Relationship Between Relative Density and Weight/mL

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

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