Platinum Black

Platinum Black consists of platinum in a very fine state of division, and contaminated with traces of foreign substances which vary according to its method of preparation. This usually consists of precipitating the metal from solutions of its salts by reducing agents. To this end alcohol has been used successfully in the presence in alkalies; sodium formate, glucose in alkaline solution, sodium acetate, hydrazine hydrate in alkaline solution, formaldehyde in sodium hydroxide solution, and glycerine with potassium hydroxide have also been used as reducing agents. Instead of these reagents, the more highly electropositive metals may be used, as, for example, magnesium, zinc, or aluminium. The last-named metal is recommended by McDermott in sheet form for the reduction of platinum chloride solution since the precipitated platinum is relatively pure, very finely divided, and contains no aluminium, whereas, when zinc is used, traces of this metal contaminate the precipitate.

Platinum black is a powerful catalyser, effecting the union of hydrogen and oxygen with explosive violence. It readily absorbs hydrogen. Although the hydrosol of platinum enables carbon monoxide to unite with oxygen to form the dioxide when shaken with the mixed gases at ordinary temperatures, this reaction does not appear to take place in the presence of platinum black.

An interesting reaction consists in the oxidation of atmospheric nitrogen in air by the agency of platinum black in the presence of potash or baryta water, whereby nitrous acid or ammonium nitrite are produced.

Not only are oxidising reactions accelerated by platinum black, but reductions are similarly hastened. In the presence of dextrose it reduces free nitric acid to ammonia, potassium chlorate and perchlorate to chloride, and potassium iodate to iodide. By heating platinum black to between 300° and 500° C. its catalytic hydrogenation activity is appreciably reduced.

Hydrazine hydrate is converted by platinum black into free nitrogen and ammonia, the reaction proceeding according to the equation:

3N₂H₄.H₂O = 4NH₃ + N₂ + 3H₂O.

Neither hydrogen nor nitrous oxide appears to be formed during the process.

Platinum black slowly absorbs 60 times its volume of carbon monoxide at ordinary temperature, and appears to form a chemical compound with it, although efforts to isolate any such substance have hitherto proved abortive. The absorption can hardly be a purely mechanical effect, since other gases such as hydrogen do not effect its elimination. Upon heating to 250° C. the carbon monoxide is suddenly liberated.

Platinum black, if it has not been ignited, dissolves in concentrated hydrochloric acid in the presence of hydrogen peroxide, yielding chlor-platinic acid, H₂PtCl₆, which, from its method of preparation, is entirely free from nitro compounds.

When exposed to red heat, platinum black shrinks and becomes spongy, no longer blackening the fingers.

Platinum Sponge is obtained by igniting ammonium chlorplatinate, preferably in hydrogen, when metallic platinum is left behind as a dull, grey, soft and porous mass. Its density is approximately 21.16. When heated to whiteness it may be welded by hammering into compact platinum foil.

Platinum sponge readily absorbs hydrogen, and when charged with this gas becomes incandescent in air owing to the induced combustion. This is the principle of the Doebereiner Lighter used for igniting coal gas without a match. Hydrogen and chlorine readily unite in the presence of platinum sponge, as also do hydrogen and iodine, although not quite so rapidly. Platinum sponge prepared at a relatively low temperature will cause hydrogen and oxygen to unite with explosive violence.

When spongy platinum or even the compact foil is boiled with sulphuric acid containing ammonium sulphate, sulphur dioxide is formed, and some free nitrogen evolved. The platinum apparently acts as a catalyst by alternately yielding the disulphate and free metal, thus:

Pt + 4H₂SO₄ = Pt(SO₄)₂ + 2SO₂ + 4H₂O
3Pt(SO₄)₂ + 2(NH₄)₂SO₄ = 2N₂ + 3Pt + 8H₂O + 8SO₂.

Nitrogen is not evolved if the platinum is replaced by gold or iridium.

Explosive Platinum is obtained by dissolving an alloy of the metal with excess of zinc in hydrochloric acid. The zinc passes into solution, leaving metallic platinum in the form of a finely divided residue.

As obtained in this way the platinum is frequently explosive, but not always; its behaviour is very erratic in this respect.

The explosive property appears to be due to the union of occluded oxygen and hydrogen, for the metal is not explosive if obtained in the entire absence of air.