Density and Specific Gravity PDF

Density and specific gravity  Objectives Define density, specific gravity, and specific volume and determine each through appropriate calculations. Calculate specific gravity from data derived from the use of a pycnometer. Apply specific gravity correctly in convertin...

Density and specific gravity  Objectives Define density, specific gravity, and specific volume and determine each through appropriate calculations. Calculate specific gravity from data derived from the use of a pycnometer. Apply specific gravity correctly in converting weight to volume and volume to weight.  Density Density (d) is mass per unit volume of a substance at a fixed temperature and pressure. It is usually expressed in CGS system, i.e., grams per cubic centimeter (g/cc) or grams per milliliter (g/mL). In SI units, it may be expressed as kilograms per cubic meter. Density may be calculated by dividing mass by volume, that is: Example: if 10 mL of sulfuric acid weighs 18 g, its density is: Density = 18 (g)/ 10 (mL) = 1.8 grams per milliliter  Specific Gravity Specific gravity (sp gr) is a ratio, expressed decimally, of the weight of a substance to the weight of an equal volume of a standard substances at the same temperature. For the specific gravities of liquids and solids, water used as the standard. The most useful standard for gases is hydrogen. Specific gravity has no units. Specific gravity may be calculated by dividing the weight of a given substance by the weight of an equal volume of water, that is:  Example: If 10 mL of sulfuric acid weighs 18 g, and 10 mL of water, under similar conditions, weighs 10 g, the specific gravity of the acid is: Specific gravity = 18 (g) / 10 (g) = 1.8 Substances that have a specific gravity less than 1 are lighter than water. Substances that have a specific gravity greater than 1 are heavier than water. In the USP, the standard temperature for specific gravities is 250C, except for that of alcohol, which is 15.560C by government regulation.  Density Versus Specific Gravity Density Specific Gravity Density of a substance is a Specific gravity, being a ratio of concrete number (1.8 g/mL in like quantities, is an abstract the example) number (1.8 in the example) Density varies with the units of Specific gravity has no measure used dimension and is therefore a constant value for each substance (when measured under controlled conditions) Density of water may be Specific gravity of water is variously expressed as 1 g/mL, always 1 1000 g/L, or 621⁄2 lb/cu ft  Calculating the Specific Gravity of Liquids Known Weight and Volume Examples: If 54.96 mL of an oil weighs 52.78 g, what is the specific gravity of the oil? 54.96 mL of water weighs 54.96 g Specific gravity of oil = 52.78 (g) / 54.96 (g) = 0.9603, answer. If a pint of a certain liquid weighs 601 g, what is the specific gravity of the liquid? 1 pint = 16 fl. oz., 16 fl. oz. of water weighs = 473 g Specific gravity of liquid = 601 (g) / 473 (g) = 1.27, answer.  Pycnometer or Specific Gravity Bottle A pycnometer is a special glass bottle used to determine specific gravity. It is available in the lab. Its volume ranging from 1-50 ml. In using a pycnometer, it is first weighed empty and then weighed again when filled to capacity with water.  Example: A 50 mL pycnometer is found to weigh 120 g when empty, 171 g when filled with water, and 160 g when filled with an unknown liquid. Calculate the specific gravity of the unknown liquid. Weight of water: 171 g - 120 g = 51 g Weight of unknown liquid: 160 g - 120 g = 40 g Specific gravity = Weight of substance / Weight of equal volume of water Specific gravity of unknown liquid = 40 (g) / 51 (g) = 0.78  Displacement or Plummet Method The specific gravity calculation is based on Archimedes’ principle, which states that 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. In this method weigh a plummet when suspended in water and when suspended in a liquid the specific gravity of which we want to determine; Then by subtracting these weights from the weight of the plummet in air, we get the weights of equal volumes of the liquids needed.  Example: A glass plummet weighs 12.64 g in air, 8.57 g when immersed in water, and 9.12 g when immersed in an oil. Calculate the specific gravity of the oil. – 12.64 g - 9.12 g = 3.52 g of displaced oil – 12.64 g - 8.57 g = 4.07 g of displaced water – Specific gravity of oil = 3.52 (g) / 4.07 (g) = 0.865  Use of Specific Gravity in Calculations of Weight and Volume Specific gravity is a factor that expresses how much heavier or lighter a substance is than water, the standard with a specific gravity of 1.0. If we had 50 mL of a liquid with a specific gravity of 1.2, it would weigh 1.2 times as much as an equivalent volume of water. An equivalent volume of water, 50 mL, would weigh 50 g, and therefore the liquid would weigh 1.2 times that, or 60 g. Calculate the weight, Knowing the volume and specific gravity Grams = Milliliters x Specific gravity Grams (other liquid) = Grams (of equal volume of water) x Specific gravity (other liquid) Examples: What is the weight, in grams, of 3620 mL of alcohol with a specific gravity of 0.820? 3620 mL of water weigh 3620 g 3620 g x 0.820 = 2968 g, answer.  Calculating Volume, Knowing the Weight and Specific Gravity Example: What is the volume, in milliliters, of 492 g of nitric acid with a specific gravity of 1.40? 492 g of water measure 492 mL 492 mL / 1.40 = 351 mL, answer  Pharmaceutical Applications ✓ Specific gravity is employed to convert the weight of an ingredient or preparation to volume or vice versa. ✓ Specific gravity is also used to calculate the equivalent strength of a preparation on the basis of either weight or volume. ✓ Specific gravity is in automated pharmaceutical equipment used by pharmacists to prepare total parenteral nutrition (TPN) admixtures. ✓ The purpose of the specific gravity of the large-volume liquids being mixed is to determine the weights of components (e.g., dextrose, amino acids, and water).  Clinical Application ✓ Specific gravity is an important factor in urinalysis. ✓ In normal adults, the specific gravity of urine is usually within the range of 1.010 and 1.025 with a normal fluid intake (this range may vary with the reference source). ✓ The specific gravity of urine generally decreases with age. ✓ In newborns, it is generally within the range of 1.001 to 1.020. ✓ Specific gravity is an indicator of both the concentration of particles in the urine and a patient's degree of hydration. ✓ A higher-than-normal specific gravity indicates that the urine is concentrated. ✓ This may be due to the presence of excess waste products or electrolytes in the urine, the presence of glucose (glucosuria) or protein (proteinuria), low fluid intake, excessive water loss, decreased fluid intake, ect. ✓ A low specific gravity indicates that the urine is dilute, which may be a result of diabetes insipidus, renal disease (by virtue of the kidney's reduced ability to concentrate urine), increased fluid intake, intravenous hydration, or other factors. ✓ In the modem clinical laboratory, the specific gravity of urine is determined (using the refractive index method) as a component of a comprehensive urinalysis performed by sophisticated, fully automated equipment that determines, in seconds, urine chemistry, specific gravity, pH, color, and clarity. Urine Specific Gravity Refractometer