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# Analytical Chemistry3

الكلية كلية العلوم للبنات     القسم قسم الكيمياء     المرحلة 1
أستاذ المادة محمد حامد سعيد الدهيمي       12/01/2017 17:40:53
LECTURE 3
Salts of Weak bases and Strong acids
Salts of Weak acids and Weak bases
In this type of salt, both the anion of weak acid (X–) and the cation of weak base (B+) undergo hydrolysis simultaneously
Acid-Base Indicators

Acid - Base indicators (also known as pH indicators) are substances which change colour with pH. They are usually weak acids or bases, which when dissolved in water dissociate slightly and form ions.
Consider an indicator which is a weak acid, with the formula HIn. At equilibrium, the following equilibrium equation is established with its conjugate base

The acid and its conjugate base have different colours At low pH values the concentration of H3O+ is high and so the equilibrium position lies to the left. The equilibrium solution has the colour A. At high pH values, the concentration of H3O+ is low - the equilibrium position thus lies to the right and the equilibrium solution has colour B
We define pKlni = - log(Kln), and the pKln value is also the pH value at which the color of the indicator is most sensitive to pH changes.
Taking the negative log of Kln gives,
[In-]
-log Kln = -log[H+] - log------
[HIn]
or
[In-]
pH = pKln + log-----
[HIn]
This is a very important formula, and its derivation is very simple. Start from the definition of the equilibrium constant K, you can easily derive it. Note that pH = pKln when [In-] = [HIn]. In other words, when the pH is the same as pKln, there are equal amounts of acid and base forms. When the two forms have equal concentration, the color change is most noticeable.
Colors of substances make the world a wonderful place. Because of the colors and structures, flowers, plants, animals, and minerals show their unique characters.
Many indicators are extracted from plants. For example, red cabbage juice and tea pigments show different colors when the pH is different. The color of tea darkens in a basic solution, but the color becomes lighter when lemon juice is put into a tea. Red cabbage juice turns blue in a basic solution, but it shows a distinct red color in an acidic solution