CHE 226 EXPERIMENT 3
Complexometric Titration of Zn(II)
D. A. Skoog, D. M. West, F. J. Holler, and S. R. Crouch, Analytical
Chemistry: An Introduction, 7th ed. Chapter 15, pp. 345-381.
Submit a clean 250-mL volumetric flask
to the instructor so that an unknown zinc solution may be issued.
Your name, section number, and your locker number should be written
legibly on this flask. Note that the flask must be turned in at
least 1 lab period before you plan to do the experiment, so that
the Teaching Assistants will have time to prepare the unknown.
Preparation of Solutions
USE ONLY DEIONIZED WATER (NOT
DISTILLED WATER!) IN THIS EXPERIMENT.
1. EDTA, 0.01 M. (Prepare at least
one day ahead of time to make sure that the solute is dissolved.)
Dissolve about 3.8 g of the dihydrate of the disodium salt
and 0.1 g MgCl2 in 1 liter of water. Store in a plastic
bottle. A small amount of sodium hydroxide may be added
if there is any difficulty in dissolving the EDTA. Try not to
exceed 3.8 g of Na2H2Y•2H2O because
much more than this dissolves only with difficulty. The EDTA solution
should be filtered using suction filtration. See TA for the apparatus.
NOTE: Break the suction before you turn off the water flow
on the vacuum aspirator.
2. Buffer, pH 10. Each titration
will require the addition of pH 10 ammonia buffer. The stock buffer
solution has been prepared for you, and the appropriate quantity
is dispensed directly into your titration flask from the "Repipet"
repetitive dispenser located in hood #7. The buffer should be
added immediately before you titrate a sample.
3. Calcium Standard Solution. A
Ca2+ solution is prepared as the primary standard.
Obtain from the TA approximately 0.7 g of predried analytical-reagent-grade
CaCO3. Accurately weigh a 0.25-g sample by
difference into a 100-mL beaker. Add about 25 mL water and
then add dilute HCl dropwise until the sample dissolves, then
add 2 drops more. Mild heating will speed dissolution if necessary.
Transfer quantitatively to a 250-mL volumetric flask (rinse
well with water) and carefully dilute to the mark. (eye
dropper!) Mix thoroughly. Because this Ca2+ standard
solution is used to standardize the EDTA titrant, it must be prepared
very carefully so you know its exact molarity, and therefore the
exact (to ± 0.1 mg) mass of Ca2CO3
Standardization of EDTA Solution
1. Pipet 25-mL aliquots of the standard
Ca2+ solution into three or four 250-mL Erlenmeyer
flasks. Remember that each aliquot contains one-tenth of the
CaCO3 weighed out to prepare the standard solution.
2. Take each sample to completion before
starting next sample. Add 7-8 mL of pH 10 buffer, 15 mL of water,
and 3 drops of Eriochrome Black T and titrate immediately with
EDTA until the light red solution turns a LIGHT SKY BLUE.
Titrations must be performed swiftly (but
carefully) because ammonia will evaporate and thus the pH of the
solution will change. In general, the faster the titrations are
performed the better the results will be (as long as the endpoint
is not overshot due to the speed). It is advantageous to perform
a trial titration to locate the approximate endpoint and to observe
the color change. In succeeding titrations, titrate very rapidly
to within about 1 or 2 mL of the endpoint, then titrate very carefully
(dropwise) to the endpoint. (See Note 1 at the end of the report.)
3. Calculate the molarity of the EDTA
from the volume of EDTA used in the titration of each aliquot.
The values (MEDTA and titration
volumes) should all agree very closely. If not, titrate additional
aliquots until agreement is reached, and any spurious values can
be rejected with confidence.
Determination of Zinc
1. Dilute your unknown sample in the 250-mL
volumetric flask to the mark with water. Mix thoroughly.
2. Pipet 25.00-mL aliquots into 250-mL
erlenmeyer flasks. Add 15 mL of water, 9-10 mL of pH 10 buffer,
and 2 drops of Eriochrome Black T immediately prior to titrating
3. Titrate with standardized EDTA until
the red solution turns blue. (See note at the end of the report.)
4. Calculate the number of milligrams
of zinc in the total sample. Remember that each aliquot
represents one-tenth of the total sample volume.
= MEDTA x (V, mL EDTA) x (molar
mass Zn) x 10
Hazardous Waste Disposal
1. Empty all titrated Ca and Zn solutions
into the proper Hazardous Waste Bottle for this experiment.
2. When completely done with the
experiment mix any remaining EDTA titrant and any remaining Ca
stock solution together in a large beaker. Pour down the drain
with copious amounts of cold tap water. The two solutions are
slightly basic and slightly acidic, respectively; when mixed,
they will be near neutral. There are also no toxic chemicals present,
so disposal directly down the drain is allowable and safe.
1. Eriochrome Black T Indicator.
The color change of Eriochrome black T at the endpoint is rather
subtle. It is not an abrupt change from bright red to a
dark blue; but rather it is from a light red (or pink) to a pale
blue. At least one trial titration is recommended. (You can always
discard a "bad" value when you know there is a definite
reason for its being bad.)
If you have trouble distinguishing the
endpoint, a "before" and an "after" flask
are recommended. Prepare two 250-mL flasks exactly the same as
for the samples, except use distilled water instead of a sample;
add additional distilled water to approximately equal the volume
of EDTA titrant that would be titrated into the flask for the
sample. Add the indicator. To one flask (the "after"
the endpoint flask) add a small amount of EDTA to just past the
color change at the endpoint. Stopper the flasks and keep them
nearby for comparison of the colors. Titrate against a white background
for better discrimination of colors. (Extra flasks can be checked
out from the stockroom.)
Sometimes the Eriochrome black T solution
goes bad because of air oxidation. If the endpoints seem very
indistinct to you, try a fresh bottle of indicator. Alternatively,
try adding a small amount of solid Eriochrome black T mixture
(1 g indicator ground with 100 g NaCl); a small amount on the
end of a spatula is sufficient.
2. pH 10 Ammonia Buffer: Dissolve
64.0 g of ammonium chloride in 600 mL of concentrated ammonia.
Slowly and carefully add 400 mL deionized water. This should
be sufficient for over 120 titrations.
This page was last updated August 28, 2003