Vinegar and Alkalinity: Problem?

[Randy Holmes-Farley]

In preparation for an article on alkalinity, I got to thinking about how the vinegar that many use may be causing problems in terms of alkalinity. We had an extensive discussion here about whether the acetate that's formed eventually becomes available alkalinity for calcification:

http://archive.reefcentral.com/vbull...2&pagenumber=1

The point of interest now is that the acetate that forms will interfere with alkalinity testing. The pKa of acetate/acetic acid is just below 4 (4.75 in fresh water but somewhat lower in seawater). If here is appreciable acetate in the tank water it will be partially "counted" as alkalinity.

An alkalinity measurement (or titration) usually drives the pH down to about 4-5. Consequently, some of the acetate present will become protonated during the titration, and show up as alkalinity.

FWIW, I did the test myself by adding acetate to my tank water (in a beaker) and then measured the alkalinity. It is appreciably raised by the acetate present (assuming, of course, that you add enough acetate).

So how can this be a problem?

Well, I'd like to throw this open for discussion, but here's one possibility:

That in some situations, the conversion of acetate to OH- to HCO3- may be slow enough that it is limiting the availability of HCO3- to corals (assuming, for the moment, that that is what they want when they calcify). And that by measuring alkalinity only, you may not know this.

Now, the conversion of acetate to OH- to HCO3- must happen at some appreciable rate or else those using vinegar/limewater would see the alkalinity rise over time as acetate built up. Have we seen enough cases to know this isn't happening?

The more vinegar that people use, the worse this problem would be. I've heard of a few people that use far more than Craig suggested in his Aquarium Frontiers paper. These would be the people most at risk for having a problem.

In many cases, however, it is possible that the people using vinegar have appreciably lower carbonate alkalinity than they think, and this might possibly be an issue for the corals.

What can you do?

Here's one suggestion if you are chemically inclined: do a carbonate only alkalinity titration. Using a pH meter, titrate from your usual pH to 7.5 or so. That counts all of the carbonate present, but not any of the acetate or bicarbonate. If that value is in line with expectation then you are OK (say, 0.3 to 0.6 meq/L, I got 0.6 meq/L in a tank with a total alkalinity of 3.6 meq/L and a starting pH of 8.45, and also got 0.6 meq/L in the same tank water to which had been added enough acetate to drive the total alkalinity to 10-26 meq/L (depending upon what pH one thinks of as the endpoint)).

If the result is appreciably lower than 0.3 meq/L, then you may have an alkalinity dominated by acetate.

[Guy]

I am chemically challenged so my answer has nothing to do with chemistry.

I had thought that acetate was bacterial candy and would be consumed at an alarming rate in a reef aquarium environment. If so I don't understand how Alkalinity could be dominated by acetate.

On the other hand, i was recently asked how KH could be low and ALK way high (off the scale). I may have incorrectly answered that the test kit may be bad. Possibly this was an instance of acetate dominated ALK ie. ADA syndrome.

[6-line]

I am chemically challenged so my answer has nothing to do with chemistry

I fall under this catagory as well.
So.... I hope to keep checkling back and all you chemically competent people will sort this senario out and I can decide whether to keep doing my kalk with vinegar or not.
I'm thinking of suspending the practice of adding it for the time being...

[Randy Holmes-Farley]

It is clear that bacteria can convert acetate into CO2 and OH-. In fact, I read it again in a new book that I just got for Christmas: "Aquatic Chemistry".

The question is how fast? If people are dumping acetate for most or all of their alkalinity, is it being decomposed fast enough that it is not a significant factor in alkalinity determinations?

I'm not sure if we know the answer to that question, though maybe someone does.