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Re: Reconstituting RO/DI Water


Here is an e-mail I saved for reference from the Aquatic Plant list.  Lucky
for me my water is pretty good (14 ppm TDS) now I have to find some decent

Victor Eng
Vancouver, British Columbia, Canada
 > On 12 June 1999, Roger Miller posted a great note on reconstituting RO
> water (copied at the end of this message).  In this post, he makes
> reference to an earlier post by Neil Frank, Paul Krombholz, and Paul
> Sears, which contained similar info on chemical dosing.  I can't seem to
> find the original post -- perhaps it's under a PMDD instead of an RO
> subject line, in which case I'll never find it!  Can anyone point me in
> the right direction?

It's at http://www.thekrib.com/Plants/Fertilizers/dosing.html

> One thing about Roger's recreation of the formula
> that confuses me (I'm no chemist!) is how those ingredients can generate
> a GH of ca. 3d.  Using recommendations posted numerous times on this
> list, we know that 2 teaspoons of calcium carbonate will raise 13
> gallons by 4d (both KH and GH).  Roger's formula requires 6600 mg of
> calcium carbonate (6.6g, approx. 1 tsp) for 50 gal.  That's 1/2 the
> chemical dose for 4x the water, or 1/8 the previously recommended dose,
> which would seem to then yield 0.5 d GH.

I don't really know the origin of your dosing advise, but in order for 2
tsp of CaCO3 to create 4 dGh in 13 gallons of water it would have to weigh
1.76 grams per teaspoon, which I think is only about a quarter to a third
of the actual weight of a teaspoon of powdered CaCO3.  By my estimates it
would only take something like 1/2 teaspoon of powdered CaCO3 to generate 4
dGH in 13 gallons of water.

You may not be a chemist, but you did a pretty good job of catching a
couple problems I had with calculating the dose from the calcium tablets.

First, the tablets described in Neil's paper contain 600 mg of *calcium*.
I treated them as if they contained 600 mg of *calcium carbonate*.  To
contain 600 mg of calcium the tablets would actually contain 1500 mg of
calcium carbonate.

Second, I mishandled the calculation of Kh from the tablets.  To be clear
I'll state this precisely; I will assume the CaCO3 is dissolved in water
creating a solution with a high pH and that the solution will be allowed
to react with air or with added CO2 until the CO3-- ions are all reacted
to bicarbonate (HCO3-) ions and the pH drops to more reasonable numbers
(say, below 8.5).  My confusion on this point also altered the amount of
sodium bicarbonate needed to make 3 dKH.

> Granted, there is also
> magnesium in the formula (epson salt), which will contribute to GH, but
> clearly less than the calcium (as evidenced by thier final
> concentrations in mg/l, 4.64 vs. 13.95).  So where am I losing it?

The magnesium provides about 1 dGH.  The calcium should provide 2 dGH to
total 3 dGH.

> Similarly, I don't see how KH can be ca. 3d.  We get 0.5dKH from the
> calcium carbonate, plus 2dKH from the sodium bicarbonate (if 1 tsp gives
> 4dKH to 13 gallons, as per previous recommendations, then the 2 tsp / 50
> gallons in this recipe should yield 2dKH, yes?).

According to Neil's paper, a 1/4 teaspoon of sodium bicarbonate weighs 1.3
grams, so a full teaspoon would weigh 5.2 grams (+/-, I'm sure).  5.2 grams
of sodium bicarb would give 3.78 grams of bicarb.  In 13 gallons of water
that's 76.7 mg/l of bicarb, or 3.5 dKH.  Two teaspoons in 50 gallons would
give 2*3.5*13/50 = 1.8 dKH.  Close.

> 2.5dKH is closer to
> the predicted 3 than we got with GH, but still short of the mark.  Can
> anyone put me in touch with the original post, Neil Frank, or an error
> in my calculations?

Kevin, thanks for giving the recipes the careful review they need.  The
corrected recipes follow.

 Chemical                dose/           dose/           measurement
                         100 liters      50 gallons      unit

 epson's salt            3.5             6.5             1/4 teaspoons
 calcium carbonate       2.5             4.5             600 mg tablet
 baking soda             2.5             4.5             1/4 teaspoons
 potassium chloride      1.5             3               1/4 teaspoons

The composition (after the pH drops) should be

                         dose/           dose/           measurement
                         100 liters      50 gallons      units

 calcium                 15.0            14.3           mg/l
 magnesium              4.7             4.6           mg/l
 sodium                   8.9             8.5           mg/l
 potassium              11.8            12.4           mg/l
 bicarbonate            69.4            66.0           mg/l
 sulfate                    18.4            18.0           mg/l
 chloride                 10.7            11.3           mg/l

general hardness    3.2             3.1           degrees
                           57.1            55.0           ppm as CaCO3
 alkalinity               3.2             3.0           degrees
                            56.9            54.1           ppm as CaCO3
 Na/(Ca+Mg+K)            0.27            0.26           molar ratio
 total dissolved solids  79.8            78.9           mg/l

And of course, all the same instructions and cautions that went with the
first draft could be repeated here.

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