IONIC STRENGTH: Some properties of dissolved solids, such as the solubility of calcium sulfate and calcium carbonate, are affected by ionic strength.
CONDUCTIVITY: The ability of an aqueous solution to carry electric current depends on the presence of ions in the solution. Conductivity is a quantitative measure which describes this ability. Solutions of inorganic ions are relatively good conductors (and exhibit high conductivity), whereas solutions of organic molecules are rather poor conductors (and exhibit low conductivity). Highly purified water is also a poor conductor. Conductivity is expressed in units of Siemen/cm (previously known as mhos/cm). Conductivity measurements are typically encountered in monitoring the performance of reverse osmosis equipment. Conductivity is temperature dependent and should be measured with a temperature-compensated meter. The usual reference temperature is 25ºC. Conductivity measurements are sometimes used to estimate total dissolved solids in water. While convenient, this practice is imprecise. (See also Resistivity.)
RESISTIVITY: Resistivity is a measure of the ability of a substance, such as water, to carry an electric current. It is expressed in units of ohm-cm and is the reciprocal of conductivity. Resistivity measurements are commonly used to asses the quality of water produced by deionizers. Like conductivity, Resistivity is temperature dependent and should be measured with a temperature-compensated meter. The usual reference temperature is 25ºC.
GRAINS OF HARDNESS: Although the theoretical hardness of water is the sum of the concentrations of all metallic ions, other than the alkali metals, it is commonly expressed as the equivalent concentration of calcium carbonate in grains. Ionic concentrations can be expressed in terms of their combining potential (Eq/L), the number of moles present (mol/L), or their masses in any of several conventions. In the English system masses are expressed in terms of pounds (avoirdupois) which contains 7000 grains each. Although considered outdated in most of the world, the U.S. water purification industry continues to express hardness in units of grains/gal expressed as calcium carbonate. Grains/gal expressed as calcium carbonate can be converted into metric units (mg/L) by multiplying the former by 17.1. Grains/gal expressed as calcium carbonate can also be converted into mEq/L of a univalent ion, such as sodium (Na+) by multiplying by 0.342.
ALKALINITY: Capacity of water to accept acid, ordinarily because of its bicarbonate content. In some cases a part of the alkalinity in water may be do to carbonate, hydroxide, phosphate, or silicate. It is expressed in terms of mg/l of calcium carbonate. Total alkalinity is determined by titration with acid to a pH of 4.3. Methyl orange indicator is frequently used as the end point, but potentiometric titration is preferable.
If the pH of the water is greater than 8.3, it is probable that carbonate is present. Carbonate alkalinity is determined by titration with acid to pH 8.3 (phenolphthalein indicator)
ANION: A particle of matter dissolved in water that has a negative charge. In each system of matter the number of anions is equal to the number of cations (positively charged ions).
BICARBONATE: Bicarbonate is formed in water by the reaction of carbon dioxide with mineral matter, such as limestone or dolomite. The carbon dioxide is absorbed from air or from decaying organic matter, and the bicarbonate that is formed imparts alkalinity to water. See Alkalinity. Calcium and magnesium bicarbonate do not exist in solid form. When water is removed from solutions of these salts, they lose carbon dioxide and form calcium and magnesium carbonate.
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