Determination of chloride ion in natural water

Objectives:

1. To apply Mohr method to the measurement of chloride ion in natural water

2. To determine the chloride concentration in unknown water sample

3. To understand the titration theory of chloride with silver nitrate solution

 

Introduction:

Titration is a common analytical technique in which the concentration of a substance of solution can be determined by adding standard solution that reacts with the unknown. By using the stoichiometry of the reaction and the number of moles of standard solution needed, the concentration of analyte can be determined through titration.

Argentometric titration is a type of titration that based upon silver (I) ion, Ag⁺, for example, Mohr method. In Mohr method, the most important precipitating agent is silver nitrate in which this method determines the chloride concentration of a solution by titration with silver nitrate. In argentometric determination of chloride concentration, potassium chromate is used as an indicator by reacting with silver ions to produce brick-red silver chromate precipitate at the end point.

As the silver nitrate solution is slowly added, the chloride ion will react with silver ion to form silver chloride precipitate.

Ag⁺(aq) + Cl^-(aq) –> AgCl(s) K_sp = 1.70x10^-10

This reaction continues until all the chloride ion in the system has been used up to form precipitate and this approaches the end point of titration. After all the chloride ion has been precipitated, any more silver(I) ion will react with the chromate ion, CrO₄^2- to form a red-brown precipitate of silver chromate as shown in the following equation.

2 Ag⁺(aq) + CrO₄^2-(aq) –> Ag₂CrO₄ (s) K_sp = 9x10^-12

Mohr method can be used to determine the chloride ion concentration of water samples from many sources such as seawater, stream water, river water and estuary water. Two unknown water sample are tested by using the particular method. The Mohr method works well in a slightly acidic condition, hence the pH of the water sample should be adjusted to the range of 6~8 by adding dilute acid or dilute base as needed.

Apparatus:

Conical flask, burette, pipette, measuring cylinder, volumetric flask

Materials:

Deionized water, unknown A water sample, unknown B water sample, 0.075M silver nitrate solution, pH paper, sodium hydroxide solution, calcium carbonate, 0.25M potassium chromate solution

Procedures:

 

Discussion:

In this experiment, the titration of unknown sample was conducted in the pH range of 7 to 8. This is because at pH lower than 7, the chromate ion will be converted to dichromate ion.

CrO₄^2-(aq) –> Cr₂O₇^- (aq)

Eventually, this dichromate ion cannot form a brick red silver chromate precipitate with silver ion and hence end point of the titration cannot be detected. At pH higher than 8, the silver ion will tend to react with the excess hydroxide ion to form brownish silver hydroxide. Formation of silver hydroxide will cover the colour of silver chromate precipitate and hence end point also cannot be seen.

Ag⁺(aq) + OH^-(aq) --> Ag(OH) (s)

Mohr method can only roughly show the concentration of chloride ion in the water sample. This is because excess silver nitrate is needed to produce sufficient silver chromate precipitate to be observed in the solution with heavy white precipitate of silver chloride. Besides, the intense yellow colour of chromate solution causes the brick red silver chromate that formed is hardly to be observed.

 

Precaution steps:

1. All solution mixture that involved silver nitrate solution must be discarded into waste bottle because silver is heavy metal.

2. The burette must be rinsed with silver nitrate solution before titration starts.

3. Wear gloves when handling silver nitrate solution since it will cause skin staining and chemical burn.

4. Chromate solution needs to be used with care as chromate is a known carcinogen.

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