Density and Pycnometry Flashcards

Flashcards for reviewing density, relative density and pycnometry.

Density (ρ)

ρ = m/V (kg · m−3; kg · dm−3; g · cm−3)

Relative Density (d 20 20)

Ratio of the mass of a given volume of a substance to the mass of the same volume of water at 20 °C. Dimensionless.

ρ20

d 20 20 × 998.203 (kg · m−3)

Significance of Density in Pharmacy

Determination of solution concentration, purity criterion (waxes), qualitative data for tinctures and extracts; according to ČL: determination of density for liquids and solids.

Density (ρ) Dependence on Temperature

Dependent on temperature (for most substances: T↑ – V↑ – ρ↓). Use tables and maintain same conditions.

Methods for Density Determination

Pycnometry, hustometer (Archimedes' principle), submersible body (direct measurement of buoyant force using scales) = hydrostatic scales.

Pycnometry - Liquids

Fast measurement, small sample consumption; limited accuracy due to hard to maintain measurement temperature; pycnometer always weighed with the stopper!

Pycnometry Formula (Volume Known)

Volume known (V), ρ = (𝑚2 − 𝑚1) / V, where m1 is the mass of the pycnometer, and m2 is the mass of the pycnometer filled with the liquid.

Pycnometry Formula (Volume Unknown)

Volume unknown. ρ = ((𝑚2 − 𝑚1) / (𝑚3 − 𝑚1)) * ρs, where ρs is the density of the reference liquid (distilled water, 20 °C).

ρ20 Calculation

ρ20 = 998.203 * d 20 20 = 998.203 * m / m(H2O)

Pycnometry - Ethanol Solutions

Determination of ethanol solution concentration; ČL: Table of density dependence on ethanol content (alcohol table) – for 20 °C; allows to read volume and mass % of alcohol based on the determined density of alcohol; online: “lihová tabulka lékopis”.

Pycnometry - Solid Substances

Solid substance never fills the entire volume of the pycnometer; use a liquid with known density for determination: wets the solid well (low surface tension), has ↓ρ than PL and does not dissolve PL (does not react with PL); liquids with ↓σ (ether) are used → ether evaporates quickly! – measurement errors.

Pycnometry Formula - Solid Substances

ρ = (𝑚2 − 𝑚1) / ((𝑚4 − 𝑚1) − (𝑚3 − 𝑚2)) * ρs, where ρs is the density of the reference liquid (ether).

Pycnometry Formula (Solid Substances with Air Density Correction)

ρ = (𝑚2 − 𝑚1) / (𝑚4 − 𝑚3 + 𝑚2 − 𝑚1) * ρs - 0.0012 + 0.0012, where 0.0012 is the density of air (g · cm−3).