Table 1.
Comparison of Continuous and Batch Methods on Four HCI-Containing Gas Streams
Continuous analyzer
1000 cc./min. gas flow rate at 15' C. I , Ira. HC1, peq./hr. p.p.m. 270 10.1 4.0 585 21.8 8.6 1,060 39.6 15.6 20,900 778 306
The pH chosen for the reference point of the titration of a strong acid or base does not affectthe level indicated by the generating current other than to influence the degree of cycling. As with all potentiometric titration methods, best results are obtained when temperature fluctuations are not great. Changes in temperature will cause shifts in pH and in the reference electrode POtential. These changes, if they occur rapidly and in one direction, can be troublesome. Cycling temperatures with a frequency of several minutes will cause minor fluctuations; but the average titration current read will be a true indication of the amount of acid or base
Batch samples: 283 liters of gas scrubbed by NanSOa HC1, peq. p.p.m. 46.1 107 184 3870
3.85 8.9 15.4 323
%
Deviation +4.1 -3.5 +1.3 -5.3
being determined. It was shown experimentally that application of heat to the titrating vessel a t a uniform rate of 5' F. per hour would result in a reduction in observed titrating current of 10 pa. (when determining an acid). Lining out at an increased temperature, however, would result in the *titration current's returning to its original level. These figures enable one to estimate the degree of temperature constancy required for any given level of acidity being determined. Normal ambient temperature variations have not caused dficulties with our apparatus within the over-all &5% limits of accuracy which we have experienced with the equipment. This
level of accuracy was determined by comparison with batch methods on the same gas stream during periods of constant HCl level. The batch analyses were conducted by passing 10 cubic feet of sample gas through a sodium carbonate scrubber and subsequently analyzing the solution for chloride by potentiometric titration with silver nitrate. Table I shows the results of four such comparisons. We have had one of these analyzers in continuous service on a refinery gas stream for eight months. During that time, only replenishment of evaporated water from the titration chamber of the cell has been required for continued troublefree operation. LITERATURE CITED
(1) Briglio, A,, Jr., Brackman, J. A. Jr.,
Shaffer, p. A., Jr., u. 8, ~ ~&m-~ merce, Office of Publication Board, OSRD 6183 PB5940, 1945. ( 2 ~ ~ ~ ~ L*~*., (3) FU&, w., Qua&, we, 2. ~ ~chern. ~
147, 184 (1955). R~~~~~~~ for review Accepted July 16,1963.
J~~~
17, 1963.
A Rapid Radiochemical Procedure for Tin ALLEN E. GREENDALE and DANIEL L. LOVE U. S. Naval Radiological Defense laboratory, San Francisco 24, Calif.
b A very rapid radiochemical procedure has been developed for the isolation of radioisotopes of tin from their fission-product isobars. An irradiated uranium solution containing tin and antimony carriers is added to a solution of sodium borohydride. The volatile stannane (SnHd) formed is decomposed in a hot quartz tube to the metal, which i s collected on a cold surface. Stibine (SbHa), which is also formed under these conditions, is removed by absorbtion on an Ascarite column. The tin chemical yield ranges between 15% for an antimony de10' to contamination factor of 2 60% for an antimony decontamination factor of lo3. The time required for separation of the tin metal from the other fission product elements is about 10 seconds. Decontamination factors of other antimony descendents are: I = 7 X lo4, and Te 2 X lo4. Arsenic is also volatilized as the hydride; however, it is not necessary to eliminate it in this work for the determination of the Sn fission yield.
x
>
A
VERY RAPID radiochemical procedure for the separation of tin from mixed fission products (MFP) was required to determine the fission yields
1712
ANALYTICAL CHEMISTRY
and radiation characteristics of shortlived tin isotopes. A simple one-step procedure was sought which would have the required speed and would give an exact time for the tin separation. The decontamination factors for antimony, tellurium, and iodine had to be large since the fission yield of the tin nuclide is finally determined from the amount of a longer-lived radioactive iodine descendent arising from the decay of this separated tin. The Nuclear Science Series Monograph, "The Radiochemistry of Tin" (II), lists a number of radiochemical methods for separating tin from MFP. These procedures include separations by multiple sulfide precipitations, distillation of the halides, solvent extraction, and combinations of these methods. The time for separation varies from 20 minutes to 4 hours, and chemical yields vary from 50 to 95%. Decontaminsi tion factors for descendent and parent elements are not given. Since these methods are unsatisfactory for studies of short-lived tin isotopes, other methods were mvestigated. Production of tin hydride with an akali borohydride has been reported (1, 2, 6, 1.2). This paper describes an application of these methods to give a
rapid: clean radiochemical separation of tin from MFP. Slight modifications were made to this general tin procedure so that irradiated samples could be handled rapidly for determination of tin fission yields. EXPERIMENTAL
Reagents. The concentration of carriers and reagents used are: antimony(III), 10 mg. of Sb per ml. in concentrated HCl; tin(IV), 400 mg. of Sn per ml. in 3 N HC1; and sodium borohydride solution, 120 mg. of NaBHl per ml. in 0.2N NaOH. Apparatus. The apparatus for the determination of tin is shown in Figure 1. A simpler apparatus may be used if a rapid separation is not necessary. Unit A holds the solution of carriers, acid, and mixed fission products. It is fitted with a three-way stopcock t o provide vacuum for introducing the solution of MFP into the vessel and nitrogen for forcing the solution rapidly into unit B. Unit A also has a side arm with a needle valve in series to introduce nitrogen for flushing the air out of the system before starting the reaction. This unit and the method of introducing the sample to it are the same as unit A described previously in the procedure for the rapid separa-
t
~
.
~ l
.
w,j N2
VACUUM
D - S A M P L E SOLUTION
TO HOOD
NEEOLE VALVE
IFigure 1 .
Apparatus for tin separation
tion of antimony and arsenic hydrides from M F P (4). Stannane is formed in unit B, which is a borosilicate glass tube of 18-ml. volume with a grounc female joint that fits onto section A . The gases from unit B pass to a calciLm chloride drying tube, of 12-mm. i.c. and 100 mm. long, which contains Drierite (8-mesh anhydrous CaS04). 'This tube is fitted with a three-way stopcock, one leg of which is vented to a hood. The other leg is in series with :L 100-ml. syringe, unit C, fitted with E, weighted piston to give a pressure of about 1 p.s.i.g. The syringe acts as a surge chamber to hold the rapidly evolved gzs from unit B. A needle valve in line directly after unit C is used for controlling the flow rate of the gas through the rest of the system. Next, the gases pass through a borosilicate glass tube of 11-mm. i.d. and 150 mm. long, of which 110 mm. is filled with Ascarite (20- to 30-mesh sodium hydroxideimpregnated asbesto 3 absorbent) ; a quartz tube heated 5y a h'leker-type burner; and a bubbbr to indicate the flow rate of the flushing gas. Rubber tubing carries the waste gas from the bubbler to the hood. Since stibine and stannane are extremely toxic, it is imperative that they be vented to a hood and thus diluted with air to a nontoxic concentration. Procedure. I n Figure 1, unit B contains 1 ml. of sodium borohydride (NaBH4) solution. This unit and the train of apparatus following i t is flushed for a few minutes with nitrogen before starting the analysis. The needle valve between unit C and the Ascarite column is adjusted to a flow rate of 10 ml. per second. This adjustment is made in the following manner. With nitrogen flowing through the system, the rubber tubing leading out of the bubbler is pinched so that no gas escapes past the bubbler. The
valve which lets the flushing gas into B is turned off when the piston of the syringe reaches the 100-ml. graduation on the barrel. If the plunger remains stationary, the system is gas-tight. A stopwatch is started at the same time the gas is allowed to flow out of the bubbler. I t should take the syringe 10 seconds io empty. In a few trials the correct adjustment of the needle valve can be made. This adjustment should have to be made only once after the apparatus is assembled. Unit A contains 1 ml. of 0.6N HCl and 1 mg. of antimony(II1) carrier. The quartz tube is heated until it just starts to glow. (It is best to heat the central portion of the quartz tube just prior to use so the ends will remain cool. This prevents damage to the rubber tubing and provides a cool portion for the deposition of the tin mirror. While the temperature of the tube does not seem to be critical, overheating will cause apparent migration of the tin into the quartz.) The vacuum is applied (unit A ) and the M F P solution containing 10 mg. of tin(1V) carrier is drawn into unit A followed by 1 ml. of 0.6N HCl rinse solution. It is convenient but not necessary to have both tin tracer and tin carrier present during the irradiation. The vacuum line is turned off, and the inert gas (under 2 lb. pressure) is applied by a half-turn of the three-way stopcock. The reagents are then added to the XaBH, solution in unit B. The tin metal deposits on the quartz tube within about 2 seconds from the start of the reaction. After 10 seconds the three-way stopcock between the Drierite and unit C is given a half-turn. This vents any residual gas to the hood so that the quartz tube with the tin deposit can be safely and quickly removed and that the time from the end of irradiation to the tin separation can be better known. The collected sample is dis-
solved from the tube in a closed system with cold 50% HISOa containing a carrier of the appropriate descendent element (iodine in the present studies). Other reagents may be used for this solution step or the radiations from the separated tin may be measured directly from the quartz tube depending upon the purpose of the analysis. Tin-1 13, which is not formed in fission, may be used for the chemical yield determination if it is added with the tin carrier. An aliquot of the dissolved tin mirror is counted or it is assayed through gamma pulse-height analysis after the fission product tin and its descendents have decayed or have been otherwise separated. The tin-113 counting rate, after correcting for aliquot size and any decay of the tin-113, is a measure of the chemical yield. Luke's (8, 9) photometric method for the determination of tin in microgram amounts is an alternative method of finding the chemical yield. Except for the small amount of gas vented to the hood, all fission product liquids and gases are retained within the apparatus. Since the hydrides of tin and antimony are extremely toxic, a fully enclosed system such as has been described, affords a maximum of safety. RESULTS
Effect of Tin Oxidation State. Our experiments showed that the oxidation state of the tin carrier solution had no detectable effect on the production of stannane. This is not unexpected since nTaBH4 is a very strong reducing agent. I n the application of this procedure to independent fission yields, tin(1V) was used because most of the fission-product tin is expected to be initially in its highest oxidation state, and stannous chloride is readily oxidized by the air (7) dissolved in irradiated solutions. Separation of Volatile Materials from Stibine and Stannane. A clean separation of tin from iodine was essential t o determine the independent fission yields of tin. The desiccant Drierite, anhydrous calcium sulfate, which works well in removing iodine activities from stibine, (4) was used. It holds back other volatile fission product activities as well. A glass wool plug just before the desiccant stops most of the spray and enables one to make several runs before changing the desiccant. A small spring between the one-hole rubber stopper and the Drierite maintains constant pressure on the column so that the gases cannot channel. Separation of Stibine from Stannane. While the reaction of a tin salt with sodium borohydride gives a good yield of stannane, stibine, which has similar physical properties, is also formed in significant amounts. The major effort in developing this procedure was directed toward a separation of the gases stannane and VOL 35, NO. 11, OCTOBER 1963
e
1713
stibine. As mentioned previously, the quantity of iodine descendent is a measure of the cumulative tin yield. Therefore, a high decontamination from all elements in the mass chain was required. In previous work (4) it was found that arsine (ASH,) and stibine (SbH,) can be separated from each other by passing the mixture of gases through a quartz tube a t 600" C. where only the stibine is decomposed to the metal. The arsine passes through this tube and is decomposed to the metal in a second quartz tube heated to a higher temperature. The contamination of antimony in the arsenic mirror was only 0.05%. It was not possible to make a satisfactory separation of tin and mtimony in this manner, From Mellor (10) a list of possible materials which viould preferentially absorb stibine from a stibine-stannane mixture was compiled and studied. At the high flow rates needed to give a rapid separation of the tin from MFP, liquid traps were ineffective. The gas was not scrubbed well enough to reduce the stibine contamination to an acceptable level. Solid traps found to retain varying amounts of stibine were: mercurous salts, freshly-reduced copper powder, silver nitrate, potassium hydroxide, and sodium hydroxide. These solids were mixed about 1:l with asbestos to maintain a fast flow of gas through the absorbant. Of these reagents, potassium and sodium hydroxide gave the best separation of the hydrides under the experimental conditions established. Since these hydroxides are hygroscopic and difficult to handle, a commercially-available mixture of S a O H and asbestos, developed by J. B. Stetser (IS) and called Ascarite, was used. A trap 11 mm. in diameter and 80 mm. long passed only 0.039% of the original amount of stibine entering. The amount of antimony holdback carrier was 1 mg. Figure 2 illustrates the absorption of stannane and stibine individually on an Ascarite trap. Initially, 1 mg. of tin carrier was used in this separation since Schaeffer and Emilius (12) have shown that the tin yield has an inverse dependence upon the concentration of tin in solution. Figure 2 shows that by increasing the tin concentration a better relative separation of stannane from stibine can be obtained. These curves show the absorption of the independent gases. T i t h a mix-ture of stannane and stibine, the absorption behavior is somewhat different. When 33 mg. of tin(1V) c*arrierwas used vith 1 mg. of antimony (111) carrier, more stibine came through the Ascarite than when 11 mg. of tin (IV) was used with 1 mg. of antimony (111). [One milligram of Sb(II1) carrier was used because with only 1 pg. of Sb (111) carrier, 0.25% of the antimony activity deposited with the tin, and with
1714
8
ANALYTICAL CHEMISTRY
H4 from 33 3 m g
Sncm
H4 from I 1 1 m g S n ( E i
SnH4 from 1 rng S n i E : )
SbH, from 1 mg Sb(I!J)
FttO
O 0
l i
U 2
L 3
I 4
I
I
5
6
I
7
I 91
8
I
I
I C ' i
I
I
1 2 1 3
Clil C F A S C A R I - E
Figure 2. trap
Absorption of stannane and stibine on Ascarite
larger amounts of Sb(II1) carrier the Ascarite column might be overloaded.] In the procedure adopted, 10 mg. of tin(1V) carrier is used with 1 mg. of antimony(II1) holdback carrier. To get a still better decontamination from antimony, the column length was increased to 11 cm. with a resultant sacrifice in tin yield. The Ascarite column was filled in the following manner: a plug of glass wool was placed in the constricted end of the 11-mm. i.d. glass tube; the Ascarite was poured into the column up to a mark 11 em. from the glass wool plug with gentle tapping to settle the Ascarite: a short glass wool plug was placed over the Ascarite filling; and, a small steel compression spring was placed between the glass wool plug and the one-hole rubber stopper to prevent channeling. DISCUSSION
A specific application of the rapid separation of tin from its fission product descendents has been made in the determination of the fission yields of tin-131, 132, and 133 in the thermal neutron irradiation of uranium-235. Use was made of the same pneumatic system mentioned in a previous paper ( 5 ) . After leaving the reactor, the pneumatic sample carrier containing the MFP solution, 10 mg. of tin(1V) carrier, and tin-113 tracer (for chemical yield determination) impales itself upon two hypodermic needles. The solation
is drawn through a hypodermic needle by vacuum into A (Figure 11, which contains 1 nil. of 0.6N HCl and I mg. of antimony(II1) carrier. The rabbit and needle assembly are then quickly rinsed with 1 ml. of 0.6N HCI, which is added through the second hypodermic needle. Emptying and rinsing the rabbit takes The tin fission about 2 seconds. products are then separated as described in the tin procedure. The time of the tin separation is determined a t five seconds after the addition of the fissionproduct solution to the borohydride solution. Five seconds is chosen because the SnH4 is passing through the Ascarite column a t a constant rate over the 10-second period. In the determination of the independent fission yield of a given tin isotope, the separated tin was allowed to decay to its known iodine descendent. The tin was then dissolred in cold 50% H9SO4containing a measured amount of iodine carrier, and a radiochemical determination of iodine was carried out similar to Glendenin's (3). The amount of iodine found is a direct measure of the amount of tin originally present. The chemical yield of tin (which varied between 10 and 14%) was determined by counting the added tin-113 tracer after alloiving the fission product tin and its daughters to decay to an insignificant level of activity. The interference of the tin-113 produced from the tin-112 in the tin carrier
amounted to only disintegrations per second. It has been possihle by the above technique t o determine independent fission yields of tin fssion products in thermal neutron fission of uranium-235, tin half-lives, generic relationships, and also prominent gamma photopeak energies from pulse-height distributions taken of the rapidly separated tin fractions. The results in Table I have been obtained from many different tin qeparations made at times after fission varying from 10 secoiids to 30 minutes. 1ndependent evperiniental values of these tin half lives were obtained through continuous wolution of SnH4 from a basic KaBII4scllution of uranium235 during neutron irradiation and collection of the tin descendents on a charged mire. Tentative values by this method are 44 seconds for tin-131, 50 seconds for tin-132, 43 seconds
Table I.
Half Lives and Fission Yields of Tin Isotopes
Half life, Fission Suclide seconds yield, yo Snl31a 65 zt 10 0.7 Sn132 50 f 10 0.5 SnlS3 39 d= 15 0.2 The 1.6-hour isomer is not observed. Its fission yield would be less than 0.037,.
for tin-133, and 20 sec,onds for tin-134. LITERATURE CITED
(1) Gaylord, N. G., J . Chem. Educ. 34, 367 (1957). ( 2 ) Gaylord, ?;. G., “Reduction With Complex RIetal Hydrides,” pp. 1013, 1022. Interscience. Kew York. 1956. ( 3 ) Glendenin, L. E., RIetcalf; R. P., National Nuclear Energy Series, Vol. 9, Book 3, p. 1625. (4) Greendale, A. E., Love, D I,., ANAL. CHEU.35, 632 (1963).
(5)-Greendale, A. E., Love, D. L., U. S. haval Radiological Defense Laboratory Technical Rept., USNRDL-TR-601, Nov. 13. 1962. ( 6 ) Jolly,-’W. L., J. Am. Chem. SOC.83, 335 (1961).
( 7 ) Latimer, W. AI., Hildebrand, J. H., “Reference Book of Inorganic Chemistry,” p. 259, Macmillan, New York,
1936. _ ._. (8) Luke, C. L., A N ~ LCHEM. . 28, 1276 (1956). (9) Luke, C. L., Ibid., 31, 1803 (1959). (10) Mellor, J. W., “A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. VII, pp. 324-5; Vol. X I , pp. 394-400, Longmans, Green and Co., London, 1947.
(11) Nervik, W. E., “The Radiochemistry of Tin,” Sational Academy of Science, Xuclear Science Series, Monograph NAS-NS 3023, 1960. (12) Schaeffer, G. W., Emilius, M., J . A m . Chem. SOC.7 6 , 1203 (1954). (13) Stetser, J. B., Norton, R. H., Iron Age 102,443-5 (1918).
RECEIVEDfor review Rfay 20, 1963. Accepted August 5, 1963.
Auto matic, Am pero metric, Cupfe r ron Titration of Zirconiium in Highly Radioactive Solutions HISASHI KUBOTA arid
J. G. SURAK’
Analytical Chemistry Division, Oak Ridge National laborafory, Oak Ridge, Tenn.
b The amperometric titration of zirconium with cupferron reported b y Olson and Elving hcis been modified and adapted for hot cell analysis. A titration has been devised in which the current is monitored continuously while the titrant i s fed in a t a constant rate. A stirred mercury pool i s the indicating electrode which senses the point where cupferron comes inyo excess, and a platinum wire i s used for the reference. Precision of 1 relative standard deviation i s obtainable for benchtop analysis and 1 .5% for titrations in a hot cell. This titration was specifically devised for the analysis of zirconium in molten salt reactor fuel, and the titration can be performed directly following dissolution without the need for separations.
yo
T
of the Xolten Salt Reactor ExperLnent (commonly referred to as PVISRE: program brought out the need for a method to determine zirconium in highly radioactive solutions. The activity oi’ the sample material makes it mandatory to perform all the operations in a hot cell, thereby imposing certain limitalions on the procedure to be wed. Zirconium analysis is routinely carried out by x-ray fluorescence. The relative e:ase of this analysis coupled with the good results obtainable have given little incmtive to develop chemical methods for zirconium. Since HE INTIATION
the installation of an x-ray fluorescence unit into a hot cell involves serious problems related to cost, operation, and maintenance, attention was directed to the development of a chemical method of analysis. Titrimetric procedures are preferred to gravimetric or spectrophotometric procedures in hot cell work from the standpoints of the ease of installation, manipulation, and replacement. Furthermore. procedures utilizing electrometric end point devices are best because the components within the hot cell require a minimum of space and moving parts, and the bulk of the instrumentation can be located outside. Therefore, it was natural that the search for a suitable method was directed toward a \-olumetric method utilizing an electrometric end point device. Most of the available volumetric procedures for zirconium depend upon the complexation reaction with EDTA. The direct titration with EDTA a t a pH of 1.5 has been described by Fritz and Fulda (3). At a pH of 1.5, the hydrolysis of zirconium becomes a serious problem and a back-titration of excess EDTA (4, 5) is generally more reliable. Olson and Elving (8) reported the amperometric titration of zirconium using cupferron for titrant and a dropping mercury electrode for the indicator electrode. The amperometric titration has many
attractive features which can be utilized to good advantage for this analysis. The titration is performed in a 2iM sulfuric acid medium in which only a few metals beside zirconium can react with cupferron. The high acid medium also minimizes errors due to the partial hydrolysis of zirconium. Best of all, this titration is feasible in the presence of moderate amounts of fluoride, and even large excesses of fluoride can be tolerated by the simple expedient of adding some aluminum ion. This is important t o this particular analysis because the starting material is a mixture of fluoride salts, and the repeated fuming necessary to remove the last traces of fluoride can be avoided. On the other hand, the entire process is tedious and lengthy and is not suited for adaption to hot cell use. -4small increment of titrant has t o be added, the solution deaerated, and the current determination made. This series has to be repeated several times for a single titration. Finally, the current us. titrant volume graph has t o be constructed for the location of the end point. It was thought that if the principle of this titration could be adapted to one in which the titrant is fed in at a constant rate and the current monitored Present address, Department of Chemistry, Marquette University, Milwaukee 3, Wis. VOL. 35, NO. 1 1 , OCTOBER 1963
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