Lecture 2: Pharmaceutical Measurement; Density, Specific Gravity (9.9.24)

Pipette

Volumetric

Cylindrical graduate

Graduated cylinder

Conical graduate

Graduated cone

Smallest measurement tool

Used to measure desired volume

Reading error

Error in matching meniscus

Volume error

Surface area of the meniscus magnifies reading error

SENSITIVITY REQUIREMENT

≤ 6mg (load that causes smallest change of smallest division of measurement)

± 5%

Max % error

Minimum weight

Minimum quantity that can be measured from device

Calibrate

Regularly to reference/standard

Readability

Smallest division that can be read from instrument (e.g. 0.001 g)

Analytical balances

More accurate weighing

100% × SR (mg) / Acceptable error (%)

Smallest quantity (mg) =

6 mg SR, 5% error

Typical class A prescription balance

Aliquot

What if dose < minimum weight (smallest quantity)?

Aliquot

(Method to create/weigh dose with accuracy, precision

Step 1

Determine multiplication factor of does that is > min. weight

Step 2

Determine weight of inert diluent (when added to drug dose, must be greater than smallest quantity/minimum weight. Multiply by factor in #1. Mix with drug.

Step 3

Weigh aliquot of mixture corresponding to dose

Limitation

Always error in measurement

Max potential error × 100% / quantity desired

Percent error =

SR (sensitivity requirement)

Max potential error =

D (density)

Mass per unit volume of the substance

Density

Mass / volume = g / cc ≈ g / mL =

Sp gr (specific gravity)

Weight of target per weight equal vol of reference substance

Water, Sp gr = 1

Reference substance =

Weight target / weight equal vol water

Sp gr =

Specific gravity < 1

Specific gravity lighter than water (floats on water)

Specific gravity > 1

Heavier than water

Pyncometer

Device to determine specific gravity