Measuring alkalinity: Filtered vs. Unfiltered

[JCDelbeek]

I just read Habib Sekha's comments in the ReefKeeping Online magazine concerning alkalinity measurements and the effect of phytoplankton and bacteria on alkalinity.

It is standard practice to filter seawater samples through filter discs to remove fines when doing alkalinity measurements. With the use of "milky" kalkwasser additions and calcium reactors that can introduce calcium carbonate fines into the water, I think it is very important to take the same action in home reef aquariums. I have a sneeking suspicion that alkalinity measurements in reef tanks are probably measuring higher values than really exist due to the inclusion of this micro particles. Anyone ever test this??

Aloha!
Charles

 

[Randy Holmes-Farley]

The second part is a good question. I commented on it here:

http://www.reefcentral.com/forums/showthread.php?s=&postid=6880324#post6880324

But I'll comment here on the original article:

http://reefkeeping.com/issues/2006-02/snn/index.php

"Phytoplankton and bacterial cells suspended in seawater make a significant contribution to the measured alkalinity of unfiltered seawater; their contribution is probably next to that of borate ion in most seawater samples. "

Second to borate would put that contribution of suspended organisms to alkalinity at less than that of borate, which puts it at less than about 0.07 meq/L, or 0.2 dKH. So that part may be insignificant to a reef aquarists.

The contribution of suspended CaCO3 will depend, of course, on how much particulate calcium carbonate is present in the sample.