KD McMahon
Chemistry
Lab: Determining the Concentration of an Acid by Titration
Background
What are Acids and Bases?
The Bronsted-Lowry theory of acids and bases, and acid is a proton donor while a bas is a
proton acceptor. Strong acids donate their protons readily where as weak acids donate
their protons with complexity. Similarly, the strong bases accept protons easily and weak
bases do not.
What is Titration?
Volumetric analysis is a technique for determining the amount of a certain substance by
doing a titration. A titration involves delivery (from a buret) of a measured volume of a
solution of known concentration, the titrant into a solution containing the substance being
analyzed, the analyte. The titrant contains a substance that reacts in a known manner
with the analyte. The point in the titration where enough titrant has been added to react
exactly with the analyte is called the equivalence point or the stoichiometric point.
This point is often marked by an indicator, a substance added at the beginning of the
titration that changes color at or very near the equivalence point. The point where the
indicator actually changes color is called the endpoint of the titration. The goal is to
choose an indicator such that the endpoint (where the indicator changes color) occurs
exactly at the equivalence point, where just enough titrant has been added to react with all
the analyte.
When the analyte is a base or an acid, the required titrant is a strong acid or strong base,
respectively. This procedure is called an acid-base titration. An indicator very commonly
used for acid-base titrations is phenolphthalein, which is colorless in an acidic solution
and pink in a basic solution. Thus, when an acid is titrated with a base, the
phenolphthalein remains colorless until after the acid is consumed and the first drop of
excess base is added. In this case, the endpoint, the solution changes from colorless to
pink, occurs approximately one drop of base beyond the stoichiometric point.
An acid may contain one or more protons or ionizable hydrogen atoms (H) per molecule.
The equivalent mass or normality of an acid is the mass that provides one mole of
hydrogen ions. The equivalent mass can be calculated by dividing the molecular mass of
an acid by the number of hydrogens.
The equivalent mass or moles of hydrogen present in an unknown acid can be determined
by titration with a standardized solution of sodium hydroxide. Since one mole of sodium
hydroxide (NaOH) will react completely with one mole of H+, a volumetric analysis,
using either a pH indicator such as phenolphthalein or a pH meter, can be employed to
determine the moles of H+ present.
The standardized base will be titrated against monoprotic unknown acid to determine the
molarity of the acid. A pH indicator and/or a pH meter will be used to determine the
equivalence point.
Materials & Reagents
Sodium hydroxide, 6M
Unknown: Hydrochloric acid
2X50mL burets
200mL beaker
Buret stand
pH meter
1500mL) boiled and cooled de-ionized water)
Phenolphthalein
1 Liter Volumetric Flask
Pipette
Goggles
Procedure:
1.) Place about 1.7mL of the 6M NaOH in a volumetric 1 liter flask and QS to one liter
with boiled and cooled de-ionized water. Use this 0.01M NaOH to fill a buret.
2.) Refill the 0.01 M NaOH buret and set up a second buret containing the unknown acid.
3.) From the second buret dispense 5.OmL of the unknown acid in a 250mL Erlenmeyer
flask. Add 25mL boiled and cooled de-ionized water and 1-2 drops of phenolphthalein
indicator solution.
4.) Use a pH meter to record the pH after the addition of every 5mL of titrant.
4.) Titrate the unknown acid with the standardized 0.01 M NaOH. If the end point is
missed, back titrate with the unknown acid. Record initial and final readings of both
burets.
5.) Clean and dry the Erlenmeyer flask and repeat the titration two additional times.
6.) Record your data in your lab manual. Use the spreadsheet to produce graphs to
determine the equivalency point and to determine the concentration of your unknown
HCl solution.