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Percentage, Ratio Strength, and Other Expressions of Concentration Lecture (6)

This document discusses various methods of expressing concentration in pharmaceutical preparations, including percentage, ratio strength, parts per million, and parts per billion. Percentage can be expressed as weight-in-volume (w/v), volume-in-volume (v/v), or weight-in-weight (w/w). Ratio strength is another way to express percentage concentration. Examples are provided for calculating amounts of ingredients using these different concentration expressions.

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Ahmed Hassan Mina Hamad
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© © All Rights Reserved
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254 views

Percentage, Ratio Strength, and Other Expressions of Concentration Lecture (6)

This document discusses various methods of expressing concentration in pharmaceutical preparations, including percentage, ratio strength, parts per million, and parts per billion. Percentage can be expressed as weight-in-volume (w/v), volume-in-volume (v/v), or weight-in-weight (w/w). Ratio strength is another way to express percentage concentration. Examples are provided for calculating amounts of ingredients using these different concentration expressions.

Uploaded by

Ahmed Hassan Mina Hamad
Copyright
© © All Rights Reserved
Available Formats
Download as PDF, TXT or read online on Scribd
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University of Sulaimani

College of Pharmacy
1st Stage
Pharmaceutical Orientation

Percentage, Ratio Strength, and


Other Expressions of Concentration
Lecture ( 6 )
Shahen S. Mohammed
BSc pharmacy
MSc pharmaceutics

1
Percentage

• The term percent and its corresponding


sign (%) mean ‘‘by the hundred’’ or ‘‘one part in
every hundred,’’

• percentage means ‘‘rate per hundred’’; so 50 percent (or 50%) and a


percentage of 50 are equivalent expressions.

• A percent may also be expressed as a ratio, represented as a common or


decimal fraction.
• For example, 50% means 50 parts in 100 of the same kind, and may be
expressed as ⁄ or 0.50.

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Percentage Preparations

• The percentage concentrations of active and inactive constituents in


various types of pharmaceutical preparations are defined as follows
by the United States Pharmacopeia:

• Percent weight-in-volume (w/v) expresses the number of grams of a


constituent in 100 mL of solution or liquid preparation and is used
regardless of whether water or another liquid is the solvent or
vehicle. Expressed as: ------- % w/v.

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• Percent volume-in-volume (v/v) expresses the number of milliliters
of a constituent in 100 mL of solution or liquid preparation. Expressed
as: --------% v/v.

• Percent weight-in-weight (w/w) expresses the number of grams of a


constituent in 100 g of solution or preparation.
Expressed as: ------% w/w.

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Percentage Weight-in-Volume
Taking water to represent any solvent or vehicle, we may prepare
weight-in-volume percentage solutions or liquid preparations by the SI metric
system if we use the following rule:

Multiply the required number of milliliters by the percentage strength,


expressed as a decimal, to obtain the number of grams of solute or constituent
in the solution or liquid preparation.

The volume, in milliliters, represents the weight in grams of the solution or


liquid preparation as if it were pure water.

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Examples of Weight-in-Volume Calculations

• How many grams of dextrose are required to prepare 4000 mL of a


5% solution?
4000 mL represents 4000 g of solution
5% = 0.05
4000 g × 0.05 = 200 g

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• How many grams of potassium permanganate should be used in
compounding the following prescription?
R Potassium Permanganate 0.02%
Purified Water ad 250 mL
Sig. as directed.
250 mL represents 250 g of solution
0.02% = 0.0002
250 g × 0.0002 = 0.05 g

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Percentage Volume-in-Volume

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Examples of Volume-in-Volume Calculations

• How many milliliters of liquefied phenol should be used in


compounding the following prescription?
R Liquefied Phenol 2.5%
Calamine Lotion ad 240 mL
Sig. For external use.
Volume (mL) × % (expressed as a decimal) = milliliters of active
ingredient
240 mL × 0.025 = 6 mL

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Percentage Weight-in-Weight

• Percentage weight-in-weight (true percentage or percentage by


weight) indicates the number of parts by weight of active ingredient
contained in the total weight of the solution or mixture considered as
100 parts by weight.

15
Examples of Weight-in-Weight Calculations

• How many grams of phenol should be used to prepare 240 g of a 5%


(w/w) solution in water?

Weight of solution (g) × % (expressed as a decimal) = g of solute

240 g × 0.05 = 12 g

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• What weight of a 5% (w/w) solution can be prepared from 2 g of
active ingredient?

• How many grams of benzocaine should be used in compounding the


following prescription?

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• How many milligrams of hydrocortisone should be used in
compounding the following prescription?
R Hydrocortisone ⁄%
Hydrophilic Ointment ad 10 g
Sig. Apply.

⁄ % = 0.125%
10 g × 0.00125 = 0.0125 g or 12.5 mg

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Ratio Strength

The concentrations of
weak solutions are
frequently expressed in
terms of ratio strength.
Because all percentages
are a ratio of parts per Concentration =
hundred, ratio strength
is merely another way of
expressing the
percentage strength of
solutions or liquid
preparations.

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Example Calculations Using Ratio Strength

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• To change ratio strength to percentage strength, it is sometimes
convenient to ‘‘convert’’ the last two zeros in a ratio strength to a
percent sign (%) and change the remaining ratio first to a common
fraction and then to a decimal fraction in expressing percent:
1:100 = ⁄ % = 1%
1:200 = ⁄ % = 0.5%
3:500 = ⁄ % = 0.6%
1:2500 = ⁄ % = 0.04%
1:10,000 = ⁄ % = 0.01%

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Simple Conversions of Concentration to ‘‘mg/mL’’

1. To convert product percentage strengths to mg/mL, multiply the


percentage strength, expressed as a whole number, by 10.
• Example:
Convert 4% (w/v) to mg/mL.
4 × 10 = 40mg/ml

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2. To convert product ratio strengths to mg/mL, divide the ratio
strength by 1000.
• Example:
Convert 1:10,000 (w/v) to mg/mL.
10,000 ÷ 1000 = 1mg /10ml

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Milligrams Percent

The term milligrams percent (mg%) expresses


the number of milligrams of substance in
100 mL of liquid.

It is used frequently to denote the


concentration of a drug or natural substance
in a biologic fluid, as in the blood.

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Parts per Million (PPM) and Parts per Billion (PPB)

• The strengths of very dilute solutions are commonly expressed in


terms of parts per million (ppm) or parts per billion (ppb), i.e., the
number of parts of the agent per 1 million or 1 billion parts of the
whole.

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Example Calculations of Parts per Million and Parts per
Billion
• Express 5 ppm of iron in water in percentage strength and ratio
strength.

5 ppm = 5 parts in 1,000,000 parts = 1:200,000, ratio strength, and


=0.0005%, percentage strength

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