factory to replace the standard method for boron, further study was omitted. Application. T o test the practical applicability of the method, a quantitative determination of boric acid in a Japanese tile glaze was attempted. By an ordinary analysis its composition was: SiOn, 47.10; A1203, 1.12; FezOa,0.05; CaO, 12.10; Na20, 12.06; K20,0.34; Bz03, 26.81%. The sample was fused with 5 grams of anhydrous sodium carbonate , and the melt was heated with 100 ml. of hot water. The insoluble matter was filtered off through rapid filter paper and washed with hot water. The combined filtrate and washings were heated in a 300-ml. beaker covered with a watch glass, after addition of a few drops of a 1% alcoholic solution of phenolphthalein. During heating, hydrochloric acid was added in small portions to discharge the red color and then until the red color was slowly restored. The solution was filtered through rapid filter paper, and after the addition of a small excess of dilute hydrochloric acid, it was heated with a small excess of calcium carbonate under a reflux condenser for about 30
Table II.
Sample Taken, G. 1
0.5
Analysis of a Glaze
Rotatio: Angle, $4.65 +3.85
B, Mg. 16.6 8.38
Bz03, % 26.8 27.0
The results, shown in Table 11, agree well with that of the chemical method. Silicon, calcium, and copper were detected as impurities in the final solution by spectrographic test; the calcium content was 0.1 mg. in a 25-ml. aliquot and probably did not interfere. ACKNOWLEDGMENT
minutes, cooled, filtered, and titrated as usual with 0.1N sodium hydroxide in the presence of mannitol. Another 1-gram portion of the sample was treated as above until the second filtrate was obtained, and was evaporated to about 70 ml. After the addition of a few drops of a 1% alcoholic solution of methyl red, the solution was heated and made just acidic with dilute hydrochloric acid and then just basic with dilute aqueous ammonia. The solution was filtered through a small rapid filter paper, caught in a 100-ml. volumetric flask, and diluted to the mark with washings and water. A 20-ml. aliquot was then pipetted into a 50-ml. beaker containing 1.000 gram of solid tartaric acid, and boron was determined by the above polarimetric method.
The authors are particularly indebted to Teruyuki Kanie and Yohe Tsuchiya for assistance in carrying out a part of analysis of the glaze. LITERATURE CITED
(1) Darmois, E., J. chim. phys. 23, 130, 649 (1926); 27, 179 (1930). (2) DeFord, D. D., Blonder, A. S., Braman, R. S., ANAL. CHEM.33, 471 (1961). (3) Kodama, K., Shiio, H., Bunseki Kagaku 9, 685 (1959). (4) Rosenheim, A., Leyser, F., 2. anorg. u . allgem. Chem. 119, 1 (1921). (5) Sciarra, J. J., Zapotocky, J. A., J. Am. Pharm. ASSOC.,Sci. Ed. 44, 373 (1955). RECEIVEDfor review March 27, 1961. Accepted September 20, 1961.
New Fire Assay for iridium G. G. TERTlPlS and F. E. BEAMISH Department of Chemisfry, Universify of Toronfo, Toronfo, Canada
This research is part of a general investigation of a new fire assay procedure for the platinum metals. Iridium is satisfactorily collected in the copper-nickel-iron buttons, in amounts from 9 pg. to 4.59 mg. Losses to slag and pot walls are negligible. A determination of iridium in the presence of aluminum is also reported. This modification involves complexing with tartaric acid.
B
and coworkers recently reported procedures for new fire assays of ores and concentrates for platinum and palladium (11), ruthenium and osmium (Y),and rhodium (14). This investigation was undertaken to examine the applicability of the copper-iron-nickel button for collection and subsequent determination of iridium. Researches on the lead collection have shown that in addition to serious mechanical losses because of the insolubility of the iridium in solid lead, significant losses also occur to the slag, necessitating re-assays of the slag ( 1 ) . Cupellation losses of up to 4% of iridium in a 75-mg. silver bead have been recorded (10). Barefoot and Beamish EAMISH
108
ANALYTICAL CHEMISTRY
(1) found very poor recoveries of iridium from copper and synthetic nickel ores, by lead fusion, using sub- and monosilicate slags, whereas for synthetic iron ores with bisilicate slag the recoveries were satisfactory after a slag re-assay. In the present method, collection of 9 fig. to 4.59 mg. of iridium is shown to be satisfactory. On dissolution of the button by concentrated hydrochloric acid, iridium is dissolved, subsequently separated from the base metals by cation exchange technique, and then determined either gravimetrically or spectrophotometrically, depending upon its quantity.
by A. P. Green Co., Weston, Ontario, Canada. Roasted artificial sulfide concentrate prepared as described by Plummer and Beamish (11). Unless otherwise specified, all other chemicals were of reagent grade. IRIDIUM STANDARD SOLUTION
Ammonium iridium chloride solution, containing 10 ml. of concentrated hydrochloric acid per liter, was standardized gravimetrically by 2-mercaptobenzothiazole ( 2 ) . More dilute solutions of iridium were obtained by dilution of aliquots and maintaining the acidity as above. FIRE ASSAY FOR IRIDIUM
APPARATUS, REAGENTS AND MATERIALS
Furnaces, Pyro, optical pyrometer (The Pyrometer Instrument Co., Ken. York, N. Y.), and spectrophotometer described previously ( 7 ) . Beckrnan Model A2 glass electrode pH meter. %Mercaptobenzothiazole, practical (Eastman Kodak Co., Rochester, N. Y.), recrystallized from %yoethyl alcohol to a 1' C. melting point range. Technical grade borax and soda ash. Graphite powder, 200-meshJ supplied
Preparation of Cu-Ni-Fe Button. Ninety-five grams of the artificial sulfide concentrate were roasted and the cooled calcine was crushed and mixed with 11 grams of graphite and with the flux as previously recommended (11). About one third of the charge was placed in a 4-inch evaporating dish lined with two 8 X 8 inch cellophane sheets and salted with the standard iridium solution. The mixture was dried overnight in a steam oven a t 50' to 60" C. The dried material was
carefully pulverized in a mortar and mixed thoroughly. Five grams of the unsalted charge were placed on the bottom of a 30-gram assay crucible, follonTed by the cellophane lining and the salted charge, the latter being wrapped in a new 8 X 8 inch cellophane sheet. Finally, the mixture was covered with the remaining unsalted charge, which was also used to "clean" the mortar. The crucible was placed in the air-gas furnace, preheated to 1204' C. When the volume of the mixture was reduced by one third (temperature a t about 980" C.), the heat was increased slowly to reach 1230' C. This procedure was necessary t o avoid spilling the charge through vigorous boiling and also to ensure reduction. Finally, the temperature was raised to 1450' C., a t which temperature the gas and the air were turned off and the crucible n-as carefully removed from the furnace. The cooled crucible was broken, and the slag adhering to the button Fvas removed by gentle tapping with the end of an iron rod. The slag and pots were retained and tested for losses of iridium. Analysis of Button. The button (25 t o 30 grams) was placed in a 600-ml. beaker with 200 ml. of concentrated hydrochloric acid. The beaker, tightly covered with a watch glass, was placed on a steam bath upon a cover glass until vigorous reaction ceased and then was placed directly over the steam until the button m s almost dissolved. Two or three 5-ml. portions of concentrated nitric acid were added slowly, followed by repeated treatment with concentrated hydrochloric acid to expel nitrous fumes. When reaction ceased, glass hooks were placed on the beaker edge, thus lifting the watch glass to permit evaporation of the solution. To the cool residue, 8 ml. of concentrated hydrochloric acid were added and the residue was taken up with several portions of water. The solution was filtered through a Whatman No. 42, 7-cm. filter paper. After lvashing with mater, the filter paper with the residue was retained in an A2, medium-porosity, porcelain filter crucible for further treatment. The filtrate was brought up to 1.5 liters and its pH was within the range of 1.3 to 1.5. Then 500 mg. of solid sodium chloride were added to the solution. The solution was passed through Doweu 50-X8, 20- to 50-mesh cation exchange resin, 70 em. deep and 4 em. in diameter, to remove the base metals (8),and the effluent was collected in a 4-liter beaker. The exchanger was washed with acidulated water whose pH mas adjusted to 1.5 with hydrochloric acid. For buttons over 25 grams the solution was divided roughly into two equal parts and after waporation the effluents were combined and finally transferred to a 250-ml. lira ker , Khen milligram amounts of iridium were to be determined, the iridium in the large column effluent was precipitated hydrolytically in the presence of sodium bromate (6) and after reduction in hydrogen the residue was subjected to dry chlorination. Dry chlorination
was carried out in the presence of excess sodium chloride in a borosilicate glass tube heated to about 700' C. for 6 to 7 hours (6). The chlorination product was leached with dilute hydrochloric acid, The solution was filtered through Whatman No. 42, 7-cm. filter paper, the filtrate evaporated to dryness, the residue dissolved with water, and the resulting solution filtered through the same filter paper (Filtrate A). This filter paper was placed in the A2 crucible, and after ignition and reduction in hydrogen, the residue was chlorinated and the chlorination solution was filtered through a 5-em. filter paper. The filtrate was combined with Filtrate A. The combined solution mas evaporated to 30-ml. volume, its p H adjusted to 1.5 with hydrochloric acid, and finally passed through a small cation exchange column, 5 em. deep and 1 cm. in diameter, to remove traces of base metals (8). The column was washed with 150 ml. of pH 1.5 water. For micro amounts of iridium the same technique was followed, but the filtration prior to passing the solution through the small exchanger was made through a 5-cm. glass fiber filter paper. Iridium was determined in the effluent either gravimetrically or colorimetrically as described below. Prior to analysis, organic matter was removed by treating the sample with concentrated nitric acid and 30% hydrogen peroxide, repeating three times, and taking to dryness on a steam bath each time in the presence of sodium chloride. After this treatment, the sample was evaporated t o dryness several times with concentrated hydrochloric acid to ensure reconversion of iridium to the chloride. The results are listed in Table I.
base metals by cation exchange technique. Spectrographic analysis of the effluent from the cationic exchangers revealed that along with iridium, aluminum and silicon pass through the exchangers under the conditions applied for the removal of the base metals. Incomplete precipitation of iridium by 2-mercaptobenzothiazole in this solution is possibly due to masking or to a colloidal form of iridium brought about by the aluminum and silicon present. The presence of these elements in the exchanger feeding solutions results from the silicates of the slag which are dissolved during the dissolution of the button; complete removal of the adhering slag by mechanical means-Le., gentle tapping-is dependent upon the slag composition and the shape of the button.
T a b l e 1. Determination of Iridium by a Fire A s s a y with Cop per-Nickel -Ikon Buttons
Iridium Taken, Mg.
DETERMINATION OF IRIDIUM IN PRESENCE OF ALUMINUM
During this research difficulties were encountered in the complete precipitation and purification of the iridium by 2-mercaptobenzothiazole in the resulting solution, after the removal of the
Difference, Mg.
9.18
8.81 8.66 8.57
--0.38 -0.53 --0.62
4.59
4.48 4.41 4.41
-0.11 --0.18 --0 18
1.84
1.75 1.84 1.77 1.76 1.80 1.85
--0.09
LOSSES OF IRIDIUM TO SLAG AND POT WALLS
The slag and pot walls from the 9.18mg. iridium samples were analyzed by the classical fire assay with lead as the collector. The slags were fused with flux No. 2 ( I ) and extra litharge and flour to produce a lead button of 28 to 32 grams. The resulting slag was re-assayed twice. The lead buttons were analyzed for iridium by nitric acid parting and chlorination of the residue (1). Iron was visually noted in the lead buttons. The pot walls \\-ere treated according to the method of Hoffman and Beamish (6); the slag and pot m-alls were reassayed using flux No. 10 (1). The lead buttons were scorified and the final button was cupelled to a silver bead, then parted with 1 to 4 nitric acid, and the residue was analyzed for iridium. In both cases no significant amount of iridium was found.
Iridum Found, Mg.
Y
--".b7 -0.08 --0.04 +0.01
Y
Y
92.9
88.8 86.5
--4.1 --6.4
36.8
37.3 35.0 34.0 36.0
-0.5 --1.8 --2.8 --0.8
18.2
18.2 15.0 16.2
-3.2 -2.0
9.0 10.0
--0.3 +O.T
9.3
...
Experience in this laboratory has shown that dry chlorination eliminates silicon under the conditions used for the platinum metals. Further, to avoid the difficulties due to the presence of aluminum and silicon, the iridium was precipitated hydrolytically ( 5 ) , ignited, reduced in hydrogen, dry chlorinated ( 5 ) , and finally precipitated by 2-mercaptobenzothiazole. Traces of aluminum were found in the iridium precipitate. Leaching of the ignited or hydrogen-reduced residue with hot VOL 34, NO. 1, JANUARY 1962
109
1 to 4 nitric acid results in dissolution of iridium; with 1 to 1 hydrochloric acid leaching is incomplete. Tartaric acid solution added to the chlorination filtrate did not affect the gravimetric or colorimetric determination of iridium, while it eliminated contamination or interference from aluminum.
Mixtures of iridium standard solution with aluminum trichloride solution were placed in a 250-ml. beaker and evaporated to dryness ir, the presence of 3 or 4 ml. of 2% sodium chloride solution. Thirty milliliters of water were added, then tartaric acid solution, acetic acid, and ammonium acetate, in this order, and the solution was heated on a hot plate. The rest of the procedurz has been described (8). TKO glass beads were used to aid boiling, rather than a filter tablet. The precipitate was washed with 200 to 250 ml. of hot washing solution to \vhich 175 mg. of tartaric acid were added. Tartaric acid solution was also used in the colorimetric determination of the iridium; it was added prior to the addition of the hydrobromic acid before the iridium samples were heated in a boiling water bath. The Marks-Beamish (8) modification of the Berman-AIcBryde (3) method was used for the colorimetric determination of the iridium. Results are listed in Table 11. DISCUSSION
From a study of Table 11, it is noted that collections of iridium by the new fire assay are satisfactory for amounts from 9 fig. to 4.59 mg. For amounts larger than 4.59 mg. the recoveries of iridium by the new fire assay method were 94.5‘3,, while the lead classical fusion yielded about
Table II.
Iridium Taken, Mg. 7.32
phere, but with slower velocity of reore fusion for which a bisilicate slag action. From this point of view one was used ( I ) . may expect losses of iridium due first Various explanations may be offered to failure to reduce iridium completely, for losses of iridium in crucible fusions. or failure to reduce a t the optimum colThese may be due to a restricted mislection temperature, or mechanical cibility of iridium in the separated failure to collect iridium by the button. base metal alloy. One may assume that There can be little doubt that iridium recovery of iridium from the crucible is, to a t least some degree, incorporated fusion by the formation of metal alloys in the slag. It is not improbable that is encouraged to the degree that iridium all the iridium may be included in the is dissolved in the alloys. Although slag. In any case, it is to be expected the requirements for the formation of a that some of the slagged iridium n.ill solid solution-size, crystal structure, become incorporated n-ith the wall and yalence factor-are fulfilled in the material. Barefoot and Beamish ( I ) group VI11 elements, except in 01found that iridium can be so incoriron, ruthenium, and osmium (4, porated and at times irrecoverable by Rauh ( I S ) , dealing with the binary the lead fire assay. The authors’ alloys of iridium with other platinum failure to locate the lost iridium for metals, reported a miscibility gap in high valued samples may be due to every case; and in many alloys of the these various factors. Furthermore, in face-centered cubic platinum metals the present case, where the slag losses n i t h iron, cobalt, and nickel, transwere ascertained by a lead collection, formations occurred; for Ir-Xi alloys some dissolution of iridium could be the continuous solid solutions segreexpected by the parting acid, pargated at low temperatures. Yao (16) ticularly when the button contained an stated that the system Cu-Fe s h o w appreciable amount of iron. Methods neither a continuous solid solution nor for the determination of small amounts intermediate phases. of iridium in the lead nitrate parting Plummer et al. (22) believed that solution are not yet available. collection of iridium by Cu-Si-Fe may be a mechanical collection of iridium ACKNOWLEDGMENT metal or a collection of insoluble metallic compounds, depending on the amount The authors express their appreciaof the metal involved. tion to Electric Reduction Co. of -4 second explanation for iridium Canada, Ltd., for a fellowship given losses may involve other chemical or to G. G. Tertipis n-hile he was engaged mechanical processes. The literature in this work. (9, I S ) reveals that, in a reducing atmosphere, even a t relatively low temperatures, the platinum metals react rapidly LITERATURE CITED with all refractory materials-Le., alu(1) Barefoot, R. R., Beamish, F. E., ANAL. mina, lime, silicon, etc-this also occurs CHEM.24, 840 (1952). under vacuum or in a neutral atmos(2) Barefoot, R. R., hIcDonnel1, W. J., Beamish, F. E., Ibid., 23, 514 (1951). (3) Berman, S. S., McBryde, W. -4. E.,
Determination of Iridium in Presence of Aluminum
Tartaric Acid Added, Mg. 70
70 70 70
3.66
SOY0, except in the case of iron synthetic
70
Aluminum Added, Mg.
Iridium Found, Mg.
Iridium in Filtrate, Mg.
..
7.30 7.29 7.34 7.29
-0.01
