May 26, 2006
Chloride,
in the form of the Cl ion, is one of the major inorganic anions, or negative ions, in saltwater
and freshwater. It originates from the dissociation of salts, such as sodium
chloride or calcium chloride, in water.
NaCl(s)
----> Na+(aq)
+ Cl(aq)
CaCl2(s)
----> Ca2+(aq)
+ 2 Cl(aq)
These salts, and their resulting chloride ions, originate from natural minerals, saltwater intrusion into estuaries, and industrial pollution.
There are many possible sources of
manmade salts that may contribute to elevated chloride readings. Sodium chloride
and calcium chloride, used to salt roads, contribute to elevated chloride levels
in streams. Chlorinated drinking water and sodium-chloride water softeners often
increase chloride levels in wastewater of a community.
In drinking water, the salty taste produced by chloride depends upon the concentration of the chloride ion. Water containing 250 mg/L of chloride may have a detectable salty taste if the chloride came from sodium chloride. The recommended maximum level of chloride in U.S. drinking water is 250 mg/L.
Some sources of Chloride Ions |
River streambeds with salt-containing minerals |
Runoff from salted roads |
Mixing of seawater with freshwater |
The Use of Water softeners |
Irrigation
water returned to streams |
Chlorinated drinking water |
Salinity
is the total of all non-carbonate salts dissolved in water, usually expressed
in parts per thousand (1 ppt = 1000 mg/L). Unlike chloride (Cl)
concentration, salinity is a measure of the total salt concentration, comprised mostly
of Na+ and
Cl ions. Even though there are smaller quantities of other ions
in seawater (e.g., K+, Mg2+, or SO42),
sodium and chloride ions represent about 91% of all seawater ions. Salinity
is an important measurement in seawater or in estuaries where freshwater from
rivers and streams mixes with salty ocean water. The salinity level in seawater
is fairly constant, at about 35 ppt (35,000 mg/L), while brackish estuaries
may have salinity levels between 1 and 10 ppt. Since most anions in seawater
or brackish water are chloride ions, salinity can be determined from chloride
concentration. The following formula is used:
salinity
(ppt) = 0.0018066 5 Cl (mg/L)
A Chloride
Ion-Selective Electrode can be used to determine the chloride concentration,
which is converted to a salinity value using the above formula.
Salinity can
also be measured in freshwater. Compared to seawater or brackish water, freshwater
has much lower levels of salt ions such as Na+ and Cl;
in fact, these ions are often lower in concentration than hard-water ions such
calcium (Ca2+) and bicarbonate (HCO3). Because
salinity readings in freshwater will be significantly lower than in seawater
or brackish water, readings are often expressed in mg/L instead of ppt (1 ppt
= 1000 mg/L).
Salinity is
also of interest in bodies of water where seawater mixes with freshwater, since
aquatic organisms have varying abilities to survive and thrive at different
salinity levels. Saltwater organisms survive in salinity levels up to 40 ppt,
yet many freshwater organisms cannot live in salinity levels above 1 ppt.