Carbonate hardness
Encyclopedia
Carbonate hardness, or carbonate alkalinity
is a measure of the alkalinity
of water caused by the presence of carbonate
(CO32-) and bicarbonate
(HCO3-) anions. It is usually expressed either as parts per million (ppm or mg/L
), or in degrees KH (from the German "Karbonathärte"). One German degree of carbonate hardness (dKH) corresponds to the carbonate and bicarbonate ions found in a solution of approximately 17.848 milligrams of calcium carbonate
(CaCO3) per litre of water (17.848 ppm). Both measurements (mg/L or KH) are usually expressed "as CaCO3" – meaning the concentration of carbonate expressed as if calcium carbonate were the sole source of carbonate ions. Bicarbonate ions only contribute half as much carbonate hardness as carbonate ions, so bicarbonates that are present in the water are converted to an equivalent concentration of carbonates when determining KH. For example:
An aqueous solution
containing 120 mg NaHCO3
(baking soda) per litre of water will contain 1.4285 mmol/L of bicarbonate, since the molar mass
of baking soda is 84.007 g/mol. This is equivalent in carbonate hardness to a solution containing 0.71423 mmol/L of carbonate, or 71.485 mg/L of calcium carbonate (molar mass 100.09 g/mol). Since one degree KH = 17.848 mg/L CaCO3, this solution has a KH of 4.0052 degrees.
The term carbonate hardness is also sometimes used as a synonym for temporary hardness, in which case it refers to that portion of hard water
that can be removed
by processes such as boiling or lime softening
, and then separation of water from the resulting precipitate.
Carbonate alkalinity
Carbonate alkalinity is a measure of the amount of carbonate and bicarbonate anions in solution. Carbonate and bicarbonate anions contribute to alkalinity due to their basic nature, hence their ability to neutralize acid...
is a measure of the alkalinity
Alkalinity
Alkalinity or AT measures the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate. The alkalinity is equal to the stoichiometric sum of the bases in solution...
of water caused by the presence of carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
(CO32-) and bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
(HCO3-) anions. It is usually expressed either as parts per million (ppm or mg/L
Gram per litre
A gram per liter or litre is a unit of measurement of mass concentration that shows how many grams of a certain substance are present in one litre of a usually liquid or gaseous mixture. It is not an SI unit because it contains the non-SI unit litre...
), or in degrees KH (from the German "Karbonathärte"). One German degree of carbonate hardness (dKH) corresponds to the carbonate and bicarbonate ions found in a solution of approximately 17.848 milligrams of calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
(CaCO3) per litre of water (17.848 ppm). Both measurements (mg/L or KH) are usually expressed "as CaCO3" – meaning the concentration of carbonate expressed as if calcium carbonate were the sole source of carbonate ions. Bicarbonate ions only contribute half as much carbonate hardness as carbonate ions, so bicarbonates that are present in the water are converted to an equivalent concentration of carbonates when determining KH. For example:
An aqueous solution
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
containing 120 mg NaHCO3
Sodium bicarbonate
Sodium bicarbonate or sodium hydrogen carbonate is the chemical compound with the formula Na HCO3. Sodium bicarbonate is a white solid that is crystalline but often appears as a fine powder. It has a slightly salty, alkaline taste resembling that of washing soda . The natural mineral form is...
(baking soda) per litre of water will contain 1.4285 mmol/L of bicarbonate, since the molar mass
Molar mass
Molar mass, symbol M, is a physical property of a given substance , namely its mass per amount of substance. The base SI unit for mass is the kilogram and that for amount of substance is the mole. Thus, the derived unit for molar mass is kg/mol...
of baking soda is 84.007 g/mol. This is equivalent in carbonate hardness to a solution containing 0.71423 mmol/L of carbonate, or 71.485 mg/L of calcium carbonate (molar mass 100.09 g/mol). Since one degree KH = 17.848 mg/L CaCO3, this solution has a KH of 4.0052 degrees.
The term carbonate hardness is also sometimes used as a synonym for temporary hardness, in which case it refers to that portion of hard water
Hard water
Hard water is water that has high mineral content . Hard water has high concentrations of Ca2+ and Mg2+ ions. Hard water is generally not harmful to one's health but can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling...
that can be removed
Water softening
Water softening is the reduction of the concentration of calcium, magnesium, and certain other metal cations in hard water. These "hardness ions" can cause a variety of undesired effects including interfering with the action of soaps, the build up of limescale, which can foul plumbing, and...
by processes such as boiling or lime softening
Lime softening
Lime softening, also known as Clark's process, is a type of water treatment used for water softening. In the USA, it is used primarily in the Midwest, Florida and Texas. It utilizes the addition of lime to remove hardness ions by precipitation...
, and then separation of water from the resulting precipitate.