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Water Hardness

by larry/creative.net (Larry Frank)
Date: Tue, 16 Dec 1997

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^- ; carbonate CO₃^2- ; and sulfate SO₄^2-.

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

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 (KH).

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.

Units

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: (GH)

By definition ppm Ca is not for elemental calcium but for ppm CaCO₃.

(KH)

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):

Therefore 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:

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:

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

CO₂, pH And Alkalinity from Carbonates

CO₂, pH and carbonates are all related by the following three equations:

Example of how increasing carbonates will either raise the pH or need counterbalancing CO₂ added to maintain the pH level.

If NaHCO₂ is added to aquarium water the additional carbonate ions will cause a shift to the left side of equation (2). This will form more carbonic acid extracting a H+ ion and thus raising the pH. The additional carbonic acid will drive equation (1) to the left creating CO₂ which will dissapate out of solution bringing equalibrium at a higher pH. In order to maintain equalibrium at the original pH equations (1) and (2) must be shifted back t the right. This can be accomplished with the addition of more CO₂ into the aquarium. (Equation (3) becomes important as pH approches 10.25)

These relationships are expressed in the familliar Chart with KH/pH and the amount of CO₂ needed in solution to maintain a specific pH. The higher the amount of carbonates/bicarbonates in the aquarium, the more CO₂ is needed to maintain a specific pH.

    pH   6.0   6.2   6.4   6.6    6.8   7.0   7.2   7.4   7.6   7.8   8.0
 .........___________________________________________________
KH
0.5 |      15   9.3   5.9   3.7   2.4   1.5   .93   .59   .37   .24   .15
1.0 |      30  18.6  11.8   7.4   4.7   3.0   1.7   1.2   .74   .47   .30
1.5 |      44   28   17.6  11.1   7.0   4.4   2.8   1.8   1.11  .70   .44
2.0 |      59   37    24   14.8   9.4   5.9   3.7   2.4   1.48  .94   .59
2.5 |      73   46    30   18.5   11.8  7.3   4.6   3.0   1.85 1.18   .73
3.0 |      87   56    35    22    14.0  8.7   5.6   3.5   2.2  1.40   .87
3.5 |     103   65    41    26    16.4 10.3   6.5   4.1   2.6  1.64  1.03
4.0 |     118   75    47    30    18.7 11.8   7.5   4.7   3.0  1.87  1.18
5.0 |     147   93    59    37     23   14.7  9.3   5.9   3.7   2.3  1.47
6.0 |     177  112    71    45     28   17.7  11.2  7.1   4.5   2.8  1.77
8.0 |     240  149    94    59     37    24   14.9  9.4   5.9   3.7   2.4
10. |     300  186    118   74     47    30   18.6  11.8  7.4   4.7   3.0
15. |     440  280    176  111     70    44    28   17.6  11.1  7.0   4.4
20. |     590  370    240  148     94    59    37    24   14.8  9.4   5.9

CO₂ in excess of 40mg/liter is harmful to fish. Using this chart the alkalinity of the water from bicarbonates is balanced against a desired pH using CO₂ to control the pH, making sure a unhealthy amount of CO₂ is not called for.

Hardness

by George Booth <booth/hpmtlgb1.lvld.hp.com>
Date: Wed, 17 Dec 1997

Great summary, Larry. I would only like to add the following for those who don't read between the lines:

1. The pH/KH/CO₂ table is based solely on "carbonate hardness".
2. All KH test kits that I know of measure alkalinity, not true KH.
3. The CO₂ value you get from the table will NOT be accurate if there are other sources of alkalinity in your water besides bicarbonate.
4. Even CO₂ test kits are not accurate in the presence of other sources of alkalinity (phosphates, for sure).

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