randy holmes-farley
Advanced Reefer
- Location
- Arlington, MA
This is a copy of a post that I put up in my reef cemistry forum ( http://www.reefcentral.net/vbulletin/forumdisplay.php?s=ea0da5e35bff855a83d32dc79c349ae6&forumid=112 ), but since many of you don't go there, I thought I'd start up a discussion here as well.
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 previously had an extensive discussion here about whether the acetate that's formed eventually becomes available alkalinity for calcification.
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/bicarbonate 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.
[ December 26, 2001: Message edited by: Randy Holmes-Farley ]
[ December 26, 2001: Message edited by: Randy Holmes-Farley ]</p>
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 previously had an extensive discussion here about whether the acetate that's formed eventually becomes available alkalinity for calcification.
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/bicarbonate 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.
[ December 26, 2001: Message edited by: Randy Holmes-Farley ]
[ December 26, 2001: Message edited by: Randy Holmes-Farley ]</p>
