Pharmacy Concentration Calculations

Concentration Expressions

• Concentration is the ratio of the amount of an ingredient to the amount of product.
• Concentration is measured in various ways.
• In a solid ingredient in a liquid vehicle, the ratio is expressed as weight-in-volume (w/v).
• If the liquid ingredient is in a solid vehicle, the ratio is expressed as volume-in-weight (v/w).
• When both the drug and vehicle are liquids, the ratio is expressed as volume-in-volume (v/v).
• If both the drug and vehicle are solids, the ratio is expressed as weight-in-weight (w/w).

Percentage Strength

• Percentage describes the number of parts of the active drug relative to 100 parts of the total.
• % w/w: grams of active drug per 100 grams of product.
• % w/v: grams of active drug per 100 mL of product.
• % v/v: milliliters of active drug per 100 mL of product.
• Example: 10% w/v = 10 grams of drug in every 100 mL of total.

Special Considerations in Percentage Calculations

• The nature of the ingredients in the pharmaceutical preparation dictates the basis of the calculation.
• A powdered substance dissolved or suspended in a liquid vehicle is calculated on a weight-in-volume (w/v) basis.
• A powdered substance mixed with a solid or semisolid (e.g., ointment base) is calculated on a weight-in-weight (w/w) basis.
• A liquid component in a liquid preparation is calculated on a volume-in-volume (v/v) basis.
• Weight may need conversion to liquid or vice versa (e.g., grams to milliliters).

Ratio Strength

• Ratio strength is another method of expressing drug concentration.
• It's expressed in terms of parts of active drug related to any number of parts of the entire product.
• Ratio strength is expressed as a ratio, unlike percentage, which is calculated relative to 100 parts.
• Example: A 1:1000 ratio strength indicates one part of active drug to 1000 parts of the product.

Interconversion of Percentage and Ratio Strength

• Percentage concentration to ratio strength: Convert to the correct units (e.g., grams to milligrams), set the denominator of the ratio proportion equation to 100 and adjust the numerator based on the original concentration
• Ratio strength to percentage strength: Set the numerator to 1, and adjust denominator to 100. Calculate and adjust accordingly.

Other Concentration Expressions

• Milligram per Milliliter (mg/mL): Expresses concentration as milligrams of solute per milliliter of solution.
• Parts per Million (ppm): Represents the parts of a substance per one million parts of the whole.
• 4 ppm = 4 parts per 1,000,000
• Conversion examples provided for calculating mg/mL from Percentage and ratio strength.

Examples of Calculations

• Include numerous examples to demonstrate calculations for different types of concentration expressions (v/v, w/v, w/w, mg/mL ppm, ratios).
• Show steps for problem-solving using percentages, ratios, and conversions.

This quiz focuses on understanding concentration expressions and percentage strength calculations in pharmacy. It covers various ways to express ratios of ingredients and special considerations in percentage calculations. Test your knowledge on weight-in-volume, volume-in-weight, and other measurement techniques.