1. Relative Density (RD) or Specific Gravity (SG) :
The ratio of the density of a substance to the density of water at a specified temperature (usually 25°C or 15°C).
Since the density of water at 25°C = 0.997 g/mL (approximately 1 g/mL), relative density is dimensionless.
Formula:
RD = Density of Liquid / Density of Water at the Same Temperature.
2. Weight per mL (Wt/ml):
The weight (in grams) of 1 milliliter (mL) of a liquid at a specified temperature.Expressed in g/mL.
Formula:
Wt/ml = Weight of Liquid Sample/ Volume in mL
Relationship Between RD and Weight per mL:
Since the relative density is based on water’s density, the relationship can be written as:
Wt/ml = RD × Density of Water at that Temperature
For example, at 25°C:
Wt/ml = Relative Density × 0.997 g/mL
Key Takeaways:
- If Relative Density = 1, then Wt/ml ≈ 1 g/mL (like water).
- If Relative Density > 1, the liquid is heavier than water.
- If Relative Density < 1, the liquid is lighter than water.
Significance in Pharmaceuticals:
Ensures correct formulation of syrups, suspensions, and liquid injectables.
Helps in dose calculation for liquid medications.
Important in quality control for consistency and compliance with pharmacopoeial standards.
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Resource Person: Susmita