Precipitation at very high pH

[norskfisk]

Here is a chemical question maybe Randy or some of you other buffs can help me with: I am running a special phytoplankton refugium to feed my non photosynthetic tank. I dose F/2 algae growth nutrient in the refugium. The pH there can be very high, 9.5 or even higher. I have noticed that when I dose the macro part of the F/2 (potassium nitrate and sodium phosphate solution) there is a white precipitate forming. I can also measure that nitrate is building up in the system, but there never seems to be any phosphate. Is it logical to assume that it is the phosphate that precipitates out?

 

[Randy Holmes-Farley]

Magnesium hydroxide (fast) and calcium carbonate (slow) will precipitate under those conditions. Calcium and/or magnesium phosphate will also be more likely to precipitate at higher pH. There should be no nitrate precipitates.