V O L U M E 27, NO. 6, J U N E 1 9 5 5 n eights encountered. Heretofore the heaviest published mass bpectra did not extend much beyond molecular weight 600. Apart from the obvious limitation in resolving power and mass nieasurement in a n instrument designed for molecular weights of 350 or less, no difficulties were encountered in extending the mass innge above 1700. The success of the experiment depends on the unusual properties of the material. T h e thermal stability of the molecules is so great that chains with as many as eleven phenylene I ings evaporate without degradation and the stability of the ions I. sufficient t o make the molecule ion the predominant mass peak i n the spectrum. These circumstances make it possible t o identify qualitatively all the different molecules and radicals liqted in Table 111. I n most mixtures of heavy and light molecules fragment ions from heavy molecules can mask the lighter
a77
molecules in a spectrum and it is necessary t o know the spectra of the pure compounds t o derive an analysis. LITERATURE CITED (1) Ani. P e t r o l e u m Inst., “API Catalog of Mass S p e c t r a l D a t a , ” Research P r o j e c t 44, 1949 t o d a t e . (2) B r a d t , P a u l , D i b e l e r , V. H., a n d 3 I o h l e r . F. L., J . Research N u t l .
Bur. S t a n d a r d s , 50, 201 (1953). (3) H e l l m a n n , Max, Bilbo, A . J., a n d P u m m e r , W. J., J . Am. Chem. Soc.. s u b m i t t e d for p u b l i r a t i o n . (4) S i e r . -1.O., Rea. Sci. Instr., 11, 212 (1940). ( 5 ) O’Xeal, 31. J., a n d K i e r , T. P., ;Is.~L. CHEY.,23, 830 (1961).
RECEIVED for review November
4, 1954. Accepted February 17, 1955. Work performed as part of a research project on high temperature-resistant polymers sponsored by the Ordnance Corps, Department of the .Irmy.
Direct Spectrophotometric Determination of Uranium in Aqueous Solutions R. G. CANNING and P. DIXON Geological Survey Laboratories, Department o f Mines,
A rapid, direct method was required for estimating uranium in sulfate solutions containing uranium, vanadium, chromium, and rare earths in concentrations of 1 to 2 grams per liter of the respective oxides, titanium up to 10 grams per liter of titanium dioxide, and ferrous and ferric iron up to a total of 40 grams per liter of ferric oxide. Existing chemical methods were too lengthy, while rapid fluorimetric and colorimetric methods were not sufficiently precise. A two-component spectrophotometric method was developed, utilizing the reduction of uranium and vanadium by ferrous phosphoric acid solution. sulfate in 40 volume Estimations may be completed in 1 hour with reasonable precision. Using a Hilger Uvispek spectrophotometer, standard deviations in precision of &lqo were obtained over the range 0.5 to 5.0 grams per liter of uranium oxide in the presence of other components in the concentrations mentioned above. Standard deviations in accuracy were better than 2’70 over the same range. Concurrent with the determination of uranium, a less accurate estimation of vanadium is possible.
P
RELIMINARY examination of the absorbance curves of all the known components of the solutions under investigation showed that the most feasible method of determining uranium directly was to make use of the strong absorbance peak of the u ~ a n o u sion a t a wave length of 660 mfi. It mas necessary, therefore, to produce two solutions-one a reduced solution and the other an unreduced reference solutionfrom the sample in such a way that the components, other than uranium, remained unaltered in the two solutions. A large number of conditions of reduction and oxidation were studied to this end. One method for obtaining specific reduction of uranium was found. This was the copper-catalyzed reduction by hydroxylamine in hot 40 volume% phosphoric acid solutions, but the method was discarded because of the slow rate of reduction. I t \vas found that ferrous sulfate in hot 40 volume % phosphoric acid reduced uranium and vanadium rapidly and completely from the uranium(V1) and vanadium(1T’) t o the uranium( IS‘) and vanadium(II1) valence states, respectively, without affecting the other components of the solutions, and this was the procedure finally adopted. The reduced solutions xere
South Australia perfectly stable over long periods. The characteristics of the absorbance curves of hexavalent and tetravalent uranium and tetravalent and trivalent vanadium made it possible to apply a two-wave-length method for determining uranium in the presence of vanadium, using wave lengths of 660 and 700 mp. h satisfactory reference solution was obtained in the same strength of phosphoric acid by treating with hydrogen peroxide to oxidize all components fully, boiling to destroy peroxides of vanadium and titanium, then treat’ing mith sodium sulfite t o destroy any remaining peroxide and to ensure that no false absorbance readings were caused by oxygen evolution in the solution while in the spectrophotometer cells. -1final modification M-as to treat both solutions with hydrogen peroxide, the excess of which was then boiled off. In addition to oxidizing all components, this destroyed oxidizing agents such as chlorate and permanganate, which could interfere indirectly by consuming some of the ferrous sulfate reducing agent. A small addition of solid sodium sulfite was then made t o both solutions. Ferrous sulfate was added to one solution to form the reduced solution and an equivalent volume of sulfuric acid was added t o the reference solution, in order to preserve the same acidity in each solution. REAGESTS
Hydrogen peroxide, 30% 17,/v. Phosphoric acid, 90% w./w. Sulfuric acid, lA\7. Ferrous sulfate, analytical reagent grade, 0.5.U in 1N sulfuric acid. Sodium sulfite, analytical reagent grade, 4PP4R4TUS
-1Beckman Model DU spectrophotometer, with 1-em. Corex cells, was used for developmental work and the method was finally applied on a Hilger rvi3pel; ,spectrophotometer using 1cm. and 4-em. quartz cells. T o other special equipment x-as used. PROCEDURE
A sample volume of 20 ml. or lejs is selected where possible to yield between 10 and 50 mg. of uranium oxide. This amount of uranium gives absorbance readings between 0.1 and 0.6 in 4em. cells. Two samples are pipetted into beakers and diluted with water,
ANALYTICAL CHEMISTRY
818 where necessary, to 20-ml. volume. Twenty milliliters of phosphoric acid and 0.5 ml. of hydrogen peroxide (30% w./v.) are added to each, and boiled for approximately 10 minutes or until all evolution of oxygen ceases. The solutions are partly cooled and approximately 0.1 gram of sodium sulfite is added to each, Then to one solution 10 ml. of 1N sulfuric acid is added, and to the other, 10 ml. of 0.5M ferrous sulfate in 1N sulfuric acid. The solutions are boiled for approximately 5 minutes to ensure complete reduction of the uranium and vanadium. After cooling, the solutions are made to volume in 50-ml. volumetric flasks, and after filtering through fine paper, are ready for measurement. The absorbance of the reduced (ferrous sulfate) solution is read against the unreduced solution as a reference, a t wave lengths of 660 and 700 mp, using 4-em. cells. Where higher grade solutions are assayed, an aliquot containing 40 to 200 mg. of uranium oxide is selected and 1-em. cells are used for the absorbance measurement. The concentrations of uranium and vanadium are calculated using equations which are the solutions of simultaneous equations involving the standard absorptivities of the two components a t each wave length. A, - ay2 A1 c, = ayl.ax2 - ab2 axi a,ul
(1)
where
C
= concentration of particular component
2, y =
a
components
= absorptivity of component a t particular wave length
1, 2 = wave lengths used A = measured absorbance a t particular wave length
The absorbance due to the reagent blank is incorporated into the equation as a constant, as in the following example. Using 4-em. cells in a Hilger Uvispek spectrophotometer:
Reagent Blank Uranium oxide
(U80S)
Vanadium oxide
;VPod
Absorptivity a t 660 &hi 0.028 0.570 per gram per liter in final solution -0.148 per gram per liter in final solution
Absorptivity a t 700 M p 0.048 0 , 0 0 8 per gram per liter in final solution -0.262 per gram per liter in final solution
From these standard absorptivities, subdtuting in Equation 1:
U308 = 1.77 A660- 1.00
- 0.002 gram per liter in final solution
(2)
+ 0.185 gram per liter in final solution
(3)
V20s= 0.054 A663- 3.85 Ai00
DEVELOPMENT OF METHOD
Reagents Investigated. I n an endeavor to obtain selective reduction of the uranyl ion to the uranous state, or alternatively, complete reduction of the solution followed by selective oxidation of the components other than uranium, the effects of the following reagent,s on the known components were examined in different concentrations of hydrochloric, sulfuric, and phosphoric acids.
ZISC A N D ZINC~ ~ X ~ L G A MI n. 0.1-V sulfuric acid reduction was slow, while in 1N sulfuric acid, or stronger, titanium was reduced as well as iron and uranium and could not be reoxidized reliably. Reduced titanium absorbed strongly in the region of 660 mp, the wave length of the peak absorbance of uranium(1V). STPNNOUS CHLORIDE.Titanium JTas reduced in addition to uranium. STAXNOUS SULFATE.Uranium was not reduced in the absence of chloride. HYDROXYLAMINE HYDROCHLORIDE. In sulfuric, hydrochloric, and phosphoric acid solutions greater than 5 volume %, boiling with hydroxylamine was found to oxidize iron( 11),titanium(III), vanadium(II1) [to vanadium(IV)] and uranium(1V). Hydroxylamine reduced vanadate and chromate and other strong oxidants, and could be used to produce the valence states uranium(VI), vanadium( IV), iron(III), titanium( IV), and chromium(II1). I n the presence of copper, uranium(V1) n-as reduced to uranium( IV) by prolonged boiling with hydroxylamine in 40 volume % phosphoric acid, while no other component shon-ed any alteratCo6 of Galence. FERROUS SULFATE.I n strong phosphoric acid solutions ferrous sulfate reduced uranium(V1) and vanadium(1V) to uranium( I V )
and vanadium( 111),respectively, while the valences of chromium (111) and titanium(1V) were not altered. ~~VMONIUM PERSULFATE. Reduced solutions were slo~vlyoxidized cold and more quickly when heated. The reagent was not selective, and also oxidized manganese to permanganate. POTASSIUM CHLORATE.Vanadate was produced and all other components fully oxidized. SODIUMNITRATEA X D SITRITE. Vanadium was partly oxidized to vanadate. HYDROGEN PEROXIDE. I n strong phosphoric acid the valence states uranium(VI), iron(III), and chromium( 111)were produced together with the peroxides of titanium and vanadium. On boiling, the peroxides of titanium and vanadium were destroyed, leaving titaniumilv) and vanadium in a valence state approximating to vanadium(1V). SODIUNSULFITE. I n phosphoric acid solutions, no effect was found on the valences iron(II), iron( 111), uranium(IV), uraniuni(VI), vanadium(III), vanadium(IV), titanium(IV), but sulfite was found useful for destroying any remaining free oxygen in the solution after boiling off hydrogen peroxide. Vanadium was also brought to the pure vanadium(1V) state by this treatment. SoDImf THIOSULFATE. -4precipitate of sulfur formed. Investigations into the absorbance of uranium in phosphoric acid solutions confirmed the observation reported by Rodden (1) that the absorbance was enhanced in strong acid, and that a t approximately 50 volume % phosphoric acid the absorbance was not affected by small variations of acid strength. For this reason and also because selective reduction of uranium and vanadium by ferrous sulfate was obtainable in this solution, a medium of 40 volume % phosphoric acid was finally chosen. Procedures Tested. Three procedures for obtaining reduced and reference solutions in 40 volume yophosphoric acid were fully examined. The first, using the specific reduction of uranium by hydroxylamine catalyzed by copper, in boiling 40 volume yo phosphoric acid solution, gave erratic results due to the slow rate of reduction. The reference solution in this procedure consisted of all the same reagents in a cold solution. The second procedure used ferrous sulfate for the reduced eolution and boiling with hydroxylamine for the reference solution. Copper had to be absent; otherwise uranium was reduced in the reference solution. In the third procedure, which was finally adopted, both assay and reference solutions were treated with hydrogen peroxide followed by sulfite. Reduction was then effected in the assay solution by ferrous sulfate. This method, as described above, is designed to have the most general application possible for all uranium solutions. Optimum Conditions for Reduction. Tables I and I1 summarize the effects of varied acid strength and varied concentra-
Table I. Effect of Phosphoric Acid Strength on Reduction of Uranium by Ferrous Sulfate (10 ml. 0.5M ferrous sulfate per 50 ml. of final so!ution used for reduction. Final concentration of uranium, 4 grams per liter of UaOs)
Concn. of Phosphoric Acid, Vol. %
n
Absorbance a t 660 Mp 0.007
J
10
15 20 30 40
Table 11. Effect of Ferrous Sulfate Concentration on Reduction of Uranium in Phosphoric Acid Solution (Final strength of phosphoric acid, 40 yol.%. Final concentration of uranium 4 gram per liter of UaOp) Ferrous Sulfate Concn. in Final Solution, Absorbance a t 660 M 0.01 0.02 0.05 0.1
0.199 0.385 0.568 0.568
879
V O L U M E 2 7 , N O . 6, J U N E 1 9 5 5
Copper, nickel, cobalt, bismuth, manganese, chromium, and molybdenum a t 0.5 gram per liter of the metal ion. Titanium dioxide and combined rare earth oxides a t 5 grams per liter. Ferric oside a t 10 grams per liter.
400
450
500
WAVE
Figure 1.
550
LENGTH
600
650
700
m,u
Of these elements, only molybdenum showed any change of valence state under the assay procedure. Although the peak absorbance for molybdenum was found a t 440 mp, some absorbance was found a t 660 and 700 mp, hence molybdenum would interfere if present. A concentration of molybdenum of 1 gram per liter of the metal ion cauEed a positive error equal to 0.2 gram per liter of uranium oxide (U308). In the sulfate solutions used for these investigations no molybdenum was detected. Copper interfered indirectly, when present together with chloride, by forming a precipitate of cuprous chloride on the addition of ferrous sulfate to the 40 volume % phosphoric acid solution. Filtering off this precipitate was not feasible because copper was thus removed from the reduced solution while an equivalent amount remained in the reference solution. This caused a negative error to the absorbance readings. If more than traces of copper and chloride were present together, the interference was overcome by removing copper from the sample by a preliminary treatment with aluminum powder and filtering off the precipitated copper.
Absorbance curves of uranium and vanadium in 40% v./v. phosphoric acid Beckman Model DU spectrophotometer: 1-cm cells Reference solutions of appropriate reagents 1. U(VI),0 . 8 gram per liter of UaOs 2. U(IV) 0 8 gram per liter of UsOs 3. V(IV): 0 : s gram per liter of V2Oa 4. V(III), 0 . 8 gram per liter of VZOS
tion of ferrous iron, respectively. Final concentrations of 40 volume % phosphoric acid and 0.1M ferrous sulfate were chosen to allow a reasonable margin in excess of actual requirements. These tests were carried out on a higher concentration of uranium than was normally present in samples. Absorbances were read on a Beckman Model DU spectrophotometer, in 1-cm. cells against a reference of the appropriate reagents. Selection of Wave Lengths. Absorbance curves of hexavalent and tetravalent uranium and tetravalent and trivalent vanadium solutions in 40 volume yophosphoric acid are shown in Figure 1. The curves developed experimentally for uranium were found to be in substantial agreement with those illustrated by Rodden (I). Absorbances were measured in each case against the appropriate reference solution of reagents. Figure 2 s h o w the absorbance “difference” curves obtained by measuring reduced solutions against unreduced solutions according to the assay procedure described. The absorbance curve due to ferrous sulfate in the concentration used in the assay is also shown in Figure 2, and has been deducted from the measured absorbances for uranium and vanadium to obtain the “difference” curves. From these curves the trvo wave lzngths of 660 and 700 mp were chosen. A possible second wave length of approximately 430 nip was discarded because interferences were found at wave lengths less than 500 mp. The absorptivities of uranium and vanadium a t each wave length were determined using uranium solutions prepared from XY-ST “standard black oxide” and vanadium solutions prepared from sodium vanadate of known purity. It was found that each component obeys Beer’s law a t each wave length, as shown in Figures 3 and 4. Blank solutions of reagents were prepared by the standard procedure to determine the absorbance due to ferrous sulfate. INTERFERENCES
Absorbance curves were developed, using the standard assay procedure, for the following elements a t concentrations in the final phosphoric acid solution as indicated:
__
400
450
500
WAVE
550
600
LENGTH
mp
S O
700
Figure 2. Absorbance “difference” curves of uranium and vanadium, together with absorbance curve of reagent blank (ferrous sulfate) in 40% v./v. phosphoric acid against a reference of the same strength acid Beckman Model D U spectrophotometer, 1-em. cells 1. Uranium, 0 8 gram per liter of UaOs 2 . Vanadium, 0.8 gram per liter of VZOS 3. Reagent blank (FeSOd)
Some interference was experienced when the samples were relatively rich in titanium. A precipitate of titanium phosphate formed in one or both solutions during boiling. This interference was overcome by taking a smaller sample such that all the titanium remained in solution. The removal of bulky precipitates by filtering was considered time-wasting and likely to cause loss of uranium by coprecipitation. The final filtration step in the procedure served merely to clarify the solutions before reading the absorbances. Xitrate interfered by forming in the reduced solution a colored compound with a high absorbance. Nitrate solutions were converted to sulfates by a preliminary fuming of the pipetted samples with 1 to 2 ml. of excess sulfuric acid.
880
ANALYTICAL CHEMISTRY
Table 111. Precision and Accuracy of Method on a Hilger Uvispek Spectrophotometer, Using 4-Cm. Cells Uranium Oxide (UaOs), Standard Deviationa, Replicate assays 0.03 0.04 0.03 0.04
Solution Barren
Spiked barren No. 1
KO. 2
0.035
0.23 0.20 0.23 0.23
0.22
0.205
7
11
0.51 0.51 0.51
0.515
0.505
2
2
1,005
1.005
0 6
0.6
1.98
2.005
0.9
1.7
4.94
5,005
1.1
1.7
1 01 1.01 1.00
No. 4
2.00 1.96 1.99 1 97 5 03 4.88 4.93 4.95 4.89 4.95
KO. 5
%
Known content 0 005
0.53 1 .oo
KO. 3
Gram/Liter Mean assay€
Precision 16
bccuracy
..
-
---700 600
05
1.0
1.5 “205
Figure 4. PRECISION AND ACCURACY
I
I
I
I
I
3 0
Vanadium standard curves
Hilger Uvispek spectrophotometer; 4-cm. cells
Table 111 indicates the precision and accuracy obtainable on B series of spiked “barren” sulfate solutions which were obtained by adding known quantities of uranium t o a solution containing normal concentrations of all components other than uranium. A fluorimetric assay of the “barren” solution showed the uranium concentration to be 0.005 gram per liter of uranium oxide. Table I V shows the precision obtained on a series of assay samples in which the concentrations of all components varied I
,2,5
20
9/[
Table IV. Precision of Method on a Hilger Uvispek Spectrophotometer Using 4-Cm. Cells Uranium Oxide (UsOs), Standard Assay Sample KO. 1
Gram/Liter Replicate Mean assay assay 2.27
Deviation in Precision,
%
1.1
I No. 2
5-0.3
KO.4
1.81 1.82 1.80 1.80 1.81 1.77 2.10 2.07 2.06 2.05 2 06 2.07 1.67 1.68 1 io 1.66 1.66 1.64
1 80
1 0
2.01
0.8
1.67
1.2
within the normal limits experienced in the solutions used for these investigations. -4standard deviation in precision of 1.1% was obtained overall and a standard deviation in accuracy better than 2% for concentrations greater than 0.5 gram per liter of uranium oxide. ACKh-OWLEDGMENT
0
I
I
1
I
I
I
0.2
0.4
0.6
0.8
1.0
1.2
“3 0 8
Figure 3.
9/!
Uranium standard curves
Hilger Uvispek spectrophotometer; 4-cm. cells
1.4
The authors are indebted to T. W. Dalwood, superintendent of laboratories, Department of Mines, and to S. B. Dickinson, director of mines, South Australia, for permission to publish this work. LITERATURE CITED
(1) Rodden, C. J., “Analytical Chemistry of the Manhattan Project,” ~ p 78-80, . McGraw-Hill, New York, 1950. RECEIVED f o r review Rlay 15, 1954. Accepted January 10, 1955.
