Specific gravity

What is this for?

Specific gravity is the ratio of the weight of a substance to the weight of an equal volume of water. Because 1 mL of water weighs 1 g, specific gravity is numerically equal to density in g/mL. Compounders use it constantly to convert between the volume a formula specifies and the weight a balance measures.

How to use it

1. Fill in any two of the three fields and leave the third blank.
2. Weight is in grams and volume in millilitres; specific gravity is dimensionless.
3. If a formula specifies a liquid by weight but you can only measure volume, solve for volume using the published specific gravity.

Worked example

A formula calls for 60 mL of glycerin (specific gravity 1.25). What weight should you place on the balance?

Answer: 1.25 × 60 = 75 g. Measuring 60 g instead would under-deliver glycerin by 20%.

Clinical pearls & pitfalls

• Viscous liquids such as glycerin and mineral oil are far more accurately measured by weight than by volume, because a substantial film clings to the graduate. Convert to weight whenever practical.
• Specific gravity is temperature-dependent; published values are usually at 25 °C. The variation is small for compounding purposes but matters in analytical work.
• Specific gravity is a ratio and therefore dimensionless. Density carries units (g/mL). They are numerically identical only because water is the reference and weighs 1 g/mL.

Assumptions & limitations

• Uses the convention that 1 mL of water weighs exactly 1 g. Water is actually at maximum density at 4 °C (0.99997 g/mL), so this introduces a negligible error at room temperature.
• Requires a published or measured specific gravity for the specific liquid. Do not assume 1.0 for anything other than dilute aqueous solutions.

References

• United States Pharmacopeia. General Chapter <841> Specific Gravity.
• Allen LV. The Art, Science, and Technology of Pharmaceutical Compounding. American Pharmacists Association.

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