Iron nitrate dissolving metals

When the pulverulent gold has entirely deposited, the liquid must be decanted or filtered off with the greatest precaution. Care must bo taken that not the smallest particle of the gold powder is allowed to pass away with tho liquid, A little hydrochloric acid, which must ho quite free from any admixture of nitric acid, is then to be poured upon the precipitate. This will remove any iron or other metallic impurities without dissolving the gold. The latter is then to be washed, at least six times, with successive portions of distilled waterj and lastly, it is transferred to a email porcelain Or platinum crucible, in which it is heated over a spirit lamp, till the last portions of water are expelled. It ought, in fact, to bo raised to a red heat, or even to be fused with a small quantity of borax and nitrate of soda, as formerly recommended, to expel the last traces of ohlorlde of silver. 

Saturating the electrolyte with iron(lll) hydroxide (e.g., by addition of aqueous solutions of ferric nitrate) and simultaneously adding cobaltous salts leads to in situ formation of a mixed Fe(llI)/Co(ll)/Co(IIl) deposit, which exhibits catalytic activity comparable to that of Fe304 shown by the current voltage curve in Fig. 11. Such mixed oxidic catalyst coatings are composed of very small oxide crystals, which evidently are dissolved upon current interruption due to dissociative oxide dissolution. The transfer of dissolved metal ions to the cathode followed by cathodic deposition of the metal, however, can be completely prohibited, if the potential of the cathode due to optimal electrocatalysis of cathodic hydrogen evolution proceeds with an over-... 

Cold, dilute sulphuric and nitric acids had very little action on the metal, but on warming the iron readily dissolved. Aqueous hydrogen chloride attacked the metal even in the cold. Saturated solutions of the sulphate or nitrate of copper exerted no action at ordinary temperatures even after an exposure of several months to copper sulphate solution, no change could be detected in the iron when examined under the microscope. On raising to 100° C., however, the iron gradually dissolved, copper being simultaneously deposited. 

To achieve a more intense reaction the alchemists of old increased the ammonia and nitrate in the solution with niter. Ammonium nitrate is very soluble and a concentrated solution will dissolve metals, although the reaction is very slow with metals less active than iron. 

Water can also contain ammonium (NH4), nitrates (NOJ), phosphates (PO ), dissolved metals such as copper as (Cu ) and iron as (Fe ), generally as traces (a few milligrams per litre). 

Nitric acid reacts with all metals except gold, iridium, platinum, rhodium, tantalum, titanium, and certain alloys. It reacts violentiy with sodium and potassium to produce nitrogen. Most metals are converted iato nitrates arsenic, antimony, and tin form oxides. Chrome, iron, and aluminum readily dissolve ia dilute nitric acid but with concentrated acid form a metal oxide layer that passivates the metal, ie, prevents further reaction. 

Heavy phosphate coatings do not necessarily have better corrosion resistance than lighter coatings. Even with a single process, e.g. zinc/iron/ phosphate/nitrate, no consistent relationship has been found between corrosion resistance and either coating weight or weight of metal dissolved. 

---