Out of curiosity, whats the difference between total hardness CaCO3 and total alkalinity CaCO3? I use the the total alkalinity one in the EZ water calculator right.
"as CaCO3" is an (unfortunate, IMO) unit for measuring the electronic charges of the species (ions) that result when limestone (calcium carbonate) is dissolved in water by means of carbonic acid.
If one places 100 mg of calcium carbonate in a liter of water very little dissolves but if one bubbles CO2 through the water the following reactions takes place:
H2O + CaCO3 --> Ca++ + CO3--
CO2 + H2O + Ca++ + CO3-- --> 2HCO3- + Ca++
If the CO2 is bubbled through the solution until its pH reaches about 8.3 the net overall reaction is
CO2 + H2O + CaCO3 --> 2HCO3- + Ca++
which says that all the calcium carbonate will be separated into its components which are the calcium ion and carbonate ion and that the carbonate ions will convert to bicarbonate (HCO3-) ions. In fact only 99% of them do but we can ignore that.
The gram molecular weight of calcium carbonate is 100 mg/millimole thus we have put 1 mmol of CaCO3 into this liter of water and have obtained 1 mmol of Ca++ ions with charge 2 and, therefore, 2 mmol of + charges. A mmol of charge is referred to as a 'milliequivalent' so we have 2 mEq of + charges. We aso got 2 mmol of bicarbonate ions each with its single negative charge of -1 so we have 2 mmol (2 mEq) of negative charge as well.
When alkalinity is measured acid (a source of H+) is added to a sample until all the bicarbonate is converted to CO2 and leaves the solution.
H+ + HCO3- --> H2CO3 --> H2O + CO2
The point where nearly all the HCO3- has been converted and removed is detected by measuring the pH of the solution and the reaction is generally considered complete when the pH is in the low 4's. The sample we have prepared, with its 2 mmol of bicarbonate, will clearly require 2 mEq of H+ to clear that bicarb and so we say its alkalinity is 2 mEq/L. In Europe they stop there but we don't in north America.
When hardness is measured a special agent called a chelating agent (EDTA) is added to the sample untill all the Ca++ is 'chelated' (chela means 'claw') which is indicated by a colored dye changing from red to blue. The chelating agent has to capture 2 mEq of positive charge (from the 1 mmol of Ca++). Just as the number of mEq of H+ provided by a mL of calibrated acid solution is known so is the number of mEq of Ca++ charge grabbed by a mL of a calibrated chelating solution. A mL of 1N acid releases 1 mEq of H+ and a mL of 1N EDTA chelates 1 mEq of Ca++. So just as we would add 2 mL of 1 N acid to our 1 L sample to reach pH near 4 and conclude the alkalinity is 2 mEq/L we would have to add 2 mL of 1 N EDTA to the sample to get the hardness indicator to turn blue and would call the hardness 2 mEq/L. Again in most of the world that's the end of the story but in the US somebody decided given the way in which bicarbonate gets into most natural water is by the mechanism we just described and as each mEq corresponds to 50 mg of calcium carbonate dissolved that it would be convenient to multiply the mEq/L of alkalinity and hardness both by 50 which tells us immediately how much limestone went into the water. In our case we have 2 mEq/L alkalinity, which derived from 100 mg/L limestone dissolved, and 2 mEq/L calcium harndess which derived from 100 mg/L limeston. So we multiply by 50 and have 100 mg/L alkalinity, 'as calcium carbonate' and 100 mg/L calcium hardness 'as calcium carbonate'.
That is convenient but only works when the limestone is dissolved by CO2. If we dissolve 100 mg of limestone with hydrochloric acid we would have
CaCO3 + H2O + 2 HCl- ---> Ca++ + HCO3- + Cl-
Now there is still 2 mEq/L calcium hardness (100 mg/L as calcium carbonate) but only 1 mEq/L alkalinity (50 mg/L as calcium carbonate).
This leads to confusion when 'as calcium carbonate' numbers are given. I advise people to not to even try to remember all this and just, when they see 'as CaCO3' divide by 50 and work in mEq/L.
To add further to the confusion there are spreadheets and calculators abroad the tell you, among lots of other things, of course, what happens if you add a mmol (74 mg) of slaked lime (Ca(OH)2) to a liter of water. The result is 2 mEq/L of hydroxyl ions which result in 2 mEq/L alkalinity and 2 mEq/L calcium hardness. Most of these spreadsheets will list both the alkalinity and harndess for this case as 100 mg/L as calcium carbonate even though no calcium carbonate was used. Note that this is not incorrect. Just confusing though one takes the confusion to an even greater height by telling you that your solution contains 122 mg/L bicarbonate ion in this case when clearly it contains none.
So that's the relationship between alkalinity and hardness 'as CaCO3' in all its glory. 'As CaCO3' is here to stay. It's been used in the water industry for too long though I've noticed that hardness is usually reported in mg/L of the metal (calcium or magnesium) and seldom as CaCO3. Perhaps alkalinity will start to be reported in mEq/L. Until that happens, if it ever does, just divide by 50 and think mEq/L.
