Environmental Analysis 2002
Argentometric Titration for Chloride Ion
(Chloride titration based on measurement of silver)
Today’s lab is the well-known Mohr titration. It is an example of a precipitation titration
that relies on competitive equilibria for detection of the end point. The end point of the
titration occurs after the equivalence point, so a blank is required to obtain the correct volume
of titrant. You will enjoy it.
You are now analyzing anion content of water samples from central Washington lakes
using ion chromatography. It will ease your workload to know the chloride ion concentrations
from another method so you will have a good idea of what dilution to use. It is also nice to
have two analyses for chloride, because chloride is the ion most likely to suffer from
interference in the ion chromatograph.
Theory. Chloride ion reacts with silver ion to make silver chloride.
AgCl (s) ↔ Ag+ + Cl- Ksp = 1 x 10-10
If silver nitrate solution is used to titrate a solution containing chloride ion, a white precipitate
is formed. The chloride ion concentration decreases as the titration progresses and the silver
ion concentration increases. At the equivalence point the concentrations of silver and
chloride in solution are equal and their concentrations described by the solubility product.
Ignoring ionic strength effects, calculate the silver ion concentration at the equivalence point.
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Chromate ion is used as an end point indicator. Chromate forms a red precipitate when it
reacts with silver ion.
Ag2CrO4 (red, s) ↔ 2Ag+ + CrO42- Ksp = 2 x 10-12
We will use 0.002M chromate in your sample to detect the end point. Ignoring ionic strength
effects, calculate the silver ion concentration silver chromate is in equilibrium with 0.002M
chromate ion. ____________________________________________
As long as chloride ion remains in solution, the silver ion concentration is too low to
cause silver chromate to precipitate. Once the chloride ions are titrated, the silver ion
concentration rises rapidly allowing the silver chromate to precipitate and turning the
precipitate red to signal the endpoint of the titration.
Since the end point comes after the equivalence point, it is necessary to run a blank to
determine the excess volume of silver nitrate needed for the analyst to observe the endpoint.
The blank volume is then subtracted from each titration to obtain the volume of titrant needed
for titrating the chloride sample. You will use a calcium carbonate suspension for a blank.
With potassium chromate indicator added to the blank, you will carefully titrate the
suspension until you observe a color change that you can precisely repeat by titration. You
will record the volume of titrant needed for the blank to subtract from later titrations and you
will save the blank sample as a color comparator for your chloride titrations.
Method. The method is based on Standard Method 4500-Cl B and EPA Method 9253
Chloride (Titrametric – Silver Nitrate).
Blank titration:
1. Add 50 mL water to each of two 125 mL Erlenmeyer flasks.
2.
3.
4.
5.
To each flask add 3 drops of CaCO3 suspension (30 g/100 mL).
To each flask add 4 drops 0.5 M K2CrO4.
Fill a burette with 0.01 M AgNO3.
Carefully add AgNO3 solution dropwise to the calcium carbonate suspension with constant
stirring until you observe the color change. Record the volume of AgNO3 solution
recorded. Add 2 drops extra titrant to one of the flasks so you see the color produced
from over titrating.
6. You can use these two flasks as color references for the chloride titration. Remember to
subtract the volume of the blank from the volume of AgNO3 required to titrate your soluble
chloride samples.
Standardization of silver nitrate solution.
1. Pipette 25.00 mL of a standard NaCl solution (about 500 mg/L Cl-) into a 150-mL beaker.
2. Dilute the sample to about 50 mL.
3. To each beaker add 4 drops 0.5 M K2CrO4.
4. Titrate the sample with 0.01 M AgNO3 with constant stirring. Rinse the walls of the flask
with water to make certain that no sample has been splashed and retained on the sides.
Use your calcium carbonate blanks as color references. Subtract the black volume from
the volume of titrant.
5. Calculate the molarity of the silver nitrate solution.
Chloride analysis of a Soap Lake Sample.
In 1963 (Eugster and Hardie) Soap Lake was reported to contain 4680 mg/L Cl-. Each 1 mL
of the silver nitrate solution will titrate about 500 g of chloride. So 1 mL of Soap Lake water
will require about 9.3 mL of silver nitrate titrant. Here is a suggested method which you may
need to modify for Soap Lake and will certainly have to modify for other lakes. If you do
water at or below the chemocline you may want to titrate a 1 mL sample first.
1. Pipette 5.000 mL of Soap Lake Water into a 150-mL beaker.
2. Dilute the sample to about 50 mL.
3. Add 1 mL 30% hydrogen peroxide and stir for 1 min to oxidize any sulfide that would react
with silver ion to form silver sullfide.
4. To each beaker add 4 drops 0.5 M K2CrO4.
5. Titrate the sample with 0.01 M AgNO3 with constant stirring. Rinse the walls of the flask
with water to make certain that no sample has been splashed and retained on the sides.
Use your calcium carbonate blanks as color references. Subtract the blank volume from
the volume of titrant.
6. Calculate the chloride concentration in moles/L and mg/L of each water sample from the
volume of sample, volume of titrant used and the concentration of the titrant. Calculate
the mean, standard deviation, and 95% confidence limits from you work.