The change in transmittance with pyrophosphate was 57%; this ion forms a colorless complex with iron.
(7) Peters, C. A,, MacMasters, bl. XI.,
State Chamber of Mines for their permission to publish this paper.
French, c. L., IND. ENG. CHEM., ANAL.Ed. 11,502 (1939). (8) Rakestraw, N. W., Mahncke, H. E., Beach, E. F., Ibid., 8, 136 (1936). (9) Snell, F. D., Snell, C. T., “Colorimetric Methods of Analysis,” 3rd ed., Vol. 11, p. 306, Van Nostrand. Nostrand, New York. York, 1949. (10) Stokes, H,-N., H, N., Cain, J.-R., J. R., J . Am. Chem. SOC.29,409 (1907). (11) Walden, P., Z. physik. Chem. 147A,
LITERATURE CITED ACKNOWLEDGMENT
(1) Bernhard, A,, Drekter, I . J., Science 75. 517 (1932). .. (2) Berdlius, J., “Lehrbuch der Chemie,” Vol. 11, p. 771, 1826. (3) . . Cole, R. H., J . Chem. Phw. . 9,. 251 (1941). (4) Lister, M., Rivington, D., Can. J. Chem. 33,1572 (1955). \ - - -
The author wishes to acknowledge the assistance of N. G. Harvey, Morag Mullins, and Fay Pein in the experimental determinations and preparation of the graphs. Appreciation is expressed to members of the st& of this laboratory for helpful suggestions and to the Transvaal and Orange Free
I .
’
1 (1930). (12) Winsor, H. W., IND.ENQ.CHEX., ANAL.ED.9,453 (1937).
(5) Marriott, W. M., Wolf, C. G. L., J . Bid. Chem. 1,451 (1906). (6) Ossian, P., Pharm. Zentr. 13, 205
RECEIVED for review December 3, 1956. Accepted April 27, 1957.
(1837).
r
I
n
.*
r
Spectrographic Evaluation ot beparation ot Platinum from Palladium, Rhodium, and Iridium GILBERT H. AYRES and HERBERT J. BELKNAP’ Department of Chemistry, The University o f Texas, Ausfin, rex. F A spectrographic method was developed to evaluate the sharpness of the conventional separation of platinum from palladium, rhodium, and iridium by double precipitation of the latter elements as their hydrous oxides. Solutions of the different fractions were evaporated to dryness with a weighed amount of pure powdered graphite, aliquots of which, in a shallow-cup electrode, were burned with a direct current arc; cobalt, added as cobalt(l1) chloride, was used as the internal standard. The relative analysis error was about 5%. The separation was found to be 99.94% complete, except in samples in which only small amounts (20 mg. or less) of the elements were present.
T
HE S E A R P N E S S of the separations of the platinum group elements by the method of Gilchrist and Wichers (4) has been under study in this laboratory for some time. Evaluation of the separations has taken advantage of the specificity of emission spectrographic methods for determining the amounts of other platinum elements coprecipitated with the desired constituent, and the amount of the desired constituent remaining in solution. Ayres and Berg reported the application of the spark, porous-cup electrode technique to the spectrographic determination of palladium, platinum, iridium, and rhodium (I), and also evaluated the separation of palladium from the other three elements by precipitation as palladium dimethylglyoximate 1 Present address, E. I. du Pont de Nemours & Go., Inc., Memphis, Tenn.
1536
ANALYTICAL CHEMISTRY
(2). Ayres and Maddin (3) studied the separation of rhodium (111) from iridium (IV) by reduction of the former to the metal with titanium (111). In the previous work it was shown that both of these separations were a t least 99.7’70 effective; there was evidence of slight compensation of errors in the gravimetric separations. The present work concerns a similar study of another separation in the GilChrist and Wichers scheme-namely, the separation of palladium, rhodium, and iridium by hydrolytic precipitation from bromate solution buffered with sodium bicarbonate, leaving platinum in solution. The required sensitivities were obtained by the use of a modified direct current arc technique and by careful selection of line pairs. APPARATUS
The spectrographic equipment, film developing procedures, and film calibration methods were the same as used in the previous studies (1). In the present work, however. direct current arc excitation n‘as used, instead of the high voltage spark; the power source could supply up t o 15 amperes of current through a 4-mm. electrode gap maintained a t approximately 50 volts across the gap. High purity A’ational graphite 0.25-inch spectrographic electrodes were used. The sample was contained in the bottom electrode (anode) in a cup 2 mm. deep, made by a special mandrel, the counter-electrode was pointed with a pencil sharpener. REAGENTS
The platinum metals and their compounds, obtained from A. D. Mackay,
Inc., were stated by the supplier to be of 99.5% purity or better. All samples were subjected to direct current arc analysis to detect any impurities of other platinum elements. No other platinum elements could be detected in the metallic platinum. Rhodium(II1) chloride showed traces of platinum and palladium, but no iridium. Palladium showed only faint traces of platinum by its most sensitive lines, and iridium chloride contained faint traces of rhodium and palladium, and platinum estimated to be about 0.03%. Solutions of palladium, platinum, and rhodium were the same stock solutions prepared by Ayres and Berg (1); the iridium stock solution was prepared according to Ayres and Maddin (3) by removing traces of rhodium and platinum from the hydrated iridium(1V) chloride source material. Cobalt(I1) chloride solution, containing 8.00 grams of cobalt per liter, was used as the stock solution for the internal standard. Potassium bromate, sodium bicarbonate, and other required reagents were of C.P. or ACS specifications quality. EXPERIMENTAL
Spectrographic Procedure. Because of the relatively low spectrographic sensitivity of some of the platinum group elements. only the most sensitive lines are generally useful, Using high voltage spark excitation and the porous-cup electrode technique, Ayres and Berg ( 2 ) reported the lower limit of detection t o be palladium, 5 p.p.m.; platinum. 20 p.p.ni.; rhodium, 10 p.p.m.; and iridium, 25 p.p.m. It was anticipated that the present work would require greater sensitivity for platinum and iridium than that given above. Direct current arc excitation was used for increased
sensitivity, even though at eome possible sacrifice of precision. Arc excitation is not feasible for porous-cup electrode work because excessive heating expels the liquid from the electrode. Sample Preparation and Burning. The solution containing the platinum group elements (standards or samples for analysis) was evaporated t o about 20 ml. in a porcelain dish; 30 y of cobalt, the internal standard, and 100 mg. of finely powdered graphite were added. The graphite was obtained as dust from the preparation of the electrodes. It was ground in a mortar for 1 hour, and only the portion passing a 100-mesh screen was used. The solution and graphite mixture was evaporated under a heat lamp, with frequent stirring, until completely dry. The dry solid was mixed thoroughly with a mortar and pestle, and exactly 10 mg. were placed in a shallow-cup electrode. A small amount of lithium carbonate was packed evenly on top of the sample, which was burned completely by a direct current arc at 9 amperes for 60 seconds, using a 10-second prearc period and an electrode gap of 4 mm. A primary slit width of 45 microns was used on the spectrograph. Under these conditions the background transmittance of the developed film was 90 to 98%The lithium carbonate cover on the electrode charge prevented the sample from being expelled from the electrode cup when the arc was struck; it also greatly reduced the background due to cyanogen band spectra (6) and enhanced the spectral lines of the platinum group elements, perhaps because of a more uniform vaporization rate. Selection of Line Pairs. Many line pairs of the platinum group element and cobalt internal standard were measured in the wave length region from 2800 to 3000 A., using eight spectra for each of four different amounts (2, 4, 8, 12 y) of each platinum
2o
c z
y
W -1 W
k5
t
108-
6-
v)
5
4-
W
@
=0 2 -
Table I. Precision of Line Intensity Ratio Measurements Intensity Ratio Amount of Pd 3027.9 Rh 3263.1 Ir 2924.8 Pt 2998.0 Metal, y Co 3013.6 Co 3013.6 Co 3013.6 Co 3013.6
-
2 4 8 12
0.192 f 0 . 0 1 4 0.308f0.017 0.656 f 0 . 0 2 7 0.840f0.029 Average relative deviation for all
0.327f0.022 0.665f0.011 1.15 i 0 . 0 0 8 1.59 f 0 . 0 1 9
0.251f0.012 0.477i0.022 0.422f0.028 0.968f0.070 0.664 f 0 . 0 4 2 1.59 f 0 . 0 8 0.982f0.048 2.12 f O . 1 0 measurements, 4.7%
Table II. Accuracy of Spectrographic Determinations Amount of Metal, Micrograms Palladium Rhodium Iridium Platinum Taken Found Taken Found Taken Found Taken Found 2.0 2.0 4.0 4.0 8.0 8.0 12.0 12.0
2.0 2.0 2.0 2.15 4.0 4.1 4.0 4.45 8.0 8.3 8.0 8.5 12.0 11.5 12.0 11.7 Average relative error for all determinations, 5.3%
2.05 2.05 4.3 4.1 8.3 8.0 11.6 12.0
2.0 2.0 4.0 4.0 8.0 8.0 12.0 12.0
2.2 1.9 4.4 3.8 8.5 8.8 13.0 12.3
group element. Final selection of the line pair to be used for each element was based upon freedom from self-absorption and interference from lines of other elements, and upon the smallest relative deviation in the measured intensity ratios. The cobalt 3013.6 line was satisfactory as the internal standard for all four platinum group elements. The most sensitive lines of platinum showed self-absorption t o varying degrees; the platinum 2998.0 line was chosen for use. Iridium was measured by its 2924.8 line. Palladium 3027.9, although of only medium intensity in the concentration range used, was completely free from interference. Many lines of rhodium were either too heavy or too faint; the rhodium 3263.1 line, of medium intensity, had some interference from the cobalt 3263.2 line, but a cor-
2.0 2.0 4.0 4.0 8.0 8.0 12.0 12.0
2.1 1.95 4.5 4.5 9.0 8.2 11.5 11.8
rection could be applied by determining the intensities of several other cobait lines of about the same intensity in this region. The validity of the correction was proved by the linearity and slope of the analytical working curve. Although not suitable for high concentrations on account of self-absorption, the rhodium 3396.8 line was used, in conjunction with rhodium 3263.1, for amounts of rhodium in the range from 2 to 4 y. The precision of intensity ratio measurements is shown in Table I ; the data were obtained from duplicate exposures of each concentration of a given element on each of four or more films. Table I1 shows the accuracy obtained with duplicate samples of four different concentrations of each element, using duplicate exposures of each standard sample and concentration, requiring several films. Preparation of Analytical Curves. A suitable aliquot of stock solution of the platinum group element, with 30 y of cobalt internal standard added, W&S evaporated with powdered graphite as described earlier. For each element four standards were prepared, containing 2, 4, 8, and 12 y of the element. Spectra of duplicate 10-mg. portions of each prepared standard were recorded, and four films were made on different days to include day-to-day and film-to-film variations. The intensity ratios (intensity of desired line/intensity of internal standard line) were used t o construct the analytical working curves, of which those in Figure 1 are typical. These curves had a slope on the log-log plot of about 55 O, rather than the 45 O slope of a normal working curve when background corrections were made; by applying background corrections the working curves were rotated to the 45' slope. KO serious error was involved in using working curves without background corrections, because only a proportionality constant was introduced; background was either relatively constant or proportional to total exposure ( 5 ) . VOL. 29, NO. 10, OCTOBER 1957
1537
Table 111.
Results for Separation of Platinum
Amount Taken, Mg. Pd Rh Ir Pt 100
100 80 80 70
20 20 20 20 20
20 10
50 50 40
50
50
100
20
0.10 0 010
20
0 024 0 032
40 40 35
5 100
35 ~.
~.
20
_..
100 10
10
20 20 20 20 20 10
40
20 20 20 20 20 20
Found in Solution, Mg. Pd Rh Ir
100
5
10 50
10
Av.
0.032
0 . K
0.14 0.032 0.032 0.048 0.045
0.032
0.052
Separation of Platinum. I n the Gilchrist-Wichers method for separation of the platinum group elements, the solution remaining after the volatilization of osmium and ruthenium contains the other four elements, along with the excess reagents used in these separations. Palladium, rhodium, and iridium are then removed by double precipitation as their hydrous oxides from a hot bromate solution, the pH of which is adjusted with sodium bicarbonate, first to about 6, and finally to about 8. In the present study the Gilchrist-Wichers procedure was followed in detail, except for the use of smaller samples of the platinum group elements. The precipitate was filtered m-ith gentle suction in a sintered-glass filter crucible. The precipitated hydrous oxides were dissolved in a small amount of hot concentrated hydrochloric acid; the solution was evaporated just to dryness to remove excess acid. The residue was dissolved in water, diluted to exactly 100 ml. (Solution l), and reserved for determination of platinum present as a contaminant of the precipitate. The combined filtrates from the double precipitation were treated with hydrochloric acid to decompose the excess bromate and were evaporated to dryness. The residue was taken up with 5 ml. of concentrated hydrochloric acid and diluted to 250 ml. for use in determining the trace amounts of palladium, rhodium, and iridium that had failed to precipitate. The solution containing the platinum and traces of the other elements contained also about 1.5 grams of soluble salts that were added during the precipitation separation. It was necessary to separate the platinum group elements from the bulk of the soluble salts before the samples were arced for analysis. I n the Gilchrist-Wichers method, platinum is recovered from the filtrate by precipitation as sulfide; of the other platinum group elements, palladium and rhodium are precipitated quantitatively, but it is difficult to precipitate iridium quantitativeiy as the sulfide (7'). Each of the solutions analyzed contained 5 mg. or more of platinum, 1538
ANALYTICAL CHEMISTRY
030 020 020 015
08
Pt Pt SeparaFound in tion, % by Ppt., Mg. Difference
0.08
0.06
0.036
0.08 0.05 0.03 0.053
0,054 0.034 0.025 0.025 0.034
99.94 99.7 98.5 99.94 99.8 99,94 99.94 99.5 99.3 99,75 99.95 99,66
0.022
0.058
0,049
99.74
0 0 0 0
0 023
0.OZ0
0.030 0.020 0.020 0.010 0.023
0 0 0 0
08
053
05
0 05
0.053 0.062
0.06 0.06 0.076
0.056 0.041 0.06
and it was hoped that iridium would coprecipitate or postprecipitate with platinum sulfide. However, the iridium was not carried by platinum sulfide to any appreciable extent in acid solution, but was precipitated (or coprecipitated) more or less completely from alkaline solution after the main precipitation from acidic solution. For the sulfide precipitation the acidic solution containing the platinum, along with small amounts of the other elements in a volume of about 250 ml., was heated to boiling and saturated with a rapid stream of hydrogen sulfide. The platinum sulfide was highly colloidal, which in this instance was perhaps desirable to increase the possibility that it might carry the iridium. The mixture was made alkaline with ammonium hydroxide and digested for several hours, then filtered through a sintered-glass crucible. The precipitate was not washed on account of its great tendency to peptize. The precipitate mas dissolved in hot aaua regia. then diluted t o known volume *for stectrographic analysis (Solution 2). In testing the reliabilitv of the sulfide precipitatioun, the resu1ts"of many analyses of known solutions showed palladium and rhodium to be precipitated quantitatively; however, in samples in which the amount of iridium varied from 40 to 200 mg. and the quantities of the other platinum elements each varied from 40 to 500 mg., the recovery of iridium varied from 55 to 1OOyo; the average recovery was 75y0. There appeared to be no correlation between the percentage recovery and the amount of iridium or of the other elements in the solution from which precipitation was made. Spectrographic Analysis and Evaluation. Solutions 1 and 2 were analyzed spectrographically. I n most cases half of the solution was taken for analysis, although in some cases the entire solution was used to prepare the spectrographic sample by evaporation with 100 mg. of powered graphite. Spectra of duplicate 10-mg. portions of each prepared sample were recorded; the amounts of the elements were determined from the line intensity ratios
by use of the analytical curves, and the aliquot fraction factor was applied for converting to amounts in the solutions analyzed. It was possible to determine as little as 1 y of palladium, rhodium, and platinum, and 2 y of iridium. In estimating the amount of iridium, the results were corrected on the basis of a 75% recovery in the sulfide precipitation. Typical results are shown in Table
111; each line is the average of duplicate portions of each of four samples of the composition stated, the eight spectra being recorded on the same film. Results were calculated to per cent separation of platinum; per cent separation yalues for the other elements have no significance because the amounts of these elements left in solution were not dependent upon the amounts taken. DISCUSSION
The data of Table I11 show that there xi-as a slight compensation of errors in the separation; however, the errors were small except where small amounts (20 mg. or less) of the elements were involred. In the case of 10 mg. or less of platinum, the error can be of significance. The amount of palladium, rhodium, and iridium failing to precipitate was not a function of the amounts of these elements taken nor of the amount of platinum taken, and was probably a solubility loss. When only a single precipitation of the hydrous oxides was made, the amount of platinum carried down by the hydrous oxides was dependent upon both the amount of the precipitate and the amount of platinum present, and was significant unless the amount of platinum was less than 5 mg. From several experiments in which the first precipitate was analyzed, it was estimated that 3 to 5% of the platinum present was carried down with the precipitate. When double precipitation was used, as prescribed in the GilchristWichers method, the amount of platinum carried down with the second hydrous oxide precipitate was essentially independent of the amounts of the various platinum elements taken. The spectrographic tests showed that the separation method effectively removed 99.770 of the platinum on the average, and when used for weights of 50 to 100 mg. of platinum it was 99.94% effective. The spectrochemical method developed showed a precision of 4.7% and an accuracy of 5.3%. ACKNOWLEDGMENT
Part of this work was supported jointly by The University of Texas and the U. S. Atomic Energy Commission under the terms of Contract No. AT(40-1)-1037
(5) Harvey, C. E., “Spectrochemical RECEIVED for review June 21, 1956. h c Procedures,” p. 238, Applied Recepted May 20, 1957. Condensed from a search Laboratories, Glendale, Calif., dissertation submitted by Herbert J . Ayres, G. H., Berg, E. \V., * I x ~ L . 1950. Belknap t o the faculty of the Graduate CHEM.24, 465 (1952). (6) Keenan, R. G White, C. E., ISAL.School of The University of Texas in parIbid., 25, 980 (1953). CHEM.25, 8& (1953). tial fulfillment of the requirements for the ilyres, G. H., Maddin, C. hI., Ibid., degree of doctor of philosophy, August ( 7 ) Soyes, A. A , Brav, W.C., “Qualita2 6 , 671 (1954). t h e Analysis for the Rare Ele1955. Portions presented at Tenth SouthGilchrist, R., Wichers, E., J . -4t1z. ments,” p. 133. Macmillan. S e n west Regional Meeting, ACS, Fort Worth, Tork, 1927. Teu., December 1954. Chein. SOC.57, 2565 (1935). LITERATURE CITED
Quantitative Determination of Porosity by X-Ray Absorption G. L. CLARK and C. H. LIU Department of Chemistry and Chemical Engineering, University o f Illinois, Urbana, 111. ,The x-ray absorption method for measurement and control of porosity, which involves penetration through specimens with no discrimination between open and closed pores, has been critically compared with the pycnometric method in tests with commercial rubber-composition storage battery separators. Measurements b y both methods were in satisfactory agreement. For the x-ray method, after the linear absorption coefficient i s once determined for the complex mixture, each specimen can be nondestructively tested in a few minutes. The x-ray method is reliable as well as speedier and simpler than pycnometer density measurements. It also permits a rapid test of uniformity of porosity.
T
PROPERTY of porous texture, which may often have great practical value, is possessed in varying degrees by many natural and manufactured solid materials. The nieasurement and control of porosity are often difficult, inaccurate, and time-consuming by older methods of density measurement. The porosity of a solid may be defined as the volume of interstices measured as a fraction of the apparent volume. Thus, HE
where P is the porosity, V , is the apparent volume of the solid, and Ti, is its true volume. The most generally used method for measuring porosity involves pycnometric determinations (1-3). The volume of the pores is calculated from the increase in weight of a porous solid after immersion in a penetrating liquid of known densitj-:
THEORY
n-here ITl is the \\-eight of sample before immersion, WZis its weight after immersion, p is the density of the liquid, and T7= is the apparent volume of the sample. Another method based upon the same general principle is the gas absorption method. These methods are applicable only to open pores which are accessible to the liquid or gas. For closed porosity, the solid must be ground into small particles to expose all pores. With such a sample the true density of the solid material can then be determined. From this value and the weight, the true volume of the solid can be calculated and in turn, the porosity, from Equation 1. I n all these methods long and difficult procedures are involved; often the sample has t o be destroyed during a determination. The x-ray absorption method is a simple and efficient means of measuring porosity. This application is by no means a new one, for it has been used with some success in this laboratory to evaluate the porosity of storage battery plates. There are also reports of measurements of the gage of compressible or porous materials, such as leather, cellophane, cloth textiles, rubber blankets, plastic films, paper, and the like (4). The determination of the porosity of sandstone in oil field work has also been suggested. Hoviever, the present work represents probably the first highly critical comparison of classical and x-ray techniques, and is designed to test thoroughly the reliability of the latter as a routine, commercial procedure for assuring constancy of optimum porosity in fabricated rubbercomposition separators for storage batteries now supplanting the fragile Food separators used for so many years.
The absorption l a v of x-rays is well known : I 2.303 log - = - p X (3) IO where IO is the intensity of the unabsorbed beam, I is the intensity of the absorbed beam, p is the linear absorption coefficient at a given wave length, and X is the sample thickness. The basic assumption is that there is no preferred orientation in the sample under consideration, a condition which may be tested by diffraction analysis. When an x-ray beam of large cross section passes perpendicularly through a porous solid sheet, before the intensity of the absorbed beam is measured the beam will have swept through a known apparent volume of the substance. Specifically, this volume is the product of the apparent thickness of the sample and the area of the slit or of the detection device in direct contact with the specimen, so that no beam divergence is involved. It can easily be seen that X t = -17,
x, and
X,
=
T,’
X,(l - P )
(4)
where X 1 is the true thickness of the sample, X,is its apparent thickness, V , is the true volume swept through by the x-ray beam, V ais the apparent volume, and P is the porosity of the sample. The absorption law as applied t o the porous solid will then become 1
2.303 log- = - p X a (I - P) 10 Hence, I 1 2.303 log Io P =
(5)
+
rxa
(6)
If the value of the absorption coeffiVOL. 29,
NO. 10, OCTOBER 1957
1539
