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Titrimetric Determination of KHP

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					                              Titrimetric Determination of KHP


Introduction
       In this experiment you are to standardize a NaOH solution and use that solution to
determine the weight percent of KHP in an unknown sample. With care, it is possible to
determine the weight percent of KHP to an accuracy within one part-per-thousand.


Preparation for the Experiment
        In a previous laboratory period, you should have dried between 5 and 6 grams of primary
standard grade potassium hydrogen phthalate (KHP) in a weighing bottle with its top off at 105o
in the oven. After drying the primary standard KHP, allow the standard to cool in your desicca-
tor.
        Also obtain your KHP unknown sample and dry the entire sample in a weighing bottle at
     o
105 . Cool it in your desiccator as described above. Mark the two bottles to clearly indicate the
identities of the contents.


Procedure
Sodium Hydroxide Solution. To make one liter of about 0.1 N NaOH solution, fill your plastic
one-liter bottle to approximately 90% full with distilled water. (Leave a small air space to
facilitate mixing.) In the hood, add 5.0 ml (measured from 10 ml graduated cylinder) of 50%
NaOH to the bottle. Seal the bottle and shake. Mix well by repeatedly inverting and shaking.


Standardization of the NaOH Solution
        You will be using the 0.1 M NaOH solution prepared in a previous experiment. This
solution will be standardized by titrating primary standard KHP to a phenolphthalein end-point.
Weigh by difference 0.6 to 0.8 grams of primary standard grade potassium hydrogen phthalate
(KHP) into a 250 ml or 300 ml flask. Remember that to weigh by difference means to record the
weight of the weighing bottle containing your primary standard grade KHP from the analytical
balance, spoon some of the primary standard grade KHP sample into the Erlenmeyer flask using
your micro-spatula, and then to re-weigh the weighing bottle. The difference in the two weights
is the weight of KHP in your flask. Do not weigh samples on paper and do not weigh the Erlen-
meyer flask on the analytical balance. You may use your distilled water wash bottle to rinse
down the sides of the Erlenmeyer flask, if necessary. All sample that leaves the weighing
bottle must end up in the Erlenmeyer flask. Do not lose any material! The sample weight
must be correct and recorded in your laboratory notebook to the nearest 0.1 milligram; therefore,
each weighing must be made to the fourth decimal place. This weight of 0.6 to 0.8 grams is
about 3 to 4 milliequivalents (meq); and it should require 30 to 40 ml of 0.1 M Na0H for titration
to the phenolphthalein endpoint.
        Dissolve the KHP in 50 to 75 ml of distilled water, warming if necessary. Add two drops
Titrimetric Determination of KHP                                                            page 2


of phenolphthalein indicator solution to the flask. Titrate the colorless solution of your sample
with your 0.1 M NaOH to the first change from no color to a faint pink endpoint color that
persists after the solution is thoroughly mixed. For a more precise endpoint determination, split
drops at the endpoint to determine the smallest volume increment necessary to change the
solution from no color to the first faint pink color. Each buret reading must be made to the
nearest 0.01 ml by estimating the value between each 0.10 ml marking on the barrel.
         Titrate at least two additional samples, making the necessary adjustments in the weight of
the KHP so that ideally between 30 and 45 ml of the 0.1 M NaOH solution is required for each
titration. Split drops at the endpoint to locate the color change more precisely. This procedure
should produce the same faint pink color at the endpoint for each titration. Calculate the nor-
mality of the sodium hydroxide for each titration. A precision of one part-per-thousand (ppt) is
expected. Repeat until good precision is obtained. Consult the instructor for assistance if your
precision is above three ppt.


Titration of the KHP Unknown
        Weigh out as accurately as possible about between 1.0 and 1.1 g of your dried KHP
unknown sample into a 250 ml flask. Dissolve the unknown in 50 to 75 ml of distilled water,
warming if necessary. Add 2 drops of phenolphthalein indicator, and titrate the sample with
your standardized NaOH solution to the first faint pink endpoint. Follow the same titration
procedures described above that you used for the standardization of your NaOH solution.


    Weigh out two more samples of unknown, using (if possible) samples large enough to require
at least 25 ml of NaOH in each titration. Titrate each sample as you did the first sample.
Calculate the results as the weight percentage of potassium hydrogen phthalate in the original
sample. Agreement to within one parts per thousand is ideal. You can titrate more than three
samples if you have enough sample; however, no additional amount of sample can be given. So,
do the best you can with what you have.


Results
        Determine the molarity of the NaOH solution and report the mean, standard deviation,
and relative standard deviation of the concentration.
        Determine the weight percent of KHP in the unknown sample and report the mean,
standard deviation, and relative standard deviation of the weigh percent. (The uncertainties
require a propagation of error calculation.)
                                     Laboratory Report
Name: ____________________


Standardization of NaOH
                             1st Titration        2nd Titration   3rd Titration
      mass KHP (g)           __________           __________      __________
      volume titrant (mL)    __________           __________      __________
      concentration (M)      __________           __________      __________


      mean concentration NaOH (M)            __________
      standard deviation (M)                 __________
      relative standard deviation (ppt)      __________



Analysis of the Unknown Sample
                          1st Titration           2nd Titration   3rd Titration
      mass unknown (g)    __________              __________      __________
      volume titrant (mL) __________              __________      __________
      Vt/m (mL/g)         __________              __________      __________
      weight percent KHP __________               __________      __________


      mean Vt/m (mL/g)                       __________
      standard deviation                     __________
      relative standard deviation (ppt)      __________


      mean weight percent KHP                __________
      standard deviation                     __________
      relative standard deviation (ppt)      __________

				
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Description: Titrimetric Determination of KHP