DETERMINATION OF CHLORIDE BY PRECIPITATION
TITRATION
BACKGROUND
In a precipitation titration, the species being titrated forms a sparingly soluble compound
with the titrant. This experiment involves the titration of chloride ions with silver ions.
The indicator senses the surface charge of the precipitate, and when excess silver ions are
present in the solution after the complete reaction with free chloride ions the indicator
undergoes a color change.
Such titrations can be used for the determination of chloride in an unknown sample or
certain other anions that form sparingly soluble silver salts, and for silver ion.
PROCEDURE
PART I. Preparation of a 0.05 M solution of silver nitrate
Silver nitrate (AgNO 3) can be weighed as a primary standard. However, a solution of
approximately the desired molarity is usually prepared and then standardized using a suitable
indicator. Prepare 250 mL of 0.05 M silver nitrate solution in a volumetric flask. Protect the
solution from direct light by wrapping the flask with an aluminum foil. (Caution: Silver
nitrate is very corrosive to skin and clothes. It produces a dark, almost permanent mark on
skin. If you spill any silver nitrate on your skin or clothes, wash the exposed areas with
plenty of tap water.) Silver nitrate is also very expensive, so use it wisely.
PART II. Standardization of silver nitrate
Analytical grade sodium or potassium chloride can be used as a primary standard for silver
nitrate standardization.
Fajans Method: Weigh precisely three samples (comprising ca. 1.0-1.5 mmol) of pure, dry
sodium or potassium chloride into 250-mL Erlenmeyer flasks. Dissolve each sample in 50.0
mL of distilled water and add 2 drops of dichlorofluorescein indicator and 0.1 g of dextrin.
The function of dextrin is to prevent the coagulation of colloidal silver chloride. Titrate the
first sample with silver nitrate to the point where the color of the dispersed silver chloride
changes from yellowish white to a definite pink. The endpoint is easier to detect in diffuse
light. The color change is reversible and back titration can be done with a standard NaCl
solution if desired. Titrate the other two samples in the same manner for a total of three
reliable measurements. Calculate the molarity of the silver nitrate solution along with the
%RSD. The %RSD of at least three good titrations should not exceed 0.5%.
Do a Q-test to reject the suspected data if more than three titrations are necessary. Finally,
calculate the range of true mean at 95% CL.
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PART III. Analysis of unknown soluble chloride (Unknown from instructor)
Obtain the unknown soluble chloride sample from the instructor. Chloride can be
determined by titration with standard silver nitrate. Start the titration with ca. 0.1 g of
unknown sample (in a 250 mL Erlenmeyer flask) and change the amount so as to obtain 3040 mL of titrant. Dissolve the sample in approximately 50 mL of distilled water. Obtain at
least three reliable measurements and calculate the weight percent of chloride ion, with its
%RSD, and µ at 95% CL in the unknown sample.
(Alternative unknown)
Determination of Total Soluble Chloride in Comme rcial Chip Snacks by
Potentiometric Titration of Chloride with Standard Ag Procedure
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Weigh 10.000 g of chips into a beaker. Add 40.0 mL of water and steep for few minutes
with occasional gentle swirling. Filter the supernatant liquid into a 100.0 mL volumetric
flask. Add another 40.0 mL of water into the beaker. Filter the same way. Add another
30.0 mL of water in the beaker and filter it again until the filter volume is up to the mark of
the volumetric flask. This is your corn-chip water extract for Cl - determination. Titrate 10.0
mL of the aliquot with standard silver nitrate solution that you have prepared by the Fajans
method as described in the lab supplement.
For potentiometric titration, mix 2.00 mL of the extract with 40.00 mL (with measuring
cylinder) of deionized water and titrate with your standard silver nitrate solution. The
following is an example of potentiometric titration. The derivative plot shows how to obtain
the end point volume.
Example calculation
The end point volume from the derivative plot is 5.00 mL (see plot on the following page).
Knowing that moles of chloride equals moles of silver, moles of chloride =
0.005 L x 0.0100 M = 5.0 x 10 -5 mol x 35 g/mol = 175.0 x 10 -5 g
which is present in 2.0 mL of aliquot.
Therefore, 100.0 mL extract has 175.0 x 10 -3 g total chloride (i.e., 175.0 mg). Therefore,
percent chloride in the chip = 0.175g *100/10.0g = 1.75 % by weight.
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Example spreadsheet (part) and data plots
QUESTIONS
1. Identify three factors that must be considered in choosing a proper adsorption indicator
for a precipitation titration. Explain briefly.
2. What would happen if dextrin were not used in the titration? Explain.
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