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Experiment 4
QUANTITATIVE ACID DETERMINATION BY NaOH TITRATION
REFERENCES: Harris textbook, Chapters 7, 11; read especially pages 121-124.
INTRODUCTION
This experiment is a simple acid-base titration. You will prepare ~0.1 M NaOH solution and
standardize this solution against pure primary standard potassium hydrogen phthalate, KHP. This is how to
make a basic solution of precisely known concentration (a standard solution). The unknown, consisting of a
mixture of KHP and inert materials, is then titrated with the standardized NaOH. Since you know the weight of
the unknown, the concentration of NaOH solution, and the volume of titrant needed to reach the endpoint, you
have the necessary information to calculate % KHP in your unknown. (KHP and NaOH react 1:1, Molar Mass
of KHP = 204.2 mg/mmol).
SAMPLES
Dry about 4 g of primary standard KHP in an open weighing bottle at 110 C for 1-2 hours using the
same procedure as always. KHP is an organic compound which can decompose, so do not allow the sample
to remain in the oven for the week. Allow to cool before weighing. Also obtain and dry your unknown in the
same manner. Do this during the lab period prior to this one.
PROCEDURE
Boil 1 L of deionized water using 500 mL beakers. Use ice to cool down to room temperature.
Using a 1-L polyethylene bottle, prepare an approximately 0.12 M solution of NaOH using
concentrated NaOH, usually 19 M and the boiled, cooled deionized water. Make sure your solution is well
mixed by first adding about 500 mL of water, then the proper amount of concentrated NaOH solution. Cap
the bottle and swirl to mix. Finally, add more water to nearly fill the bottle. Cap and mix thoroughly.
CAUTION: USE EXTREME CARE IN HANDLING CONCENTRATED NaOH. IF the
reagent comes into contact with your skin, IMMEDIATELY flush the area with copious amounts of water.
This solution is not yet a standard solution. It will be standardized in the next step by titrating with KHP.
It is therefore not necessary to try to make exactly 0.12 M NaOH, it's not even possible! You will
determine the exact concentration of this solution momentarily.
Standardization of NaOH solution using primary standard KHP. Weigh accurately (by difference)
three portions of the dried KHP, about 0.6-0.8 g each, and transfer to clean (not necessarily dry) Erlenmeyer
flasks. Dissolve each sample in about 50 mL distilled water.
Rinse your buret with three small portions of the ~0.1 M NaOH solution, fill, and adjust to near zero.
If you have water or something else in the buret, then when you add your titrant to the buret it is not the
same concentration as it was in the bottle. This is a common mistake and is avoided by rinsing your buret
with the solution you will titrate with as just described in the procedure. Record the initial volume.
Add 2-3 drops of phenolphthalein indicator to each KHP sample and titrate with ~0.1 M NaOH to a
faint pink end point. The color should persist at least 30 seconds. Split drops at the end of the titration.
Record the final volume. Remember that you should estimate the last digit, so record the volume in the
buret to the nearest 0.01 mL. You now have the data you need to determine the concentration of your
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NaOH solution. In other words, once you've done the calculations, you have standardized your NaOH
solution.
To perform the quantitative analysis of your unknown, weigh accurately three portions of the dried
unknown (split the unknown in three portions), and transfer to clean Erlenmeyer flasks. Dissolve each sample
in about 50 mL distilled water, warming if necessary. Add 2-3 drops of phenolphthalein indicator to each
unknown sample and titrate with the standardized NaOH to a faint pink end point. The color should persist at
least 30 seconds. Split drops at the end of the titration. Record the initial and final volumes of NaOH for each
titration.
For next week: Dry 1-2 g Na2CO3 in the oven at 110 °C for at least two hours (or leave in the oven
until next week). Also obtain and dry your unknown in the same manner. Unlike KHP, Na2CO3 will not
decompose in a week at 110oC.
Fill out and hand in the results sheet.
Error Analysis Hints
In the following discussion the term “uncertainty” will generally refer to the standard deviation.
A. 95% confidence interval for the NaOH solution molarity
There are two ways to approach this. The first is easy. Simply determine the NaOH molarity for
each run and use the resulting answers to compute the absolute standard deviation, confidence limit, and
confidence interval.
The second approach is a little trickier to understand but is conceptually useful. If you think about the
analyses you carried out, each one had a unique mass of KHP. The amount of NaOH solution used correlated
to the KHP amount. In other words, the ratio of the mass of KHP to the volume of NaOH solution should,
theoretically, have been constant. Most importantly, since all of the significant uncertainty is from the mass
and volume measurements, the relative uncertainty in the mass/volume ratio is the same as the relative
uncertainty in the molarity of NaOH. So, first compute the (mass KHP to volume NaOH solution) ratio for
each run, then determine the relative uncertainty associated with this ratio. This is the same as the relative
uncertainty in the molarity of the NaOH solution. It is a trivial matter to convert this to the absolute
uncertainty of your NaOH solution (using the mean value of the NaOH molarity from all runs).
Try both approaches—each should result in the same absolute uncertainty in the molarity of
the NaOH solution.
B. 95% confidence interval for the %KHP in the Unknown
The overall uncertainty in your final %KHP answer is derived from both the uncertainty of the NaOH
molarity (determined above) as well as the titrations of your unknown. Fortunately, you can independently
determine the uncertainty of the second set of titrations by considering, as described above, the relative
uncertainty in the mass unknown to volume NaOH ratio.
Go ahead and do this for your unknown runs. Which set of titrations shows better precision?
(Compare the relative uncertainties)
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Now, how might we propagate these uncertainties to arrive at the overall relative uncertainty for the
% KHP answer? This turns out to be easy, because when you calculate the final % KHP value you are
multiplying the NaOH solution molarity with data from the second set of titrations. Confirm this by going
ahead and using the mean NaOH molarity with each unknown run to find individual %KHP answers. Notice
how all of the steps are multiplication and/or division? This means we can use the multiplication rule for
propagation of uncertainty (see p. 45 in text. Harris uses the percent relative uncertainty, but it is not
necessary to convert the relative uncertainty to percent relative uncertainty) to determine our overall
uncertainty:
R%KHP = [(RNaOH Molarity)2 + (RUnknown Titrations)2]0.5
Where R%KHP, RNaOHMolarity, and Rmass/volume are the relative uncertainties in the mean %KHP answer, the
NaOH solution molarity (from part A), and the mass unknown to volume NaOH solution ratio of the 2nd
(unknown) set of titrations (first paragraph in part B).
After you find the relative uncertainty in your %KHP answer, convert and report the absolute
uncertainty. To do this you will need, of course, to first determine the mean %KHP value by simply
averaging the individual %KHP answers obtained from each unknown run. This gives the standard deviation.
To compute the 95 % confidence interval use this standard deviation and the confidence interval equation
with n-1 degrees of freedom, where the degrees of freedom are the number of titrations of the unknown that
were carried out.
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KHP Acid-Base Titration Results Sheet
Name: _______________________________
Unknown Number:
________
KHP Mass (mg)
% KHP:
________ (Filled in by grader)
Standardization Titrations
NaOH Volume (mL)
Average NaOH Molarity ± absolute standard deviation:
NaOH Molarity (mmol/mL)
______________
Relative uncertainty in NaOH Molarity: _____________
Unknown Mass (mg)
Unknown Titrations
NaOH Volume (mL)
Mass % KHP
Relative uncertainty in unknown titrations: _______________________
Propagated relative uncertainty associated with overall analysis: _____________________
Average % KHP in unknown (reported result):
___________________
Absolute standard deviation of % KHP in unknown:
_______________________
95% Confidence Interval of % KHP in unknown:
_______________
Grade _________________