Calcium Hardness ?

Total hardness and calcium hardness in pool water.
Scale, calcium buildup, hard water and scaling problems.
jeff6898
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Calcium Hardness ?

Postby jeff6898 » Thu 25 Jun, 2009 10:24

My Calcium hardness reading is way low 20 I was told I really don't need to worry about this since I have a above ground vinyl pool. Is this correct? Should N leave it alone or bring it up? Does it effect alergy or water clarness at all?


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Calcium Hardness ?

Postby chem geek » Thu 25 Jun, 2009 12:02

To avoid foaming, you can raise it to 100-120 ppm. It doesn't have to be very high unless you have exposed grout in tile, plaster, etc. A vinyl pool does not need a higher calcium level. There is controversy over whether it is needed to prevent metal corrosion, but pH is the primary factor for metal corrosion (i.e. don't let the pH get very low -- keep it near 7.5 and not below 7.0 for any extended length of time).
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Calcium Hardness ?

Postby Toe In The Water » Fri 14 Aug, 2009 06:55

Hi everyone - I've found your forum and it looks like it could be a goon 'un. :thumbup:

Anyway, one of the pools I look after here in Italy is inground and cement lined 60m3. I have historically disinfected the pool with TriClor 90 200g tablets via the skimmers, but having observed some light white cloudyness coming from the walls and tested for Calcium Hardness i have switched to Calcium Hypochlorite 200g tabs x 4 or 5 per week, under advice from local "experts". My maintenance visits are weekly and in 6 weeks the Calcium test has nudged up from 130-140 to 220 where it has remained constant for for 3 weeks.

I've read that I should be aiming for around 350. Is this A) correct and B) something which can be achieved just by keeping going as i am? TA is around 100 and pH varies between 7.6 and 7.2, the latter being actively managed weekly.

Any thoughts please? :wave:
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Calcium Hardness ?

Postby chem geek » Fri 14 Aug, 2009 10:10

You should use The Pool Calculator to calculate the saturation index which should be somewhat close to zero. You should understand the side effects of the chlorine chemicals you are using. The following are chemical rules of fact that are independent of concentration of product or of pool size:

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by 7 ppm.

So unless you have significant water dilution, continued use of Trichlor pucks/tabs will increase CYA causing chlorine to become less effective (unless you proportionately increase the FC level). Continued use of Cal-Hypo will cause the CH to rise.

You could use chlorinating liquid (or 6% unscented bleach) instead as this will not add to either CYA nor CH.
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Calcium Hardness ?

Postby srlewis76 » Sun 04 Oct, 2009 11:24

I have an above ground pool as well. I have read on the pool calculator basic pool chemistry that calcium is not needed. I have a salt water chlorinator and know that calcium is bad for the electrode plates, but does it hurt the liner or pump or is it just a ploy for you to purchase yet another product?
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Calcium Hardness ?

Postby chem geek » Sun 04 Oct, 2009 15:00

You definitely need calcium and a balanced saturation index if you have any exposed plaster/gunite/grout and probably if you have fiberglass with a calcium carbonate based gelcoat. You do not need it for a vinyl pool or acrylic spa. However, as noted, some hardness will prevent foaming.

There used to be the thought that saturating the water with calcium carbonate would prevent metal corrosion, but that hasn't been definitively determined (see here ) and instead it is low pH and high oxidizer levels that are far more likely to corrode metal (as well as high chloride levels, especially for stainless steel and aluminum).
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Calcium Hardness ?

Postby Entropy » Wed 07 Oct, 2009 19:12

chem geek wrote:...it is low pH and high oxidizer levels that are far more likely to corrode metal...


Richard, it makes sense that low pH will cause metal corrosion for any metals that have Standard Electrode Potentials that are lower than 2H+ + 2e− <> H2(g). The hydrogen ions oxidize the metal and are reduced to hydrogen gas. However, why does a lower pH cause increased corrosion of metals that a have Standard Electrode Potentials that are greater than 2H+ + 2e− <> H2(g), such as copper?

Is it because, while hydrogen ions can't oxidize metals with Standard Electrode Potentials that are greater than 2H+ + 2e− <> H2(g), they still exert a "pull" on the electrons, which makes it easier for other oxidizers to remove electrons?
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Calcium Hardness ?

Postby Entropy » Wed 07 Oct, 2009 19:22

I think that a thin film of calcium carbonate can reduce corrosion due to several reasons. First, the film is an electrical insulator, which decreases galvanic corrosion. Second, it reduces ion migration of oxidized metal ions away from the metal. Third, it provides a buffer to protect the metal from acids. And, fourth, it protects the metal from oxidizers.
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Calcium Hardness ?

Postby chem geek » Wed 07 Oct, 2009 20:43

Entropy wrote:Richard, it makes sense that low pH will cause metal corrosion for any metals that have Standard Electrode Potentials that are lower than 2H+ + 2e− <> H2(g). The hydrogen ions oxidize the metal and are reduced to hydrogen gas. However, why does a lower pH cause increased corrosion of metals that a have Standard Electrode Potentials that are greater than 2H+ + 2e− <> H2(g), such as copper?

Is it because, while hydrogen ions can't oxidize metals with Standard Electrode Potentials that are greater than 2H+ + 2e− <> H2(g), they still exert a "pull" on the electrons, which makes it easier for other oxidizers to remove electrons?

The oxidation or reduction potentials are concentration dependent. The following are reduction potentials for oxygen gas (including dissolved oxygen in water) a pH 0 and pH 14. Remember that these potentials in tables are for ions and aqueous molecules at 1 molar concentration and gases at 1 atmosphere pressure.

O2 + 4H+ + 4e- ---> 2H2O ..... Eo = +1.229V
O2 + 2H2O + 4e- ---> 4OH- ..... Eo = +0.401V

The oxidation potential of solid copper is as follows and remember that this is when the concentration of copper ions is at 1 molar:

Cu(s) ---> Cu2+ + 2e- ..... Eo = -0.3419V

Copper staining usually occurs when the copper ion level is above 0.3 ppm though this is dependent on pH. 0.3 ppm copper is 4.7x10-6 moles/liter concentration. So at this concentration, the half-reaction has an oxidation potential of -0.18V.

The reduction reaction for hypochlorous acid is as follows, again at 1 molar concentrations (so pH 0):

HOCl + H+ + 2e- ---> Cl- + H2O ..... Eo = +1.482V

Even at normal pool chlorine levels with CYA in the water and normal pool pH, the theoretical reduction potential from the chlorine is +1.15V though in practice ORP sensors measure around 700 mV (+0.7V) for reasons I have yet to be explained to me (some of it may be the reference electrode at 230 mV, but that doesn't completely explain it). The dissolved oxygen at pool pH has a theoretical reduction potential of +0.27 V. However, just lowering the pH to 6.0 has the oxygen potential rise to +0.45V and the chlorine potential rise as well.

The net of all this is that lower pH increases the corrosion tendency from oxidizers, not necessarily from the acidity by itself. If you had water free of oxidizers including dissolved oxygen, copper would not corrode even under acidic conditions (see this link , for example). In addition, there may also be catalytic effects at lower pH that speed up the reaction -- remember that thermodynamics only tells you what is possible, not how quickly it will occur.

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Calcium Hardness ?

Postby chem geek » Wed 07 Oct, 2009 20:49

Entropy wrote:I think that a thin film of calcium carbonate can reduce corrosion due to several reasons. First, the film is an electrical insulator, which decreases galvanic corrosion. Second, it reduces ion migration of oxidized metal ions away from the metal. Third, it provides a buffer to protect the metal from acids. And, fourth, it protects the metal from oxidizers.

This is the theory, but obtaining and maintaining that "thin film" may not be as easy as simply saturating the water with calcium carbonate. If the water is saturated enough to maintain that film under normal pool temperature, then firing up the gas heater could increase the tendency for scaling and a buildup of scale is very bad for heaters. As I had linked to earlier ( here ), the concept of having calcium carbonate saturation protect metal from corrosion is not definitive by any means.

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