Table 1.
Distribution of Lead-212 Tracer
Recoverv. “ , 7% ,” Run Run Run A B C
Fraction 1 2 3
1.0 0.7
5
0.0 0.0
0.3
4
97.8 0.1 0.3
6
7
8 9 10
0.1 0.3 0.0 1.0 1.1 0 . 2 0.0 0.0 0 . 0 97.8 0.0 0.4 0.0 0.0 0.2 0.0 1.5 0.1 0.2 0.0
92.8 4.1 0.1
11
12 13
Material balance
100.2 100.1 99.8
Some of the data obtained in connection with other work are shown in Table I to demonstrate the effectiveness of lead-212 as a tracer. Dithizone was being used to separate lead and bismuth both from the samples and from each other. Every fraction obtained through the entire analytical procedure was counted separately so that the distribution of lead could be studied. About 1.5 X lo6 c.p.m. of tracer was taken initially for both the experiment and standard. All solutions were counted the following day at which time the standard counted approximately 4 X 105 c.p.m. The data shown are not specially selected but represent the first three experiments performed using the lead-212 tracer. The material balances obtained show the excellent precision with which the tracer could be accounted for even over the large
number of fractions obtained. hll subsequent work has further confirmed the effectiveness and convenience of this tracer. I3y using gamma spectrometry, information on the distribution of bismuth can be obtained simultaneously. Use of a gamma-emitting tracer has proved particularly useful in demonstrating the losses of lead produced by volatilization, ion exchange to glass surfaces and silica, and reduction and consequent alloying during fusions in platinum dishes. The relatively short-lived lead-212 can also be used to determine the yield in the radiochemical determination of lead-210. LITERATURE CITED
(1) Gibson, W. hI., “The Radiochemistry of Lead,” National Academy of Sciences, Xuclear Science Series, NAS-SS 3040 (1961).
Zone Refiner with Temperature Control Fred Ordway,’ National Bureau of Standards, Washington 25,
the heat is usually radiation (1, 3). The temperature of the sample is not directly controlled. For many samples, however, elevated temperature endangers purity because of rearrangement, decomposition, or reaction with atmosphere. Therefore, it is often desirable to control the maximum temperature to which any part of the sample is subjected, or at least to know it accurately. When the zone refining process is in smooth operation it may be possible to estimate the temperatures to which the sample is being exposed. When the process is starting up, on the other hand, the temperature in any given region of the sample may be highly unpredictable. If the container is initially filled with granular material, or even melted material with voids, the isolated portions of the sample may often reach significantly higher temperatures than does any portion during the ideal steady-state process. After all voids have been eliminated the subsequent passes may or may not remove impurities resulting from the overheating. With some samples the complete elimination of voids may be difficult or impossible. The fundamental problem is that the temperature is determined by the accidental value of thermal resistance, together with a fixed hcat flow from a source of very high temperature. To
I applied ’ by
N ZONE REFININQ,
Present address, Melpar, Inc. Falls Church, Ya. 1178
ANALYTICAL CHEMISTRY
D. C.
control the temperature properly the heat should flow to the sample, from a source whose controlled temperature is close to t h a t of the sample, through a comparatively low thermal resistance. The maximum sample temperature is then limited to the source temperature required for the steady state of the process. This objective has been attained by arranging to circulate a fluid of controlled temperature over a narrow band around the sample tube. The constant-temperature band is made capable of motion along the tube by the use of shaft seals. DESCRIPTION OF APPARATUS
The construction of the fluid carrier is shown in Figure 1. The assembly consists of alternating partitions, P , (bearing shaft seals that closely fit the sample tube) and collars, C, (each with two tubes for fluid circulation) , with a window section, W7, for each chamber. These parts are separated by gaskets, G . The assembly is aligned by the internal shoulders on the collars and by three symmetrically disposed bolts, B, and is clamped together by nuts, N , bearing on two end plates, E . Outside diameter of the end plates is 2.5 inches (63 mm.). ’4 series of three annular compartments is formed by four of the partitions. Each of the compartments has two connecting tubes for fluid circulation and a glass wall for viewing the sample inside. The shaft seals were automotive-type oil seals (Garlock “Klozure” or equivalent), but O-ring seals or other types
might possibly have been used. Interchangeability of units designed for different-sized shafts is desirable. A suitable tolerance to the sample tube’s deviations from perfect centering and roundness is necessary. The fluid carrier was made by assembling a set of parts, all clamped together with gaskets, to permit variations in the number and lengths of the compartments. Any future units having just three compartments can be constructed of fewer pieces and require no more than three gaskets. The compartments may be as short as 1-2 cm. each. The present unit was slipped over the sample tube while loosely assembled, and then given the final tightening, to ensure correct alignment of the seals. The fluid carrier was attached to the translation mechanism of a Fisher zone refiner. The upper end of the sample tube, protected by a short length of elastomer tubing, %as clamped in a three-jaw drill chuck of 0.5-inch capacity, which projected downward from the 10: 1 worm gear reducer of a 1/15-h.p. variable-speed motor mounted on top of the zone refiner cabinet. The sample t’ube was in contact only with the driving chuck and the elastic seals in the fluid carrier. Temperature-controlled heating fluid was circulated through the center compartment and coolant through the top and bottom compartments. The fluids were supplied, through silicone tubing, from two small baths by immersiontype centrifugal circulating pumps. The fluid pressure was slight, flow rates being less than 0.1 liter/minute. The hot bath was filled with DowCorning 510 silicone fluid. The cooling
I
TUBE
PLUG
P
C T
W Figure 2. Sample tube arrangement to prevent breakage by expansion of sample in melting
-G P
Figure 1 . refining
Fluid carrier for temDerature control in zone
fluid was water containing a colorless emulsifiable oil ordinarily used with machine tools. T a p water, used directly as a coolant, corrodes the metal shells of the shaft seals. Therefore, the tap water was instead run through a copper coil in the emulsified oil bath. The connecting tubes, gaskets, and seals did not deteriorate seriously in contact with the silicone and emulsified oil, although the hot bath was used a t temperatures above 160' C. for a number of months. The hot bath was maintained at constant temperature within less than 0.5' C . by a proportioning controller with thermocouple sensor. The sample tubes were made from stock lengths of ordinary borosilicate glass tubing, usually 8 mm. in diameter, In normal operation the tube was rotated at 10-30 r.p.m. The rotation made the molten zone quite symmetrical and possibly reduced the sliding friction of the seals along the tube. The tube was so well lubricated by the circulating fluids that friction was not a strain on the translation mechanism even without rotation. Periodic stopping or reversing of the rotation would improve mixing in the molten zone and thus might improve purification by reducing the concentration of rejected impurity a t the advancing solid-liquid interface. PERFORMANCE
The apparatus was capable of several months' operation with a total fluid leakage of only a few milliliters when used with glass tubing selected to fit the seals. I n a single lot of stock tubing, about 80-90% appeared usable for this purpose. The end of the tubing inserted in the shaft seals was always sealed off, or at least fire-polished.
Breakage of the sample tube did occur a t times, with deleterious outpouring of the circulating fluids. The inconvenience of such events was reduced by placing the entire zone refiner in a large tray. It is believed that this danger could be minimized, and leakage in routine operation eliminated, by a change in the circulating system. Suction pumps should be used, so t h a t a fluid leak stops the flow and all fluid is automatically removed, and the baths should both be filled with the same fluid and interconnected with a siphon, so that any flow of fluids between adjacent compartments is automatically compensated. Using a single fluid would eliminate the tendency toward foaming in the present baths when leakage occurs from one compartment into another. The present apparatus has been used for zone refining a t speeds from the zone refiner's maximum, about 60 mm./hour, to 1 mm./hour or less (obtained by connecting an auxiliary interrupter in the motor circuit). It has been run continuously for 1-3 weeks with samples of naphthalene and for periods of several months with the straightchain hydrocarbons C32He6 (melting point 89.5' C.) and &HISO (melting point 113.9' C.). The naphthalene samples eventually formed single crystals while the hydrocarbons apparently remained polycrystalline. Passage of the molten zone upward through the sample appeared to eliminate voids more dependably than downward passage, and the upward motion was therefore used throughout. There appeared to be no hindrance to the use of the apparatus
at a n y temperature for which a suitable heating fluid is available. Special shaft seals can be obtained for considerably higher temperatures, and the temperature of the seals in a n y event can be held well below the melting point of the sample. For work at considerably higher temperatures with a circulating fluid, the elastomer tubing can be substituted by metal sylphon tubing. hlternatively, a n arrangement of telescoping delivery and receiving tubes can be used, with gravity flow through the fluid carrier, to eliminate the flexible connecting tubes entirely. The stability of the size of the molten zone depended as much on constancy of flow as on temperature control. The bath temperature for zone refining of naphthalene, for example, was 118' C., about 38' higher than the melting point. Connecting tubes without the restricted bore of the present design would be desirable to reduce this differential and thus the sensitivity to variations in pump performance. The length of the molten zone was kept between about 2 and 10 mm., and did not fluctuate rapidly enough to produce a motion of the advancing solid-liquid interface comparable with the slowest translation rates used. SAMPLE TUBES
The major cause of breakage was not a fault of the apparatus, but a property of the sample itself-its tendency to create pressure on melting by expansion against a column of solid adhering to the sample tube. When experiments were made with naphthalene, it was early observed that sample tubes broke near the bottom, where the new molten zone was formed. I n fact, the sealed end of the glass tubing was frequently pushed off and a length of the solid sample extruded. Clearly the 10-30 VOL. 37,
NO. 9 ,
AUGUST 1965
0
1179
cm. of crystalline material above adhered to the sample tube so well that the expansion during the melting of the new zone produced high pressures. The problem did not occur with the straight-chain hydrocarbons, and may or may not be serious in a given case depending on the substance and the dimensions of the sample tube. Joncich and Bailey (2) have solved this problem by using polytetrafluoroethylene sample tubes. Such tubes would be unsuitable for the present apparatus because rigidity is required for proper rotation, dependable functioning -of the shaft seals, and protection of the final crystal from deformation. The problem was eliminated by the arrangement shown in Figure 2. A tapered glass plug, sliding loosely and supported by a spring, forms the end of the sample space. The molten zone initially extends down to the point where the annular space between the
tube and the plug attains its minimum width, but not far beyond. If the sample requires added space for each new zone, the plug moves downward gradually and the spring is compressed. The initiation of crystal growth in the narrow annular space may encourage formation of a single crystal, as does the pointed tip frequently used on sample containers in the Hridgman method. To avoid excessive pressures no rapid change in length of the molten zone, such as that which occurs on restarting the apparatus after a power failure, should be permitted except a t the end of the traverse. If the solid adhers to the sample tube, the multiple-zone oscillating-heater technique of Sloan and McGowan (4) could in principle be used with a fluid carrier of many compartments. Such an arrangement would present mechanical problems because of the large total frictional torque of all the shaft
seals. If the solid does not adhere well, the body of solid between two liquid zones might tend to rise or sink and disrupt the continuity of the process. In this case a multiplicity of zones would be undesirable. ACKNOWLEDGMENT
The author thanks Harold Johnson for his loyal assistance throughout the development and testing of this apparatus and C. P. Saylor for helpful discussion. LITERATURE CITED
(1) Baum, F. J., Rev. Sci. Znstr. 30, 1064 (1959). ( 2 ) Joncich, M. L., Bailey, D. R., ANAL. CHEM.32, 1578 (1960). (3) Lawson, W. D., Nielsen, S., "Preparation of Single Crystals," Butterworths, London, 1958.
(4) Sloan, G. J., NcGowan, N. H., Rev. Sci. Znstr. 34,60 (1963).
High Precision Conductivity Bridge Robert L. Wershaw and Marvin C. Goldberg, U. S. Geological Survey, Denver, Colo. 80225
r
N THE DEVELOPMENT of methods for
the measurement of solubilities of pesticides in water, a conductivity bridge of very high precision for measuring small differences in conductivity was required. Commercially available instruments could not meet all of the required specifications. Therefore, the bridge which is described below was designed and built in our laboratory. A schematic diagram of the bridge circuit is given in Figure 1. The ratio arms of the bridge are formed by a variable tap autotransformer (ESI Model IlT72.4 or Gersch 1000 Series Inductive Divider). A standard cardwound resistor of low capacitance and inductance, and a Jones conductivity cell make up the comparison arms of the bridge circuit. The two leads from the oscillator are connected directly to the comparison arms of the bridge. The resistances of the leads to the inductive divider are, therefore, effectively eliminated from the comparison arms of the bridge and are incorporated into the ratio arms of the bridge, where the resistance of the leads is negligible compared with the input impedance of the divider (approximately 1,000,000 ohms at 1000 cycles/second). An appreciable error is eliminated by this type circuit, for it was found that errors of the order of 50-100 p.p.m. in ratio measurements resulted from having the oscillator connections at binding posts '1 '1M)
ANALYTICAL CHEMISTRY
of the divider. A complete discussion of four terminal resistance measurements is given by Thomas ( 2 ) . The quadrature correction is made with two adjustable air capacitors connected across the over-windings of the autotransformer.
This conductivity bridge offers the following advantages over the Jones Bridge ( I ) which has been used almost universally up to this time for high precision conductivity measurements : the measurements are made using a fourterminal resistance measuring circuit
CONSTANT TEMPERATURE BATH
r - 1 2
Figure 1 .
Circuit diagram of conductivity bridge
