Experiment 33
Quantitative Analysis of a KHP Unknown
KHP is an abbreviation for potassium hydrogen phthalate (see structure), a salt that can
be of primary standard purity.
O
C
C-OK
OH
O
As a reactive, monoprotic acid, it will neutralize an equivalent amount of dilute NaOH
solution. The goal of this experiment is to test your quantitative technique. Each person will be
issued a solid containing an unknown percentage of KHP. By titrating accurately - - weighed
samples with standardized NaOH, one can determine the KHP composition. The titration
reaction in H2O is:
HP- + OH-
H2O + P-2
Procedure
Prepare about 300 ml of well-mixed 0.12 M NaOH from the available NaOH stock
solution.1
(Be certain to add the washings from your graduated cylinder.) Rinse a buret with at
least three 10 mL portions of the NaOH, fill, and record the initial reading to the nearest 0.05
mL. Weigh to the nearest one mg about 0.7 g sample of primary standard KHP and dissolve it
with 50 ml H2O in a 250 ml Erlenmeyer flask.2 Add 3 drops phenophthalein as indicator and
titrate with the NaOH until a faint pink color persists.3
1
Keep NaOH solutions well-capped when storing to minimize deterioration by CO2 from the air. For this reason,
“standard” NaOH solutions should be checked periodically by titration.
2
KHP samples may dissolve slowly; give them enough time to.
3
Swirl the solution gently near the endpoint because of the potential CO2 problem mentioned in footnote 1. The
endpoint may continuously blur:
CO2 + H2O
HCO 3- + H+.
Record this final reading and determine the volume of NaOH used. Repeat the
standardization with two more KHP samples. Adjust the sample weights, if necessary, so
that between 25 and 50 ml NaOH is used. Calculate the average molarity of NaOH from
the titrations, and the average deviation. If this precision exceeds 5 parts per thousand,
repeat the standardization until this criterion is achieved.
With your NaOH now standardized, obtain an unknown from the instructor. (Be
certain to record its number.) Accurately weigh about 1 g of the unknown, then dissolve
it, add indicator, and titrate as in the standardization procedure.2 Do at least two more
analyses, adjusting the samples sizes if the first analysis did not require 25 to 50 ml
- average deviation. (The precision
NaOH. Calculate the average weight % KHP +
should be as in the standardization.) At the end of the experiment rinse the buret well, fill
it with distilled H2O, and return it to its container.4
4
If you intend to do Experiment 40, save your NaOH standard solution
QUANTITATIVE ANALYSIS REPORT OF A KHP UNKNOWN
Name
Date
Sample Number
Average of % KHP
+/Average Deviation
Molar mass of KHP (potassium hydrogen phthalate, KHC8H4O4)
Data and Results
Table 1. Standardization of NaOH
Trial
1
2
3
4
2
3
4
Grams KHP
Moles KHP
mL NaOH used
(4 sig. figs.)
Molarity
(4 sig. figs)
Average molarity
+/-deviation
Table 2. Unknown KHP Sample
Trial
1
Grams unknown
mL NaOH used
Moles NaOH
used
Moles KHP in
unknown
% (m/m) KHP in
unknown (4 sig.
figs.)
Average % KHP
(4 sig. figs)