Very good point!!! I have to dose momentarily about 0.045ppm Po4 over the cause of the day, a single shot of so much P/Po4 will usually harm corals like a nutrient shock that many people have heard about and seen.Hello everybody!
A few hours offline, and im missing all the action in this thread ;-)
We do measure total phosphorus (via ICP-OES or ICP-MS, depending on which method the customer chooses) and also orthophosphate (PO4) using a photometric method based on the molybdenum blue/ascorbic acid method. We are using a Shimadzu Lab photometer with 4 cm optical path to have low detection limits. We do not calculate a phosphate value from the ICP data.
Phosphate is not very sensitive on ion chromatography (since it has a low specific conductivity), thus IC is not very useful for detection of phosphate in reef tanks. Photometry is the superior method in this case.
In case of higher phosphate levels total phosphorus and orthophosphate often agree very well - so most of the phosphorus in the sample is actually phosphate. This is not always the case, especially at lower nutrient levels, where a significant proportion of total phosphorus can be something else (might be oligophosphates or DNA or other phosphorus containing molecules).
We are using an orthophosphate based salt for phosphorus dosing, and i would be surprised if other products/brands would use other phosphorus sources. In my opinion it is important to dose phosphate spread out over several dposings per day, to achieve a constant availability. Otherwise this important nutrient might be depleted fast - either by consumption/metabolism, or by adsorption onto surfaces. Cycling between "available state" and "limitation" is imo stressful for corals, and should thus be avoided.
Best regards,
Christoph