However, the reproducibility of these manipulations offset some of this disadvantage. I n studies employing solvents for the more quantitative transfer of samples, several worth-while observations were made. Methanol, methylene chloride, and aqueous solutions were used. The volumes transferred to the powdered potassium bromide were between 0.01 and 0.1 ml. The routine evacuation time was sufficient to remove the solvents but longer periods were applied in the case of water. No solvent could be detected by its absorption in the infrared curve. The crucial test for the presence of solvent not detectable by its own characteristic absorption was the appearance of alterations in the spectrum of the compound under investigation. Barker and coworkers (4) have demonstrated the formation of the monchydrate of glucose in potassium bromide disks. Farmer (7) has evidence for the involvement of the bromide or chloride in pressed pellets containing organic acids. In the present study opportunity was available for recording spectra of the same steroid prepared in a potassium bromide prism as a dry and a solution admixture. Some steroid spectra, such as cortisone, dehydroepiandrosterone, and etio acid of cortisol, were altered by exposure to methanol; others, such as cortisol and etio acid of ll-dehydrocorticosterone, were not. Caution should be exercised when sample transfer via a solvent is involved. This does not seem to be necessary for all substances, but no
set rule is available for segregating different molecules according to their ability to bind solvents. As the spectral alterations observed could be ascribed to the use of a solvent, it was felt that binding of the steroid with the bromide probably did not occur in these compounds. Another advantage of this procedure was that the total time required for the quantitative preparation of a rectangular prism was about 15 minutes, and the pressed forms could be stored or the sample recovered easily. Application of the method to chromatography eluates indicated that fractions which give poor infrared curves because of contamination do not readily lend themselves to analysis for the steroid components. Eluates which yielded crystalline mixtures of steroids permitted estimations of each compound to within 20% of the true concentration. Apparently, relatively clean residues must be obtained before quantitative estimations can be made. After this work was completed Kirkland (12) reported the use of a similar rectangular die which afforded good quantitative results and similar handling advantages. ACKNOWLEDGMENT
The authors wish to thank W. A. Patterson of Baird Associates, Inc., for the loan of a rectangular die which was used as a model for improvement for the particular needs discussed here. Thanks are also due to Dwight Priest of the Parker Manufacturing Co. for use of the facilities of his plant.
LITERATURE CITED
(1) Anderson, D. H., Smith, R. G., ANAL. CHEM.26, 1674 (1954). (2) Anderson, D. H., Woodall, N. B., Ibid., 25, 1906 (1953). (3) Barker, S. A,, Bourne, E. J., Neely, W. B., Whiffen, D. H., Chem. & Znd. (London) 1954, 1418. (4) Barker, S. A,, Bourne, E. J., Weigel, H., WhifFen, D. H., Zbid., 1956, 318. (5) Browning, R. S., Wiberley, S. E., Nschod. F. C.. ANAL.CHEM.27. 7 (i955j. Corbridge, D. E. C., Lowe, E. J., Ibid., 27, 1383 (1955). Farmer, V. C., Chem. & Ind. (London) 1955, 586. Hayden, A. L., ANAL. CHEM.27, 1486 (1955). Ingebrigtson, D. N., Smith, A. L.? Zbid., 26, 1765 (1954). Jones, R. N., J . Am. Chem. SOC.74, 2681 (1952). Kirkland, J. J., ANAL. CHEM.27, 1537 (1955). Ibid., 29, 1127 (1957). Rosenkrantz, H. in Glick’s “Methods of Biochemical Analysis,” Vol. 11, p. 21, Interscience, Kew York, 1955. Schiedt, U., Reinwein, H., 2.Naturforsch. 7B, 270 1952). Schwarz, H. P., hilds, R., Dreisbach, L., Mastrangelo, S. V., Science 123, 328 (1956). Stimson, M. M., O’Donnell, M. J., J . Am. Chem. SOC. 74, 1805 (1952).
6
RECEIVED for review June 21, 1956. Accepted January 8, 1958. Supported in part by a grant from the Medical Research and Development Board, Office of the Surgeon General, Department of Army under Contract No. DA-49-007-MD-184, and by contract AT(30-1)-918, U. S. Atomic Energy Commission.
Extraction and Colorimetric Determination of Chromium with 1,5-Diphenylcarbohydrazide JOHN A. DEAN and MARY LEE BEVERLY Departmenf o f Chemistry, Universify o f Tennessee, Knoxville, Tenn.
b A rapid, selective, and accurate colorimetric determination of chromium is based upon the solvent extraction of chromium(V1) from aqueous 1 N hydrochloric acid with 4-methyl-2-pentanone and development of the magenta color of the chromium-l,5-diphenylcarbohydrazide complex in the extract. The absorbance of the color body is measured 15 minutes after mixing a t 540 mp. Beer’s law is followed. The optimum concentration range extends from 1.0 to 10.0 y of chromium in 6 ml. of solution. Very large amounts of iron and moderate amounts of
vanadium offer no interference. The method is particularly applicable to cast iron and steel samples whose chromium content is low.
T
REATXENT Of a weakly acidic SOlUtion of chromium(V1) with 1,5diphenylcarbohydrazide (6) has found wider application than other colorimetric methods for chromium. Several elements interfere, including vanadium(V) and iron(II1) which give brown colors with the reagent. Bernhardt (3) extracted the chromium-1,5-
diphenylcarbohydrazide complex with 1-hexanol or cyclohexanol, but all the metals that interfere with the usual aqueous color development remained troublesome. The solvent extraction of chromium(VI) from aqueous 1N hydrochloric acid with 4-methyl-2pentanone is a convenient and rapid method for isolating chromium from many elements (4, 11). Following the extraction step, the magenta color of the chromium(VI)-1,5-diphenylcarbohydrazide reaction is developed in situ and measured 15 minutes after mixing. A similar treatment after exVOL 30, NO. 5, MAY 1958
* 977
*
Table I. Chromium Analyses on NBS Samples Certified Cr Sample (Ratio) Cr Values Found, 7% Value, 7% N-Cr cast iron 82a 0.32310.008 0.324,0.312,0.323 (1 Mn, 1Ni) 0.325,O. 325,O. 323 Av. Std. dev. B.O.H. steel 152 0.050 i0.002 0.051,0.051,0.049 ( 1 Mn, 0.1 Cu) 0.049,0.051,0.049 Av. Std. dev. Cr-Mo-A1 steel 106a 1.14,1.15,l. 15 1.15 f 0.01 (0.5 Mn, 1 AI,0.2 M o ) 1.13, 1.14,1.13 sv. Std. dev. Monell62 0.227,0.225,0.225 0.23i0.007 0.232,0.227,0.233 Av. Std. dev. 0.23110.004 0.230,O. 231,O. 230 0.236,0.236,0.236 .4v. Std. dev. Aluminum alloy 86c 0.02910.004 0.0288,0.0301 0.0295,O. 0286 (2 Zn, 1Fe, 8 Cu) 9v. 0.0297,O. 0294 Std. dev. Aluminum alloy 87 0.170,0.170,0.170 0.17 10.00 (2 Zn, 1Fe, 1Nil 1 M g ) 0.170,O. 171,O. 171 Av. Std. dev. traction of chromium with trioctylphosphine oxide has been reported (6). The method has been successfully applied to a variety of steels, cast irons, and nickel- and aluminumbase alloys. The entire procedure requires approximately 10 minutes of the operator's time following dissolution of the sample. REAGENTS AND EQUIPMENT
4Methyl-2-pentanone1 practical grade. Equilibrate with an equal volume of aqueous 1N hydrochloric acid before use. To recover the used solvent, distill, and collect the fraction boiling between 114O and 116' C. 1,5Diphenylcerbohydrazide (s-diphenylcarbazide) solution, 0.25% (weight per volume) in 4methyl-2pentanone. Prepare a fresh solution as soon as the stock solution acquires an amber color. Sulfuric acid solution, 0.072N. Dissolve 1.00 ml. of 18M sulfuric acid in 4 methyl-2-pentanone and dilute to 500 ml. with additional solvent. Standard aqueous solution of chromium, 1.00 ml. equivalent to 2.00 y of chromium. Dilute 10 ml. of a stock solution (containing 0.4566 gram of primary grade potassium dichromate per liter) to 1liter. Any suitable colorimeter or spectrophotometer. Pieliminary studies were done using a B e c k m h Model DU spectrophotometer; subsequent work using a Klett-Summerson photoelectric colorimeter, with 1.2-cm. tubes. PROCEDURE
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 thousandths of 1%to 1% or more. The sample weight and final volume should be chosen so that with a 5- to 15-ml. aliquot the concentration of chromium in the extract falls in the range from 0.25 to 1-00y per ml.
978 *
ANALYTICAL CHEMISTRY
0.228 0.0035
The calibration curve is linear and passes through the origin when allowance is made for the blank whose absorbance is approximately 0.070. The molar absorbancy index, calculated on the basis of molarity of the chromium solution, is 27,000 a t 540 mp. Following Ayres treatment (0,the concentration range of best accuracy extends from about 1.0 to 10.0 y of chromium per 6ml. volume when using the 1.2-cm. tubes supplied with the Klett-Summerson colorimeter. The relative analysis error per 1% absolute photometric error over this concentration range is 3.0%.
0.233 0.003
RESULTS AND DISCUSSION
0.322 0.005 0.050 0.001
1.14 0.01
The validity of the foregoing procedure is substantiated by the results shown in Table I, obtained with a va0.170 riety of National Bureau of Standards 0.001 samples. At least two samples of each type were dissolved and replicate portions of these were carried through Weigh samples containing 0.03 to the procedure. Allen (1) found that 0.5 mg. of chromium into a 250-ml. the most important variables were the beaker or flask, add 30 ml. of water and 10 ml. of 15M nitric acid. Boil gently quality of the reagent and stock soluuntil decomposition is complete or the tions of the reagent. Other operareaction subsides. Add 2 ml. of 85% tional variables may be changed within phosphoric acid and adjust the volume wide limits in aqueous solution. The to about 40 ml. Add 1 to 2 ml. of 0.1N operational variables pertinent to thie silver nitrate solution and 2 grams of investigation are discussed below. pure potassium peroxydisulfate in small Extraction of Chromium. I n view portions. Swirl the flask until most of the practical work of Weinhardt of the salt has dissolved and heat to and Hixson (If) and Bryan and Dean boiling. Keep a t the boiling point for 15 minutes (1). Cool, transfer to a (4, the extraction conditions found 250-ml. volumetric flask, and dilute t o satisfactory by these authors were the mark with distilled water. adopted. A hydrochloric acid conTransfer a 5.0- to 15-ml. aliquot of centration of 0.6N or larger provides each sample to individual 60-ml. separacomplete extraction of chromium. tory funnels and dilute to about 18 ml. However, 1N hydrochloric acid is recwith distilled water. Add 2 ml. of ommended, particularly when the or10M phosphoric acid and exactly 10.0 ganic extract is backwashed to remore ml. of Pmethyl-Zpentanone. Stopper iron(II1) chloride, to avoid any slight and place the funnel in an ice bath for 5 minutes; then add 2 ml. of 10N hydroloss of chromium. chloric acid and shake for 1 minute. A single extraction removes all the When the phases have separated, draw chromium and no loss occurs if the soluoff and discard the aqueous layer. If tion is backwashed with a fresh soluthe amount of iron in the aqueous phase tion of 1N hydrochloric acid. The is expected to exceed 8 mg., wash the ketone must be equilibrated beforeextract with a fresh 20-ml. portion of hand with aqueous lhT hydrochloric aqueous 1N hydrochloric acid and disacid. Failure to do so results in erratic card the aqueous phase. From the orextraction conditions. ganic extract, transfer exactly 5.00 ml. A temperature of 0" to 10' C. is to individual absorption cells. Add exactly 1.00 ml. of 0.25% 1,5-diphenylsatisfactory; the rate of reaction becarbohydrazide solution. Mix and tween chromium(V1) and chloride ion allow to stand 15 minutes. Read the is negligible. Kevertheless, conabsorbance a t 540 mp within 10 minutes. centrated hydrochloric acid must never Carry a reagent blank through the enbe added directly to the sample aliquot tire procedure to compensate for the or standard solution until the volume slight increase in color of the blank has been adjusted to 18 ml. Small upon standing. amounts of chromium(V1) are easily Prepare a calibration curve in the reduced by concentrated hydrochloric concentration range from 0 to 10 y (0 to 1.67 y of chromium per ml.). Transacid. fer 2.5, 5.0, 7.5, and 10.0 ml. of the Color Development. The optimum standard chromium(V1) solution to range of acidity in the organic phase individual 60-ml. separatory funnels and for the chromium-l,5-diphenylcarboproceed to develop the color following hydrazide color development is 0.0006 the extraction step as described with this to 0.003N. At lower acid concentraexception: To 5.00 ml. of the organic tion, the color develops more slowly extract add exactly 0.1 ml. of 0.072N sulfuric acid and 0.9 ml. of 0.25% 1,s and incompletely. Increasing the acid concentration hastens color developdiphenylcarbohydrazide. 0.0293 0.0006
ment but decreases the duration of constant absorbance. I n the recommended range of acid concentration, the maximum color intensity is attained within 15 minutes and will remain constant a t least 20 additional minutes before fading begins. When samples are processed according to the procedure, sufficient acid accompanies the chromium in the estraction step so that additional acid is not needed. The acid concentration of many sample extracts will fall slightly above the recommended range and, therefore, the absorbance readings should be taken within 10 minutes after adding 1,5-diphenylcarbohydrazide t o the organic extract. Apparently a slight amount of phosphoric acid is estracted by 4methyl-2-pentanone and some hydrochloric acid accompanies the iron(II1) chloride, which is known to esist as solvated HFeCI4 in the organic phase. Interference Studies. From solutions 1N in hydrochloric acid, the extraction of metals other than chromium is slight (7, 8, 10). Amounts of vanadium(V) not exceeding 0.5 mg. per aliquot can be present before sufficient amounts extract and cause interference. The amounts of mercury(II),
molybdenum(VI), and tin(1V) should not exceed 50 y in the aliquot taken for analysis. All these limits can be extended by backwashing the extract. The tolerance limit for iron is only 2 mg. when the extraction from aqueous solutions 1N is conducted in hydrochloric acid and, although much larger than the amount able to be tolerated in aqueous medium (Q), is not large enough for many applications, Inclusion of 1M phosphoric acid as masking agent in the aqueous phase extends the tolerance limit to 8 mg. For larger amounts of iron, a single backwash of the 4-methyl-2-pentanone extract with a fresh portion of aqueous 1N hydrochloric acid removes 975% of the iron. Iron(II1) chloride is quickly stripped from the organic phase and by this modification, as much as 0.320 gram of iron can be present initially in the sample aliquot. Undoubtedly this limit could be extended even further by including phosphoric acid in the backwash solution. The percentages of iron extracted from solutions of various hydrochloric acid concentrations, as determined by flame spectrophotometric measurements on the organic phase, are: at 0.6hT,0.6%; a t 0.8N, 1.2%; a t lN,
2.5%; a t 1.2iV, 5%; and from the literature (10)a t 2N, 25%, and a t 3N,
77%.
LITERATURE CITED
(1) Allen, T. L., ANAL CHEbf. 30, 447 (1958). (2) Ayres, G. H., Ibid., 21, 653 (1949). (3) Bernhardt, H. A., U. S. Atomic Energy Comm. Rept. MDDC1541 (1947).
(4) Bryan, H. A., Dean, J. A,, ANAL. CHEX29, 1289 (1957). (5) Cazeneuve, .4.,Bull. soc. chim. (3), 23, 701 (1900). (6) Mann, C. K., White, J. C., Abstracts of Papers, 132nd Meeting, ACS, New York, 1957, p. 29B. ( 7 ) Morrison, G. H.? Frejser, H., “SoIvent Extraction in Analytical Chemistry,’’ pp. 128-9, Wiley, New York, 1957. (8) Mylius, F., Hiittner, C., Ber. deut. chem. Ges. 44, 1315 (1911). (9) Sandell, E. B., “Colorimetric Determination of Metals,” 2d ed., pp. 262-3, Interscience, New York, 1950. (10) Specker, H., Doll, W., 2. anal. Chem. 152, 178 (1956). (11) Weinhardt, A. E., Hixson, A. N., Ind. Eng. Chem. 43,1676 (1951). RECEIVED for review September 23, 1957. Accepted December 23, 1957. Abstracted from a portion of a M. S.thesis to be submitted by Mary Lee Beverly to the Graduate School of the University of Tennessee, Knoxville, Tenn.
Auto matic Cryoscopic Determination of Molecular Weights EDWARD L. SIMONS General Elecfric Research I aborafory, Schenectady,
b A completely automatic recording cryoscopy apparatus has been assembled for the determination of molecular weights. The temperaturesensing element is a thermistor that forms one arm of a Wheatstone bridge circuit. The cooling curve of the liquid in the freezing cell is traced out by a millivolt recorder which measures the unbalance of the bridge produced b y changes in the thermistor temperature. Provision has been made for automatically relieving the supercooling of the liquid.
C
in benzene and cyclohexane are used in this laboratory for the routine determination of molecular weights. The conventional Beckman technique (3) has three operational drawbacks: It requires continual attention by the operator during the entire cooling process; it is difficult t o make accurate temperature readings while the merRYOSCOPIC MEASUREMENTS
N. Y.
cury column in the Beckman thermometer is falling or rising; and manual plotting of the cooling curve of temperature us. time is necessary. These drawbacks can be eliminated by the use of automatic recording cryoscopic equipment. Such equipment has been described by Stull (16, 17) and Witschonke (18), who used platinum resistance thermometry, and by Herington and Handley (7, 8), who used a thermistor as the temperature-sensing element. More recently, Zeffert and Witherspoon (20) described an automatic thermistor temperature recorder that can be used for cryoscopic measurements. Methods based upon such equipment, however, provide no automatic means for the control of supercooling. If supercooling is persistent, as it is for benzene solutions, attention of the operator is required to note the onset of this phenomenon and to take steps to relieve it. The apparatus described, like one
developed earlier in this laboratory by Zemany ( a l ) , uses a thermistor rather than a platinum resistance thermometer as the temperature-sensing element because of the simpler instrumentation involved. Its unique feature lies in the method developed for the automatic control of supercooling. APPLICATION OF THERMISTORS T O CRYOSCOPY
Thermistors are semiconductors whose large temperature coefficients of resistance make them particularly useful for the measurement of small temperature changes (2, fa). I n the reported applications of thermistors to cryoscopic measurements (4, 7,8, IO,11,14, f9-21), the thermistor has been used as one arm of a Wheatstone bridge circuit, and in most cases the resistance of the element has been measured manually, by a null method, as a function of time during the freezing process. VOL. 30, NO. 5, MAY 1958
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