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Hardness alkalinity

Here is a post I sent in December on hardness and alkalinity

After reading all the posts for hardness and alkalinity, I tried to go back
through the aquarium literature that I have, and make some sense of all the
different definitions, also tried to understand how all this affects CO2
addition. The following is what I came up with. My thanks to Dave
(eworobe at cc_UManitoba.CA) for his help with understanding alkalinity. Any
errors are mine not his.


Water Hardness

All fresh water sources contain calcium and magnesium in varying
quantities. These are cations with a +2 charge.  They form salts with
anions which have a negative charge. The most important of these are
bicarbonate (HCO3(-1) ; carbonate CO3(-2) ; and sulfate

General Hardness (GH) measures the cations (+ charge);  for calcium and

Carbonate Hardness (KH) Refers to only the bicarbonate, and carbonate
anions(-charge); it does not measures the sulfates and other anions.
        Carbonate Hardness is a confusing term because it does refer to
hardness, but rather to the alkalinity (the ability of a solution to resist
a Ph change with an addition of an acid.) from the carbonates and
bicarbonates. Other anions  (such as hydroxide, borates, silicates, and
phosphates) can contribute to the alkalinity. To be absolutely correct, you
should NEVER use the term 'kh'; however, this is often refered to in
aquarium literature. It should be noted that it is the
bicarbonate/carbonate buffering system which provides the majority of the
alkalinity in aquariums plant aquariums.

Kh and Gh are usually are close two each other, but the GH can be the same,
higher or lower to the KH depending on the Cations and Anions in the
sample.  For example, a large amount of NaHCO3 would raise the (KH) and not
effect the (GH).  A large amount of MgSO4 would raise the (GH) and not the

Usually, in fresh water most of the cations are calcium and magnesium (In a
3:1 ratio) and most of the anions are carbonates. The  levels for (GH) and
(KH) will often be similar.


It would make sense to measure the general hardness in # of ions/liter or
molarity, but this is not used. The common units found in the literature
are degrees of general hardness dgh (GH) from the German system or ppm Ca
from CaCO3 .  Carbonate hardness (KH) is a term which has nothing to d with
hardness, rather it is the amount of carbonate or bicarbonate equivalents
which effect the alklinity or acid buffering capacity. (KH) is equated to
ppm CO3 from CaCO3

Converting from dgh and dkh to ppm CaCO3 can be accomplished by multiplying
by 17.86

How  the conversion factors were derived:
By definition 1dgh = 10mg/liter CaO
        Atomic Weight  Ca= 40  O=16  CaO= 56
        So 10 mg/liter CaO contains 40/56 *10 = 7.143 mg/liter of Ca

By definition ppm Ca is not for elemental calcium but for ppm CaCO3 .
        Atomic weight CaCO3=100
        So 7.143 mg/liter of elemental Ca would be expressed as
        100/40 * 7.143 = 17.8575 mg/liter(ppm)CaCO3.

1dgh = 17.86 ppm CaCO3  and 7.143 ppm Ca+2

1 dgh is defined as 10mg/lit CaO this can be related to ppm of CaCO3 as in
above. Now the definition for dKh must have come from the amount of
carbonate in 17.86 ppm CaCO3 which has nothing to do with GH wich is
defined by CaO!
Historically GH must have been defined first in terms of CaO; hardness in
terms of ppm CaCO3 second, then KH third?

1dKh = 17.86 ppm CaCO3

From above; 1dkh = 17.8575 mg/liter CaCO3.  7.143 mg/liter of this is Ca,
the rest ;(17.8575-7.143)= 10.7145mg/liter CO3

1dKh = 10.7145 ppm CO3

For bicarbonate:

CaCO3 forms Ca(HCO3)2 in water at ph less than 10.25 . (Two bicarbonates
are formed from each carbonate ion):

CaCO3 + H20 + CO2 ---> Ca(HCO3)2

CO3 mw = 60
HCO3 mw = 61

10.7145mg/liter CO3 from CaCO3 (each CO3 carbonate anion forms two HCO3
bicarbonate anions; 61/60*2 *10.7=21.8 mg/liter HCO3

Another way to calculate this is using molarity:

1dkh = 17.86 mg/liter CaCO3
                        mw CaCO3 =100
17.86 mg/liter CaCO3 = .179 m Mole CaCO3

This will form 2* .179 m Mole = .358 m Mole
Multipling moles *mw will give mg:
0.358*61(mwHCO3) = 21.8 mg/liter HCO3

1dkh= 21.8 ppm HCO3

How to use these conversion factors:

If you have alkalinity in ppm or hardness in ppm divide by 17.86 to get degrees.

If you want to raise the alkalinity by 1dkh using CaCO3:
        use 17.86 mg CaCO3
If you want to raise the alkalinity by 1dkh using NaHCO3:
mw Na =23
mw HCO3= 61
mwNaHCO3= 84
        1dkh= 21.8 ppm HCO3
        21.8 *84/61=30 mg/liter of  NaHCO3

using molarity:
        0.358 mMoles * 84(mwNaHCO3) = 30 mg/liter of  NaHCO3