Chloride and water quality
Chlorides are salts resulting from the combination of the gas chlorine with a metal. Some
common chlorides include sodium chloride (NaCl) and magnesium chloride (MgCl2). Chlorine
alone as Cl2 is highly toxic and it is often used as a disinfectant. In combination with a metal
such as sodium it becomes essential for life because small amounts of chlorides are required for normal
cell functions in plant and animal life.
Methodology: Although analysis can be performed by potentiometric titration using a
computer aided titrimeter (CAT) with an ion-specific chloride electrode, Ion Chromatography
(IC) is the preferred method of identification. Essentially, an analytical column is used to separate out
various anions. The time required for the anion to pass through the column indicates its
concentration. To determine the identification of the anion, the IC uses a conductivity meter.
Since each anion has a different conductivity, its identity can easily be determined.
Environmental Impact: Chlorides are not usually harmful to people; however, the sodium part
of table salt has been linked to heart and kidney disease. Sodium chloride may impart a salty
taste at 250 mg/L; however, calcium or magnesium chloride are not usually detected by taste
until levels of 1000 mg/L are reached.
Chlorides may get into surface water from several sources including:
1) rocks containing chlorides;
2) agricultural runoff;
3) wastewater from industries;
4) oil well wastes;
5) effluent wastewater from wastewater treatment plants, and;
6) road salting.
Chlorides can corrode metals and affect the taste of food products. Therefore, water that is used
in industry or processed for any use has a recommended maximum chloride level. Chlorides can
contaminate fresh water streams and lakes. Fish and aquatic communities cannot survive in high
levels of chlorides. The table below shows the effects of chlorides on fish:
Chloride effects on fish
Chloride Above These Levels Can Be Toxic | mg/L (PPM)
| | Short Exposure
| Long Term
Exposure
| Species
| 2,540
| 400
| Snail | 6,570
| 430
| Fathead minnow | 6,740
| 900
| Rainbow trout | 8,000
| 800
| Channel catfish | 8,390
| 850
| Carp |
Criteria: Public Drinking Water Standards require chloride levels not to exceed 250 mg/L.
Criteria for protection of aquatic life require levels of less than 600 mg/L for chronic (long-term)
exposure and 1200 mg/L for short-term exposure.
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