Chloride (Cl) anions are usually present in natural waters. A high Cl concentration occurs in waters
that have been in contact with Cl-containing geological formations. Otherwise, high Cl content may
indicate pollution by sewage or industrial wastes or by intrusion of seawater or saline water into a
freshwater body or aquifer.
A salty taste in water dependents on the ions with which the Cl are associated. With Na ions the taste is detectable at about 250 mg/L, but with Ca or Mg the taste may be undetectable at 1,000 mg/L. Chlorides being highly soluble is present in all waters but the amount is often very low in natural waters. High Cl content has a corrosive effect on metal pipes and structures and is harmful to most trees and plants.
The determination of Cl is done by AgNO3 (Mohrs’s titration) method, which is based upon the fact
that in solution containing Cl and chromate (Cr). Silver reacts with all the Cl and precipitates before
the reaction with chromate begins. The appearance of the brick-red colour of the silver chromate
precipitate is the end-point of the titration.
- Burette and stand
- Volumetric flask
A. Potassium Chromate Solution (K2CrO4), 5% in water
Dissolve 5 g K2CrO4 in 50 mL DI water.
Add dropwise 1 N silver nitrate (AgNO3) until a slight permanent red precipitate is formed.
Filter, and bring to 100-mL volume with DI water.
B. Silver Nitrate Solution (AgNO3), 0.01 N Dry about 3 g AgNO3 in an oven at 105 °C for 2 hours, cool in a desiccator, and store in a tightly stoppered and brown bottle.
Dissolve 1.696 g dried AgNO3 in DI water, and bring to 1-L volume.
C. Sodium Chloride Solution (NaCl), 0.01 N
Dry about 3 g NaCl in an oven at 140 °C for 2 hours, cool in a desiccator, and store in a tightly
stoppered and brown bottle.
Dissolve 0.585 g NaCl in DI water, and bring to 1-L volume.
- Pipette a suitable aliquot of water sample (10 mL natural water sample) into a 250-ml Erlenmeyer flask.
- Add 4 drops potassium chromate solution.
- Titrate against AgNO3 solution until a permanent reddish-brown color appears.
In order to standardize the AgNO3 solution used in the determination of Cl:
Titrate 10 mL 0.01N NaCl solution against 0.01 N AgNO3 after adding 4 drops potassium
chromate solution until a permanent reddish-brown color appears.
Take the reading, and calculate AgNO3 normality:
N AgNO3 = Normality of AgNO3 solution
V AgNO3 = Volume of AgNO3 solution used (mL)
N NaCl = Normality of NaCl solution
V1 = Volume of 0.01 N AgNO3 titrated for the sample (mL)
N = Normality of AgNO3 solution
V = Volume of water sample used for measurement (mL)
Natural waters are often low in Cl, and 10 mL is a suitable aliquot in most cases.
Saline waters may be high in Cl and 5 mL (or even less than 5 mL) may then be more appropriate aliquots.
The Cl estimation has two different purposes:
If the test is done regularly on a water supply and there is a sudden increase, it may indicate pollution due to ingress of sewerage or other chloride-rich wastes.
Many groundwaters have Cl content high enough to be of objectionable taste. By using the Cl test, the well with the lowest amount of Cl can be identified. If several well are being pumped, it can be planned in such a way that the lowest Cl content is obtained.