Lecture 2 METRIC SYSTEM

Transcription

1 University of Kerbala College of Pharmacy Department of Pharmaceutics Principles of Pharmacy Practice Lecture 2 METRIC SYSTEM Assistant lecturer Zahraa Alaa Hasan MSc. Drug Delivery

2 Objectives To demonstrate an understanding of the major systems of measurement used in pharmacy. To convert measures between the International System of Units and other systems of measurement. To apply the International System of Units correctly in pharmaceutical calculations. 2

3 Overview The knowledge and application of pharmaceutical and clinical calculations are essential for the practice of pharmacy and other related health professions. The International System of Units (abbreviated SI) is the modern form of the metric system. In addition to the International System of Units, the pharmacy student should be aware of other systems of measurement: the avoirdupois system, the apothecary system and the household system. 3

4 Avoirdupois system Is a very old system which is widely used in the United States in measuring body weight and in selling goods, ex: the ounce or pound. It is no longer used by pharmacists. Therefore, practice problems will not be provided. 4

5 Apothecary system It has units for weight and volume only. The basic unit for weight is grain (gr) and that of volume is minim (m). The amount is explained in roman numerals. Ex: gr ss. However, Arabic numbers could also be used. It is an old system but could be seen in some pharmaceutical preparations and few drug labels. Refer to Table 1 and Table 2 for Apothecariesʻ units used for measuring weights and volumes, respectively. 5

6 Apothecariesʻ units Table 1: Table 2: 6

7 Household system Though inaccurate, the use of the household or domestic system of measurements is on the rise because of an increased home health care. In this system the patients use household measuring devices such as the teaspoon, tablespoon, glass, etc. See Table 3 below. Table 3: 7

8 Cont. Household system Household system of measures is designed for the convenience to the patient. Therefore, this system is used for the directions on labels for the patients. In the past, a drop has been used as an equivalent of a minim. The official dropper delivers 20 drops per ml of water at 25 ᴼC. 8

9 Cont. Household system Such a measure should be discouraged because of many factors affecting the drop size which include: the density of the medication temperature surface tension diameter of the dropper opening of the dropper the angle of the dropper and the applied pressure 9

10 Metric system The International System of Units was first developed by the French. It is the world's most widely used system of measurement, both in everyday commence and in science. In the metric system, the three primary or fundamental units are: the meter for length, the liter for volume, and the gram for weight. In addition to these basic units, the metric system includes multiples and subdivisions of the basic units with a prefix to indicate its relationship with the basic unit. The process of changing from the common system (pound, gallon) to the metric SI is termed metric transition or metrification. 10

11 Table 4: Shows the metric prefixes and their meanings Prefix Subdivisions: piconanomicromillicentideci- Multiplies: kilomegagigatera- Meaning One trillionth (10-12 ) of the basic unit One billionth (10-9 ) of the basic unit One millionth (10-6 ) of the basic unit One thousandth (10-3 ) of the basic unit One hundredth (10-2 ) of the basic unit One tenth (10-1 ) of the basic unit 1 thousand (10 3 ) times the basic unit 1 million (10 6 ) times the basic unit 1 billion (10 9 ) times the basic unit 1 trillion (10 12 ) times the basic unit 11

12 Cont. metric system Today the pharmacopeias, pharmaceutical industry and the practice of pharmacy depend on the SI. The reasons for the transition from the common system to the SI are: 1. The simplicity of the decimal system. 2. The clarity provided by the base units and the prefixes of the SI. 12

13 Guidelines for the Correct Use of the SI 1. The short forms for SI units such as centimeter (cm) are termed Symbol, not abbreviation. 2. Unit names and symbols generally are not capitalized except when used at the beginning of a sentence or in headings. However, the symbol for liter (L) may be capitalized or not (to prevent the misleading with number 1). Examples: 4 L or 4 l, 4 mm, and 4 g; not 4 Mm and 4 G 3. The decimal point is placed on the line with the number. The comma are not preferred to be used. Examples: 4.5 ml; not 4,5 ml 13

14 Cont. guidelines for the correct use of the SI 4. Periods are not used following SI symbols except at the end of a sentence. Examples: 4 ml and 4 g, not 4 ml. and 4 g. 5. A compound unit that is a ratio of two units is indicated by a solidus (/). Examples: 5 ml/hr, not 5 ml per hour. 6. Symbols should not be combined with spelled-out terms in the same expression. Examples: 3 mg/ml, not 3 mg/milliliter 7. Plurals of unit names, when spelled out, have an added (s). Symbols for units are the same in singular and plural. Examples: 5 milliliters or 5 ml, not 5 mls 8. Two symbols exist for microgram: mcg and µg. 14

15 Cont. guidelines for the correct use of the SI 9. Decimal fractions are used, not common fractions. Examples: 0.25 g, not 1/4 g 10. A zero should be placed in front of the decimal point. Example: 0.5 g, not.5 g 11. In selecting symbols of units, the choice generally is based on selecting the unit that will result in a value between 1 and Examples: 500 g, not 0.5 kg, 1.96 kg, not 1960 g, 750 ml, not 0.75 L 12. Symbols are written in upright type so as to differentiate from the italic type that used for variables. Example: m for meter and m for mass 15

16 Measurement of length The meter is basic unit of length. The following table shows the common units that used in the measurement of length: Table 5: length units conversion 1 Kilometer (km) 1000 meters 1 decimeter (dm) 0.1 meter 1 centimeter (cm) 0.01 meter 1 millimeter (mm) meter 1 micrometer (µm) 0.000,001 meter 1 nanometer (nm) 0.000,000,001 meter 16

17 Measurement of volume The liter is the basic unit of volume. In SI, one milliliter (ml) is regarded equivalent to one cubic centimeter (cm 3 ). The following table shows the commonly used units in the measurement of volume: Table 6: volume units conversion 1 deciliter (dl) 0.1 liter 1 milliliter (ml) liter 1 microliter (µl) 0.000,001 liter 17

18 Measurement of weight The gram is the basic unit of weight. In SI, one gram (g) of water is regarded equivalent to one cubic centimeter (cm 3 ). The following table shows the commonly used units in the measurement of weight: Table 7: weight units conversion 1 kilogram (kg) 1000 g 1 milligram (mg) g 1 microgram (µg) g 1 nanogram (ng) g 18

19 Practice 1 1. Add 0.5 kg, 50 mg, and 2.5 dg. Reduce the result to grams. 2. Add 7.25 L and 875 cl. and express the answer in milliliters. 3. Aspirin tablets generally contain 325 mg of aspirin. How many such tablets may be prepared from 5 kg of aspirin? 4. If a chlorpheniramine maleate tablet weighs 0.26 gram, one-fourth of the same tablet weighs how many milligrams? 19

20 Cont. Practice 1 5. One milligram of streptomycin sulfate contains the antibiotic activity of 650 µg of streptomycin base. How many grams of streptomycin sulfate would be the equivalent of 1 g of streptomycin base? 6. If a vial of gentamicin contains 80 mg of drug in 2 ml, how many micrograms of the drug are present in ml? 7. How many grams of digoxin would be required to make 25,000 tablets each containing 250 mcg of digoxin? 20

21 Relation of SI to Other Systems of Measurements (Interconversions) It is necessary to convert all quantities to the same system of measurement. Additionally, the pharmacist must still appreciate the relationship between the various systems of measurement. For example, when there is need to convert fluidounces to milliliters or kilograms to pounds and vice versa. See Table 8 for conversion values. 21

22 Table 8: Inter-conversion between the Apothecary and metric systems. Apothecary Metric 1 gallon 3785 ml 1 fluidounce 30 ml 1 fluidram 4 ml 1 minim 0.06 ml 1 pound 454 g 1 ounce 30 g 1 grain g 2.2 pound 1 Kg 1 inch 2.5 cm 22

23 Practice 2 1. How many ml of a topical keratolytic solution are contained in a xiv fluidram bottle? 2. If a prescription calls for gr ii of drug X tablets and the pharmacist has gr ss tablets in stock, how many tablets of gr ss should be provided? 3. A child received 5 fluidounce of amoxicillin suspension containing 125 mg/5 ml. If one teaspoonful was taken three times a day, how many grams of amoxicillin did the child take in one week? 23

24 Learning outcomes The major systems for measurement used in pharmacy were understood. Pharmaceutical calculations were practiced to employ the International System of Units and other systems. Answers of: Practice 1: 1= g, 2= 16,000 ml, 3= 15,384 tablets, 4= 65 mg, 5= g streptomycin sulfate, 6= 1000 µg, 7= 6.25 g digoxin Practice 2: 1= 56 ml, 2= 4 tablets of gr ss, 3= g 24

25 Homework Join a group of 4-6 students. Answers should be handed in a single paper. Deadline is Wednesday 28/11/ Suprax suspension contains 100 mg/ 5 ml of the drug cefixime. If the patient takes one teaspoonful of the suspension twice daily for ten days, how many grams of the drug does the patient consume? 2. Tylenol with Codeine elixir contains 12 mg of codeine per 5 ml of the elixir. If a pharmacist dispenses 4 fluidounces of the elixir to a patient, how many grains of codeine does the patient receive? 25

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