I'm not sure which thread you pulled that from, He and I have debated some issues around limewater in some threads here at reefs.org. I don't recall debating adding the lime directly to the vinegar, nor do I remember seeing anyone do it until recently, but I may have simply overlooked that issue.
I don't expect that he shows any equation that demonstrates that putting lime directly into vinegar is better than doing so in the presence of more water, so I cannot show you an equation that is wrong.
This assertion, however, is clearly wrong if it is being used to justify not adding the water first:
First, it will get more Calcium ions (Ca++) into the solution because you are dissolving the Ca(OH)2 in an acid instead of water, and forming Calcium Acetate, which exists as a dissociated equilibrium of free Calcium ions and Acetate ions.
In fact, more calcium hydroxide will dissolve in 1 L of water to which has been added a certain amount of vinegar than will dissolve in that vinegar alone (unless it completely dissolves in both, in which case they are the same).
Second, the Acetic Acid (Vinegar) provides an equivalent of all the CO2 you need to avoid precipitating the newly-added Calcium ions as useless white Calcium Carbonate powder.
I think this statement is a little confusing. As written, it must mean the CO2 generated when the vinegar is metabolized in the tank. If so, then
he is adding way more than enough (twice as much as necessary), but however much it is, it isn't reduced by mixing them in water first.
I think what he meant was that it adds enough acid to prevent precipitation of calcium carbonate if CO2 entered the system from the air. Personally, I think that concern is overblown, but regardless, if you mix in enough vinegar to lower the pH to the point where CaCO3 cannot form in the vinegar/lime mixture when exposed to air, as he does, then he has effectively neutralized the mixture, and has calcium acetate remaining.
That is where the impurity problem comes in. Lime can contain all kinds of stuff that you don't want enetering your tank, and the high pH of normal limewtaer causes it's precipitation as, say, copper oxide/hydroxide. Just look at the colored gunk that is often left behind. These concerns apply to heavy metals, phosphate, arsenic, etc.
The amount of vinegar that Craig recommends is less, and the pH does not drop as much, keeping much of this stuff precipitated (but not quite as efficiently as limewater without vinegar). Full neutralization won't do much if any of this precipitation.
Third, after all the cool Calcium ion chemistry is over, the leftover Acetate ions from the broken-down Vinegar leaves you with free organic Carbon in the water that feeds the bacteria in your tank so that it converts more poisonous Nitrates to NO2 gas (a very good thing).
I don't think that much NO2 gas is made relative to N2, but that aside, the effect is exactly the same whether the mixing is done in water or directly in the vinegar. I also believe that this is an undemonstrated theory. Ive not seen anyone add acetate to a reef tank and observe a sudden decrease in nitrate (by this, or by the more liklely mechanism of simple increased bacterial growth taking up nitrate). It theoretically can happen, but isn't shown that I know of.
