76 WATER AND SOLUTIONExperiment 6
STANDARDIZATION OF SOLUTIONS
If we have a solution of an acid of known normality we can
determine the concentration of an unknown basic solution by
finding what volumes of the two solutions are required to produce
neutrality. For example, it is found that 60 cc. of a 0.5N hydro-
chloric acid solution are required to neutralize 25 cc. of a sodium
hydroxide solution of unknown concentration. The normality
of the base is 1.2, or the solution contains 48 grams of solid NaOH
per liter. In general
NA : NB = VB : VA
VA = volume of acid and NA = normality of acid.
VB = volume of base and NB = normality of base.
Instead of a hydroxide a carbonate may be used to neutralize an
acid Na
2 CO 3 + 2HC1 -» 2NaCl + H 2 O + CO 2A liV solution of sodium carbonate (molecular weight = 106)
106
contains -~- grams of solid in a liter of solution. Since sodium
carbonate can be obtained in a high state of purity and can be
accurately weighed it is frequently used to determine the exact
concentration of an acid solution. Any solution, the concentration
of which has been accurately determined, is known as a " standard
solution."
In this experiment hydrochloric acid will be standardized against
a known weight of pure sodium carbonate. A solution of methyl
orange will be used as an indicator to tell when the solution is
neutral. This indicator is pink in acid solution and yellow in
alkaline solution (see page 135).
The process of comparing the concentration of solutions is
known as titration. The apparatus used is a burette, a glass tube
accurately graduated in cubic centimeters.
Apparatus: two 50-cc. burettes.
2 burette clamps.
500-cc. graduated cylinder,
bottle with glass stopper,
bottle with rubber stopper.
300-cc. Erlenmeyer flask,
large vial.