EXPERIMENT 10
Volumetric Analysis I
Standardization of NaOH Solution
Outcomes
After completing this experiment, the student should be able to:
1.
2.
3.
4.
Demonstrate the concept of quantitative analysis.
Make solution and standardize it.
Explain the difference between primary and secondary standard solutions.
Quantitatively determine the concentration of a base.
Introduction
Titration is a common method of quantitative analysis used to determine the concentration of
an unknown substance in a solution. The method is easy to use if the quantitative relationship
between two reacting solutions is known. It is particularly well-suited to acid-base and
oxidation-reduction reactions. In this course, we will analyze commercial products such as
household vinegar and bleach.
In the case of acid-base titrations, these permit the determination of concentrations of unknown
acids or bases with a high degree of accuracy. In order to analyze unknown acids or bases, we
must have a “standard solution” to react with the unknowns. A standard solution is one in
which the concentration is accurately known. We will first prepare a standard solution of
NaOH. One way to prepare a standard solution is to dissolve an accurately massed amount of
the substance and dilute it to a measured volume. In this way, the concentration can be
calculated exactly. However, it is usually impossible to obtain NaOH of sufficient purity to use
it as a primary standard. Moreover, NaOH is highly hygroscopic, therefore absorbs water
during the weighing process. An indirect method is more practical for obtaining a standard
solution of NaOH. We will prepare a solution of NaOH of approximate molarity and
standardize it against a primary standard of known purity. A primary standard is a substance
that should:
* be of high purity
* remain unchanged in air during massing and remain stable during storage
* have a high molar mass to reduce massing errors
* react with the solution to be standardized in a direct, well-defined reaction.
Potassium hydrogen phthalate is used as a primary standard in this experiment. This is a large
molecule (KHC8H4O4) with a molar mass of 204.2 g/mol. Instead of writing the whole formula,
we abbreviate it as KHP, where "P" stands for the phthalate ion, (C8H4O4)2−, not for
phosphorus. KHP is an acidic substance, with the ionizing hydrogen placed placed separately in
the formula for emphasis. Therefore, KHP is monoprotic (one ionizable proton, H+) and will
react with NaOH in a simple 1 to 1 relationship according to the following reaction:
56
In this experiment, phenolphthalein will be used as an indicator which helps in determining
endpoint and sodium hydroxide will be standardized.
Safety Precautions
Wear safety goggles while working in the laboratory. NaOH is corrosive so avoid skin contact.
Follow all safety rules described in experiment 1.
Materials and Equipment
Buret (50 mL), beaker (200 mL), graduated cylinder, Erlenmeyer flask (250 mL), funnel,
balance, 10 mg or 1 mg accuracy
Solid potassium hydrogen phthalate (KHP), NaOH
Procedure
You will be provided NaOH with approximate molarity.
1. Accurately weigh about 0.8 g KHP (when using 50 mL buret) into a 250 mL Erlenmeyer
flask. Add about 50 mL of water and swirl the flask until the sample is dissolved. Add 3
drops of phenolphthalein indicator (colorless in acidic solution; pink in basic solution).
2. Titrate the KHP solution with NsOH solution to be standardized. Buret readings should be to
two decimal places, with the second decimal place estimated between the two lines. The
titration should proceed until the faintest pink color persists for 30 sec. after mixing. The
color will fade upon exposure to the air (WHY?). After completing a trial, breathe into the
flask and swirl. What has happened?
3. Repeat the titration three times (this is called doing the measurement in triplicate). For
accurate work, the three molarities must be within 1% of one another. Then calculate the
average molarity, M, of the NaOH.
55
Experimental General Chemistry 1
Experiment 10: Standardization of NaOH Solution
Laboratory Data Sheet
Name: _____________________________________________
Trial
Mass of KHP
Moles of KHP
Section:
Moles of NaOH
____________
V1 (mL)
V2 (mL)
VNaOH (mL)
VNaOH (L)
Molarity of
NaOH (mol/L)
1
2
3
4
Show your calculation:
Average molarity of NaOH =
Calculations:
V1 = buret reading before. V2 = buret reading after. VNaOH(mL) = V2 – V1 Report two decimal places for V1, V2 and VNaOH and at least three significant figures for
MNaOH. Thus, volumes should be recorded e.g. as V = 21.36 mL or 18.50 mL, (not 18.5)
Moles of KHP = mass of KHP/molar mass of KHP , then moles KHP = moles NaOH (KHP is monoprotic)
moles NaOH
MNaOH = V
= ____________ mol/L
NaOH (L)
Repeat this for each trial. Then calculate the average MNaOH from your 3 best trials:
MNaOH(average) = (M1 + M2 + M3)/3
56