🧪 Introduction
Platinum (Pt) is a highly valuable noble metal widely used in catalysts, pharmaceuticals, and chemical industries. Accurate determination of platinum content in ash samples is essential for quality control, recovery efficiency, and economic evaluation.
One of the most reliable classical techniques is the Gravimetric Method using Ammonium Chloride (NH₄Cl). This method ensures high accuracy by isolating platinum as a solid and measuring its mass.
🧠 Principle of the Method
The gravimetric determination of platinum involves:
- Dissolving platinum from ash using aqua regia (HCl + HNO₃)
- Precipitating platinum using ammonium chloride
- Converting it into pure platinum sponge
- Weighing the final product for calculation
This method is based on the principle of quantitative precipitation and mass measurement, ensuring precise analytical results.
🧫 Required Chemicals & Reagents
- Formic Acid (AR Grade)
- Hydrochloric Acid (HCl)
- Nitric Acid (HNO₃)
- Ammonium Chloride (NH₄Cl)
- Distilled/DM Water
- Ice
⚗️ Equipment Needed
- Silica Crucible
- Muffle Furnace (up to 700°C)
- Hot Plate & Water Bath
- Beakers (250 ml & 500 ml)
- Whatman No. 41 Filter Paper
- Measuring Cylinder
- Watch Glass
🧾 Step-by-Step Procedure
🔹 Sample Preparation
Weigh the platinum ash sample (~0.3 g Pt content)
Add formic acid and evaporate at 80–90°C
🔹 Digestion with Aqua Regia
Add aqua regia (3:1 HCl:HNO₃)
Heat for 30 minutes to dissolve platinum
Cool and filter to remove impurities
🔹 Removal of Nitric Acid
Concentrate solution
Add HCl repeatedly to expel nitric acid
Confirm removal using starch paper test
🔹 Precipitation of Platinum
Cool solution in ice bath
Add saturated ammonium chloride solution
Yellow precipitate forms (Pt complex)
🔹 Filtration & Ignition
Filter precipitate using Whatman 41 paper
Wash thoroughly with NH₄Cl solution
Ignite at 700°C to obtain platinum sponge
🔹 Final Treatment
Treat with dilute HNO₃
Re-ignite and weigh final platinum sponge
🧮 Calculation
Pt (%) = Weight of Pt Sponge * 100 / Weight of Sample
⚠️ Important Precautions
- Ensure complete removal of nitric acid before precipitation
- Maintain proper temperature during ignition (700°C)
- Use double filtration for accuracy
- Avoid contamination during transfer and weighing
📊 Advantages of Gravimetric Method
✅ High accuracy and reliability
✅ No need for expensive instruments
✅ Suitable for standardization and validation
✅ Ideal for QC laboratories
🧠 Applications in Industry
Precious metal recovery plants
Catalyst manufacturing industries
Pharmaceutical intermediates
Chemical QC laboratories