Qualitative tests to Determine Platinum Group Metals
Try to differentiate and determine the presence of platinum group metals is not simple, but it is possible to practice some qualitative tests. Prospectors recognize that platinum, iridium and palladium are the three more important. Basically, the first two come mainly from placer deposits, the last occurring more frequently, though in small quantities, with gold and silver in rock formations. As compared with rock and silver, heir important assaying properties are as follow, melting point: silver, 960 oC; gold, 1,063 oC; palladium, 1,550 oC; platinum; 1755 oC; and iridium, 2,300oC. If cupellation is finished close to the minimum permissible temperature for gold and silver, 5-7% of palladium and less of platinum and iridium will produce a frosted bead. Frosting may also be produced on an ordinary bead by sudden chilling just before the finish, but solid litharge is usually present on the cupel surface.
It has been noted that large quantities of platinum group metals produce a rough bead of leady appearance. Nitric acid dissolves silver and platinum, giving with the latter a yellow to reddish brown solution depending on concentration. 10-15 mL of diluted nitric acid, containing 0.10-0.12 mg of palladium in a parting cup has sufficient color to be detected when matched against distilled water. When alloyed with silver, platinum is soluble in nitric acid, though no totally. The solution is only slightly colored, but may appear dark due to colloidal platinum. Gold and iridium are not attacked. Hot concentrated sulphuric acid dissolves silver and slowly attacks palladium. The other metals don’t react.
Aqua regia converts silver to chloride and dissolves palladium, gold and platinum, but not iridium, except with vigorous treatment or when alloyed with platinum. All these elements, except silver, yield colored solutions that range from yellow through orange to dark red, depending on the combination and concentration of the metals. Oxalic acid precipitates gold from slightly acid solutions, but not the others. Potassium or ammonium chloride in concentrated solutions will precipitate platinum and iridium, but not gold or palladium in its usual condition. Potassium iodide reacts with gold and produces free iodine, with platinum brownish red color of potassium platonic iodide, with palladium, a black precipitate soluble in excess. The last reaction is extremely delicate.
Dimethylglyoxime precipitates palladium completely, platinum less readily. Silver precipitated as chloride in presence of palladium and platinum is contaminated by these metals. Platinum metals concentrate in lead buttons with gold and silver in the usual method of assay, but unless present in sufficient amounts to affect cupellation or parting are apt to be overlooked. If parting acid is evaporated to small bulk, a few hundredths of a milligram of platinum will produce coloration. When platinum is present the parted residue is likely to be finely divided. Iridium may be present as dark spots on the gold particle.