of dimethylsulfoxide. By increasing the amount of silver perchlorate, the amount of interfering solvent which can be tolerated may be increased.
Intermediate Strength Acids
and
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Bases. Acids with Ka values of 10-4 to 10-1 are acidic to screened thymol blue but are not quantitatively titrated by tris (hydroxymethyl) aminomethane. The solution is thus strongly buffered and the end point cannot be obtained. Aromatic amines, with KB values of 10-9 to 10~12 are only partially neutralized by perchloric acid in methanol. These interfere and cause erroneous results. The effect of amines with
KB values in the range of 10~7 to 10-s has not been investigated. If the amine is so weak that it is neutral to the indicator (KB 10~13), results are quantitative.
this manuscript and to the Air Reduction Co., Inc., for permission to publish. LITERATURE CITED
=
ACKNOWLEDGMENT
The author is indebted to C. A. Wamser for his aid in the selection of tris- (hydroxymethyl) aminomethane as the amine titrant and alphazurine as the screening agent for the thymol blue indicator. Acknowledgment is also extended to A. H. Taylor and C. A. Wamser for their critical examination of
Lucien, Molinini, L. Anal. Chem. 27, 1025-7 (1955).
(1) Barnes,
J.,
(2) Critchfield, F. E., Johnson, J. B., Ibid., 26, 1803-6 (1954). (3) Fossum, J. H., Markunas, P. C., Riddick, J. A., Ibid., 23, 491-3 (1951). (4) Koulkes, Michel, Marszak, Israel, Bull. soc. chim. France 1952, 556-7. (5) Miocque, Marcel, Gautier, J. A., Ibid., 1958, 476-9. (6) Siggia, Sidney, Anal. Chem. 28, 1481-3 (1956). Received for review June 13, 1958. Accepted October 13, 1958.
Thermal Decomposition of the Rare Earth Metal Oxalates WESLEY W. WENDLANDT
Department of Chemistry and Chemical Engineering, Texas Technological College, Lubbock, Tex.
The thermal decomposition of the hydrated oxalates of terbium, dysprosium, thulium, ytterbium, and lu-
tetium was studied on the thermobalance. The oxalates were precipitated from solution as the 10- or 5-hydrates and when subjected to pyrolysis, began to lose weight in the 45° to 60° C. temperature range. After various stages of intermediate hydrate formation and decomposition, the metal oxide levels were obtained in the 71 5° to 745° C. temperature range.
thermal decomposition of the
The scandium, yttrium, and the lighter
earth metal oxalates has been described (2, ). This report is concerned with the thermal decomposition of the heavier metal oxalates—dysprosium, terbium, ytterbium, thulium, and rare
lutetium.
Except for degrees of hydration, little known about the chemistry of the heavier rare earth metal oxalates. According to Vickery (3), even the optimum conditions for their preparation have never been defined. To circumis
Table I.
Compound Terbium oxalate,“ curve A
Dysprosium oxalate, curveB
Formula
First Wt. Loss, 0
C.
Tb2(CA)=.10H2O
45
Dy¡(CA)>. 10H2O
45
EXPERIMENTAL
Reagents. The source of the dysprosium, terbium, and ytterbium oxides was the St. Eloi Corp., Newtown, The thulium and lutetium Ohio. oxides were obtained from the Lindsay Chemical Co., West Chicago, 111. The purity of these compounds was listed as 99.9% by the supplier. The methyl oxalate was prepared as
Thermal Decomposition Data Breaks in Curve 0
C.
140s
265 140=
220 0
Thulium oxalate, curveC Ytterbium oxalate, curve D Lutetium oxalate,
vent this question, the metal oxalates were prepared by homogeneous precipitation with methyl oxalate.
Formula Tb2(C204)3.5H20 Tb2(C204)3.lH20 Dy2(C204)3.4H20 Dy2(C204)3.2H20
Anhydrous Oxalate ° Formula C. 435
Tb2(C204)3
Loss of CO and CO; Formula C.
°
725
Tb407
745
DyaOs
Horizontal Wt. Level C. Formula
Tm2( C204)3 5H20
55
220-295 195-335
Tm2(C204)3.2H20
730
Tm203
Yb2(C204)3.5H20
60
175-325
Yb2(C204)3.2H20
730
Yb203
Lu2(C204)3.6H20
55
190-315
Lu2(C204)3.2H20
715
Lu203
·
Thermal decomposition curve similar to that found for gadolinium oxalate ( ). Because horizontal weight levels were not obtained, amount of hydrate water may be fortuitous. terbium oxalate is the only rare earth that shows such behavior. = This amount of hydrate water may be fortuitous. 11
6
408
·
ANALYTICAL CHEMISTRY
If
a
1-hydrate is actually formed,
Table II.
Composition Data for Metal Oxalates (% metal oxide)
Mi(CiO()a. Rare Earth
Metal
ioh2o
Theor. Found
48.94
Dysprosium
Terbium
49.05
49.2 49.1
49.8 49.6
M2(C204)3.sH20
M2(C204)3
X
Theor.
Found
Theor.
Found
4
56.42
57.2 (140° C.)
63.32
64.7 (415° C.) 64.8
2
59.67
60.7 60.4
5
55.63
56.0 56.0
64.24
65.0(435°)
64.9
WEIGHT
1
62.32
62.2(265°)
61.9
Ytterbium
Thulium Figure 1. curves
of
Thermal rare
decomposition
5
56.28
55.6 55.9
o
60.99
61.0 61.4
5
55.76
55.2 55.2
2
60.48
61.4 61.3
6
55.08
55.1 55.5
2
61.19
61.8 61.1
earth metal oxalates A. £.
Terbium Dysprosium
C. D.
Thulium
E.
Lutetium
Lutetium
Ytterbium
previously described (1). All other chemicals used were of c.p. quality. Thermobalance. The automatic recording thermobalance has been described (4). A heating rate of 5.4° C. per minute was employed with sample sizes ranging in weight from 70 to 90 mg. A slow stream of air was passed through the furnace during the pyrolysis. The metal oxalates were prepared as previously described ( ).
Decomposition Temperatures of Rare Earth Metal Oxalates Temp., ° Transition Metal C.
Table III.
Rare Earth
Lanthanum
55-380 380-550 735-800
10-hydrate — anhydrous Anhydrous —> La203.C02
Cerium
50-360 40-420 420-790 50-445 445-735 45-300 410-735 60-320 320-620 45-120 120-315 375-700
oxide 10-hydrate —* 10-hydrate anhydrous Anhydrous —* oxide 10-hydrate —* anhydrous Anhydrous —> oxide 10-hydrate -* anhydrous Anhydrous —*· oxide 10-hydrate —* anhydrous Anhydrous —> oxide 10-hydrate — 6-hydrate 6-hydrate -* anhydrous Anhydrous — oxide 10-hydrate — 5-hydrate 5-hydrate —* 1-hydrate 1-hydrate — anhydrous Anhydrous —* oxide
Praseodymium
Neodymium DISCUSSION
The thermal decomposition curves are given in Figure 1 and thermal decomposition data and composition data in Tables I and II.
Samarium
Europium Gadolinium
GENERAL OBSERVATIONS
The thermal decomposition patterns of the metal oxalates can be classified into two of the three classes previously assigned ( ). Dysprosium and terbium can be placed in class II, with samarium, europium, and gadolinium, because the formation of intermediate hydrates was indicated but weight levels were not obtained. Ytterbium, thulium, and lutetium can be placed in class III, with yttrium, holmium, and erbium, because stable hydrate weight levels were obtained. As in the decomposition of the other metal oxalates, the 2-hydrates appear to be surprisingly stable. The anhydrous metal oxalates are apparently too unstable to exist, and decompose with the evolution of carbon monoxide and carbon dioxide. It may be possible to determine the presence of a class III compound in a mixture with a class I metal oxalate because of the stability of the class III metal oxalate 2-hydrate.
Terbium
Dysprosium
Holmium Erbium Thulium Ytterbium Lutetium
Yttrium Scandium
45-140 140-265 265-435 435-725 45-140 140-220 295-415 415-745 40-200 240-400 400-735 40-175 265-395 395-720 55-195 335-730 60-175 325-730 55-190 315-715 45-180 260-410 410-735 50-185 220-635
La203.C02
—
oxide
—*
10-hydrate — 4-hydrate 4-hydrate —* 2-hydrate 2-hydrate —* anhydrous Anhydrous — oxide 10-hydrate —*· 2-hydrate 2-hydrate — anhydrous Anhydrous —* oxide 6-hydrate —* 2-hydrate 2-hydrate —anhydrous Anhydrous -* oxide 5-hydrate — 2-hydrate 2-hvdrate —oxide
5-hydrate —* 2-hydrate 2-hydrate — oxide 6-hydrate — 2-hydrate 2-hydrate —>· oxide 9-hydrate —> 2-hvdrate 2-hydrate — anhydrous Anhydrous —* oxide 6-hydrate —> 2-hydrate 2-hvdrate —* oxide
VOL. 31, NO. 3, MARCH 1959
·
409
The decomposition temperatures for all the rare earth metal oxalates are given in Table III.
of the Davison Chemical Co. and the Lindsay Chemical Co. for the samples of thulium and lutetium oxides.
ACKNOWLEDGMENT
LITERATURE CITED
The author would like to thank James R. Slagle for the preparation of the methyl oxalate, and Richard M. Handle
(1) Blatt, A. H., “Organic Syntheses,” Coll. Vol. 2, p. 414, Wiley, New York, 1943.
(2) Caro, P., Loriers, J., J. recherches
centre natl. recherche sci. Labs. Bellevue (Paris) 39, 107 (1958). (3) Vickery, R. C., “Chemistry of Lanthanons,” pp. 250-1, Academic Press, New York, 1953.
(4) Wendlandt, W. W., Anal. 30,56(1958). (5) Ibid., p, 58.
Received for review7 April 28, Accepted October 7, 1958.
Chem.
1958.
Determination of Tin in Inorganic and Organic Compounds and Mixtures MARIE FARNSWORTH and JOSEPH PEKOLA Research Laboratory, Metal & Thermit Corp., Rahway, N. J.
The standardization of potassium iodate for the volumetric determination of tin and its determination in inorganic and organic compounds have been critically studied. Various methods for destroying organic matter, if
present, are given. Inaccuracies inherent in certain methods are pointed out and a method is given for the determination of high tins colorimetrically after combustion in a Parr bomb. is eliminated by hypophosphorous acid as the reductant; in other cases iron powder and nickel are recommended.
Copper interference using
the
volumetric determination of
Intin it is customary to titrate the
duced tin with
re-
standard iodate solution with starch for the indicator. Potassium iodate is a primary standard and for most work it is satisfactory to use its theoretical equivalent, although this is not true for tin. Here it is customary to standardize the solution against a known amount of tin, and the tin equivalent thus found is higher than the theoretical value. This is attributed partially to the oxygen dissolved in the potassium iodate solution, and it would be difficult to remove this dissolved The experience oxygen complete^7. gained in this laboratory in determining both stannous and total tin in very pure stannous salts should throw further light on the subject. While a number of metals and other reducing agents have been recommended for use in the volumetric determination of tin, the work reported here has been confined principally to nickel or to a combination of iron and nickel. Using nickel reduction alone in the analysis of very pure stannous compounds gave a value for stannous tin which was higher than the value for total tin. This is
410
·
a
ANALYTICAL CHEMISTRY
clearly impossible without an analytical Such results indicate that reduction with nickel alone is incomplete, although reproducible results are obtained. Incomplete reduction would give a tin equivalent for the iodate solution that is too high. When total tin is determined, the reduction is also incomplete to the same extent and correct values for tin are obtained. When stannous tin is determined and no reduction is involved, the high tin equivalent for the solution gives a value for stannous tin that is too high. To check this, a combination of iron and nickel was next used for the reduction. The following data show that reduction with nickel alone is incomplete, resulting in a higher tin equivalent for the potassium iodate solution: 0.006004 and 0.006054 gram per ml. for nickel error.
Table
1,
Total Tin in Inorganic Compounds Total Tin, % Sn Eq.
31N 93W 92W 26V
SnS04
Table II.
Sn Eq.
from + Ni
Fe
Gravimetric
reduc-
tion
tion
54.74
54.74 62.41 54.49 57.39
54.73 62.40 54.51 57.40
SnCL SnS04 SnC204
from Ni
57.41
Stannous Tin
1
in
reduc-
Inorganic
Compounds Stannous Tin, % Sn Eq.
Sn Eq.
from Ni
31N 93W 92W 26V
reduction 54.88 SnS04 62.90 SnCl2 54.73 SnSÓj SnC204 57.51
from + Ni
Theoretical
tion 54.55 62.40 54.30 57.12
Value 55.27 62.60 55.27 57.42
Fe
reduc-
Sn +
+
reduction and 0.005956 and 0.006010 gram per ml. for the corresponding iron and nickel reduction. Table I shows that nickel reduction is satisfactory when only total tin is sought. Table II shows that nickel reduction alone is not satisfactory for the standardization when stannous tin is being determined. It can be seen from Table II that the tin equivalent obtained by nickel reduction actually gives stannous values higher than theoretical for the very pure stannous chloride and stannous oxalate. Aside from the values for the tin equivalent thus obtained, other factors enter into the selection of a reducing agent. Nickel reduces only in a hot solution and it does not need to be removed when the solution is cooled to room temperature or lower for titration. Metallic iron must be entirely removed before the solution can be titrated, for it reduces in both the hot and the cold. Using up a reductant completely and not allowing the solution to reoxidize introduces some mechanical difficulties, when dealing with more than a few samples at one time. The combination of iron and nickel is to be preferred to iron alone. Stannous tin is very easily reoxidized and must be protected from the atmosphere at all times. Any water or acid used in the determination of stannous tin must be thoroughly boiled to remove all air. The number of samples involved will, to some extent, determine the mechanical devices used. If a number of stannous tin samples are to be analyzed concurrently the hydrochloric acid (1 to 2) required should be prepared at one time. DETERMINATION OF TOTAL AND STANNOUS TIN IN INORGANIC COMPOUNDS
Solutions.
Standard potassium io-
