Study Set Content:
- used in analytical methods must have a sharp contrast between two colors which will exhibit the acidity & alkalinity of the mediums.
Mixed Indicators
Mixed Indicators examples
- bromocresol green-methyl red, bromocresol green-chlorophenol red-methyl orange, cresol red-thymol blue, and thymol blue-phenolphthalein
Malachite Green pH range Acid Base
0.0-2.0, yellow, green
Methyl Yellow pH range Acid Base
2.9-4.0, Red, Yellow
Bromephanol Blue pH range Acid Base
3.0-4.6, Yellow, Blue
Methyl Orange range Acid Base
3.2-4.4 pink, yellow
Bromocresol green range Acid Base
4.0-5.4 yellow, blue
Methyl Red pH range Acid and base
4.2-6.2 red, yellow
Bromocresol purple pH range Acid and base
5.2 - 6.8 yellow, purple
Bromothymol blue pH range Acid and base
6.0-7.6, yellow, blue
Phenol red pH range Acid and base
6.8-8.2, yellow, red
Cresol Red pH range Acid and base
7.2-8.8, yellow, red
Thymol blue pH range Acid and base
8.0-9.2, yellow, blue
Phenolphthalein pH range Acid and base
8.0-10.0 Colorless red
Thymolphthalein pH range Acid and base
9.3-10.5, colorless, blue
Rules in using your indicators:
a. use 3 drops of indicator test solution for a titration unless otherwise directed
b. when a strong acid is titrated with a strong alkali, or a strong alkali with a strong acid – Methyl orange, methyl red or phenolphthalein may be used
c. when a weak acid is titrated with a strong alkali, use phenolphthalein as indicator
d. When a weak alkali is titrated with a strong acid– Use methyl red as indicator
e. a weak alkali should never be titrated with a weak acid, or vice versa since no indicator will give a sharp end point.
f. the appearance of a color is more easily observable than the disappearance. Therefore always titrate where possible to the appearance of a color.
- attributes the color to certain ions an increase in which causes the appearance of a new color, and a decrease in which causes the disappearance of a color or the appearance of a different color
The Physicochemical Theory
- attributes the color of indicators to certain groupings of the elements in a compound and the change in color to a change in molecular structure
The Organic Theory
- attributes that indicators form colloidal solutions the change in color of which is dependent upon change in size of the colloidal particle
The Colloidal Theory
- process of determining Molarity and Normality of a solution
Standardization
