SPECIFIC GRAVITY
INTRODUCTION
Specific gravity often becomes a part of the solution to a pharmaceutical calculation. Hence, the main use of specific gravity is to solve for a liquid s volume when the weight of the liquid is known. Because of the difficulty which may be encountered in trying to weigh a liquid, it is often advantageous to calculate the liquid s volume and measure it in a graduate as opposed to weighing it.
DEFINITION
a. Specific Gravity. Specific gravity is the ratio of the weight of a substance to the weight of an equal volume of distilled water at 25?C.
b. At 25?C and 1 atmosphere of pressure, one milliliter of distilled water weighs one gram. Therefore, the specific gravity of water is established as one.
c. Formula.
Specific gravity = Weight of the substance/ Weight of an equal volume of water
d. Because one milliliter of water weighs one gram:
Specific gravity = Number of grams of the substance/ Number of milliliters of the substance
e. Specific gravity has no units. Because specific gravity has no units, only the numbers must be placed in the formula providing the units of weight and volume are grams and milliliters. If units are other than grams and milliliters, using the conversion factors as discussed in the metric system and common systems of measure should change them.
f. Example Problem. What is the specific gravity of 10 milliliters of mineral oil,
which weighs 8.5 grams?
Specific gravity = # of grams of substance/ # of milliliters of the substance
Specific gravity= 8.5 Sp gr= 0.85
SPECIFIC GRAVITY TRIANGLE
a. The triangle below is an excellent aid in the solving of specific gravity problems.
b. To use the triangle, cover the unit for which you are solving; then, perform the operation indicated.
c. For example: What is the specific gravity of an alcohol that has a volume of 1000 milliliters and weighs 810 grams?
Cover specific gravity:
d. Perform the operation that is left: grams/milliliters
Sp gr = grams / milliliters
Sp gr = 810 /1000
Sp gr = 0.810
e. Example problem.
Water Soluble Ointment Base
Calcium citrate………………………………………………… 0.05 g
Sodium alginate………………… 3.00 g
Methylparaben………………………………………………… 0.20 g
Glycerin……………………………………………………… 45.00 g
Distilled Water……To make……………………………… 1000.00 ml
(1) In this formula, the amount of glycerin is given as grams, a measure of weight. Because glycerin is a liquid, it would be easier to change the grams to milliliters and measure the glycerin in a graduate. The specific gravity of glycerin is 1.25. How many milliliters of glycerin would be required?
(2) Using the triangle:
Therefore: milliliters = grams /specific gravity
milliliters = 45/1.25
milliliters = 36 (answer)
(3) In the place of weighing 45 grams of glycerin, 36 milliliters of glycerin may be measured.
f. Example problem. What is the weight in grams of 240 milliliters of light liquid petrolatum having a specific gravity of 0.81?
g = sp gr X ml
g = 0.81 x 240
g = 194.4 (answer)
240 milliliters of light liquid petrolatum weighs 194.4 grams.
g. Example problem. The following prescription has all of the amounts indicated as grams with the exception of mineral oil. Because mineral oil is a liquid, the units, as written on this prescription, are milliliters. In compounding this prescription, all of the substances must be weighed or measured separately and then combined by a certain procedure. It is impractical to "q.s." with an ointment base. Instead, the weights of all the other ingredients must be totaled and this combined weight must be subtracted from the total amount of the preparation to find the amount of white petrolatum needed. To accomplish this, the weight of the 8 milliliters of mineral oil must be calculated.
Specific gravity of mineral oil = 0.85.
(1) grams = specific gravity x milliliters
grams = .85 x 8
grams = 6.8
(2) Then, add the weights of all ingredients except the white petrolatum.
14.0 g zinc oxide
+4.0 g sulfur, ppt
6.8 g mineral oil
24.8 g
(3) Subtract this from the total volume.
100.0 g total volume
-24.8 g other substances
75.2 g white petrolatum required
(4) The above problem is a typical pharmaceutical calculation problem involving specific gravity.