composition products were absent from the catalyst a t that time. The infrared spectrum of thiophene adsorbed on cobalt molybdate, Figure 9, is in accord mainly nith the presence of the flat structure for thiophene. On a surface saturated with hydrogen detectable concentrations of the fourpoint species IT-ere observed, along with large quantities of hydrogenated species (Figure 10). Hydrogenated species are present whrn thiophene reacts with the hydrogen covered surface from the appearance of infrared bands in the 2930 cm.-l region of the spectrum. The influence of temperature on the relative amounts of the surface forms of thiophene on molybdenum disulfide is rather marked. Some spectroscopic data bearing upon the adsorption equilibria are found in Figure 11. With 9 mm. partial pressure of gas in the infrared cell, a net increase in the
two-point species is observed on lowering the temperature from 300' to 150' C. Simultaneously, there is a decrease in the concentration of the four-point species. This observation may arise partially, a t least, from a crowding effect on the surface as the temperature is lowered, since the gravimetric data indicated an increase in the weight of thiophene adsorbed as the temperature was lowered. The noting of these phenomena strengthens the infrared assignments. A blank run is recorded in Figure 12. ACKNOWLEDGMENT
The author thanks J. T. Richardson and L. W.Vernon for providing samples of molybdenum disulfide in conjunction with their study of the electrical, magnetic, and catalytic properties of these metals. He also thanks Haywood
Kinsey for performing experimental work during the initial phases of the spectrometric study. LITERATURE CITED
(1) Eischens, R. P., Division of Petroleum Chemistry, 133rd Meeting, ACS, San Francisco, Calif., April 1958. (2) Kurbatov, L. N., NueImin, G. G., Dokladu A k d A'ni~kS - R .S _ _R_ 68. 241 _-(1949): C.A., 44, 435f(1950). (3) Nicholson, D. E., ilx.4~. CHEY. 32, 1365 (1960). ( 4 ) Nicholson. D. E.. Nature 186. 630 (1960). ( 5 ) Waddington, Guy, Knowlton, J. W., Scott, D. W., Oliver, G. D., Todd, S. S., Hubbard, W.N., Smith, J. C., Huffman, H. M., J.Am. Chem. SOC.71,797 (1949). (6) Yaroslavski, N. G., Terenin, A. I%., Doklady Akad. Nauk S.S.S.R. 6 6 , 885 (1949))C.A. 43,7343h (1949). --I
RECEIVEDfor review February 28, 1961. Resubmitted November 24, 1961. Accepted December 20, 1961.
Determination of Oxygen in Zinc, Cadmium, and Magnesium by Carbon-Reduction in an Inert Gas Stream BEN D. HOLT and HARVEY
T.
GOODSPEED
Argonne National laborafory, Argonne, 111.
b Microgram quantities of oxygen have been determined in zinc, cadmium, and magnesium b y the inert gas fusion technique after separation of the metal from the oxide b y evaporation onto a removable cold surface inside the inert gas fusion furnace. The oxygen was measured manometrically as carbon dioxide. Recoveries of known quantities of oxide added to for each metal were 100 f 470 for cadmium, and zinc, 94 84 f 9% for magnesium. Bias correction factors were applicable to cadmium and magnesium data. The lowest concentration of oxygen that could b e satisfactorily determined on 1-gram samples was about 10 p.p.m. for zinc, 20 p.p.m. for cadmium, and 50 p.p.m. for magnesium. The optimum range for manometric measurement was 200 to 300 pg. o f oxygen. The time required per analysis was 1.0 to 1.5 hours.
*
N
270
available for the determination of micro quantities of oxygen in ~ O K melting metals such as zinc, cadmium, and magnesium appears to be applicable when both sample size and analytical 374
ONE OF THE FEW METHODS
ANALYTICAL CHEMISTRY
time are to be conserved. Hartmann and Hofmann (4) developed a method, characterized by high accuracy, for the analysis of zinc in n hich the water produced in the reaction, ZnO H2S = ZnS H20, was measured gravimetrically. About 90 grams of sample was used per analysis. A high-temperature fluorination method (3, 6) was applied in our laboratory to the analysis of zinc and magnesium. Relatively high blanks of about 150 wg. of oxygen made the method undesirable for use in the microgram range. Furthermore, zinc metal did not react appreciably during the analysis, eren though it melted and floated on molten potassium bromotetrafluoride for tn o overnight heatings a t 500" C. Methods involving the separation of magnesium from MgO by volatilization with subsequent analysis of the residue have been reported (1, 2 ) . Allsopp (1) determined oxygen in the range of 1 to 2% with very good accuracy by dissolving the residue and measuring its magnesium content by titration with a solution of disodium (ethylenedinitri1o)tetraacetate. Berry, Walker, and Johnson ( 2 ), working in the parts per million range, transferred their residue to a vacuum fusion ap-
+
+
paratus and extracted the oxygen as carbon monoxide, which was oxidized and measured as carbon dioxide. The transfer and fusion were made in iron capsules, the oxygen content of which depended upon the source and cleanliness of the iron. A mean recovery of 91% was reported on a group of oxide standards carried through the vacuum fusion part of the procedure but apparently not through the metil sublimation procedure. The time required was about 21 hours per 12 samples. I n the inert-gas-carbon-reduction method reported here, the metal (zinc, cadmium, or magnesium) was also separated from the oxide by evaporation, but in a stream of argon a t a pressure of 100 mm. of Hg. Volatilized from an inductively heated graphite crucible, the metal was condensed on a split tantalum cylinder and subsequently removed from the crucible area. The oside remaining in the crucible was reduced in situ by the inert gas "fusion" technique, but in the absence of a metal flux. The entire analysis was performed in a single, analytical train, and the total time per sample was about 1 hour for zinc or cadmium and about 11/2 hours for
56
URANIUM
TO PUMP
DESICCANT
P
-ARGON
A7 ~~
.-
I
j = -
Figure 1.
MANOMETER
Analytical train
()
Figure 2.
magnesium. Of the three metals tested for oxygen in the microgram range, the best results vere obtained on zinc, with 100 + 27, recovery of oxygen added as the oxide to the metal. APPARATUS AND REAGENTS
An over-all diagram of the analytical train is shown in Figure 1. Modified for the present application, this inert gas fusion line contained a specially designed reaction tube, and a grooved stopcock, Sp,for adjusting low pressures in the reaction tube. A glass wool filter was included to collect small, smoke-like particles of metal produced by the condensation of metal vapor in the argon stream. Figure 2 shows the details of the reaction tube. The tantalum tube (8 inches long, 1 inch in diameter, and 0.02 inch in thickness) was suspended by three platinum wires from a glass cylinder, the flared edge of which rested on the male section of the 40/35 standard taper joint. A slit, cut in the tube from the lower end upward (about inch wide by 7 inches long) minimized the energy pick-up from the induction coil. The fused silica liner, supported on the lower concave surface of the reaction tube, enveloped the tantalum tube to prevent metal vapors, escaping through the slit, from condensing on the inner wall of the reaction tube. It was necessary to keep this inner wall clear of magnesium; otherwise, subsequent crucible-heating caused the magnesium to etch the silica surface, forming a yellowish brown ring. The ring was probably caused by SiO, proSiOz = duced in the reaction, Mg MgO SiO. Control of the site of metal condensation within the tantalum tube was dependent upon the argon stream a t a pressure of 100 mm. of Hg. Both the tantalum tube and the fused silica tube were easily removed from the reaction tube for cleaning in dilute acids by a pair of 10-inch tweezers, the tips of which were bent outward to engage into holes in the tubes. Another pair of tweezers, extended in length, with suitable contour tips, was employed to handle the crucible.
+
+
The dimensions of the crucible mere the same as those reported by Smiley (6). The crucible and the 20-mesh crushed material used to cover the oxides were made from pile-grade graphite. Induction heating \vas supplied by a 10-kw. generator. The Schutze reagent (silica gel impregnated with iodine pentoside) was prepared as described by Smiley (7'). PROCEDURE FOR ZINC
Set up the train as in Figure 1, with argon flowing out through the manostat and through the reaction tube to bubbler B. Turn the four-way stopcock 83 to bypass the Schutze reagent, and the grooved stopcock Sz to adjust the rate of flow to about 150 cc. per minute. Insert in the reaction tube a graphite crucible filled to a depth of l / 4 inch with crushed graphite fragments (passed by a 20-mesh and retained by a 40-mesh sieve). To condition the graphite, replace the cover on the reaction tube lightly; close stopcock S4;and with argon flowing out from beneath the cover of the reaction tube, heat the crucible to 1900' C. for 10 minutes. Shut off the power, tighten the cover, and open stopcocks Sz, S3, Sa,and Sa, to direct the argon flow through the Schutze reagent to the pump. Partially open the grooved stopcock S6 to adjust the rate of flow such that the manometer reading is 100 mm. below the zero reference mark. Place liquid nitrogen on the U-tube cold trap of the manometer and turn on the power for a 10-minute line blank run. The line blanks should fall to about 3 mm., which on our manometer is 2 pg. of oxygen, within two or three such runs; if not, make one low-pressure bakeout run by throttling the flow a t Sp instead of a t 86. This serves to remove moisture contamination in the reaction tube. Repeat the blank runs described above if necessary. Open the reaction tube a t the top and insert the cleaned and dried %inch quartz liner, and then the slit tantalum tube. Insert within the tantalum tube a funnel (made from 18-mm. glass tubing) for directing the sample into the
'-2nrn
83R0SIdCATE CAPLL4RY
Diagram of reaction tube
crucible. Drop in the weighed sample, withdraw the funnel, and replace the cover. Close the grooved stopcock SZand open the others, downstream toward the pump, with the Schutze reagent bypassed. When the pressure in the reaction tube has diminished to about 5 mm. of Hg, as indicated by the manometer of the capillary trap, partially open SZ to give a manometer reading of about 100 mm. of Hg. Energize the induction heating coil a t low power and increase the power to raise the temperature of the crucible rim to 820' C., as read by an optical pyrometer through the optical window in the cover of the reaction tube, Notice that the contents of the crucible remain a t a lower temperature until all the zinc has vaporized. When the evaporation is complete, (usually about 25 minutes) as evidenced by an optical field of uniform brightness, cut off the power and allow 5 minutes for cooling. Increase the argon flow to an excessive amount through the manostat. Close SBand slowly open Szto increase the pressure in the reaction tube to atmospheric. Loosen the cover and, without delay, remove the tantalum tube and quartz liner. Replace the cover immediately but do not tighten. Allow argon to flow out under the cover for 1 minute to flush the reaction tube of entrapped air and moisture, and then tighten. Turn on the Schutze reagent. Close S5 and open &. Immerse the manometer U-tube in liquid nitrogen and partially open SSto adjust the flow of argon through the cold trap to give a reading of 50 mm. on the manometer. Raise the temperature of the crucible to 1850' C. The rate a t which the temperature is raised appears to have no noticeable effect on the results in zinc analysis. After the crucible has been heated for 10 minutes and cooled for three minutes, close S 5 ; evacuate the manometer and close 86. Warm the condensate to room temperature and take a pressure reading. Discard the collected COz and repeat the carbonVOL. 34, NO. 3, MARCH 1962
* 375
~~~~~
Table 1.
Oxide ZnO
~
~
Recovery of Oxygen in Oxide Samples
Oxygen, r g . Recovered
Added
CdO
Table II.
~
Deviation
165 227 221 193
166 231 226 191
f l +4
221 224 237 257 277 214 247
204 200 226 241 243 207 232
- 17 - 24 -11 - 16 - 34 -7 - 15
286
283
314
302
+j
-2
-3 +1 0 -2 -21 -5 - 12
Recovery, % 100 101 102 99 Av. 101 += 1, Std. dev. 92 89 96 94 88 97 91 Av. 93 i 3, Std. dev. 99 101 100 99 92 98 96 Av. 98 & 3, Std. dev.
Recovery of Oxygen in Metal Capsules Spiked with Respective Oxides
Oxygen, r g . Added In metal
In oxide
(Av.) 5
86
0
Total
Recovered Deviation Recovery, %
ZINCOXIDEIN ZINCCAPSULES -2 91 89 5 9 +4 58 52 -6 21 23 5 3 297 306 f9 191 194 +3 5 2 -3 +I 195 196 180 182 +2 5 5 0 218 219 +1 -2 195 193
t;
0
5
199 164 0 175 232
4 4 4 4 4 4 4 4 4 4 4
CADMIUX OXIDEIN CADMIUM CAPSCLES 152 151 -1 501 477 - 21 4 1 -3 260 240 - 20 216 202 - 14 4 8 +4 203 177 - 26 . 168 167 -1 4 3 -1 179 169 - 10 236 219 - 17
219 204 283 369 263 268 224 0 0 250 175
12 12 12 12 12 12 12 12 12 12 12
MAGNESIUM OXIDEIN MAGNESIUX CAPSULES 231 223 -8 216 170 - 46 295 215 - 80 381 296 - 85 275 261 - 14 280 244 - 36 236 181 - 55 12 12 0 12 11 -1 262 206 - 56 187 175 - 12
148 497
0 256 212
0
a
98 a D
103 101 100 101 100 99 100
376
ANALYTICAL CHEMISTRY
l l o o c.
PROCEDURE FOR CADMIUM
Cadmium, having lower melting and boiling points, was evaporated a t a lower temperature. The crucible was maintained a t a temperature giving dull redness, too weak to be measured by the optical pyrometer. Due to the greater thermal instability of cadmium oxide, provisions were made for better contact with hot carbon during the reduction process. The sample was added, not by dropping through a funnel on top of the bed of graphite fragments, but by first removing the crucible, pouring out the chips, adding the sample, returning the chips, and then replacing the crucible in the tube. Thus, the oxide residue remained buried in the graphite chips after evaporatjon. For reduction of cadmium oxide, the temperature was raised as rapidly as possible to 1900' C., so that any oxygen produced by the thermal decomposition of the relatively unstable oxide would have a minimum opportunity to escape without interaction with hot carbon. PROCEDURE FOR MAGNESIUM
-f
2 Std. dev.
99 95 92 94 87 99 94 93 94 i 4-Std. dev. 97 79 73 78 95 87 77
79 94 84 i 9 Std. dev. Sample size waa considered to be too small to yield a representative value for per
cent recovery.
reduction procedure until the readings drop to the blank level (usually only one repeat run is necessary). The readings are converted to micrograms of oxygen, using a calibration curve prepared for the manometer, and the procedure blank, expressed in micrograms, is deducted. Successive net quantities of oxygen are added for calculation of the total in the sample. Dissolve the condensed zinc metal from the tantalum tube and fused silica liner by submerging them in dilute hydrochloric acid. Rinse with water and then with acetone. Dry in an oven a t
Magnesium samples also were buried in grarhite chips. This was not because the oxide residue was thermally unstable like cadmium oxide, but because it was refractory. Carbon reduction proceeded too slowly when the oxide was not thoroughly surrounded by and in good contact with carbon. Another advantage for covering the magnesium sample was to minimize the physical displacement of the light, fluffy magnesium oxide out of the crucible by convection currents of metal vapors. Although the melting and boiling points of magnesium are the highest of these three metals, the evaporating temperature was about BOO' C. so that the rate of evaporation was slow enough to prevent serious losses of magnesium oxide by entrainment. Accordingly, more time was required for evaporation than for that of zinc or cadmium. For the carbon-reduction of MgO, a higher temperature (about 2100O C.) was needed than for ZnO or CdO. The method was tested by analyzing samples of oxides and samples of metals to which varying amounts of oxide had been added. Having available no standard samples of the three metals of known oxygen content, oxide-spiked snmples were prepared. Into small,
cleaned, and weighed capsules, machined from pure rod stock of each metal, were added microgram quantities of the respective oxides. The oxide weights were obtained by difference on a microbalance, before and after charging the capsules. The open end of each capsule was closed by crimping with cleaned wire pliers. The outside dimensions of the 1-gram zinc capsule were 4 by 12 mm., and the inside, 2 by 6 mm. Capsules of cadmium and magnesium, fabricated similarly, weighed approximately 1 gram and 0.4 gram, respectively. RESULTS AND DISCUSSION
Recovery data are given in Table I for the oxides n i t h no metal present, and in Table I1 for oxides sealed in metal capsules. These results show the successful application of the method to the analysis of zinc in which the average recovery was approximately 100% in both groups of experiments. Recoveries on cadmium averaged 6 to 7y0 lover, suggesting that by using a compensating correction factor, an accuracy of about 4y0may be achieved. Although good recoveries were obtained from magnesium oxide samples alone, encapsulated samples gave low and variable values. I n studying factors which may have contributed to incomplete recoveries, experimental conditions Fere varied to accentuate low results. For example, recovery of oxygen from magnesium oxide was readily dropped to a n unacceptable range by permitting the evaporation to proceed so rapidly as to intensify the entrainment of the light, fluffy oxide in the metal vapors. Similarly, results on cadmium could be lowered by raising the carbonreduction temperature too slowly800" say, 200" C. per minute-from to 2000' C., or by holding the reduction temperature a t some relatively lorn value (about 1200' C.) for several minutes. These conditions apparently permitted the oxygen, produced from the thermal decomposition of the oxide, to escape without quantitative reaction with the carbon. Best results nere obtained when the crucible temperature m-as raised as rapidly as possible to 2000" C. There was evidence that gettering n a s a problem in the analysis of magnesium. During the reduction procedure in which the crucible containing the evaporation residue was heated to 2000' C. in an argon stream a t about atmospheric pressure, the magnesium, released both from the reduction of the oxide residue and from the decomposition of magnesium carbide formed in the evaporation step, came off, not as a vapor as in a high vacuum system, but as a white smoke. Most of the smoke particles
were swept by the argon stream to the glass wool filter where they were collected. Some, however, adhered to the quartz walls of the reaction tube, remaining there until the next disassembly and cleaning of the tube. There was no evidence of gettering by these deposited smoke particles as long as they remained a t room temperature, but when heated, as were those closest to the crucible, there was apparently some interaction with the carbon monoxide to give low results. Table I11 shows recovery data of oxygen added as carbon monoxide to the analytical train and allowed to pass through the previously smoked reaction tube. On alternate runs the crucible was heated to 2000' C. so t h a t the small amount of magnesium n-hich had condensed on the quartz walls of the reaction tube in previous runs was heated and probably became more active. On the other runs the crucible was unheated. Oxygen recoveries on the carbon monoxide were about 5 to 12% lower for the hot runs than for the cold. The apparent improvement in recovery with each succeeding hot run may have been an indication that the capacity of the heated smoke layer to react with carbon monoxide was being diminished. I n agreement with the results of this experiment was the observation that generally higher recoveries were obtained on magnesium oxide when the reactior tube was freshly cleaned than when it had accumulated smoke deposit from previous runs. Such high-temperature gettering did not appear to be a problem in the analysis of zinc and cadmium. A pressure of about 100 mm. of Hg was selected for the argon stream in the reaction tube during evaporation. At much higher pressures the rate of evaporation was diminished and the metal vapors tended to condense into smoke, while a t much lower pressures, say, less than 5 mm., condensation of the vapors \vas not confined to the desired area in the tantalum tube, and a heavy corona of ionized gas interfered with temperature readings by optical pyrometer. The procedure blanks varied from 7 to 10 pg. of oxygen for zinc and from 10 to 20 pg. for cadmium and magnesium. These were obtained by going through the respective procedures for sample loading, metal evaporation, and oxide reduction. The higher values for cadmium and magnesium reflect the effect of removing the crucible from the furnace for sample loading. However, this exposure of the graphite crucible and chips to room atmosphere did not produce blanks intolerably high for analysis in the microgram range. Apparently most of the adsorbed contaminants, such as moisture and car-
bon dioxide, were removed from the system during the period of metal evaporation, and therefore were not collected for measurement during the carbon-reduction process. Platinum, commonly used as a flux in inert gas fusion techniques for the determination of oxygen in higher melting metals, was omitted in this work. I n the absence of such a carbon-bearing flux, the rate of reduction of the oxides was improved by covering the sample with the graphite chips. This proved to be more effective than to cover the graphite crucible with only a graphite lid through which was drilled a l/8-inch hole.
Table 111.
through
Consecutive Run No. 1
n
Recovery of Oxygen Passed Reaction Tube as Carbon Monoxide
Recovery, 7 0 Crucible Crucible a t 30" C. at 2000" C. 100
L
88
3
93
4
5 6 7
I n an effort to prevent loss of magnesium oxide by entrainment at a more rapid evaporation rate, a covered crucible made of very porous, filtergrade graphite was tested. Even though entrainment loss was presumably diminished by this technique, loss by gettering appeared to be greatly increased. The porous graphite soaked up much larger quantities of magnesium metal than did the compact graphite during the evaporation. The result was an increased evolution of smoke and attendant gettering during reduction. The slit tantalum condenser tube n as replaced a t one time during the investigation by a slit fused silica tube to test its usefulness in this application. It proved to be unusable. When the crucible temperature was elevated to about 1000" C. to complete the evaporation of the metal, the fused silica was severely attacked, not only by the adhering metal, but apparently also by the hot carbon crucible in close proximity. Large quantities of carbon monoxide were evolved and light yellow crystals of silicon carbide grew on the crucible surfaces. Kitrogen was tested as the carrier gas instead of argon in a n attempt to retard high-temperature gettering. It was assumed that if the hot magnesium vapor could be converted to the nitride, it would not be available to getter carbon monoxide. The success of this technique was precluded, howVOL. 34, NO. 3, MARCH 1962
@
377
ever, by the formation of cyanogen which condensed out in the capillary U-tube cold trap, interfering with carbon dioxide measurement. The technique of spiking metal samples with oxygen by adding weighed amounts of the oxides to metal capsules did not necessarily approximate the physical distribution nor the particle size of oxide impurities in the metals. It is possible that the degree of entrainment of the oxide particles in the metal vapor was not the same for the two types of oxide distribution. However, it seems improbable that this difference was significant. I n either case the twophase system of liquid metal and solid oxide had opportunity to approach the same physical distribution a t the begin-
.
ning of the evaporation, regardless of the original solid-metal-solid-oxide distribution. This method may be applicable to other metals of relatively high vapor pressures, provided the conditions of the procedure can be made to comply with the respective melting and boiling points, and with the temperatures of thermal decomposition and carbon reduction of the oxides.
LITERATURE CITED
(1) AlLsopp, H. J., Analyst 81,469 (1956). (2) Berry, R., Walker, J. A. J., Johnson,
R. E., DEG Report I11 (c), United Kingdom Atomic Energy Authority, 1960. (3) Goldberg, G., Meyer, A. S., White, J. C., ANAL.CHEM.32, 314 (1960). (4) Hartmann, H., Hofmann, W., Strole, G., 2.Metallk. 49, 461 (1958). ( 5 ) Sheft, I., hfartin, A. F., Katz, J. J., J. Am. Chem. SOC.78, 1557 (1956). (6) Smiley, IT.G., ANAL.CHEM.27, 1098 (1955).
(7) Smiley, W. G., Nuclear Sn'. Abstr. 3, 391 (1949). RECEIVEDfor review October 30, 1961. Accepted January 10, 1962. Presented a t the XVIII International Congress of Pure and Applied Chemistry, Montreal, Canada, August 1961. Based on work performed under the auspices of the U. S. Atomic Energy Commission.
ACKNOWLEDGMENT
The authors acknowledge the helpful suggestions of R. J. Ackermann and E. G. Rauh regarding the design of the apparatus, and the laboratory assistance of A. Venters and C. E. Plucinski.
A mperometric Titration
Of
.
Thorium in Monazite Sands
J. J. BURASTERO and R. W. MARTRES Administracio'n Nacional de Combustibles, Alcohol y Portland, Divisi6n lnvestigaciones Cienti,ficas, Pando, Uruguay
b A practical method for the separation and amperometric determination of thorium in monazite sands is proposed. The attack is carried out with sulfuric acid on 1 0-gram samples; thorium and the rare earths are separated b y a single precipitation with oxalic acid, and the final amperometric titration is made with ammonium paramolybdate as titrant. The composition of the thorium molybdate precipitated, as a function of the initial concentration of thorium, is verified. Possible interferences of some impurities that frequently occur in these sands are determined. The results agree with those of the iodate chemical method and other more laborious amperometric techniques.
S
workers have used ammonium paramolybdate as a precipitating agent for the determination of thorium by volumetric and electrometric methods (1, 4,5,15,22, 25). Kevertheless, few of these methods are used for the determination of thorium in the presence of the rare earths, as under certain conditions thorium molybdate may be precipitated, leaving the rare earths in solution (5, 15, 23). Direct titration has been questioned because of the difficulty of establishing the end point ( I , 9). Titration of the molybdenum combined with the thorium in the precipitate has the disadvantage of being rather lengthy (I). As a result, electrometric methods for EVERAL
378
ANALYTICAL CHEMISTRY
the determination of the end point appear desirable (1,16). ilpplication of the procedure proposed by Gordon and Stine (5) to monazite sands includes a prior treatment of the sample and separation of the thorium and rare earths from other metals occurring in the ore. This is a useful method, but it is very complicated in the early stages. The present work was carried out on monazite sand samples obtained from Uruguayan deposits, to find a method for amperometric determination, which would involve easier manipulation, less time, and easily obtainable and cheaper reagents. For this purpose, a method of sample processing is proposed which does not require mechanical stirring for the precipitation of the thorium oxalate, temperature control and adjustment of pH, nor reprecipitation under the same conditions. The number of washings and filtrations are reduced. The use of methyl oxalate as a precipitating agent is eliminated and also treatment with hydrogen peroxide prior to precipitation. Perchloric acid is not used, as the sample is decomposed by the classical sulfuric acid method. Once the sulfuric acid solution has been obtained, the only steps required are a single precipitation with oxalic acid, decomposition with nitric acid of the oxalates obtained, and conversion to chlorides prior to the final ampero-
metric determination. These steps can be easily performed in a few hours. The reliability of the method is further confirmed by its application to samples from other sources and of different compositions. EXPERIMENTAL
Apparatus. The molybdenum(V1) polarograms were obtained with a Sargent Polarograph, Model XXI. This polarograph was also used in the amperometric titrations, working a t constant voltage. An H-type polarographic cell, like that of Lingane and Laitinen (IS),with a working initial volume of 35 ml. of solution, was used. The conventional dropping mercury electrode was used as a cathode, with the capillary and mercury reservoir of Linnane and Laitinen arrangement -
US).
-
Beckman p H meter, Model N-1.
Monazite Sand. The previous tests were carried out on a monazite sand fraction separated by the magnetic process from a black sand concentrate obtained from Balneario San Luis, Departamento de Canelones, Uruguay (sample I). The total oxide (thorium and rare earths) content of the monazite fraction was determined gravimetrically by the oxalate method (20). The result obtained was 41.4%. The high content of impurities tests the effectiveness of the method under unfavorable conditions. Commercial samples generally contain about 60% of total oxide.
