Hardness & PH.

[Lotusman]

Hello to all,
Had a question the other day about water. "How does hardness support PH"? I was not quite sure of the answer, so I ask the question here. Thanks.

[KoiCop]

One simple answer would be that sufficient carbonate hardness (KH, or alkalinity) keeps the pH from dropping too low, while sufficient general hardness (GH, especially the calcium and the magnesium) keep the pH from climbing too high.  So when properly supported by both hardnesses (KH and GH), the pond will establish and maintain the pH it wants to.

And don't worry: Once Rick sees this thread, you'll have a proper answer. 

[MCA]

...from my water quality presentation at the Koi Health Seminar in Feb.


GH is the symbol used for General Hardness of the water.  This is the level of “lite metals” such as calcium and magnesium
DH/dH is another scale for general hardness and stands for German degrees of hardness:  1DH = 17.9ppmGH
Ultra soft water makes it harder for nishikigoi to replace electrolytes lost in urine
GH ranges:
0-25 mg/l   very soft
25-75 mg/l   soft
75-150 mg/l   medium
150-300 mg/l   hard
Over 300 mg/l   liquid rock


KH is the symbol used to represent alkalinity or carbonate hardness.  It stands for the German word Karbonathärte.  The alkalinity is a measurement of the water’s buffering ability, or its ability to neutralize acid.
Ways to increase KH:
Water changes to replace spent KH
1 pound of sodium bicarbonate (baking soda) per 1000 gallons will raise the KH by 71ppm
Crushed/powdered oyster shells, coral, limestone, marble in water column



Either GH or KH in the 50-100mg/l (or ppm) is a gracious plenty.   For good skin quality...you do NOT want hard water.    Hard water is not good for the house plumbing either!!! 

[RickF]

KH (carbonate hardness) is a measure of the ability of the water to resist a change in pH.  Although KH is called "carbonate hardness" and is expressed in terms of the parts per million of carbonate that would be required to explain the buffer capacity (risistance to change in pH) of the water, the reality is that any buffer (any combination of a weak acid and its salt) that is present in the water contributes to the KH.  There can be absolutely zero carbonate (CO3--) in the water, and yet the water can have a very high KH.  The higher the KH, the more stable the pH will be, but KH tells you nothing about what the pH is - only how difficult it will be to change the pH.

GH (general hardness) is the measure of all of the divalent ions (e.g., Ca++, Mg++, Fe++, etc) in the water.  GH is also expressed in terms of parts per million, but it does not matter what the divalent ions are - they all contribute to the GH.

Where KH, GH, and pH come together is when baking soda (sodium bicarbonate or NaHCO3) is used to artificially raise the KH.  This is a common practice in areas where the source water has little or no naturally occurring KH.  When the KH is less than 100 ppm, it does not take much to change the pH drastically, so those of us in areas with extremely soft water add baking soda to raise the KH.

Baking soda, though, is an incomplete buffer.  It will prevent the pH from dropping below 8.3, but by itself, it does nothing to prevent the pH from going higher than 8.3.  This is where GH, and specifically Ca++ comes to the rescue.  In water, baking soda becomes Na+ (sodium ion) and HCO3-(bicarbonate).  If acid is added to the water, the extra H+ is taken up by the HCO3- to form carbonic acid (H2CO3), which dissassociates to H20 and C02.  With adequate aeration, the CO2 is driven out of the water, and the pH remains at 8.3.  When an H+ is removed from the water or an OH- is added (either of which would increase the pH), the HCO3- gives up an H+ to form CO3-- (carbonate).  As long as there is much more HCO3- than CO3--, the pH will be 8.3; however, as the CO3-- concentration increases in relation to the HCO3-, the pH will start to go up.  If there is Ca++ in the water, as soon as CO3-- is formed, it will combine with Ca++ to form CaCO3 (calcium carbonate, or chalk).  Calcium carbonate has very limited solubility in water, so it precipitates out as a soft white crystal, and what remains in the water is HCO3-, so the pH remains at 8.3.

Since GH is a measure of all divalent ions, it is possible to have a high GH with no Ca++, but it is not possible to have Ca++ in the water without having a measurabel GH.  If the source water has no KH and no GH, then the usual practice is to add baking soda to raise the KH to about 150 and equal parts of Epsom salts (MgSO4) and calcium chloride (CaCl) to raise the GH to about 100 ppm.  Both the Mg++ from the Epsom salts and the Ca++ from the calcium chloride contribute to the GH, but only the Ca++ is helping to prevent the pH from going above 8.3.

[Lotusman]

Thank you all, gentlemen. I will pass this information along.