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. A 2-mg. do\mthrust flow minute-'. 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, A U G U S T 1963
e
1307
Table I.
Compound 92-Dinitrobenzene p-Nitroaniline o-Yitrocinnamic acid o-Kitrobenzaldehyde
4-Chloro-1,3-dinitrobenzene
Nitrobenzene 3,5-Dinitrobenzoic acid p-Nitrobenzoic acid w9itrobenzoic acid
Percentage of Nitro Group Present
Calculated 54.76 33.33 23.88 30.46 45.45 37.40 43.39 27.54 27.54
Cyclohexanone-2,4-dinitro-
Nitro group, % Found4 Semimicro- Alean Microscale error, % scale 54.77 +0.01 54.14 33.64 +0.31 33.79 23.55 23.84 -0.33 30.21 29.96 -0.25 45.97 +0.52 45.76 37.26 -0.14 36.99 43.72 43.81 +0.33 -0.27 2i.27 27.51 27.28 27.03 -0.26
phenylhydrazone 33.09 32.70 ... 4-Bromo-3-nitrobenzoic acid 18.70 3-Nitrophthalic anhydride 25.44 ... I ,3,5-Trinitrobenxene 64.79 ... a Values are average of two determinations.
3.0 ml. of sulfuric acid (50%) is added. The color of the reaction mixture becomes darker because of the release of titanous sulfate which had been complexed with potassium citrate. The excess reducing agent is back-titrated against a 0.05.V ferric sulfate solution. K h e n the dark color of titanous sulfate diminishes, 5.0 ml. of potassium thiocyanate solution are added and the titration is completed to a red end point.
-0.39
...
32.68 18.32 25.69 64.24
Mean error, % -0.62 +0.46 -0.40 -0.50
+0.31 -0.41 +0.42 -0.03 -0.51
-0.41 -0.38 $0.25 -0.55
the presence of 5.0 ml. of potassium citrate solution. After the reduction, 2 ml. of sulfuric acid are added and the excess reducing agent is back-titrated against 0.033N ferric sulfate solution, using 3.0 ml. of potassium thiocyanate solution as indicator. RESULTS
The per cent of nitro group present is calculated in the normal manner from the difference between the milliliters of ferric sulfate solution required for the sample and for the blank. Results of the determinations on both the semimicro- and microscales are given in Table 1. LITERATURE CITED
(1) Siggia,
During all this period, nitrogen gas is passed through the flask a t the rate of 1 to 2 bubbles per second. X blank is run under identical conditions using all the reagents except the sample. AIicro determination. The procedure is essentially the same as that described above. Thk reduction of 2 to 10 mg. of sample is carried out using 15.0 ml. of 0,033-Y titanous su!fate solution in
S., “Quantitative Organic
Snalysis via Functional Groups,” 2nd ed , p. 125, Wiley, New Pork, 1954. (2) Tiwari, R. D., Sharma, J. P., 2. Anal. Chem. 191, 329 (1962). R. D. TIWARI J. P. SHARMA Department of Chemistry University of .Ushabad .Uahabad, India One of the authors (J. P. Y.)expresses his thanks to the Ministry of Scientific Research and Cultural Affairs, Government of India, for the award of a Scientific Research Training Scholarship.
Diglyme as Solvent in the Gasometric Determination of Silanols by the Lithium Aluminum Hydride Method SIR: Some improvements result from the substitution of bis(2-methoxyethy1)ether (diglyme) for the di-n-butyl ether usually used as solvent in the lithium aluminum hydride method (4,6). The lithium aluminum hydride solutions are much easier to prepare in diglyme than in di-n-butyl ether. -A simple corrective technique allows this method to be applied to samples containing certain types of siloxanes from which interference ha3 been previously reported. Silanols have also been determined by the Karl Fischer method (a) and the Zerewitinoff method (3). The addition of lithium aluminum hydride to di-n-butyl ether is followed by a period of approAimately a day before gas evolution stop;. Prior purification of the ether does not shorten this time. By contrast, the addition of lithium aluminum hydride to diglyme leads to rapid solution and only a short period of gas evolution after which a determination can be started a3 soon as a constant temperature is reached. 1 stock solution can be prepared and portions tramferred to the reaction flask as required. If a stock solution is allowed to stand. the suspended material settles out and a clear solution can be withdrawn. 1308
ANALYTICAL CHEMISTRY
Diglyme of practical grade may be used with no further purification. Diglyme should not be dried by distillation from lithium aluminum hydride as this may lead to an explosion ( 1 ) . I n addition to the greater convenience in preparing solutions, the modified method gives good results in the presence of siloxanes in which it is possible for cleavage to take place to gi1-e volatile silicon hydrides. Guenther (3) has stated that such cleavage takes place with lithium aluminum hydride in di-n-butyl ether, and for this reason has recommended the Zerewitinoff method for such samples. EXPERIMENTAL
Apparatus. A 50-nil. flask with a side a r m adapted t o take a serum cap, stirred with a magnetic stirrer, and connected to a mercury manometer of 50-ml. capacity was used. Reagents. T h e silanols were prepared in connection with another Phenyldimethykilanol, diproject . phenylmethylsilanol, and triethylsilanol were diatilled under reduced pressure before analysis as were the two new compounds: l-ethyltetramethyldisiloxan-3-ol,b.p. 41’ to 42’ C./about 1 mm., n”,”1.4050-4,
dZ5 0.8852; found: R , 0.2770, OH (this method) 9.63, 9.70%, Si 31.75, 31.49%; calcd. for Si2C6H1802: R , 0.2772, OH 9.54%, Si 31.50%. Heptamethyltrisiloxan-3-01, b.p. 50.5’ to 57.5’ C./about 1 mm., n‘,j 1.3977-80, dZ5 0.9044; found: R , 0.2667, OH (this method) 7.28, 7.28%, Si 35.17, 35.26%; calcd. for Si?C-H&:. R ,- 0.2662. OH 7.13%, Si 35133%; p - Bis (dimethylhydroxysily1)benzene was recrystallized from toluene; tetramethyldisiloxan-l,3-diolwas recrystallized from hexane; and triphenylsilanol was dried by heating to 160’ C. a t 25 mm. for 20 minutes. The diglyme was Eastman practical grade and the dioxane was Eastman (m.p. 10.5’ to 11’ C.). Lithium aluminum hydride solutions were prepared by adding 0.3 gram (0.4-11) or 0.75 gram (1.OJI) of the finely ground reagent (caution) to 20 ml. of diglyme with stirring. Procedure. -1pproximately 1 meq. of the silanol was introduced from a hypodermic syringe and sample weight was determined by weighing the syringe before and after injection. T h e final volume of gas obtained was corrected for the displacement b y the liquid volume of the sample and for the gas volume derived from t h e dioxane if used (a blank determination on one sample of dioxane was 1.5 ml. of gas per 0.50 ml. of dioxane).
