ANALYTICAL EDITION
MARCH 15, 1940
cedure described above for the natural concentrate. The quantities of platinum metals recorded roughly represent the various compositions encountered by the authors. No. 7 illustrates the fact that with high percentages of insolubles the dry assay is unsatisfactory. This is due to both retention of lead and loss of precious metals. Direct examination by means of chromate always revealed considerable lead where the composition and weight were similar to KO.7 . I n many cases beads similar to S o . 2 yielded small amounts of lead. This was found to be caused by the high proportion of palladium in a bead weighing about 40 mg. or more.
Summary With ores or concentrates containing high proportions of nickel oxide, one or two fusions of the slag are necessary for complete collection of platinum metals in a lead regulus. High proportions of copper and iron oxides have very little deleterious effect on the lead collection of platinum metals. Various monosilicates, bisilicates, trisilicates, and excess litharge and niter charges subjected to wide temperature treatments did not materially improve the first fusion recovery of the platinum metals contained in concentrates of high nickel content.
171
I n the absence of high proportions of rhodium, iridium, and ruthenium proper washing of the lead regulus permits the a p plication of the all-dry assay. If the proportion of insoluble to soluble platinum metals is very much greater than that usually present in lode deposits, t,he assay bead is no longer coherent and lead is retained even after I to 3 hours’ treatment a t 1300’ C. in oxygen. If the proportion of palladium to platinum is about 1 to 1 or greater, a weight of concentrate should be chosen to produce a bead weighing about 10 or 15 mg. Even where a small amount of lead is retained, the absence of silver facilitates wet analysis of the bead. The all-dry assay has a limited application. It should not ordinarily be used for the original discovery of platinum metals.
Literature Cited ( l j Adam,
H. R., J . Chem. M e t . Mining Sac. S. Africa, 29,106-15
(1928).
F. E., and Scott, M., IND.ENG.CHEM.,Anal. Ed., 9, 460 (1937). (3) Davis, C. W., U. S. Bur. Mines, Tech. Paper 342 (1924). (4) Rusden, H., and Henderson, J., .I. Chem. )Vet. Mining Sac. S. Africa, 28, 181-98 (1926). (5) Savelsberg, W., and Fischer, A., Mctall u . Erz , 31, 451 (1934).
( 3 ) Beamish,
Dropping Electrode with a Constant Head of Mercury E. F. RIUELLER, National Bureau of Standards, Washington, D. C. OST of the dropping electrodes used in polarographic measurements have no provision for automatically maintaining a constant head of mercury, although it is obvious that such a constant head can be obtained very simply by using the principle of the Mariotte flask. To one unfamiliar with polarographic literature, it seemed probable that the device had already been used but on discussing the m a t t e r w i t h E. R . Smith of the Physical Chemistry Section of this bureau, i t appeared that this was not the case. He accordingly made and tried the device, and has kindly supplied the following comment and description. Measurements made with the polarograph generally require comp a r i s o n s of w a v e heights which are recorded a t different times. These wave heights depend on a number of variables, among which is the rate of dropping of the merFIGURE1
cury electrode. For special purposes it is adtantageous to arrange the electrode so that its dropping rate can be varied arbitrarily, u-ithin limits, by changing the height o,f the column of mercury or by changing the external pressure on the column, but for most analyses it is necessary and sufficient to have substantially a constant pressure acting on the droplets a t the tip of the capillary both during successive determinations and during an initial calibration. For this purpose, a constant head of mercury may be very simply obtained by usin the principle of the Mariotte flask, as shown in Figure 1 (not firawn to scale). The reservoir is filled with mercury through tube A , which is open to the atmosphere and drawn to a bore of about 1 mm. at D. Electrical connection to the dropping electrode, C, is made through E. By gentle suction at B , the level of mercury in A is brought down to tip D and then stopcock B is closed. As long as the level of the mercury in the reservoir is above D, a constant head, H , is maintained without attention to the height or amount of mercury in the reservoir, Instead of sealing the capillary, C, to the reservoir, it may be attached by a short piece of rubber tubing, as indicated by the dotted lines at F , or by means of an interchangeable ground joint. With a suitable set of interchangeable capillaries different d r o p ping rates may be obtained. Stopcock B should be lubricated, but the stopcock above F and the ground joint need not be. They might be partially lubricated in such a way as to avoid contact between the lubricant and the mercury. If used dry, some kind of spring would be needed to keep the parts in position. Even without lubricant, there should be no leakage, since the mercury would not enter the narrow spaces, and air should not leak in because the pressure is above atmospheric. A dust cap for tube A adds a toutrh of elegance. The same device may also be used where it is preferable to use a single capillary sealed to the mercury reservoir and an adjustable head. In this case t,he mercury reservoir is made in the form of a tube, perhaps 8 or 10 mm. in inside diameter and somewhat less then 76 cm. long, fitted with a perforated stopper at the top. A long glass tube, corresponding to A in the figure, passes through the stopper. If this tube is of small bore and if the reservoir of mercury is nearly filled at the start, the level in the tube will fall to the point corresponding to D after a. small quantity of mercury has been allowed to escape through the capillary. Capacity for a supply of mercury can be provided in a bulb at the top of the long reservoir. By using a two-hole stopper, a stopcock corresponding t o B may be included.
