How to guess your waters Ca and Mg if all you know is its Hardness

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Silver_Is_Money
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How to guess your waters Ca and Mg if all you know is its Hardness

Post by Silver_Is_Money » Sun Jan 19, 2020 9:49 pm

If all you know with regard to your water is its total hardness, you can make an educated stab at determining its ppm Ca++ and Mg++ ion concentration as follows:

First, convert whatever units your total hardness is reported in into units of ppm (mg/L) of total hardness "as CaCO3".

Next, it appears as if an aggregate average of all of the worlds fresh water indicates that "on average" roughly 70% of fresh waters "total hardness" is derived from Calcium, with roughly 30% from Magnesium.

MW of CaCO3 = 100.0869
MW of Ca++ = 40.078
MW of Mg++ = 24.305

100.0869/40.078 = 2.4973
100.0869/24.305 = 4.118

Total Hardness (as CaCO3) = 2.4973(Ca++) + 4.118(Mg++)

Therefore, on average:

Ca++ ~= (0.70*TH)/2.4973
Mg++ ~= (0.30*TH)/4.118
Developer of 'Mash Made Easy', a free and complete mash pH adjustment assistant spreadsheet

https://mashmadeeasy.yolasite.com/

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Re: How to guess your waters Ca and Mg if all you know is its Hardness

Post by IPA » Mon Jan 20, 2020 8:50 am

You could check if this is correct by entering the same information into Graham's calculator on this forum.
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Silver_Is_Money
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Re: How to guess your waters Ca and Mg if all you know is its Hardness

Post by Silver_Is_Money » Mon Jan 20, 2020 11:18 am

IPA wrote:
Mon Jan 20, 2020 8:50 am
You could check if this is correct by entering the same information into Graham's calculator on this forum.
It will never be "correct" sans for blind luck. But it is fairly close often enough. All it can tell you is that if your water is "average" for its given level of total hardness this is what its calcium and magnesium levels would be. But individual sources of water are rarely "average" in their mineral makeup. That said, if it was all I had to go on, I'd brew with it (if I also knew my waters alkalinity).

An inexpensive fish tank GH/KH water testing kit will give you both GH (Total Hardness) and KH (alkalinity).

If, OTOH, you know your actual Ca++ ppm value, the formula seen above can be solved for actual Mg++, or visa-versa.

But to answer your question, if you were to enter the Ca++ and Mg++ figures derived from the formula seen above, the Total Hardness should indeed match the Total Hardness value as seen for the Graham Wheeler calculator.
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Re: How to guess your waters Ca and Mg if all you know is its Hardness

Post by aamcle » Mon Jan 20, 2020 11:59 pm

United Utilities and probably most other suppliers post a water report on their website, it has the details. I usually use the rolling average in my calculations, my supply is very consistent so it's a reasonable choice.

It's worth checking your suppliers site.


Atb. Aamcle

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