(9) Pharmaceutical Calculations

Viscosity

Definition:

 resistance to flow

Viscosity

Examples:

• Toothpaste → high viscosity

• Mouthwash → low viscosity

Types of fluids:

Newtonian

• Viscosity = constant

• Does NOT change with shear

Non-Newtonian

• Viscosity changes with force/shear

What type of viscosity we need?

• Need optimal viscosity:

  • Too high → drug release ↓

  • Too low → unstable / melts

Example of not having optimal viscosity

• Suppositories melting in package = ❌ bad

Arrhenius equation relates:

• Viscosity ↔ Temperature

• Temperature ↑ → Viscosity ↓

• Temperature ↓ → Viscosity ↑

 VISCOSITY vs TEMPERATURE (ARRHENIUS)

Example:

• Heat suppository → melts → viscosity ↓

• Refrigerate → becomes thicker

VISCOSITY vs TEMPERATURE (ARRHENIUS)

APPLICATION

• Used for:

  • Shelf life determination

  • Stability testing

 DRUG RELEASE FROM OINTMENT

equation

Variables:

• Q = drug released

• C₀ = initial concentration

• D = diffusion coefficient

• t = time

 

💡 KEY RELATIONSHIPS ✅⭐

• C₀ ↑ → Drug release ↑

• Time ↑ → Drug release ↑

• D ↑ → Drug release ↑

 SUPPOSITORIES DRUG RELEASE

• Depends on:

• Viscosity

• Type of base

SUPPOSITORIES DRUG RELEASE

Base types:

Base	Mechanism
Cocoa butter	melts
PEG	dissolves

SUPPOSITORIES DRUG RELEASE

 KEY IDEA

• High viscosity → slow release

• Low viscosity → unstable

DRUG RELEASE MEASUREMENT

• Need to measure:

How much drug released

DRUG RELEASE MEASUREMENT

Methods:

• UV spectroscopy (common)

• HPLC (more sensitive)

 

BEER-LAMBERT LAW

calculations

Variables:

• A = absorbance

• c = concentration

• k = constant

• l = path length

BEER-LAMBERT LAW

 KEY RELATIONSHIP

• Concentration ↑ → Absorbance ↑ (linear)

 

💡 EXAM LOGIC

• If A doubles → C doubles

CHARACTERIZATION TECHNIQUES - overview

• Qualitative:

• Quantitative:

CHARACTERIZATION TECHNIQUES

 Qualitative:

• IR

• Raman

• X-ray

• DSC

• Microscopy

👉 Used for:

• Structure

• Impurities

CHARACTERIZATION TECHNIQUES

 Quantitative:

• UV spectroscopy

• HPLC

👉 Used for:

• Drug concentration

SUPPOSITORY CALCULATIONS

  WHEN TO IGNORE CALCULATIONS

• Drug < 100 mg

• Suppository ≈ 2 g

👉 Volume negligible

SUPPOSITORY CALCULATIONS

WHEN TO USE FACTORS

• Drug > 100 mg

• OR suppository < 2 g

DOSAGE REPLACEMENT FACTOR (f)

MATH:

ON EQUATION SHEET

DOSAGE REPLACEMENT FACTOR (f)

EXAMPLE STEPS

1. Convert drug to %

2. Plug into equation

3. Solve for G

DENSITY FACTOR

MATH:

ON EUQATION SHEET

PULMONARY (AEROSOL CALCULATIONS)

MATH

RAOULT’S LAW

• Total pressure = sum of partial pressures