is the same, they may be wired in parallel as well as series. The lower end of plate I is a convenient place t o mount a socket for making the electrical connections. Teflon or additional glass insulation should be used on the wires between the coils and plug connections. I n the present case about 7 or 8 feet of Constantan wire taken from S o . 28 glass-covered thermocouple wire was used in each heating unit. The units were wired in parallel, and the heated cell was plugged into a Variac, nhich was in turn connected to the secondary of a 10-volt 8-ampere filament transformer. This arrangement gave a temperature of 120' C. a t a maximum Variac setting of 130. Over-all temperature variability without special control devices was about 4" C. during about 100 minutes. To reduce the temperature drop when the cell is put into the instrument, the ways were insulated from the instrument proper by Bakelite. I n this iTay reduction in temperature on insertion into the ways was limited to less than 3" C. in the region near 70" C. The temperature
could be raised from 40' C. t o 110" C. in about one hour. For certain purposes this performance could be improved with a revision of the heating circuit and special devices for controlling temperature] but for most purposes i t was found adequate. This cell has been used mostly between 70" and 90" C. Richards and Thompson [Trans. Faraday SOC.41, 183 (1945)l discuss the use of a heated cell a t temperatures up t o 200" C. The temperature of the cell may be measured by attaching a thermocouple to one of the heating units. This point may be calibrated with respect to the sample by inserting the flattened end of another thermocouple between the cell windows a t the well. Temperature variations over the section of the cell exposed to the infrared beam should ordinarily cancel out in much quantitative work; in some studies they may become important. The mounted cell, heated to the desired temperature, is loaded with the hot liquid sample; after measurement
the cell is cleaned and dried without demounting] using an appropriate solvent and a n air stream. As a safeguard] the thickness of the cell should be checked occasionally using interference patterns. Limitations imposed on this simple procedure by the use of highly elevated temperatures have not been studied. This heated cell has been used in a quantitative method for oil and stabilizer in oil-extended SBR synthetic rubber in which the semisolid extract of the sample to be analyzed must be heated before analysis to form a homogeneous liquid. A cell of this type has been used in quantitatively comparing transmittances of highly viscous and semisolid fractions of high molecular w i g h t oils [Linnig, F. J., Stewart, J. E., J . Research Natl. Bur. Standards 5 9 , 27 (1957); RP 27711. WORK performed as part of a research project sponsored by the Federal Facilities Corp., Office of Synthetic Rubber, in tonnection with the Government Synthetic Rubber Program.
Simple Thermostatic Device for the Beckman DU Spectrophotometer Harold J. Martin' and George Gorin, Department of Chemistry, Oklahoma State University, Stillwater, Okla. HE absorption spectra of many Tchemical systems are significantly altered by temperature changes. For the spectrophotometric analysis of these systems some means of temperature control is desirable, and such control is necessary for studies of reaction rate. An apparatus is described that can be constructed a t low cost and M ill provide temperature control of 10.5' C. in the range from 20" to 50' C . for cells in the Beckman DU spectrophotometer.
The parts that must be qpecially made are a cell compartment cover and two hollow metal cell. shaped like the IO-mm. rectangular cells commonly used in the spectrophotometer. The cover is milled from a 1 X 1.1 X 5 inch block of aluminum and fits the cell cornpartment in place of the standard cover. The hollow metal cells. each 0.5 X 0.5 X 1.5 inches, are made from t n o pieces of brass soldered at the seams. The water inlet and outlet of each cell are made of copper tubing, 3'16 inch in outer diameter, and soldered into the top of each cell; the inlet tube evtends through to the bottom of the cell. Water connections to the cells are made through holes in the cowr with thin-walled black rubber tubing inch in outer diameter. Care should he taken in making these connection. t o Present address, Connors State -4gricultural College, Warner, Okla. 892
ANALYTICAL CHEMISTRY
forestall the possibility of a connection's coming loose inside the instrument. The tubing must fit tightly in the holes through the cover, so light will not enter; a simple check can be made n-ith a flashlight with the phototube shutter open. Finally, the tubes must have enough slack so the cells can be moved in and out of the light path. The metal cells are placed in locations 1 and 4 of the standard Beckman cell holder; locations 2 and 3 are used for the blank and sample cells. The temperature in the blank or sample cell is determined by inserting one junction of a thermocouple through the compartment cover into either cell. The junction has flexible leads and is clipped to one side of the cell to avoid interference with the light beam. The cover, the tn-o cells, and the thermocouple junction are assembled as shown diagrammatically in Figure 1. The temperature in the cell compartment is regulated and maintained by circulating water from a constant temperature reservoir through the metal cells. The exchange of heat is not sufficiently efficient to establish temperature equilibrium between the sample cells and the reservoir. which must therefore be maintained several degrees above or below the temperature desired, if this is substantially different from the normal operating temperature of the instrument. Little or no regulation can be achieved by varying the rate of flow of nater through the cells.
hC_i
Figure 1.
Cell compartment unit 1. 2.
3.
Cover Cell Thermel
The constant temperature reservoir can be of conventional type. The reservoir used in this work was a 4-liter Deaar flask, with an immersion heater placed inside it. Water was circulated from the reservoir to the metal cells to a cooling coil or a bypass tube and back into the reservoir by means of a small centrifugal pump (Eastern Engineering Co.. S e w Haven, Conn., Model B-I)] run a t 60 volts. The cooling coil was made from a 12-foot length of ','r-inch copper tubing. The flow of water through the cooling coil could be regulated by diverting part of the flow through the bypass and the temperature i n the reservoir could thus be maintained below room temperature.
The operating characteristics of the apparatus depend on its exact construction, and must be determined in preliminary experiments. The following results indicate the performance to be expected. First, the flow of water n-as adjusted to a convenient rate, approximately 1 liter per minute. Secondly, a calibration curve was constructed by maintaining the reservoir a t various temperatures, and determining the temperature attained in the sample cells after steadystate conditions n ere established. As the data gave a straight line, fire measurements n ould in general be sufficient for delineating it. From the line, it could easily be determined what reservoir temperature would give any desired temperature in the cell compartment. Minor additional adjustments, if needed, were then made by changing the reservoir temperature further. I n the apparatus used in this work,
sample-cell temperatures of 20°, 30°, 40°, and 50" C. required reservoir 29", 45.5", and temperatures of Eo, 62" C., respectively. When the reservoir temperature was set a t 0" by filling with ice and water, the temperature established and maintained in the sample cells was 12.5'; however, no regulation was possible in this case (when absorption measurements are done below room temperature. condensation of moisture on the faces of the cells must be guarded against; this was not serious a t 12.5"). When the instrument was originally a t its normal operating temperature, the time required before the sample cells reached the required temperatures was about 15 minutes for 20" and 30°, and as long as 40 minutes for 50". Once attained, the temperature could be maintained within k0.5' for 1 hour or longer. I t is not good practice to keep samples in the cell compartment
for a long time. If measurements are desired a t long intervals, it is preferable to keep the sample in a thermostat, and to transfer it to the cell compartment when the measurements must be taken. The parts of the instrument next to the cell compartment are heated only very slightly even when water from a reservoir a t 70" C. is passed through for some hours. The wave-length scale was checked with the mercury line a t 436 mp, and found to vary by less than 1 mp during this time. ACKNOWLEDGMENT
Heinz Hall and Frank Hargrove constructed the special apparatus parts described and their assistance is gratefully acknowledged. WORKsupported by the National Science Foundation through Grant G-5966 and the Research Participation Program, Oklahoma State University, summer 1959, of which Harold J. Martin was a participant.
X-Ray Diffractometer Gear-Changing Mechanism A. C. Lilly, 1. H. Milne, W. T. Caneer, and J. A. Dalzell, Gulf Research and Development Co., Harmarville, Pa.
E
operation of the Philips high angle diffractometer for semiquantitative analysis of mineral mixtures requires that the scanning speed be as fast as possible. However, a certain amount of resolution and intensity accuracy is sacrificed a t high scanning speeds and it is frequently necessary to record portions of the diffraction spectrum a t slower rates. Changes in scanning speed are normally accomplished by manually changing a pair of gears on the front of the goniometer. This procedure becomes timeconsuming when a large number of samples with differing requirements are involved. The problem has been solved in this laboratory by installation of a mechanical gear-changer which permits rapid selection of three different gear ratios. The details of the gear changing mechanisms are illustrated in a cross section of the apparatus shonm in Figure 1. FFICIENT
The driving shaft of the goniometer motor has been extended to support a movable sleeve upon which are mounted three gears increasing in diameter from front to back. The driven gear shaft has been extended also to carry three gears arranged in decreasing diameter from front to back. Spacing of gears on the shafts is arranged so that one pair of gears is engaged a t a time by forward or backward movement of the sleeve on the motor shaft. The sleeve is fixed to the motor shaft in a rotational sense by a slot arrangement. .4 knob
2.7083 0 D
C'
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S C A L E IN INCHES
0
Figure 1 .
1/2
I
+
2
Layout drawing of gear-changing mechanism
Area between A and A' i s cross-sectioned to indicate internal construction. ratios. AA' 1 :4; BB' 1 : 1; CC' 4: 1
is available on the outside of the gear box to move the sleeve, and ball-ingroove stops are provided to locate correct positions. The gear-changing mechanism has been operated for a year with the goniometer motor running, without noticeable damage to the aluminum gears. However, if more expensive gears were used, it would be advisable to stop the
Gear
motor before changing gears so as to prolong gear life. Specific scanning rates may be selected on the basis of the gear ratios of the three pairs of gears which are installed. At present the mechanism is installed on two diffraction units in this laboratory, one of which provides scanning speeds of 4O, lo, and per minute and the per minute. other 2", 1/20, and VOL. 32, NO. 7, JUNE 1960
893
