Zinc Oxide Assay Quiz & Flashcards: Titration Method

Study Notes

Definition: A technique where excess standard solution reacting with an analyte is determined by titration with a second standard solution.

Aim: To determine the amount of zinc oxide within a pharmaceutical preparation.

Zinc Oxide: A white insoluble powder in water, used in many industries including as a food additive and topical agent.
Calamine: A combination of zinc oxide and iron(III) oxide, used as a lotion.
Zinc oxide applications: Ointments and creams for sunburn and UV protection.

Reaction:
- ZnO + H2SO4 → ZnSO4 + H2O
- H2SO4 (excess) + 2 NaOH → Na2SO4 + 2 H2O
Process:
- Zinc oxide is dissolved in excess sulfuric acid, forming a turbid solution.
- The mixture is heated until a clear solution forms.
- The remaining acid is titrated with a standard sodium hydroxide solution.

Chemicals:
- Zinc oxide: unknown weight (g)
- Sulphuric acid: 1N
- Sodium hydroxide: 1N
- Ammonium chloride: 2.5 g
- Methyl orange indicator
Equipment:
- Sensitive balance
- Beaker
- Fume hood
- Heater
- Burette
- Conical flask
- Hot plate magnetic stirrer

Weighing:
- Accurately weigh approximately 0.5g of ZnO.
- Weigh 2.5g of ammonium chloride in the same beaker.
Dissolving:
- Dissolve the mixture in 30ml of 1N H2SO4 in a fume hood, noticing the development of a turbid solution.
Heating:
- Gently heat the mixture until a clear solution is obtained.
Titration:
- Add two drops of methyl orange indicator (pH 3.2).
- Fill the burette with 1N NaOH.
- Titrate the solution until the color changes from pink (acidic) to yellow (basic, pH 4.4).

Dissolving:
- Dissolve the sample with 50ml of 1N H2SO4, stirring until a turbid solution forms.
Heating:
- Gently heat the mixture until a clear solution is obtained.
Titration:
- Transfer 10ml of the solution to a conical flask.
- Add two drops of methyl orange indicator.
- Fill the burette with 1N NaOH.
- Titrate until the color changes from pink to yellow.
- Repeat the titration process.

Formula: mEq.of ZnO = N H2SO4 * V H2SO4 - N NaOH *( V NaOH * 50/10)
- Where:
  - N refers to the normality of the solution.
  - V refers to the volume of the solution.
Weight of ZnO:
- Wt.of ZnO mg = mEq.of ZnO * Eq.Wt.of ZnO
- Wt.of ZnO g = Wt.of ZnO mg / 1000
Percentage of ZnO:
- Wt.% = wt.practical / wt.Theoretical *100
- Assume wt.Theoretical = 0.5 g ZnO

Formula: % ZnO = ( Vtot–Vrem) x meq.wt x100 / wt of sample
- Where:
  - Vtot = Total volume of H2SO4
  - Vrem = volume of H2SO4 reacting with NaOH

Zinc oxide:
- Inhaling zinc oxide is hazardous.
- It can cause skin and eye irritation.
Ammonium Chloride:
- Decomposes upon heating, producing toxic fumes.
- Irritant to the eyes and skin.
Sodium Hydroxide:
- Highly corrosive and dangerous to skin and eyes.
- Toxic if ingested or inhaled.