Pharmacy-I Lab: Pharmaceutical Calculations 2nd Year

Objectives of Practical Laboratory

• Define solutions and their preparation methods.
• Understand concentration and expression methods.
• Learn dilution techniques for prepared solutions.

Definitions

• Solute: Substance that dissolves in a solvent (usually solid).
• Solvent: Substance that dissolves the solute, typically a liquid like water or alcohol; must be greater than 50% of the solution.
• Solution: Homogeneous mixture of two or more substances.

Solution Preparation Steps

• Weigh the appropriate solute and add it to a beaker.
• Add sufficient solvent and stir until the solute dissolves completely.
• Transfer the solution to a volumetric flask, rinsing the beaker with additional solvent.
• Add more solvent until the final calibrated volume is achieved.
• Stopper the flask, shake well, and label appropriately.

Concentration Definitions

• Concentration refers to the amount (mass/volume) of solute within a specified amount (mass/volume) of solution.

Methods to Express Concentration

Qualitative Methods

• Concentrated solutions are darker, stronger in taste/odor, and have more solute and less solvent.
• Diluted solutions are lighter in color, milder in taste/odor, and contain less solute and more solvent.

Quantitative Methods

• Concentrations commonly expressed as molarity, normality, and percentage.

Molarity (M)

• Defined as moles of solute per liter of solution (mol/L).
• Requires the molecular weight of the solute for calculations.
• Example: To prepare 50 ml of 0.5 M NaCl (m.w. NaCl = 58.5 g/mol), weigh 1.46 g of NaCl and dissolve in water to 50 ml.

Normality (N)

• Indicates the number of gram equivalent weights of solute per liter of solution.
• Equivalent weight is the molecular weight divided by the number of replaceable hydrogen or hydroxyl ions.
• Example: To prepare 50 ml of 0.5 N Na₂CO₃ (m.w. = 106 g/mol), weigh 1.325 g of Na₂CO₃ and dissolve in water to 50 ml.

Percentage Concentration Expressions

• Percent weight/volume (w/v): grams of solute in 100 ml of solution.
• Percent volume/volume (v/v): milliliters of solute in 100 ml of solution.
• Percent weight/weight (w/w): grams of solute in 100 g of solution/mixture.

Percent Example

• 0.9% NaCl solution contains 0.9 g of sodium chloride in 100 ml of solution.
• 5% v/v methanol solution involves 5 ml of methanol in 100 ml.

Dilution and Concentration Law

• To dilute a concentrated solution: pipette an appropriate amount of stock solution into a volumetric flask, adding solvent up to the mark.
• Dilution Law: C1V1 = C2V2
  • C1: initial concentration (stock solution).
  • C2: final concentration (desired solution).
  • V1: initial volume from stock.
  • V2: final volume desired.

Example of Dilution

• To prepare 250 ml of 4% w/v glucose from 20% w/v glucose, measure 5 ml of 20% solution and dilute with distilled water to reach 250 ml.

Experimental Work

• Prepare the following solutions using a volumetric flask:
  • 100 ml of 0.5 M NaCl
  • 100 ml of 2 N NaCl
  • 100 ml of 0.1 N Na₂CO₃
  • 100 ml of 0.1 M Na₂CO₃
  • 100 ml of 2% w/v NaCl
  • 100 ml of 5% v/v Alcohol