of 15 pa. I n eight determinations the range was from 240.3 to 242.6 seconds with a relative standard deviation of =t0.3%. Three to five increments of titrant were automatically added before a constant potential was maintained. More precise results could have been obtained by manually adding the last few seconds of titrant.
The detector may be used with any constant current source provided with the necessary relays to control the current and the timer. It has a midscale sensitivity of approximately 15 nanoequivalents of double bond per microampere and thus is quite useful for microanalyses. By using higher currents (100 to 200 ma.), assay amounts can be
determined with good precision and accuracy. LITERATURE CITED
(1) Roberts,
C. B., in “Titrimetric Methods,” D. S.Jackson, ed., pp. 21-31, Plenum Press, S . Y., 1961.
Division of Analytlcai Chemistrv, 144th Meeting, ACS, L~~ ~ ~Calif , ~*4pril ~ 1963
A Simple, Inexpensive, Heated Infrared Gas Cell Eugene A. Burns,‘ Propulsion Sciences Division, Stanford Research Institute, Menlo Park, Calif.
SIMPLE,
inexpensive, heated in-
A frared gas cell has been designed
and fabricated for the purpose of elucidating the infrared-active gases evolved on thermal decomposition of high energy compounds. A survey of leading manufacturers of infrared equipment at the inception of these studies revealed that there was no heated gas cell commercially available. Because of the potentially explosive nature of some of the high energy fuel additives which were to be examined, it was desirable to fabricate a n inexpensive cell. Several possible cell designs were considered and evaluated, and the best features of these were incorporated in the design shown in Figure 1. The chemical requirements imposed by the high energy compounds, in general, permit use of a glass container. The method of attaching the optical windows to the cell was best served by “O”-ring closures. -4 simple glass section was constructed with a stopcock connected to a male 9/18 ball joint attached in the center and two glass (size 15) “0”ring joints a t either end. The gas volume of the cell is approximately 15 ml. with an optical path of 8 em. Vacuum seal is made between the optical plate and the glass section with a choice of Silicone, Seoprene, Viton A, or Teflon “0” rings. The optical plates are held in place with lead gaskets maintained by the furnace itself. The furnace is fabricated in two sections and the closure is ensured by adjusting the two sections until they are as close together as practical. Each section of the furnace is prepared from an aluminum block, machined, and wrapped with 30 inches of No. 20 insulated Nichrome wire. This wire is held in place and uniform heating is ensured by a coating of KO.6 Sauereisen. The Sauereisen is applied in a paste form and heated to a hard finish. Thermocouple and thermistor wells are Present address, Space Technology Laboratoriee, Inc., One Space Park, Redondo Beach, Calif. 1106
ANALYTICAL CHEMISTRY
drilled in the block for measurement and regulation of the temperature. When 115 volts are applied across the heater terminals, the cell temperature is 225’ C. To use readily obtained inexpensive “O”-ring groove glass and optical windows, the size of the cell was relatively small and hence some of the energy from the source is lost. It is necessary to align the cell in the light path to give maximum energy, and then attenuate the reference side of the spectrometer. T o obviate this difficulty, construction of a larger diameter cell would be required, which in turn would require a higher capacity furnace and a larger sample size. The disadvantages encountered with a larger furnace, larger windows (inefficient furnace and heat losses from the windows), and a larger sample size (when dealing with potentially explosive compounds the smaller the sample the better) offset the alternative attenuation requirement. The furnace may be powered from a 115-volt variable transformer (Variac or equivalent n-ith a maximum capacity of 7.5 amp.), with manual temperature control. For precise temperature control in the order of 0.02’ C., a thermistor temperature regulator similar to that of Gray and VanDilla (3) was constructed. A Victory Engineering
Co. Type 51A1 glass bead thermistor was used as the sensing device. Figure 2 shows the schematic of this device. By adjustment of the bridge balance with helipot R1 and ratio arms Ra, R,, and Rs to a prescribed setting, regulation of the temperature is effected by the phase sensitive control system. This system (3) operates in the following manner: The output of the bridge is fed directlv to a 6 SL7-GT dual triode connected as a two-stage resistance-capacitance coupled amplifier. The amplifier output is coupled through Ce and Rl1 to the grid of a 2050 Thyratron. The plate circuit of the thyratron is supplied with ax. power through switch Sz and the coil of the relay, so that contacts on the relay close when the thyratron conducts. If the bridge is unbalanced, a n alternating voltage having a phase dependent on the direction of the imbalance is supplied to the grid of the thyratron. When the temperature of the thermistor is below the preset value on the balance resistor, the phase of the thyratron grid voltage is then a few degrees ahead of the alternating supply to the plate of the thyratron. Hence, when the plate voltage goes into ita positive half cycle the grid voltage is already positive and the thyratron starts conduction a t the beginning of
WINDOW GASKET
I t -2
Figure 1.
I” 116
Cross-section diagram of heated infrared cell
l
'z%-y---j
i: 1
40,000 2QOOO
f
:
8000
?'
4000
2
2000
0
E
, , ,
,/::I
,
, , , ,
,
, , ,~
200 0 100
200 TEMPERATURE -%
300
Figure 3. Resistance setting-temperature curve of temperature regulator Figure 2.
Schematic diagram of temperature regulator
RI = 1000-R ten-turn helipot; Rz = R 3 = 500 9,1% W.W.; Rd = 5K, 1% W.W.; Rg = 50K, 1% W.W.; Re = R r = R l o = Pi1 = 500K, 1/2W; Rs = 25K, 1/2W; R O = Rid = 5K, 1/2W; Rlz = 1OK,l/2W; R1g = 2.5K,W.W.l0W Ci = 10 pf. 50 Volt; C? =: Cq = 8 pf., 450 volt; C 3 = Cs = 0.01 pf., 600 volt; Cg = 0.05 pf., 600 VOI t
VI = 6 SL7-GT; V - = 2050; Vg = 6 X 5 GT; T = Victory 51 A1 Thermistor
each positive half-cycle of plate voltage. As the temperature of the cell increases the amplitude of the hridge output and thyratron-grid voltagl?s decreases, and finally reads zero when the preset temperature is attained because the bridge is then balanced. If the temperature continues to increase above the preset value, the bridge is sgain unbalanced but 180" out of phase. The grid is then positive during only the last few degrees of each positive cycle of plate voltage; hence, the conduction time of the thyratron is very short, and the average current through the relay coil is not sufficiently large to close the relay. The calibration curve of the setting on the helipot and ratio arms as a function of temperature was determined and is shown in Figure >. To set a prescribed temperature ,ill that is necessary is to evaluate the resistance from this curve and set the value on the helipot and the ratio arms. It should be mentioned that although the temperature regulation is capable of 3=0.02O C., and when the steady state equilibrium has been established, this regulation is attained, little is known concerning the precise heat loss in the vicinity of the salt n-vndows. Hence, a small conical thermal gradient is expected a t each end of .-he cell. The cell is designed t o minimize this gradient; however, a reasonable estimate of the temperature a t the inier surface of the salt window is low, -0.5% of the temperature of the cell. When the gradient is integrated over the volume of the cell a t steadystate equilibrium, a reasonable estimate for the error in the temperature of thl? cell is -0.0801,. This gas cell is quibe versatile for it can be used over all the infrared region
from 2 microns to 50 microns \Tith proper selection of optical windows and spectrometer capability. The temperature range utilized at the long wavelengths may be limited because some infrared materials coldflow a t elevated temperatures. Any temperature-dependent phenomenon in the gas phase which concerns infrared active gases may be explored. An example of a use of this cell is the infrared determination of the heat of vaporization of liquids. Specifically, 0.25 ml. of recently distilled nitrobenzene was placed in the gas cell. The cell was evacuated for 5 minutes, and then was filled to a total pressure of 760 mm. with dry nitrogen. The temperature of the cell was raised to specified values and at each temperature the total pressure of the cell was adjusted to 760 mm. After the temperature was in control, the infrared spectrum was obtained between 1750 and 1425 cm.-l From the combination of the relationship AH log p = - 2.3 RT -I-
(1)
where p is the vapor pressure of a component, AH is the latent heat of vaporization, R is the gas constant, and T is the absolute temperature and the Beer's Law A = apl
(2)
where A is the absorbance a t a particular wavelength, cs is the absorbancy index, p is the pressure of the absorbing species, and 1 is the optical path length, we find log A
=
- 2.3AHR l
+k
(3)
for constant path length. Plots of the base-line absorbance of the peak near 1540 cm.-' as a function of 1 / T gave good straight lines. The average heat of vaporization of nitrobenzene calculated from Equation 3 is 11.36 kcal. per mole which compares favorably with that of 11.72 kcal. per mole reported by Bruckner (1). This technique is not an inexpensive way to determine heats of vaporization. However, it has a major advantage over conventional manometric methods in that the sample need not be pure, provided the impurity does not absorb the same frequency, for by use of infrared discrimination only the component of interest will be observed. It is conceivable then, that the heat of vaporization of several compounds could be determined simultaneously by measure ment a t different wavelengths. The heated infrared gas cell affords the opportunity to measure interactions and reaction kinetics in the gas phase. Results of thermal decomposition studies of ammonium perchlorate, hexanitroethane, and other high energy compounds will be presented elsewhere. Because of the small cell volume to path length ratio, this technique is quite sensitive; for example, the amount of ammonium perchlorate that decomposes to give rise to a quantitatively measurable hydrogen chloride spectrum is in the order of 100 wg. LITERATURE CITED
Briickner, A., 2. Anorg. Allgem Chem. 199, 91-2 (1931). ( 2 ) Gray, T. S., VanDilla, M., Report N o . 180A, Climatic Research Laboratory, Quartermaster Corps, Lawrence, M~EB., October 10, 1945.
(1)
PRESENTED at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 9, 1962.
VOL. 35, NO. 8, JULY 1963
1107
