pH buffers

[WoodsN0]

Could someone explain what is meant by "a buffer system cannot buffer itself"

in relation to the carbonic acid:bicarbonate buffer system of the body?

 

thank you

[Immunologist]

Ok, yeah, in fact, sounds confusing...

But, read your sentence as follow:

 

A buffer system can not buffer the addition of one of its component.

 

Now for the explanation. Imagine your context of carbonic acid (H2CO3)and bicarbonate (NaHCO3, that gives HCO3-).

H2O + CO2 <-> H2CO3 <-> H+ + HCO3-

 

If you add one of these constituants, you are changing the equilibrium of the equation and therefore changing the pH of the solution. Only if you add different acids that you provide only H+ or OH- then you can buffer them because their H+ or OH- will taken and buffered while their radical (other part of the molecule) will form a salt and not enter the equation.

 

More details:

http://www.chemistry.wustl.edu/~edudev/LabTutorials/Buffer/Buffer.html

 

Hope I helped!

[WoodsN0]

ok - I must still be be missing something...

 

so... this buffer system can adjust easily and quickly for changes in pH brought about by causes other than CO2 generated H2CO3. Why can't it handle elevated CO2 from a respiratory problem? If there is an abundance of both carbonic acid and bicarbonate - why cant the system handle extra H+ generated by the elevated CO2?

[Immunologist]

the problem is not the extra H+, but the extra CO2 that will transform to HCO3-. This one also enters your equilibrium. If it was HCl... H+ would be buffered and Cl- would form a salt and re-equilibrate the equation, no?

 

Wow... this is quite far for me though, I don't want to lead you to something wrong...

[WoodsN0]

Would it be safe to say the carbonic acid/bicarbonate buffer system is self contained and H+ is only added minimally to the system but mostly just changes form?

[Nashyboyo]

Carbon dioxide is catalysed to bicarbonate by carbonic anhydrase in red blood cells. The CO2 concentration gradient is maintained by the bohr effect of haemoglobin.