# The Naked Scientists Forum

### Author Topic: What is buffer capacity and its unit?  (Read 19542 times)

#### sasha44

• Jr. Member
• Posts: 16
##### What is buffer capacity and its unit?
« on: 07/11/2010 08:11:09 »
What is buffer capacity?
What is the unit of buffer capacity?
and what does dn means in this equation?

B=dn/dpH

Where B=Buffer Capacity
and dpH =change of pH

says "n" is "where n is number of equivalents of added strong base"

What does that mean?

I'm at total loss because none of the books I'm supposed to refer doesn't even have this equation!

#### Bored chemist

• Neilep Level Member
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##### What is buffer capacity and its unit?
« Reply #1 on: 07/11/2010 09:30:28 »
The idea of a buffer s that it maintains a nearly constant pH even if you add a little acid or base.
In practice, they are never perfect, the pH changes a bit, and some buffers are better than others.
The buffer capacity is a measure of the ability of the buffer to keep the same pH when it reacts with a small amount of added base (or acid it doesn't matter much with most buffer systems).

As for the meaning of the little "dn", do you know about calculus?

#### sasha44

• Jr. Member
• Posts: 16
##### What is buffer capacity and its unit?
« Reply #2 on: 07/11/2010 10:28:40 »
No sir I don't know any calculus. I know that d stands for delta which is a small change. Isn't it?

I got this question for Biochemistry

"The buffer capacity of a solution containing ammonium chloride and ammonium hydroxide maintained at pH 10 was 10.0 cmmol dm^-3 Calculate the concentrations of Ammonium hydroxide and ammonium ions in the buffer solution"

I don't understand the unit used for buffer capacity here
or the way to solve this problem

How does the buffer capacity relate to the actual pH or the concentration of ions?

This is rather Greek to me because I don't see any change in pH in the question or any kind of acid or base being added.

#### Bored chemist

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##### What is buffer capacity and its unit?
« Reply #3 on: 07/11/2010 15:30:45 »
That's not an easy question.
If you had a very dilute buffer solution then adding a little base to it would change the pH more than adding the same base to a more concentrated buffer.
In this case they tell you how much it changes. They say it takes 10 mMol to change the pH by one unit.
They also tell you it starts at pH 10.
There is enough information there to answer the question.
You also need this
http://en.wikipedia.org/wiki/Henderson%E2%80%93Hasselbalch_equation

#### sasha44

• Jr. Member
• Posts: 16
##### What is buffer capacity and its unit?
« Reply #4 on: 07/11/2010 18:47:14 »
Thanks! I asked one of my seniors and he explained it

NH4OH NH4+ + OH-

At equilibrium

Kb = [NH4+][OH-]/[NH4OH]
pKb = pOH - log [NH4+]/[NH4OH]

Assuming the solution is at room temperature and it is dilute

pKw = pH + pOH

pKb = pKw - pH - log [NH4+]/[NH4OH]

Assuming that pKb for NH4OH is 4.74

for pH = 10

4.74 = 14 - 10 - log [NH4+]/[NH4OH] -----------------(1)

for pH = 9

4.74 = 14 - (10-1) - log [NH4+ + 10]/[NH4OH - 10] ----(2)

And using (1) and (2) you can get the NH4+ and NH4OH concentrations

But they hadn't given the Kb value for NH4OH in the question

and

I still am puzzled with the unit cmmol dm-3

what does cm stand for ?

#### Bored chemist

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##### What is buffer capacity and its unit?
« Reply #5 on: 07/11/2010 19:50:45 »
I think the c is a typo and it should be millimoles per decimetre cubed.
BTW, just to confuse things, ammonium hydroxide practically doesn't exist.

#### sasha44

• Jr. Member
• Posts: 16
##### What is buffer capacity and its unit?
« Reply #6 on: 08/11/2010 16:55:02 »
okay

Its NH3 that practically exist right?

#### Bored chemist

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##### What is buffer capacity and its unit?
« Reply #7 on: 08/11/2010 18:48:16 »
Yes.

#### sasha44

• Jr. Member
• Posts: 16
##### What is buffer capacity and its unit?
« Reply #8 on: 13/11/2010 16:28:37 »
thanks!
Thank you very much!!

#### The Naked Scientists Forum

##### What is buffer capacity and its unit?
« Reply #8 on: 13/11/2010 16:28:37 »