loss observed for various metallic oxides upon ignition and the increased assay value for the ignited oxides tend to substantiate this. -4s shown in Table I, each of the three oxides, europium oxide, indium oxide, and neodymium oxide, showed appropriate weight losses on ignition, and. in all cases, the observed purities of the ignited oxides were better than 99%. When using spectrographically analyzed standards, there are several precautionary measures which could be used to guard against unnecessary and costly errors. The following are recommended : 1. Independent assay of all spectrographic standards by other reliable analytical methods 2 . Drying or ignition of all spectrographic standards before use 3. Cleansing of all metallic spectrographic standards to remove surface films 4. Use of proper weighing techniques
*All spectrographic standards should be assayed independently whenever possible. Quite frequently, the chemicals contain an EDTA-titratable
metal ion. For these chemicals, the readily available EDTA methods of analysis (2) which are reliable to k0.5% or better are recommended. Spectrographic standard chemicals are often blended with other chemicals in such forms as alloys, powder mixtures, and solutions. With appropriate manipulations (for example, pH control, selective masking, and preliminary separations) EDTA methods can be used to analyze such multicomponent samples ; however, for the sake of simplicity and economy, the unaltered spectrographic standards should be analyzed initially. The iniportance of proper pretreatment and proper weighing techniques (precautionary measures 2 , 3, and 4) is aell recognized, but all too frequently these important details are overlooked. Before use, all spectrographic standards should be dried at the highest permissible temperature below that a t which decomposition, chemical reaction, or volatilization occurs. Simple drying in a conventional oven at 110-160" C. may not be adequate. As noted in the remarks for neodymium oxide, Table I. the observed weight loss was lesq than 1% a t 110" C., while the ohserved total
weight loss was 13.3y0a t 950-1000° C. DuvaI's comprehensive work "Inorganic Thermogravimetric -4nalysis" ( I ) , is especially useful as a guide to the selection of appropriate temperatures. ,111 metallic standards should be cleaned before use-particularly the reactive metals, such as aluminum, zinc, and zirconium which are oxidized readily. Finally, proper weighing techniques should be used to reduce the absorption of common contaminants such as carbon dioxide and water. Diligent use of these precautionary measures will eliminate many unnecessary and costly errors which may be encountered in the use of spectrographically analyzed standards. LITERATURE CITED
(1) Duval, C., "Inorganic Thermogravimetric Analysis," 2nd ed., Elsevier, Kew York, 1963. (2) Meites, L., "Handbook of Analvtical Chemistry," Section 111, pp. 167-284, McGraw-Hill, New York, 1963. WORKdone under Contract X o . A T ( 10-1)205 yith the U. S. Atomic Energy Commission.
Apparatus for Recording Kinetics of Absorption or Evolution of Small Volumes of Gases at Constant Pressure by liquid Systems L. R. Mahoney, R. W. Bayma, A. Warnick, and C. H. Ruof, Scientific Laboratory, Ford Motor Co.3 Dearborn, Mich. is engaged in a the kinetics of compounds in liquid solutions (4,5 ) . An improved apparatus has been developed which permits the aut'oinatic recording of the volume of oxygen gas absorbed at constant pressure and temperature as a function of time and which is readily adaptable to wide ranges of rates of absorption. This apparatus is equipped with a servo-controlled pressure stabilizer and can be used without modification for recording the rates of evolution of gases as well. The present apparatus is somewhat similar to that described by Edgeconibe and Jardine ( 2 , 3). Their apparatus, however, was designed for rapid, relatively large, changes in volume and utilized a glass spiral manometer which was sensitive to vibrations of the building causing a "noise" level of about 100 to 500 microns. The present apparatus has an overall accuracy of k 17 microns and utilizes a rugged, capacitive differential micromanometer which is insensitive to building vibrations. With the particular gas absorption cell design and volumes of solution dexribed below, it is possible to record the volume changes accompanying the oxidation of small amounts of rare and expensive HIS
LABORATORY
Tdetailed study of oxidation of organic
2516
ANALYTICAL CHEMISTRY
Figure 1. Gas absorption apparatus
i ,
materials and to determine the slow initial rates encountered in highly inhibited systems. The average uncertainty amounts to about 0.5 p1. per minute. DESCRIPTION
OF
THE APPARATUS
The apparatus, Figure 1, consists of a reaction cell, a means for evacuation and charging of the cell to the desired pressure, the micromanometer and associated electronics, the infusion-withdrawal pump system, and the linear potentiometer-recorder system. Gas Absorption Cell. The cell, Figure 2 , consists of a borodicate glass vessel surrounded by a water jacket which is supplied from a constant temperature (60.00' i 0.02" C.)
circulating pump. A second water jacket a t the top of the cell is held constant at 25.00" 0.02" C. by a similar pump: this cold jacket prevents excessive evaporation of solvent during the evacuation. The 2-nim i.d. capillary lines of the apparatus are i n d a t e d from localized transients in the teniperature of the air-conditioned laboratory by foamed plastic (.Arniaflcx 2 2 , *Armstrong Cork Co.). Micromanometer Pressure Transducer and Associated Electronics. A sensitive capacitive sensor and associated electronics are available commercially ( 1 ) . For greatw long term stability, the follon.ing micromanometer, a.c. bridge, and servo-system have been constructed in this labora-
*
To
Rubber Syring* COP
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Figure 3.
Figure 2.
Capacitive differential micromanometer
direction and speed of the servomotor of the pump. Pumping System. T h e imbalance to the pressure transducer is corrected by raising or lowering the mercury level in the calibrated pipet by means of an Infusion-Withdrawal P u m p (model #OOO-900 Basic Pump Mechanism, Harvard Apparatus Co.). This system can readily accommodate syringes from 2 ml. to 53 ml. in size and can be readily equipped with servomotors of different speeds or with an available multi-speed transmission to
in pressure creates a movement of the membrane which alters the electrical capacities of the two sides. These two capacitances make up two arms of a bridge circuit which is excited with 60cycle a.c.; the other two arms are connected to a center-tapped transformer, Figure 4. The output voltage of the bridge is coupled through an impedance matching semiconductor preamplifier to a servo-amplifier (Brown N o . 357926-1) from which the input chopper had been eliminated; this servo-amplifier in turn controls the
Gas absorption cell
tory. The pressure transducer, Figure 3, which is used as a null device, is somewhat similar to that of Opstelten and Warmoltz ( 6 ) . I t consists of two chambers separated by a 0.001-inch stainless steel membrane. The capacitor plates are spaced 0.002 inch from the diaphragm giving a capacitance of 162 pf. per side. K h e n the reference chamber is isolated by closing the appropriate stopcock, a slight difference
f I2
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(INSULATED FROM
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-_Figure 4.
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Circuit diagram, constant pressure volume measuring servo VOL. 36,
NO. 13, DECEMBER 1964
2517
provide a wide range of pumping speeds. This mechanisin has been modified as follows to minimize any dead zones which together with temperature gradients are the principal sources of error. The we ght-loaded half nut resting on the lead screw was converted to a springloaded mechanism. The head of the syringe was prevented from moving within the adapter by an axial set screw inserted through the carriage and withdrawal adapter. The adjustable ramp was removed and two micro-
Rate of Decomposition of AlBN at 60.0" C. (in 10 ml of chloroben7me) Rate of ~ _evolution _ _ _ _ _ _of S2 [AIBS], ml min -l moles moles liter-' at N T P liter-' eec - 1 k,, see - l 0 2470 0 0425 2 818 x 10-6 1 14 x 10-5 1 12 x 10-5 0 3208 0 0373 2 4 i 2 X 10-6 0 1341 0 0253 1 680 x 1 09 x 10-9 liean = 1 12 x 10-5
Table I.
~~~
\.T't of AIBS, gram 0 4037 0 3623 0 2530
switches were added m liiiiit the travel at the ends to prevent mechanical damage. A stainless steel bellows-type positive coupling was used to connect the motor to the lead screx. The carriage was linked mechanically to the slide of the linear potentiometer. Linear Potentiometer-Recorder System. The axial displacement of the infusion-withdraLya1 syringe is converted to a voltage change by the linear potentiometer (Model 108, Bourns, Inc.). The output signal is recorded on a strip chart recorder equipped with an adjustable span, an adjustable zero, and dual chart speeds (Speedomax H, Model S, Leeds & Northrup Co.). TYPICAL MEASUREMENTS
,
Figure 5. Typical record for determination of rates of evolution of nitrogen from AlBN
251 8
ANALYTICAL CHEMISTRY
Gas Evolution. The rate constant for thermal decomposition of azobis(2-niethylpropionitrile), (*4113X)) has been determined in this apparatus from the rate of evolution of nitrogen. The desired amount of . 1 I B S dissolved in 10.00 ml. of chlorobenzene was inserted into the cell. As much of the oxygen was removed from the apparatus as possible by four cycles of evacuating and flushing with prepurified nitrogen (99.996Tc) (-4irco). Hot vater was admitted to the jacket of the cell, the zero of the recorder was adjusted, and recording was started. X typical record is shown in Figure 5 . The induction period on this record results from the absorption of the last traces of oxygen. Following this induction period, recording was continued for three complete traverses of the chart. The rate of gas evolution was computed as the average of the slopes of these three The rates from three such ts are summarized in Table I. The rate constant, k d , agrees well with that of 1.15 X 10-5 .sec.-l adopted by Russell ( 7 ) from a compilation of literature yalues. Gas Absorption. T o ascertain t h e repeatability in the rates of absorption of oxygen iii autoxidation psperiments in thir apparatus, the same autoxidizt e m was measured in quade. .I solution of 1.00 nil. of the osidizablc hydrocarbon tetralin in 9.00 nil. of chlorobenzene wa:, inserted into the cell. pres'ured t o 740 rnm. wit,h oxygen, and allowed to
Table
II.
Oxidation of Tetralin in Chlorobenzene
Run 1
[Tetralin], moles liter-' 0.670
2
0 670
3
0.670
4
0.670
u =
Rates of O2 absorption, ml. min.-' at N.T.P. 0.01598 0.01500 0.01551 0 00293 0.01552 0,01545 0 01503 0 00293 0 01629 0 01634 0 01578 0,00293 0.01575 0,01505 0,01487 Mean = 0.01555 5.0 X l o w 4ml./min. [AIBX], moles liter-' 0,00293
reach thermal equilibrium at 60.00" C. as indicated by no further changes in gas volume. A hypodermic needle was then inserted through the syringe cap and 0.30 nil. of AIIBKsolution in chlorobenzene was injected into the cell. Oxygen absorption commenced almost iristantly and reached a steady rate in less than two minutes as indicated by the constant slope of the recorder trace. The results of the first three traverses for the quadruplicate runs are summarized in Table I1 where the initial tetralin and AIBK molarities are given in terms of concentration in the total liquid present-Le., 10.30 ml. The standard deviation of these runs amounts to 5.0 X lo-' ml. per minute at S . T . P . which corresponds to an error in the rate of absorption of about 3.0 X lo-* mole liter-' sec.-l LITERATURE CITED
(1). Decker LIicro-differential-High
Sensitivity Pressure Meter, Model 306, The Decker Corporation, Bala-Cynwyd,
Pa. ( 2 ) Edgecomhe, F. H. C., Jardine, D. A., Can. J . Chern. 39, 1728-30 (1961). ( 3 ) Edgecombe, F. H. C., Jardine, D. A., X w . Sei. Instr. 33, 240-1 (1962). ( 4 ) Liahoney, L. R., J . A m . Chem. Soc. 86, 444-9 (1964). ( 5 ) Ahhoney, L. R., Ferris, F. C., Ibid., 85, 2345 (1963j .
(6) Opsteltent J. P., Warrnoltz, x., d p p l . Sei. Res., Sect. 5,4,329-36 (1055). ('7) Russell, G. A, J . A m . C h e m . Soc. 79, 38'71-7(195'7).
