remain closed only long enough to recharge the capacitor until the working electrode reaches the limiting potential set for the electrolysis. However, a small “time lag” prevents the tripper circuit from responding immediately t o any sudden changes in the potential of the rvorking electrode and the current source to cell circuit remains closed for longer periods of time than are now required for charging the capacitor. The response time lag for the relay operation may account for the deviations from linearity at low currents for the curves shown in Figures 4 and 5 . No attempt was made to determine the causes of the errors in these determinations. Hon-ever, to obtain accurate results, the optimum electrolysis conditions given by Lingane (6) should be determined for the reaction of each given species. Advantages, Improvements, and Applications. B y using t h e capacitor
for storage of a portion of t h e current delivered by t h e constant current source, a very efficient electronic coulometer is obtained. The recorded electrolysis time a n d t h e value of t h e constant current may be used t o calculate t h e number of coulombs of electricity delivered to the electrolysis cell. The coulometer system for the determinations reported was used t o measure the number of coulombs for currents ranging approximately from 35 ma. to as lorn as 5 ha.; or, for a current range of 7000 t o l. The instrumental circuit may be improved, the electrolysis time decreased, and the accuracy of the individual determinations increased by modifying the circuit in several mays. An increase in the value of capacitor, C, decreases the number of tripping cycles per unit time and increases the accuracy of measuring the recorded
time. The charging time error for the capacitor is increased; however, this error may be taken into account by Equation 6. The errors associated with relay operation may be decreased by substituting for the relay an electronic circuit using thyratrons or high vacuum tubes. Such a circuit will be shortly described. It will eliminate all moving parts but will require two to four additional tubes. One main difficulty is finding suitable circuits and tubes to switch the higher currents desired in many determinations. Faster relays could be used in place of the one specified by Ehlers and Sease. A4n increase in the attainable range of output currents would be advantageous in some cases. By using a high current, tripping can be initiated at the beginning of the electrolysis and the total time for electrolysis decreased. For other purposes, such as the determination of small amounts of materials, a smaller current output from the constant current source is advisable. Smaller, high-voltage, lowleakage capacitors can be employed. The time pulses can easily be of a higher frequency, should more precise time measurements be necessary. Frequency standards in the range 200 to 3000 cycles per second are simpler and cheaper than those below this range. Improvements in the design of the apparatus along these lines are being investigated in these laboratories. The automatic controlled-potential instrumental system and the electronic coulometer may be used for either controlled-potential or constant current coulometry. The controlled-potential instrumental circuit may be used for any reactions now carried out by constant controlled-potential electrolysis. I n addition t o the applications de-
scribed, controlled-potential coulometry may be used to determine the number of electrons, n, involved in an electrode reaction. W , i, T,F , and M are measured or known and n may be calculated from Equation 1. New and useful preparative methods, particularly of organic compounds, may be developed by the application of controlled-potential coulometry. By using the appropriate instrumental system and electrolysis conditions, this method may be used in any desired concentration ranges. ACKNOWLEDGMENT
This work was supported by the U. S. Atomic Energy Commission under contract At(ll-1)-120. Their support is gratefully acknorvledged here. LITERATURE CITED
(1) Booman, G. L., AN?LL.CHEM. 29, 213-8 (1957). ( 2 ) Delahay, P., ‘%‘em, Instrumental
Methods in Electrochemistry,” Part
4, Interscience, New York, 1954.
( 3 ) Ehlers. V. B.. Sease. J . W.. ANAL. CHEhf. 26, 513-6 (1954). ’ ( 4 ) Elmore, W. C., Sands, M., “Electronics,” pp. 390-3, McGram-Hill, Sew York, 1949. ( 5 ) Gerhardt, G. E., Lawrence, H. C., Parsons, J. S., ASAL. CHEW 27, 1752-4 (1956). ( 6 ) Lingane, J. J., “Electroanalytical \
I
Chemistrv,” Interscience, New York, 1953. ( 7 ) Lingane, J. J., J . -4m. Chem. SOC.67,
1916 (1945). 18) Meites. L.. -4s.4~. CmM. 27. 116-9 (195h). ’ ( 9 ) Reilley, C. S.,Adams, R. N., Furman, Tu’. H., Ibid., 24, 1044-5 (1952). RECEIVEDfor reviem May 24, 1957. Accepted December 16,1957. Abstracted \
I
from theses presented to the Graduate School of Indiana University by Ernest L. Martin, Jr., and Ram Dev Bedi in psrtial fulfillment of the requirements for the degree of doctor of philosophy. Contribution No. 812 from the Chemistry Lab-
oratories, Indiana University.
Differential Therma I Ana lysis Apparatus for Heating and Cooling Data D. D. WILLIAMS, R. D. BAREFOOT, and R. R. MILLER lnorganic and Nuclear Chemisfry Branch, Chemistry Division,
b A versatile apparatus for differential thermal analysis, based on the concept of Rosenhain, has been developed. A constant reproducible temperature gradient is established in a heavy-walled metal tube 32 inches long. The sample holder, containing the base and differential thermocouples, i s pulled in either direction through this tube at the rate 492
ANALYTICAL CHEMISTRY
U. S.
Naval Research Laboratory, Washingfon, D.
required to produce the desired rates of temperature change. Individual runs require approximately 1 hour, but consecutive runs may b e made immediately. Simple modifications permit controlled atmosphere work.
A
thermal analysis (DTA) system based on the thermal gradient produced in a metal DIFFERENTIAL
C.
tube has been developed to test salt systems rapidly for phase changes and reaction temperatures. The system, distinguished by its versatility, is based on the fire-clay furnace developed by Rosenhain (4). It was recently modified by Evans, Fromm, and Jaffee ( 2 ) to determine the melting points of low melting alloys. T h e principle of the apparatus was also
14
P -1
-3
32"
/I
-3 I
- -3 "*
0
1 2
I
I
I
3
5
4
-
MV. ( P t P i 10% R h )
Figure 3.
Differential thermal anal-
ysis curve for ferrous sulfate hepta-
-3
hydrate showing reproducibility Six runs, 0 . 0 3 7 gram, 10' C. per minute
-7
'C
Figure 1, Differential thermal analysis tube (left) and (right) sample holder
described by Brace ( 1 ) . I n contrast t o the usual method of heating a furnace through a temperature range a t a constant rate (8, b), this system establishes a constant gradient in a long metal tube with heavy walls by heating the top portion and cooling the bottom portion. The sample is exposed to this graded temperature by being pulled through the tube a t a constant rate. APPARATUS
The DTA tube and sample holder (Figure 1) map be of any metal, consistent with individual need. For temperatures up to 600" C., an aluminum tube has proved satisfactory. Silver and gold tubes-a large but recoverable investment-extend the range to 900" and 1000" C., respectively. These materials exhibit nearly linear gradients. For higher temperatures, and a t the expense of gradient linearity, stainless steel may be used. The nonlinear thermal characteristics of stainless tubes can be compensated for by machining a tapered tube or by programing the rate of sample travel through the tube. Sickel would be an excellent tube material, except for machining problems and temperature-sensitive phase changes that could affect the gradient. The dimensions in Figure 1 are nominal. The diameter of the sample holder must be adjusted to suit the tube diameter (clearance should not exeeed 0.010 inch), The complete furnace assembly is s1ion-n in Figure 2. Associated recorders and controllers are not shown, as any convenient system may be used,
3
Figure 2. Differential thermal analysis furnace and tube assembly 1. 2. 3.
Furnace shell Insulation (Silocel) Marinite sheet 4. Alundum tube 5. Silver tube 6. Sample holder 7. Alundum cement 8-9. Main heater 9-1 0. Control heater 1 1 . Control thermocouple 12. Differential thermocouple 13. Base temperature thermocouple 14. Motor, gears, and pulleys for moving sample holder
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3'
1
I
2
3
__ 4
5
6
7
8
Y V (Fe-CONST )
Figure 4. Heating and cooling curves for ammonium nitrate
depending upon the sensitivity rpquired and budget considerations. The sample pulling assembly used by the authors is simple, but adequate. It consists of a heat-resistant wire on each end of the sample holder for travel in the furnace proper; a nylon cord attached to these wires forms an endless-belt system which operates over two Teflon pulleys (one 3 feet above and one 3 feet below the furnace tube) and a motor-driven take-up drum. The take-up drum is attached through an appropriate gear train to a constant speed motor which is reversible. The apparatus uses a Bodine 1/150-hp. motor (7.2 r.p.m.) and two radio dial drive gears (a 16 to 1 reduction worm gear and a 5 to 1 concentric shaft). The concentric shaft gear contains a friction clutch which is desirable for positioning the bucket a t the start of a run. Automatic run termination is accomplished by a microsidch in the motor circuit, activated by a knot in the nylon cord. This knot engages a hole in a Teflon washer mounted on the arm of the switch.
0 . 0 5 0 gram, 10' C. per minute
Top temperature control may be accomplished by supplying the exact wattage from a constant voltage source. For routine work, a simple on-off arrangement gives & l o o c. control. A modified arrangement, whereby a small portion of the wattage required to maintain the top temperature is cut in and out as needed, gives excellent control. The sample holder, ideally, should be of the same material as the tube, to eliminate stray gradients within the holder. However, negligible errors result from the use of dissimilar metals, because the location of all thermocouples in the sample holder presents an accurate picture of base temperature us. differential temperature. The authors have used a variety of thermocouple materials and arrangements with success. The current and most versatile array consists of Chromel-Alumel \vires (Brown & Sharpe, VOL. 30, NO. 4, APRIL 1 9 5 8
493
28 gage) sealed into cup inserts of borosilicate glass for the sample and reference differential thermocouple, and 10-mil platinum-platinum, 10% rhodium for the base temperature thermocouple. The recording system consists of an x-y recorder with a direct current amplifier of multiple range in the differential thermocouple system. The available amplification ranges in six steps from =kt25 to = t l O O O ~ v . DISCUSSION
A plot of base temperature against time is essentially linear, using a silver bar and saniple holder and 10% wattage control. A maximum deviation of 5’ C. has been observed which is related t o the on-off cycle of the control heater. The heating and cooling curves are identical. Figures 3 and 4 contain representative heating and cooling curves for single salt systems. Ferrous sulfate and ammonium nitrate illustrate the behavior of the DTA apparatus, but the curves do not represent investigations of the compounds themselves. The ordinates
are intentionally shonn as relative units, because the sensitivity of any given run \\-ill depend on many factors: sample size, thermocouple material, amplification, etc. Each relative ordinate unit is equal to about 2.5’ C. and was recorded with a ChronielAlumel differential thermocouple feeding the +5OO-pv. range of the preamplifier. The abscissas are in millivolts to emphasize the use of different thermocouple materials to extend or conipress the base temperature range. The reference material in all runs was prefired aluminum oxide. The described method possesses many features not inherent t o present apparatus. Foremost is its ability t o record cooling data under the exact reverse gradient used for heating. The gradient is varied by changing the control temperature of the hot zone; the rate of change of sample heating is varied by the same method or by changing the size of the take-up drum. Consecutive runs may be made immediately by changing sample holders or by cooling the same holder. Replacing the cooling
coil with a controlled heater extends small temperature zones to full tube range for minute scrutiny. Raising or lowering of the holder redraws a sample through any desired zone. A slow constant-rate purge of the tube with inert gas controls the atmosphere. Improved, but more complicated, control is obtained by modifying the tube t o prevent back diffusion or entrainment of air. LITERATURE CITED
(1) Brace, P. H., U. S. Patent 1,558,828 (Oct. 27, 1925). (2) Evans, R. M.,Fromm, E. O., Jaffee, R. I., J . M.etals Trans. 4, 74 (1932). (3) Norton, F. H., J . Am. Ceram. SOC.22, 54-63 (1939). (4) Rosenhain, IT., J . Inst. Xetals 13, 160 (1915). (5) Smothers, IT. J., Chiang, Y., Wilson, A., “Bibliography of Differential Thermal Analysis,” Research Series KO. 21, University of Arkansas, Fayetteville, Ark., 1951. RECEIVEDfor review July 5, 1937. Accepted December 7 , 1957.
S pect rochemicaI An a Iys is of Nonmeta IIic Sa mpIes Pellet-Spark Technique with a Multicha n ne1 Photoelectric Spectrometer W. H. TINGLE and C. K. MATOCHA Alcoa Research laboratories, New Kensington, Pa.
b A comprehensive spectrochemical method uses a multichannel photoelectric spectrometer for the quantitative analysis of nonmetallic samples unlike in origin, physical structure, and chemical composition. For the determination of the major constituents the method provides for precision to 1 to 2% of the amount present. Quantitative accuracy may be obtained with either synthetic or chemically analyzed standards. A fusion technique reduces all materials to a common form. The fused sample is pulverized, mixed with graphite, and pelleted. A low-inductance spark discharge is used for excitation. Limits of detection are given for 21 oxides. spectrochemical methods n/ll have been developed the analysis nonmetallic samples, but .41i~
for of there has been a need for a single method that simultaneously satisfies the requirements of routine, control, and general analytical laboratories. Although any universal method has
494
0
ANALYTICAL CHEMISTRY
compromises and limitations, the following method furnishes accurate analysis for major constituents and also limits of detection significant to the geologist or chemical engineer. It is universal in that the procedure is independent of sample origin or physical form, yet it may be applied to routine analysis of a specific product. Through the use of synthetic standards, it can be an absolute method independent of chemically analyzed samples for reference. Simplicity and speed are achieved through mechanical equipment, which can be used by relatively inexperienced personnel. The method is economically acceptable to the routine laboratory n-here man-hours per sample are a concern, and it also provides for an elapsed time per sample which is compatiblr with the requirements of a control laboratory. To analyze a variety of materials without excessive standardization, all samples must be reduced to a common form. This may be accomplished by disqolving the sample in a n appropriate solvent. All samples are then similar
in chemical state, and by dilution all elements are placed in a coninion environment. Aqueous solution methods, such as the porous-cup and spray techniques, are impractical because of the insolubility of many samples in acid or alkaline solutions. Fusion of the sample Ivith a fluying agent provides the most practical method. The advantages of a fusion technique have been adequately set forth by Price and others (4-6, 8-11). Price ( I O . I I ) investigated the use of sodium carbonate and borax as flusing agents and found that samples fused 11-ith borax produced clear, glasslike fusions. For analysis of miscellaneous materials, the determination of sodium is often required and borax cannot be used as a flus. Another fluxing agent which has also been used is a niisture of lithium carbonate and boric oxide (4-6.8, 9 ) . During development of this method, it n-as found that n hen used in proper proportions, this mixture produces clear, glasslike fusions with a wide variety of materials. As this fusion does not wet graphite, the bead formed is
