PHARMACEUTICAL MEASUREMENT P2

Q: Define Accuracy in the context of pharmaceutical measurements.

A: Accuracy is a measure of the capability of a balance to approach a true or absolute value.

Q: Define Precision.

A: Precision is the relative degree of repeatability, meaning how closely the values within a series of replicate measurements agree with each other.

Q: What is Tolerance (or 'limits of permissible errors')?

A: Tolerance is the extreme value of an error permitted by specifications for a measuring instrument.

Q: Describe the characteristics of a measurement series that exhibits both High Accuracy and High Precision.

A: A measurement series with High Accuracy and High Precision has values that are close to the true value (high accuracy) and are close to each other (high precision).

Q: Define Density and state its common pharmaceutical unit.

A: Density is the mass per unit volume of a substance⁵. It is usually expressed as grams per cubic centermeter (g/cc) or, according to the USP equivalence of 1 cc= 1ml, as grams per millimeter g/mL

Q: State the formula for calculating Density.

A: Density is calculated by dividing mass ($M$) by volume ($V$):

\text{Density} = \frac{\text{Mass}}{\text{Volume}}

Q: Define Specific Gravity (SG) and the standard substance used for liquids and solids.

A: Specific Gravity is the ratio, expressed decimally, of the weight of a substance to the weight of an equal volume of a substance chosen as a standard. Water is used as the STANDARD for the specific gravity of liquids and solids

Q: What is the USP standard temperature for specific gravity determination?

A: The standard temperature for specific gravity according to USP is 25C, except for alcohol, which is 15.56C

Q: Define Specific Volume (SV) and explain its relationship with Specific Gravity.

A: Specific Volume is an abstract number representing the ratio of the volume of a substance to the volume of an equal weight of another substance (water) taken as a standard¹¹. Specific gravity and specific volume are reciprocals; their product is 1

Q: What is the primary use of specific gravity for a pharmacist regarding ingredients and preparations?

A: Specific gravity is employed when a pharmacist wishes to convert the weight of an ingredient or preparation to volume or vice versa.

Q: How is specific gravity used in the preparation of Total Parenteral Nutrition (TPN) admixtures using automated equipment?

A: The specific gravity of the large-volume liquids (e.g., dextrose, amino acids) is used to determine the weights of components. The automatic compounder then uses these calculated weights to make the correct mixture, rather than measuring the volume

Q: In urinalysis, what does a higher-than-normal specific gravity indicate, and what are some possible causes? A: A higher-than-normal specific gravity indicates the urine is concentrated. Possible reasons include:

• Presence of excess waste products or electrolytes.

• Presence of glucose (glucosuria) or protein (proteinuria).

• Low fluid intake or excessive water loss.

Q: Name the three general methods for determining the specific gravity of liquids

. A: The three methods mentioned are:

1. When known weight and volume are available.

2. Using a Pycnometer/Specific Gravity bottle.

3. Displacement or Plummet method.

Q: Explain the principle behind the Displacement or Plummet Method.

A: This method is based on Archimedes’ principle: A body immersed in a liquid displaces an amount of the liquid equal to its own volume and suffers an apparent loss in weight equal to the weight of the displaced liquid.

Q: For solids heavier than and soluble in water (e.g., a chemical salt), what must be used to determine its specific gravity?

A: The solid must be weighed in an insoluble liquid (like oil or naphtha)^. The final specific gravity is calculated by multiplying the weight quotient (ordinary weight / loss of weight) by the specific gravity of the insoluble liquid²⁵.

Q: What are the formulas used to convert between weight and volume of a liquid when its specific gravity (SG) is known?

A:

• To determine Weight (W) from Volume (V)

  W = \text{SG} \times \text{Volume}

  ²⁶

• To determine Volume (V) from Weight (W)

  \text{Volume} = \frac{\text{Weight}}{\text{SG}}