Names of elements common to water and wastewater technology along with their symbols, atomic weights, common valence, and equivalent weights are given in Table 4.1. Certain groupings of atoms act together as a unit in a large number of different molecules; these refer to radicals, are given special names, such as the hydroxyl group (OH−). The most common radicals in ionized form are listed in Table 4.2. Radicals themselves are not compounds, but they join with other elements to form compounds. Data on inorganic compounds common to water and wastewater chemistry are given in Table 4.3. The proper name, formula, molecular weight, and equivalent weight are included for all of the chemicals listed. Popular names, for example, alum for aluminum sulfate, are included in brackets. For chemicals used in water treatment, one common use is given; many have other applications which are not included. Equivalent weights for compounds and some combinations (e.g. Ca(HCO3)2) involved in treatment are provided.
Table 4.1 Basic information on common elements.
| Name | Symbol | Atomic valence | Common weight | Equivalent weighta |
| Aluminum | Al | 27.0 | 3+ | 9.0 |
| Arsenic | As | 74.9 | 3+ | 25.0 |
| Barium | Ba | 137.3 | 2+ | 68.7 |
| Boron | B | 10.8 | 3+ | 3.6 |
| Bromine | Br | 79.9 | 1− | 79.9 |
| Cadmium | Cd | 112.4 | 2+ | 56.2 |
| Calcium | Ca | 40.1 | 2+ | 20.0 |
| Carbon | C | 12.0 | 4− | 3.0 |
| Chlorine | Cl | 35.5 | 1− | 35.5 |
| Chromium | Cr | 52.0 | 3+; 6+ | 17.3 |
| Copper | Cu | 63.5 | 2+ | 31.8 |
| Fluorine | F | 19.0 | 1− | 19.0 |
| Hydrogen | H | 1.0 | 1+ | 1.0 |
| Iodine | I | 126.9 | 1− | 126.9 |
| Iron | Fe | 55.8 | 2+; 3+ | 27.5 |
| Lead | Pb | 207.2 | 2+ | 103.6 |
| Magnesium | Mg | 24.3 | 2+ | 12.2 |
| Manganese | Mn | 54.9 | 2+; 4+; 7+ | 27.5 |
| Mercury | Hg | 200.6 | 1+; 2+ | 100.3 |
| Nickel | Ni | 58.7 | 2+ | 29.4 |
| Nitrogen | N | 14.0 | 3−; 5− | |
| Oxygen | O | 16.0 | 2− | 8.0 |
| Phosphorus | P | 31.0 | 5+ | 6.0 |
| Potassium | K | 39.1 | 1+ | 39.1 |
| Selenium | Se | 79.0 | 6+ | 13.1 |
| Silicon | Si | 28.1 | 4+ | 6.5 |
| Silver | Ag | 107.9 | 1+ | 107.9 |
| Sodium | Na | 23.0 | 1+ | 23.0 |
| Sulfur | S | 32.1 | 2− | 16.0 |
a Equivalent weight (combining weight) equals atomic weight divided by valence.
Table 4.2 Common radicals found in process water.
| Name | Formula | Molecular weight | Electric charge | Equivalent weight |
| Ammonium | NH4 + | 18.0 | 1+ | 18.0 |
| Hydroxyl | OH− | 17.0 | 1− | 17.0 |
| Bicarbonate | HCO3 − | 61.0 | 1− | 61.0 |
| Carbonate | CO3 − | 60.0 | 2− | 30.0 |
| Orthophosphate | PO4 − | 95.0 | 3− | 31.7 |
| Hydrogen phosphate | HPO4 − | 96.0 | 2− | 48.0 |
| Dihydrogen phosphate | H2PO4 − | 97.0 | 1− | 97.0 |
| Bisulphate | HSO4 − | 97.0 | 1− | 97.0 |
| Sulfate | SO4 − | 96.0 | 2− | 48.0 |
| Bisulfite | HSO3 − | 81.0 | 1− | 81.0 |
| Sulfite | SO3 − | 80.0 | 2− | 40.0 |
| Nitrite | NO2 − | 46.0 | 1− | 46.0.0 |
| Nitrate | NO3 − | 62.0 | 1− | 62.0 |
| Hypochlorite | OCl− | 51.5 | 1− | 51.5 |
EXAMPLES 4.1
Calculate the molecular and equivalent weights of ferric sulfate.
SOLUTION
The formula from Table 4.3 is Fe2(SO4)3. Using atomic weight from Table 4.1
The ferric (oxide of iron) atom has a valence of 3+, thus a compound with two ferric atoms has a total electrical charge of 6+. (Three sulfate radicals have a total of 6− charges.)
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