Pharmaceutical Calculations Lab 2

Pharmaceutical Measurements

• Weight is measured using a balance, which comes in different types:
  • Double beam balance for rough weights
  • Electronic balance for accurate and sensitive weights

Accuracy and Precision

• Accuracy refers to the closeness of a measurement to the true value
• Precision refers to the closeness of agreement among a set of results

Rules for Using and Caring for a Balance

• Locate the balance in a well-lit, vibration-free area, away from dust, moisture, and corrosive vapors
• Keep the balance clean at all times; remove any chemical spills immediately
• Use a watch glass for weighing solids and a stoppered bottle for corrosive substances or liquids
• Never place materials to be weighed directly on the pan
• Check the balance's adjustment, including zeroing, before each weighing

Sources of Errors in Weight Measurement

• Air currents
• Failure to zero the balance
• Incorrect posture of the balance (must be on a smooth surface)

Systems of Measuring Weight

• Imperial system: units of weight include grain (gr), drachm (ʒ), ounce (oz), and pound (lb)
• Metric system: units of weight include gram (g), kilogram (kg), and milligram (mg)

Relationships between Imperial and Metric Systems in Weight Measurement

• 1 grain = 0.065g
• 1 gm = 15.432 grains
• 1 drachm = 4 gm
• 1 ounce = 30 gm
• 1 kg = 2.2 pounds

Bias (Systematic) and Random (Accidental) Errors

• Bias errors are consistent and repeatable, while random errors arise from random fluctuations in measurements
• To differentiate between the two: random errors are reduced when the experiment is repeated many times, while bias errors remain constant
• Absolute error = measured value - theoretical value
• Percent error = (measured value - theoretical value) × 100% / theoretical value

Example of Error Calculation

• A prescription calls for 800mg of a substance, but the measured value is 850mg; the percentage of error is 6.25%