phenolphthalein alkalinity

[eroan]

Why do some alkalinity test kits include a phenolphthalein test?
Does a saltwater reef tank running below a ph of 8.3 have any carbonate alkalinity as opposed to a bicabonate alkalinity? Thiel was a fan of adding sodium carbonate to boost the ph, does this get converted to bicarbonate or is it that once the ph is above 8.3 we now have a carbonate component in the tank water?

 

[randy holmes-farley]

Seawater has a substantial amount of carbonate, and this amount is critical for the buffering of the water. The amount of carbonate is largely fixed by the pH, and varies from about 7% total the bicarbonate/carbonate at pH 7.8 to 28% at pH 8.5.

FWIW, the phenolthalein endpoint is of little use to marine aquaria keepers. It has no simple interpretation in seawater, though it does in fresh water (becasue the pKa of bicarbonate/carbonate is shifted low enough in seawater that the phenolthalein endpoint misses much of the carbonate titration). So don't bother with it.

Adding sodium carbonate (washing soda) is a fine way to supplement alkalinity if you also want the pH to rise, but I discourage people from adding things just to adjust pH, because they frequently screw up alkalinity that way.