V O L U M E 2 2 , NO. 3, M A R C H 1 9 5 0
45 1
If the sample of sodium nionofluophosphate weighs more than 0.7 gram, sufficiently small aliquots must be taken for analysis to ensure a final concentration of sodium monofluophosphate below 7 mg. per 100 ml. of color medium. Above this concentration the linear relatioilship no longer holds because of too rapid hydrolysis.
sample of the calcium salt furnished by the Ozark-Manhoning Company, Tulsa, Okla.. These samples contained only sodium fluoride, sodium hexametaphosphate, and sodium carbonate as impurities.
Monofluophosphate Analysis. -1 s a l . aliquot of thc 1-litcr sample solution, prepared as for determination of fluoride ion impurity, was transferred to a 100-ml. volumetric flask. Ten milliliters of 5 AV hydrochloric acid were added and the sample was allowed to stand a t room temperature for at least 1 hour. If the original sample weighed less than 0.4 gram, 2 hours were allowed for hydrolysis. The sample was analyzed for fluoride ion impurity during this time interval. After sufficient time had been allowed for complete hydrolysis, 10 ml. each of the color reagent and 6y0 hydrogen peroxide were added and the total volume was made up to 100 ml. This solution was thoroughly mixed and the per cent transmittance was determined. The total fluoride ion concentration corresponding to this transmittance reading was taken from Figure 4,and was corrected for the amount of impurity and converted to per cent sodium or calcium monofluophosphate. In the analysis of the calcium salt, no attempt was made to determine the fluoride ion impurity, as the salt was not soluble enough in pure water to allow determination in the usual manner.
The Steiger ( 5 ) and Merwin ( 2 ) colorimetric procedure for fluoride ion analysis has been adapted for the analysis of commercial samples of monofluophosphate salts. Because the monofluophosphate ion hydrolyzes in the color medium, a graphical extrapolation has been used to obtain the amount of fluoride ion present in the form of fluoride salts. The monofluophosphate ion has been determined by hydrolysis to fluoride and orthophosphate ions, followed by determination of the fluoride ion and ralculation to monofluophosphate. The limits of concentration to which this method are applicahle are 0.1 to 1.0 mg. of fluoride per 100 ml. of final solution.
ANALYTICAL RESULTS
Table 1 presents the rcsults obtained from the analysis of four coniniercial samples of $odium monofluophosphate and one
SUMMARY
LITERATURE CITED
(1) Dahle, .I.d s s o c . Ob’ic. Agr. Chemists, 20, 505 (1937). ( 2 ) Merwin, A m . J . Sci., (4) 28, 119 (1909). (3) hlonnier, Vaucher, and Wenger, Helv. Chim. Acta, 81, 929 (1948). (4) Oeark-Manhoning Co., Tulsa, Okla., unpublished work. ( 5 ) Steiger, J. A m . Chem. SOC.,30, 219 (1908). (6) Willard and Winter, IND.ENG.CHEM.,ANAL.ED.,5, 7 (1933). R E C E I V EJuly D 29, 1949.
Fire Assay and Gravimetric Determination of Rhodium W.F. .ALLEN
AND
F. E. REAXIISII, University of Toronto, Toronto, Ontario, Canada
T w o methods for the determination of rhodium in lead-rhodium buttons are described. These were applied to buttons obtained by the fire assay of salted rhodium ores under various conditions, and slag losses were determined. A method for the determination of rhodium in the presence of lead, zinc, and nickel is reported.
I
N THE analysis of ores and concentrates for platinum metals, fire assay procedures are usually recommended for a preliminary separation of the precious metals from the gangue (4,8, 12. 15, 16). .4lthough much work has been published on methods for the separation and determination of the metals following pot assay and cupellation (2-4, 8, 12, 16), and some work has been reported dealing with cupellation difficulties (15, 2O), little attention has been paid to the efficiency of the lead collection of platinum metals in the crucible fusion itself; it is generally accrpted as satisfactory by analogy nith gold and silver. A survey of the literature revealed the follou ing reports on the lead collection of the precious metals. Fulton and Sharwood (8) reported combined slag and cupel losses up to 370 for gold and 5% for silvrr in copper-bearing ores, and slag losses up to 1% for each metal in simple ores. Shepard and Dietrich (16) state that slag losses should be less than 0.5%. Adam and Westwood say that the platinum metals are not collected as well as gold and silver ( 1 ) . Seath and Beaniish (15) found losses up to 1 5 q in the recovery of precious metals (chirfly platinum and palladium) from nickel ore concentrates. Thiers, Graydon, and Beamish (19) recorded ruthenium losses up to 30% under various assaying conditions. The present investigation deals a i t h the collection of rhodium, and forms part of a systematic examination of the analytical chemistry of tho platinum metals. DETERMINATION OF RHODIUM IN ASSAY BUTTONS
Cupellation of lead-rhodium alloys is unsatisfactory ( Z O ) , as lead cannot be completely removed because of thc high melting
points of rhodium-rich alloys. The customary addition of silver fails, as rhodium is insoluble in silver (6). Rhodium may bc lost during cupellation because of oxidation ( 1 , 11) and suhscquent mechanical loss of oxide particles. Consequently, n e t methods of button analysis were used. Nitric acid parting of lead-rhodium alloys is recommrndcd by a number of authors, and procedures are given for thc determination of rhodium following such parting (14, 1 7 ) . In the course of this parting, a variable amount of rhodium is dissolved with the lead, while the residue consists of rhodium contaminatcd with small quantities of lead. Lead is precipitated from the parting solution as the sulfate (13, 83). The parting residue is dissolved in sulfuric acid ( 2 2 ) and combined with the filtrate from the lead precipitation, and rhodium is precipitated as the sulfitic for determination as the metal. There are several weaknesses in such a procedure: Lead sulfate is appreciably soluble under the conditions of its precipitation, so that separation of lead from the rhodium is incomplete. Precipitation of large amounts of lead sulfate from a solution containing small quantities of rhodium involves the occlusion of considerable rhodium, and a double precipitation is a very unsatisfactory operation with large amounts of lead. Precipitation of rhodium sulfide from solutions of the sulfate is incomplete. Various reagents for rhodium were tested in an effort to find B satisfactory means of determining rhodium in the presence of milligram quantities of lead. It was found that thiobarbituric acid ( 5 )did not precipitate lead from solutions of the chloride or
452
nitrate, but that the reagent did not precipitate rhodium quantitatively from yellow sulfate solutions; asain the sulfide method, rhodium had to be present as the pink chloride complex. The conventional method for converting the sulfate to the chloride is to boil with concentrated hydrochloric acid (9, 10). This was unsatisfactory with thiobarbituric acid, and has been reported to give inconsistent results with hydrogen sulfide (I?).
ANALYTICAL CHEMISTRY Table I. Determination of Rhodium in Sulfuric Acid Solutions by Means of Thiobarbituric Acid R h Taken MQ.
R h Found
10.68 10.68 10.68 10.68 10.68 10.68 10.68 10.68 10.68 10.68 10.68
11.78 13.83 10.60 10.76 10.70 10.84 10.62 10.67 10.63 10.65 10.72
THIOBARBITURIC ACID PRECIPITATION OF RHODIUM FROM SULFURIC ACID SOLUTION
Fuming to a very small volume (
