Physical PharmacyPOST-LAB DISCUSSIONfor Preliminary Period

Gillian O. Velasco,RPh

EXPERIMENT 1: Specific Gravity Determination of Liquids

• Density (ρ) – mass per unit volume at a fixed temperature and pressure

ρ = M / V

- unit in cgs system: g/cm3SI unit: kg/m3

• Relative Density – ratio of the density of a substance to the density of water, the values for both substances being determined at the same temperature unless otherwise specified

EXPERIMENT 1: Specific Gravity Determination of Liquids

• Specific Gravity – ratio of the mass of a substance to the mass of an equal volume of water at 4°C or at some other specified temperature.

*Water attains its maximum absolute density 0.999973 at 3.98°C* For gases, the standard used is Hydrogen

• Notations: 25°/25° ; 25°/4° ; 4°/4° *First Figure: temperature of the air at which substance was weighed*Second Figure: temperature of water used

Determination of Specific Gravity defined by the USP: “Unless otherwise stated, specific gravity basis is 25°/25°…”

*Exceptions: 1. Theobroma oil – solid at 25°, that’s why specific gravity is determined at 100°/25°2. Alcohol – determined at 15.56°/15.56° because many years ago, the US

government adopted 60°F as the temperature at which alcoholometric measurements are to be made for government control of alcoholic liquids.- USP/NF Monograph for Alcohols – it complies with the test for Specific Gravity <841> (Method I – Pycnometric, Method II – Oscillating transducer density meter)

EXPERIMENT 1: Specific Gravity Determination of Liquids

• Apparent Specific Gravity – when measurements of masses for specific gravity determination is conducted in air.

• True Specific Gravity – also known as absolute specific gravity; when measurements of masses for specific gravity determination is conducted in a vacuum.

*As a rule, the difference between the two specific gravities is very small.

EXPERIMENT 1: Specific Gravity Determination of Liquids

• Methods of Determining Specific Gravity- by the use of various types of pycnometer- Mohr - Westphal Balance- Hydrometer

EXPERIMENT 1: Specific Gravity Determination of Liquids

• PYCNOMETER – fixed volume vessels used to be filled with sample liquid.

Advantage:- If correctly used, provides fast and

accurate measurements of specific gravityDisadvantages:- Great care must be exercised to obtain

accurate results.- High precision pycnometers are

expensive. - They require precision weighing scale and

controlled laboratory environment.- Their good performaces depend on the

skill of the operator.

EXPERIMENT 1: Specific Gravity Determination of Liquids

Archimedes’ Principle “ Any object placed in a fluid

will experience an upward or bouyant force equal to the weight of the fluid it displaces.”

- This principle applies to both liquids & gases and to objects which are completely or partially submerged

- Reason for Bouyant Force: Pressure increases with depth, so the upward force on the bottom of the object is larger than the downward force on top of the object.

EXPERIMENT 1: Specific Gravity Determination of Liquids

• HYDROMETER - is an instrument that is calibrated to indicate the specific gravity of a liquid by the extent to which it is submerged into the liquid.

Precautions:- There should be no air bubbles in

the surface of the liquid and of the jar.

- There should be sufficient sample on the jar to allow the hydrometer to float freely.

- Allow the hydrometer to be completely stationary before noting the reading.

EXPERIMENT 1: Specific Gravity Determination of Liquids

• WESTPHAL BALANCE – is an instrument in which the up thrust on a small sinker (a glass plumet) immersed in the liquid at a particular temperature is compared with the up thrust on the same sinker when immersed in water up to the same extent at the same temperature.

*It directly gives the relative density of the liquid but the results is slightly less accurate than pycnometric measurements.

EXPERIMENT 2: Alcohol Determination by Specific Gravity Method

USP Methods for Alcohol Determination <611>:Method I – Distillation Method II – Glass Chromatographic Method

EXPERIMENT 2: Alcohol Determination by Specific Gravity Method

DISTILLATION- is the process of heating a solution to its boiling point, passing the vapors through a cooling device called the condenser and collecting the liquid which condenses.

Ground Glass Joints – are used to quickly fit leak-tight apparatus.- Its limitation is that, once original smooth surface the glass has been damaged by grinding, the advantage would be lost.

EXPERIMENT 2: Alcohol Determination by Specific Gravity Method

• Physical Property that is considered for distillation: BOILING POINT

Alcohol has a lower boiling point than water (78.5°); therefore it will boil first. Thus, the distillate that can be collected is expected to contain alcohol.

Because of the alcohol it contains, the distillate will have a specific gravity lower than that of distilled water. By the use of the USP Alcoholmetric Table, the percentage of alcohol in a solution can be determined by knowing the specific gravity of the solution, assuming that no interfering substances co-distill with ethanol which would affect the specific gravity of the distillate.

EXPERIMENT 3: Melting Point Determination

• Melting Point – the temperature at which the pure liquid and solid exists in equilibrium.

Importance of Determining the Melting Point: - it is used as indicator of purity of chemical substances (a pure substance must have a sharp melting point and a lower melting point range; impure substances may undergo decomposition prior to reaching its theoretical melting point)- used in the identification of an unknown substance- used to determine the proper procedures in formulating drugs because drugs with low melting point tends to soften during processes in which heat is generated - used to determine the compatibility of various substances before inclusion in the same dosage form

EXPERIMENT 3: Melting Point Determination

Methods of Determining Melting Point

EXPERIMENT 3: Melting Point Determination

Methods of Determining Melting Point

EXPERIMENT 4: Freezing Point Determination of Isotonic Solutions

• Freezing Point – the temperature at which liquid passes into the solid state.- The temperature at which solid and liquid exists in equilibrium.

• Isotonic Solutions – solutions that has essentially the same salt concentration and hence the same osmotic pressure as the RBC.

* Pharmaceutical solutions intended to be applied to delicate membranes and blood should be isotonic so that there would no contraction or swelling of the tissues with which they come in contact. Thus, preventing discomfort.

EXPERIMENT 4: Freezing Point Determination of Isotonic Solutions

• Methods of Determining Tonicity1. Hemolytic Method

- the effect of various solutions of the drug is observed on the appearance of RBC suspended in the solutions.

*Hypertonic Solution – causes the cell to shrink and become wrinkled or crenated.

*Hypotonic Solution – causes the cell to swell and finally burst

Hemolysis – a phenomenon wherein the cell swells and burst with liberation of hemoglobin

EXPERIMENT 4: Freezing Point Determination of Isotonic Solutions

2. Methods that Determine Colligative Properties- based on the measurement of the slight

temperature differences arising from differences in the vapor pressure of thermally insulated samples contained samples contained in constant-humidity chambers

- Referred from the freezing point of blood and tears

- at present, it is established that -0.52°C is the freezing point of blood and lacrimal fluid and it corresponds to the freezing point of 0.09% NaCl solution