Ratio Strength & Other Expressions of Concentration

Dr. Amani Elsayed

Ratio Strength

• The concentration of weak solutions or liquid preparations is frequently expressed in terms of ratio strength.

• Ratio strength is another way of expressing the percentage strength of solutions or liquid preparations (and, less frequently, of mixtures of solids).

• For example, 5% means 5 parts per 100 or 5:100.• Although 5 parts per 100 designates a ratio strength, it

is customary to translate this designation into a ratio, the first figure of which is 1; thus, 5:100 = 1: 20.

Ratio Strength

• 1/1000, used to designate a concentration, is to be interpreted as follows:

• For solids in liquids = 1 g of solute or constituent in 1000 mL of solution or liquid preparation.

• For liquids in liquids = 1 mL of constituent in 1000 mL of solution or liquid preparation.

• For solids in solids = 1 g of constituent in 1000 g of mixture.

• The ratio and percentage strengths of any solution or mixture of solids are proportional, and either is easily converted to the other by the use of proportion.

Ratio Strength

• Ratio Strength Given Percentage Strength.• Express 0.02% as a ratio strength

0.02 100 1 X X = 100 *1 = 5000. 0. 02Ratio strength 1: 5000

Ratio Strength

• Percentage Strength Given Ratio Strength.• Express 1: 4000 as a percentage strength. Answer 1 4000 X 100 X = 100/4000 = 0.025%

Ratio Strength

• NOTE:• To change ratio strength to percent strength, it is sometimes

convenient to "convert" the last two zeros in a ratio strength to a percent sign (%), change the remaining ratio to a common fraction, and then to a decimal fraction in expressing percent.

• Examples:• 1:100 =1/1% = 1%• 1:200 = ½% = 0.5%• 3:500 = 3/5% = 0.6%• 1:2500 = l/25% = 0.04%• 1:10,000 = 1/100% = 0.01%

Ratio Strength1) Ratio Strength of Solution or Liquid

Preparation, Given Weight of Solute in a Specified Volume.

1) A certain injectable contains 2 mg of a drug per milliliter of solution. What is the ratio strength (w/v) of the solution?

2) What is the ratio strength (w/v) of a solution made by dissolving five tablets, each containing 2.25 g of sodium chloride, in enough water to make 1800 mL?

Answers

1) 2 mg = 0.002 g 0.002 g 1 ml 1 g X ml X = 1 *1 = 500. Ratio strength 1: 500 0.0022) 2.25 g * 5 = 11.25 g of sodium chloride 11.25 g 1800 ml 1 g X mlX = 1 *1800 = 160 ml. 11.25Ratio strength 1: 160

Problems Involving Ratio Strength

3) How many grams of potassium permanganate should be used in preparing 500 mL of a 1:2500 solution?

4) How many milligrams of gentian violet should be used in preparing the following solution?

℞ Gentian Violet Solution 500 mL 1:10,000 Sig. Instill as directed.

Answers

3) 1 g 2500 ml X g 500 mlX = 1 *500 = 0.2 g 25004) 1 g 10000ml X g 500 mlX = 1 *500 = 0.05 g = 50 mg 10000

Problems Involving Ratio Strength

5) How many milligrams of hexachlorophene should be used in compounding the following prescription?

℞ Hexachlorophene 1:400 Hydrophilic Ointment ad 10 g Sig. Apply.

Simple Conversions of Concentration TO mg/ml

• 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 X 10 =40 mg/mL, answer. Proof or alternate method: 4% (w/v) = 4 g/100 mL = 4000 mg/100 mL = 40 mg/mL

Simple Conversions of Concentration TO mg/ml

• To convert product ratio strengths to mg/mL, divide the ratio strength by 1000.

Example: Convert 1:10000 (w/v) to mg/mL. 10000 ÷ 1000 =1 mg/10 mL, answer. Proof or alternate method: 1:10000 (w/v) = 1 g/10000 mL = 1000 mg/10000 mL = 1 mg/10 mL

Simple Conversions of Concentration TO mg/ml

• Convert product strengths expressed as grams per mililiter (g/mL) to mg/mL

• Convert the numerator to milligrams• Divide by the number of milliliters in the denominator. Example: Convert a product concentration of 1 g per 250 mL to mg/mL. 1 g=1000 mg 1000 ÷ 250= 4 mg/mL, answer. Proof or alternate method: 1 g/250 mL = 1000 mg/250 mL = 4 mg/mL

MILLIGRAMS PERCENT (mg%)

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

• Is used frequently to denote the concentration of a drug or natural substance in a biologic fluid, as in the blood.

• The concentration of nonprotein nitrogen in the blood is 30 mg% means that each 100 mL of blood contains 30

mg of nonprotein nitrogen.• As noted in the following section, "Expressing Clinical

Laboratory Test Values," quantities of substances present in biologic fluids also commonly are stated in terms of milligrams per deciliter (mg/dL) of fluid.

PARTS PER MILLION (PPM) ANDPARTS 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.

• Fluoridated drinking water, used to reduce dental caries, often contains 1 part of fluoride per million parts of drinking water

(1:1,000,000).

PARTS PER MILLION (PPM) ANDPARTS PER BILLION (PPB)

• May be used to describe the quantities of trace impurities in chemical samples and trace elements in biologic samples.

• Depending on the physical forms of the trace substituent and the final product, a concentration expressed in ppm or ppb could, in theory, be Calculated on a weight-in-volume, volume-in-volume, or weight-in-weight basis.

• For all practical purposes, the unit-terms of the solute and solution are generally considered like units (i.e., the same type of "parts").

PARTS PER MILLION (PPM) ANDPARTS PER BILLION (PPB)

• Equivalent Values of Percent Strength, Ratio Strength, and Parts per Million or Parts per Billion.

Example: Express 5 ppm of iron in water in• percent strength and ratio strength.• 5 ppm = 5 parts in 1,000,000 parts = 1:200,000, ratio strength,• = 0.0005%, percent strength, answers.

PARTS PER MILLION (PPM) ANDPARTS PER BILLION (PPB)

• Using Parts per Million or Parts per Billion in Calculations.• Example: The concentration of a drug additive in an

animal feed is 12.5 ppm. How many milligrams of the drug should be used in preparing 5.2 kg of feed?

12.5 g (drug) 1,000,000 g (feed) X 5200 gX = 12.5 *5200 = 0.065 g = 65 mg 1000000

Bonus Question

1) If a patient is determined to have a serum cholesterol level of 200 mg/dL

a) What is the equivalent expressed in terms of milligram percent, and

b) how many milligrams of cholesterol would be present in a 10-ml sample of the patient’s serum?