cousinkenni said:
Why do you divide by 21.8 instead of what Edward says to divide by which is 17.86. There is quite a difference between the two numbers. So what number is correct and why would Edward post the incorrect number if your number is correct?
Which is heavier: one ton of bird feathers or one ton of steel? To put it another way, which is longer: 1 meter or 100 centimeter? They all express the same quantity right? :mrgreen: ...except that each is measured in different units. The same concept applies here:
1dKH = 17.86 ppm CaCO3
1dKH = 21.8 ppm HCO3
KH (and GH) is usually expressed as CaCO3 equivalents so that's why a lot of people omit the CaCO3, assuming that it is understood/implied. You used CO3 so 17.86 is the appropriate conversion factor, whereas I used HCO3 so 21.8 is my conversion factor. They're all the same thing.
cousinkenni said:
From my calculations (once again I weighed 1/8tsp) 0.5217g NaHCO3 should give 4.78ppm rise in 78L. Is this the same number that you get or does NaHCO3 dissociate to form another ion inconjunction with CO2? [...] Another way of thinking about it is that 4g NAHCO3 would give a 2.66dKH rise in 50L Correct?
That is absolutely correct Ken. [smilie=k:
Techinically: yes, the HCO3-- does go on to react with CO2 but that's a different calculation: one that leads us to the pH/KH/CO2 relationship that everyone uses to measure CO2 concentration. ;-) But for our purpose of calculating KH, no, that's not important because NaHCO3 is VERY soluble in water, forming Na+ and HCO3- (which is stable at our pH) directly. This is why everyone in this thread advises grandmasterofpool to use baking soda instead of CaCO3.
[b]NaHCO3 => Na+ + HCO3-[/b]
Conversely, CaCO3 is not very soluble in water. It needs an acid to do so. In our tanks, that acid is the carbonic acid (H2CO3), formed from the injection of CO2 gas. That is why it is included in the equation below and not the NaHCO3 equation above.
CaCO3 + H2CO3 => Ca(HCO3)2 => Ca++ + 2(HCO3--)
CO2 + H2O <=> H2CO3 <=> HCO3- + H+ (I skipped this part in my previous post and just shorthanded the whole thing. Sorry. But this is the source of the carbonic acid that is required to dissolve CaCO3.)
***Recall the solubility rules from general chemistry (I don't recall but I've just googled the net) that (1) compounds of CO3-- are not soluble in water except those with NH4+ and Group IA; but (2) compounds of HCO3- with Groups IA & IIA metals are. So, looking back, we see that Ca
CO3 is not soluble until it becomes Ca(
HCO3)2 whereas Na
HCO3 is immediately soluble. If my teacher had an aquarium in class, then I would have paid more attention. Live and learn. [smilie=l:
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Could you have asked a more involved question? Speaking fo which, please don't ask me where babies come from. ::laughs::