Unit 3: Analysis of Cosmetics

General Background and Classification

Cosmetics are substances or products used to enhance or alter the appearance or fragrance of the body. They are generally applied to the skin, hair, nails, and lips.

Analytical chemistry is crucial for cosmetics to:

• Ensure the quality and consistency of ingredients.
• Test for the presence of harmful substances (like heavy metals).
• Verify the advertised claims (e.g., "contains vitamin E").

Classification: Cosmetics can be classified based on their use:

• Skin Care: Creams, lotions, talcum powder, moisturizers.
• Hair Care: Shampoos, conditioners, dyes.
• Make-up: Lipstick, foundation, eyeliner.
• Fragrances: Perfumes, deodorants.

Analysis of Talcum Powder

Talcum powder is a common cosmetic product used to absorb moisture and reduce friction.

Major and Minor Constituents:

• Major Ingredient (Base): Talc. This is a mineral, hydrated magnesium silicate (Mg₃Si₄O₁₀(OH)₂). It provides the smooth, "silky" feel.
• Additives (Minor):
  • Adsorbents: Starch, kaolin, or calcium carbonate (CaCO₃) to increase moisture absorption.
  • Antiseptics: Boric acid (H₃BO₃) or zinc oxide for mild antiseptic properties.
  • Fragrance: Perfume oils for scent.
  • Other: Magnesium carbonate (MgCO₃) is often used as a carrier for the perfume.

Determination of Constituents of Talcum Powder

This involves separating the components. A common procedure is Ashing, where the sample is heated to a high temperature to burn off all organic material (like starch and perfume), leaving behind an inorganic "ash" (talc, CaCO₃, Boric acid). This ash is then dissolved in acid to create a solution for analysis.

1. Determination of Magnesium (Mg)

Principle (Complexometric Titration): The sample is ashed, and the ash is dissolved in acid to get Mg²⁺ ions in solution. This is the same principle as water hardness.

The solution is buffered to pH 10, and Eriochrome Black-T (EBT) indicator is added. It is then titrated with a standard EDTA solution. The endpoint is a color change from wine red to blue.

2. Determination of Boron (as Boric Acid, H₃BO₃)

Principle (Acid-Base Titration with Polyol):
Boric acid is a very weak acid, so it cannot be titrated directly with a base (like NaOH) because the endpoint is not sharp.
However, when a polyol (a compound with multiple -OH groups, like glycerol or mannitol) is added, it forms a stable complex with the borate ion. This complex is a much stronger acid.

Boric Acid (weak) + Glycerol → Boric-Glycerol Complex (strong)

Method:

1. The sample is treated to isolate the boric acid.
2. A large amount of glycerol is added.
3. The mixture is now titrated with a standard NaOH solution using phenolphthalein indicator. The endpoint is a sharp color change from colorless to pink.

3. Determination of Calcium (Ca)

Principle (Complexometric Titration): This is used if the talc powder contains chalk (calcium carbonate, CaCO₃).
The titration is done at a high pH (12-13) with a strong base (NaOH). At this pH, all the Magnesium (from the talc) precipitates as Mg(OH)₂ and does not interfere with the titration.
A specific indicator (e.g., Murexide) is used. The solution is titrated with standard EDTA. The endpoint is a color change (e.g., from pink to purple).