Question

What is the best method to determine the amount of Fe ion in boiler water?

Answer 1

Conveniently measured by Inductively coupled Plasma. Can also get other important boiler species like Al, Na, Ca,Mg,S etc

Answer 2

If you have access to a modern lab equipment you can certainly use spectroscopic techniques. If you dont, then probably the best way would be to precipitate it with NH4OH then filter it out. Cook it in a muffle furnace and weigh the remaining oxide as Fe2O3.