Carbonate hardness, is a measure of the
water hardness
Hard water is water that has high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, bicarbo ...
caused by the presence of
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
() and
bicarbonate
In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula .
Bicarbonate serves a crucial biochemic ...
()
anions
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
. Carbonate hardness is usually expressed either in degrees KH (
dKH
Degrees of carbonate hardness (dKH or °KH; the K is from the German ''Karbonathärte'') is a Units of measurement, unit of Hard water, water hardness, specifically for temporary or carbonate hardness. Carbonate hardness is a measure of the conce ...
) (from the German ''"Karbonathärte"''), or in ''parts per million
calcium carbonate'' (
ppm or grams per litre, mg/L). One
dKH
Degrees of carbonate hardness (dKH or °KH; the K is from the German ''Karbonathärte'') is a Units of measurement, unit of Hard water, water hardness, specifically for temporary or carbonate hardness. Carbonate hardness is a measure of the conce ...
is equal to 17.848 mg/L (ppm) , e.g. one dKH corresponds to the carbonate and bicarbonate ions found in a solution of approximately 17.848 milligrams of calcium carbonate() per litre of water (17.848 ppm). Both measurements (mg/L or KH) are usually expressed as mg/L – meaning the concentration of carbonate expressed as if calcium carbonate were the sole source of carbonate ions.
An
aqueous solution
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...
containing 120 mg
NaHCO3 (baking soda) per litre of water will contain 1.4285
mmol
The mole, symbol mol, is the unit of amount of substance in the International System of Units (SI). The quantity amount of substance is a measure of how many elementary entities of a given substance are in an object or sample. The mole is define ...
/l of bicarbonate, since the
molar mass
In chemistry, the molar mass of a chemical compound is defined as the mass of a sample of that compound divided by the amount of substance which is the number of moles in that sample, measured in moles. The molar mass is a bulk, not molecular, p ...
of baking soda is 84.007 g/mol. This is equivalent in carbonate hardness to a solution containing 0.71423 mmol/L of (calcium) carbonate, or 71.485 mg/L of calcium carbonate (molar mass 100.09 g/mol). Since one degree KH = 17.848 mg/L CaCO
3, this solution has a KH of 4.0052 degrees.
Carbonate hardness should not be confused with a similar measure
Carbonate Alkalinity
Carbonate hardness, is a measure of the water hardness caused by the presence of carbonate () and bicarbonate () anions. Carbonate hardness is usually expressed either in degrees KH (dKH) (from the German ''"Karbonathärte"''), or in ''parts per mi ...
which is expressed in either
illi[equivalent.html" ;"title="quivalent.html" ;"title="illi[equivalent">illi[equivalent">quivalent.html" ;"title="illi[equivalent">illi[equivalentper litre (meq/L) or ppm. Carbonate hardness expressed in ppm does not necessarily equal carbonate alkalinity expressed in ppm.
whereas
However, for water with a pH below 8.5, the carbonate, CO
32− will be less than 1% of the
HCO3− so carbonate alkalinity will equal carbonate hardness to within an error of less than 1%.
In a solution where only CO
2 affects the pH, carbonate hardness can be used to calculate the concentration of dissolved CO
2 in the solution with the formula CO
2 = 3 × KH × 10
(7-pH), where KH is degrees of carbonate hardness and CO
2 is given in ppm by weight.
The term ''carbonate hardness'' is also sometimes used as a synonym for
temporary hardness
Hard water is water that has high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, bicarbo ...
, in which case it refers to that portion of
hard water
Hard water is water that has high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, bicarbo ...
that can be
removed by processes such as boiling or
lime softening Lime softening (also known as lime buttering, lime-soda treatment, or Clark's process) is a type of water treatment used for water softening, which uses the addition of limewater (calcium hydroxide) to remove hardness (deposits of calcium and magnes ...
, and then separation of water from the resulting precipitate.
See also
*
dGH or DGH or Degree of General Hardness, a measure of the hardness of water
*
Hard water
Hard water is water that has high mineral content (in contrast with "soft water"). Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, bicarbo ...
References
{{Reflist
External links
Water Hardness definitions
Carbonates