Percentage, Ratio Strength, and Concentration Expressions PDF

Percentage, ratio strength, and other expressions of concentration  Objectives Define the expressions percent weight-in-volume, percent volume-in -volume, and percent weight-in-weight. Define the expression ratio strength. Convert percent strength to ratio strength and r...

Percentage, ratio strength, and other expressions of concentration  Objectives Define the expressions percent weight-in-volume, percent volume-in -volume, and percent weight-in-weight. Define the expression ratio strength. Convert percent strength to ratio strength and ratio strength to percent strength. Calculate the percentage strength and ratio strength of a pharmaceutical preparation. Apply percent strength and ratio strength to calculate the quantity of an ingredient present in a pharmaceutical preparation. Apply percent strength and ratio strength to calculate the quantity of an ingredient to use in compounding a pharmaceutical preparation.  Concentration Concentration: is the expression of the ratio of the amount of an ingredient to the amount of product. Concentration can be measured in many ways: – In the case of a solid ingredient in a liquid vehicle, the ratio is expressed as weight in volume or w/v. – If liquid ingredient is formulated into a solid vehicle, the ratio is expressed as v/w. – If both drug and vehicle are liquids, it is expressed as v/v. – If both drug and vehicle are solids, it is expressed as w/w.  Percentage Percent describes the number of parts of active drug relative to 100 parts of the total. In pharmacy, Simply stating % is incomplete ; you must attach the appropriate description from the following list: – % w/w: g of active drug per 100 g of product – % w/v: g of active drug per 100 mL of product – % w/v: mL of active drug per 100 mL of product Examples: – 10% w/v = 10 g of drug in every 100 mL of total – 10% w/w = 10 g of drug in every 100 g of total  Special Considerations in Percentage Calculations In general, the nature of the ingredients in a pharmaceutical preparation determines the basis of the calculation. – a powdered substance dissolved or suspended in a liquid vehicle would generally be calculated on a weight-in-volume basis; – a powdered substance mixed with a solid or semisolid, such as an ointment base, would generally be calculated on a weight in weight basis. – a liquid component in a liquid preparation would be calculated on a volume-in-volume basis  Special Considerations in Percentage Calculations Many formulations are presented on the basis of weight, even though some of the ingredients are liquids. It may be necessary to convert weight to liquid or, in some instances, vice versa.  Ratio strength Ratio strength is another way of expressing concentration, in terms of parts of active drug related to any number of parts of the whole. Ratio strength is usually expressed in terms of 1 part of active drug relative to the total number of parts of the product, as opposed to percent, which is any number of parts of active drug relative to 100 parts of product.  Percentage Weight-in-Volume Example: How many grams of dextrose are required to prepare 4000 mL of a 5% solution?  Percent volume-in-volume (v/v) Example: How many milliliters of liquefied phenol should be used in compounding the following prescription?  Percent weight-in-weight (w/w) Example: How many grams of phenol should be used to prepare 240 g of a 5% (w/w) solution in water? 240 g X 0.05 = 12 g, answer.  Percentage and Ratio Strength Interconversion Example: Express 0.02% as ratio strength. Example: Express 1:4000 as a percentage strength.  A certain injectable contains 2 mg of a drug per milliliter of solution. What is the ratio strength (w/v) of the solution?  How many milligrams of hexachlorophene should be used in compounding the following prescription?  Other Concentration Expressions Milligram Per Milliliter (mg/mL) Parts Per Million (PPM) Example: Convert 4% w/v to mg/mL. Example: Convert 1:10,000 (w/v) to mg/mL.  Parts Per Million (PPM) The strengths of very dilute solutions are commonly expressed in terms of parts per million (ppm) ppm means the number of parts of the substance per 1 million parts of the whole (e.g. 4 ppm = 4:1,000,000) Example: Express 5 ppm of iron in water in (1) percentage strength and (2) ratio strength