408
ANALYTICAL CHEMISTRY
follorved by 3.0 ml. of sodium nitrite reagent from a fastrdelivery pipet. The mixture should be swirled vigorously for 5 seconds and left standing exactly 60 seconds. Two milliliters of potassium hydroxide reagent is then added and mixed, followed by roughly 10 ml. of distilled water, a small spatula-tipful of anhydrous sodium sulfate to facilitate the separation of the layers following extraction, and 10.0 nil. of iso-octane. The mixture is stoppered, shaken vigorously for 30 seconds, and allowed to settle. The iso-octane layer is then read on the Cary recording spectrophotometer a t 450 to 350 mp in the visible region and 340 to 280 mp in the ultraviolet, either absorption maximum being used for reference. A blank, prepared in exactly the same fashion, is subtracted from these readings. Spectral stability has been observed by the author for a t least 1 hour in the absence of daylight and has been reported to persist for a day or more (8). ACKNOWLEDGMENT
The author is grateful to James F. Mead and David R. Howton, School of Medicine, University of California a t Los Angeles,
for their helpful criticism and suggestions in the preparation of this paper. LITERATURE CITED
(1) Emmerie, A., and Engel, C., Nature, 142, 873 (1938). (2) Emmerie, A., and Engel, C., Rec. trav. chim., 57, 1351 (1938). (3) Furter, M., and Meyer, R. E., Helv. Chim. Acta, 22, 240 (1939). (4) Karrer, P., Escher, R., Fritzsche, H., Keller, H., Ringier, B. H., and Salomon, H., Ibid., 21, 939 (1938). (5) Karrer, P., and Keller, H., Ibid., 21, 1161 (1938). (6) Mead, J. F., and Polister, B. H., University of California, Los Angeles, unclassified report, UCLA-253 (May 6, 1953). (7) Quaife, 31.L., J . A m . Chem. Soc., 6 6 , 308 (1944). (8) Quaife, -M. L., J . Rzol. Chem. 175, 605 (1948). (9) Scudi, J. V., and Buhs, R. P., Ibid., 146,l (1942). (10) Weisler, L., Robeson, C. D., and Baxter, J. G., 4 x . 4 ~ .CHEM., 19,906 (1947). RECEIVED for review June 22, 1953. Accepted September 21, 1953. Based on work performed under Contract AT-04-1-GEN-12 between the Atomic Energy Commission and the University of California a t Los Angeles
Cadmium Thiourea Reineckate Procedure CHARLES L. RULFS, E. P. PRZYBYLOWICZ, and C. University o f Michigan, A n n A r b o r , M i c h .
T
H E classical determinative forms for cadmium are of limited utility, being characterized by moderate accuracy and no selectivity (3, 4). Quinaldic acid, anthranilic acid, and 8quinolinol give good weighing forms ( 2 , 9, 16) but contribute very little to the selectivity of the determination. Separations based upon the precipitation of cadmium iodide complexes with various organic bases such as 2-naphthoquinoline ( I , 9) or brucine (14) give more selective procedures; but these precipitates are unstable, low-melting compounds. Walter and Freiser (15) have recently described a gravimetric procedure employing 2(o-hydroxyphenyl)-benzoxazolewhich gives good results and has excellent separation features. Optimum selectivity results in alkaline media, hence tartrate is employed to complex the heavy metals. Of the commoner metal ions, only cobalt, nickel, copper (which may be previously separated by precipitation from acid with the same reagent), and lead interfere. I n 1937, Mahr and Ohle described a cadmium separation from zinc in acid solution; their procedure appeared to be simple, to give good (though not highly precise) results, and to be highly selective ( 7 ) . The anion of Reinecke’s salt [diammine tetrathiocyanatochromium(IIZ)] forms an insoluble salt in 0.1 to l.OLV mineral acid with a complex cadmium-bithiourea cation. The cadmium content of the precipitate is only 12.47%. The constitution of the precipitate, the accuracy of the procedure, and certain questions regarding the interfering ions seemed to warrant fiiller investigation. REAGENTS AND APPARATUS
Several brands of C.P. grades of thiourea were employed a t various times. The shelf-life of this reagent is somewhat limited and fresh, small stocks should be obtained a t reasonable intervals. Solutions should be stored for no more than several days. Eastman Reinecke’s salt vias employed without further purification. The saturated (about 5 % ) solution of the salt was filtered and used for 1 week before replacing. All other chemicals were of C.P. grade and distilled water was used throughout. Porcelain frits (1.5-ml.) and glass frits (8-ml. and 30-ml., medium porosity) were employed as the filtering media. Conventional drying ovens or micro hot-plate ovens ( 1 1 ) were used for drying. Weighings on a semimicrobalance to the nearest 0.01 mg. were made in a portion of this work. A Fischer Elecdropode was used for the polarographic work. Half-wave potentials were determined from actual curves whose E-scale was corrected to the nearest 0.5 mv. on the basis of external potentiometer readings. The potential of the dropping
E. SKINNER
mercury electrode was read us. a saturated calomel reference electrode (S.C.E.). The temperature was controlled to within zko.05°
c.
COYSTITUTION OF CADM1U.M THIOUREA REISECKATE
Mahr and Ohle ( 7 ) assumed that the cadmium ion in acid solution complexed two neutral thiourea molecules to give a bipositive, cadmium bithiourea cation. This cation unites with two uninegative reineckate anions, diammine tetrathiocyanatochromium(III), to give the salt:
This formulation was justified on the basis of: 1. The method of preparing the salt. 2. Reported assays of 11.54% for chromium (theory, 11.54%) and 35.5% for sulfur (theory, 35.6). 3. The cadmium results by their rocedure (the gravimetric method gave values precise to i 0 . 6 & , absolute, and accurate to +0.3%). This much information would seem to establish the essential correctness of the empirical composition of the salt, despite its very high molecular weight (901.2). Its structural formulation, however, is of special interest in view of the remarkable specificity for cadmium in the presence of zinc. The existence of weakly bound addition compounds of lead salts with variable amounts of thiourea (8, 9) as well as the recent work on urea and thiourea adducts (IO,I d ) , are suggestive of alternative structures. Microdeterminations of carbon, hydrogen, nitrogen, and residue were obtained to supplement the available cadmium, chromium, and sulfur assays. The salt was prepared and dried for analyses in exact conformity with the analytical procedure for cadmium. Carbon values of 13.40, 13.35, and 13.40% (theory, 13.32%) were obtained with corresponding hydrogen values of 2.24, 2.31, and 2.14% (theory, 2.24%). The boat residues in two of the carbon-hydrogen runs were weighed without further treatment, and gave 39.3 and 39.8% [which must be assumed to consist primarily of chromium oxide (CmOs) plus cadmium sulfate (CdSOI), theory 39.98% 1. A Dumas nitrogen determination gave 24.967, (theory, 24.87%). The aggregate analytical data leave no uncertainty regarding the correctness of the empirical composition, nor does it seem possible (in view of the use of three
V O L U M E 26, NO. 2, F E B R U A R Y 1 9 5 4
409
different preparations for these analyses) that the thiourea content could be subject to any appreciable variability. The validity and nature of the thiourea complexation of cadmium ion were examined polarographically by determining the shift in half-wave potential observed for the cadmium wave as a function of the concentration of added thiourea. All solutions xere 0 . 2 5 in potassium nitrate and ImJf in cadmium, and contained 0.004cc gelatin. -4s a result of the modest strength of the complexation, no significant shifts are observed n-ith less than 0.02.1f thiourea present. K t h 0.03 to 0.12Jf thiourea, the number of coordinating thioureas, p , per cadmium ion was calculated through the relation (4),
which is also 1% in thiourea, is added in excess (strong pink color remains in the supernatant liquid). The mixture i s allowed to stand in an ice bath, with frequent stirring, for 0.5 hour (full hour with less than 1 mg. of cadmium). The cold suspension is filtered with suction, washed once with cold 1% (2%, for low-cadmium) thiourea, and three or four times with cold absolute ethyl alcohol. The precipitate is dried a t 110" to 120" C. for 1 hour (or, 90 minutes for 50 mg. or more of cadmium). The cadmium factor is 0.1247.
and proved to be 1.0 i 0.1. The dissociation constant of the cadmium monothiourea complex, h, may be estimated from measurements to be 2.63 X With 0.12 to 0.5OV thiourea, p proves to be 2.0 i 0.2, and the over-all dissociation constant, K1.2, may be estimated as 2.34 X (the second successive constant, kz, can be approximated as 8.9 X 10-2).
Table 11. Cadmium Results on Standard Zinc Spelters
Table I. Cadmium Thiourea Reineclcate Results in Various iMedia Principal Acid (ca. 0 . 5 N )
H2S04
Hh-08
Metal Ions Present (ca. 15 Mg.)
i$+Y+ lIn++ Ki++ co++ Cr-+FeC+ I + + i Fe+++ AS+++ Ai+++
HzSOI CrD-With added thiourea HK03 Pb++
HzS04 Plus tartrate
(PbS04 filtered) Sb-++
Cd Taken, Mg.
Cd Found, Mg.
11.02 11.02 11.02 11.02 11.02 11.02 13.71 13.71 13.71 13.71 11.02 11.02 13.71
11.07,11.00 11.05, 11.07 11.08,11.08 11.01,11.11 11.09,11.08 11.06,10.89 13.70, 13.70 13.69, 1 4 . 0 1 13.64, 13.59 13.72,13.78 10.72,10.55 11.10 13.78, 1 8 . 4 1 19.73, 19.92 11.53, 11 .59 18 82, 19.49 11,04,11,09
11 02 11 02
11 02
This information, in conjunction with the analytical data, confirms the structural formulation inferred hy Mahr and Ohle. I n the absence of thiourea, the reineckate anion alone will precipitate certain unipositive cations such as cuprous ion (6) (which can be utilized for the preliminary determination of copper, prior to cadmium by the present procedure), but brings down cadmium or zinc ions only from very concentrated solutions. Hence the specificity of this method for cadmium in the presence of up to 20,000 parts of zinc, is to be attributed (apparently) to the existence of the cadmium-bithiourea complex ion and the nonesistence of any corresponding zinc complex. The halfwave potentials of ImM zinc ion in 0.2N potassium nitrate with and without thiourra concentrations up t o 0.2M were measured a t 25" f 0.05OC. -411 values were -1.008 volts LS. saturated calomel electrode, i 1 mv. PROCEDURE
Standard stock solutions of cadmium ion containing about 1.0 and 0.2 mg. of cadmium per ml. were prepared by weighing selected crystals of cadmium sulfate octahydrate; their concentrations were assayed by precipitation and ignition to the pyrophosphate. Aliquots of the cadmium stock solutions were made about 0.5N in free mineral acid. Sulfuric or hydrochloric acids appear to be equally suitable; in the case of nitric acid, prolonged contact with thiourea a t room temperature should be avoided. The acidity may vary from 0.1 to l.ON, the lower value being pTefefable in the case of separations from large amounts of forei n ions. Sufficient freshly prepared and filtered 5% thiourea sofution is added to bring the thiourea concentration up to 1% in the sample solution. A saturated solution of Reinecke's salt,
Results on pure cadmium solutions containing 1 to 55 mg. of cadmium show the procedure to have a precision of i 0.6276, 0.38%. Mahr and Ohle absolute, and to be accurate within found approximately the same limitations.
+
Natl. Bur. Standards No.
Wt. of Ppt., Mg.
Sample, Grams
110
43,09 43.02 43.24
1.00 1.00 1.00
0.538 0,537 0.539
0.56
108
7.35 7.51
1.00 1.00
0.0916 0.0937
0.092
94
1.7 2.2
5.00 5.00 1.00 2.00
0.0042 0.0055
0.004
0.0026 0.0029
0.0018
109
0.20 0.46
Cd Found, %
Natl. Bur. Standards Value
SELECTIVITY
Aliquots of the standard cadmium solution, equivalent to 10 to 15 mg. of cadmium, x-ere tested in the presence of 15 mg. of various ions. The results of these trials are recorded in Table I. I n general, it was found that the mineral acids and alkali and alkaline-earth cations cause no interference. Magnesium, aluminum, arsenic(III), chromic, zinc, ferrous, or ferric ions cause no trouble. Manganese, nickel, and cobalt ions tie up some of the thiourea by complexation, but are not disturbing in small amounts (nor in Iarge amounts if the thiourea concentration is made 2%). Bichromate is reduced to chromic ion with thiourea; since chromic ion does not interfere, restoration of the thiourea concentration will rectify this situation. JVhile lead WAS not emphasized by the original authors ( 7 ) as an interference, its presence in moderate to large amounts is detrimental in view of its ability to form loose addition compounds with thiourea. Lead ion may be conveniently removed as lead sulfate. Antimony interferes, through hydrolysis, but can be held up by the addition of tartrate. Results are slightly high in the presence of tin: again, probably through hydrolysis. Copper and mercury interfere, but either of these may be previously determined with Reinecke's salt alone (6). The interference of bismuth can be evaded by its prior separation as bismuthyl iodide ( I S ) . As shown in Table 11, very small quantities of cadmium (as little as 0.03 mg.) may be sepsrated from large excesses (up to 20,000 parts) of zinc by a single precipitation, this remarkable specificity is the most striking feature of this technique. CONCLUSIONS
The thiourea reineckate procedure for cadmium employs readily available reagents. The method is short and direct. Tin, antimony, copper, bismuth, mercury, and lead will interfere, but suitable preliminary separations and/or determinations are available for these ions. Few other ions are harmful, and the technique permits a direct separation of cadmium from almost any quantity of zinc. The favorable gravimetric factor makes the method particularly attractive a t the semimicro level. An accuracy of 0.4y0 makes the method compare favorably with
A N A L Y T I C A L CHEMISTRY
410 alternative procedures. The character and reasons for the specificity of this precipitation have been examined. ACKNOWLEDGMENT
The authors are indebted to Wilfred Byrd and Goji Kodama for the microanalyses. LITERATURE CITED
(1) Berg, R., and Wurm, O., Ber., 60, 1664 (1927); 98,287 (1934). (2) Flagg, J. F., “Organic Reagents,” Kew York, Interscience Publishers, 1948. (3) Furman, N. €I., “Scott’s Standard Methods of Chemical Analysis,” New York, D. Van Iiostrand Co., 1939. (4) Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganir .4nalysis,” New York. John Wiley & Sons, 1929.
(5) Kolthoff, I. AI,, arid Lingane, J. J . , “Polarography,” 1-01. I. New Tork, Interscience Publishers, 1952. (6) Mahr, C . , Z. anorg. u . allgem. Chem., 225, 386 (1935). (7) Mahr, C.. and Ohle, H., Z. anal. Chen., 109, 1 (1937). (8) hlahr, C., and Ohle, H., 2. anorg. u . allgem. Chem., 234, 224 (1937). (9) Prodinger, W., “Organic Reagents Used in Quantitative Inorganic Analysis,” Sew York, Elsevier Publishing Co., 1940. (10) Redlich, O., et al., J . .4m.Chenf.Soc., 72, 4153, 4161 (1950). (11) Rulfs, C. L., Anal. Chim. A c t n , 5 , 4 6 (1951). (12) Schiessler, R. W., and E‘litter. D., J . dm. Chem. Soc., 74, 1720 (1952). (13) Ytrebinger, R., and Ortner. G., Z. a m l . Chem., 107, 14 (1936). (14) Thompson, T. L., I m . E x ; . CHEM., A x . 4 ~ED., . 13, 164 (1941). ~ ~ ~24,~984 . (1952). (15) Walter, J. L., and Freiser, H., A 4 CHEM., (16) Welcher, F. J., “Organic Analytical Reagents,” Iiew Y o r k , D. Van Sostrand Co., 1947. RECEITED for review September 25, 1952.
Accepted September 19, 1953.
Reduction by Aluminum Powder in Aqueous Solution Titrimetric Determination of Molybdenum E. RAYMOND RIEGEL’ and ROBERT D. SCHWARTZ’ University o f Buffdfalo, BuffaTo, N. Y.
S
EVERAL reducing agents have been proposed for use in the titrimetric determination of molybdenum (1,S, 7,9, 10). Although Willard and Furman (10) list the action of aluminum on molybdenum(V1) in acid solution, no journal reference t o this reaction was located. Boulanger’s ( 2 ) work, in alkaline solution, was the only reference to the reduction of molybdenum(V1) by aluminum. PRELIMINARY STUDIES
A sample of C.P. ammonium molybdate was used as the source of molybdenum(V1) for this work. This material was assayed by the method of Birnbaum and Walden (1). The Moo3 contcnt was 81.0%. The first reductions were performed in 2111 hydrochloric acid solution. Alcoa (99.4%) aluminum foil was used as the reducing agent. As the mixtures were heated, the colorless molybdcnum(VI) solutions became orange and then green. The green solutions became orange if they were exposed to air. From the information given by Hiskey (4), it was possible to decside that the orange solutions contained molybdenum(V), and that the green solutions were molybdenum(IT1) materials. These results confirmed the data given by Willard and Furman (IO). In order to determine whether the reduction could be carried past the (111) state, samples were reduced in an atmosphere of nitrogen by a large excess of aluminum. N o color change after the green (111) state was observed. The reaction between aluminum foil and molybdenum(V1) in hydrochloric acid solution leads to the formation of molybdenum(II1). Molybdenum(\‘) was the only intermediate in the reduction and in the air oxidation of molybdenum(II1). The nonexistence of molybdenum(1V) was confirmed by POtentiometric studies of the reaction. Time-potential curves were plotted during the reduction using platinum-calomel electrodes and a Beckman Model G pH meter. Additional confirmation was obtained by potentiometric measurements made during the oxidation of molybdenum(III), in 2J1 hydrochloric acid, by ceric sulfate. Sidgwick (8) indicates that molybdenum(1V) exists only as certain complexes. Other molybdenum(1V) compounds are unstable in aqueous solution by virtue of reaction to givc a mixture of molybdenum(II1) and molybdenum(V). REDUCTION USING FINE ALUMIh-U>.I POWDER
A fine aluminum powder (Reynolds No. 400) was found advantageous by the authors (6) for iron determinations. Samples of molybdenum(V1) in 1.25-21 sulfuric acid were Present address, R . D. 91, Deep River, Conn. Present address, Exploration and Production Research Laboratory, Shell Development Co., Houston, Tex. 1
2
heated with No. 400 aluminum powder in an atmosphere of nitrogen. As in the reduction of iron(II1) ( 6 ) , the powder yielded more rapid reaction and hetter efficiency of reduction than foil. -Uter reduction the samples werp allowed to coel in the nitrogen atmosphere. Titration with potassium permanganate yielded molybdenum(F‘1). However, st room temperature the oxidation of molybdenum( V) by permanganate waa slow. Potential measurements] taken during the titration, revealed that upon each addition of permanganate the potential rose sharply, and then dropped slowly as the oxidant reacted with molybdenum(V). Other samples were reduced with S o . 400 powder and were titrated with potassium permanganate while warm. A t the elevated temperatures the reaction was rapid and good end points were obtained. Several samples of molybdenuni(V1) in 1.25‘51 sulfuric acid were reduced with an exrcss of S o . 400 powder and titrated with permanganate. The results obtained were compared with those yielded by the procedure of Birnbaum and Walden ( 1 ) . Molybdenum
Sulfuric Acid, Moles p?r Liter 1 .2A I .25
(VI)
Taken, hleq. 0 . zoo 0.750
Aluminum,
1.500 0 . zoo 0.760 1.000
0.490 0.740 1.012 1.448 0.499 0.752 1,003
0 ,O D
11.1 11.1 Reductor Reductor Reductor Reductor
Silvrr Silver Silver Silvrr
2.150
hleq.
?.??
1.26 1. 2 5
1.010
Molybdemini ( V I ) Found,
Meq.
2.144
The agreement with results obtained by the silver reductor procedure ( 1 ) was satisfactory. XIETHOD OF DETERWINING MOLYBDENUM
Samples of C.P. ammonium molybdate were analyzed for molybdenum by the silver reductor procedure, the Jones reductor procedure, and by the use of Reynolds No. 400 aluminum powder as the reducing agent. The results obtained are listed below. Silver reductor
Sample 1
81.0
2
80.7
81.03 81.09 80.74 80.71
hIOOl, % Jones reduetor 81.1 80.8
81.12 81 .OS 80.83 80.78
Aluminum 81.1 80.7
81.16 81.11 80.73 80.67
