Using hypochlorite dilution to measure pool volume

[chem geek]

[Ben, feel free to move this thread to The China Shop if it's too technical, though the point of just using rough measurements at some point is valid for all.]

With all of the "roughness" and inaccuracies of the testing methods and percentage of chlorine in chlorine sources, the estimate of "about 30,000 gallons" for you pool will be fine for figuring dosing amounts. At this point, if I were you I would just use the 30,000 number and then note if there seems to be a consistent undershoot or overshoot when you add chemicals to your pool. However, the only chemical you will be adding consistently to your pool will be chlorine (in whatever form).

Calcium Hypochlorite in most sources for pool use is 65-75% while sources for scientific (and some commercial) use it's 100%. It is true that it is less likely for solid calcium hypochlorite to breakdown and if you can find a source that has an MSDS (or labeling) that is accurate, then this would give you a more accurate chlorine concentration value. Then you would have to adjust for the molecular weight of Ca(OCl)2 which is 142.98366 vs. Cl2 gas of 70.906, but then also have a factor of 2 since there are two OCl in each Ca(OCl)2. Lo and behold -- 70.906 * 2 / 142.98366 = 0.992 so you can pretty much just treat the "weight" of calcium hypochlorite as equivalent to the ppm "weight" of FC. Just convert the weight to milligrams and divide by the volume of your pool in liters to get ppm (mg/l).

As for sodium bicarbonate, the conversion of it into alkalinity is not straightforward since this is dependent on the pH of your pool. Only the bicarbonate that remains HCO3- directly contributes one-to-one to TA. At a pH of 7.5, about 5% of the sodium bicarbonate (NaHCO3) that you add (molar percentage) is converted to H2CO3 which does not contribute to TA (and a very small amount is coverted to CO3(2-) which contributes double its amount to TA). At a pH of 7.0, about 15% of the sodium bicarbonate you add (molar percentage) is converted to H2CO3 and is not counted towards TA. Converting from sodium bicarbonate to TA also requires the conversion factor of (100.0892 g/mole CaCO3) / (84.0069 g/mole NaHCO3) / (2 CaCO3 TA / NaHCO3 TA) = 0.596 since TA is measured in units of ppm of Calcium Carbonate (which has twice the TA molecule for molecule as NaHCO3 due to CO3(2-) being able to take two hydrogen).

The Cyanuric Acid (CYA) appears to be 100% pure, but the turbidity test is not very accurate.

Muriatic Acid (31.45% HCl) and possibly Borax would be accurate sources, but their effect on pH in pool water is dependent on the TA (and CYA since CYA also acts as a pH buffer) and the pH test is not very accurate (+/- 0.1 or possibly 0.05 if you are really good) unless you were willing to force your pool to have rather large pH swings.

I hope I have impressed upon you the hopelessness of trying to get an "accurate" answer for the volume in your pool just by using chemicals added to your pool. And this is coming from someone who just loves being accurate!

Richard