and were never greater than f1.08 volt. Davis and Everhart reported a value of + l . l O volt. If we assume that the difference between their value and ours is iR drop and if we take a value of 40-mv. for this iR drop, an electrode resistance of approximately 50 ohms is calculated with their current value of 0.846 ma. This calculated resistance value falls within the range of resistances observed by us in our unmodified electrode holder. The possibility of the inclusion of iR drop in the potential values of Davis and Everhart is thus suggested. Another possibility which cannot be discounted from the available data as a source of a t least part of the disagreement between El,, values is differences in the properties of the carbons used. However, in both cases graphitic carbons were used and their properties should be similar enough to minimize this source of disagreement. That this possibility can be raised lends support to our previous statement that well defined carbons should be used for electrochemical studies involving carbon electrodes. I n ‘the anodic stripping voltammetry of gold and silver reported by Jacobs
( 5 ) , large area carbon paste electrodes were used also. With a variation of electrode areas from 0.196 sq. cm. to 1.32 sq. cm. he found that the plating rate decreased as the area of the electrodes increased. This variation of plating rate with electrode area suggests a possible resistance effect. However, from the magnitudes of the currents given, iR drops would be negligible unless the electrode resistances were high. Some other effect is probably the main cause of the variation. Although carbon paste electrodes have been used successfully for many studies, the brief observations reported here show that there are many things that need to be learned about the nature of carbon paste electrodes and about the nature of carbon electrodes in general. One still must approach results obtained with carbon electrodes with care.
ACKNOWLEDGMENT
Graphon carbon black was obtained through the courtesy of Walter R. Smith of the Cabot Corp. Electronic equipment was designed and constructed by Richard Mueller.
LITERATURE CITED
(1)Adams, R. N., Rev. Polarog. (Japan) 11, 71 (1963). (2) Beilby, A. L., Brooks, W., Jr., Lawrence, G. L., ANAL.CHEM.36,22(1964). (3) Davis, D. G., Louisiana State Uni-
versity at New Orleans, New Orleans, La., private communication. (4) Davis, D. G., Everhart, ST. E., ANAL.CHEM.36. 38 (1964). ( 5 ) Jacobs, E. S., ibzd.; 35, 2112 (1963). (6) Malmstadt, H. V., Enke, C. G., Toren, E. C., Jr., “Electronics for Scientists,” p. 148, W. A. Benjamin, Inc., New York, 1962. (7) Morris, J . B., Schempf, J. M., ANAL. CHEM.31, 286 (1959). (8)TNational Carbon Co., New York, Tu. Y., Catalog- Section 5-7655 (February 1960). (9) . . Olson. C.. Adams. R. N.. Anal. Chim. ’
Acta 22: 582 Cl96Oi. (10)Olson, C., Adams, R. N., Zbid., 29, 358 (1963). ( 1 1 ) Walker, P. L., Jr., Am. Scientist 50, 259 (1962). I
\ -
ALVINL. BEILBY BRUCER. MATHER Department of Chemistry Pomona College Claremont, Calif. 91713 Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Undergraduate Research Participation Program of the Xational Science Foundation for support of this research.
Errors in Vacuum Thermogravimetry SIR: For a research program which involves evaluating kinetics of decomposition of polymers, the author recently built a vacuum microthermobalance. Small samples were used to minimize diffusion controlled weight loss and to promote better homogeneity of temperature than can be obtain-d with larger samples. The value of vacuum is its tendency to remove decomposition gas molecules from the area of the polymer so that secondary reactions are reduced. A sketch of the thermobalance system is shown in Figure 1. I t is based on the
VACUUM SYSTEM WATER-COOLED .OlNT
SAMPLE PAN THERHOCWPLE
REFERENCE PAN CERAMIC THERKICRRLE INSULATOR bND PAN SUPPDRT
SCREEN W W R T ASS SUPPORT F W
Figure 1.
768
Cahn Model R G electrobalance. The furnace is the Hevi-Duty Type MK1012s. The temperature controller and recorder is a Minneapolis-Honeywell system composed of a programmed temperature controller-recorder with proportional control output to a magnetic amplifier and saturable reactor. Data are recorded with an Electronic Associates, Inc., Model 1110 Variplotter. The only unusual features of the system are that the thermocouple which is used to measure and control temperature was spot-welded to a pan which was placed so as to be in a zone where the temperature would be equal to that of an empty sample pan, and that a grounded Type 316 stainless steel screen cylinder liner was used to reduce electrostatic effects. Several types of errors including bouyancy effects, radiometric effects, and thermomolecular flow, can influence vacuum thermogravimetry. Some of these have been discussed in the three volumes on Vaccuum RIicrobalance Techniques (5-7) and by Duval ( 3 ) . Others nere demonstrated by Cahn and Schultz ( 1 ) . Curtiss ( 2 ) pointed out that momentum transfer effects could be important in vacuum gravimetry, according to the relationship
~ = m - 1- - dm Thermobalance system
ANALYTICAL CHEMISTRY
g
cyv
dt
where weight as read by the balance, grams m = actual weight of sample, grams g = acceleration due to gravity, cm./sq. second ff = geometric factor 21 = velocity of ejected gas, cm./second dm/dt = rate of change of actual weight, gram/second w
=
* -0.21
1
\
SAMREBELWMN
W HEATINGRATE: IO‘CIMIN.
400
5 00
600
700
TEMFfRATURE.’C
Figure 2. Effect of Teflon pyrolysis gases on empty sample pan
Therefore, w > rn for any time when weight loss occurs. Potential errors due to this are serious for polytetrafluoroethylene a t certain stages of decomposition (4). To measure this effect, film samples of polymer were subjected to thermogravimetry while supported both above and below the sample pan. Results of these experiments showed that molecular drag played an important role. T o approximate the magnitude of molecular drag, 7-mg. films of polymer were decomposed with linear programmed temperature rise while being supported at the top surface of the reference pan, and a b v e , but not touching the sample pan, with no sample in the sample pan. Results of these experiments are shown in Figure 2. The results indicate that errors are caused by lift effects caused by molecules which migrate to regions below the
pan as they are pumped p a t pan and support wire. With sample placed on the reference pan, more molecules would have found their way to this region. Improved design could probably reduce such errors and faster pumping could reduce the tendency of molecules to migrate helow the pan. Increasing the space between the furnace tube and the pan could be helpful, as could reducing the surface area of the pan when possible. Concurrent rapid pumping from both the top and bottom of the furnace tube could be beneficial. Because it was not possible to reprw duce molecular lift effectswhich occurred during experiments designed to memure momentum transfer effects, the experiments described herein were not able to help in the evaluation of momentum transfer. Momentum transfer effects can be calculated in some cases.
LITERATURE CITED
(1) Cahn, L,,sehultz, H,, cHEM 35, 1729 (1963). (2) C??tiss,xr">;""" C . F., ,Z,L University ^,^of Wis.
~"-
munieation, 1963. (3) Duval, C., "Inorganic Thermogravi-
metric Analysis," Elsevier, Amsterdam, 1963.
(4) Friedman, H. L., U. S. Air Force
Materials Lab. Rept. ML-TDR-64.274, August 1964. (5) "Vacuum Microbalance Techniques," M. .l. Kata. ed.. Vol. 1 . Plenum Press.
HENRYL. FRIEDMAN Space Sciences Laboratory General Electric Co. King of Prussia, Pa. WORKperformed under auspices of U. S. Air Force Materials Laboratory under contract ?To. AF 33(657)-11300.
Elimination of Carbonates from Aqueous Solutions Prior to Organic Carbon Determination C. E. Van Hall,
Dennis Earth, and V. A. Stenger, Special Services laboratory, The Dow Chemical Co., Midland, Mich.
N THE DETERMINATION
bustion-infrared method ( l o ) ,the result obtained for total carbon includes inorganic ss well m organic carbon. Because carbon dioxide and carbonate are not considered as pollutants in waste water analysis and are not included in methods based upon the consumption of an oxidant, it is desirable to eliminate , I ~ c-... . ~ ~ - ~ ~ n L . 1-2 L-. 1L^ _-__:-I me.= irom TBUIM r e p ' MU U J YLK ~ n p u method., Procedures for avoiding the effect of carbonate are also of interest in connection with other methods of analysis for total carbon. Several possibilities exist: carbonate may be precipitated with barium hydroxide (3); a separatedetermination of carbon dioxide may be made (6, 8,9)and a correction applied; or carbon dioxide may be expelled by acidification and boiling or purging with an inert gas ( 5 ) . The last route seemed most promising in regard to simplicity and rapidity. Displacement of carbon dioxide with a carrier gas raises questions as to the most suitable conditions (acidity, flow rate, time) and the possibility of losing volatile organic matter. Although the method has been used, experimental data in the literature are meager (5, 10). Preliminary mention hss been made (7) of some work done under contract with the U. S. Public Health Service. Removal of carbon dioxide is accomplished ~~~~
grams of long-fiber asbestos in a porcelain dish with a solution of 20 grams of cobalt nitrate in 50 ml. of water. The mixture is evaporated to dryness, placed in a cool muffle, and gradually heated to 950" C. After an hour at 950" C., it is cooled and any large lumps are broken up. About a gram is added to the combustion tube in small amounts with forceps or tweezers to provide a loosely packed wad 4 to 6 em.
of carbon com-
I pounds in water by the rapid com-
~
in . ..
.._^_
Fiaure 1 .
Diffusion cell
Millimeter rule shown
readily, but losses of certain volatile compounds can occur. Therefore a diffusion technique was developed whereby volatile materials can he kept confined. This paper includes detailed procedures from the contract work, as well as some more recent experimental results. EXPERIMENTAL
Apparatus. The comhustion-infrared analyzer previously described (10) and a modified model constructed by neckman Instruments, Inc., were both used in this study. One change was made in thecomhustion tube packing-asbestos impregnated with cobalt oxide (1s) was substituted for the asbestos-platinum gauze system originally used. The new packing is prepared by treating 15
1mwt.h
With ".. thir "
.....
n a ~ l r i ~, na r_"
nn
platinum gauze is needed. The diffusion apparatus (Figure 1) is essentially an enlarged Conway cell (2) modified to permit sampling of liquid and vapor phases with syringes. The cell is made from 70-mm. o.d. borosilicate glass tubing closed nt the ends to form a compartment 6 em. tall. From the top, a 40-mm. 0.d. reservoir with a 9-mm. rim is suspended by a rod, so that the distance between the bottom of the cell and that of the inner reservoir is 3.0 em. One neck of 15-mm. diameter and two of 8-mm. are provided, the latter being fitted with No. 18 SS syringe needles (sealed in with epoxy resin) which accommodate Hamilton No. 1001 and No. 705 N syringes for sampling vapor and liquid. The larger neck is closed with a cork or rubber stopper wrapped in aluminum foil. The total internal volume of the cell is determined by calibration with water. An ordinary reducing valve and a Brooks flowmeter are used to control cylinder nitrogen for purging. Gas is dispersed in samples through a Corning No. 39533 EC glass frit bubbler tube. VOL. 37. NO. 6. M A Y 1965
769
