Buffer solution دکتر امید رجبی دانشیار گروه شیمی دارویی. شیمی عمومی. Buffer solutions are solutions that resist change in Hydronium ion and the hydroxide ion concentration (and consequently pH ) upon addition of small amounts of acid or base , or upon dilution. .
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Buffer solution دکتر امید رجبیدانشیار گروه شیمی دارویی
A buffer solution
CH3COONa CH3COO- + Na+
CH3COOH + H2O CH3COO- + H3O+
small amount Large amount
CH3COOH + OH-CH3COO- + H2O
CH3COO- + H3O+CH3COOH + H2O
Weak acid + H2O H3O+ + Conjugate base
Ka = [H3O+][conjugate base]
[H3O+] = Ka x [weak acid]
pH = -log [H3O+]
The weak acid H2PO4- in a blood buffer H2PO4-/HPO42- has Ka = 6.2 x 10-8. What is the pH of the buffer if it is 0.20 M in both H2PO4- and HPO42-?
[H3O+] = Ka x [H2PO4-]
[H3O+] = 6.2 x 10-8 x [0.20 M] = 6.2 x 10-8
pH = -log [6.2 x 10-8] = 7.21
The Henderson-Hasselbach Equation: A practical application
Let's begin our discussion of the Henderson-Hasselbach Equation with a continuation of the dissociation constant derivation.
should be a familiar relationship . If we rearrange our equation to provide the [H3O+] on the left side, we get
[H3O+] =Ka (HA)/(A-)
(Notice that the ionized species of the acid is now in the denominator.)
Taking the negative log of both sides gives us easier numeric values to work with:
which can be rewritten