cause of this, an exact comparison of the two sets of data is not possible. A comparison of the literature values for the melting points of the picrates with the maximum decomposition rate of these compounds as taken from the curves, gave the following values.
Temp.
of iMax. Melting
Compound Hydrazine picrate A;-Methylguanidine picrate N-Ethylguanidine picrate Guanidine picrate Guanylurea icrate Aminoguanixine picrate
Decompn. Point Fte, (I$.), C. C. 115 20 1 180
20 1
195
180 333 265
222
190 160
available
Sot
The decomposition temperatures obtained in this laboratory are, except for the N-ethylguanidine salt, considerably below the reported literature values. This is to be expected, because a t rapid heating rates thermodynamic equilibrium is probably not attained and the existence of large thermal gradients in the sample is probably conducive to the formation of “hot spots” which
lead to rapid decomposition, or in the case of the styphnates, to detonation. For the six picrates examined, the order of increasing thermal stability under rapid heating rates is: hydrazine, aminoguanidine, N-ethylguanidine, guanylurea, A’-ethylguanidine, and guanidine. For the styphnates the rate and violence of the decomposition were such that these compounds could be considered to undergo detonation. The relative stability in terms of increasing temperature of detonation is: hydrazine, N-methylguanidine, N-ethylguanidine, guanidine, guanylurea, and aminoguanidine. It was considered desirable from the standpoint of safe handling of the styphnates to determine the sensitivity to impact as indicated by the 5-kg. drop test. The results obtained are given in Table I. When listed with the most sensitive (least stable to impact) first and the least sensitive a t the end, the order is: hydrazine, guanylurea, N methylguanidine, N-ethylguanidine, guanidine, and aminoguanidine.
by melting points and other properties obtained by slow heating. The styphnates of these organic bases are easily prepared, beautifully crystalline compounds, which can be obtained in pure form and used for the identification of these bases, but these conipounds should be considered rather sensitive to detonation both by impact and by rapid heating. LITERATURE CITED
(1) American Cyanamid Co., “Guanyl-
urea,” New Product Bull. 35 (1952).
(2) American Cyanamid Co., Nitrogen Chemicals Digest 4 (1950). (3) Audrieth, L. F., ,?gg, B. A., “Chemistry of Hydrazine, pp. 167-80, Wiley,
New York, 1951.
(4) Deshusses, L., Mitt. Lebensm. u. H y g .
CONCLUSIONS
&f04235 (1929). allant, W. K. A “Ap aratus for Differential Thermd a n 8 Thermogravimetric Analysis,’’ Diviaion of Anal tical Chemistry, 133rd Meeting, ZCS, San Franciso, Calif., April 1958. (6) Lieber, E., Smith, G. B. L., Chem. Revs. 25, 213 (1939). (7) Migrdichian, V., “Or anic Synthesis,” Vol. I, pp. 408-9, Rein%old, New York, 1957. (8) Schenk, M., Kirchhof, H., 2. physiol. Chem. 154, 292 (1926). (9) So11, G., Stutzer, A., Ber. 4 2 , 4532 (1909). (10) Traube, W., Gorniak, K., 2. angew. Chem. 4 2 , 379 (1929).
At rapid heating rates considerably lower decomposition temperatures may be expected than would be indicated
RECEIVED for review December 9, 1958. Accepted December 24, 1959. Division of Analytical Chemistry, 134th Meeting, ACS, Chicago, Ill., September 1958.
Differential Thermal Analysis of Hydroxides in Reducing Atmosphere WILLIAM LODDING and LAURENCE HAMMELL Bureau o f Mineral Research, Rutgers, The State University, New Brunswick, N. J.
,The reactions and phase changes occurring during thermal analysis of iron hydroxides and oxides in reducing and oxidizing atmospheres were identified. Exotherms due to transformations can be used to obtain rapid information on the heats involved and the rates at which these changes proceed under controlled conditions of atmosphere and pressure to 400 p.s.i.g. The amount of gibbsite can be determined from the dehydration peak regardless of the iron hydroxides present, by heating to 450” C. in a hydrogen atmosphere and then in air to 1000” C. The exotherm due to conversion of 7- to cr-FenOa is proportional to the iron hydroxides present.
D
thermal analysis (DTA) is a very sensitive tool for the detection and measurement of gibbsite, Al(0H)a. The analysis is often hampered by the presence of hydrated iron oxides. Goethite, aFeO.OH, and lepidocrocite, y-FeO.OH, dehydrate a t approximately the same temperature as gibbsite (1, 2, 4, 6, 7). The closeness of the resulting endothermic peaks does not allow separate measurement of the areas under the peaks. The reaction cannot be used, therefore, to measure the amount of gibbsite present, unless the percentage of iron hydroxides is known. A new method for the determination of iron hydroxides was based on differential IFFERENTIAL
thermal analysis in a strongly reducing atmosphere. EXPERIMENTAL
Apparatus. A new high temperature-pressure vacuum furnace has been described (6). The pressure vessel (Figure 1) consists of a recrystallized alumina tube 11/4 inches in inside diameter and 12 inches long. (The apparatus will be manufactured by Testing Equipment Sales Corp.) Pressures to 500 p.s.i.g. a t 1200” C. have been used. The tube is gas-tight and relatively easy to evacuate because of its small volume and the absence of voluminous insulating material inside the vessel. The heating element is outside of the tube and, therefore, not in contact with the furnace atmosphere. VOL. 32, NO. 6, MAY 1960
0
657
Figure 1 . Left, A. Right, 8. A.
Pressure-vacuum furnace and DTA assembly
Furnace assembled
Furnace in loading porltlon, insulating jacket removed
Pieriure vessel
D. ubper
cally shows the dehydration, reduction, oxidation-recrystallization sequence. Ground hematite heated in hydrogen to 450' C. also shows the endotherm a t 120' C. and two exotherms a t 390' and 360' C. An endothermic peak is observed a t 670' C., but not the exotherm a t 775' C. The endotherm a t 520' C. in samples containing gibbsite is due to the dehydration of some diaspore which forms by the stepwise dehydration of some of the gibbsite (7). Figure 7 shows thermograms of synthetic ferric hydroxides 1001 and 1015 in hydrogen, followed by heating in air, Both samples show the endotherm of dehydration, followed by an exotherm due to crystallization of the amorphous FelOs to magnetite. The exotherm takes place a t a higher temperature in sample 1015 than in sample 1001. After cooling, flushing with nitrogen, and introduction of air, the sudden pyrophoric reaction can be observed followed by exotherms a t 145' to 175' C. due to oxidation to maghemite. Sample 1001 shows two more exotherms a t 470' and 560' C. which were not identified. Sample
1015 has a small exotherm a t 320' to 330' C. and the characteristic conversion peak a t 760' C. Both coldprecipitated hydroxides show an exothermic peak a t 410' and 580' C., respectively, if they are heated in air without previous reduction. This exotherm does not occur in goethite (a-FeO.OH), but has been observed in lepidocrocite (6). This difference in behavior suggests that the exotherm is due to the considerable difference in lattice energy in the dehydration of amorphous iron hydroxide 'to hematite and also in the dehydration of lepidocrocite to hematite. The difference in lattice exiergy between the goethite and the hematite structure is not large enough. Lepidocrocite and the cold-precipitated hydrous iron oxides dehydrate to maghemite first and convert to hematite a t a higher temperature. This assumption is supported by measurement of the exothermic peak areas. I n samples 1001 and 1015 the areas were 5.2 and 4.8 sq. cm., as shown in Table 11, compared to 4.5 sq. cm. for goethite after reduction. In air the conversion takes place a t lower temperatures than after reduction in hydrogen. In sample 1001 the exotherm follows the dehydration endotherm immediately. In sample 1015 the exotherm comes a t 580' C. It was possible, therefore, VOL. 32, NO. 6, MAY 1960
661
to dehydrate sample 1015 and to stop heating before the start of the exotherm. X-ray diffraction of the dehydrated material heated to 300" C. gives a completely amorphous pattern, After the same sample is heated t o 600' C., a strong hematite pattern is found. The conversion peak is always a doublet, suggesting that it takes place in two steps. The exotherm may be attributed, therefore, to the conversion of the lepidocrocite defect structure or of amorphous ferric oxide to hematite. Lepidocrocite reduced by hydrogen at 400' C. and reoxidized a t room temperature shows two exotherms upon heating in air: one a t 400' C. and the other a t 725" C. The combined areas under these exotherms are of the same magnitude as that of the y- to a- conversion of goethite. Heat of y- to a- Conversion of FezOa. The heat of the y- to CY- conversion of FezO, can be determined from the area under the high-temperature exotherm given in Table 11. Integrated area under peak for 400 mg. of goethite: 4.5 =k 0.1 sq. em.
Sensitivity of thermal analysis between 700" and 900" C. (from calibration with CaCOJ: 3.8 i 0.1 cal. per sq. cm. 3.8 X 4.5 = 171.1 & 0.83 cal. for 360 mg. FenOs
Weissman and his associates of the Bureau of Engineering Research of Rutgers, who also graciously permitted the use of their x-ray diffraction facilities. REFERENCES
or 47.5
2.3 cal. per gram
Similarly, the heats of other transformations can be calculated from the peak areas. Effect of Hematite i n Raw Sample. Heating in hydrogen t o 450' or 500' C. during DTA reduces hematite particles only at t h e surface. Because even finely ground hematite has very little surface area compared to that of dehydrated iron hydroxides, the effect of hematite present in the raw samples on the size of the y- to a- conversion peak is negligible. ACKNOWLEDGMENT
Valuable advice was given by Sigmund
( 1 ) Bernal, J. D., et al., Clay Minerals Bull. 4 (21), 15-30 (1959). (2) Francombe, M. H., Rooksby, H.P., Ibad., 4 (21), 1-14 (1959). (3) Kopp, G. C., Kerr, P. F., Am. Mineralogist 42, 445-54 (1957). (4) Kulp, J. L., Kerr, P. F., Ibid., 36, 2344 (1951). (5) Lodding, W., Hammell, L., Rev. S C ~In&. . 30, 10 885-6 (1959). (6) Mackenaie, R. b., "Problems of Clay and Laterite Genesis," Am. Inst. Min. and Met. Engrs., 1952. (7) Paulik, F., Erdey, L., Acta Chim. Acad. Sci. Hung. 13, 117-39 (1957). (8) Wells, A;, F., "Structural Inorganic
Chemistry, Oxford Univ. Press, Oxford, 1950.
RECEIVED for review August 31, 1959. Accepted December 28, 1959. Published by permission of Helgi Johnson, Director of the Bureau of Mineral Research, R u b pers, The State University.
A n a Iys is of Bis muth-Ant imo nyTe I I urium- Se Ie nium Co mbina t io ns JAMES F. REED Technology Depaffmenf, Westinghouse Research Laboratories, Pittsburgh 35, Pa. In the analysis of mixtures containing bismuth, antimony, selenium, and tellurium each element interferes with the determination of one or more of the others. However, suitable separations are possible. Selenium and tellurium are separated at different acidities with sulfur dioxide and weighed as the free elements. Bismuth is titrated with (ethylenedinitrilo]tetraacetic acid with thiourea as the indicator. Antimony is titrated potentiometrically with potassium permanganate. Accuracy and precision are within 1 to 2 parts per thousand.
T
HE analysis of combinations of bismuth, antimony, selenium, and tellurium is useful for the investigation of their thermoelectric properties. The determination of any one of these elements is straightforward, but there are mutual interferences and other intrinsic difficulties. For example, selenium and tellurium interfere in the permanganate titration of antimony. Antimony interferes by hydrolysis in
662
ANALYTICAL CHEMISTRY
the titration of bismuth with (ethylenedinitrilo) tetraacetic acid (ethylenediaminetetraacetic acid, EDTA), and tellurium reacts with the indicator, thiourea, The reducing agents used for the separation of selenium and tellurium must be removed before antimony can be titrated. Finally each element except bismuth forms a volatile Chloride, yet hydrochloric acid is required to maintain all four elements in solution. Hillebrand and coworkers (4) separated selenium and tellurium at different hydrochloric acid concentrations with sulfurous acid, and dried both elements in air a t 110' C. Duval(1) showed that tellurium oxidizes a t temperatures above 40' C. This work shows that tellurium may be successfully dried in vacuo at room temperature. Fritz (8) titrated bismuth with EDTA using thiourea as a n indicator at a p H of 2.0. Gronkvist (3) recommended a pH range of 2.5 to 4.0 and a higher concentration of thiourea, In this work a p H of 2.4 is preferred when the titration is carried out in the presence of antimony. Also,
with antimony present, values are erratic above p H 2.8. Potentiometric titration of antimony with permanganate eliminated the difficulties associated with ice baths and fading visual end points. EXPERIMENTAL
Reagents. Sulfurous acid, saturated solution of sulfur dioxide in water. Hydrazine hydrochloride, 15% (w./w.) in water. Thiourea. (Ethylenedinitri1o)tetraacetic acid (EDTA), disodium salt, 0.01M in water, standardized against pure bismuth. Potassium permanganate, 0.01 or 0.05N, standardized against sodium oxalate. Acid sulfide wash solution, 1.1N sulfuric acid saturated with hydrogen sulfide. Apparatus. Fisher Titrimeter o r equivalent with calomel platinum electrodes. Procedures. DETERMINATIONOF SELENIUM. Dissolve about 0 . 5 gram of sample in 10 ml. of nitric acid and evaporate t o dryness. Dissolve t h e residue in 100 ml. of concentrated
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