cobalt, gallium, and tin, it has a n advantage over the carrier distillation procedure, because certain elements forming highly refractive oxides, such as the rare earths, can be determined with a high degree of sensitivity. The simplicity of the ion exchange procedure makes it ideal for spectrographic purposes because the elemental contamination can be held to very low levels. The manipulations can be easily carried out in a glove box. The number of additives is held to a minimum. The visual comparison procedures used in the preceding n-ork have already provided valuable information and were adequate for the present application. The desirability of extending the usefulness of the method through the increased accuracy of densitometric procedures is also recognized. These procedures have been applied and are being prepared for publication. It appears entirely possible to develop a method for plutonium using nitric acid exclusively except for the initial dissolution. Hov-ever, hydrochloric acid prepared as described above had
smaller amounts of the common impurities, such as calcium, magnesium, sodium, aluminum, and boron, than the nitric acid used. Consequently, hydrochloric acid was preferred for the determination of these elements. Furthermore, the division of the sample into hydrochloric and nitric acid fractions reduces the possibility of matrix effects in the excitation of a n impure sample. In specific instances the ion exchange technique used in conjunction with conventional spectrographic methods may provide the means of extending the concentrational limits of detection another order of magnitude. ACKNOWLEDGMENT
The authors rrish to express their appreciation to D. C. Stewart and LI. H. Studier for helpful discussions during the course of this work. They are also grateful to J. -4.Goleb for the carrier distillation results of Table I. This paper is based on rvork performed under the auspices of the U. S. Atomic Energy Commission.
LITERATURE CITED
(1) Bierlein T. K., Kendall, L. F., Van
Tuyl, H. k.,General Electric, Hanford Atomic Products Operation, Richland, Wash., Doc. HW-25074 (July 25, 1952). (2) Daniel, J. L., Ibid., HW-25859 (Oct.
7, 1952). (3) Ibid., HW-43353 (March 3, 1956). (4) Fred, M., Nachtrieb, N. H., Tomkins, F. S.,J. Opt. SOC.Am. 37, 279 (1947). (5) Kraus, K. A., Nelson, F., Proc. Intern. Conf. Peaceful Uses Atomic Energy, Geneva, 1966 7, 113 (1955). (6) Metz, c. F., ANAL. C H m . 29, 1748 (1957). (7) Metz, C. F., Los Alamos Laboratory, Los Alamos, N. M., private communication. June 1958. (8) Reinschreiber J. E., Langhorst, -4.L.,
Jr., Elliot, M. b., U. S.Atomic Energy Comm., Rept. LA-1354 (1952). 19) Scribner, B. F., Mullin, H. R., J . Research h'atl. Bur. Standards 37, '
379 (1946). (10) Van Tuyl, H. H., Doc. HW-28530, General Electric, Hanford Atomic Products Operation, Richland, Wash., 1953. (11) Wish, L., Rowell, M., San Francisco, Calif., USNDRL-TR-117(October 1956).
RECEIVEDfor review April 25, 1958. Accepted July 21, 1958.
Plant-Type Polarographic System for Determining Uranium in Radioactive Waste Streams G. J. ALKIRE, KARL KOYAMA, K. J. HAHN, and C. E. MICHELSON' Hanford laboratories Operation, General Elecfric Co., Richland, Wash.
b An industrial-type polarographic system has been developed for the automatic determination of uranium in radioactive process solutions. The equipment is designed for in-line use and records the uranium concentration in the range from to M every 71/2minutes. The instrument has been used for nearly a year in pilot plant operation and i s considered suitable for use in highly radioactive streams in processing plants. The detection of other constituents, off-standard process conditions, and material affecting the precision are discussed.
C
processing plants separating or purifying uranium by solTent extraction must keep the uranium content of primary waste streams to a minimum. The long-standing practice of periodically taking a sample and sending it to the laboratory for chemical analysis is not sufficient for a continuous HEIIICAL
1 Present address, General Engineering Laboratories, General Electric Co. , Schenectady, K. Y.
1912
ANALYTICAL CHEMISTRY
process. A plant stream analyzer is necessary if high waste losses, rework volumes, and essential materials are to be kept to a minimum. A survey of analytical methods and techniques suitable for the continuous detection of uranium concentrations in the range from t o 10-4M shows that the polarographic method has certain advantages over other electrometric and more classical volumetric methods, The polarographic method measures concentration directly in any volume of solution. It has a relatively high sensitivity and is capable of detecting other conditions that may be of process significance. Several types of automatic systems using the polarographic principle have been described. One instrument has been developed by Wilson and Smith (5) to determine sulfur dioxide in corn steep liquor. Lewis and Overton (3) hare described a system which uses two polarographic cells and two dropping mercury electrodes for determining uranium in a flowing stream. The potential of one electrode is adjusted to the bottom of the reduction wave, while
the voltage of the other electrode is set a t the top of the tvave. The difference in current flow between the t n o cells is a measure of the uranium concentration. Bertram and coworkers ( I ) reccntly reported a n automatic derivative polarograph which determines high concentrations of uranium in a process stream. A sample from the process stream is diluted with a suitable supporting electrolyte to bring the uranium concentration within the range of the polarograph, and to dilute out interferences. The dilution is accurately carried out n-ith a specially constructed proportioning s j stem. This paper describes an automatic polarograph used to determine uranium directly in a radioactive process stream of essentially 2.0 i 0.lM nitric acid, without the addition of other reagents. The method uses nitric acid in the stream as the supporting electrolyte for t h e polarographic reduction of uranium(V1) to uranium(V). Xormally the uranium concentration a a s less than 0.01 gram per liter, but during off-standard conditions, rose to as much as 10 grams per liter. I n addition to uranium, the
ro
POTENTIOMETER
f-STANDARD S O L U T I O N N O I STANDARD S O L U T I O N NO. 2
A
DROPPING
If
3
MERCURY RESERVOIR
SATURATED KCI
dgI
Figure 2. MERCURY T R A P
.,
I
, '.- - -' PROCESS T A N K OR LINE
Figure 1. ograph
USED MERCURY STORAGE
-I
R E T U R N TO PROCESS
Piping arrangement for uranium polar-
stream contained nitrite, iron, and tributyl phosphate. DESCRIPTION AND FUNCTION OF APPARATUS
The direct current or polarograph circuitry is conventional in design, similar to that used in the Sargent Model XXI polarograph. However, the slidewire for the voltage sweep consists of a 360" Helipot of 1000-ohm resistance driven by a '/8-r.p.m. synchronous motor. One revolution of the Helipot is necessary t o obtain a polarogram between fO.05 and -0.25 volt zw. S.C.E. Four ranges of current sensitivities of 12.5, 50, 100. and 200 pa. full scale on the 0- to 10-mv. strip chart recorder are available. Damping of oscillations, when needed, is done by switching in either 2000 or 4000 pf. of capacitance. The electrolysis cell, mercury trap, standard solution lines, and general arrangement of 1/8-inch pipe used in the automatic polarograph are shown in Figure I . The system is located in an enclosure provided with adequate shielding against radioactivity. Periodic use of standard solutions eliminated the necessity for temperature control and the temperature in the unit was normally in the vicinity of 27" C. During cold periods in the winter, it dropped to 20" C., and during hot periods of the summer, rose to 31" C. Even under these conditions, the temperature variation over a Deriod of 1 hour was less than +2"C.' The sample cell (Figure 2) is constructed of stainless steel and holds the
conventional dropping mercury electrode and reference electrode. The cell is 13/4inches in outer diameter, and 43/*inches high. Hypalon gasket material is used to seal the electrodes tightly in place. A porous-glass plug ( 2 ) , attached to the reference electrode with polyethylene tubing, serves as the salt bridge. This type of salt bridge has been satisfactory for periods of 9 to 12 months. -4sample of about 20 ml. is provided for the analysis by locating the inlet and side outlet above the bottom of the cell. An air-aspirator sampler of the type described by Pierce (4) and Ivith a capacity of 400 ml. per minute is used to circulate the process solution through the cell. Although the holdup volume of the process lines to and from the cell is about 175 ml., the 3-minute sampling period ensures a complete exchange of the previous sample with ne\v solution. The mercury trap is bypassed during sampling. As it is undesirable to allow mercury from the dropping mercury electrode to enter the process stream, it is removed from the sample by a mercury trap. The trap is made from a 5-inch length of 3'/*-inch stainless steel pipe, and has inlet and outlet lines spaced 90" apart, provisions for flushing and draining, and conductivity probes for remote indication of mercury level. An inert solvent with a density lower than mercury, but greater than the process solution, is placed in the trap to inhibit the reaction of nitric acid with mercury. Either carbon tetrachIoride or trichloro-
POLYETHYLENE TUBING
Sectional view of polarographic cell
ethylene works well. At the completion of each analysis, the bottom drain valve of the cell is opened to alloiy the niercury and sample to enter the trap, n here the mercury settles while the sample continues on through to be returned to the process stream. The mercury is removed from the trap through the bottom drain valve when the level reaches the upper conductivity probe. The trap is then filled m-ith the inert solvent before being used again. The removal of mercury from the trap presents a potential radiation hazard, because, although the metal itself is free from fission products, there may be radioactive particles and liquid at the mercury solution interface. Thi. hazard is minimized by leaving some residual mercury in the trap.
Automatic Operation. The concentration of uranium is recorded a t 7','?minute intervals, though other convenient times may be used. The analyzer has the folloning sequence of operation: Beginning at zero time, the valves controlling the air-jet, helium, and sample flow are energized to circulate process solution through the sample cell. At 3 minutes these valves are closed, leaving a fresh sample in the cell. The recorder chart drive is then started a t 3'/* minutes. The polarographic scan is started a t 31/2 minutes and completed a t 61/2 minutes. At 6 3 / 4minutes the recorder chart drive iq turned off, and the cell drain and air-jet valves are opened to remove the analyzed sample. The drain valve is closed a t 71/4minutes, but the air-jet valve is left open for the next cycle which begins a t the end of i 1 / 2 minutes. -4Nulticam timer was uqed to program the various operations. The standardization cycle, which is actuated manually, opens the selected standard solution valve instead of the sample valve. The standardization VOL. 30, NO. 12, DECEMBER 1958
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cycle can be run only during the first 33/4 minutes of the regular cycle, because a t this time, a latch relay is closed to prevent either of the two standard solution valves from being energized. This feature precludes mixing of the standard with the process solution in the cell. The uranium concentration of the standard solutions is made to bracket the concentration of uranium normally present in the process stream. Removal of Oxygen and Nitrite. Oxygen is reduced a t nearly t h e same potential as uranyl ion. The presence of nitrite ion not only enhances t h e height of t h e uranium wave, but also catalyzes the oxidation of mercury in nitric acid, producing interfering reaction products. Both oxygen and nitrite must be removed from the process solution prior to the polarographic analysis. Oxygen n as removed by displacement with helium, and nitrite was made to react with methanol to form a volatile methyl nitrite. These two steps n ere acconiplished simultaneously by sparqing the sample with helium saturated with methanol. Deaerating the sample with a conreiitional sparge tube in the sample cell was difficult. Cnless the pressure in the helium line !vas always greater than in the sample cell, the process solution occasionally entered the sparge tube and helium line. This resulted in a n accumulation of radioactive material in the helium line outside of the samplecell enclosure and became a radiation hazard. The problem was circumvented by introducing the helium, saturated with methanol in the line normally used for the sampler air lift. This eliminated a helium line into the polarographic cell and utilized a line which was enclosed within a shielded region. By this method the oxygen and nitrite vr-ere removed before the solution reached the sample cell. The displaced oxygen and volatile methyl nitrite were sm-ept out of the system nith the escess helium through the jet. Valves. Valves rvere required t o control t h e filling and draining of t h e sample cell. The type of 1/8-inch valve ivhich met all requirements was t h e diaphragm-operated (air t o open), bellows-sealed valve equipped with Teflon seats and manufactured by Hoke, Inc. Commercial l/s-inch stainless steel solenoid valves were also tried for this purpose. Although the solenoid valves operated satisfactorily in the laboratory, they were unsatisfactory in plant use. Some difKculties encountered n-ere valve armatures sticking in either the open or shut position, leakage past the seat, short circuit of the solenoid coil, and corrosion failure because the stainless steel parts were silver-soldered together. Solenoid valves were used to control the air-operated valves, but are not rec1914
ANALYTICAL CHEMISTRY
F i g u r e 3. R e c o r d e r chart showing uranium concentration during process upset and correction
75
-
6o
-
45
-
w v1 3
-
z
w
5 30
15
0
-
-
-.
ommended for liquid service in process streams.
-
RESULTS A N D DISCUSSION
A calibration curve (wave height us. uranium concentration) prepared with synthetic process solutions was nearly linear up to 5 grams per liter of uranium. Above this level the slope gradually decreased. The over-all precision of the analysis, determined with synthetic soluhowever, tions, was better than =!=2y0; with process solutions, it was no better than ilo%, because of interferences. Small deviations in the nominal 2 M nitric acid concentration have very little influence on the diffusion current of uranyl ion. The lower limit of detection is determined by the nature and quantity of the other ions present. Ferric iron, for example, interferes with the lower portion of the uranium w a l e and causes the drop oscillations t o become excessively large if the mole ratio of iron to uranium is 5 or greater. For the typical curves (Figures 3 and 4) obtained in pilot plant operation, the ratio was less than 5 . The major portion of the iron in the process solution comes from the corrosion of process equipment. The presence of tributyl phosphate in the waste stream introduces a break in
-
30
SiD. SOL". 0 . 8 7 q,A, NC. 2 RANGE
m
W
-
3
E
4 W
15-
0-
Figure, 4. Uranium polarogram obtained during normal process operation
the polarogram near the uranium halfwave potential as shown in some of the scans in Figure 3. This effect, which is caused by a n adsorption-desorption process on the mercury drop, accounts for most of the low precision obtained. The amount of tributyl phosphate dis-
solved in the aqueous stream from the tributvl phosphate-kerosine extractant is normally less than 0.2 gram per liter; however, during certain types of process ups&, additional tributyl phosphate becomes entrained in the stream and a larger break in the polarogranl is obtained, Interferences due to fission products, such 3s molybdenum, may present a problem. These have not been investigated. Clear presentation of the polarographic data is a problem in the process plant. Engineers usually have no difficulty in interpreting the conventional uranium waves, as long as a predetermined manner of measurement is followed. However, nontechnical plant
operators often have difficulty in making such a measurement. A derivative presentation n hich would be a partial solution has been tested, but is not practical because of the break caused by tributyl phosphate in the uranium wave. The removal of oscillations from the scan lyith a filter system mould be another aid, Nonever, the use of the primitive polarographic wave as now done has certain inherent advantages. Information other than the concentration of iron in the stream is readily detectable from the increase in the length of the drop oscillations, and a n increase in the size of the break in the wave suggests a n unusual amount of tributyl phosphate in the stream.
LITERATURE CITED
(1) Bertram, H. W., Lerner, XI. K., Petretic, G. J., Roszkowski, E. S., Rodden, C. J., AXAL. CHEW 30, 354 (1958). (2) Carson, W.N.! Jr., hfichelson, C. E., Koyama, K., Ibzd., 27, 472 (1955). (3) Len.is, J. A,, Overton, K. C., Analyst 79,293 (1954). (4) P F e , C. E., Ind. Eng. Chem. 48, No. 3, , / A (1956). (5) Wilson, L. D., Smith, R. J., AKAL. CHEM.25, 334 (1953). RECEIVEDfor review January 28, 1958. Accepted May 26, 1958. Division of Industrial and Engineering Chemistry, Symposium on Xuclear Technology in the Petroleum and Chemical Industries, 131st Meeting, ACS, hfiami, Fla., ilpril 1957.
Spectrophotometric Determination of Copper in Titanium and Titanium Alloys by Dithizone Extraction HOWARD W. PENDER Research and Development Department, Chase Brass and Copper Co., Inc., Waterbury, Conn.
,A rapid spectrophotometric method using a dithizone extraction for the determination of copper in titanium and its alloys is presented. The procedure involves no preliminary chemical separation and is adaptable in the range from 0,0001 to 1%. Variables in the procedure and the effect of possible interfering elements have been investigated. Accuracy and precision data from analysis of synthetic samples, and comparison of results on other available samples a r e presented.
T
determination of small amounts of copper in titanium and its alloys is a fairly recent problem. Electrodeposition has been suggested ( I ) , but a large sample is required, and preliminary separations must be made if the common alloying elements used with titanium are present. Nikula and Codell (3) have reported a polarographic procedure for the simultaneous determination of copper, nickel, and cobalt; hovw-er, the useful concentration range is from 0.2 to 57, for each component. Frank, Goulston, and Deacutis ( 2 ) have reported a spectrophotometric method in which a chloroform-alcohol extract of a copper-neocuproine complex is used for the determination of copper in titanium. Chromium interferes and must be removed. This involves repeated fuming with perchloric and sulfuric HE
acids, filtrations, and finally, a double extraction. Shortly before the Frank, Goulston, and Deacutis method was published, work on a dithizone extraction method of analysis was started in this laboratory for the purpose of establishing spectrographic standards. When a solution of dithizone in carbon tetrachloride is shaken with a n aqueous solution of a reacting heavy metal, an internal complex salt, dithizonate, is formed. This complex salt generally is soluble in the organic solvent, to which it imparts a violet. red, orange, or yellow color, depending upon the metal involved ( 5 ) . Although dithizone reacts with nearly a score of metals, the reaction can be made specific for certain metals by adjusting the p H of the solution to be extracted. For example, copper may be extracted from a n aqueous solution containing a certain combination of metals with a dithizonecarbon tetrachloride solution, by adjusting the aqueous solution to p H 2 . The titanium samples used in this investigation, for comparison, were obtained from the Watertown Arsenal Laboratories, Watertown, Mass. They were prepared for cooperative analysis under the direction of the Panel on Methods of Analysis, Rletallurgical Advisory Committee on Titanium, and distributed by the Watertown Arsenal for reference purposes. These samples were not primary standards.
APPARATUS AND REAGENTS
Transmittance measurements were made in matched 5-cm. Corex cells with a Beckman Rlodel B spectrophotometer. A Burrell wrist-action shaker was used for all extractions. Hydrion short range p H paper (1.4 to 2.8 and 6.0 to 8.0) was used for p H measurements. Dithizone (diphenylthiocarbazone) was obtained from the Eastman Kodak Co. and used as a n 8 mg. per liter solution in C.P. carbon tetrachloride. This solution was prepared weekly and stored under refrigeration. Standard Copper Solution. A0.1000gram portion of 99.99% vacuum-melted, vacuum-cast copper w3s dissolved in nitric acid (1 to 1) and diluted with water to 1 liter (1ml. = 100 of copper). Then 10 ml. of this solution !vas diluted to 1 liter and used as the standard copper solution (1 ml. = 1 y of copper). Standard Titanium Solution. A 1.0000-gram portion of crystal bar titanium (copper-free) w3s dissolved in sulfuric acid (1 to 1) and oxidized Kith hydrogen peroxide (30%). The excess peroxide was destroyed by boiling and the s-olunie made u p to 100 ml. n ith n-ater. Citric Acid Solution. A 50% (weight per volume) solution of citric acid was used. Hydroxylamine Hydrochloride Solution. A 20% (weight per volume) solution of hydroxylamine hydrochloride was prepared daily. Distilled water. Double - distilled water Tvas used throughout the investigation. All reagents were of C.P. quality VOL. 30, N O . 12, DECEMBER 1958
1915
