A general buffer system (1 Viewer)

[mr EaZy]

Can someone tell me the general idea/concept of a buffer system. i dont need examples, just that i cant find info on what a general buffer system is.

i'll start off:
1) they contain equal ammounts of conjugate acids/bases..
2) all soluble in water.

Thats about it from me!

 

[mystify]

3) A buffer system is one which resists large changes in pH when small amounts of either an acid or base is added.
4) Buffer systems usually consist of a weak acid and the salt of the weak acid (i.e its conjugate base).

 

[hipsta_jess]

Can someone tell me the general idea/concept of a buffer system.

are you asking what they do, or how they do it?

 

[Tommy_Lamp]

A buffer controls the level of acidity or basicity in a solution. If an acid or a base is added to a buffer solution, there is hardly any change in pH.
A buffer solution is usually a mixture of a weak acid and its conjugate base, such as hydrogen carbonate ions, HCO3-, and carbonate ions, CO32-.

 

[xiao1985]

u can also use weak base and its conjugate acid...

u need comparable amount of both substance tho...

 

[nit]

Yeh, you need to have roughly the same amount of the conjugate/base pair, as the buffering capacity is largely dependent on the ratio acid/base according to henderson-hasselbalch equation which governs the system. If this ratio changes significantly as a result of addition of acid/base, then you've got a crap buffer.

 

[mitochondria]

The concentrations of the acid and it's conjugate base do not necessarily have to be the same, it will depend on the application of the buffer and the desired starting pH of the buffer.

Just a note: as nit has mentioned about the henderson=hasselbalch equation, which is given by:

pH = pKa + log10([conjugate base]/[acid])

The final pH of the buffer is dependent on the pKa of the conjugate base/acid pair you are using and the concentration of both substances. You can design buffer with different pH's by knowing the pKa and solve for [conjugate base]/[acid]

Back to the point, when it's not necessary for [A-] = [HA], it is preferable to have [A-] = [HA] if you want the highest buffering capacity - a buffer's ability to resists to a change in pH is maximised when [A-] = [HA]. Alternatively, the more the pH deviates from the pKa the less effective the buffer.

I have done a lab on this topic today and for reference:

When you add 5 drops of 0.1M HCl to 10 mL of NH4CL/NH3 (pH = 9.43, pKa = 9.24), it gives you an ~0.1 change in pH. The first 5 drops of such HCl in water will give you a ~4.0 shift in pH.

 

[mr EaZy]

thnx all
let me clear up somethings
wats the difference between pH and pKa??

"a buffer's ability to resists to a change in pH is maximised when [A-] = [HA]"
so the if equation goes like this right:

HA ------> (H+) + (A-)
so if [A-] = [HA] there must be excess h+ ions. So what ur trying to say is that buffers (good ones) should be polyprotic right?????


strictly speaking this would be in equilibrium ofcourse. Am i right here?

 

[mitochondria]

pH is a gerneric term where as pKa is specific. pH is the measurment of [H+] at any desired time whereas pKa is the pH at equilibrium. Ka is the acidic constant (also dissociation constant) of a system and it is given by:

Ka = [H+][A-]/[HA]

at equilibrium.

For an acidic buffer, when an acid is added to the buffer, the conjugate base [A-] reacts with the [H+] and form the acid HA:

A- + H+ -----> HA

conversely, if a base is added to such a buffer, it reacts with the acid and we have:

OH- + HA -----> A- + H2O

producing it conjugate base and water:


Usually buffers (for an acidic buffer) are made by having both the acid and a salt of the its conjugate base in a solution. A more correct representation would be:

A- + H+ <-----> HA


Analyse the following data:

H3PO4/H2PO4, pKa = 2.13, e.p.r = 1.1-3.1

CH3COOH/CH3COO-, pKa = 4.76, e.p.r. 3.8-5.8

(values taking form my lab manual)(e.p.r. - effective pH range). You will see that both acidic buffer have an effective pH range of 2. Phorphoric acid is triprotic whereas acetic acid is monoprotic - the point is that the effectiveness of a buffer is not determined by the proticity of its acid. I am assuming you are thinking that since a polyprotic acid can donates more H+ therefore it can react with more base? The ability of the buffer to resist change in pH depends on both the conjugate base and the acid. When you analyse the examples given above, you will see that the conjugate base and acid in both cases have a monoprotic relationship - therefore being a polyprotic acid does not mean that the buffers made form it are "better" (more effective).

Remember:

The buffer's ability to resist to change in pH is maximised when [A-]=[HA]
And the larger the [A-] and [HA] the more effective the buffer is (as more acid or base can be put into the system).