TA: CHEMISTRY QUESTION

[roger888]

Why is it that one adds sodium bicarbonate to increase total alkalinity: but that one tests total alkalinity by measuring calcium carbonate? It's the bicarb which provides the buffering capacity to resists pH bounce etc. But CaCO3 is chalk. Confused.

[Théière]

Good question, we test the alkalinity of the water and that is actually a test for all forms of carbonates present (calcium and bicarbonates) it also gets affected by high cyanuric acid levels (another reason to get a good tester) we use bicarb to adjust the Alk (it also has an effect on the Ph) but out gassing of carbonates will occur and cause Ph rises hence the buffer required but that will not have a major effect on the alkalinity of the water. It is also important to test the hardness of the water which will also measure the calcium carbonate and magnesium levels which although different they do effect one another and the overall balance of the water. too high and you get scale deposits and that can ruin a sand filter (becomes a chaux blanche filter) [:)] too low and the liner or tiled/plaster surfaces will degrade and repair is expensive.

Again it comes down to testing so dip strips don't help as they are not accurate enough. The water in our area is around 220-240ppm Alk and very hard so it needs careful attention to set up a pool.

[chem geek]

The "ppm CaCO3" is just a unit of measurement.  It is convenient because the molecular weight of calcium carbonate is 100.0869 g/mole.  Total Alkalinity (TA), Calcium Hardness (CH) and often Total Hardness (TH) are all expressed in this unit of measure even though the TA test is measuring mostly bicarbonate (plus some carbonate), the CH test is measuring calcium, and the TH test is measuring magnesium and calcium.  A better unit of measurement that would not require any reference to a specific compound would be molar concentration as in moles/liter (or millimoles/liter) but only chemists use that sort of measuring system.

[roger888]

Now that makes a lot of sense. I've never seen that explained before. (And yes, I remember moles from my chemistry days at school!) Many thanks

[chem geek]

One more annoying fact specifically about TA is that since the units are ppm CaCO3 equivalent, the TA is about carbonate equivalent, not bicarbonate, and carbonate can accept TWO protons (hydrogen ions) so if you are converting from sodium bicarbonate weight to ppm CaCO3, you still need to divide by 2 to calculate the TA change because it would take half as much CaCO3 as bicarbonate (in moles) for the same TA change.  I know this is very confusing, but that's the way it's defined.

For CH, no such factor of 2 is needed since there is only one calcium in CaCO3, the same as in calcium chloride (CaCl2).