Specific Gravity Determination of Liquids

Physical Pharmacy LAB MODULE 1: SPECIFIC GRAVITY DETERMINATION OF LIQUIDS PRE LABORATORY DISCUSSION SPECIFIC GRAVITY ratio of mass of a substance to mass of equal volume of another substance taken as STD '! !". $ = '( unitless numerically equal in metric and English system varied value (1 for water) apparent specific gravity true/absolute specific gravity DENSITY mass of substance per unit volume * )= + has unit (g/mL) the same in metric and English system (62.4 lb/ft3) apparent density absolute density relative density Specific Gravity Specific Gravity (Relative density / Specific weight) • unitless expression of the ratio of the mass of a substance (Ws) to the mass of an equal volume of another substance taken as standard (Ww) !"#$%&%$ ()*+%,- = !"#$%& () *+,*&-./" !"#$%& () "0+-1 2(1+3" () !-&"4 5* !". ( = 5! • • • • • • Ratio of one density to another density It tells us whether the sample will float or sink in water Plays an important role in determining the performance of materials (flow properties) in dosage forms. Monitoring the relative density of solid pharmaceutical materials can be useful during the design optimization and scale up of manufacturing process for solid dosage forms and may help achieve robust drug product manufacturing process. Specific gravity is always given first in defining the character of chemical liquids in the descriptive text of the USP While not always of itself sufficient to prove the identity, etc., It is always first and essential step, which in conjunction with the determination of the boiling or congealing points and other physical attributes, comprised in the term character, discloses the identity, purity or otherwise of the substance. Temperature • A very important variable in specific gravity determination. • The common practice with regards to the determination of sp.gr. defined by the USP: o "Unless otherwise stated, the sp. gr. basis is 25 deg./25 deg., i. e., the ratio of the weight of a substance in air at 25deg. to that of the equal volume of water at the same temperature. o 25 /̊ 25 ̊ Specific Gravity Bottle (Pycnometer) • Special glass bottle used to determine sp. gr. of liquids by weighing of the defined volume • Fitted glass stopper with a capillary tube through it, so that air bubbles and excess fluids may escape from the apparatus. • Enables a liquid's density to be measured accurately by reference for an appropriate fluid, such as water, using an analytical balance • Available in volumes ranging from 1mL to 50mL How to get specific gravity using pycnometer Step 1. Measure the weight of a clean, dry and empty pycnometer. Step 2. Weigh pycnometer filled with distilled water. Step 3. Weigh pycnometer filled with the sample liquid Step 4. Get the weight of the water/sample liquid - Weight of Pycnometer with (distilled water) or (sample liquid) - Weight of empty pycnometer Step 5. Solve for the Specific Gravity of the substance: !"#$%&%$ ()*+%,- = !"#$%& () *-361" !"#$%& () !-&"4 MODULE 1: SPECIFIC GRAVITY DETERMINATION OF LIQUIDS Mohr Westphal Balance • The scale relies on Archimedes Principle of buoyancy • Operates by suspending a glass tube into a sample of a solution of unknown density via a thin platinum wire • For lighter liquids, the plummet will sink while for the heavier liquids, it will float • Can quickly provide very close approximation of the specific gravity of any liquid • Incapable of highly accurate readings, as large temperature ranges in laboratories can affect its ability to duplicate results and the alignment of its pointers is based on human sight and therefore full of potential for human error. Hydrometer • Instrument used to measure the density of a liquid as compared to that of water. • Usually consist of a calibrated glass tube ending in a weighted glass sphere that makes the tube stand up right when placed in a liquid. • The greater the density, the tighter or closer the molecules are packed inside the substance, therefore the greater the density/specific gravity of a liquid the higher a hydrometer will be buoyed by it. Types of Hydrometer A. Baume hydrometers – calibrated to measure specific gravity of evenly spaced scales 1. Baume heavy hydrometer – for liquids heavier than water 145 !" $, = 145 − °23 2. Baume light hydrometer – for liquids lighter than water 140 !" $, = 130 + °23 B. Brix (BX) hydrometer – for determining the percentage by weight of sucrose POST LAB DISCUSSION SAMPLE LIQUIDS STANDARD SPECIFIC GRAVITY CLOVE OIL 1.038 – 1.060 ETHYL ALCOHOL NMT 0.816 LIQUID PETROLATUM 0.860 – 0.905 GLYCERIN NLT 1.249 SYRUP 1.313 If experimental sp gr is less than std sp gr it means it has impurities or is less than the standard.