RE: [APD] Re: Nutrient Dosing

["Liz Wilhite" <liz_wilhite at hotmail_com>]

> TW: "In mg/L: Calcium 12, Magnesium 15, Total Hardness (as CaCO3) 93, Total 
> Alkalinity (as CaCO3) 66"

> But if Ca+2 and Mg+2 are the most common, why don't their values add up to 
> most of the other two values? (12+15 versus 93 and 66) Sorry if that's a 
> dumb question, just trying to grok the relationships here. And, "given 
> those readings" where do these values fit in along the spectrum of 
> possiblities? Are they high or just average?

The atomic weight of Ca is 40
at. wt. Mg = 24
At. wt. C = 12
At Wt O = 16
The molecular weight of CaCo3 = 40+12+(3x16) = 100.
mwt MgCO3 = 24 + 12 + (3x16) = 84

If you have 40 mg of Ca in a L of solution and all of that Ca comes from 
CaCO3 then there are 100 mg of CaCO3 in that solution.  In other words, 100 
mg of CaCO3 contains 40 mg of Ca and 60 mg of CO3.

Your water company decided to report ionic concentratons as molecular 
species.  (Don't ask me why -- I have no idea but that reporting standard 
may be required in your state.  WA state has weird reporting requirements, 
too.)  So, what they did was say "assuming that all the CO3 is CaCO3 then 
the water would contain 66 mg of CaCO3."

There are 60 mg of CO3 for each 100 mg of CaCO3.  So your water contains 66 
mg CaCO3 x (60mg Ca)/(100 mg CaCO3) = 40 mg CO3 in each L or 40 mg/L or 
40ppm = (0.056 x 40) KH = 2.2 degrees


Total hardness is different -- many ions contribute to hardness but Ca and 
Mg are most common.  If all the Ca and Mg in your water were existed as 
Carbonates the yiu would have

CaCO3 12 x 100/40 =  30 mg/L
MgCO3 15 x 84/24 = 53 mg/L

53 mg of MgCO3 has the same hardness as 53 x 100/84 = 63

So, the hardness of your water from Mg and Ca (when reported in the insane 
units of CaCO3) is 93 mg/L -- just what they reported.  Note that you can 
calculate what they reported as hardness from the concentrations of Mg and 
Ca they gave.  You just have to jump through hoops to get there. Also, 
according to this, your water has essentially no ions other than Ca and Mg 
contributing to total hardness.

93 mg/L CaCO3 = 93 ppm CaCO3 = 0.056 x 93 GH = 5.2 degrees

HTH

Liz


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