Hi fellow coffee nerds, I thought it would be interesting to get a sense of the GH/KH ratios as well as the particular Mg2+ and Ca2+ concentrations across the United States (and beyond?) using the Reef Foundation® Pro Test Kit advocated by Christopher Heddon, one of the author's of 'Water for Coffee'.

I'll start off:

Location: Upper West Side, NY, NY, USA
GH/KH: 0.65

KH: ~20 ppm HCO3- as CaCO3
[20 ppm first time]
[22.5 ppm second time]
[both times used 10cc of sample water with precision of 0.01 cc = 2.5 ppm HCO3- as CaCO3]

Mg2+: ~10 ppm Mg2+ as CaCO3
[20.5 ppm first time using 16cc of sample water with precision of 0.01 cc = 10.25 ppm]
[10.25 ppm second time using 32 cc of sample water with precision of 0.01 cc = 5.125 ppm]

Ca2+: <3 ppm Ca2+ as CaCO3
[6.25 ppm first time using 10 cc of sample water with precision of 0.01 cc = 6.25 ppm Ca2+ as CaCO3]
[3.125 ppm second time using 20 cc of sample water with precision of 0.01 cc = 3.125 ppm Ca2+ as CaCO3]

TDS 49 ppm (used HM digital TDS-3 TDS/TEMP probe a friend lent me)

Alex

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KH:
if you use 10 cc of sample water, why does 0.01 cc KH titrant = 2.5 ppm HCO3- as CaCO3?

per the kit, 0.01 cc KH titrant = 0.05 mEq / L = 0.05 mmol / L since an equivalent is just the number of moles x the valence, which is 1 if KH = HCO3-. to convert from moles of HCO3- to mg of HCO3- as CaCO3, you need to remember that 1 mol CaCO3 is proportionate to 2 mol HCO3- because CaCO3 + H2O + CO2 <-> Ca(HCO3-)2.

now for some stoichiometry:

0.01 cc KH titrant = 0.05 mmol / L HCO3- * (1 mol HCO3- / 1000 mmol HCO3-) * ( 1 mol CaCO3 / 2 mol HCO3- ) * ( 100 g CaCO3 / 1 mol CaCO3 ) * ( 1000 mg CaCO3 / 1 g CaCO3 ) = 2.5 mg/L HCO3- as CaCO3 = 2.5 ppm HCO3- as CaCO3

Mg2+:
if you use 32 cc of sample water , why does 0.01 cc Mg2+ titrant = 5.125 ppm Mg2+ as CaCO3?

per the kit, 0.01 cc Mg titrant = 20 ppm Mg2+ using 2 cc of water. since I used 32 cc of water, 0.01 cc Mg titrant = 20/16 = 1.25 ppm Mg2+ = 1.25 mg / L Mg2+. now for some stoichiometry:

1.25 mg / L Mg2+ * ( 1 g Mg2+ / 1000 mg Mg2+ ) * ( 1 mol Mg2+ / 24.3 g Mg2+ ) * (1 mol CaCO3 / 1 mol Mg2+) * ( 100 g CaCO3 / 1 mol CaCO3) * ( 1000 mg CaCO3 / 1 g CaCO3) = 5.125 mg / L Mg2+ as CaCO3

Ca2+
if you use 20 cc of sample water, why does 0.01 Ca2+ titrant = 3.125 ppm Ca2+ as CaCO3?

per the kit, 0.01 cc Ca2+ titrant = 5 ppm Ca2+ using 5 cc of water. since I used 20 cc of water, 0.01 cc Ca2+ titrant = 5/4 = 1.25 ppm Ca2+ = 1.25 mg / L Ca2+. now for some stoichiometry:

1.25 mg / L Ca2+ * ( 1 g Ca2+ / 1000 mg Ca2+ ) * ( 1 mol Ca2+ / 40.078 g Ca2+ ) * (1 mol CaCO3 / 1 mol Ca2+) * ( 100 g CaCO3 / 1 mol CaCO3) * ( 1000 mg CaCO3 / 1 g CaCO3) = 3.125 ppm Ca2+ as CaCO3

Global Customized Water AB Formula

GH/KH: 3.3

KH: ~30 ppm HCO3- as CaCO3
[used 10cc of sample water with precision of 0.01 cc = 2.5 ppm HCO3- as CaCO3]

Mg2+: ~25 ppm Mg2+ as CaCO3
[used 32 cc of sample water with precision of 0.01 cc = 5.125 ppm Mg2+ as CaCO3]

Ca2+: ~75 ppm Ca2+ as CaCO3
[used 20 cc of sample water with precision of 0.01 cc = 3.125 ppm Ca2+ as CaCO3]

TDS 155 ppm (used HM digital TDS-3 TDS/TEMP probe a friend lent me)

icantlactate wrote:GCS Water

? Not sure what that is. Perhaps is water made from Global Customized Water's AB formula kit?

Whoops, you're right, I meant Global Customized Water AB Formula.

If that's the case, I suspect the Mg number may actually be zero. Check the vials for the contents - If it lists calcium chloride, potassium bicarbonate, sodium bicarbonate, and nothing else, then I don't think it contains any magnesium. When you have significant calcium, I suspect the Reef Foundation kit may make it look like you also have a small amount of magnesium when there actually is none.




P.S.
Your TDS, alkalinity, and Ca hardness measurements look reasonable, since I think those formula were developed by David Beeman, and he recently recommended something like 30 ppm alkalinity, and 77 ppm hardness as an ideal for coffee and tea (See pg 10: https://issuu.com/kerrigoodman9/docs/ctmagazine.2015.06 ) He also recommends 150 TDS, and I'm confident that he is referring to a TDS meter reading. (If you calculate an actual dry residue TDS for this formula at GH/KH of 77/30, you get a value of 115 or less, but the conductivity would probably be up in the ~150ppm range on a typical meter.)

Thanks homeburrero! Your input is always appreciated.

Your hypothesis of a false positive measurement of magnesium by the Reef Foundation Pro Titration kit is interesting.

So I used the kit to measure my bespoke water which I made by pouring out 40g from a 1 gallon jug of distilled water and then adding back ~20g of mg2+ concentrate (225 g epsom salts dissolved in 1 gallon of distilled water) and ~20g of hco3- concentrate (75 g baking soda in 1 gallon of distilled water):

GH/KH: 2.5/1

KH: ~57.5 ppm HCO3- as CaCO3
[10cc of sample water with precision of 0.01 cc = 2.5 ppm HCO3- as CaCO3.]

Mg2+: ~143.5 ppm Mg2+ as CaCO3
[16cc of sample water with precision of 0.01 cc = 10.25 ppm]

Ca2+: didn't measure

Based on my calculations I expected 120 ppm Mg2+ as CaCO3 and 60 ppm HCO3- as CaCO3. It is funny that the Mg2+ measurement was higher than expected and the HCO3- measurement was pretty much spot on.