How much chloride is in the water?

Over 97% of the worlds total reservoir of water is found in the oceans and is too salty fro drinking, irrigation, and most industrial and household needs. For fresh water, we depend on the 0.7% of water that is found underground and in inland seas, lakes and rivers. Water that contains up to 1000 ppm of dissolved solids is generally termed 'fresh' water, but assuming it is safe in other respects, such water is not necessarily  suitable for drinking. For drinking, the U.S.  Public Health Service recommends a solid content of no more than 0.05% (500 ppm) The concentration of dissolved solids in seawater averages 3.5% (35,000 ppm) Water is termed 'brackish' if the solid content lies between that of fresh water and seawater.

In both fresh and seawater, there are lots of dissolved ions. The ions present in freshwater are essential for the normal growth of most life forms. However, if the water contains abnormally high concentrations of of ions (saline water) it is unfit for drinking and can destroy crops, fish, and other aquatic organisms. Irrigation is the main cause of increased salinity in natural waters. Because land that is irrigated is located where rain is sparse, and the land is hot and dry, the rate of evaporation from the soil is high and the salts tend to accumulate. As a result, thousands of acres of crop land around the world have ceased to be productive. Runoff from the soil and subsurface drainage, when returned to the water supply, can carry and increased load of salt.

Chloride is one of the major anions found in water and wastewater. The recommended maximum contaminant level is 250 mg/L (250 ppm), since the chloride ion imparts a salty taste to the water. If ions of Calcium and Magnesium are present, the chloride ion may not impart a salty taste until over 1000 mg/L. In addition to human and animal waste, sources of chloride can include natural geological formations, road salt storage and applications, oil / natural base drilling, and saltwater intrusions. High levels of chloride can attack and weaken metallic piping and fixtures and inhibit the growth of vegetation.

As you well know, chlorine ions react with silver ion to form an insoluble precipitate, AgCl:
AgNO₃ + Cl^- ¦AgCl + NO₃^-

This reaction can be used to quantitatively determine the amount of chlorine in water using a titration technique. A solution containing a known amount of AgNO₃ is prepared and is slowly added to a precisely measured amount of water to which a small amount of indicator has been added. The indicator (a 5% sodium chromate solution) signals when all the Cl in the sample has reacted with AgNO₃. The silver ion would rather precipitate with the Cl ion in solution. Once all the Cl ion is gone, the silver reacts with the chromate forming red silver chromate. The solution must be stirred continually, otherwise the silver chloride solid will cause problems. Knowing the volume and concentration of added Ag ion required to precipitate the Cl ions in a known amount of water, the concentration of Cl- can be calculated. There will be different concentrations (0.01 M, 0.005 M, 0.0001 M) of Ag+ solutions available, and your choice will depend on the strength of the sample.

The purpose of this experiment is to compare the chloride levels in the water around our community. You will bring in two samples, one from your house, one from a local water source.

What happens when you misrepresent chloride ion data.

Michigan Dept. of Environmental Quality Information

This lab requires a full boat formal lab report, everything included. You need to include data from 4 sources, and include in your discussion general conclusions about the collective data (see map outside teh lab)

Prelab Questions (Where should I write them??)
1.Review the MSDS for both silver nitrate and sodium chromate. Briefly describe their hazards.

2. Dream up a data table for the experiment. You will be measuring at least four samples.

3. You must bring in two samples of water to be tested, one from the 'tap' in your home, and another from a body of water (lake, stream, ocean) Label the containers. For the body of water, make sure you know WHERE the sample was taken from (A map will be provided to pin down the levels)

~MEO 04.07.04 10:05