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WEIGHTS AND MEASURESLecture – 11

02-04-2012

DR. SHAHNAZ USMAN

Associate Professor

Dept. of PharmaceuticsRAKMHSU

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WEIGHT Weight is a measure or the gravitational force

acting on a bodyWeight is directly proportional to the body's

mass. The body’s mass being a constant based on

inertia, whereas weight varies slightly with latitude, altitude, temperature, and pressure.

The effect of these factors usually is not considered unless very precise weighing and large quantities are involved.

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MEASUREMeasure is the determination of the volume or

extent of a body. Temperature and pressure have a pronounced

effect, especially on gases or liquids.

These factors, therefore, are considered when making precise measurements.

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Methods of Metrology

1. Metric System

2. English Systems

a. Avoirdupois System

b. Apothecary System

Both the Metric and English systems of weights and measures are in use in the US.

The USP of 1890 adopted the metric system of weights and measures to the exclusion of all others except for equivalent dosage statements.

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The British Pharmacopoeia adopted it in 1914.

In 1944 the Council on Pharmacy and Chemistry of the American Medical Association adopted the metric system exclusively.

Advantage of Metric SystemThe advantages of the metric or decimal system,

its simplicity, brevity, and adaptability to everyday needs.

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The English SystemsIn the US, both the avoirdupois and apothecary

systems of weight measurement sometimes are used in handling medicines.

It must be emphasized that pharmacists may buy their drugs by avoirdupois weight and sale in apothecary.

These two systems differ:1pound avoirdupois = 7000gr and is abbreviated lb1 pound apothecary = 5760gr and is abbreviated lb

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1ounce avoirdupois = 437.5gr and is abbreviated oz1 ounce apothecary = 480 gr and is abbreviated

Important differenceThe grain avoirdupois is exactly the same as the

grain apothecary.

The apothecary pound is therefore 1240 gr lighter than the avoirdupois pound

The apothecary ounce is therefore 42.5 gr heavier than the avoirdupois ounce.

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TABLE 11-2 METRIC WEIGHT

= 10-6g

= 10-3g

= 10-2g

= 10-1g

= 1g

= 10g

= 102g

= 103g

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10

= 10-6L

= 10-3L

= 10-2L

= 10-1L

= 1L

= 10L

= 102L

= 103L

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THE RELATIONSHIPS OF WEIGHTS ANDMEASURES

When the systems of weights and measures in use in the US are examined, the lack of dose relation between the different units is found at once.

Practical EquivalentsSo it is necessary to keep the Tables of weights

and measures and a Table of practical equivalents, in a conspicuous and convenient place in the Prescription Department

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Practical Equivalents

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Practical Equivalents

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PHARMACEUTICAL CALCULATIONSThe problems generally consist of determining the

quantity or quantities of materials required to compound prescriptions properly and make products used to aid the compounding of prescriptions.

The materials used to compound prescription orders may be pure or mixtures of substances in varying strengths.

The strengths of mixtures may be denoted in different ways.

Conversions may be necessary between systems of varying strengths or between different measuring systems.

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ADDITIONQ. Add 3 kg, 33 g, and 433 mg.Convert to a common unit. The gram is convenient

because it is the unit of weight.

3 kg x 1000 = 3000g

33g = 33g

433mg x 1 g/ 1000mg = 0.433g

Answer = 3033.433 g

Problems

1. Add 25mg, 25g, 210mg, 2kg, 1.75g, 215mg, 454g, and 30 mg.

2. The following quantities of a drug were removed from a container: 31g, 225g, 855.6g, and 45,4 g. What is the total weight removed from the container?

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SUBTRACTIONQ. Subtract 285 ml from 1 L. Convert to a common

unit. 1000 ml

- 285 ml

715 ml

Answer: 715 mlProblems

1. How much is left in a 5 L container after the removal of 895 mL?

2. A pharmacist buys 5 g of a potent drug and at different times dispenses 0.2 g, 0.85 g, 90 mg, and 150 mg on prescription orders. How much of the drug remains?

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MULTIPLICATIONQ. What will be the total weight of the ingredients in a prescription order for 25 units, each unit containing 0.4 g of Solid F, 0.01 g of Solid G, and 5 mg of Solid H? First, convert to a common unit such as grams.

0.4g + 0.01g + 0.005g = 0.415g total weight of one unit0.415g/unit x 25 units = 10.375g total weight of all units

Answer = 10.375g

Q. Multiply 22.4 ml by 2.65

22.4 ml X 2.65

59.36mlAnswer = 59.36 ml

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Problems

1. Multiply 48.5 ml by 3.24.

2. A certain preparation is to contain 0.0325 g of a chemical in each ml of solution. How much must be weighed out to make 5 L of the Solution?

3. How much cod liver oil is necessary to make 2500 capsules, each containing 0.33 ml?

4. How many mg are used to make 1500 units, each of which contains 250 µg of a drug?

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DIVISIONQ. Divide 3 by 25

3L = 0.120L = 120 ml25

Answer: 120ml

ProblemsI. How many 65 mg capsules can be made from 50g of a drug?3. The dose of a drug is 0.1 mg. How many doses are contained in 15mg of the drug?5. How many 325 mg Capsules of a drug can be filled from a 454 g amount?

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APOTHECARY CONVERSION There are only three conversions necessary to memorize in

order to convert between the apothecary, avoirdupois, and metric systems.

These are• 1 gr (avoir) = 1gr (apoth)• 1 gr = 64.8 mg• 1 fluid ounce = 29.6 ml

Q. Convert 1ounce (apoth) to weighable quantities in the avoir system: where

1 ounce (apoth) = 480 gr1 ounce (avoir) = 437.5gr

480 – 437.5 = 42.5grAnswer: 1 ounce (apoth) = 1 oz, 42.5gr (avoir)

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Q. How many mg equal 1 gr?

1g = 15.4 gr

1g / 15.4 gr = 0.0648 g / gr = 64.8 mg / gr or 64.8 mg = 1 gr

Answer: 1gr = 64.8mg

Q. How many grams are in 1 (ounce)?1g = 15.4 gr

1 =480 gr

480 gr /15.4 = 31.1 g

Answer: 1 = 31.1 g

Q. How many grams are in 1 oz(avoir)?

Remember: 1 gr(apoth) = 1 gr(avoir).

1g/ 15.4 gr x 437.5 gr = 28.4g

Answer: 1oz = 28.4 g

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Q. Convert 1 pt,4 f into ml

First, convert into f ; 1 pint = 16 f + 4 f = 20 f

whereas 1f = 29.6ml

20 f x 29.6ml = 592ml

Answer : 1 pt,4 f = 592 ml

Q. What is the weight of 1200 g in the apothecary system?

1200g x 15.4 gr = 18480gr

Answer: 1200 g = 18480 gr

Q. How many minims are in 1 ml?

1 fluid ounce = 480 minims

1fluid ounce = 29.6 ml

480mx / 29.6 ml = 16.2 mx

Answer: 1 ml = 16.2 mx

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Problems (page : 115 Remington)I. Convert:

a. 6.50 grains into milligrams.

b. 3/10 grain into milligrams.

c. 3 ½ apoth ounces into grams.

d. 2 into mg.

e. 3 ½ avoir ounces into grams.

f. 1 Ib avoir into grams.

2. Convert:

a. 550 g into weighable quantities in the avoir system.

b. 450 mg into grains.

c. 550 g into weighable quantities in the apoth system.

d. 100 µg into grains.

e. 1 kg into Ib (avoir).

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3. Convert the following doses into metric weights:

a. 1/100 gr.

b. 1/320 gr.

c. 1/6 gr.

d. 5 gr.

e. 20 gr.

4. Convert:

a. 200 minims into ml

b. 3 f into ml

c. 8 f into mI

d. 1 pt into ml

e. 5 minim into mI

f. 0.1 mg into gr.

g. 5 mg into gr.