That’s a great question, as the testing process for pharmaceuticals is critical for quality control. The procedure outlined in the image you provided describes the chemical assay for Dried Ferrous Sulphate (FeSO4), which is a standard redox titration method known as Cerimetry.
Here is the detailed testing process, focusing on the steps mentioned in your notes, along with the correct calculation to determine the percentage purity.
Testing Process: Assay of Dried Ferrous Sulphate
The goal of this assay is to determine the percentage of FeSO4 (Ferrous Sulphate) present in the dried sample by titrating the iron(II) ions (Fe2+) with a strong oxidizing agent, Ceric Sulphate (Ce(SO4)2).
I. Preparation and Reaction
| Step | Description | Rationale |
| 1. Sample Preparation | Accurately weigh approximately 0.5 gm of the Dried Ferrous Sulphate (DFS) powder. | Ensures precision in calculating the final percentage purity. |
| 2. Dissolution | Transfer the sample to a conical flask. Add 20 ml of Dilute H2SO4 (Sulfuric Acid) and 30 ml of distilled water. | The acid medium is essential for the titration reaction to proceed correctly, and it helps prevent the immediate air-oxidation of Fe2+ to Fe3+. |
| 3. Indicator Addition | Add a few drops of Orthophenanthroline TS (Ferroin Solution). (This is the “Ferrianic solution” mentioned in your note, a common redox indicator). | Ferroin forms a stable, complex red-orange color with Fe2+. |
| 4. Titration | Immediately titrate the solution with a standardized 0.1 N Ammonium Ceric Sulphate (or Ceric Sulphate) solution. | The Ceric ion (Ce4+) oxidizes the Fe2+ to Fe3+. The volume of titrant used is recorded as the Burette Reading (BR). |
| 5. Endpoint | Continue titration until the red-orange color of the indicator abruptly changes to a very faint pale blue/colorless solution. This indicates that all Fe2+ has been consumed. | This is the point at which the titration is complete, and the slightest excess of Ce4+ oxidizes the indicator itself. |
II. Calculation of Percentage Purity
The formula is used to convert the volume of titrant consumed into the equivalent weight of FeSO4 in the sample, expressed as a percentage.
Correct Standard Assay Formula:
%Purity(of FeSO4)=WeightSampleBR×N×0.01519×100
Where:
- BR: The volume (in mL) of Ce(SO4)2 solution consumed by the sample (Burette Reading).
- N: The exact Normality of the Ce(SO4)2 titrant solution.
- 0.01519: The milliequivalent factor. This is the weight in grams of anhydrous FeSO4 that is chemically equivalent to 1 ml of a 1 N oxidizing solution.
- WeightSample: The accurate weight of the Dried Ferrous Sulphate sample in grams (≈0.5 g).
- 100: To express the result as a percentage.
The formula from your note: (weight of sample)÷0.1BR×0.01519×N of Normality×100 follows the same structure, confirming the use of the 0.01519 factor.
