V O L U M E 19, NO. 8
612
mittance correspond to that of the standard within the limits of experimental error with very few exceptions. If the solution is very concentrated in sulfurit: acid before neutralization (5 ml. of 5 N sulfuric acid per 25 ml. of final solution) there is a consistent slight depression of the color of the solution amounting to approximately 37, transmittance. If the solution contains more than about 0.25 gram of thorium a depression of the color occurs. This amounts to about 3 to 47, transmittance, if the solution being analyzed contains approximately 1.25 grams of thorium and the standard contains no thorium. However, if a standard curve is prepared with solutions contairiing approximately the same thorium content throughout (within 0.25 grams of thorium per 25 ml. of solution), a straight line is obtained when the uranium content is plotted against the light transmittance on semilogarithmic paper. The blank solution should also contain the same amount of thorium as that used in preparation of the standards. Curve 2, Figure 1, was obtained when uranium solutions were “salted” with approximately 1 gram of thorium (2.4 grams of thorium nitrate tetrahydrate). This curve also indicates the change in sensitivity which occurs Then a light filter of longer wave length is used. These measurements were made with a Fisher Electrophotometer with a light filter having its maximum transmittance at a wavs length of 425 mp.
I n all these experiments the colors were measured soon after the addition of the thiocyanate reagent. Although no practical difficulty has been encountered because of the change of transmittance of a solution with time, it is worth noting that after a period of about 20 hours the transmittance of a given solution will have decreased about 1541, when compared with a similar solution to which the thiocyanate reagent has just been added. SUMMARY
The procedure described has been used successfully for the rapid colorimetric determination of small quantities of uranium in the presence of large quantities of thorium and small amounts of iron and copper. The method depends on the development of an intense yellow color with uranium and thiocyanate. Stannous chloride is used to reduce iron to the divalent state and prevent the formation of an interfering color. LITERATURE CITED
(1) Sandell, E. B., “Colorimetric Determination of Traces of AMetals,” pp. 330-8, New York, Interscience Publishers, 1944. (2) Ibid., pp. 433-8. (3) Skey, W., Chem. S e t u s , 15, 201 (1867).
Direct Colorimetric, Method for Phosphorus in All Types of HENRY L. KATZ AND KENNETH L. PROCTOR Industrial Test Laboratory, Philadelphia Naval Shipyard, Philadelphia 12, Pa.
Hague and Bright’s method for the colorimetric determination of phosphorus in steel and cast iron has been modified to eliminate interferences incurred with high-chromium, columbium, and tungsten steels. Insoluble carbides are decomposed by prolonged digestion w-ith perchloric acid and oxides of columbium and tungsten are removed by filtration.
H
AGUE and Bright’s method ( 1 ) for determining phosphorus by means of an ammonium molybdate-hydrazinc sulfate reagent yields excellent results when applied to plain carbon and low alloy steel and to cast iron. However, the need for a compensatory blank to correct for high amounts of chromium, and the fact that the procedure becomes unreliahlv in the presence of tungsten and columbium, have made it deairable to introduce modifications which make the method applicable to all types of steel, employing a single reagent blank and reference curve. Hague and Bright ( 1 ) found that the presence of more than nickel leads 2% chromium, 15% copper, 5 % vanadium, or to appreciable positive errors. To overcome this difficulty t.hep suggested the use of a compensatory sample blank in Tvhich the phosphomolybdenum blue color was not developed. The use of this sample blank, however, was objectionable because of the increase in work and loss of time incurred and i t was considered desirable to attempt to eliminate these interferences directly. h s copper, vanadium, and nickel are very rarely encountered in steel in excess of the percentages listed above, the problem of determining phosphorus directly in the presence of high chromium became a chief concern of the authors. I t was also desired to eliminate the objectionable turbidity caused by columbium or
The absorption of the phosphomolybdenum blue complex is measured photometrically with a filter having a mean transmission of 690 millimicrons, which eliminates interference due to chromium. Only a single reagent blank and reference curve are required for all types of steel. The sensitivity of the proposed method is better than 0.0004%.
tungsten n.hen present in the amounts commonly found in corrosion-resistant and high-speed tool steels. EXPERIMENTAL
The spectral transmittance characteristics of chromium and the phosphomolybdenum blue complex were studied using a General
Table I.
Comparison of Chromium Interference at 660 and 690 m,u Phosphorus Found miurn K.S. K.S. Content Present No. 66 S o . 69 Chro-
SarnDle
%
55
%
76
4
0 087
S.B.S. 73a
14
0.015
S.B.S. 121
18
0.016
T-7746
23
0.043
T-3101
28
0.021
0 087 0.088 0 017 0 017 0.018 0 019 0 047 0.048 0 026 0 027
0.086 0.087 0.015 0.014 0 015 0.016 0.043 0.044 0.022 0.023
T-248
Difference K.S. K.S. S o . 69
No.66 %
0,000
+0.001 +0.002 +0.002
+0 002 +0.003 f0.004 +0.005 f0.005 +0.006
x
-0 001
0.000 0 000 -0,001 -0 001 0.000 0.000 +0.001 + O 001 +0 002
613
AUGUST 1947 Table 11. Precision and Accuracy of Procedure for High-Chromium, Columbium-, and Tungsten-Bearing Steels
Mean
Difference (Mean from Amount Present)
%
7%
Phosphorus Sample
Xominal composition
Present
%
.4nalyst l a
%
Found Analyst Analyst 20 3a
%
%
and selenium in amounts normallv encountered in steel. Interferences due to high silicon and arsenic can be eliminated as proposed by Hague and Bright ( 1 ) . APPARATUS AND REAGENTS
All measurements were made with a S.R.S.133 14y0 C r 0.022 0.020 0.021 0.021 0.021 - 0 001 Klett-Summerson photoelectric colorimS.B.9.101b 18% C r , 9 % Ni 0.018 0.018 0.019 0.019 0.019 + O 001 eter using a 10-ml. test-tube cell apS.R.9.121a 18% Cr, 11% S i , 0.4% T i 0 . 0 2 3 0.022 0.024 0 024 0 023 0.000 X.B.S.123a 1 8 % C r , l l % S i , 0 . 7 5 % C b 0.035 0.034 0.033 0 033 0 033 - 0 002 proximately 12.5 mm. in internal diam18% W, 4% Cr, 1% V 0.020 0.020 0.021 0.020 0.020 0,000 S.B.S.50a eter. The absorption of the phosphoS.B.8I53 8% Mo. 8% Co, 4% Cr, molybdenum blue solution was read 2 % V, 1.5% W 0.026 0.027 0 027 0.027 0 027 f 0 001 with a Klett-Summerson filter S o . 69, T-248 14% M n , 4 % C r , 3% N i 0.087 0 087 0.086 0.087 0 087 0 000 having a mean transmission of 690 T-103 15 22% Cr, 10% S i , 0.4% Se 0 . 1 3 0 0 . 1 3 3 0.126 , 0 . 1 2 8 0 129 -0 001 millimicrons. S-170 21% Cr. 10% S i , 0.2% Cb, Dilute nitric acid, specific gravity 1.20. 0 2% w 0 . 0 4 8 0 049 0.047 0 048 0 048 0 000 X-li3 ' 23% Cr, 11% Ni.0.4% C b , Perchloric acid. 70%. 0.095 0.095 0 096 0 4 % W 0.094 0 095 + O 001 Sodium sulfite solution, 10%. T-1758 19% Cr, 9 % Xi, 0.2% Cb, Hydrazine sulfate solution, 0.15%. AsvJ, 0.1% Zr, 0.012 0.012 0 012 . ,. ,012 0.000 Ammonium molybdate solution, 2% in 11 S sulfuric acid. Add 300 ml. of a iverage of 3 independent determinations, no determination deviating from another by more t h a n sulfuric acid (specific gravity 1.84) to 0.003 % . 500 ml. of water and cool. Dissolve 20 grams of ammonium molvbdate in the solution and dilute to 1000 ml wlth water. .lmmonium molybdate-hydrazine sulfate reagent. Dilute Electric recording spectrophotometer with a slit width of 10 125 ml of the ammonium molybdate solution to 400 ml. w t h millimicrons. I t was observed that chromium exhibited maximater, add 50 ml. of the hvdrazine sulfate solution, and dilute to mum absorption in the region of 580 millimicrons which thereafter 500 ml. xvith w,ter. This solution is not stable and should be steadily decreased until absorption became negligible a t apprepared as needed. proxiniately 700 millimicrons. The absorption of t,he phosphomolybdenum blue complex was found to increase gradually PROCEDURE from the region of 420 millimicrons until it reached its peak a t Dissolve 0.100 gram of the sample in a 200-ml. Erlenmeyer 830 millimicrons. This led to the belief that the use of a filter flask in 10 ml. of dilute nitric acid (specific gravity 1.20); for having a mean transmission of close to 700 millimicrons could steels not attacked by nitric acid, dissolve in a freshly prepared red acid mixture of equal parts of hydrochloric acid (specific directly eliminate the interference due to chromium and simulgravity 1.19) and nitric acid (specific gravity 1.42). Add extaneously obtain greater absorption of the phosphomolybdenum actly 3.0 ml. of 70Y0 perchloric acid from a buret. Evaporate blue complex. Obsrrvations of the data obtained from subsethe solution over a medium flame to fumes of perchloric acid and, quent experiments by making measurements with a Klett-Sumfor those samples not containing high chromium, tungsten, or nierson photoelectric colorimeter in the regions of 660 and 690 columbium, fume gently for 3 to 4 minutes and remove from the hot plate. [An alternate method, which facilitates the handli.1.. millimicrons, disclosed that, TThile an appreciable chromium interof large numbers of plain carbon and low-alloy steel samples, and ference was encountered in the former region, a t 690 millimicrons ensures expulsion of all the nitric acid, is to evaporate the soluan error of less t'han 0.002% was incurred when as much as tion until fumes of perchloric acid fill the flask, then fume gently 28% chromium was present (Table I). Moreover, a t t.he for 30 seconds longer. Cool slightly, add a few milliliters of water and 1 ml. of hydrochloric acid (specific gravity 1.19), again latter wave length the absorption readings of the phosphomolybevaporate until fumes fill the flask, and fume gently for 30 seconds. ] denum blur complex increased by approximately 25% thus For high-chromium steels heat at such a rate that the pergiving the method a sensitivity of better than 0.0004% based chloric acid refluxes on the walls of the flask while only slight fumes on a 0.1-gram sample. escape from the mouth, and continue the refluxing until red chromic acid condenses on the walls and reaches the neck. This When a Klett-Rummerson filter S o . 69, having a mean transusually requires 20 to 30 minutes. For tungsten- or columbiummission of 690 millimicrons, was initially applied to Hague and bearing steels which contain insufficient chromium to obtain this Bright's carbon steel procedure (1) results for phosphorus in highcharacteristic indication, reflux at the same rat? for 20 to 30 chromium steels generally tended to be low. It was suspected minutes. Allow the solution to cool someTThat and if tungsten and columthat the prescribed perchloric acid fuming period was insufficient bium are absent dissolve the salts in 15 or 20 ml. of water. (If to decompose the chromium carbides entirely, and resulted in an either tungsten or columbium is present dissolve the soluble salts entrapment of some of the phosphorus (3). A complete oxidation in 5 ml. of water and filter on a retentive paper into another 200of all the phosphorus to the ortho acid, therefore, could not bc ml. flask; then rinse the flask and wash the residue with small portions of hot water, keeping t,he volume of the filtrate below achieved. 40 ml.) Add 15 ml. of 1OYo sodium sulfite solution, boil the soluTo overcome t,his difficulty, a carefully controlled fuming, tion gently for 30 seconds, and immediately add 50 ml. of freshly so as not to lose excessive perchloric acid, was employed for a prepared ammonium molybdate-hydrazine sulfate reagent. period of 20 to 30 minutes to ensure a complete dissolution of Heat on a steam bath at 85" to 90" C. for 20 minutes, then cool rapidly to room temperature in running water. Dilute to exthe carbides and oxidation of the chromium. Excellent results actly 100 ml. with water in a graduated mixing cylinder and mix were obtained with this modification (Table 11). thoroughly. Attention was then directed toward eliminating the object>ionTransfer a portion of the colored solution to the photoelectric able turbidities caused by the presence of columbium and tungcolorimeter cell and read the absorption of the solution, after having previously set the instrument a t zero with a blank on resten. I t was found that these interferences could be removed agents carried along simultaneously with the sample. Convert by filtration after the columbium and tungsten carbides had the reading to per cent phosphorus by reference to a calibration been completely decomposed by fuming in the manner prochart prepared from National Bureau of Standards samples. posed for high-chromium steels. Subsequent results indicated that no significant amounts of phosphorus were retained by the ACKNOWLEDGMENT residues ( 2 ) . The authors desire to express their gratitude to Thomas F . Further investigations showed that no interferences were Boyd for preparation of the transmission curves used in the caused bv manganese, molybdenum, cobalt, titanium, zirconium,
tyJZ
V O L U M E 1 9 , NO. 8
614 spectrophotometric study and to thank I. Weber and I. Geld for their assistance in obtaining the experimental data. LITERATURE CITED
(1) Hague, J. L., and Bright, H. A,, J . Research *YaatZ.Bur. Standards,
26, 405-13 (1941). (2) Lundell, G. E. F., Hoffman, J. I., and Bright, H. A., “Chemical
Analysis of Iron and Steel,” p. 214, S e w York, John Wiley & Sons, 1931. (3) U. S. Steel Corp. Chemists, “Sampling and Analysis of Carbon and iilloy Steels,” p. 92, New York, Rheinhold Publishing Corp., 1938. THEopinions expressed are those of the authors and are not to be construed as reflecting the official views of the Xavy Department, through whose permission this article is published.
Carotenoids of Stored Dehydrated Carrots GORDON AIACKINNEY AND W. E. FRATZKE Division of Food Technology, University of California, Berkeley, Calif. A procedure has been devised for determining whether significant changes occur in the relative proportions of the carotenoid pigments of dehydrated carrots on storage. Present methods of carotenoid assay are ill adapted to accurate determination of all components where large numbers of samples are involved. A critical analysis of spectrophotometric data indicates no major change in the
I
N ADDITION to a- and p-carotenes, the carrot contains numerous carotenoids present in minor proportions. One frequently needs to know whether significant changes have occurred in the relative abundance of these components in processed carrots on storage in order, ultimately, to assess the effect of a given processing treatment, and this paper is concerned with methods that are applicable. h detailed study of the absorption spectra of crude extracts and of the chromatographed components, in the range 510 to 310 mp, is reported. MATERIALS
Five samples of Imperator carrots were selected, four of which had been in storage for years or more, under unfavorable conditions. The initial carotene content of all samples ranged from 90 to 100 mg. per 100 gram of dry weight, and four samples had lost from 40 to 80% of their original endowment. Significant trends should therefore be discernible. The samples were prepared as follows: 1. Sulfited and dehydrated in the laboratory, winter of 1942, stored (a) a t room temperature, (b) for six months a t 48“ C., in air, in air-tight jars. Sample l a was light in color, sample l b a dark brown. 2. Dehydrated in the factory in 1943, (a) sulfited, (b) control; in air a t room temperature. 3. A 5-gallon can, commercial pack, in nitrogen a t room temperature. PREPARATION OF EXTRACT
Samples are passed through a Wiley mill, 60-mesh, and used immediately, unless otherwise noted. Sormally a 1- to 2-gram sample is taken, covered with 50 ml. of acetone, stirred, decanted, and washed two to three times with small portions of acetone. At this stage, about 80% of the carotene has been removed. The residue is then covered with water and stirred gently to ensure absorption, and the acetone treatment is continued until the residue is colorless. The combined filtrates, about 200 ml., are then transferred to petroleum ether which is washed and dried in the usual way, and made to 50- or 100-ml. volume. Twenty minutes suffice for this operation. One half is then chromatographed directly, and an aliquot of the remainder is diluted for direct measurement of the optical density. It has been found more satisfactory to rehydrate the sample after a preliminary acetone extraction. If no water is added, prolonged grinding is necessary. If it is added a t the start, minute carrot particles will form a faint emulsion, and lower values are obtained. CHROVATOGRAMS
Micron-brand magnesia and Hyflo-Supercel were used for adsorption (3). A column filled to a height of 15 to 20 cm., 2.2 cm. in diameter, was used. The chromatogram was developed with petroleum ether containing 3y0 acetone.
proportions of cis-trans isomers on storage. Oxidation of phytofluene and r-carotene certainly proceeds no faster than that of other components. This is of interest in view of their marked instability on extraction. Products or processes for which significant anomalies in spectrophotometric data are found would, in addition, require chromatographic examination.
The pigments of the stored samples were fractionated as follows: 1. Phytofluene, discovered by Zechmeister and Sandoval (5, 6), with maxima in petroleum ether about 368, 347, and 330 mp. (Since the relative heights of the bands are identical, differences of 1 mp or less in the positions are ascribed to solvent.) 2. a-Carotene, maxima about 475 and 445 nip. 3. p-Carotene, maxima about 480 and 450 mp. 4. r-Carotene described and provisionally named by Strain (3) and by Xash and Zscheile ( 2 ) , maxima about 425, 400, and 375 mp. 5 . 7-Carotene, maxima about 490, 460, and 435 mp. 6. hliscellaneous minor components, including xanthophylls, eluted from the column with acetone and transferred to petroleum ether. In all cases where acetone had been used, even in eluates from the mixed solvent, it was removed by washing with water, prior to measurements of optical densities, made a t suitable dilutions, with a Beckman quartz spectrophotometer and a tungsten light source. The petroleum ether containing 3% acetone gives excellent and rapid separation of the a- and p-carotenes with a colorless eluate betaeen them. The effectiveness of this solvent is limited to the first five components mentioned above, which pass into the eluate within 2 hours. Beyond this point, additional washing is ineffective. Shorter columns, while effectively separating the CY- and p-carotenes, do not permit clean-cut separation of the last of the 8-zone from the p- and y-zones. The latter was normally not estimated because it represented an insignificant fraction of the total. I n fact, spectroscopic identification of the y-carotene required 5 to 10 grams of carrot powder. Severtheless, the band served as a useful reference marker on the Tsn-ett column. The r-carotene was considerably more abundant, and was routinely estimated. I t occupies an intermediate position on the column between the p- and y-zones and is pale yellow in color. Three problems are presented: 1. The proportion of the original that can be accounted for by the separated components. 2. The homogeneity of a given zone. 3. Changes in the proportions of the various components. RECOVERY O F PIG.MEIITS
The first problem was to determine whether adequate recovery of chromatographed material could be obtained. Four wave lengths were selected: 450, 445, 400, and 345 mp. The last two were chosen because of the influence of carotenoids with maxima in the’ near ultraviolet. Two crude extracts 11-ere prepared, and
