TEST FOR SALTS OF IRON.
In the fourth test tube we will search for iron salts. A solution of potassa or ammonia will produce, should iron be present, a whitish precipitate, which presently turns to a dirty green, and, ultimately, a reddish-brown color, owing to its absorption of oxygen from the air. A more delicate test for iron is ferro-cyanide of potassium, or sulpho-cyanate of potassium; but other mineral combinations might deceive a novice using these reagents into the belief that iron was present when it was not. A light trace of iron in water is very common; and, so long as it is in minute quantities, the salt seems quite innocent. But occasionally water is found saturated with iron in the form known by chemists as FeSO4. Then it is very objectionable for boiler-feeding; since it exercises a strong corrosive action on the plates, pitting and furrowing being a common result of its action. Scale formed by water of this character is nearly always a hard, thin substance, that sticks with intense tenacity, and generally takes away part of the skin of the iron when it is taken off.