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Unformatted text preview: EAS 44600 Groundwater Hydrology Lecture 14: Water Chemistry 1 Dr. Pengfei Zhang Concentration Units Molar concentration ( M ) defines the number of moles of a chemical species in one liter of solution (mol/L). One mole (mol) of a substance is its formula weight in grams. For example, one mole of NaCl is 58.44 grams. Molal concentration ( m ) defines the number of moles of a chemical species in one kilogram of solution (mol/kg). For dilute solutions, molal concentrations are almost the same as molar concentrations because a one-liter solution has a mass of about 1 kilogram. Notice that molal concentrations will not change as temperature changes, whereas molar concentrations will due to the dependence of solution density (and hence solution volume) on temperature. Therefore, in chemical thermodynamics, molal concentrations rather than molar concentrations are used. Mass per unit volume concentration is the most common unit for concentration. It defines the mass of a chemical species in one liter of solution, typically reported in units such as mg/L or g/L. Equivalent charge concentration is the number of equivalent charges of an ion in one liter of solution (meq/L or eq/L). The equivalent charge for an ion is equal to the number of moles of an ion multiplied by the absolute charge of the ion. For example, with singly charged species such as Cl- and Na + , 1 M = 1 eq/L = 1,000 meq/L, and with doubly charged species such as SO 4 2- and Ca 2+ , 1 M = 2 eq/L = 2,000 meq/L. Mass per unit mass concentrations define the mass of a chemical species per total mass of the system (mg/kg or g/kg). Older analyses often used concentration units of ppm (parts per million) or ppb (parts per billion). These units are no longer commonly used. Routine Water Analysis A routine water analysis involves measuring the concentration of a standard set of constituents, as well as pH, total dissolved solids (TDS), and specific conductance. The major constituents reported in a routine water analysis include Ca 2+ , Na + , Mg 2+ , K + , HCO 3- , SO 4 2- , Cl- , NO 3- , and dissolved SiO 2 , and the minor constituents include Fe 2+ /Fe 3+ , Cu 2+ , F- , etc. Table 14-1 lists the chemistry of some natural waters and the drinking water standards for major ions. The major ions in most ground waters meet the drinking water standards (Table 14-1). In fact, most of the groundwater contamination is caused by constituents in trace amounts (e.g., heavy metals and organic compounds), not the major ions. organic compounds), not the major ions....
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This note was uploaded on 12/29/2010 for the course EAS 44600 taught by Professor Pengfeizhang during the Spring '10 term at CUNY City.
- Spring '10