T o determine the effect of temperature on gas release, samples of UOZ were heated to 1000 "C and held until gas release was complete. This procedure was repeated at 200 "C intervals up to 1600 "C, at which temperature the material was at least partially sintered. On the material tested the gas release was 0.15 ml/gram (STP) at 1000 "C and increased linearly to 0.31 ml/gram at 1600 "C. One obvious problem is the possible loss of gas from the sample while it is in the vacuum system but before it is heated. If there is an appreciable loss, one would expect the apparent gas content to decrease as a function of the time the sample was under vacuum before analysis. Four of the samples included in the precision data were left under vacuum for extended periods of time (20,42,54,and 79 hours) before being analyzed. Results from these four samples were in the same
range as the remainder of the eleven samples which were under vacuum less than twenty hours before analysis. The apparatus as described provides a reasonably precise and accurate measure of gas content in ceramic fuel material. Because of problems related to the use of glove boxes, all crucible bake out is done during normal working hours. With crucible bake out and sample loading time, approximately 16 analyses can be performed in two days. The powder containers and associated glassware are a major improvement and greatly simplify the handling of powdered material.
RECEIVED for review November 12, 1968. Accepted December 27, 1968. Research supported by the U S . Atomic Energy Commission under Contract No. AT(45-1)-1830.
Versatile Temperature-Jump Cells with Long Light Path Rufus Lumry and Richard Legare1 Laboratory for Biophysical Chemistry, Department of Chemistry, Unicersity of Minnesota, Minneapolis, Minn. 55455 MOSTTEMPERATURE-JUMP CELLS use windows made of quartz or glass slugs, and the heated optical path is usually 1 cm long. Even the highest quality silica slugs display stars caused by crystallization and other imperfections when placed between crossed Nicol prisms. Thick glass windows contain strains built-in during manufacture and additional strains are introduced when the windows are rigidly glued into the cell. As a result of the strains and imperfections, it was not possible to obtain a dark field-i.e., a suitable null condition, when attempting to measure chemical relaxation by changes in optical rotation. When thin windows were used in cells even approximating conventional design, the shock wave produced by rapid heating of the cell contents destroyed the windows or produced vibrations in the windows causing spurious signals caused by the strain-induced birefringence. Furthermore, the conventional cell design can be modified for light paths of more than 5 cm only with difficulty, and then with a large total cell volume ( I ) . CELL DESIGN
The most generally satisfactory type of cell design we have developed is shown in Figure 1. For work with water solutions, any insulating plastic is satisfactory. For nonaqueous solvents Teflon by Du Pont or Kel F is recommended, and, compared with the other plastics, seems to reduce the tendency to arc through the solution and along the plastic walls at high potential gradients. The cell shown has a 10-cm heated path length and a 20-ml total volume. The heated volume is 5.3 ml. It is important to avoid reflection of the light beam from the walls in optical-rotation work, and shorter cells simplify the optical problem. However, cells of this design with at least 20 crn of heated path length appear to be practical. The inter-electrode distance is 0.6 cm and the beam width should be no greater than 3 mm. The heated area is outlined by longitudinal plastic ridges extending up from the bottom of the cell and down from the top. Contact to the electrodes Present address, Allegany Ballistics Laboratory, Cumberland, Md. (1) R. Legare, Dissertation, University of Minnesota, 1962.
Figure 1. Cell design
may be made in any number of ways as required by the construction of the cell housing. The high-voltage electrode in our arrangement is connected through the long rod shown emerging from the cell. The plastic portion of the cell rests in a brass and plastic block which is thermostated, and the block itself rests on pins which center it and provide contact to the ultimate ground. The outside configuration of the cell can be modified to adapt it to other versions of T-jump apparatus. The novel feature of the cell as well as its most important feature is the provision of wells at either end of the cell connected to the main compartment by holes of 4- to 5-mm diameter. The windows are small quartz flats of 1-mm thickness or pieces of microscope slide glass, and are held in place against the outer face of the wells either by grease or by wedgVOL. 41,NO. 3, MARCH 1969
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ing with small nylon or rubber pieces. The well and hole design totally damps the shock wave before it can reach the windows and thus provides mechanical protection for the windows. Round quartz slugs placed in the inner holes between the wells and the main compartment and thus in contact with the inner contents of the cell did not crack during temperature-jump heating, but did develop transient strains which appeared as spurious oscillating 0.d. transients persisting for several milliseconds. The wells haye the disadvantage that they introduce nonheated segments of the solution in the light path which reduces the percentage change in measured parameter. Hence, for spectral-change measurements, conventional quartz slugs can be inserted into the inner holes to eliminate the wells. When relatively long heating times are used ( > l o p s ) , the well length can be reduced from 8 mm t o as little as 4 mm. Our electrodes were made of platinum-coated silver on brass. Stainless steel electrodes arced badly, but solid gold or heavy gold plated electrodes can be used. It is essential that all corners and edges, excepting back edges, be rounded. A radius of curvature of about 3 / 3 2 in. worked in our cells. The two electrodes must be of the same length and oriented symmetrically about the axis of the light path. Imperfect orientation produces a prismatic effect in the heated solution caused by the failure of the isothermal lines t o cross the light-path axis at 90". With a 0.6-cm separation between electrodes, n o arcing occurs up to a potential gradient of at least 35 kV/cm. Higher potential gradients can probably be used if suitable attention is paid to electrode smoothness and plating. Arcing tends to occur along the shortest plastic-surface path between electrodes, but this path can be extended by cutting vertical grooves in the end walls of the inner compartment. Care must be taken to remove air bubbles under the electrodes when the cell is filled. Because of the large metal area, cooling is apparent after a few hundred milliseconds and the cells are not suitable for transient processes of that length. USE OF THE CELL
Although fluorescence transients can be observed with a photomultiplier placed on top of the cover, modified EigenCzerlinski-Diebler cell designs (2, 3) are usually better for this observable. The particular utility of the new cell lies in the long light path, which allows a factor of ten in dilution of reactants over conventional cells, and in its suitability for optical rotation studies. In studies using spectral changes, Halpern, Legare, and Lumry have been able to measure second-order rate constants as large as 4 X 109 M-1 sec-1 in electron-transfer reactions with a 6-cm heated path length ( 4 ) . In optical-rotation measurements, the cell is placed between crossed Glan prisms. The light beam may be split before being collimated for passage through the cell, or it may be split after passing through the cell. In either case, the beams are collected on separate photomultipliers, the outputs of which are then fed to a good differential preamplifier. When the beam is split before the cell, two experiments are necessary to measure the absolute value of the change in optical rotation produced by heating, and t o allow rejection of effects not caused by such (2) M. Eigen and L. de Maeyer, Technique of Organic Chemistry, Ed. by S. L. Friess, E. S . Lewis, and A. Weissberger, Interscience Publ., 8, 969 (1963). (3) G. H. Czerlinski, "Chemical Relaxation," Marcel Dekker, Inc., New York, 1966, p 214. (4) J. Halpern, R. Legare and R. Lumry, J . Amer. Chem. SOC.,85, 680 (1963).
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ANALYTICAL CHEMISTRY
changes. The prisms are first set to the best null position. Depending on the size of the transient to be observed, the analyzing prism is then rotated through a n angle of 0.5 to 2" on one side of the null position by a suitable gear and scale system and a temperature jump is made. The analyzing prism is then set at the same angle on the other side of the null position and the second picture is taken. Relaxations which are mirror images of each other are caused by opticalrotation changes; relaxations due t o birefringence changes or changes in optical density are not mirror images of each other. Dilution of the cell contents on heating decreases the rotation change symmetrically on the two sides of the null position. If there are no other changes, a n optical-rotation change in the cell will provide two trace recordings symmetrical about the zero line. However, changes in scattering and light absorption also occur on heating and these move both traces in the same direction, so that the pattern is not symmetrical. Birefringence effects with polymers and proteins and changes in light absorption or scattering produce nonsymmetrical patterns and are thus readily separated from effects caused by opticalrotation changes. In our apparatus, total optical-rotation changes of 0.01 O at scanning times of 10 psec per cm of scale are easily measured (5). The practical limit appears to be a total change of about 0.002" a t this scanning time with 10-cm heated paths. Eigen (6) has designed a cell which splits the beam after it emerges from the cell. The two beams pass through Glan prisms set at small equal and opposite angles on either side of the null position. The resulting differential input to the oscilloscope contains only optical-rotation changes so that only a single experiment is necessary. The following information summarizes the important design and operational characteristics of this T-jump cell. Light source: Osram HBO 100 mercury lamps with batterystabilized constant voltage were satisfactory for both opticalrotation and absorbance change in the visible and ultraviolet regions. Heating: (for 0.5 pF storage capacitor). An aqueous solution of KNOpof 0.2M ionic strength gave an 8" temperature rise in less than 10 psec with the capacitor charged to 16 kV. A nonaqueous solution of 0.2M tetramethylammonium bromide in 76z dichloroacetic acid and 24% 1,2 dichloroethane gave a cell resistance of 200 ohms and a heating time of 100 psec with capacitor charged to 10 kV. The temperature jump was estimated to be 7 "C. Sensitivity and noise level : Optical-rotation measurements. At a bandwidth of 0-200 kc (and larger) the sensitivity is 0.01" per cm of oscilloscope deflection with a n rms noise equivalent of 0.01 ". With the bandwidth reduced to 0-10 kc the sensitivity was 0.005" cm with a 0.005" rms noise level. Absorbance measurements: At a bandwidth of 0-200 kc (and greater) the sensitivity was better than 0.01 absorbance unit/cm with rms noise equivalent of 0.005 absorbance unit. ACKNOWLEDGMENT Our thanks t o Warren Ibele for Figure 1. RECEIVED for review July 31, 1968. Accepted November 29, 1968. This work supported by grants 9360 and 9629 from the National Science Foundation. ( 5 ) R. Legare, W. Miller, and R. Lumry, Biopolymers, 2,489 (1964). (6) M. Eigen, Max-Planck-Institiit fur Physikalische Chemie,
Gottingen, Germany, personal communication, 1968.
