Automatic Oxidimetric Micromethod for Uranium KENNETH A. ALLEN
Oak Ridge National Laboratory, Oak Ridge, Tenn.
A method has been developed which
uses a chemical and mechanical system for the rapid and convenient determination of 10 to lOOy quantities of uranium. Microliter amounts of standard ceric sulfate are automatically delivered to reduced sample solutions and the volumes are rewrded on the chart of a recording potentiometer. Replicate titrations of 0.5-pmole quantities of uranium show a standard (959%) deviation of ; t l . Z % . The relative ermr for smaller samples is proportionally larger, and, therefore, the method is not rewmrnended for samples containing less than 10 y of uranium. The procedure should be helpful in dealing with pure solutions of elements which are conveniently determined by oxidimetrio titration, suoh as iron and vanadium. The meohanieal features of the apparatus m a y be altered to conform to various requirements both as to expected sample sizes and redox i
TITRATION
BURET DRIVE MOTOR
COUPLING
POTENTIOMETER
IO -10rnv.l
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I MANUAL OR
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AUTOMATIC OPERATION
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HE determillation of from 10 to lOUr qumtlties of uran~um
usually accomplished by colorimetric methods, the accuracies of which are a t best limited to those of the readings obtainable an a spectrophotoineter. In addition, such methods are generally inconveniently dependent on pH, reagent stability, and the like, and the required sample preparations are often tedious and time consuming. For samples containing larger amounts of uranium the axidimetric titration of reduced strongly acid solutions is rapid, convenient, and accurate, and the usual standard oxidimetric reagents are stable indefinitely. Application of the latter method to less than milligram quantities of uranium would therefore be advantageous, provided the equipment necessary for the retention of the desired accuracy nere not unduly cumbersome, costly, and operationally complex. .4 ohernical and mechanical system which meets these requirements is described here. 18
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There have been several otbor investigations dealing with tho titration of small amounts of uranium. The methods described by Rodden (7) depend on the use of miniature Jones reduetars and colorimetric end point indicators, and would appear to have a lower limit of about 0.1 mg. of uranium with an associated accuracy within i27”. Disadvantages inherent in these methods include the large volume increases necessitated in washing the amalgam, the high blanks resulting from peroxide formation (9), and the use of indicators. Indicator blanks can be entirely eliminated by potentiometric end point detection, and an automatic microtitrator described by Kelley ( 4 ) is reported to titrate SOr samples of iron (the approximate ohemicd equivalent of 0.1 mg. of uranium) with a standard (95%) error of ~ t l . 2 7 7 ~ . The use of chromous and titanous ions far the initial uranium reduction has been reported (7), and offers the best means of retaining small sample volumes. With this modification and the use of ferric sulfate for the intermediate oxidation instead of ferric chloride, the present procedure is essentially that of Kolthoff and Lingane (5). Hahn and Kelley have described a Bimilar method in which interferenci from small amounts of iron (up to B maximum of about half the weight of uranium present) is removed by complexing with l,l0-phenanthroline (3). Ceric sulfate was chosen as the standard oxidant because of its wellknown stability in 0.5 to 1M sulfuric acid (IO) snd because this oxidant establishes a stable potential against inert electrodes such as platinum. EXPERIMENTAL
Figure 1. Automatic titration equipment
Materials. Standard uranyl sulfate solutions in 1M sulfuric acid were prepared by the usual methods from black oxide (U,O,) equivalent t o National Bureau of Standards material.
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V O L U M E 2 8 , NO. 7, J U L Y 1 9 5 6 Ceric sulfate (G. Frederick Smith Chemical Co.) was prepared as a 0.1M solution in 0.5.21 sulfuric acid in order to keep the density of the titrating liquid below that of the samples. This limited mixing in the buret tip, which dipped into the samples (1M in sulfuric acid) during the runs, to true diffusion rather than the extremely rapid t'hermal and gravitational convection processes which might otherwise have occurred. Chromic sulfate, 0.1M in 0 , l M sulfuric acid, was stored in a Jones reductor with a capillary tip and an amalgam bed measuring approximately 30 X 1 cm. The effluent chromous solution was used dropwise directly from the reductor. Ferric sulfate, 0.1M, was made in 1M sulfuric acid and stoied in a small reagent bottle equipped with a dropper cap for convenience. Apparatus. An automatic titrator was developed for this determination which embodies, with several simplifications, the features described by Robinson (6). The apparatus includes a motor-driven microburet ( I ) , synchronized with a 0- to 10-mv. recording potentiometer, and an interval timer. The physical arrangement is shown in Figure 1, and a block diagram of the electrical system is shown in Figure 2. The buret drive was geared such that with the motor on continuously, titrant was ejected a t the rate of about 1 pl. per minute, during which time the chart paper, graduated in 0.1-inch divisions, moved a little less than 1 inch. The titration cup, with a t'otal capacity of about 1 ml., was cut from the end of a 15-mm. test tube. During a run, the cup was rotated at 300 r.p.m. with the buret tip (inside diameter, 0.05 to 0.1 mm. j and a platinum electrode dipping well into the sample solution at one side of the cup. A second platinum wire sealed into the buret complet'ed the cell. The internal resistances of the latter varied from 20 to 50 kilo-ohms; its output was therefore shorted across a 1-megohm voltage divider from which the recorder read potentials corresponding to 3.9 or 8.2 kilo-ohms, depending on the desired scale coverage. A mercury switch on the recorder was set to turn on the interval timer at the reproducible potential corresponding to the point of unit slope on the titration curves between the horizontal plateaus and the nearly vertical end point, breaks. The timer then turned on the buret motor and the recorder chart drive for 1 second and off for 5 seconds, repeatedly, until the end point was completed. This cycle provided sufficient equilibration without undue loss of speed. Procedure. The following procedure was used to obtain the data reported below, except where otherwise noted. Set up the apparatus as shown in Figure 1 and allow a 2-minute warm-up for the recorder. Pipet' 0.5 ml. of a solution from to 10-3 Jf in uranyl sulfate in 1-If sulfuric acid (about 12 to 120 -J of uranium) into the titration cup. Turn on the stirring motor and add 1 drop of chromous sulfate solution from the Jones reductor. Turn the operating switch to the manual "on" position until t,he potentiometer needle has travelled 80 to 90yc of the full-scale distance to the right, then turn the swit'ch back to :'off." Allow 10 minutes for air oxidation of uranium(II1) and chromium(I1j, then add 1 drop of ferric sulfate solution and turn the operating switch to "automatic." After the recorder has plotted the ferrous-ferric end point, turn off the operating sn-itch and the stirring motor. The vertical distance on t'he chart paper bet\Teen the point a t which automatic operation was started and the mid-point of the iron break is proportional to the total amount of uranium in the sample. The precision of delivery of a standard 1-ml. measuring pipet (Mohr type) can be increased considerably by dran-ing out the tip such that the last 1 to 2 cm. is of 0.1- to 0.2-mm. inside diameter and 1- to 2-mm. outside diameter. This slows the rate of delivery effectively and, more important, limits liquid pickup after delivery to a reproducible minimum. K i t h the 8200-ohm resistance in the voltage divider, the $1.6volt potential of the tit,ration cell immediately after addition of chromous ion is off the scale of the recorder. Titration curves showing the complete process viere obtained with the 3000-ohm resistance. For routine determinations it is unnecessary to observe the initial reduction and aeration steps fully, and use of the 8200-ohm resistance results in a highly magnified iron break. This initial temporary operation of the system serves the purposes of cocking the mercury sxitch which turns on the interval timer as the ferrous-ferric end point is approached, and of providing an immediate visual check on proper operation of the apparatus and complete reduction of t'he sample. A titration requiring 10 pl. of oxidant (equivalent to about 120 y of uranium) is completed in about 20 minutes, including the aeration step. This time consideration is the reason for indicat-
ing the upper limit of uranium. The system is capable of titrating much larger quantities, but because the time required for a single determination is roughly proportional to the amount of uranium present, it is desirable to take only enough uranium to provide the necessary accuracy. DISCUSSION
It was originally intended that the results of this determination would be obtained from the difference between the two end pointe corresponding to the oxidation of uranium( 111)and chromium(I1) (first) and of iron(I1) (second). A plot of such a titration is shown in Figure 3, 9. In this run the buret drive was operated continuously in the hope t h a t overshooting a t the breaks n-ould be cancelled, or nearly enough to provide the desired accuracy.
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Figure 3.
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Comparison of first end points
0.5 pmole of uranium in 0.5 ml. of 1M sulfuric acid A . Buret drive on continuously B. Buret drive turned on a t time of adding iron(II1)
The results of these runs mere generally poor, however, with regard to both precision and linearity with respect to total uranium. For the run shown in Figure 3, B, the buret drive was not turned on until air oxidation of the uranium(II1) and chromium(I1) was complete; there is no great difference in the shape of these first breaks from that shown in Figure 3, A . Thus, it became apparent that no cancellation of overshooting effects could be expected. Therefore, the interval timer was installed and the procedure given in the preceding section adopted. Figure 4 shows a series of runs a t varying uranium levels obtained by this procedure.
ANALYTICAL CHEMISTRY
1146 The results of a large number of replicate determinations, conducted primarily to test the buret, are given in Table I . The buret had an inside diameter of "8 inch (precision bore glass tubing) and a total possible piston travel of 1 inch. More than 100 determinations involving 10 PI, quantities of titrant each can therefore be made a t one filling and it was necessary to check the linearity of the bore over a considerable distanw. A statistical analysis of the resultp shovin in Table I using the F test (11) indicated that the data could be pooled, and the standard 95% confidence interval deviation for a single determination a t this uranium level was found from the pooled set t o be fl.270. -
Table I.
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Chart Inches for 0.500-pmole Samples of [Tranium
Buret Initially Ftill 8.46 8.53 8.42 8.54 8.48 8.42 8.43 8.47 8.52 8.48 8.48 8.54 8.48 8.49 8.56 A>.. 8 . 4 9
tables. The effects of interfeiing substances on this titration have been thoroughly investigated ( 2 , 5, 7 , 9). The present method was developed specifically for pure uranium samples; it would probably be applicable to any unknown in which metals reducible by chromous ion are either absent or removable. This is particularly true in the case of iron, although it should be borne in mind that the tolerance levels for such interfering ions may be unusual a t these uranium levels, especially with regard to the posqibility of catalysis of the air oxidation of uranium( IT).
Buret Half Ft111 8 , s
8.4Y 8.52 8.56 8.44
8.63 8.48 8,55 8.41 8.60
Buret Nearly E m p t y 8.44 8.46 8.53
8.51 8.58 8 52 8.54 A\.. 8 51
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8.50 .iv. 8 5 2
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TRAVEL
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The results of several runs at other uranium levels are shown in Table 11. Linearity with respect to total uranium is exhibited, and it is qualitatively apparent that the absolute deviations are comparable to those shown in Table I. As was pointed out in the preceding section, therefore, the per cent accuracy of a given determination is proportional to the amount of uranium present. On this basis the titrations of 127 quantities of uranium compare favorably in accuracy with both the colorimetric and fluorometric methods in this range.
Table 11. Chart Inches for Varying Amounts of Uranium Uranium Taken, @mole 0,250 4.23 4.25 4.26 4.24 4.25 4.22 4.25 4.26 4.32 4.25 4.24 Av. 4 . 2 5
0.100 1.72
0,050
1.67 1.69 1 72 AT. 1.70
0.83 0.84
Av. 0.86
This method is not recommended for samples containing less than 10 y of uranium. Such samples give readable titration curves, but the per cent error becomes extremely high (about 25$4 for 5 y ) and the fluorometric method is a t least as good as and usually better than this a t these levels. The sources of error inherent in this method can be divided into two categories, chemical and mechanical. The former includes ( a ) variations in potentials caused by differences in the sample solutions, ( b ) air oxidation of uranium(1T') after the addition of chromium(I1) and before the addition of iron(III), and (c) interfering substances. Of these, (a)has not been observed and ( b ) was shown to be unimportant by aerating 0.05and 0.5-pmole samples for 10, 20, and 40 minutes before adding iron(II1); the results were in accord with those shown in the
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Figure 4.
Titration for varying amounts of uranium
Amount of uranium given a t breaks as rmole in 0.5 ml. of 1M sulfuric acid a. Add chromium(I1) b. Begin titration after aeration c . Potential a t which incremental operation is automatioally triggered d . Potential at which distance from base line defined by b is read
The only commonly encountered anion which could give difficulty is phosphate (chloride and nitrate should be removed by fuming with sulfuric acid). If phosphate is present, the sulfuric acid concentration should be raised to 4 to 531 to avoid precipitation of uranium(1V) phosphate (8). Mechanical errors include pipetting errors. temperature changes affecting the volume of the titrating liquid or the metal parts of the buret, variations in the length of the Teflon piston caused by squeezing and relaxation, differences in coast time betv-een the buret drive and the chart drive during the incremental addition cycle, chart paper length changes due t o humidity variations and/or binding in the recorder, and variations in the buret bore. Of these, pipetting errors can be controlled by using the same pipet for standards and unknowns; temperature changes affecting the volume of the titrating liquid v-ere rendered negligible by a water jacket, kept a t 25.00' =k 0.01" C.. for the buret. The remaining sources of error are not easily subject to evaluation. If any of them were sufficient to lead to consistent errors greater than those apparent from the deviations noted in Tables I and 11, then an equally consistent compensating effect must have been operative. This is regarded as highly unlikely in view of the nature of these sources of error, and it is therefore considered reasonable to include them in the over-all indeterminate error implied by the statistical analysis of the data. LITERATURE CITED
(1) Allen, K. A., ANAL.CHEM.28, 277 (1956). (2) Furman, N. H., Schoonover, J. C., J . Am. Chem. SOC. 53, 2561
(1931).
V O L U M E 28, NO. 7, J U L Y 1 9 5 6
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( 3 ) Hahn, R. B., Kelley, M. T., Anal. Chim. Acta 10, 178 (1954). (4) Kelley, M. T.,J. Inst?. Soc. Am. (Proc.) 7, 63 (1952). (5) Kolthoff, I. PI.,Lingme, J. J., J . Am. Chem. SOC.55, 1871
(1933). (6) Robinson, H. A., Trans. Electrochem. SOC.92,445 (1947). (7) Rodden, C. J.. ".4nalytical Chemistry of the Manhattan Project," McGraw-Hill, New York, 1950. (8) Schreyer, J. M., Baes. C. F., Jr., ANAL.CaBm. 25, 644 (1953).
(9) Sill, C. W., Peterson. H. E.. U.S. Bur. iWines Rept. Invest. 4882 (1952). (IO) Willard, H. H., Young, Philens, J. Am. Chem. Soc. 51, 119
(1929). (11) Youden, W. J., "Stntistioal Methods for Chemists," Wiley, New York, 1951. R~cr;rv~ for o review December 7,1955. Accepted April 23, 1956. Division oi Anhlytiod Chemistry, 129th Meeting, ACS, Dallas, Tex., Ami1 1950.
Micromethod for Determining Viscosity of High Viscosity Materials J. W. A. LABOUT and W. P.
OORT
VAN
KoninklijkeJShell-Laboratorium. N. V. De Bataalrche Petroleum Maatschappij, Amsterdam, The Netherlands
In studying the durability of asphalts by exposing thin asphalt films to the atmosphere, viscosity determinations on minute quantities of material are necessary. For that purpose a miorovisoometer has been developed which requires only 12 to 30 mg. of asphalt. The method is hasod on simple shear of the substance hetween two parallel plates and can be used for the determination of the visoosity of thermoplastic materials in the range of IO4 to 10'poises with an accuracy within 5%. T h e mioroviseometer is suitable for investigations in which it is essential to work with small quantities of material and is a valuable aid in the analysis of asphalts recovered from bituminous constructions, such as road carpets. The method may also offer advantages in the study of fractionating processes, as viscosity measurements may he made on small fractions.
principle of simple sheer of the material between tN0 parallel planes under the action of a constant shearing stress. Several other methods founded on the same principle have been published (1,S, 5 ) ,hut that described here differs from the others in requiring only very small amounts of material.
The two glass plates me arranged in a horisontal position.
with divisions of 0.02 mm. T i e wgole device is ulaced in an air thermostat, in which the temperature can he kkpt constant to within 0.1' C. hetween 25' and 40' C. Representations of a double apparatus without thermostat are given in Figures 1 and 2. OPERATING PRINCIPLE
Under the influence of weight P a horizontally directed force is exerted on the upper glass plate so that shear takes place in the layer of the materid under investigation. The displacement of the upper plate is read from the scale by means of a microscope. The viscosity can then be calculated immediately from the formula defining the viscosity of Newtonian liquids:
where r = shearing stress, dynes per sq. em. 'I = viscosity, poises y = shear = tangent of angle of shear t = time, seconds
CONSTRUCTIOP
The micraviscometer consists of two pdished glssa plates, both =.-"- a thin l r r i o ~ measuring 20 X 30 X 7 mm. Between thazn nli+as of the substanoe under investigation is applied: the thickness of this layer may vary from 20 to 50 microns, so that a measurement of the viscosity requires only 12 to 30 mg. of material. 1111-
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Figure 1. General view of mieroviseometer
Figure 2.
Measuring equipmen1 for microscope reading
