from orange to green is sharp, although the indicator blank is relatively large. I n Table I are shown data for the titration of o-phenylphenol with 0.02M tetrabutylammonium hydroxide using p -nitro-p '-aminoazobenzene indicator. Correction for the indicator blank gives excellent agreement with the results of a potentiometric titration. Comparison of the precision of the two methods is not valid. I n the indicator titrations, the transitions are very distinct, owing to the reaction of a very small amount of dye. The potentiometric end point is, in both cases, subject to a relatively large variation due to the small potential break at the equivalence point. Replicate potentiometric titrations would be subject to errors that are larger than those associated with the
indicator titrations, and hence the precision would be poorer. The potentiometric values were included to show that the indicator end points are almost identical to the inherently more accurate potentiometric end points. The titration of malonic acid using o-nitroaniline was also studied. Here also, the results agreed well with the potentiometric values. Use of indicators that contain sodium or potassium salts, or indicators that are in the form of a sodium or potassium salt should be avoided unless the transition range is more positive than -0.3 volt. 4 t potentials more negative than this, the transference of the alkali ion is so large that the color change interval cannot be established (with any certainty). This does not preclude the
use of an indicator that contains sodium or potassium, since a trial and error method may still be used to establish its usefulness. LITERATURE CITED
(1) Fritz, J. S., Marple, L. W., h A L . CHEM.34,921 (1962). 12) Kolthoff. I. 11..Guss. L. S..J. Am. Chern. Soc: 60. 2518 11938). (3) Marple, L. 'W., Fiitz, J. S., AISAL. CHEM.34, 796 (1962). LELAKD W. MARPLE JAMESS.FRITZ Institute for Atomic Research and Department of Chemistry Iowa State University Ames, Ion-a Contribution S o . 1229. Work performed in the Ames Laboratory of the U. S. Atomic Energy Commission.
Modification of a Thermogravimetric Balance for Pyrolysis Experiments in a Controlled Atmosphere and after the pyrolysis, as has been proposed in the literature. A similar difficulty has arisen in the modification of the newer recording thermogravimetric balances for pyrolysis in a controlled atmosphere. For example, the otherwise satisfactory technique of Vassallo ( 9 ) employs a downward flow of inert gas which may encourage condensation on the lower part of the weight transfer rod-the rod which transmits the load of the sample in its holder to the balance mechanism.
SIR: The quartz spring balance of LVladorsky (3, 4) has been criticized by Jellinek ( 2 ) . He points out the danger, in any suspended system, of condensation of volatiles on the (cooler) support and spring early in the pyrolysis, resulting in too low a measured weight loss. Later in the pyrolysis, the condensate may volatilize, giving too high a weight loss at long times of heating. To show the absence of such condensation effects, it is clearly not sufficient to weigh the support and spring before si I ico ne
rubber
I
f
StOS
A similar criticism can probably be made of the more elaborate system described by Stonhill (8). Tn an attempt to overcome this disadvantage, Soulen and RIockrin ( 7 ) have modified the Chevenard balance by passing the weight transfer rod through a sleeve, with most of the outflow gas passing down the outside of the sleeve where condensation may occur without influencing the recorded sample weight. They claim that, in the absence of the sleeve. the thermogravimetric curveq may be seriously in error. While we have no experience with the Chevenard balance, we have recently had occasion to modify a Stanton TRI thermogravimetric balance for a study of polymer pyrolysis in an inert atmosphere (6). Using the technique of Soulen and Mockrin, we found it difficult to center the rod accurately enough to prevent rubbing on the inside of the sleeve and consequent distortion of the weight-loss curve. Instead, an upward gas flon- Tvas used, which has the advantage of carr! ing corrosive pyrolysis
/p
/' tinplate
push f i t
lid
alloy
0
2
4
U
I inch
YITROGEN
balance
Figure 1 . Design of gland for introduction of inert gas to vertical furnace
1306
ANALYTICAL CHEMISTRY
FLOW
Figure 2. Relationship between apparent weight gain at 20' C. and nitrogen flow rate
products away from the mechanism of the balance. The inert gas was admitted at the base of the furnace through a gland of the form shcm-n in Figure 1. The gland has a n advtntage over the simpler device suggested by Gilbert et al. ( I ) in that it is less susceptible to back-diffusion of air a t the base of the furnace. I n setting up a pyrolysis run, the furnace was pulled up vertically away from the balance to its full height of travel. The metal lid was placed on the heat shield of the balance and threaded by the metal rod which was located in a socket on ihe beam of the balance. The silica ro1-l was threaded through the body of the gland and centered in the socket a t the top of the metal rod by three grub screws. The sample, in its holder, W L S placed in the support ring a t the top of the silica rod and the furnace drawn down sufficiently for the body of the gland to be pushed on to the flange. Furnace and gland were then lowered further until the body of the gland could be pushed into the lid. T i e whole operation of assembly took only a few seconds. The use of updraft g t s avoided condensation problems. Back-diffusion of air during the pyrolyses m-as effectively overcome by deliberate loss of considerable quantities of inert gas flowing down past the weight transfer rod. Escape of gas from the top of the furnace was effected through a small vent. The gas flow down through the gland caused an apparent weight gain c f a nonvolatile test specimen (Figure 2), and the downthrust recorded by the balance was used as a convenient ineasure of gas flow rate when setting u p each run. Buoyancy and convection corrections were obtained for each f ow rate using a
P
-
10
E
.-$
1
I
c
-
3.5
0
203
400
600
temperature
800
'C,
Figure 3. Buoyancy and convection correction: apparent weight gain for silica basin ( A ) and flat nickel capsule
(6) nonvolatile load, as suggested by Xewkirk ( 5 ) . Typical correction curves are given in Figure 3. The system was evaluated using a method suggested by Gilbert et al. ( 1 ) . A 300-mg. sample of super abrasionfurnace type carbon black (Vulcan 9) was heated in the sample holder under conditions of constant temperature and gas flow. X temperature of 650' C. was chosen for convenience of rate measurements and to avoid being too far above normal pyrolysis temperatures (300' to 500' (3,). Weight-loss data fitted zero order kinetics, the rate coefficient in convected air-Le., ivith the nitrogen turned off-being 12.0 mg. minute-'. With three different nitrogen floiv rates (corresponding to downthrust measure-
ments of 1, 2, and 4 mg.), the rebpective zero order rate coefficients for weight loss were 0.08, 0.06, and 0.04 mg. minute-'. A 2-mg. do\mthrust flow rate of dry, oxygen-free nitrogen of commercial purity was chosen for the polymer pyrolyses, assuming that the reduction of the oxidation rate of the black b y a factor of 0.005 was a sufficient criterion of inertness of the atmosphere for technological purposes. The validity of this assumption was supported by the identical polymer thermograms obtained a t this and a t higher gas flow rates. ACKNOWLEDGMENT
The author thanks R. J. Aldred for his help in constructing the gland and P. I. Gayapersad for experimental assistance. LITERATURE CITED
(1) Gilbert, J. B., Kipling, J. J., McEnaney, B., Sherwood, J. Y., Polymer 3, l(1962). (2) Jellinek, H. H. G., J . Polymer Sci. 10, 506 (1953). (3) Madorsky, S.L., Ibid., 9, 133 (1952). (4) Madorsky, S. L., Straus, S., J . Res. Natl. Bur. Std. 63A, 261 (1959). (5) Newkirk, A . E., ANAL. CHEBI.32, 1058 (1960). (6) Smith, D. A,, Trans. Inst. Rubberlnd., in press. ( 7 ) Soulen, J. R., Mockrin, I., ANAL. CHEN. 33. 1909 119611. (8) Stonhill; L. G:, J . Znorg. S u c l . Chem. 10, 153 (1959). (9) Vassallo, D. -I.,ASAL. CHEM. 33, 1823 (1961). DEREK-4.SMITH National College of Rubber Technology London, England
A Rapid Procedure for the Determination of Nitro Groups on Semimicro- and Microscales SIR: A method for the semimicrodetermination of nitro groups by reduction with titanous wlfate was reported earlier ( 2 ) . Further work showed that the reduction could be carried out almost instsntaneously a t room temperature in t i e presence of 10.0 ml. of a 75% poi,assium citrate solution. Thus, the reduction period of about 30 minutes to 1 hour could be reduced to 2 to 3 minutes. This, therefore, offers a rapid procedure for the determination of nitro groups in organic compounds. The modified procedure has also been applied to the micro scale, using lesser amounts of the reagents. The sample, dissolved in glacial acetic acid, is reduced with a n excess of titanous sulfate solution in the presence of potassium citrate as alkaline buffer. The excess reducing agent is estimated b y back-titration against standard ferric
sulfate solution using potassium thiocynate solution as indicator. Potassium citrate appears to catalyze the reaction. I n a few cases, slight warming for about 1 minute is necessary for accurate results. Thus, the use of a reflux condenser is no longer necessary. One should employ about a 100% excess of the reducing agent for good results. Titanous sulfate solution in 3 to 4N sulfuric acid was quite stable when maintained in a n inert atmosphere. EXPERIMENTAL
Reagents. Titanous sulfate solutions, 0.05 a n d 0.033N, stored in a container similar t o one described b y Siggia ( I ) , 0.05 and 0.033N ferric sulfate solutions (standardized iodometrically), 50% (v./v.) sulfuric acid, 20% (w./v.) potassium thiocyanate solution,
and 757, (m./v.) potassium citrate solution were used. Except for the titanous sulfate solutions, all the reagents were prepared from analytical grade materials in airfree distilled water. Titanous sulfate solutions were prepared from a 15% (w./v.) solution supplied from British Drug House, London. Procedure. Semimicro Determination. T h e sample, 10 t o 25 mg., is weighed accurately and transferred t o a 150-nil. Jena glass conical flask with a n arrangement for flushing with nitrogen gas. The sample is dissolved in 3 to 5 ml. of glacial acetic acid and flushed with nitrogen gas for about 5 minutes to remove air from the flask. Potassium citrate solution, 10.0 ml., is added, followed by 20.0 ml. of 0.05N titanous sulfate solution. The contents of the flask are shaken for 1 minute and then warmed for 1 minute over a hot plate. The flask is rapidly cooled and VOL 35, NO. 9, AUGUST 1963
e
1307
