V O L U M E 2 4 , NO. 7, J U L Y 1 9 5 2 The test has also been successfully applied to a variety of apples, peaches, grapes, and beans. ANALYTICAL PROCEDURE
The recommended procedure for applying this test to the determination of SR-406 residues on foliage or fruit involves the following stpps; 1. Residue Removal. For best results the size of sample (fruit or foliage) should be such as to yield a total SR-406 residue of a t least 0.1 mg.-for example, a 100- to 2OO-gram sample is satisfactory for estimated residues in the range of 1 to 10 p.p.m. The sample is washed with two 150-ml. portions of warm chloroform, the chloroform solutions are filtered, and the filter paper is rinsed with an additional 10 ml. of solvent. The combined filtrates are evaporated to a volume of 3 ml., which is transferred to a graduated cylinder. The container used for evaporation ip washed with sufficient fresh chloroform to give a total volume of 5 ml. For higher residue values this final volume may be correspondingly increased, 2. Resorcinol Reaction, One-gram samples of resorcinol are placed in each of two 2 x 15 cm. test tubes. A 1-ml. sample of the chloroform solution from Step 1 (for residues of less than 5
1175 p.p,m. on a 100-gram sample 2 ml. of chloroform should be used) is pipetted onto the resorcinol in one of the test tubes. To ensure contact of all of the SR-406 with resorcinol, the chloroform solution should not be permitted to run down the sides of the test tube. Both test tubes are inserted to a de t h of about 7 cm. for 20 minutes in a constant temperature oil %ath maintained at 135" to 138" C. The test tubes are removed and 5 ml. of absolute ethyl alcohol added. After the reactants have completely dissolved, the alcohol solution is transferred to a volumetric flask. The test tubes are then washed with additional quantities of alcohol to remove all the reactants. The total volume of alcohol is adjusted t o a known volume sueh that the per cent transmittance is greater than 20 but less than 85. 3. Spectrophotometric Analysis. The spectrophotometric determination may be made with a Beckman spectrophotometer using a wave length of 425 mp. Portions of the alcohol solutions from Step 2 are placed in each of two matched 1-cm. quartz cells and the instrument is adjusted to 100% transmittance with the blank sample. The reading obtained on the unknown sample may be translated into parts per million of SR-406 by referring to a semilog graph as in Figure 2 prepared from known concentrations of SR-406. RECEIYED for review August 24, 1951.
Accepted April 26, 1952.
Volumetric Determination of Milligram Quantities of Uranium CLAUDE W. SILL1 AND HEBER E. PETERSON Bureau of Mines, Salt Lake Experiment Station, Salt Lake City, Utah Because most of the uranium mined in the United States comes from low-grade ore deposits, a method was needed that would accurately determine both small and large amounts of uranium in ores and metallurgical products in a routine manner. Generally, methods have been developed for the determination of either very large or very small quantities of uranium. The accuracy attainable with the method described ranges from about 2% with 1 mg. of uranium to within =kO.ly~ with 20 mg. or
A
LTHOUGH the Jones reductor is used extensively in the determination of uranium, the conditions under which
errors nil1 or will not be produced are rather vaguely known and information regarding them varies with the source. This is especially true in the volumetric determination of small quantitiee of uranium in ores. The present paper s h o w under what conditions and to what e\tent errors are produced, and recommends a combination of procedures that has proved most satisfactory for ore analysis in over 5 years of practical application in this laboratory. The methods described differ in one or more of the following respects from similar procedures already in the literature ( 1 1 ) : A lead. reductor is used in place of the conventional Jones reductor; sufficiently large volumes of solution are used to accommodate the salt3 from relatively large samples: ferroin is used as indicator because of its sharper color change, more reproducible blanks, and independence of volume in comparison with diphenylamine sulfonic acid; the use of ferrir sulfate or phosphoric acid allows the titration to be made with sulfatoceric acid at room temperature in an essentially colorless solution; and no encumbrances such as inert atmosphere, elevated temperature, special microapparatus, or techniques or expensive equipment are required. 1 Present address. U. S. Atomic Energy Commission, P. 0. Box 1221. Idaho Falls, Idaho.
more. Separations are given for essentially all interfering elements. The method is well adapted to routine application to large numbers of samples and no expensive or unusual equipment is required. This method is designed to cover the intermediate range w-ith the greatest accuracy, but is also applicable to larger quantities. The analyst is provided with an accurate procedure to be followed, and, of equal importance, with information as to the sources of error and how they are produced. Standard solutions of uranium were prepared by dissolving
Uno8 (99.96%) in boiling perchloric acid and by diluting a stock solution of uranvl sulfate that had been standardized gravimetrically according to the method of Lundell and Knowles (9). Solutions of sulfatoceric acid were prepared according to the directions of Smith (15) and were standardized against arsenic trioxide in the presence of osmic acid according to the directions of Gleu (7). The cerium solutions were stored in 18-liter carboys fitted with siphon arrangements, so that the bottles did not need to be opened, and were protected from light by storing in a dark cupboard. Under these conditions, even the 0.01 N solution changed by less than 1 part per thousand in about 6 months. A 25-ml. buret was used in all tests to facilitate accurate reading, except where 50 ml. was used. TITRATION PROCEDURE USING JONES REDUCTOR
The solution to be titrated for uranium should have a volume of about 75 ml. and contain 6 ml. of concentrated sulfuric acid. h f t e r being cooled to 15" t o 20" C., the solution is reduced by passing it through an air-free Jones reductor having a bore of 12 mm. and a 30-cm. column of 20- to 30-mesh amalgamated zinc (1%mercury). The flask and reductor are then Jvashed with 50 ml. of cold 2 S sulfuric acid followed by 50 ml. of cold water, added in small portions to obtain maximum washing efficiency. Each solution should be poured down the side of the reductor tube to prevent the formation of air bubbles that might be drawn into the zinc column. It is extremely important to keep the column free of air, to prevent formation of significant amounts of hydrogen peroxide. The solution is then aerated by passing a
ANALYTICAL CHEMISTRY
1176
fairly rapid stream of clean air through it for 5 minutes. Two milliliters of 20% ferric alum in 5% sulfuric acid or 1 ml. of 85% phosphoric acid and 1.0 nil. of 0,001 Af o-phenanthroline ferrous sulfate are added, and the solution is titrated with 0.01 A: sulfatoceric acid until the indicator color is sharply discharged. About 30 seconds should be a l l o ~ e dfor all reactions t o come t o equilibrium before the last 0.02-ml. portion is added. The end point is easily discernible on addition of 0.02 ml. of 0.01 N sulfatoceric acid in 250-ml. volume, and will be stable almost indefinitely if the solution is not exposed t o direct sky light. Unfortunately, essentially no warning is given of its approach, and the entire titration must be conducted very slowly or part of t h e solution held in reserve t o prevent the extremely sharp end point from being overrun. After the present work had been completed, however, it was found more convenient t o titrate the Eolution rapidly u ithout addition of ferric alum until the indicator color has been discharged. At this point the addition of ferric alum will produce an immediate return of t h e indicator color, after which the titration can be finished by slower addition. The same procedure can be used equally well with titrations involving 0.1 N ohidant (see “Effect of Phosphoric Acid”). As is shown by tests 1 through 12 of Table I, the use of either ferric alum or phosphoric acid yields identical results, which are, however, about 0.1 t o 0.27, low. This error is shown below t o have been produced during titration. The procedure described above for use with 0.01 K solutions of sulfatoceric acid can also be used x i t h 0.025 S and 0.1 S solutions. I n the latter case, an aeration time of 15 minutes and 2 drops of 0.025 M ferroin should be used. The ferric alum method is used in all subsequent tests Kith the Jones reductor, except where noted.
Table I.
Titration of Reduced Uranium Solutions w-ith Sulfatoceric Acid (From Jones reductor) 0 004842
Test KO.
Method
0 0 1 1
5 5
10 10 25
25 50 50
IO 11
12 13 14 13 16 17
Fe Fe Fe Te Fe
18
Fe
N
UO~SOI, AIL 30 30 00 00 00 00 00 00 00 00 00 00
0 010472 .V
HaCe(SO4)p, ,111.
n
27 0.26 0 90 0 89 4.49 4 50 8 94 8 94 22.40 22.39 44.81 44.79
0.02494 N
0.02698 N
50.00 25.00 10.00 5.00 1.oo
46.23 23.11 9,21 4.62 0.93
0.09840 N
0.10305 .?‘
50.00
47.75
Theoretical, 111.
n
27
0 27 0 90 0 90 4 49 4 49 8 97 8.97 22 43 22 43 44 85 44 85 46 22 23.11 9 24 4 62 0.92 47.74
AIR OXIDATION OF TETRAVALENT URASIUM
Although uranous sulfate solutions have been reported by several investigators t o be very stable toward air, the uranium concentrations employed have generally been considerably higher than those in the present method. As reaction rates are obviously responsible in large measure for this stability, considerable errors may be introduced when small quantities of uranium a t high dilution are determined by a method of comparable sensitivity. Therefore, t o determine under what conditions oxidation will take place t o any measurable extent as applied t o milligram quantities of uranium, a rapid stream of filtered air from a compressed-air line was bubbled through solutions of uranium covering the range of the present method. The data are shown in Table 11. The variations in temperature and acidity were kept rather small, be-
Table 11. Effect of Time, Temperature, and Acidity on Air Oxidation of Uranous Sulfate Solutions Test
No.
0 009842 S
UOzSOr,
M1.
Aeration Time, Min.
Aeration Temp.,
c.
1 2 3
50.00 50.00 50.00 50.00 1.00 1.00 1.00 1.00
15 60 180 2 15 60 180
14 14 14 14 14 14 14 14
9 10 11 12
50.00 50.00 1.00 1.00
5 6; 60
26 26 26 26
13 14 15 16
50.00 50 00 1.00 1.00
5 60
14 14 14 14
60
0 010972
N TheoHzSO4 Acidity, H4Ce(SOr)r, retical, .v Rfl. MI. 44.80 44.77 44.68 44.40 0 89 0 90 0 89 0.90
44.85 44.85 44.85 44.85 0.90 0.90 0.90 0.90
2
2
44.80 44.49 0.89 0 89
44.85 44.85 0 90 0.90
0.5 0.5 0.5 0.5
44 77 44.32 0.90 0.88
44.85 44.86 0.90 0.90
2
2
cause extreme variations are known to produce serious errors and it was desirable to know the extent of errors produced by chance variations during practical application. Of special importance to the present method is the fact that the st,ability of tetravalent uranium solutions of the order of 10-6 iLf is not noticeably different from that of the stronger ones used under the same conditions. EFFECT O F PHOSPHORIC ACID
Birnbaum and Edmonds ( 1 ) have shown t,hat phosphoric acid exerts a catalytic effect on t,he cerimetric titration of uranium with ferroin as indicator. The following observations indicate that the catalysis is actually more concerned with the reaction between tetritvalent uranium and the oxidized form of the inclicator (ferriin) than with the reaction between uranium and cerium, as is generally supposed. Sulfatoceric acid can be added to dilute sulfuric acid solutions of uranous sulfate a t the full speed of delivery of a buret without producing a yellow color of excess tetravalent cerium. The addition of excess uranous sulfate to a solution of dilute sulfuric acid containing ferriin does not produce the characteristic pink color of ferroin. Furthernore, the subse uent addition of phosphoric acid still does not cause reduction o? the indicator its colored form. However, if a few drops of 0.01 sulfatoceric acid are now added, the color of the reduced form of the indicator is produced in full intensity. If a 2 N solution of sulfuric acid containing 1 ml. of 0.01 S uranous sulfate is titrated with 0.01 N sulfatoceric acid, the indicator color is discharged completely on addition of the first 3 or 4 drops of oxidant. If 25 ml. of uranous sulfate is used under the same conditions, the indicator color does not lighten appreciably until the titration is about half completed; from this point on, the indicator color becomes increasingly lighter, until it disappears completely about 1 or 2 ml. before the end point. I n either instance, the addition of phosphoric acid \vi11 cause the indicator color to return only after the addition of sulfatoceric acid is resumed. The color is not discharged again unt,il oxidation of the uranium is complete. I n contrast, the addition of ferric sulfate will cause an immediate return of the indicator color in full intensity, since the reduction of ferriin by ferrous iron is rapid and requires no catalysis. Indeed, the only function of the ferric sul.fat,e used in the present method is to provide sufficient ferrous iron to keep the indicator in its colored reduced form.
40
Apparently the cerium-uranium reaction takes place rapidly; on the other hand, ferriin is reduced by tetravalent uranium a t a negligible rate. Consequently, vihen the concentration of tetravalent uranium has been reduced sufficiently by titration, the small quantity of indicator present is oxidized gradually along with the uranium, and a premature discharge of color result’s. Phosphoric acid catalyzes the reduction of ferriin by tetravalent uranium during the oxidation of the uranium t o the hexavalent state by cerium, thus keeping the indicator in its colored form until oxidation of the uranium is complete. I t is suggested that the
V O L U M E 2 4 , NO. 7, J U L Y 1 9 5 2
1177
method a t 14" C. (tests 2 and 4). At 26" C., the e ~ r o rhas incatalytic effect of phosphoric acid can be explained by assuming creased to 0.3 and 0.6%, respectively (tests 5 and 6). The slightly that it stabilizes a n intermediate oxidation product of tetravalent low results obtained in Tables I and I1 (ca. -0.15%) are caused uranium (possibly U' ) formed during the titrat,ion, and that the by the normal method of titration employed. The presence of resuking complex has a high rate of reaction with the oxidized phosphoric acid during aeration produces no noticeable increase indicator. Fluoride and high concentrations of sulfate also in the rate of air oxidation of tetravalent uranium (test 7 ) , except catalyze the reduction. (luring a slow titration (teqts 1 and 6). As phosphoric acid catalyzes the reaction between tetravalent uranium and ferriin, it seemed entirely possilile that the same catalyst would be effective with other strong oxidants, notab]!. INDUCED OXID.4TION O F TETRAVALENT URAN1U.M oxygen. The air oxidation of tetravalent uranium is indeed Because vanadium is such a frequent component of uranium catalyzed b y phosphoric acid t o a s~iiallextent, and, as with the ores, it, seemed desirable t o determine the effect of small quantitier uranium-ferriin reaction, only while the uranium-cerium reaction is taking place-that is, during the titration. .Is shown by t h ~ of this metal on the uranium determinat'ion. Inasmuch as vanadiuni is reduced to the divalent state by a Jones reductor and data of Table 111. exactly theoretical results arc ol)tained if thc is not fully reosidized by air, its presence should lead to high retitration is carried out almost instantaneously. If the titration is sults for uranium. \Vhen the quantity of uranium involved is carried out slowly under conditions approaching violent agitation. rmall, this is the case, but, n-it'h relatively large quantities of the results are slight1)- Ion-. This error amounts to about 0.2y0 uranium, the results surprisingly are low. K h e n a solution confor the iron nicthod and to about 0.4cC for the phosphoric acid taining uranium equivalent to 25.00 ml. of 0.00997 .\-sulfatoceric acid was titrated in the presence of 0.90 nig. of vanadium, a value of only 23.7 nil. was obtained. Such a IOK value i!: even Table 111. Effect of Titration Speed in Presence and more remarkable n-hen it, is remembered that part of the eulfato.4bsence of Phosphoric Acid ceric acid was consumed by the oxidation of the reduced vana(Theoretical titrations. 44 85 and 22 43 nil.) dium present. Since the olive-green color of trivalent uraniumn-as Titrapresent, indicating complet'e reduction, air oxidation of the divation, Test lent and trivalent vanadium apparently induces thc, air oxidation Conditions of Test hll. KO. of tetravalent uranium. Thus, the results can be either high or 1 -1solution containing 50.00 ml. of 0.009842 S 44.8;' low, depending on the relative amounts of uranium and vanauranyl sulfate was reduced and aerated for 5 minutes at 14" C., 2 ml. of ferric sulfate soludium-that is, the relative effect, of the reaction of reduced forms tion and 1.0 ml. of 0.001 -11ferroin were added, of vanadium with oxidant t o give high results, as against the and the solution was titrated in the shortest lower results caused by the induced oxidation of uranium by air. possible time with 0.010972 S sulfatoceric acid. T o det,erminewhether or not t,his effect is peculiar to vanadium, This was accomplished by drawing off 44.5 ml. of the wrium solution into a beaker and then and to copper and molybdenum as shown by others ( l a ) , or adding it all at once to the uranium solution. whether i t can be produced by other substances reducible in a The titration m-as finished by careful addition Jones reductor t o forms unstable in air, solutions of known uranium > content containing known quantities of other elements were t,i44 77 Test 1 was repeated exactly, except that the entire titration was made by adding a single trated. I n the case of metals deposited in the reductor column, drop of sulfatoceric acid at a time while a additional tests were made in which the metal sulfate was placed vigorous stream of air was buhhled through the in the suction flask and the uranium solution passed through the solution rt:ductor directly into the metal .solution. I n this way, reduction :3 Test 1 was repeated-Le., fast addition44 S i to metal would still be brought about by trivalent uranium, hut using 1 ml. of 85% phosphoric acid in place of the quantity of metal causing a n y effect would be known. Obvithe ferric sulfate solution ously, this is not PO n-hen the metal was mixed with the uranium solution before it was passed through the reductor. 4 44.69 Test 2 was repeated-Le., slow additioiiusing 1 ml. of 85% phosphoric acid in place of JVhen the metal solution was passed through the reductor to the ferric sulfate solution dtltermine how completely the metal \vas retained in the column, a new reductor was always used to avoid contamination from 5 Test 2 was repeated at 26' C. (iron method, 44 71 previous tests. The quantities of each metal used in the teste slow addition) were kept rather small to determine Tvhether these small amounts 6 Test 4 was repeated at 26' C. (phosphoric 44 60 ivould interfere in the present sensitive method. Such data are acid method, slow addition) obviously useful in developing separations of the necessary tlfficiency and in understanding the manner in which the errors are l Test 3 was repeated adding the phosphoric 44 86 acid before reduction and aeration (phosphoric produced. acid method, fast addition) The data of Table IV show very clearly the tn-o opposing errors introduced b y small amounts of various contaminants on the S 1 ml. of 85% phosphoric acid was added to 44 82 volumetric determination of uranium-one giving low results the uranium solution before reduction and aeration, 2 ml. of ferric sulfate solution were oiving to induced air oxidation of tetravalent, uranium and the added, and the titration was made in a normal other giving high iesults owing to consumption of the titrating manner although rapidly solution. With ions that can exist in several valences, such as vanadium, molybdenum, and tungsten, either high or low results 9 Test 1 was repeated, titration being made in 44 80 a normal manner (test 11, Table I, 44.81 ml.) ran be obtained, depending on the metal-uranium ratio. Generally, the small quantity of metals not removed in the usual macro10 Test 1 was repeated exactly 44 85 separations will cause high results on small quant.ities of uranium 11 and low results on larger quantities. I n the case of titanium, all Test 1 was repeated using 25.00 ml. of the 22 43 uranium solution (22.2 ml. added all a t once) results will be Ion-, owing t o induced oxidation of uranium, because the titanous ion is more or less completely oxidized by air 12 Test 1 was repeated using 26.00 ml. of the 22 39 leaving nothing capable of consuming oxidant. This is also the uranium solution, titration being made in a predominant effect with copper, niobium (columbium), and chronormal manner (test 9, Table I, 22.40 ml.) mium. Silver and mercury are not appreciably oxidized by air
-
-
1178
ANALYTICAL CHEMISTRY Table IV. 0.009973
,v
Effect of Various Metals on Determination of Uranium
0.00!954 HaCeikOa)a, Variations in Experimental hfl. Conditions and Remarksa E n d points very unstable 24.7 1.4 23.7 1.2
0.18 1' 0.18V 0.90 V 0.90 v
UOzSOa, hf1. 25.00 1.00 25.00 1.00
0.17 T i 0.17Ti 0.84Ti 0.84Ti
25.00 1.00 25.00 1.00
24.80 0.74 24.28 0.52
E n d points sharp and stable
0.94 hlo 0.94 M o
25.00 1.00
17.77 1.49
E n d points slightly slow
10
11 12 13
0.52 F e 0.52 F e 0.52 Fe
25,oo 1.00
25.98 1.94 0.93
E n d points sharp i n d stable
...
14 15
2.00Cu 0.20cu
25.00 25.00
7.34 21 87
Copper sulfate placed in suction flask. Reddish solution of finely divided copper produced b y U + - + cleared on aeration. E n d points sharp and stable
Test No. 1 2 3 4
5 6 7 8
9
Metal, Mg.
16
2.00Cu
25.00
24.80
17
2.00Cu
25.00
24.61
Copper, sulfate added after aeration
wool a t bottom distinctly pink
18 19
20 21
... I
.
.
25.0cu 25.0Cu
24.99 1.00
Uranium solution put through reductor contaminated b y copper from test 17
25.00 1.00
25.01 1.01
Copper plated out uniformly on amalgamated zinc, reductor repacked, and washed thoroughly with acid. Reductor column dark gray in color
22 23
1.00 H g 1.00 H g
25.00 1 .oo
25.03 1.01
E n d points sharp and stable
24
1.00 H g
25.00
25.55
Mercuric sulfate placed in suction flask. ilfter addition of U +, solution became turbid. Turbidity cleared very little on aeration b u t cleared immediately on addition of ferric sulfate. E n d point sharp and stable
25 26
4.0 As203 4.0 A S 2 0 3
25.00 1 .oo
25.1 1.1
28 29
hletal, Mg.
...
1 00 Ag 5 00Ag
0 009973 N Uozso4, Ml. 25.00
25.00 25.00
0.009954 AHaCe(SOc)a,
MI. 25.01
25.70 28.25
+
E n d points not very sharp and indicator color returned. After addition of osmic acid, titration on test 25 continued t o 27.3 ml. with indicator color still returning
but are oxidized by ferric ion; therefore, little or no induced oxidation will take place, and the results are alxays high if any of the metal is carried out of the reductor column. On the other hand, the error caused by iron is always positive, but, unlike any of the others, is an exact stoichiometric function of the quantity present. Small quantities of arsenic produce little error, as the reduced forms of this element are not oxidized appreciablv a t room temperature by oxygen, ferric iron, or even ceric cerium in the absence of a specific catalyst such as osmic acid. Such error as may be present, however, is ahaays positive. The effect of tin also is rather small. I n sulfuric acid solution, tetravalent tin is reduced very slo~rlyin the reductor, and the small amount of divalent tin produced is oxidized very slowly by air. Moreover, owing to the slowness of the reaction of divalent tin with ferriin in the absence of chlorides or a specific catalyst such as osmic acid, the indicator color is discharged shortly after completion of the uranium oxidation. The interference of nickel in procedures involving the Jones reductor has been pointed out ( 5 ) . Cobalt appears t o be deposited in the reductor even faster than nickel. I n the present method, one solution containing 25 mg. of either nickel or cobalt may be put through a new reductor without producing error.
Variations in Experimental Conditions and RemarksQ Same reductor as in tests 25 and 26 Silver sulfate placed in suction flask. On addition of U solution became turbid. Turbidity did not clear appreciably on aeration b u t cleared immediately and completely on addition of ferric sulfate. E n d points sharp and stable + +
+
0.009842S 0.010972 2' 25.00 22.38 < 0.1% error E n d points - 5 4% error sharp and 5.00 4.25 1.00 -18.0% error stable. No 0.74 25.00 21.94 - 2.1% error interference 1 .oo -29.0% error from color 0.64 \ of C r + + -
i
30 31 32 33 34
1.00Cr 1.00Cr 1.00 Cr 5.00 Cr 5 0 0 Cr
35 36
1.00 w l.0OW
25.00 1.00
22.13 1 17
End points sharp and stable
37 38
1.00N b 1.00Nb
25.00 1.00
21 87 0.26
End points sharp, b u t color returned very slightly o n test 37
39 40
l.OOSn++ l.OOSn++
25.00 1.00
22.25 1.03
Tin added a s SnSOa after reduction but prior t o aeration. Indicator color rather sharply discharged a t end point b u t returns slowly. After addition of osmic acid, titrations were continued t o 23.05 and 1.62 ml., respectively, after which end points were sharp and stable
41
l.OOSn++
25.00
23.54
Osmic acid and tin (as SnSOa) added prior t o aeration. End point sharp and stable. 1 mg. of tin (as SnSOa solution) required 1.21 ml. when titrated directly
42 43
5.00SnIr 5 . 0 0 Sn'V
25.00 1 .oo
22 31 0.90
Tetravalent tin added t o uranium solution before reduction. E n d point on test 42 sharp but indicator color returned slowly. After addition of osmic acid, titration continued to 22.70 ml., after which end point was sharp and stable
became
25.00 1.00
+
Test No. 27
a On tests 30 through 43 theoretical values are 0.90 4.49 and 22.43 ml., respectively, for 1.00, 5.00'and 25.00 ml. of uranium 'soluti'on. However, t o eliminate effect of titration speed on higher titration, these tests should be compared with value 22.40 found in Table I rather than with theoretical value. -Similarly, tests 1 through 29 should be compared with 25.02 and 1.00.
However, an increasing error is produced by repeated passage of such quantities through the same reductor. Grimaldi (8) has pointed out that this difficulty may be overcome by the use of 10% mercury in the amalgamated zinc. One additional point t o be noted from the data of Table IV is the failure to find significant error when using a reductor contaminated by metals, especially copper, that are deposited in the column (tests 18 through 21). S o error is produced if the copper remains in the reductor column, and no evidence of incomplete reduction of uranium to the tetravalent state has been observed (tests 20 and 21). However, the effect is a cumulative one and serious errors will be produced even on pure uranium eolution if the deposition of such metals is allowed to continue-e.g., nickel ( 6 ) . Therefore, a reductor should be replaced immediate!y if it shows darkening, especially in the glass woo1 a t the bottom of the column, or if it allows an abnormal discharge of hydrogen. S o error is produced by manganese, thorium, cerium, erbium, lanthanum, neodymium, or yttrium. However, several samples known to contain rare earths exhibited interference with the present method for uranium. Large quantities of calcium interfere by occlusion of uranium in the calcium sulfate formed in sulfuric acid solution.
1179
V O L U M E 24, NO. 7, J U L Y 1 9 5 2 I n general, it may be predicted that any substance reducible by a Jones reductor to a form unstable in air will induce the air oxidation of a part of the uranium. It seems highly probable that induction reactions following imperfect separations are a major cause of instability of uranous sulfate solutions. Although the use of the Jones reductor for determining small quantities of uranium is a very accurate procedure under the proper conditions, the use of a reductor of lower energy would materially increase the reliability of the determination by lowering or eliminating the reduction of certain elements, such as chromium, niobium (columbium), cobalt, and nickel. Chromium is not removed by the methods generally used for the group separation of other heavy metals; if the quantity present is small, it will generally be unnoticed, and serious error will result. Yet small quantities of chromium have been encountered occasionally in materials being analyzed for uranium, either present in the original ore or introduced through the use of stainless steel equipment during roasting or other operations. Niobium hydrolyzes rather easily and also forms a cupferrate that is difficultly soluble in chloroform (6). Separation of this element will be incomplete unless special attention is given, and if the amount present is small, it will probably be unnoticed and serious error will result. Special procedures for complete removal of small quantities of these elements are generally time-consuming ( I O ) , and their inclusion in a general analytical procedure is undesirable. It would be desirable to eliminate the use of sulfuric acid. Uranium ores of increasingly high lime content are being utilized, and the use of sulfuric acid in the analysis of such material is both cumbersome and a source of error due to occlusion of small amounts of uranium in the calcium sulfate. Furthermore, if calcium sulfate is deposited in the reductor, error will also be produced in subsequent samples. The lead reductor would seem to be the reductor of choice for use with uranium solutions. Cooke and coworkers ( 2 ) have shown that hydrochloric acid solutions of uranium are rapidly and quantitatively reduced to the tetravalent state by lead a t room temperature through a wide range of acidities. The uranium is collected under ferric chloride and titrated with dichromate using diphenylamine sulfonic acid as indicator. I n the present investigation, the lead reductor is applied to determination of much smaller quantities of uranium than the 40 to 200 mg. used by Cooke. It is unnecessary to collect tetravalent uranium under ferric iron. If sulfatoceric acid and ferroin are used in the titration, phosphoric acid can be used in place of ferric iron to catalyze the direct oxidation of uranium. Thus, the strong yellow color of trivalent iron in chloride solution can be avoided, and the end point is rapid and sharp in a virtually colorless solution, a decided advantage when dilute oxidants are used.
the column very rapidly. The reductor recommended produces complete reduction of uranium over a wide range of conditions, and no instance of overreduction has been observed. Undoubtedly, either granulated lead or test lead may be used directly as obtained if standard screens are not available. However, a sample of test lead contained such a high proportion of very fine particles that passage of solution through the column was extremely slow. A 20- to 65-mesh fraction of this material was satisfactory. The reductor is preserved under distilled water to prevent the formation of undesirable hydrogen pockets which occur on standing under dilute acid. If a plug of glass wool is used a t the top of the column, the diffusion of oxygen to the lead surface, and hence the formation of basic lead salts, are virtually eliminated. When ready to be used, the column is washed thoroughly with 3 Ahydrochloric acid containing a little ferric iron. After the reductor has been washed free of iron with 1 N hydrochloric acid, a blank is run under the conditions of the determination to make certain that the reductor is completely free of oxygen. If the blank is appreciably higher than the indicator blank when reagent-quality chemicals are used, it must be repeated until it has decreased to the proper value before proceeding with the reduction of uranium solutions. It is recommended that a titration of a small known quantity of uranium also be included. If the value after correction for the blank deviates from the theoretical by more than 0.02 ml., contamination of the reductor by either oxygen or loose metals is indicated. For example, with the standard solutions used in Table V, values of 0.12 and 1.10 ml. are consistently obtained for the blank and 1.00 ml. of the standard uranium solution, respectively. The corrected value of 0.98 ml. is exactly the theoretical value. On the other hand, values of 0.14 and 1.07 ml. are occasionally obtained. The individual errors of 0.02 and 0.03 ml. are small but are definitely reproducible. As they are also additive, the total error of 0.05 ml. is greater than necessary and may become even larger with larger quantities of uranium. This error seems to be related to the suction a plied to the reductor and may be caused by oxygen liberated g o m the solutions in the lower part of the column under very low pressure.
PREPARATION OF LE4D REDUCTOR
Distilled lvater from copper-base storage tanks and stills will probably contain appreciable quantities of copper. Consequently, the top of t,he lead column will become black wit.h use, This deposit will do no harm so long as it is not too extensive, but must be cleaned up eventually.
Reagent-grade granulated lead is screened successively through 20- and 100-mesh screens. The middle fraction-that is, the fraction passing through the 20-mesh but not the 100-mesh screen-is then packed into a reductor tube of 12-mm. bore and of sufficient length to give a 30-em. column of lead. As the lead is very difficult to remove from the reductor tube, it is advisable to place a 1-cm. plug of glass wool a t the top of the column as well as the bottom to help protect it from contamination. A piece of thin-walled rubber tubing should be used on the aspirator to prevent oxygen dissolved in the solutions from building up in the lower part of the column under greatly reduced pressure, A reductor of this size is sufficient for reducing any quantity of uranium likely to be encountered in analytical work, including macrotitrations. The use of conventional reductor tubes of larger bore results in an undesirable increase in the volume in which the titration must be made without affecting the extent of the reduction. Hence, the use of reductors larger than that described is t o be discouraged. The size of the lead particles will have a pronounced effect on the rate of reduction. Using a 30-em. column of 20- to 35-mesh lead, the reduction of 25 mg. of uranium was incomplete if the solution was much below room temnerature or was passed through
Table V.
Titration of Reduced Uranium Solutions (From lead reductor)
Test XO.
Inltial HaPOa 1 drop
0 010036 5
0 010264 S
UOz(C10a)z, AI1 0 30
HaCe(804)4, hI1.
Theoretical, 0 29
1\11,
2
2 ml.
0.30
0.30 0.29
3 4
1 drop 2 ml.
1 00 1 00
0 98 0.98
0 98 0.98
D
6
1 drop 2 Ell.
5.00 5.00
4.89 4.88
4.89 4.89
;
1 drop
2 ml.
10.00 10.00
9.77 9.76
9.78 9.78
10
9
1 drop 2 ml.
25.00 25.00
24.4; 24.40
24.45 24.45
11 12
2 ml.
1 drop
50.00 50,oo
48.88 48.82
48.89 48.89
1
0.29
FORMATION OF HYDROGEN PEROXIDE IN LEAD A S D JONES REDUCTORS
The formation of small amounts of hydrogen peroxide during reactions of dilute acids on silver reductors ( 4 ) and liquid metal amalgams ( I f ) in the presence of air has been observed many times. -4series of tests showed that hydrogen peroxide is also formed in a Jones reductor if air bubbles are present in the column, the amount increasing with speed of passage of the solution through the reductor and with increasing amalgamation of the zinc. I n the presence of litt'le or no uranium, high results will be obtained owing to consumption of sulfatoceric acid by the peroxide. For example, using an air-free reductor, theoretically correct titrations of 0.05 ml. (indicator blank) and 0.99 ml. of 0.011 S sulfatoceric acid were obtained on a blank and a uranium solution,
ANALYTICAL CHEMISTRY
1180
respectively; using a reductor with air in the column, the corresponding titrations were 0.95 and 1.32 ml. The effect is exactly the same with a lead reductor, except that the amounts of peroxide produced are very much greater. When 80 ml. of 3 S hydrochloric acid followed by 80 ml. of 1 '\' hydrochloric acid were drawn t,hrough the 20- t,o 100-mesh reductor with free access of air, titrations of 0.3 and 5 ml. of 0.01 S sulfatoceric acid were obtained with slow and fast passage, respectively. Rapid passage of the same quantity of hydrochloric acid through a 20- to 35mesh lead reductor gave titrations of the order of 12 to 15 ml. The latter solutions also gave strong yellow colors with titanium sulfate solution. That' this error is carried over into an actual uranium determination was proved by reducing uranium solutions under the conditions described above. In all instances, serious errors were produced; when small quantities of uranium were used, the results were in no way related to t,he uranium content. Therefore, in t,itrations involving small quantit,ies of uranium and in blank determinations with either reductor, it is imperative that no air bubbles be allowed to become entrapped in the column. This is felt to be a highly desirable precaution even in a macromethod when the lead reductor is used, Before the reduction of uranium solutions is attempted, a zero reductor blank must be obtained as described above. TITRATION PROCEDURE USING LEAD REDUCTOR
The solution to be tit'rated for uranium should have a volume of 80 ml. and should contain 20 ml. of concentrated hydrochloric acid. The solution is reduced by passing it through an air-frec lead reductor at a rat'e of about 100 ml. per minute a t 25" C. Thc flask and reductor are washed with 80 ml. of 1 ;V hydrochloric acid, used in small portions to obt,ain maximum washing efficiency. The last portion of the hydrochloric acid wash should be replaced by water only when t'he reductor is not to be used for 2 or 3 hours or more. After addition of 1.0 ml. of 0.001 LII ophenanthroline ferrous sulfate (ferroin) and 1 drop of 85% phosphoric acid, the solution is titrated rapidly with 0.01 A- sulfatoceric acid until the indicator color begins to lighten noticeably. Then 2 ml. of 850/, phosphoric acid is added and the titration is resumed. Because phosphoric acid catalyzes the reduction of the oxidized form of the indicator by t,etravalent uranium only during oxidation of the latter by cerium, the first 2 or 3 drops of sulfatoceric acid will cause a return of the indicator color in full intensitmy. The tit,ration is continued sloa-1y until the indicator color has been partly but not complet,ely discharged. I s the quantit.y of indicat80rused will require 2 t,o 2.5 drops of 0.01 Aoxidant for it,s complcte oxidation, 1 drop will produce a 50% reduction in color after the uranium has been oxidized. At this point the titration is stopped for at least 30 to 60 seconds to allonall reactions to rome to equilibrium. The transfer of the solution into the 500-ml. Erlenmeyer flask that cont,ained the uranium solution before reduction (from cupferron separation described below) has an advantage, in that the end point is more clearly defined in that type of flask than in the greenish, heavy-walled suction flasks generally used with the reductor. The titration is then resumed by addition of 0.02-nil. port,ions of oxidant until the last pink tint is sharply discharged. The end point is easily discernible on addition of 0.02 nil. of 0.01 S sulfatoceric acid in 200-ml. volume and will be stable for hours if the solution is not exposed to direct sky light'. In the above procedure phosphoric acid is added in two stages in order to make the titration more rapid and convenient. If much phosphoric acid is present, the end point is so sharp that' it is likely to be overrun unless the titration is carried out almost dropwise. \Vhen large numbers of titrations must be made, as in routine work, this is obviously a slow and tedious procedure. The major portion of the uranium cannot be titrated in hydrochloric acid solution without the addition of a catalyst, as the indicator color is completely discharged during the early part of the titration, However, 1 drop of 85% phosphoric acid provides sufficient catalysis to allow the titration to be made rapidly to within a few milliliters of the end point before the indicator color is discharged. For example, in Table V, test 1, the indicator color began to decrease noticeably with the first drop of oxidant and was completely discharged with 4 drops; in test 11, the color
began to lighten a t about 47 ml. and was barely noticeable at 48.4 ml. I n both cases the addition of 2 ml. of phosphoric acid and 1 or 2 drops of oxidant produced a return of the indicator color, after which a normal, sharp end point was obtained. Of course, the warning fading will not take place if more than 5 to 10% phosphorus pentoxide (on 2-gram samples) is present in the original ore. The same procedure can be used in titrating large quantities of uranium v i t h 0.1 S sulfatoceric acid, with two exceptions. Two drops of 0.025 Jl ferroin should be used as indicator and 2 ml. of a 20% solution of ferric alum in 5% sulfuric acid should be substituted for the 2 ml. of phosphoric acid added just before t'he end point. The color change in this case is from orange to a greenish yellow, which is very distinct with 0.01 ml. of 0.1 S sulfatoceric acid. The use of phosphoric acid, other than the single drop required to catalyze the reaction nearly to the end point, is not permissible. If the uranium content is high enough (ca. 200 nig.), the addition of 2 ml. of phosphoric acid will cause precipitation of uranous phosphate, which redissolves very slowly during the tit,ration. If the phosphoric acid is added near the end point, ceric phosphate will be precipitated when the titration is resumed. The high degree of accuracy afforded by this procedure is shown in Table V. Under the recommended condit,ions, the method is accurate to 0.1% or to 0.02 ml. with the smaller volumes. If 2 ml. of 85y0 phosphoric acid is present throughout' the titration, the results are about 0.2% low.
Table VI.
Effect of Variations from Prescribed Procedure (Theoretical titration, 48.89 ml.)
Test No. 1 2
3
4 3 G
7
8 9 10 11 12
13
Conditions o i Test 4 8 . 5 nil. of sulfatoceric acid added all a t once from beaker in presence of 2 ml. of phosphoric acid Titrated b y discontinuous drops with continuous vigorous swirling in presence of 2 ml. of phosphoric acid Titrated rapidly in presence of 2 ml. o i phosphoric acid (Table V, test 12) Titrated rapidly in presence of 1 drop of phosphoric acid (Table V, test 11) Titrated by discontinuous drops with continuous vigorous swirling in presence of 1 drop of phosphoric acid Solution a t 2 5 . C. put through reductor a t 150 t o 200 nil. per minute Solution a t POo C. put through reductor a t about 40 nil. per niinute Solution a t 25O C. put through reductor a t about 100 ml. per minute. Solution aerated for 5 minutes Solution a t 11° C. passed through reductor a t 150 t o 200 ml. per minute Same as 4. Total phosphoric acid used, 0.5 rnl. Same as 4, Total phosphoric acid used, 5 nil. Solution in 1 A- hydrochloric acid. Reduced a t 25' C. a t about 50 ml. per minute Solution in 6 S hydrochloric acid. Reduced a t 25' C. a t about 40 ml. per minute
Titration. MI. 48.88 4 8 . 72
48 82
48.88 48.7i
48.87
48.87 4 8 . 89
47.0 48.87
48.88 47.5
48.83
The effects of certain variations from the prescribed procetlui e are shown in Table VI. Titration speed introduces a small error similar to that observed in sulfuric acid solution (tests 1 to 5 ) . The reduction of uranium is complete a t the highest speed of passage a t which the reductor could be used (test 6), and no overreduction could be produced either by slon- passage a t elevated temperature (test 7 ) or by slow passage a t high hydrochloric acid concentration (test 13). Incomplete reduction is pioduced by rapid passage a t low temperature (test 9 ) or by low hydrochloric acid concentration even at slow speed of passage (test 12). The stability of tetravalent uranium toward air seems to be a t least as good in hydrochloric acid solution as in sulfuric acid solution, no significant error being produced by aeration for 5 minutes a t 25" C. (test 8 ) . The phosphoric acid content a t the end point can vary a t least from 0.5 to 5 ml. without producing significant differences in the character of the end point or in the results (tests 10 and 11). However, the quantity of phosphoric acid required to maintain a rapid and sharp end point increases with increasing acidity, so that the lower limit will not provide
V O L U M E 2 4 , NO. 7, J U L Y 1 9 5 2 Table YII.
Test so.
1 2
1181
Effect of Various Substances on Reduction and Titration of Uranium
(Theoretical titrations, 0.98 and 24.45 nil.) 0.010036 N UO?(ClOd)2. Substance Added lfl. 5 nil. 9 6 7 H2SOa 1.00 5 ml. 9 6 4 H~SOI 25.00
SAMPLE PREPARATION 0 010264 5 HpCe(SOa)r, hll. 0.98 24.45
:3 4
10 ml. 72% HClOd
10 ml. 72% HClOd
23 00
1 .oo
0.98 24.43
5 6
10 ml. 85@&HaPOa 10 nil. 8370 113PO1
1.00 25,oo
0.97 24.18
7 8
1 gram C a + + 1 gram C a T +
25.00
24.44
9
1 gram A l + + -
1.00 25.00
0.97 24.43
!.OO
0 97 24.43
1.00
1.03 24.41
1 00 1 00
0 91 0 82 24 16
1
.il++-
10
1 gain
11
12
17 grams SaHS04, fused 17 grams NaHSOa, fused
13
10 nip. Xbv
.oo
25.00
0.99
14
10 mg. Xbv
2 5 00
13 16 17
1 m g . Cr+-+ 5 1ng. CI 5 mg. CI
25 00
18
50 m g . Cr volatilized with five 0.7-gram portions of solid S a C l 50 m g . C r + T +volatilized with five 0.7-grsni portions of solid XaC1
1 00
0 98
25 00
24 38
25 00
1 00
1 02 24 49
23 00
0 00 1 .oo
1 98 2 93 26 38
1 00 25 00 1 00 25 00
0 98 24 46 1 20 24 47
19
+ - +
+ - +
+
+
20 21 22 23 24
1 mg. Tilv 1 nig, TiI'
25 26 27 28
10 nig. C u f 10 mg. Cut' 300 nig. Cu
1 ing. Ti'V
+
-
Roductor from test 27
the met'al is retained completely in the column, and does not interfere appreciably with the reduction (test 28).
for niuch variation in aciditj-. I n test 13 the elid point was distinctly sloiver than a t the recommended acidity. A rapid and sharp end point was obtained when the test was repeated using 5 ml. of phosphoric acid. Table VI1 shows the effects of various added substances on the reduction and titration of uranium. A slightly slow end point was obtained in the tests with sulfuric acid, which can be overcome by the use of more phosphoric acid, as ment'ioned above. S o differences n-ere observed in the presence of 10 nil. of either perchloric or phosphoric acid. The latter case is cq)cbcially significant, inasmuch as it has been reported that phosphoric acid prevents complete reductmion. EIowever, precipitation of uranous phosphate will cause trouble when either uranium or phosphoric acid is present in relatively large quantities. S o significant salt effects were observed in the presence of relativcly large concentrations of calcium, aluminum, and sodium salts; satisfactory reduction of uranium in solutions of high sulfate content makes possible the use of bisulfate fusion for the decomposition of high-refractory materials. A s was expected, the serious induction error produced by small quantkies of niobium in the Jones reduct or has been virtually eliminated, and that of chromium hae been reduced by about 50%. lloreover, the use of perchloric acid makes identificat,ion and sulwquent elimination of the latter element a simple matter, although a small mechanical loss of solution niaj- occur (test 19). The rcduction of small quantities of titanium in a lead reductor is quantitativc, and the result.ing chloride solutions of trivalent titanium are auiprisingl!. stable toward air. As a result, t,here is 110 intluced oxidation of uranium, as is the case in sulfate solutions, and the error is a virtually ,stoichiometric funct,ion of thv quantit,? of titmiurn present. The serious evolution of hydrogen that occurs when small quantities of copper and other metals are deposited in the Jones reductor is eliminated, thus extcncling the usable life of t h r reductor. The serious error produced 1)y copper in the Jones reductor is also eliminated. Unless large quantities of this element are passed through the reductor rapidly (test 2 7 ) ,
Detailed directions for t,he treatment of ores prior to determination by both the Jones and lead reductors are given elsewhere ( 1 4 ) . Only a brief description of the perchloric acid-lead reductor method is given here. After decomposit,ion of the sample with hydrochloric, nityic, and hydrofluoric acids, 10 ml. of 72% perchloric acid is added and t,he solution is fumed nearly dry. Fifteen milliliters of perchloric acid, 50 ml. of water, and about 0.2 gram of sodium sulfite are added, and the solution is boiled until all soluble salts are in solution and sulfur dioxide has been expelled. After treatment with hydrogen sulfide or thioacetamide, the filtrate is boiled for a few minutes to remove hydrogen sulfide and then oxidized by the dropn-ise addition of 30y0 hydrogen peroxide until no further color changes are produced. Strong permanganate solution is mbsequently added as a test for completeness of oxidation. The solution is then evaporated to about 75 ml. and extracted with cupferron and chloroform, the aqueous layer is drawn off into a 500-ml. Erlenmeyer flask, and the solut,ionis evaporated to fumes. ( I f j v e r 50 mg. of U3O8is to be determined, an additional 15 nil. of (2y0perchloric acid must be added before extraction to prevent, loss of conaiderahle uranium.)
Ai very important labor-saving advantage of the present method is elimination of the fuming by hand that is required in the sulfuric acidJones reductor method to eliminate the serious bumping caused by the separation of salts. \\;hereas metallic sulfates are generally insoluble in strong sulfuric acid solution and precipitate when most of the water has been expelled, metallic perchlorates are fairly soluble in concentrated perchloric acid. If all sulfates, nitrates, and other anions have been excluded, the perchloric acid solution will generally evaporate t o fumes very smoothly with no separation of salts, and hence, no bumping. Oxidation of residual organic material from t,he cupferron separation also takes place very smoothly and without, special attention. The practice of adding nitric acid during the evaporation of cupferron filtrates is unnecessary, since the filtrate will contain very little organic mat,ter after the extraction if fresh cupferron and a minimum time are used during extraction. The practice of rinsing with water and re-evaporating to fumes one or more tinirs is time-consuming and completely needless if the evaporat,ion is carried out in an I
