CONCENTRATED ACIDS AND BASES
As discussed in paragraph 2-31, concentrated liquids are manufactured as
weight-per-weight solutions. An example of this type of solution is concentrated
hydrochloric acid that contains 37 grams of HCl per 100 grams of total solution (w/w).
These figures indicate that concentrated HCl has a percent purity of 37 percent or that
37 percent of the weight of 100 grams of solution is HCl. Since it is impractical to weigh
concentrated acids and/or bases, one needs another method to determine how much
concentrated liquid to use. The method used includes use of the specific gravity of the
solution, which is an expression of density (g/mL), and the percent purity or assay value
(%A) that indicates the proportion of the solution by weight that is the desired
substance. The specific gravity and percent purity values are found on the label of most
reagent-grade acids and bases. Using these values, one can determine the actual
amount of the substance in a given volume of the concentrated solution.
The values listed on the label of a bottle of nitric acid are specific gravity (S.G.)
1.42 and assay 70 percent. What so these values mean?
a. These values mean that 1 milliliter of the solution has a mass of 1.42 grams
and that 70 percent of this mass is nitric acid (HNO3).
b. When preparing a solution using this concentrated liquid, we are concerned
only with how much HNO3 is present. To determine this, multiply the density times the
percent assay. The answer is the number of grams of HNO3 per milliliter of solution.
X 0.70 percent HNO3
0.994 grams of HNO3 per mL of solution
c. Therefore, density times % A is an expression that yields the number of
grams of desired substance per milliliter of concentrated substance.
The % A value is equal to the assay value divided by 100. If the assay value
is 70 percent then the % A is 0.70 (70/100). The latter value or it's equivalent
is to be used in all calculations.
(g/mL)(% A) = grams of desired substance per unit volume