609
A U G U S T 1942
From a practical standpoint, for determining the azeotropic constants of a system, the plots of the above equations have been found 100 more useful and are given in Figures 1 to 5 100 80 80 for forty-five systems for which data are available. Up to this time only the curve for ethanol-halide hydrocarbons has been 40 40 published (I). A h o t h e ruse for this set of curves is for esti0 0 -GO -80 -40 0 40 80 A I20 -60 -40 -20 0 20 40 A 60 mating the aaeotropic boiling point and composition at pressures other than atmospheric. Consider the azeotrope methanol-benzene. Since the vapor pressure curves of methanol and benzene are known, the difference in boilIO0 100 ing point, A, can be obtained a t any pressure. 80 80 From this value of 1 and the C-S curve for methanol-hydrocarbons the azeotropic 40 40 qoncentration C a t that prcssure can be determined. For example, the effect of pressure 0 0 on the methanol-benzene azeotrope is show1 -120 -80 -40 0 40 80 A 120 -60 -40 -20 0 20 40 A 60 in Table I. .1plot of A as a function of C from this table IC1 I I! I! I! 1! I ! , I! I# I ! I I is shown in Figure 6. The experimental data I ! ! arc rcpresented by the five points n-bile the smooth curve is identical with the methanolhydrocarbon curve in Figure 1. Similar curves and data for other systems over the pressure range indicated are also 40 shown. I n each case the curve is the same as the general curves of Figures 1 to 5 , while the *;'t\ 0 0 esperimental points are for the particular -40 -20 0 20 40 60 A 80 system and for the pressure range indicated. In the same Kay, the 6- 1 A 1 curves of Figures Figure 6 . C - 1 CurFes for Alcohol-Hydrocarbons, Alcohol-Halide Hydro1 to 5 can be used to determine 6 and the carbons, and Alcohols-Ketones azeotropic boiling point at any pressure from Showing agreement with experimental data at various pressures C. Weight % alcohol hydrocarbon the value of I A I a t that pressure. A. Boiling point of alcohol halide ' hydrocarbon Khile the agreement between predicted minus boiling point \ketone and experimental values is far from perfect, the method has served as a valuable guide in LITERATURE CITED estimating effect of pressure on azeotropic systems. (1) Lecat. A n n . S O C . sci. Bruxelles. 55B. 43 (19363. It is reconnized that it would be more convenient to be able to i2) Lecat, Compt. rend., 183, 880 (1926) ; 184, 816 (1927) ; 189, 990 plot pressur; instead of A as a function of C and 6. However, this (1929); Ann. SOC. sci. BruseZles, 47B,39, 87 (1927); 48B, 1, would require a separate curve for each azeotrope, whereas the 105 (1928); 49B,28, 119 (1929); 55B, 43, 253 (1935); 56B, above method permits use of a single curve for a large group of 41 (1936) ; Atti accad. nail. Lincei, ( 6 ) 9,1121 (1929) : 2.a n m g . allgem. Chem., 186, 119 (1930) systems. ~
\&
B
1
Colorimetric Determination of Uranium with Thiocyanate J. E. CURRAH AND F. E. BEAMISH Department of Chemistry, University of Toronto, Toronto, Ontario, Canada A method has been de~elopedfor the rapid estimation of small quantities of hexatalent uranium in the presence of relatively large amounts of thorium and small amounts of iron and copper. The determination is based on the estimation of the color produced with thiocyanate and uranyl. This method permit5 the estimation of 0.05 to 0.80 mg. of uranium in the presence of at least 1.23 grams of thorium, 2 mg. of iron, and 50 mg. of copper per 25 ml. of solution. The ,interferenceof iron is eliminated by reduction with stannous chloride.
A
METHOD \!-as required for the determination of uranium in concentrations as low as 2 or 3 parts per million in the presence of approximately ten thousand times as much thorium. Low concentrations of iron and copper were to be considered as possible interfering elements. Becaux a rapid and simple method was desired, one that would require no preliminary separations was thought most suitable,
A number of colorimetric methods for the estimation of uranium, such as the fluorescence method, and the diethyldithiocarbamate, ferrocyanide, peroxide, salicylate, and tannic acid determinations are well known ( 2 ) . However, these methods have a poor sensitivity or require a preliminary separation to remove the interfering iron or copper. A preliminary investigation of possible new color reactions of
610
V O L U M E 19, NO. 8
organic and inorganic compounds with uranium and thorium solutions showed that thiocyanates formed an intense yellow color with uranyl solutions, but gave no color with thorium. This confirmed the work reported in 1867 by Skey (3). This report deals with the development of a colorimetric method to meet the necessary conditions. ta
I
1
I t was found that concentrated solutions of these acids, of the order of 5 N, reacted with the thiocyanate reagent rather rapidly, producing a yellow-orange solution.
If the solution were less concentrated in acid-for example, if it contained up to 2 ml. of 5 N acid per 16 ml. of solution before addition of the 2 ml. of stannous chloride reagent and 7 ml. of thiocyanate reagent-no change in absorption of light of 365 mp occurred. If greater amounts of acid were present in the 16 ml. volume before addition of the stannous chloride and thiocyanate reagents, a slight change in the absorption was noted. For example, the addition of 3 ml. of 5 S nitric acid solution decreased the absorption of a particular uranium solution from 79.5% transmittance to 78.37,, 4 ml. of the same acid gave a final transmittance of 76.4%, and 5 ml. of the same acid gave a transmittance of 75.970. In order to overcome this difficulty, the pH \vas adjusted to a definite value with potassium hydroxide solution before addition of the stannous chloride and ammonium thiocyanate reagents. I t was necessary that the pH be sufficiently low to prevent hydrolysis of the stannous chloride when added.
.I pH of 1.5 was used for most of the determinations recorded here. A lower pH-for example, pH 1.0-may be more generally satisfactory and has been used successfully. The pH should be kept approvima tely constant in all comparative determinations.
Transmittance, %
Figure 1
EFFECT OF COPPER
The optimum light absorption of the color produced occurred a t a light wave length of 365 mp with the authors' colorimeter (curve 1, Figuie 1). I t was found that the intensity of the color with thiocyanate and uranium was a function of the amount of uranium present and that relatively large quantities of thorium did not interfere with the development of this color. The intensity of the color was also a function of the amount of thiocyanate present (curve 2), and it was necessary to choose a standard amount of thiocyanate to use throughout subsequent work. I t was decided that 7 ml. of the 50% weight/volume solution of ammonium thiocyanate per 25 ml. of solution would give a suitable depth of color. The transmittance of solutions containing various amounts of standard uranium solutions with 7 ml. of thiocyanate reagent per 25 ml. !vas measured, and the effects of various chemicals which were possible interfering materials were investigated. EFFECT OF IROK
Some of the materials which were to be analyzed contained traces of iron. Since the red color of the iron"' thiocyanate complex increased the absorption of light of the wave length used, it was considered advantageous to eliminate this color, produced in the presence of amounts of iron approximately equivalent to the auantitv of uranium which would be encountered-that is, up to about 1 mg. of iron per 25 ml. of final solution For this reason 2 ml. of stannous chloride reagent were added to each solution before the addition of the thiocyanate reagent. This did not interfere with the development of yellow uranium-thiocyanate color but it was sufficient to reduce up to 2 mg. of trivalent iron and prevent its absorption of light of C,ranium 365 mp wave length. The presence of 5 mg. of Added, Mg. iron decreased the transmittance by about 2% and 0 further quantities gave greater reductions in trans0 019 mittance. The addition of a larger amount of 0 057 stannous chloride reagent will permit the elimina0.095 Q.133 tion of interference of larger concentrations of tri0.190 valent iron. 0.285 EFFECT OF MINERAL ACIDS
0 418 0 428 0 57
Since the solutions to be analyzed might contain nitric, hydrochloric, or sulfuric acids, the effects of these acids had to be considered.
h
I t was found that 10 mg. of copper in the foIm of copper nitrate did not produce any interference, but 20 mg. of copper produced a slight turbidity which increased the absorption of light, and 50 mg. produced a heavy white precipitate. If the solutions were centrifuged thoroughly these precipitates were removed and the transmittance was the same as that ohtained for similar uranium solutions containing no copper. EFFECT OF THOKIUAl
The presence of appiovimately 250 mg. of thorium (0.6 gram of thorium nitrate tetrahydrate) had little effect on the light absorption of a particular solution, but the addition of about 1.25 grams of thorium had a definite depressing action on the color and the absorption was decreased slightly. Holvever, standard curves may be prepared for solutions containing a quantity of thorium approximately the same as that of solutions to be analyzed. T h e slight depression of the yellow color in the presence of large quantities of thorium may possibly be due to the formation of some type of complex with thiocyanate which decreased the amount of thiocyanate available to develop the color x i t h the uranium. EFFECT OF MOLYBDENUM
Stannous chloride reduces molybdates to the pentavalent state, which gives an orange-red color with thiocyanate. This color
Table I. Transmittanoe Read from Standard Curve 100 95.4 86.0 77.5 70.0 60.0 46.5 32.4 31.5 21.5
Actual, used to obtain curve 100 97.7.96 0,45.0 8 6 . 0 , 8 6 . 2 , 8 60 77.7,77.4.77 7 6 9 . 6 , 6 9 . 9 , 6 97 60.8,60.2 46.6,46 4 32.4,33 0 ,
.,,.., .
Effect of Mineral Acids Per Cent Transmittance Solutions Solutions containing Containing 0.5 to 5 ml. 0.5 t o 5 ml. of 5 N HC1 of 5 N HzSOr per 25 ml. per 25 ml 100 100
........
.
,
79.8
. . . . .
......... .., . , . .
76.8,77.8a
........
63.3,60.9,60.8 62.0,64.5a 48 0 . 4 6 . 4 , 4 7 . 6 .........
........ ........
22.0,20.9,20.8 20.7,21.9,21.3
3.5 ml. of 5 S H2S01 per 25 ml. of solution. 5 ml. of 5 N H2SOa per 2 5 ml. of solution.
Solutions containing
to 5 ml. of 5 N HNO, per 25 ml. 100
........ .......
76.8,76.2
........ ........
61.4,59.9,60.9
........ 33.0,'32.i,'35.0a 3 1 . 4 , 3 1 . 7 , 3 2 . 3 38.7b
.,..,,, .,
........
AUGUST 1947
611
has been made the basis of a colorimetric method for the determination of molybdenum
Table 11. Effect of Metal Impurities Boln.
(1).
X O
REAGENTS
.ini m o n i u m Thiocyanate. Five hundred grams of am-
1 2 3 4
Transmission of '0 25 Aliquot gram 60.8,59.4,59.5 33.5,33.1,33.5 25.2,25.2,25.5 26:s 11.8,11.8
.. ..
monium thiocvanate (Baker & Idamson's reagent gGade) per liter of aqueous solution. Stannous Chloride, Fifty grams of tin" chloride dihydrate (Baker & Adamson's reagent grade) dissolved in 50 ml. of concentrated hydrochloric acid, diluted t o 500 ml. with water, and filtered to give a clear solution if necessary. Hydrochloric acid, approximately 5 S solution. Potassium hydroxide (Baker & Adamson's reagent grade), approximately 1 N solution. Solutions of uranyl nitrate accurately prepared to contain approximately 1, 0.1, and 0.01 mg. of uranium per ml. of solution. Solution of thorium nitrate containing approximately 0.5 gram of thorium per ml. of solution. PROCEDURE
Ai standard curvt' of uranium concentration plotted against light transmission is required. A blank solution containing 2 ml. of 5 N hydrochloric acid reagent and an aliquot of thorium solution equivalent to twice the approximate amount of thorium (within 0.5 gram of thorium) expected in the solutions to be analyzed is diluted to approximately 45 ml. and titrated to p H 1.0 with the 1 N potassium hydroxide reagent. I t is then diluted to 50 ml.
Thorium 05 1.25 1 gram grams mg. .. 63.2 37.1 32:7 25:s 28.3.28.1 , . .. 14.8,15.5 . .
2 mg.
32:7 25.0
..
Iron 5 mg.
Copper ~~
10 mg.
10 mg. 58.4
3 1 : 5 , ' 3 0 . 2 25:0#'25.2 32.7 2 2 . 9 , 2 2 . 2 22.0,21 2 24.8
.....
___.
.....
__
..
-
50 mg 54.9 29.9 24.2
.. __
justed to give 1 0 0 ~ otransmittance with a blank solution prepared simultaneously. The transmittance of the unknown is measured and the amount of uranium in the aliquot of unknown sample is determined by reference to the standard curve. RESULTS
Figure 2 vas prepared by measuring the approximate transmittance of a solution containing a small amount of uranium nitrate, stannous chloride, and ammonium thiocyanate, at various wave lengths after setting the colorimeter to read 100% transmittance with a blank solution of stannous chloride and ammonium thiocyanate. The maximum sensitivity with this colorimeter may be obtained with light of wave length 365 mp. Figure 3 represents the increase in absorption of light which occurs when increasing amounts of thiocyanate are added to a solution containing uranyl nitrate. This particular curve was obtained by adding varying amounts of thiocyanate reagent to samples containing approaimately 0.5 mg. of uranium, diluting to 25 ml., and measuring the transmittance of the resultant solutions. -4solution containing 0.5 mg. of uranium per 25 ml. of solution gave the same transmittance (10070) as rsater containing no uranium. I t was decided that a 7-ml. volume of thiocyanate nould be a suitable amount to produce a good depth of color for subsequent work. Curve 1, Figure 1, represents a standard curve of weight in milligrams of uranium per 25 ml. of solution plotted against light transmittance.
Light W a r e Length, m r
Figure 2
A series of solutions Containing quantities of uranium varying from 125 micrograms to 2.00 mg. is prepared, and to each are added 1 ml. of the 5 S hydrochloric acid reagent and the approximate amount of thorium (within 0.25 gram of thorium) expected in the solutions to be analyzed. One half the quantity of potassium hydroxide solution required in the titration of the blank solution is added and the solution is diluted to 25 ml. with water. Into 25-ml. volumetric flasks, 10 ml. of the blank and of each of the above uranium solutions are measured, 2 ml. of the stannous chloride reagent and 7 ml. of the thiocyanate reagent are added, and the solution is diluted to volume. These solutions should be centrifuged if there is any suspension. They are then transferred to the colorimeter cells, the instrument is adjusted to read 100% transmittance for the blank, and the transmittance for the other solutions is determined. The per cent transmittance is plotted against the weight of uranium present in the 25-ml. sample, preferably on semilogarithmic graph paper. The colorimeter used for this work was the Lumetron Model 402 E F (Photovolt Corporation). .I light filter of 365 nip wave length was used throughout and the solutions were placed in 20-mm. rectangular cells. I n order to determine the quantity of uranium in an unknown solution, an aliquot of up to about 10 ml. is measured and the pH adjusted to 1.0, then 2 ml. of stannous chloride reagent and 7 ml. of thiocyanate reagent are added, and the solution is diluted to 25 nil. and centrifuged if necessary. The colorimeter is ad-
Thiocyanate Reagent Added, 311
Figure 3
The data for this curve were obtained by taking aliquots of uranium solution which had been acidified and then adjusted to p R 1.5 with potassium hydroxide reagent. The usual procedure of addition of stannous chloride and thiocyanate and dilution to 25 ml. was carried out. The measurements were made with the Lurnetron Model 402 EF colorimeter, with a light of wave length 365 mk and with the solutions contained in rectangular cells having a 20-mm. light path. The curve was checked many times with solutions containing in addition to uranium, various amounts of nitric, hydrochloric, and sulfuric acids, thorium nitrate, ferric nitrate, and copper nitrate. Some of the results obtained in the presence of these impurities are given in Table I and 11. The values obtained for the trans-
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 2% chromium, 15% copper, 5 % vanadium, or nickel leads 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
