ph/hardness relationship - The Planted Tank Forum
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post #1 of 5 (permalink) Old 10-04-2018, 06:58 PM Thread Starter
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ph/hardness relationship

I've always understood ph as a simple acid/alkaline attribute (<7 acid, >7 alkaline). Hardness I understood to be the amount of salts/minerals in the water (0 being distilled water, 30+ being crazy-hard).


What I haven't quite wrapped my head around is the interactive relationship between the two. Generally speaking it seems that hard water is alkaline, while soft water is acidic. Is it possible to have the opposite (hard/acidic or soft/alkaline), or is there an intrinsic relationship here such that if your water is acidic, you can pretty much assume its soft?
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post #2 of 5 (permalink) Old 10-04-2018, 09:09 PM
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Hi @Ziggy,

Not necessarily. dGH measures the dissolved divalent metals in solution, most of which is usually calcium and magnesium. Dissolved calcium solutions are usually high alkaline however dissolved magnesium solutions are acidic. So you could have a high dGH that is mostly calcium and that would be alkaline or a high dGH that is mostly magnesium which would be acidic. I have a low tech 10 gallon that has a [email protected]; [email protected] and [email protected] - so soft water however alkaline.

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post #3 of 5 (permalink) Old 10-05-2018, 12:41 AM
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Quote:
Originally Posted by Ziggy View Post
Is it possible to have the opposite (hard/acidic or soft/alkaline)
Yes they are possible.

I prefer to call water acidic or basic.
Alkalinity is the buffering capability of water, the ability of an acid to change the pH.
This measurement would be KH(carbonate/bicarbonate hardness or [TA] total alkalinity).

Suppose one uses an RODI system to "make" water.
End result is pH of 5.0, GH =0, and KH = 0, and TDS is zero.
Would you call this water acidic or basic?


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post #4 of 5 (permalink) Old 10-05-2018, 11:32 AM Thread Starter
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pH of 5.0, GH =0, and KH = 0, and TDS is zero. Would you call this water acidic or basic?

Trick question? Seriously though, I dont know. At face value I would have said acidic simply because the ph is lower than 7 but I'm guessing you are going to say its neither acidic nor basic by virtue of the absence of solids?
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post #5 of 5 (permalink) Old 10-05-2018, 04:07 PM
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Originally Posted by Ziggy View Post
Generally speaking it seems that hard water is alkaline, while soft water is acidic. Is it possible to have the opposite (hard/acidic or soft/alkaline), or is there an intrinsic relationship here such that if your water is acidic, you can pretty much assume its soft?
Hard water is usually alkaline and soft water acidic because of the minerals naturally surrounding waterways.

In nature, some mineral compounds (rocks) are Anhydrite CaSO4 and Epsomite MgSO4 that will make water hard while being acidic. Because these sulfate species dissolve easier than carbonate species they are usually less common. Mostly, we have Calcite CaCO3 and Magnesite MgCO3 which create hard water and high alkalinity.

This means, that RO water can be modified in either way;

A. Hard (high GH) and alkaline (high KH, high pH) using CaCO3 and MgCO3 slowly, or CaSO4, MgSO4 and NaHCO3 faster due to solubility speed.
B. Hard (high GH) and acidic (low KH, low pH) using CaSO4 and MgSO4.
C. Soft (low GH) and alkaline (high KH, high pH) using NaHCO3.
D. Soft (low GH) and acidic (low KH, low pH) using pure RO.
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