ACID-BASE TITRATIONS 1:
Standardization of NaOH and Titration of an Unknown Weak Acid
You should review the
section in your textbook that describes proper technique for analytical
weighing and for titration. As is
usually the case, there are subtleties and pitfalls for the uninformed.
In this experiment, you will use indicator-based
titrations to standardize the stock NaOH solution
that your section will be using this term.
There is a little bit of work required and you probably would rather not
go through this procedure again in two weeks so try to be judicious in your use
of the stock solutions and make sure that the carboys are completely off after
you are through using them. You will then use the standardized base in the
titration of an unknown acid.
Solid sodium hydroxide is hygroscopic, which means
that it absorbs moisture from the atmosphere. Once it has a little moisture it
also absorbs carbon dioxide which is always present in air. The reaction is:
2 NaOH (s) + CO2 (g) à Na2CO3 (aq)
+ H2O (l)
Therefore solid reagent grade sodium hydroxide is not
pure enough to weigh directly. Furthermore, the carbonate ion interferes in
acid-base titrations because 1) it is a base, and 2) it tends to make the color
change at the end point less sharp. This
reaction also takes place in the aqueous phase, where sodium hydroxide in
solution is converted to sodium carbonate. This can change the concentration of
standard. solutions if steps are not taken to minimize
the carbon dioxide uptake. (Note 1)
It is therefore necessary to prepare sodium hydroxide solutions in such a way
that they are free of carbonate impurity. The most convenient method takes
advantage of the fact that sodium carbonate is insoluble in 50 % NaOH solution. Carbonate free solutions can be obtained simply
by diluting 50 % NaOH. (Note 2) Other methods are discussed in the textbook.
The
concentration of the 1 M NaOH solution provided to
you will not be known accurately. It is therefore necessary to measure the concentration
of a diluted solution by using it to titrate a known
amount of acid. This is called standardization of the solution. For the
titrations, you will need to prepare some 0.1 M NaOH
by using your calibrated volumetric pipet to transfer 25 mL of the nominally
1.0 M NaOH from your section’s carboy to the
calibrated 250 mL volumetric flask and then dilute to the mark with the
distilled water. After standardizing this dilute solution, you may calculate
the concentration of the standardized 1 M solution in the carboy (it will be about
10 times more concentrated.)
Primary
standard potassium hydrogen phthalate will be used to standardize the sodium
hydroxide. The acid is a crystalline solid and must be dried for 2 hours at 110°C
to remove adsorbed water. (Note 3) In order to
keep it dry it will be stored in a desiccator. (Note 4)
Calculate
the weight of primary standard potassium hydrogen phthalate (KHP = KHC8H4O4
MW = 204.2 g/mol) required to give a 35 mL titration using 0.10 M NaOH. Using the
analytical balance weigh out three samples of primary standard potassium
hydrogen phthalate into 250-mL Erlenmeyer flasks. Add about 35 mL of water to
dissolve the solid. Test your buret to make sure it runs clean. Rinse out your
buret, and its tip, three times with small portions of your NaOH
solution. (Note 5) Fill
the buret and make sure there is no bubble of air under the stopcock. Set the
initial buret reading to 0.00 mL, or if you prefer, run the liquid level down
below zero and read the initial volume. (Note 6)
Add 5
drops of 0.2 % phenolphthalein indicator solution to the first flask of potassium
hydrogen phthalate solution and titrate. Just before reaching the endpoint,
rinse the inside wall of the flask with a stream of distilled water from your wash
bottle. The end point is the first faint pink color that persists for 20 – 30 seconds.
(Note 7) If the titration
was done rapidly wait about a minute before reading the buret to allow time for
liquid on the upper walls to drain down. Read the buret to the nearest 0.01 mL.
Titrate the remaining solutions. Calculate the molarity of
your NaOH solution, the standard deviation and the
relative standard deviation. If you have done careful work your relative
standard deviation should be less than 5 parts per thousand. Otherwise you
should repeat the standardization.
Test
your 25-mL pipet to be sure it runs clean.
Obtain
about 150 mL of unknown acid in a 250-mL beaker. Rinse your 25-mL pipet with
three small portions of this solution and then pipet 25.00 mL into a 250-mL
Erlenmeyer flask. Add 5 drops of phenolphthalein indicator solution and titrate
with your standard 0.1 M NaOH solution. Repeat the
titration until you are confident that you have three good results. Calculate
the average molarity of the acid, and the standard deviation and the relative
standard deviation of your replicate results.
PreLab
calculations – with correct answers – must be submitted before you can begin
work on the experiment. At the end of
the day, you must
present YOUR results for the standardization of the NaOH
and for the titrations of the unknown acid to your lab TA before leaving. As
usual, results include the mean and relative standard deviation (‰) of your
replicate determinations of the concentrations of the solutions.
1) The water used to make standard solutions must be
carbonate free. If there is any doubt it should be boiled before use in order
to drive off dissolved carbon dioxide.
2) 50 % NaOH solution
frequently contains pieces of insoluble sodium carbonate. Usually these can be
avoided or fished out but if this is not possible it is necessary to filter the
solution through a wad of glass wool.
3) Standard operating procedure is to dry for one hour
at 110 °C. However, in a large class there is so much opening and closing of
the oven that the temperature cannot be maintained for the shorter period of
time. If the oven is not busy then one hour drying is sufficient. (Check with your TA because this might have
been done already.)
4) Check your desiccator to be sure it has good
desiccant in the bottom and that the lid seals.
5) Be careful to conserve the 1.0 M sodium hydroxide
solution. This solution will also be used in subsequent experiments. If you run
out we must prepare more and you will have to repeat the standardization.
6) If the liquid level inside the buret is changed
rapidly allow a minute for drainage of the upper wall before making a reading.
7) Absorption of atmospheric carbon dioxide causes the
color to fade over several minutes time.