gents is the basis of a general method for preparing samples in a concentration range suitable for x-ray spectrographic malysis. Heggen and Strock (3) and for example, Mitchell and Scott (4, have successfully employed this technique for concentrating trace elements for optical spectrographic analysis. The versatility of the technique is enhanced for x-ray spectrographic work by combining the concentration and briquetting techniques for the preparation of samples and standards. The real advantage of the precipitating technique is that the only separation required is that of the desired elements
from the sample matrix. Any organic reagent, or combination of reagents, can be used to concentrate the minor constituents of a sample. The organic composition of the precipitating reagents is immaterial in the final x-ray spectrographic measurements. ACKNOWLEDGMENT
The stainless steel and Monel corrosion product samples were supplied by D. L. Douglas. The copper-zinc alloys were supplied by K. G. Xinslie. The copper archeological artifacts are the property of the Oriental Institute.
University of Chicago, and were submitted for analysis by J. E. Burke of this laboratory. LITERATURE CITED
( 1 ) -Idler, I., Axelrod, J. RI., AXAL.CHEM. 27, 1002 (1955). 12) Sdler, I., Axelrod, J. M., Spectrochim. Acta 7. 91 11955).
Heggen, G. E:, Strock, L. W., ANAL. CHEX25,859 (1953). Mitchell, R. L., Scott, R. O., J . SOC. Chem. Ind. 66,330 (1947).
Mortimore, D. X,Romans, P. A., Tews, J. L., A p p l . Spectroscopy 8, 24 (1954).
RECEIVED for review September 28, 1956. Accepted -4pri115, 1937.
Extraction and Flame Spectrophotometric Determination of Chromium H. ALDEN BRYAN' with JOHN A. DEAN Department of Chemistry, University of Tennessee, Knoxville, Tenn.
b Organic solvent extraction can be applied to the flame spectrophotometric determination o f chromium in all types o f samples. Chromium in the hexavalent state i s selectively extracted with 4-methyl-2-pentanone from an aqueous solution 1M in hydrochloric acid. The organic extract i s aspirated directly into an oxyacetylene flame. This procedure circumvents interferences encountered when aspirating the bulk sample containing varying amounts o f diverse elements and increases the spectral emission of chromium fifty fold compared to an aqueous chromium solution. When the iron concentration in the aqueous phase exceeds 500 y per ml., i t accompanies chromium in the extraction but does not affect two chromium lines, including the most sensitive. Accurate measurements can be made on as little as 0.1 y of chromium per ml.
C
emission from aqueous solution has been studied by several investigators (8, 9). The determination of chromium has also been described in steels (5, I O ) , in iron-chromium alloys or slags ( 7 ) ,and in sodium citrate solutions ( I S ) . The low emission intensity obtained when aqueous solutions of chromium are aspirated has detracted from the application of the flame method. Only in the high HROMIUM
Present addrrss, Chemistry De artr ment, Davidson College, Davidssn, C.
8.
temperature cyanogen-oxygen flame (11) or hydrogen-fluorine flame (Z), in which there occurs a chromium fluoride band of high emission intensity, has sufficient sensitivity been obtained. The solvent extraction of chromium(V1) Tl-ith 4-methyl-2-pentanone is a convenient and rapid method for isolating chromium from other elements (12). The introduction of the organic solvent in place of n-ater increases the emission intensity of chromium fifty fold. -4s a result an extremely sensitive method for the determination of chromium has been developed. Operating conditions are easily adjusted t o provide emission sensitivities equal t o 0.1 y per ml. of chromium per instrument scale division. EXPERIMENTAL WORK
Apparatus. A modified Beckman Model DU flame spectrophotometer was used with a photomultiplier attachment and a n oxyacetylene burner. Rlodifications included a fine adjustment for the sensitivity control and a 7 t o 1 gear-reduction knob to facilitate positioning the wave length dial in manual operation. -4 10-mv. Bristol recorder, possessing a 2/3second pen response for full-scale deflection, was used to record spectra. Other special equipment has been described ( 3 ) . Reagents. 4-Methy1-2-pentanone, technical grade, was used as received. The solvent was equilibrated with twice its volume of 1M aqueous hydrochloric acid.
I standard solution of chromium, 1 ml. equivalent t o 10 y of chromium, was prepared by diluting to 1 liter 10 ml. of a stock solution containing 2.283 grams of primary grade potassium dichromate per liter. Flame Spectrophotometer Settings. The slit width used throughout the investigation was 0.030 mm., except when extreme sensitivity was required. Slit widths as large as 0.060 mm. have been employed for steel samples low in chromium without any difficulty. The sensitivity control was positioned a t seven turns from the clockwise limit for the chromium multiplet in the ultraviolet and a t fix-e turns for the multiplet in the blue portion of the spectrum. The corresponding adjustments on the Spectral Energy Recording Attachment are 70 and SO%, respectively. At these settings a working curve extending over the entire %!l" scale of the instrument can be prepared in the concentration range from 0 to 10 y per ml. The emission of all the chromium lines increases with increasing acetylene pressure without reaching a maximum. The background varies similarly. Neither the emission intensity of the chromium lines nor of the background is markedly influenced by small variations in the oxygen pressure. Consequently, the acetylene pressure was arbitrarily chosen a t an intermediate value (5 pounds per square inch) and the oxygen pressure (9 pounds per square inch) was adjusted until the chromium emission attained its maximum value. The phototube load resistor was 22 I
VOL. 29, NO. 9, SEPTEMBER 1957
1289
megohms and the voltage per dynode was 60 volts. Calibration Curve. Prepare a working curve in the concentration range from 0 t o 10 y per ml. as follows. Transfer 3-, 5-, 8-, and 10-ml. aliquots of the standard aqueous chromium solution to 50-ml. volumetric flasks and dilute to 18 ml. with distilled water. ,4dd exactly 2.00 ml. of 1OM hydrochloric acid and 10.0 ml. of 4-methyl2-pentanone. Cool in a n ice bath for 5 minutes, then shake for 1 minute. When the phases have separated, transfer the organic phase to sample cups or transfer the entire contents of the flask to a 30-ml. beaker. Aspirate the organic phase and measure the emission intensity a t the selected wave lengths ns described below. The working curve is linear up to 20 y per ml. of chromium, the maximum concentration investigated. Procedure. Peroxydisulfate ion in t h e presence of silver ion as a catalyst is one of t h e few oxidants which quantitatively oxidizes chromic ion t o dichromate ion in acid solution. The oxidation may be performed in solutions containing (either or both) sulfuric and nitric acids. The optimum acidity is between 0.5 and 1.5M sulfuric acid. The oxidation of chromium is inhibited at greater acidities and manganese dioxide separates at lower acidities. By appropriate choice of sample weight and final volume of the oxidized solution the following procedure may be applied to samples with chromium contents from a few hundredths of 1% up to 1% or niorc. The sample weight and final volume should be chosen so that, with a 5- to 15-nil. aliquot, the concentration of chromium in the extract falls in the range - of 3 to about 10 y per ml. ALLOYS. Weigh a samnle containing 0.3 to 0.8 mg. oflchromiim into a 2 5 6 nll. beaker or flask. Add 30 ml. of water and 8 ml. of 18M sulfuric acid. Boil gently until decomposition of the alloy is complete or the reaction subsides. Then add 5 ml. of l 5 M nitric acid in several small portions. If much carbonaceous residue persists, add 5 ml. more of nitric acid and boil down to copious fumes of sulfuric acid to ensure complete decomposition of carbides. Dilute to about 80 ml. and boil until all salts have dissolved. Cool and add 2 nil. of 0.1N silver nitrate solution. Add 2 grams of pure potassium peroxydisulfate, swirl the flask until most of the salt has dissolved, and heat to boiling. Keep at the boiling point for 8 to 10 minutes. Cool, transfer to a 100-ml. volumetric flask. and dilute to the mark. MIKERALS.For siliceous minerals, weigh a sample containing 0.3 to 1.0 mg. of chromic oxide into a platinum dish. Moisten with distilled water and add 5 ml. of 48% hydrofluoric acid and 0.5 ml. of 18M sulfuric acid. Heat to fumes of sulfuric acid. Cool, add 2 ml. of hydrofluoric acid, and evaporate to dryness. Ignite carefully until sulfuric acid ceases to be evolved. Add 2 granis of sodium carhonate and fuse 1290
ANALYTICAL CHEMISTRY
10 minutes at the full temperature of a Meker burner. Dissolve the melt in 70 ml. of 1N sulfuric acid. Add 2 ml. of 0.lN silver nitrate solution, then 2 grams of potassium peroxydisulfate. Swirl the flask or beaker until most of the salt has dissolved and heat to boiling. Keep at the boiling point for 8 to 10 minutes. Cool, transfer t o a 100ml. volumetric flask, and dilute to the mark. Flame Analyses. Transfei a 5t o 15-ml. aliquot of each sample t o a 50-ml. volumetric flask or other suitable container. Dilute to 18 ml. with water and continue as for the preparation of samples for the calibration curve. Aspirate the organic phase and measure the line emissions and flame backgrounds a t one or several of the following wave lengths: Line Emission,
Background,
mp
Ilk
357.9 359.4 425.4
358 6
Flame
Characteristics of Chro-
of chroniium in 4-methyl-2-pentanone is shown in Figure 1. The most
360.0
Sample Aluminum alloy 85a (94 AI, 2 hlg, 2 Cu. 1 h l n )
RESULTS A N D DISCUSSION
The validity of this procedure is substantiated by the results shown in Table I, obtained with a representative variety of NBS aluminum-base alloys. steel samples, cast irons, Alone1 metal, and a clay. All the elements likely to be encountered in the analysis of materials containing chromium and which might accompany chromium in the extraction step are represented by these samples. The results are in satisfactory agreement with the certificate values.
mium. The observed flame spectrum
424 4
Table I.
Bracket the unknowns with a series of standards containing chromium in 4-methyl-2-pentanone. Subtract the appropriate background readings from the unknown and standard emission readings to obtain net relative emissions and read the amount of chromium from the appropriate calibration curve.
Chromium Analyses on NBS Samples
Chromium, % Found 0,2340.228 0,227,O.235 0.230,O.223 0.229,0.231
Certified value 0 231 f 0.004
0 029 f 0 004 .iluminum allov 86c (91 Al, 2 Zi, 1 Fe, 8 Cu: 1 Si) 0 1; f 0 00 Aluminurn alloy 87 (89 Al, 2 Zn, 1 Fe, 1 M g , 1 Ni, 6 Si) 0 23 f 0 007 Alone1 162 (66 Ni, 29 Cu, 2 \In, 1 Co, 5 Fe)
0.029,O. 029 0.030,0,030
117 st 0.003
0.112,O. 104
Cast iron 4h (1 Mn, 1 Si)
U
0.172,O 179 0 170,0.172
(J
0.237,O. 237 0.226,O. 232 0 222,O.226
0.28,0.26
hv. 0.233 Std. dev. 0.008
.4V. 0 108 Std. dev. 0 010
hv. 0.38 Std. tlev. 0 05
0.049,0~049 0.048,O. 049
0 050 f 0 002
0.051,0.047 0.051,0.050
Manganese steel l0Oa 12 Rln)
0 051
Flint clay 97 (39 ;2l20,, 1Fe20s.2 Ti021
0 079 2z 0 0093"
(3203.
AV. 0.173 Std. dev. 0.004
0 . 3 7 , O .37
1 15 zt 0 01
Reported as
Av . 0.029 Std. dev. 0 001
0.35,0.38
.323 f 0 008
Cr-No-A1 steel 106a ( 0 . 5 Mn, 1-41, 0.2 510)
o
0.230
0.103,O. 100
0 . 2 8 . 0 29 13.O.H. steel 152 ( I hln, 0.1 Cu)
Av.
Std. dev. 0.0045
0.240,O. 242
0.121,O. 128 0.100.0.110
Si-Cr cast iron 82a (1 Mn, 1 Ni)
____I_
1.10,l.ll
>I I . 1 3 , l . 11
1.11,1.13
0 003
j=
Av . 0.050 Std. dev. 0 001
0.049,O. 050 0 047) 0.050
Av. 1.12 Std. dev. 0.01
Av . 0.049 Std. dev. 0 001
0.088,O.088"
0,075.0.075 0.088.0.087
.kv, 0 083 dtd. dev 0 0063
0 to 1M, especially in the low ranges of Table
II.
Major Flame Emission Lines of Chromium
Wave Length Line Peak, M p
357.9 359.4 360.5 425.4 427.5 420.0 520,6
Relative Intensity of Line, Scale Divisions per -/ per MI. of Cr5 5 4
3 10
a ti
2
a Exprewed as scale divisions out of 100 total divisions.
important chromium lines are iiicluded in two sets of triplets. A weaker group of lines around 520.6 nip appears as a single emission line with the optical system employed in this st.udp. The transition probabilities of these lines have been reported (6). I n Table I1 is given the wave length of each of these lines together with the relat'ive intensities of each. Chromium also possesses a series of oxide band systems in the yellow and red portions of the spectrum which color the pure chromium flame a pale rose (4). The background radiation in the yicinity of the chromium triplet in the ultraviolet is essentially continuous. Correction for the background radiation is easily made I J ~measuring the flame emission at the edge of the skirts of the flame emission lines or at the minimum between any pair of emission lines. By contrast, the background radiation in the vicinity of the blue triplet is continually increasing and culminates in a weak flame band attributed to C H and whose head is at 431.3 mp. The CH band does not interfere with the nieiisurenient of the chromium 425.4- and 427.5-mp lines, but does overlap the 429.0-nip line on the long wave length side. The ultraviolet multiplet offers more ideal operating conditions, the blue multiplet is more sensitive. Compared t o aqueous solutions of identical concentration, the emission inteneity of chromium from 4-niethyl-2pentanone is fifty fold larger. Extraction of Chromium. Chromium(V1) is easily a n d selectively separated from other constituents of samples by EL single extraction using 4-methyl-2-pentanone. Weinhardt and Hisson (1.2) observed no distribution of chromium(V1) in the absence of hydrochloric acid, but the presence of w e n a small amount of the acid caused a distribution. The distribution coefficient varied continuously from zero a t zero hydrochloric acid concentrations, and increased most rapidly when t h t mid concentrrttion increased from
chromium concentration ( l a , Figure 1). Weinhardt and Hixson recommended a n aqueous hydrochloric acid concentration of 3-W for dichromate concentrations in the range 20 to 300 mmoles. However, under this operating condition iron(II1) chloride is largely extracted also. Decreasing the hydrochloric acid concentration to 1M drastically decreased the distribution coefficient of iron(II1) chloride. At 1M hydrochloric acid concentrations and amounts of iron(II1) less than 500 y per ml., no iron was detected in the organic phase. At higher concentrations of iron(II1) small amounts were coextracted with the chromium. When iron predominated in samples, such as iron alloys and steels, the coneach 0.2 unit increase in concentration of acid between 0.6 and 1.4M, the amount of iron coextracted with chromium approximately doubled. Weinhardt and Hixson ( I d ) reported that the distribution coefficient of chromium(V1) increased five- to sixfold when the solution was cooled to 0" C., although in the present work a temperature of 0" to 10" C. was found satisfactory. Equally important, the rate of reaction between chromium(V1) and chloride ion is rendered negligihle at this low temperature. Nevertheless, concentrated hydrochloric* acid must
1 I
60
425.4
-
427.5 1429.0
so,
40-
I,
0
1
N
Y
-
WAVE LENGTH, mp Figure 1. chromium
Flame emission spectrum of
5 y of chromium per ml.; slit width, 0.030 mm.
0 W
I
z
2
!e w
I/-----Figure 2. Emission spectrum of iron and chromium Small arrows indicate chromium lines ond flame background free of iron interference; slit width, 0.030 mm.
never be added directly to the saniple aliquot or standard wltltion until the volume has been adjusted to 18 ml. Small amounts of chromium(V1) are easily reduced hy concentrated hydrochloric acid. A single extraction sufficed to remove all the chromium. The ketone should be equilibrated beforehand with the same volume and concentration of aqueous hydrochloric acid. Failure to do so resulted in an S-shaped calibration curve; only a very slight amount of chromium was extracted below 6 y per ml. and above 12 y per ml. the amount extracted again fell off. Chromium(T'1) slowly decomposed in the ketone phase, especially 011 contact with the aqueous phase. If the ketone extracts are kept at 0' C.. they remained stable a t least 4 hours. At room temperatures, fresh solution5 should be prepared every hour. Interference Studies. Iron is tho only element t h a t accompanies chromium in t h e extraction. Even so only spectral interference is observed from iron, and only when extracting aliquot portions in which the concentration of iron exceeds 500 y per ml. when the aqueous hydrochloric acid concentration is 1M. Direct spectral interference results with a portion of both chromium multiplets. An iron emission line at 358.1 mp coincides with the chromium 357.9-mp line and a weaker iron line appears on the long wave length side of the chromium 360.5-mp line a t 360.9 mp. The chromium 359.4-mp line is devoid of interference from iron and the flame background can be measured at 360.0 mp. VOL. 29, NO. 9 , SEPTEMBER 1957
1291
I n the blue multiplet a weak iron line a t 427.2 mp coincides with the chromium 427.5-mp line. If iron is suspected, its presence in the extract can easily be identified by the simultaneous appearance of iron emission lines a t 356.5, 357.0, 361.9, 363.2, and 364.8 mp in the vicinity of the chromium multiplet in the ultraviolet and a t 432.6 mp immediately beyond the CH band head at 431.3 mp (Figure 2). Unless these iron lines are present in the emission spectrum, no spectral interference results with the chromium lines a t 357.9 and 427.5 mp. The results shown in Table I for NBS samples lOOa and 152 were obtained in the presence of 0.1 gram of iron per aliquot portion extracted and, for the latter sample, with slit widths varying from 0.030 to 0.060 mm. The use of a recorder simplifies locating and measuring the chromium emission lines.
except very large amounts of iron. The flame spectrophotometric method can nevertheless be used in the presence of iron. The selective extraction avoids the introduction of high concentrations of diverse ions into the flame, and thus eliminates any error which might arise from their presence ( 7 ) or the need for a series of correction curves as has been reported by Burriel and coworkers (1). I n addition, the use of an organic solvent gives a fifty fold increase in the radiant power of chromium when compared to aqueous solutions of equal concentration. Consequently, accurate measurements can be made a t chromium concentrations as low as a feiv tenths of a microgram per milliliter. Extraction also permits concentration of the chromium in the organic phase, the attendant increase in sensitivity is valuable in trace analyses.
ACKNOWLEDGMENT SUMMARY
Extraction with 4-methyl-2-pentanone isolates chromium in the hexavalent state from all other elements
H. illden Bryan is indebted to E. I. du Pont de Nemours 8: Co., Inc., and Davidson College for a summer grantin-aid which made this study possible
and to the University of Tennessee for generously offering its facilities and supplies. LITERATURE CITED
(1) Burriel,
F., Ramiree-Muiioz,
J.,
Asunci6n-Omarrementeria, AI. C., Mtkrochim. Acta 1956,362. (2) Collier, H. E., Jr., Serfass, E. J., Moravian College, Bethlehem, Pa.,
private communication. (3) Dean, J. A , Lady, J. H., A N A L . CHEM.28, 1887 (19513). ( 4 ) Ghosh, C., 2. Physilc 78, 521 (1932). ( 5 ) Heyes, J., Z. Elektrochem. 42, 532 (1936). (6) Huldt, L., Lagerqvist, A., Arkiv Fysik 5 , 91 (1952). ( 7 ) Ikeda, S., J . Chenz. Soc. ( J a p a n ! , Pure Chem. Sect. 77, 463 (1956). (8) Lundegardh, H., “Die Quantitativ: Spektralanalyse der Elemente, Vol. 11, Fischer, Jena, 1934. (9) Rusanov, A. K., Kunina, S. I., Zavodskaya Lab. 9, 183 (1940). (10) Thanheiser, G., Heyes, J. Arch. Eisenhiittenw. 11, 31 (1937). (11) Vallee, B. L. Bartholomay, A. F., ANAL.CHEW28, 1753 (1956). (12) Weinhardt, A. E., Hixson, A. N., Znd. Eng. Chem. 43, 1676 (1951). (13) Wever, F. Koch, W., Wiethoff, G., Arch. Eisenhuttenw. 24,383 (1953). RECEIVEDfor review Fehruary 4, 1957. Accepted April 17, 1957.
Polarographic Behavior of the Uranyl-Cupferron System PHILIP J. ELVING and ALAN
F. KRlVlS
University of Michigan, Ann Arbor,
Mich.
Unusual current and potential phenomena disclosed in a previous polarographic examination of the uranylcupferron system in 10% sulfuric acid solution prompted the present more detailed investigation. The technique essentially involved polarographing series of solutions in which the concentration of one of the active species was held constant as the other was varied. Other pertinent experimental factors were also examined. At low concentrations of both uranium and cupferron two waves appear, corresponding to uranium(V1)-uranium(V) and cupferron-phenylhydrazine reductions. The variations in height and in Eliz of these two waves with increasing uranium and cupferron concentrations are basically related to the formation of an insoluble film of uranium(lV) cupferrate a t the mercury surface. Presence of such a film hinders the approach of uranyl ions to the electrode, facilitates the reduction of uranium(V1) past uranium(V) to uranium(IV), converts cupferron to more difficultly reducible cupferrate, and affects the capillary electrode performance de-
1292
0
ANALYTICAL CHEMISTRY
pending on the size of its lumen. The formation of the film postulated was verified by experiments in the presence of a known solvent for uranium(lV) cupferrate.
A
study (3, 15) indicated rather odd polarographic phenomena in solutions containing both uranium(V1) and cupferron. I n viea of the apparent lack of direct interaction between the uranium(T’1) and cupferrone.g., complex formation-a more intensive investigation of the system seemed warranted, as cupferron is a valuable analytical reagent for uranium-containing solutions. The polarographic behavior of uranium has been summarized ( 1 2 ) ; most pertinent to the present study is the work of Kolthoff and Harris (5, 6, 10). In 0.02M hydrochloric acid solution uranyl ion gives two waves: The first corresponds to a reversible one-electron reduction of U02++ to U02+; the second wave, which is approximately twice the height of the first, represents an irreversible reduction, probably uraRECEST
nium(V) to (111). As the hydrogen ion concentration is increased above 0.2M, the first wave height increases and the second correspondingly decreases to maintain the total current constant. In 6 M hydrochloric acid. the first wave height approaches that expected for a 2 e reduction, indicating that at high hydrogen ion concentration uranium(VI) is reduced to uranium(1V); this effect seems to be due to the increasingly rapid disproportionation of uranium(V). The optimum stability of uranium(V), with respect to its disproportionation to uranium(V1) and uranium(IV), is in the p H range of 2 to 4 (14). I n acid solution uranium(V) tends to disproportionate, because of the pH-dependence of the reactions: 2U02+
+ 4H+ UOn++
+ U+‘ + 2H20
(1)
or 2uO2++ 2H+
uo*+-+ GO-- + € 1 2 0
(la)
