VOLUMETRIC ACID DETERMINATION
The objective of this experiment is to apply the methods of volumetric
analysis to the determination of (a) the percentage of potassium hydrogen
phthalate, KHC8H4O4, in a sample, (b) the molecular weight and ionization
constant (Ka) of an unknown weak acid, and (c) the titration curve of the
same weak acid when it is titrated with a strong base, NaOH. In achieving
this objective, experience will also be gained in preparing and standardizing
a base solution and in using a pH meter.
PRINCIPLES
The theory behind the methods used in this experiment is described in your text.
The principles upon which the operation of the pH meter are based cannot be fully
understood until after the nature of electron-transfer reactions has been studied later in the
course, but a few brief notes concerning the pH meter are included in Appendix D.
Review the details on the care and attention that you should observe in the use of the
buret on pp. 22-24 of the introductory section of the lab manual and on titrations on pp.
24-25.
EXPERIMENTAL PROCEDURE
1. Preparation of Carbonate-free NaOH Solution. Boil one liter of reverse
osmosis water for about 5 minutes to remove dissolved CO2 and then cool the covered
flask without shaking (so as to avoid re-incorporation of CO2 from the air). Pour the
water into the container which will ultimately store the NaOH, leaving an air space of
approximately 50 ml. Take the bottle to the laboratory assistant who will add 4 ml of
50% NaOH to the water. Stopper the container immediately and thoroughly mix the
contents by vigorous shaking and repeated inversion of the container. The air space is
large enough to make an effective mixing bubble, but not so large as to introduce an
undesirable amount of CO2. This solution is approximately O.08 M NaOH and it should
be carefully protected against CO2 in the air at all times except when removing some of
the solution from the container. A polyethylene or Pyrex bottle is the most desirable
container, though a soft glass bottle with polyethylene stopper will be satisfactory for a
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period of a month. Label the bottle.
2. Standardization of NaOH Solution.
(a) Preliminary operations. If your NaOH solution has stood for a long period
of time (such as from one lab period to the next) some water from the solution may have
vaporized and then recondensed on the walls of the air space above the solution. If this
has happened, shake the contents thoroughly for re-mixing before using the solution.
Thoroughly rinse your buret (including stopcock and tip) with three successive small
portions of the 0.08 M NaOH solution, then fill the buret to somewhat above the "0" level
and withdraw solution from the tip in the proper manner (see p. 23). Avoid undue
exposure to the air and keep the storage bottle well-stoppered.
(b) Weighing of standard sample. Weigh to the nearest 0.1 mg three
approximately 0.5-g samples of primary standard potassium hydrogen phthalate
(abbreviated hereafter as KHP, but note that KHP is NOT the molecular formula) that has
been previously dried at 120 °C. The KHP is kept in small weighing bottles stored in
desiccators located near the analytical balances; it should be returned to the desiccator
when not in use. Weigh the samples on weighing paper and then, after sliding the
samples into clean (but not necessarily dry) 250-ml conical flasks, reweigh the paper to
take account of any crystals which remain on it. The solid should be added to the
weighing paper OUTSIDE THE BALANCE CASE. The flasks should be numbered on
the frosted spot for identification. Once weighed, the samples may be used immediately
or stored in your desk (with watch glass over mouth of flask) until another lab period.
(c) Titration of standard sample. For titration, dissolve a sample in about 50 ml
of reverse osmosis water, heating gently as necessary to aid in the dissolution , add 2-3
drops of O.l% phenolphthalein indicator, then add NaOH solution from the buret until a
faint permanent pink endpoint has been achieved. Initial and final buret readings should
be recorded to the nearest 0.01 ml, and the appropriate buret corrections applied. Being
acidic, the titration solution will not absorb any appreciable amount of CO2 until near the
endpoint. At the endpoint the color will doubtless fade after several minutes due to
absorption of CO2 from the air. To avoid a reflected pink color, do not wear pink
colored clothing to the lab.
The initial amounts of NaOH solution may be added rapidly with swirling of the
flask contents, but when the color tends to persist in solution at the point where the NaOH
mixes with the flask contents, the addition should be slowed and the subsequent
quantities added dropwise. Just prior to the endpoint the flask walls should be rinsed
down with a stream of distilled water from your wash bottle. Partial drops may be added
as discussed on p. 24.
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Once one titration has been completed, the others may be done more rapidly by
first making a rough calculation of the approximate volumes to be expected for the other
weighed samples. Almost all of the calculated volume may be added at once, followed
by the final careful procedure used with the first sample. A quick check for satisfactory
precision may be made before going on to the next section by simply comparing the ratio
of base volume to weight of KHP for each sample. In the case of poor precision (e.g., a
range of values of 2 p.p.t. or more) additional samples should be run.
3. Determination of the Percentage of KHP in a Sample. Obtain a previously
dried sample of the unknown from the desiccator on the side shelf and place it in a small
desiccator for your own use. Record the number of your unknown. Just as you did with
the primary standard, weigh out three samples to the nearest 0.1 mg. Calculate the
approximate weight of sample assuming that your sample contains about 50% KHP.
Perform the titrations in the same manner as carried out for the primary standard.
4. Molecular Weight and Ionization Constant. Titrate a sample of an unknown
weak acid, using a pH meter to follow the course of its titration, and from the titration
curve determine Ka and the molecular weight of the acid. For the purpose of the
experiment, all the unknown acids are assumed to be monoprotic.
(a) Sample preparation. Obtain an unknown weak acid and weigh a 0.4 g
sample using the same method as for previous samples, but this time transfer the sample
to a clean (but not necessarily dry) 250-ml beaker. Dissolve the sample in about 75 ml of
reverse osmosis water. Record the sample number of your unknown.
(b) Standardization of the pH meter. Normally the pH meter is standardized for
you with a standard buffer solution. Hence, do not touch the cal 2 and cal 1 controls.
(c) pH Measurements. Make sure the power switch is in the power on ()
position. Since you will be measuring pH using the automatic temperature compensation
(ATC) probe make sure that the measurement mode switch is in the ATC position. When
the ATC mode is used the manual temperature adjustment control is not used. To
measure the pH of a solution raise the combination glass electrode and calomel electrode
probe and the ATC probe from the soaking solution, rinse them thoroughly with reverse
osmosis water, and wipe them carefully with a Kimwipe. Lower the probes into the
sample solution and note the reading when relatively stable. When you are finished rinse
the electrodes and place them in the soaking solution.
(d) Titration with the pH meter. So as to avoid any splashing, carefully place a
well-rinsed magnetic stirring bar in the beaker containing your unknown acid solution.
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Position the beaker on the magnetic stirrer and lower the electrodes into the solution
being careful so that the electrodes are not hit by the stirring bar. The electrodes should be
positioned to one side of the beaker so as to provide easy access for your buret tip in the
titration operation.
Clamp your buret in position with the tip close enough to the surface of the
solution so that there is no danger of loss by splashing as the base solution is run into the
beaker. The buret should already have been properly filled with your standard base
solution, the tip readied for titration, and the meniscus level near, but below, the "O"
mark. Turn the magnetic stirrer ON and adjust the speed so as to give good mixing, but
not so fast as to risk splashing.
Record the initial buret reading and the initial pH of the solution. Add standard
NaOH solution in such quantity (but in no case more than 5 ml) as to give a change of
about 0.2 or 0.3 pH units. Record the new buret reading and the corresponding pH of the
solution, allowing sufficient mixing time after the addition so that you know you are
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observing the correct pH. Continue this stepwise addition of standard NaOH and the
recording of the volumes and related pH values until you are well past the endpoint; very
small additions must be made near the endpoint.
On the completion of the titration, turn the magnetic stirrer OFF, remove your
buret and lift the electrodes to a raised position. Rinse the electrodes with reverse osmosis
water (catching the rinsings in your titration solution), put the magnetic stirring bar in the
designated place, discard your titration solution, and replace the electrodes in the beaker
of soaking water. Rinse your buret with several portions of reverse osmosis water and
finally fill it with reverse osmosis water and stopper it for storage.
CALCULATIONS
1. Calculate the molarity of your NaOH solution from the titration of weighed
samples of KHP. Calculate the standard deviation of the molarity values and the relative
95% confidence interval of the mean in parts per thousand (p.p.t.).
2. Calculate the percentage of KHP in your unknown from the titration of weighed
samples of unknown. Calculate the standard deviation of the calculated percentages, and
the relative 95% confidence interval of the mean in p.p.t.
3. Graph the pH of the solution of your unknown acid (as ordinate) as a function
of the volume of standard NaOH solution added (as abscissa). Draw this graph on
millimeter graph paper using a convenient and sufficiently large scale so that the majority
of the graph paper is used. Draw a smooth curve that best represents all of the points; do
not draw a series of straight lines connecting each point in succession. Label both axes
carefully, including the units. Draw a vertical line corresponding to the volume of base
used at the equivalence point; draw another vertical line corresponding to half the volume
of base used at the equivalence point. Draw horizontal lines corresponding to the pH at
the equivalence point and at half-titration.
4. Calculate the molecular weight of your unknown acid and the value of its
ionization constant, Ka. Use the weight of your sample and the necessary data from the
graph prepared in 3 above.
5. Turn in the report sheet summarizing your results. Be sure to show your
sample calculations and to answer the question on the report.
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Name__________________________________Lab Section
Date Report Submitted__________________Sample No.___________
VOLUMETRIC ACID DETERMINATION
1.
Standardization of NaOH Solution
Sample No
Wt. of KHP
Moles KHP
Vol. of NaOH
Molarity
1.
__________
__________
__________
__________
2.
__________
__________
__________
__________
3.
__________
__________
__________
__________
4.
__________
__________
__________
__________
Average __________
Standard deviation __________
Relative 95% confidence interval of the mean, p.p.t __________
2.
Percentage of KHP in Unknown
Sample No.
Sample Wt.
Vol. NaOH
Moles KHP
%KHP
1.
__________
__________
__________
__________
2.
__________
__________
__________
__________
3.
__________
__________
__________
__________
4.
__________
__________
__________
__________
Average ___________
Standard deviation ___________
Relative 95% confidence interval of the mean, p.p.t ___________
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Name___________________________________Sample No.__________
VOLUMETRIC ACID DETERMINATION
(report sheet for Chemistry 51)
[You are only performing part 3 of the experiment.]
3.
Characterization of a Weak Acid
Weight of sample
________________
pH at the endpoint
________________
pH at half-titration
________________
Volume of NaOH at endpoint
________________
Ka of acid
________________
Molecular weight of acid
________________
4. Show on the back of the report sheet, sample calculations for the
calculations of the molecular weight and Ka in 3. Attach a properly
labeled graph of your titration curve.
5.
Solve the following problem.
Using the sheet on indicators with each setup and information in your text about
indicators, select an indicator which would be suitable for the titration of your
unknown weak acid. Show on your graph the pH range in which the indicator you
selected would change color and indicate on the graph what the colors are.
Indicator_______________________
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