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Re: Temp Hardness vs permanent hardness was DYING SNAILS



At 09:23 AM 7/21/2002 -0500, you wrote:


>(Snip)



>Temporary hardness in due to Calcium Bicarbonate while permanent hardness 
>is due to the amount of Carbonate present in the water.

I think this is a typo, Charles. So called permanent hardness is a measure 
of dissolved calcium ions in the water. Temporary hardness, as you state, 
is a measure of calcium bicarbonate. The latter is also referred to as 
alkalinity and buffering capacity.

Actually, that is a simplification, but serves to understand the 
principles. The term hardness comes from the old days of using real soaps, 
which are potassium and sodium salts of fatty acids (such as oleic acid). 
Soap used to be made by boiling animal fat (lards) in water with wood 
ashes. The latter are high in potassium, resulting, under these hydrolyzing 
conditions, in the formation of potassium hydroxide ("potash") and fatty 
acids. These salts are soluble in water, and act as detergents, which are 
able to emulsify fats and oils in water. That is why they work to remove 
"dirt". Salts of fatty acids with calcium, magnesium, iron and others 
cations are insoluble. Thus, if you try to wash with a real soap in water 
containing a lot of those ions, the soluble soaps will be precipitated as 
calcium, magnesium, iron (etc) salts, which are insoluble. Thus the water 
won't lather easily, and thus the term "hard water". Incidentally, modern 
"soaps" are not soaps at all, but synthetic detergents, which don't have 
these problems to the same degree.

In those good old days hardness of water could be reduced in several ways, 
but it was typically done by adding washing soda, which is sodium 
carbonate. As calcium carbonate is insoluble, this treatment precipitates 
the calcium, thus removing the source of calcium ions, which no longer can 
interact with the soaps. Thus, that treated water would be high in 
carbonates, but "soft' because the calcium would have been precipitated out.

I said alkalinity, or buffering capacity, is equivalent to temporary 
hardness. That is not quite true, either. Sodium or potassium bicarbonate 
in water will provide buffering capacity and alkalinity, but would not 
contribute to hardness. However, in "real" water, addition of sodium 
bicarbonate in the presence of calcium ions would result in an equilibrium 
in which you essentially have calcium bicarbonate in solution.

To summarize and simplify again:

Hardness, also known as permanent hardness or calcium hardness, is due to 
the presence of calcium ions (mainly) as well as magnesium, iron and other 
ions in the water.

Temporary hardness, also known as alkalinity or buffering capacity, is due 
to the presence of calcium bicarbonate in the water. Untreated, these 
calcium ions would also make the water "hard". In other words, true soaps 
would not lather. When the water is boiled, CO2 is driven off, resulting in 
calcium carbonate, which is precipitated (as "sclae"), thus softening the 
water. In other words, this form of hardness is "temporary".

I hope this, with the historical perspective, will help to clarify a set of 
terminology that aquarists, for good reasons, always find confusing.

Barry



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