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Test for Identification of Type of Emulsion

Dilution test, Conductivity test, Due solubility test, Cobalt chloride test, Florescence test, Formation of creaming, Dye test.
Both final preparations (o/w and w/o) appear identical to the naked eye; therefore, additional tests are needed to distinguish between them. A conclusion can only be reached after at least two tests have been completed.

Dilution test

During this test, either oil or water is used to dilute the emulsion. The emulsion will remain stable if the water is the dispersion medium and is diluted with water. The emulsion, however, will break if it is diluted with oil since oil is incompatible with water. Water in oil emulsions can be diluted by adding an aqueous solvent, while oil in water emulsions can be diluted through an oily liquid.

Conductivity test

Since water conducts electricity well, it can be used as a test medium. Since water is the external phase in the case of o/w emulsion, this will be a positive result. An electric bulb is connected to a pair of electrodes that are dipped into the emulsion and a pair of electrodes are dipped into an emulsion. A glowing electric bulb occurs when the emulsion is of the o/w type.

w/o type emulsion
o/w type emulsion

Due solubility test

Tests are conducted by mixing an emulsion with a water-soluble dye (amaranth) and observing the results under a microscope. In the case of a red continuous phase, the emulsion is of the o/w type, which is the case since the external phase is filled with water and the dye will dissolve in it, creating the color. The scattered globules should appear red and the continuous phase should be clear. If neither of those occurs, it is without type. A dye added to an oil emulsion (Scarlet red C or Sudan III) causes a red continuous phase, which indicates that the oil is not emulsified.

Cobalt chloride test

When filter paper is dipped in an emulsion and dried, its color turns pink when soaked in cobalt chloride solution. This shows that the emulsion is of the o/w type.

Test: To carry out this test, a Whatman filter strip is impregnated with cobalt chloride solution for 10 minutes, and when the strip dries, it becomes blue. Blue strips are soaked in sample emulsion for 5 minutes, then dried and the results are observed.

Observation
  • The blue cobalt chloride strip should become completely pink if it indicates O/W emulsion.
  • In these cases, pink spots appear against a blue background, which indicates the type of emulsion.

Florescence test

When an emulsion shows continuous fluorescence under a microscope after exposure to ultraviolet radiation, it is a w/o type. It is an o/w type if it shows only spotty fluorescence.

Formation of creaming

Testing: heating the sample emulsion in a suitable test tube for 10 minutes and observing the result

Observation
  • Whenever upward creaming is present, it indicates that the emulsion type is O/W.
  • Creaming downward indicates a water-based emulsion.

Dye test

Soluble dyes make the color according to their phase of solubility, water-soluble dyes make the color according to the water phase while oil-soluble dyes make the color according to the oil phase.

Test: Take a small amount of sample emulsion and mix it with either a water-soluble dye such as amaranth or an oil-soluble dye such as scarlet blue. Observing the result of a small sample to be mounted on a compound microscope takes place in both cases.

Observation
Case with water-soluble dye (Amaranth)
If colorless droplets are visible on a colorless background, it indicates the emulsion type is W/O, or if red-colored droplets are visible on a colorless background, it indicates the emulsion type is O/W.

Case with oil-soluble dye (Scarlet blue)
Droplets of blue color observed against colorless backgrounds indicate that the emulsion is O/W, while droplets of colorless colors observed against colorless backgrounds indicate the emulsion is W/O.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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