? Units should be mol/L then 

Recalculated my results, which had mistakes. β of dn/dpH variety is very close in values to one I determined by adding strong base until pH changed by a unit. If I add strong acid, capacity values are different, but in the same order. But then I am not too sure about the concentration of HCl I borrowed.