Experiment 9
A Volumetric Analysis
Objectives
❑ To standardize a sodium hydroxide, NaOH solution.
❑ To make a titration properly.
Chemicals
Instruments and tools
Theory
Volumetric analysis: Based upon measuring the volume. So it is performed
primarily with the aid of volumetric glassware (e.g. pipets, burets, volumetric
flasks).
Standardization: is the process of determining the exact concentration
(molarity) of a solution (secondary standard). Titration is one type of analytical
procedure often used in standardization.
Titration: is a volumetric analysis allows us to measure the amount of solution
required to react completely with another solution.
Types of titration: Acid- base, Redox titration and other types.
Standardization of a Sodium Hydroxide Solution
Solid sodium hydroxide, NaOH is very hygroscopic; able to
absorb water vapor readily, therefore, its mass cannot be
measured to prepare a solution with an accurately known
molar concentration.
To prepare a NaOH solution with a very well known molar
concentration, it must be standardized with an acid that is a
primary standard.
Standard solution: a solution having a very well known
concentration of a solute.
KHP: The primary standard acid
The properties of a primary standard compound:
• high purity
• relatively high molar mass
• High stability
• Non-hygroscopic
In this experiment, KHP, potassium hydrogen phthalate,
KHC8H4O4, solution is used as the primary standard acid for
determining the molar concentration of a sodium hydroxide
solution.
Potassium hydrogen phthalate is a white, crystalline, acidic solid
of relatively high molar mass (204.23 g/mol), and it is only very
slightly hygroscopic.
Acid- base titration
Titrant: The solution in a titration that is added from a burette to a
measured quantity of another solution; in this experiment the titrant is
NaOH solution with unknown concentration.
Analyte: a substance or sample being analyzed (usually the solution
inside the flask); in this experiment the analyte is KHP solution with
known concentration.
Indicator: A chemical substance has different colors at different pH
values.
In acid-base titration phenolphthalein is a common indicator which is
colorless in acidic and neutral solution, and turns pink in basic ones.
Equivalent point and end point
Equivalent point: the point at which an equivalent or
stoichiometric amount of titrant is added to the analyte
based on the stoichiometric equation.
.End point: the point at which the reaction is observed to be
complete; detected with the help of indicator.
The end point in volumetric analysis is the signal that
tells the analyst to stop adding reagent and make the
final reading on the burette.
5
Experimental Procedure
Using a Burette
❑ Clamp the burette carefully to a burette stand. The burette should
be secure enough so that it does not move or slide.
❑ Rinsing: Always rinse a burette before filling it with a new solution.
You should rinse the burette first with soap and tap water, then with
deionized water, and finally twice with the titrant; solution you will be
using in the burette: NaOH solution with unknown concentration.
Record unknown number on report sheet.
❑ Filling and removing Air Bubbles: using a funnel rinsed in the same
manner as the burette, fill the burette, with the NaOH solution from a
100 mL beaker, above the 0.00 mL mark and discard the excess to just
below the mark.
Making titration
1. Use a graduated pipette and a pipette filler to take 20.0 mL of
KHP (0.1M) solution.
2. Put the KHP solution in a clean 250 mL conical (Erlenmeyer)
flask.
3. Add 2-3 drops of phenolphthalein indicator to the flask.
4. Record initial burette reading before titration. Record the
volume on Report Sheet.
You should always read the volume in a burette
from the bottom of the meniscus viewed at eye
level.
5. Slowly open the stopcock so that the titrant drips
out of the burette. The titrant should come out of
the burette drop by drop. Allow the titrant to drop
into the analyte until you notice a color change in the
conical flask.
The color change may be slight, so proceed slowly
and watch carefully. If you notice a color change,
close the stopcock and allow the shake.
If the color dissipates before the 30-second mark,
open the stopcock slightly and continue to add the
titrant drop by drop until you get a permanent
change.
6. Record your final volume from your burette. Once
you’ve reached the endpoint of your titration, close the
stopcock and record the final volume of your titrant in
the burette on Report Sheet for trial 1.
Subtract your initial volume from your final volume to get
total volume of titrant added.
When reading the final volume of your burette, make sure
your eyes are at the level of the titrant meniscus. Take
your reading from the meniscus.
7. Repeat steps 1-6 for trial 2. Record the volume on
Report Sheet.
8. Calculate the concentration of the NaOH solution with unknown concentration.
Calculations:
Number of moles of KHP = Molarity X Volume of KHP (in L)
The formula equation for the reaction is:
1 : 1
Number of moles of NaOH = Number of moles of KHP
X
X
X
X
X
X