Alkaline-earth-metal buffers for increasing alkalinity?

[pszemol]

Sorry for asking, but I am still very confused with alkalinity of my reef tank water...

I have read in many places people using sodium carbonate (washing soda) to bump up alkalinity. Fine, but it also increases pH because CO3- ion attracts H+ ion and then they form more stable CO2 and we lose H+ ions resulting in pH increase, sometimes to the levels much too high to accept. Also, carbonate ion is very unstable and the change of alkalinity after droping sodium carbonate is only temporary.

Somewhere else I have read about a different method: using some natural alkaline-earth-metal buffers could be better. Some manufacturers even make formulas using alumino-silicate buffers instead of sodium carbonate... (see: Formula No 28 and No 14pH by "Sea-Lab Marine Aquarium Products", Redondo Beach, CA)

For me, not a chemist "alumino-silicate" sounds kind of scarry: is it a smart move to introduce silicates into the solution of a reef tank? If silicates are a limiting factor for diatom blooms in a common tank, aren't we risking such diatom blooms when using alumino-silicates for alkalinity increase?
Also, another concern - how do these earth-metals work? Will popular tests designed for carbonate ions detection, detect correctly alkalinity levels increased with alkaline-earth-metal buffers like alumino-silicates?

Please explain...

On a side note - I am using Sea-Lab Formula 28 as a main calcium suplement in my small tank for years and my calcium levels have been always high. Currently at 575mg/l mark. Alkalinity level measured with Seachem Magnesium/Alkalinity test turned out to be 2.5meq/l for total alkalinity and unfortunately almost ~1meq/l for borate alkalinity - the calculation of "total"-"borate"="carbonate" makes very low, about 1.5meq/l carbonate alkalinity in my tank, so I am considering a method of increasing slowly and safely my neglected alkalinity to desired levels and Sea-Lab No14pH showed up on my shelf totaly forgotten. The lecture of dusted product label rised my concerns I am sharing with you today. :eek1:

[Randy Holmes-Farley]

I have read in many places people using sodium carbonate (washing soda) to bump up alkalinity. Fine, but it also increases pH because CO3- ion attracts H+ ion and then they form more stable CO2 and we lose H+ ions resulting in pH increase, sometimes to the levels much too high to accept. Also, carbonate ion is very unstable and the change of alkalinity after droping sodium carbonate is only temporary.

Partly true, and partly not.

Sodium carbonate (washing soda) will raise pH and will also permanently raise alkalinity, at least as permanently as any other method. There is nothing temporary about it.

It is true that some CO3-- will pick up H+ at reef tank pH:

CO3-- + H+ ---> HCO3-

and that is why the pH rises, but the alkallinity in that reaction is unchanged.

If you want to raise alkalinity and not raise pH, baking soda (sodium bicarbonate; NaHCO3) is a good choice. It has a very tiny pH lowering effect, and a permanent alkalinity raising effect.

If you want a totally neutral pH effect, you can use a mixture of mostly baking soda, and a little washing soda or baked baking soda (sodium carbonate).

I discuss and show some of these effects in these articles:

Solving Calcium and Alkalinity Problems
http://www.advancedaquarist.com/issues/nov2002/chem.htm


The Relationship Between Alkalinity and pH.
http://www.advancedaquarist.com/issues/may2002/chem.htm

FWIW, I would not use any other buffers. You need to provide a ton of carbonate alkalinity to reef aquaria to resupply what is taken out via calcification. Adding other buffers (like borate) may help stabilize pH, but will mess with the standard interpretations of alkalinity, and will also result in those potentially toxic ions rising in the water. Aluminum is toxic. Silica drives diatoms. I wouldn't want either rising.

I do not recommend the Sea Lab 28 blocks for any reef aquarium. The claims of it and similar products that it can "control" the levels is just not reasonable.

FWIW, aluminum and silica are not alkaline earth metlas. Those inlcude magnesium, calcium, strontium, beryllium, radium, and barium.