Final preparation of both emulsions (o/w or w/o) looks the same in appearance with naked eyes, therefore certain tests are required to differentiate between them. It is also important that at least two tests should be performed to draw a final conclusion.
1. Dilution test:
In this test the emulsion is diluted either with oil or water. If the emulsion is o/w type and it is diluted with water, it will remain stable as water is the dispersion medium” but if it is diluted with oil, the emulsion will break as oil and water are not miscible with each other. Oil in water emulsion can easily be diluted with an aqueous solvent whereas water in oil emulsion can be diluted with a oily liquid (Shown in Fig. 12.1)
2. Conductivity Test:
The basic principle of this test is that water is a good conductor of electricity. Therefore in case of o/w emulion, this test will be positive as water is the external phase. In this test, an assembly is used in which a pair of electrodes connected to an electric bulb is dipped into an emulsion. If the emulsion is o/w type, the electric bulb glows.
(a) o/w type emulsion (b) w/o type emulsion
3. Dye Solubility Test:
In this test an emulsion is mixed with a water soluble dye (amaranth) and observed under the microscope. If the continuous phase appears red, it means that the emulsion is o/w type as water is in the external phase and the dye will dissolve in it to give color. If the scattered globules appear red and continuous phase colorless, then it is w/o type. Similarly if an oil soluble dye (Scarlet red C or Sudan III) is added to an emulsion and the continuous phase appears red, then it is w/o emulsion.
4. Cobalt Chloride Test:
When a filter paper soaked in cobalt chloride solution is dipped in to an emulsion and dried, it turns from blue to pink, indicating that the emulsion is o/w type.
5. Fluorescence Test:
If an emulsion on exposure to ultra-violet radiations shows continuous fluorescence under microscope, then it is w/o type and if it shows only spotty fluorescence, then it is o/w type.