Pharmaceutics Laboratory
Three connected activities in one practical: determine density and specific gravity with a pycnometer, relative viscosity with an Ostwald viscometer, and surface tension by capillary rise — each against water at a controlled temperature.
Density ρ = m/V (g/mL). Specific gravity is the density of a liquid divided by the density of water at a stated temperature — 25/25 °C here (USP <841>) — so it has no unit. A pycnometer holds exactly the same volume each time it is filled and stoppered, so SG = (m₃ − m₁)/(m₂ − m₁) without knowing that volume, and ρ = SG × ρ(water, 25 °C).
Viscosity is resistance to flow. In an Ostwald U-tube viscometer the time t for a fixed volume to flow between two marks is proportional to the kinematic viscosity η/ρ, so against water in the same viscometer at the same temperature, η = η(water) × ρt / ρ(water)t(water). The method assumes a Newtonian liquid in laminar flow and a flow time long enough that the kinetic-energy correction is negligible.
Surface tension draws a wetting liquid up a narrow glass tube until the weight of the column balances it: γ = ρghr/(2 cos θ). In the same clean capillary (same r, θ ≈ 0), γ = γ(water) × ρh / ρ(water)h(water).
All three properties change with temperature — density and surface tension fall slowly, viscosity falls steeply (about 2 % per °C for water) — so every liquid is measured at 25.0 °C in a thermostatic bath, after it has had time to reach that temperature.
The three activities are connected: the densities measured in A are used to calculate the viscosities in B and the surface tensions in C, so an error in A carries through.
Simulated readings, generated from published property data (CRC Handbook, IAPWS, Jasper 1972, Volk & Kähler 2018, Cheng 2008). Apparatus constants are teaching values.
Purified water · Ethanol, absolute (≥ 99.5 %) · Glycerol–water 50 % w/w
Curriculum
Listed in the UoK Pharm-D practical syllabus (PHC-406 Physical Chemistry – I (Practical), 2nd year, 2nd semester: specific gravity by pycnometer, viscosity by Ostwald viscometer, surface tension; PHT-502 Physical Pharmacy (Lab): “surface tensions, viscosity”). UoK names the drop-count method for surface tension, not capillary rise, and HEC lists only viscosity. Not faculty-reviewed.
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Simulated practice: results come from teaching models and illustrative data described on this page. Nothing you enter is sent anywhere unless you are signed in, in which case your completion is added to your progress.
