2.1c Buffers and Indicators Flashcards
What is a buffer?
a solution in which the pH remains approximately constant when small amounts of acid, base or water are added.
What does an acid buffer consist of?
a solution of a weak acid and one of its salts made from a strong base
How does an acid buffer work?
-the weak acid provides hydrogen ions when these are removed by the addition of a small amount of conjugate base
-the salt of the weak acid provides the conjugate base, which can absorb excess hydrogen ions produced by the addition of a small amount of acid
What does a basic buffer consist of?
a solution of a weak base and one of its salts
How does a basic buffer work?
-the weak base removes excess hydrogen ions
-the conjugate acid provided by the salt supplies hydrogen ions when these are removed
How can the approximate pH of an acid buffer be calculated from the acid dissociation constant?
pH=pKa-log10[acid]/[salt]
What are indicators?
weak acids
How can the dissociation of indicators be shown?>
Hin(aq)+H2O(l)=H3O+(aq)+In-(aq)
What is Kin?
The acid indicator dissociation constant
How is Kin represented?
Kin=[H3O+][In-]/[HIn]
In aqueous solution how does the colour of the acid indicator differ from that of its conjugate base?
it is distinctly different
What is the colour of the indicator determined by?
the ration of [HIn] to [In-]
When is the theoretical point at which colour change occurs?
when [H3O+] = KIn
When is the colour change assumed to be distinguishable?
when [HIn] and [In-] differ by a factor of 10
What expression can be used to estimate the pH range over which a colour change occurs?
pH=pKIn +-1
