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It's been a long time since I did any molar chemistry in school and despite doing a lot of reading on the subject this week, I am making a mess of some basic calculations. I hope that someone who understands water chemistry & molar equations a lot better than I do can point me in the right direction.

I am trying to add mineral salts to soft water to adjust the calcium, magnesium and alkalinity in a predictable manner. But I can't get past the basic equations so far.

The source water I am working with is:

12.6 mg/L Hardness as CaCO3 equivalent
11.3 mg/L Alkalinity as CaCO3 equivalent
4.8 mg/L Calcium
0.2 mg/L Magnesium
1.5 mg/L Sodium

And the desired target water is:

70 mg/L Hardness as CaCO3 equivalent
40 mg/L Alkalinity as CaCO3 equivalent
20 mg/L Calcium
4.9 mg/L Magnesium
8 mg/L Sodium

I would prefer to have most of the hardness derive from calcium, rather than magnesium if possible. I plan to try

  1. adding sufficient baking soda, NaHCO3 to the source water to obtain 8 mg/L sodium, then
  2. add sufficient calcium carbonate (chalk), CaCO3 to the source water to make 50/50 GH/KH water (general hardness/carbonate hardness) i.e. 50 mg/L hardness and 50 mg/L alkalinity, both expressed as CaCO3 equivalents
  3. add sufficient lactic acid (88% CH3CH(OH)COOH) to neutralize some carbonate and (I hope) get to 50/40 GH/KH
  4. add sufficient epsom salt, MgSO4*7H2O to obtain a final result of 70/40 GH/KH.
    This assumes that adding lactic acid won't remove (precipitate) calcium hardness.
    The solubility of calcium carbonate in water shouldn't be an issue, I can put it under pressure with carbon dioxide, C02 if necessary to dissolve it.

If the above is not feasible, then alternately

  1. add sufficient baking soda, NaHCO3 to the source water to obtain 8 mg/L Sodium
  2. add sufficient calcium carbonate, CaCO3 to reach the alkalinity target of 40 KH (40/40 GH/KH) i.e.. 40 mg/L hardness and 40 mg/L alkalinity, both expressed as CaCO3 equivalents
  3. Add non-carbonate hardness by adding sufficient epsom salt, MgSO4*7H2O to obtain a final result of 70/40 GH/KH

Any help, guidance as to molar calculations and best practices in water chemistry or well honed levity welcome

Edit to add molar amounts
Source water
12.6 mg/L Hardness as CaCO3 equivalent
11.3 mg/L Alkalinity as CaCO3 equivalent
Ca 2.4 x 10-4 M
Mg 1.7 x 10-5 M
Na 1.4 x 10-4 M

Target water
70 mg/L Hardness as CaCO3 equivalent
40 mg/L Alkalinity as CaCO3 equivalent
Ca 1.0 x 10-3 M
Mg 4.1 x 10-4 M
Na 7.3 x 10-4 M

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  • $\begingroup$ I advice to recalculate all masses to molar amounts according to the molar mass tables. You will find the ion matching easier. $\endgroup$ – Poutnik May 4 at 7:34
  • $\begingroup$ Is not better to get a mineral water of suitable composition and dilute it with this water ? As drinking water with added chemicals can have unpredictable taste and smell. $\endgroup$ – Poutnik May 4 at 7:37
  • $\begingroup$ BTW, 1 dGH/dKH are equivalent of 10 mg CaO of general/carbonate hardness, the latter known also as alkalinity. $\endgroup$ – Poutnik May 4 at 7:45
  • $\begingroup$ I have also consider the final pH of water, and that by adding of lactic acid, you create a double oH buffer of lactic acid/lactate and carbon dioxide/bicarbonate. Additionally, you will get into computation troubles in determination of the right amount of acid for dissolving CaCO3. $\endgroup$ – Poutnik May 4 at 8:05
  • $\begingroup$ Be also aware to use the right optical isomer of the acid. $\endgroup$ – Poutnik May 4 at 8:08
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The parallel thoughts:

The source water I am working with is:

12.6 mg/L or 0.126 mmol/L GH as CaCO3 equivalent
11.3 mg/L or 0.113 mmol/L KH as CaCO3 equivalent
4.8 mg/L or 0.12 mmol/L Ca
0.2 mg/L or 0.0082 mmol/L Mg
1.5 mg/L or 0.065 mmol/L Na

And the desired target water is:

70 mg/L or 0.7 mmol/L GH
40 mg/L or 0.4 mmol/L KH
20 mg/L or 0.5 mmol/L Ca
4.9 mg/L or 0.2 mmol/L Mg
8 mg/L or 0.35 mmol/L Na

What is needed to add:

0.574 mmol/L GH, as Ca+Mg
0.287 mmol/L KH as Ca+Mg as bicarbonates, I.e. 2x0.287=0.574 mmol/L of bicarbonate
0.38 mmol/L Ca
0.192 mmol/L Mg
0.285 mmol/L Na

Reaction of 0.285 mmol/L sodium bicarbonate
with 0.285 mmol/L lactic acid
releases 0.285 mmol/L carbon dioxide,
dissolving 0.285 mmol/L calcium carbonate
forming 0.285 mmol/L calcium bicarbonate.

( Not exact due equilibrium reactions but good enough )

What is needed to add:

0.287 mmol/L GH, i.e Ca+Mg

0.093 mmol/L Ca
0.192 mmol/L Mg

Adding 0.192 mmol/L MgSO4.

What is needed to add:

0.093 mmol/L GH, i.e Ca only

0.093 mmol/L Ca

This can to be added as 0.093 mmol/L calcium carbonate(optionally supported by pressured carbon dioxide) + 0.186 mmol/L lactic acid.

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