Sulfite Test KitSulfite Test Kit are commonly used to check the boiler bulk water for sulfite. Any sulfite residual means you have 0 ppm of dissovled oxygen. Sulfite is added to the feedwater tank to minimize corrosion to your boiler feed water feed pump, tanks, and boiler tubes. This test is 1 drop = 5 ppm or cut the water sample in half for 1 drop = 10 ppm. Only check your boiler water for sulfite residual. It is not necessary to check the feedwater for sulfite, since the residual are very low and difficult to detect.