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Resist change in PH despite the addition of an acid or alkali
How is a buffer soluton made?
a weak acid and one of its salts
Ethanoic acid - Acid (gives acidic PH) H donor
Example of a buffer solution made from a weak acid and one of its salts
Ammonia- (Alkaline PH ) H acceptor
Example of a buffer solution made from a weak base and one of its salts
The action of a buffer solution depends on...
The weak acid equilibrium. HA (aq) <--> H+ (aq) + A-(aq)
HA (aq) <--> H+ (aq) + A-(aq)
Assumption 1 - All A- ions come from the salt. The weak acid supplies very few A- ions.
What happens if an acidic substance is added to a buffer?
Rise in H+ disturbs the equilibrium. A- ions from the salt react with the extra H+ ions (form HA and water). Fall in PH is prevented . increased conc of H+ has caused equilibrium to shift to left.
What happens if an alkaline substance is added to a buffer?
H+ ions are removed from the solution. Buffer solution counteracts this because H+ ions can be regenerated from the acid HA. H+ can combine with the OH- ions from the alkali solution added. Equilibrium sh…
Role of HA and A- in a buffer soultion
HA ... Source of extra H+ ions (if alkali added and H+ ions are removed)
Ka = (H+(aq)) x (salt)/(acid)
Equation to work out Ka of a buffer solution
Chaneg the ratio of acid/base and the salt
How can you change the PH of a buffer solution
How do buffer solutions work
A buffer solution has to contain things which will remove any hydrogen ions or hydroxide ions that you might add to it - otherwise the pH will change. Acidic and alkaline buffer solutions achieve this in different ways.
Both acid-base indicators and pH buffers involve
mixtures of weak acids (or bases) and their salt
Acid-base Indicators are present in
low concentrations so that the ratio of the two forms of the indicator is determined by the pH
Buffer solutions are present in
high concentrations and control the pH
pH depends on the ratio of the concentrations of the acid and base, dilution should have no effect on the pH of a buffer
in order to set the pH of a solution
one needs to make up the solution with the appropriate ratio of acid and conjugate base concentrations
±1 unit of pK around the pKa value
the most effective range of the buffer solution is
Is the pH of a buffer solution truly independent of concentration?
Yes, except for low concentrations, where the contribution from autoionization of water can no longer be neglected
determined by the ratio of the acid and salt concentrations, not the volume
The more concentrated buffer
stabilized the pH better, and is said to have a greater buffer capacity
Amino acids can
Glycine as a buffer
-tends to exist in aqueous solution as a dipolar ion (a.k.a. zwitterion), due to the interaction between the acid and base components