For this purpose is required hydrochloric acid with concentration about 9 % (it can be got in household shops) and small iron filings (25 parts of acid, 1 part of filings). Filings fill up in an open ware with acid and leave for some days. Upon termination of reaction it will turn out light a green solution, which, having staid some more days becomes yellow-brown. It is a solution of iron acid.
