When reporting the concentrations of elemental ions and dissolved molecules, a commercial laboratory typically will report the results in milligrams per liter or milliequivalents per liter, or both. The former unit is useful when you need to evaluate the total mass or the "mass concentration" (i.e., milligrams per liter) of particular constituents. The latter type of measure — milliequivalents per liter — is the preferred reporting method if you need to check the quality of a water analysis or to calculate certain water quality parameters that involve electrochemistry. (One example is the SAR, described in detail later in this tutorial.)
A laboratory also may report concentrations of constituents in parts per million (ppm) or parts per billion (ppb). The ppm unit can be thought of as milligrams of solute per million milligrams of solution (water), or as milligrams of solute per kilogram of solution:
|ppm||=||mg solute / 106 milligrams solution||=||mg/liter|
|=||mg solute / kg solution|
Similarly, parts per billion (ppb) is defined as follows:
|ppb||=||μg solute / 109 micrograms solution||=||μg/liter|
|=||μg solute / kg solution|
Equating the units of mass per kilogram and mass per liter is appropriate for a dilute solution such as irrigation water. That's because, except for highly saline water, the density of water is very close to 1.00 kilograms per liter. (For brines and other waters of extremely high salinity, it is necessary to account for the higher-than-unity solution density. However, brines are not considered in detail here, as they are never suitable as irrigation water.)
Concentration data reported in milligrams per liter (mg/L) can be converted to milliequivalents per liter (meq/L), and vice versa. Simply use the following formula:
mg/L = meq/L × equivalent weight (see table)
A water chemistry report from a commercial lab shows that the sulfate concentration of a sample is 24 ppm. Assuming the sample was drawn from an irrigation well or canal, convert the ppm measurement to mg/L and meq/L.
Because the sample is irrigation water, it is of relatively low salinity and we can safely assume the solution's density is 1.00. Therefore, the concentration reported in ppm can be expressed just as well in milligrams per liter. The result is 24 mg/L.
Using the equation above and the equivalent weight for sulfate,
|meq/l||=||mg/L ÷ equiv. wt.|
|=||24 mg/L ÷ 48||=||0.50 meq/L|
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