Analyzing Chloride Ions in Environmental Sample by Precipitation Titration

Pre-Laboratory Exercise

To analyze and calculate the amount of chloride ions in an environmental sample by precipitation titration To determine the equivalence point by visual observation To become familiar with using pipettes and burettes for quantitative measurements

The chloride ion is an inorganic anion that is naturally occurring in environmental waters. Seawater contains 1.94% chloride and can be commonly found in varying forms such as potassium chloride (KCl), sodium chloride (NaCl) and magnesium chloride (MgCl2) with these salts all being water soluble (1). The Environmental Protection Agency has set a standard for the chloride concentration in drinking water to be 250 mg/L (2). Although chloride is non-toxic to humans, high levels of chloride can affect plants and corrode away infrastructure such as roads and pipes. Possible sources that increase the chloride concentration include industrial processes, sewage, road salts, and fertilizers. Titration is a technique whereby adding a measured quantity of known concentration (titrant) is reacted with an unknown substance (analyte). The goal of this process is to determine the concentration of the unknown using the stoichiometry of the chemical reaction between analyte and titrant. In this experiment, a precipitation titration will be conducted where the reaction of the analyte and titrant will give ionic compounds with limited solubility.

In today’s experiment, the amount of chloride in water from Newtown Creek will be determined by titrating the chloride analyte with silver nitrate solution and using potassium chromate as an indicator. The titrant is slowly added to the environmental water sample whereby the silver ions react with chloride ions in a 1:1 ratio to form a silver chloride precipitate:

 Ag+

(aq) + Cl-

(aq) → AgCl (s)

The end point of the titration occurs when all of the chloride ions has precipitated out of solution (as

solid AgCl) and the free silver ions begin to react with the chromate ions (CrO42- ) to give a brown-red precipitate. Knowing the volume/concentration of the titrant used to attain the equivalence point and stoichiometry of the chemical reaction, the amount of chloride ions can be calculated. Ideally, the titration should be carried out at a pH between 7 and 9 as a low pH environment will cause the chromate anion to protonate and lower the chromate concentration where an endpoint cannot be detected. Conversely, a high pH will lead to the formation of a brown silver hydroxide compound which again inhibits the observation of the endpoint. 

Pre-Laboratory Exercise

1. Define the terms equivalence point, endpoint, indicator, analyte and titrant?

2. What are the chloride ion limits for drinking water secondary standards set-forth by the Environmental Protection Agency? Experimental Procedure

1. Using a 10 mL volumetric flask, place 10 mL of filtered Newtown Creek water sample in an Erlenmeyer flask and measure the pH. The pH value should be in the range of 7-9. If your environmental sample has a pH value outside this range, adjust the acidity with nitric acid or sodium hydroxide.

2. Add 3-4 drops of potassium chromate indicator to the Erlenmeyer flask. A light yellow color solution should appear.

3. Set up a 50 mL burette:

a) Obtain a burette, burette tip, burette clamp, accompanying ring stand and assemble. A magnetic stirrer device will be used as a base for the apparatus set-up.

b) Clean the burette and burette tip with distilled water then discard. Afterwards, pre-rinse the burette with a few mL portion of standard AgNO3 solution to remove any remaining water droplets.

c) Place a funnel at the top of the burette and fill the burette with standard AgNO3 solution to the 0.00 mL mark. Allow a few mL to run (to remove air bubbles) and refill up to the 0.00 mL mark. (NOTE: You should only need to fill up the burette once)

4. Place a stir bar in the Erlenmeyer flask, turn on the magnetic stirrer so that a gentle swirl of solution is reached.

5. Record the concentration of AgNO3

6. Titrate with AgNO3 solution. A white precipitate will initially form. The endpoint will be identified by the first appearance of a permanent brownish-red precipitate (with the accompanying white precipitate). Record the volume of silver nitrate used. (Note: The first trial should be used as a “rough” titration)

7. Repeat the titration until the volume of titrant used for different trials are in agreement.

8. Calculate the concentration chloride ion concentration (Molarity, mg/L and parts per million)