the interference of mol) bdenuni increased somewhat if the solutions were allowed to stand for 1 hour. Judging by the fact that the pink and amber colors produced by the stannous chloride did not change on adding the thiocyanate (Table 111), it would seem that the stannous chloride reduced the molybdenum to the quadri- or trivalent state and that in these states molybdenum did not react with thiocyanate. The proposed procedure is not recommended for the analysir of ferrotungsten, although a very satisfactory result can be obtained for that alloy by proceeding as described and correcting for the small amount of iron in the precipitate colorimetrically. Iron is ordinarily the only interfering element present in commercial ferrotungsten in sufficient amount to interfere with the method. The results obtained for tungsten when typical samples of tungsten metal and thoriated tungsten were analyzed several times each by the proposed method are shown in Table IV. Good precision was obtained and the results compare favorably with the nominal
tungsten content as determined by difference. It is not feasible to check the absolute accuracy of a method of this type, since there are no umpire methods or primary standards (metals or compounds). ACKNOWLEDGMENT
The author is indebted to Samuel Sitelman and R. D. France for their suggestions. This project was sponsored by the Ordnance Materials Research Office. LITERATURE CITED
(1) Am. Soc. Testing Materials, Philadelphia, Pa., “ASTM Methods for Chemical Analysis of Metah,” p. 78, 1939. (2) Ibid. p. 180, 1956. (3) Am. SOC.T:!ting Materials, Philadelphia, Pa., 1958 Book of ASTM Standards.” Part 2, .746,874. (4) Blackburn, P. E., K c h , M., Johnston, H. L., J.Phys. Chem. 62, i69 ( 1958). (5) “Gmelins Handbuch der An-
organischen Chemie,” System-Nummer 54, pp. 114, 122, 123, Verlag Chemie, G.M.B.H., Berlin, 1933. (6) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A,, Hoffman, J. I., “Applied
Inorganic Analysis,” p. 692, Wiley, New York, 1953. (7) Li, K. C., Wang, C. Y., “Tungsten,” p. 277, Remhold, New York, 1955. (8) Magneli, A., Andersson, G., Blomberg, B., Kihlborg, L., ANAL.CHEM. 24,1998 (1952). (9) Mellor, J. W., “Comprehensive Treat-
ise on Inorganic and Theoretical Chemistry,” Vol. 11, pp. 749, 755, 762, 763, Longmans, Green, and Co., London, 1921
(10)hlilitary Spec. MIL-T-13827 (Ord), Tungsten Powder, December 1954. (11) Military Spec. MILT-14507A (Ord) , Tungsten Rod and Wire, January 1960. (12) Millner, T., Neugebauer, J., Nature 163,601 (1949). (13) Sandell, E. B., “Colorimetri; Determination of Traces of Metals, p. 886, Interscience, New York, 1959. (14) Scott, W. W., “Standard Methods of Chemical Analvsis.” Vol. 1. D. 1011. Van Nostrand, Ne; York, 19g9.* (15) Seybolt, A. U., Burke, J. E., “Prq;
cedures in Experimental Metallurgy, p. 292, Wiley, New York, 1953. 116) Treadwell. F. P.. Hall. W. T.. ‘ “Analytical ’Chemistry,” Vol. 2, p: 268, Wiley, New York, 1930. (17) Wohler, L., Balz, 0.)2. Electrochem. 27,406 (1921).
RECEIVED for review November 18, 1960. Arcepted May 22, 1961.
Precipitation of Platinum Metals from Solutions of Their Sulfo Salts 5. B. SANT, ARTHUR CHOW, and F. E. BEAMISH University o f Toronto, Toronto, Ontario, Canada
b Few gravimetric methods have been recorded for the determination of platinum, rhodium, iridium, and ruthenium. The published data suggest that the most accurate procedure for these metals is that recorded b y Solaria and Taimni. This method involves a sulfide precipitation, subsequent dissolution b y sodium sulfide, and reprecipitation b y the addition of acids. Because the reported technique is simple and the accuracy and precision are of a high order, this sulfide method has been examined critically, with the conclusion that precipitation b y acids from the solutions of soluble sulfo salts of the platinum metals is not satisfactory and does not produce the claimed accuracy and precision.
E
palladium, analytical methods for the platinum metals, classical or empirical, are scarce. This is particularly true for platinum, the most generally known. For standardization, the gravimetric methods, properly carried out, provide the ultimate accuracy. Precipitation as ammonium XCLUDING
hexachloroplatinate is one of the oldest methods. While there is very little interference from associated base or platinum metals, except iridium, the salt is objectionably soluble. In addition to the precipitate formed with the ammonium cation, the hesahalogen platinate anion forms precipitates with potassium, rubidium, cesium, univalent thallium, tetramethylphosphonium ( I ) , tetraphenylarsonium (6), and di(17) methylphenylbenzylanimonium cations. With the exception of the last two reagents, the precipitates are useful largely for the determination of the respective cations. Tetraphenylarsonium bromide provides a precipitate whose constant composition is doubtful. Dimethylphenylbenzylammonium chloride may be a useful precipitant but more procedural information is required. For the precipitation of platinum as a metal the most commonly used reagents are formic acid, zinc, and magnesium. The senior author’s experience (5) with formic acid used on the macro scale has been satisfactory, but for milligram amounts of platinum the
reagent is not recommended (4). Precipitation by zinc can be reasonably successful when there is some kn Jwledge of the approximate platinum content, but the tendency toward contamination or incomplete precipitation is often a n active factor ( 5 ) . The application of magnesium seems not to have been recorded with accompanying data and a detailed examination of this reductant could be of value. Although less selective than hydrogen sulfide, three organic precipitants have been used : phenylthiosemicarbazide (14, 15), mercaptobenzothiazole (23, 24, and thiophenol (8). Thiophenol has had some extended application and acceptable results are obtained when ignition is carefully controlled. However, the reagent has a n objectionable odor and is unstable, requiring storage in a nitrogen atmosphere. Precipitation by hydrogen sulfide is one of the oldest recorded methods for platinum. Used by Berzelius in 1826, the chemistry of the reaction remains largely unknown. To avoid the disadvantages of gaseous hydrogen sulfide, thioformamide (IO) and thioVOL 33, NO. 9, AUGUST 1961
0
1257
acetic acid (a) have been used with some success. However, hydrogen sulfide remains one.of the most generally used precipitants for platinum, and standard procedures, varying somewhat in detail, are recorded in most texts on analytical methods. The sulfide is always ignited to metal and usually the results are somewhat high. Methods for avoiding positive errors have been recorded (IS) and involve modifications in precipitating and ignition techniques. A new approach to the sulfide precipitation was proposed by Taimni and Salaria @ I ) , who treated the solution of platinum chloride with sodium hydroxide and sodium sulfide to produce the soluble thio salt. On the addition of acetic acid and ammonium acetate, a subsequent heating treatment, washing with water, alcohol, and ether, and drying in vacuo, the precipitate of PtS2.5Hz0 could be weighed in a sinteredglass crucible. The experimental data in the report indicated a higher degree of accuracy and precision than had been obtained by the present senior author from any recorded method for the determination of platinum. Furthermore, this approach to sulfide precipitation was extended by Salaria to include all of the platinum metals except osmium, with similarly high accuracy and precision. The use of sodium sulfide as a precipitant and separatory reagent for some of the metals of column VI11 of the Periodic Table, has received considerable attention in the present authors' laboratories and a method was developed for the effective separation and subsequent determination of iron and arsenic; the latter element, like the platinum metals, forms soluble thio salts (7). The procedure involved the treatment of the mixed solution of arsenic(V) and iron(II1) with sodium polysulfide, the removal of iron sulfide for volumetric treatment by potassium permanganate, and the conversion of the arsenic in the filtrate to ammonium magnesium arsenate. All efforts to use the precipitated sulfides as weighing forms failed. Aside from the question of constancy of composition, the precipitate contained mechanically mixed sulfur which could not be selectively dissolved by the conventional sulfur solvents. The data obtained from these investigations conflicted with the reported high accuracy and precision obtained by Salaria for comparable types of precipitates. Although the precipitant and the techniques employed by Salaria differed somewhat from those used for the above work, there seemed no explanation for the rather remarkable success in achieving suitable weighing forms. The various early attempts to corroborate the results obtained by Taimni and Salaria (21) resulted in 1258
ANALYTICAL CHEMISTRY
the simultaneous precipitation of sulfur, which was only partially removed by the various solvents recommended in the original procedure. High blanks of poor precision were encountered. It was unfortunate that the early papers dealt perfunctorily and inadequately with the most important problem of the preparation of the reagent and the probability of significant and variable blanks, Subsequent to the publication of the prescribed procedure (21) the authors studied the stability of the sulfide reagent and provided a modified method for its preparation (19). All attempts to apply the modified method indicated that while the positive errors were reduced in magnitude, they remained significant and there ww no indication of the high accuracy and precision obtained by Taimni and coworkers. The following is a report of the examination of the methods proposed by Taimni et al. (21) with summaries of their data. Since these data encourage the expectation of a simple procedure with high precision and accuracy, the present authors hope that those who have acquired experience with these sulfide procedures will not fail to report their findings. EXPERIMENTAL
Stock Solutions of Platinum Metals. A standard solution of platinum was prepared by dissolving a weighed quantity of platinum sponge in aqua regia, followed by removal of nitric acid. The platinum content was determined by thiophenol (8). A known weight of palladous chloride was dissolved in dilute hydrochloric acid and the solution was standardized by dimethylglyoxime. Sodium rhodium chloride was used to prepare a solution of rhodium, which was determined by precipitation by thiobarbituric acid (8). Sodium iridium chloride was dissolved in a 0.2N hydrochloric acid solution and the iridium content was determined by 2-mercaptobenzothiazole (3). A solution of ammonium ruthenium chloride was standardized by thionalide (16).
Reagent Solutions. Initially the sodium sulfide reagent was freshly prepared exactly as directed by Taimni and Agarwal (20). Subsequent to the report by Taimni and Salaria (21) on the atmospheric oxidation of alkali sulfides, the following modified method was used and all of the data included below were obtained with these freshly prepared reagents. Hydrogen sulfide was passed into 250 ml. of 2N sodium hydroxide solution maintained a t 0" to 5' C. by an ice bath. The stream of gas was maintained over a period of 2 to 4 minutes and the resulting clear, colorless solution was used immediately.
Procedure. The solution of platinum(1V) chloride was made slightly alkaline with sodium hydroxide solution and treated with the prescribed excesses of freshly prepared 2 N sodium sulfide reagent. The recommended excesses of acetic acid and solid ammonium acetate were added and the solution was boiled for 1 to 2 minutes, allowed to cool, and again brought to a boil. Cooled a t room temperature or by ice, the precipitate settled rapidly. The latter was filtered through the prescribed sintered-glass crucible of porosity 4 and then washed thoroughly with water, alcohol, and ether, successively. Subsequent to drying for 5 minutes with a filter pump and for one-half hour in a vacuum desiccator, the precipitate was weighed as PtSz.5Hz0. Alternatively the precipitate was dried a t 130-5" C. and weighed as PtSz.2H20.
In addition to the above procedure, a wide variation of precipitation and washing techniques failed to reduce the order of magnitude of error. The results obtained for platinum are recorded in Table I with an appended statement of the accuracy and precision reported by Taimni and Salaris (21).
Determination of Blanks. The procedure required for the sulfide determination included no directions for blanks. These were examined by the present authors for both methods of reagent preparation (19, 81). While sulfur frequently appeared with the earlier method, the later procedure produced clear solutions throughout the whole procedure. Furthermore, the gravimetric data indicated that the values for blanks were precise and of the order of normal gravimetric blanks. While these values were applied to the weights of the precipitates in all cases, they did not significantly alter the results recorded in Tables I to Salaria's investigation of the oxidation effect on the reagent showed that the sodium sulfide reagent prepared as recommended, except that samples were acidified with hydrochloric acid, revealed the presence of sulfur immediately a t 20" C. Since the procedure for quantitative determination involves a short period of boiling and some contact with air a t room temperature, one cannot exclude the possibility of osidation of sulfide to sulfur in the presence of the platinum metal sulfide. Sulfide Precipitation of Palladium, Rhodium, Iridium, and Ruthenium. Excepting palladium, few precipitants have been recorded for the above metals and particularly for iridium and ruthenium. The most suitable general method is the hydrolytic precipitation developed by Gilchrist (11). Where high accuracy is required, the method is not suitable for amounts of the order of a few milligrams.
v.
Quantitative metal precipitants are seldom used and in general are applicable only to platinum. The most acceptable gravimetric precipitanta for each of the metals are: rhodium, hydrogen sulfide, thiobarbituric acid (S), and 2-mercaptobenzoxazole (11); iridium, 2 - mercaptobenzothiazole (3); ruthenium, thionalide (16). For each of the above metals, Taimni and Salaria (21) have reported very accurate recoveries with sulfide weighing forms by slight modifications of the above procedure for platinum. The recommended procedures (21) were followed in detail and the results for each metal are recorded in Tables I1 to V, with a summary in each case of the accuracy obtained by Taimni and Salaria. Determination of Palladium a s Sulfide, PdS.2Hz0. Results included in Table I1 show that the weight of palladium obQined from that of the sulfide precipitate is variably high when compared with the weight of palladium added. Determination of Rhodium as Sulfide, RhzSs.3H2S. The results in Table I11 show a positive error of 10 to 12% between the rhodium obtained and t h a t added. Determination of Ruthenium a s Sulfide, RuzSs.2Hz0. Data presented in Table IV show t h a t the weight of ruthenium obtained by the sulfide method is 40 to 55% high. Taimni and Salaria state that “measured portions of solution were made slightly alkaline with ammonium hydroxide ...” When ruthenium solution was made slightly alkaline using a pH meter, a black precipitate appeared which did not dissolve on addition of an excess of sodium sulfide reagent. The black precipitate is perhaps due to the formation of ruthenium trihydroxide or tetrahydroxide (18). I t is generally accepted that “the disulfide is the lowest and the only certain sulfide of ruthenium; the supposed higher sulfides are probably mixtures of disulfide with free sulfur” (18). Determination of Iridium as Sulfide, Ir&. 10H20. The sodium sulfide method of determining iridium yields results 6 to 14% high (cf. Table V). Taimni and Salaria claimed (21) that the iridium sulfide, Ir2S3.10H20, so obtained is stable up to 85’ C. THERMOGRAVIMETRIC EXAMINATION OF PRECIPITATES
With the exception of those for iridium, precipitates obtained from the recommended procedures were examined thermogravimetrically . Samples of the two types of platinum sulfide precipitates placed on the pan of the thermobalance a t room temperature continuously gained in weight and these and
Table 1.
Determination of Platinum with 2N Sodium Sulfide Reagefitu (Washing with alcohol and ether)
Pt Soln., M1.
Reagent Added, M1.
8N Acetic Acid Added, M1.
Sulfide PPt., Mg.
Calcd. Wt. of Pt in Ppt., Mg.
Pt ActuaUy Present, Mg.
7%
Error
Drying a t 15’ to 20’ C. and weighing as Pt&.5&O 20 20 25 25 25 25 35 35 35
45 ~. 45 55 55 55 55 60 60 60
50 ..
50 60 60 60 60 70 70 70
65.4 65.2 80.3 81.7 84.7 84.3 117.2 116.8 115.7
36.54 .. .~ 36.43 44.86 45.64 47.32 47.10 65.50 65.27 64.65
40.12 40.12 50.15 50.15 50.15 50.15 70.21 70.21 70.21
-8.9 -9.2 -10.5 -8.9 -5.6 -6.1 -6.7 -7.0 -7.9
Drying at 130-5” C. and weighing aa Pt&. 2&0 25 25 20 20 25 25 35 35 35
60 60
45 45 55 55 60 60 60
80
96.1
50 50
67.5 67.2 81.5 82.7 121.9 120.0 118.9
80 GO 60
70 70 70
9i.i
63.53 60.23 44.62 44.43 53.88 54.68 80.58 79.34 78.63
53.78 53.78 __ .40.12 40.12 50.15 50.15 70.21 70.21 70.21
+18.1 +11.9 +11.2 +10.7 f7.4 +9.0 +14.7 +13.0 +11.9
4 Dnta for Pt recoveries recorded by Taimni and Salaria (gf) show errors of less than 0.1% over the rnnge 30 t,o 100 mg. Pt.
Table It.
Determination of Palladium with 2 N Sodium Sulfide Reagent”
Pd Solution, M1.
Reagent Added, MI.
20 20 20 20 25 25 25 25
40 40 50 50 45 45 50 50
4N Acetic Acid
Added, MI. ‘0 3)
55
55 55 55 55 55 ~~
Sulfide PPt., Mg.
Calcd. Wt. of Pd in Ppt., Mg.
Pd Actually Present, Mg.
44.0 46.0 44.6 45.0 54.0 53.5 57.9 54.5
26.87 28.09 27.24 27.49 32.98 32.68 35.37 33.29
26.34 26.34 26.34 26.34 32.92 32.97 32.92 32.92
%
Error
f2.0 +6.7 +3.4 +4.4 +0.2 -0.7 +7.4
+1.1
a Taimni and SalariR. (21) showed errors of less than 0.2% over the range of 5 to 67 mg. of palladium.
Table 111.
Determination of Rhodium with 2N Sodium Sulfide Reagent Followed by Washing with Alcohol and Ether.
Rh Solution, M1.
Rea ent Adfed, M1.
25 25
40
30
30 30 40 40
40 50 50 50 50 50
5N Acetic Acid Added, M1.
Sulfide P t pxg:)
Calcd. Wt. of Rh in Ppt., Mg.
Rh Actually Present, Mg.
Error
50 50 50 50 50 50 50
53.1 53.3 55.4 55.4 55.3 73.4 72.6
27.74 27.85 28.95 28.95 28.89 38.35 37.93
24.62 24.62 25,80 25.80 25.80 34.40 34.40
+12.7 +13.1 f12.2 +12.2 +11.9 +11.5 +lo. 3
%
a Taimni and Salaria (22)showed errors of less than 0.1% over the range of 18 to 45 mg. of rhodium.
VOL. 33, NO. 9, AUGUST 1961
1259
other precipitates exposed to the atmosphere a t room temperature contained increasing amounts of sulfate ion and increased in acidity following triturating with water. This conforms with the well-known behavior of such precipitates, which are readily oxidized by air and are usually dried in an atmosphere of hydrogen sulfide (8). The thermolysis curves were obtained by plotting the weight-temperature changes against a background of compositions. The compositions of the various hydrates were calculated from the weights used for each determination, since the recommended procedures invariably produced complete precipitation of the platinum metals. In general, the individual curves for each metal precipitate varied so much that there was no correspondence to any weighing form a t the acoompanying drying temperature. In a few instances, horizontals did appear, but these did not occur a t any of the compositions and temperatures recorded by Taimni and associates. In some instances, some coincidence with the reported values could be predicted on the assumption that the precipitate was a
Table IV.
Determination of Ruthenium with 2N Sodium Sulfide Reagent Followed by Washing with Alcohol and Ether#
Ru Rea ent Solution, Adfed, MI. MI. 35 35 35 35 35 35 50 50 50 50 50
mixture of a hydrate and some associated impurity--e.g., sulfur. The present authors were not concerned with the identification of hydrates of the platinum metal sulfates nor with their thermal characteristics but only with ascertaining whether or not the precipitates obtained by the prescribed analytical method were indeed the pure substances claimed by Taimni (21) and in any case, whether the precipitates could be used for quantitative work. The thermolysis of sulfides of platinum, palladium, and ruthenium was studied by Taimni and Tandon (22) with the conclusions that “the methods used for drying and weighing these sulphides are reliable.” An examination of the data and thermolysis curves included does not, in the present authors’ opinion, support the claim that any of the stated hydrates appear as weighing forms. Furthermore, the thermogravimetric study by D u d , Champ, and Fauconnier (9) of the iridium sulfide prepared as described by Taimni and Salaria (21) failed to reveal the weighing form Irk&.lOHzO claimed by these authors or any other hydrated weighing form. Duval found
50 50 50 50 50 50 55 55 55 55 55
8 N Acetic
Acid Added, MI.
Sulfide
60 60 60 60 60 60 60 60 70 70 70
67.2 70.9 64.8 71.1 69.9 69.5 98.5 98.5 96.9 96.6 98.7
3;:)
Calcd. Wt. of Ru in Ppt., Mg. 40.76 42.99 39.30 43.12 42.39 42.15 59.73 59.73 58.76 58.59 59.85
Ru Actually Present, Mg. 27 97
27.97 27 97 27.97 27.97 27.97 39.95 39.95 39.95 39.95 39.95
%
Error +45.7 j53.7 +40.5 +55.1 +51.5 +55.1 +49.6 +49.6 +47.0 +46.6 +49.9
S . Taimni and Salaria (,%?I)showed errors of less than 0.1% over the range of 18 to 86 mg. of ruthenium.
Table V.
Determination of Iridium with
Rea ent Solution, Ad&, M1. M1. Ir
4 N Acetic
2N
Sodium Sulfide Reagenta
Ir
Acid Added, MI.
Sulfide Ppt., Mg.
30 30 30 35 35 35 35 35 35
45.1 44.7 45.0 55.1 56.2 57.1 63.1 64.5 63.6
Calcd. Wt. Actually of Ir in Present, Ppt., Mg. Mg.
%
Error
that the sulfide of iridium not only showed no horizontal a t 85“ C. but descended abruptly. Froin room temperature to 410’ C. there waa a continuous decrease .in the weight,’ then an increase which was attributed to the oxidation of sulfur to sulfate. Thus one must conclude that platinum sulfides prepared as described by Taimni and Salari? (21) are not proved weighing forms and that the procedures recommended by these authors produce neither accurate nor precise results. Whether or not the hydrated sulfides of the platinum metals prove to be acceptable weighing forms, one may hope that Taimni and coworkers will carry further their examination of the applicability of the sulfo salts as separatory reagents. It is not unlikely that isolation of the noble metals from associated base metals in concentrates, etc., could be thus effected. LITERATURE CITED
(1) Anderson, C. J., Keeler, R. A., ANAL. CHEM.26, 213 (1954). (2) Atterburg, A., Chem.-Ztg. 22, 522-38 (1898). (3) Barefoot, R. R., McDonnell, W. J., Beamish. F. E.. ANAL. CHEM.23. 514 (1951). ’ (4) Beamish, F. E., TuZantu 1 , 3 (1958). (5) Blackmore, A. P., Marks, M. A., Barefoot, R. R., Beamish, F. E., ANAL. CEIEM. 24, 1815 (1952). (6) . , Bode, H.. Z. anal. Chem. 95, 133 (1951).’ ’ (7) Caesar, C., University of Toronto, un ublished work, 1933. (8) urrah, J. E., McBryde, W. A. E., Cruikshank, A. J., Beamish, F. E., IND.ENO.CHEM.,ANAL. ED. 18, 120 (1946). (9) Duval, C., Champ, P., Fauconnier, P., Anal. Chim. Acta 20, 152 (1959). (IO) Gagliardi, E., Pietsch, R., Monatsh. 82, 656 (1951). (11) Gilchrist, R., Bur. Standards J . Research 12, 291 (1934). (12) Haines, R. L., Ryan, D. E., Can. J . Research 27B, 72 (1949). (13) Jackson, D. S., Beamish, F. E., ANAL..CHEM.22, 813 j1950). (14) Naito,. T.,. Bunsehto Shzyaku 3, 84 (1949): (15) Naito, T., Kinoshita, Y. Y., Hayoshi, J., J . Pharm. SOC.Japan 69, 361 (1949). (16) Rogers, W. J., Beamish, F. E., Russel, D. S., IND.ENG.CHEM.,ANAL. ED. 12, 561 (1940). (17) Ryan, D. E., Analyst 77,46 !1952). (18) Sidgwick, N.. V., “Chem1cs;f Elment.9 and Their Compounds, Vol. 11. D. 1614. Oxford Univ. Press, London, 195b. ‘ (19) Taimni, I. K., Anal. Chim. Acta 13, 28 (1955). (20) Taimni, I. K., Agarwal, R.’P., Zbid., 9, 116-21 (1953). (21) Taimni, I. K., Salaria, G. B. S., Ibid., 11, 329 (1954). (22) Taimni, I. K., Tandon, S. N., Ibid., 22, 553 (1960). (23) Ubaldini., I... Gam. chim. ital. 78, 293 ’ (1948). (24) Ubaldini, I., Nebbia, L., Chimica e industria (Milan) 33,360 (1951). ~
25 25 25 30 30 30 35 35 35
30 30 30 35 35 35 35 35 35
26.29 26.06 26.23 32.13 32.76 33.29 36.79 37.61 37.09
23.60 23.60 23.60 28.32 28.32 28.32 33.04 33.04 33.04
+11.4 +6.1 +6.9 +13.4 +15.6 +17.1 +11.3 +13.8 +12.2
a Taimni and Salaria (81) showed errore of less than 0.5% over the range of 11 to 38 mg. of iridium.
1260
ANALYTICAL CHEMISTRY
RECEIVED for review March 27, 1961. Accepted May 17, 1961.
