Total Hardness mean anything?

Total hardness and calcium hardness in pool water.
Scale, calcium buildup, hard water and scaling problems.
Jeffm
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Total Hardness mean anything?

Postby Jeffm » Thu 23 Apr, 2009 10:21

When we talk about Saturation Index, does a total hardness mean anything, or are we just concerned about Calcium?


Pooltech32
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Total Hardness mean anything?

Postby Pooltech32 » Thu 23 Apr, 2009 19:04

same thing
Pool repair guy
Jeffm
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Total Hardness mean anything?

Postby Jeffm » Fri 24 Apr, 2009 07:55

Can't be the same thing:
Our lab did a test on my water: 175 Ca Hardness, and 300 Total Hardness.
Total includes Magnesium and other metals. I'm wondering if the total applies to anything on the saturation index, or if only Calcium Hardness matters.
chem geek
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Total Hardness mean anything?

Postby chem geek » Fri 24 Apr, 2009 12:34

Only the Calcium Hardness (CH) matters for the saturation index because what you want saturated in the water is Calcium Carbonate since that is what precipitates (scales) first and is what is part of plaster/gunite/grout. Magnesium Carbonate has a solubility about 9 times higher than Calcium Carbonate (i.e. the magnesium concentration has to be 9 times higher than calcium at the same carbonate level in order to precipitate). In typical water, Total Hardness is about 40% higher than Calcium Hardness, though this obviously varies depending on the specific magnesium-to-calcium content of the water. In your example, TH was around 70% higher than CH.

The saturation index is nothing more than a measure of the actual saturation level of calcium carbonate. It is a logarithmic scale of the ratio of the product of calcium and carbonate concentrations to that product when at saturation (i.e. the equilibrium constant at saturation aka saturation product). When this number is 0, the water is saturated with calcium carbonate. When the number is positive, it is over-saturated and has the potential to scale. When the number is negative, it is under-saturated so has the potential to dissolve solid calcium carbonate.

The level of calcium in the water is proportional to the Calcium Hardness (CH). The level of carbonate in the water is proportional to the Carbonate Alkalinity which is the Total Alkalinity (TA) adjusted to remove the effect of Cyanuric Acid (CYA) on TA and is also very much affected by pH and somewhat by temperature and Total Dissolved Solids (TDS).

Richard

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