INDICATORS

Indicators are complex organic compounds used:

1. Determining the end points in neutralization processes.
2. Determining the hydrogen-ion concentrations [H+] or pH.
3. Indicate that a desired change in pH has been effected.

These organic substances may be acids or bases themselves, one into the other, at given hydrogen-ion concentrations.

Table below enlists the commonly used pH indicators


Indicators                                        pH Range                                     Color Change
                                                                                              Acid                               Base

Malachite green                               0.0-2.0                            Yellow                            Green
Methyl Yellow                                 2.9-4.0                             Red                               Yellow
Bromophenol blue                           3.0-4.6                             Yellow                            Blue
Methyl orange                                 3.2-4.4                             Pink                               Yellow
Bromocresol purple                         4.0-5.4                             Yellow                            Purple
Bromothymol blue                           6.0-7.6                             Yellow                            Blue
Phenol red                                       6.8-8.2                             Yellow                            Red
Cresol red                                       7.2-8.8                             Yellow                            Red
Thymol blue                                     8.0-9.2                             Yellow                           Blue
Phenolphthalein                                8.0-10.0                           Colorless                       Red/Pink
Thymolphthalein                               9.3-10.5                           Colorless                       Blue

Three theories have been proposed for the change in color of indicators which, briefly stated, are as follows:

1. The physicochemical theory 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.
2. The organic 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.
3. The colloidal theory assumes that indicators form colloidal solutions the change in color of which is dependent upon change in size of the colloidal particle.

How to Use Indicators?

• Use 3 drops of the indicator test solution  for a titration unless otherwise directed.
• 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.
• When a weak acid is titrated with a strong alkali, use phenolphthalein as the indicator.
• When a weak alkali is titrated with a strong acid, use methyl red as the indicator.
• A weak alkali should never be titrated with a weak acid, or vice versa, since no indicator will give a sharp end point.
• The appearance of a color is more easily observable that is the disappearance. Therefore, always titrate where possible to the appearance of a color.