of dimethylsulfoxidc. By increasing the amount of silver perchlorate, the amount of interfering solvent m-hich can be tolerated may be increased. INTERMEDIATE STRENGTH ACIDSAND BASES. Acids with K a values of to 10-I are acidic to screened thymol blue but are not quantitatively titrated by tris(hydroxymethy1)aminomethane. The solution is thus strongly buffered and the end point cannot be obtained. Aromatic amines, with Kg 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
Kg values in the range of
to 10-8 has not been investigated. If the amine is so weak that it is neutral to the indicator (KB = lO-I3), results are quantitative.
this manuscript and t o the Air Reduction Co., Inc., for permission to publish. LITERATURE CITED
(1) Barnes,
Lucien, Molinini, L. J., CHEW 27, 1025-7 (1955). (2) CritcMeld, F. E., Johnson, J. B., ANAL.
ACKNOWLEDGMENT
The author is indebted to C. A. Wamser for his aid in the selection of tris-(hydroxymethy1)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. Kamser for their critical examination of
Zbid., 26, 1803-6 (1954). (3) Fossum, J. H., Markunas, P. C., Riddick, J. A., Ibid., 23, 491-3 (1951). (4) Koulkes, ilfichel, Marszak, Israel, Bull. SOC. chim. France 1952, 556-7. (5) Miocque, Marcel, Gautier,
J. A4.,
Zbid.. 1958. 476-9. (6) Siggia, S'idney, AXAL. CHEhI. 28, 1481-3 (1956). RECEIVED for review June 13, 1958. Accepted October 13, 1958.
WESLEY W. WENDLANDT Department of Chemistry and Chemical Engineering, Texas Technological College,
,The thermal decomposition of the hydrated oxalates of terbium, dysprosium, thulium, ytterbium, and Iutetium 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.
T
thermal decomposition of the scandium, yttrium, and the lighter rare earth metal oxalates has been described (2, 6). This report is concerned with the thermal decomposition of the heavier metal oxalates-dysprosium, terbium, ytterbium, thulium, and lutetium. Except for degrees of hydration, little is known about the chemistry of the heavier rare earth metal oxalates. According t o Vickery (s), even the optimum conditions for their preparation have never been defined. T o circumHE
Table
Compound Terbium oxalate," curve A Dysprosium oxalate, curve B
Formula Tb,( Cz04)~. lOHzO
Lubbock, Tex.
I.
Wt.First Loss,
c.
45
vent this question, the metal oxalates were prepared b y homogeneous precipitation with methyl oxalate. EXPERIMENTAL
Reagents. T h e source of the dysprosium, terbium, and ytterbium oxides was t h e St. Eloi Corp., Newtown, Ohio. T h e thulium and lutetium oxides were obtained from t h e Lindsay Chemical Co., West Chicago, Ill. T h e purity of these compounds was listed as 99.9% by the supplier. The methyl oxalate was prepared as
Thermal Decomposition Data
Breaks in Curve C. Formula 140b Tbz(C204)3.5HzO 265 Tbz(Cz04)3. lHzO O
Anhydrous Oxalate C. Formula O
435
Loss of CO and C o t ' C. Formula
Tbz(CzO4)3
Horizontal Wt. Level c. Formula 220-295 730 TmZ03 T ~ z ( C Z O 5~ H ) I ~. 0 55 195-335 Tmz(C204)3.2H20 Thulium oxalate, curve C 730 Ybz03 Yb?(Cz04)~.5HzO 60 175-325 Ybz(C20r)a.2HzO Ytterbium oxalate, curve D 715 LUzo3 Lu2(Cz04)3. 6Hz0 55 190-315 Luz(CzO4)r.2Hz0 Lutetium oxalate, curve E u Thermal decomposition curve similar to that found for gadolinium oxalate ( 5 ) . Because horizontal weight levels were not obtained, amount of hydrate water may he fortuitous. If a 1-hydrate is actually formed, terbium oxalate is the only rare earth that shows such behavior. c This amount of hydrate water may be fortuitous.
408
0
ANALYTICAL CHEMISTRY
Table II.
Composition Data for Metal Oxalates
(yometal oxide) Rare Earth Metal Dysprosium
715
Mz(Cz0i)a. 10Hz0 Theor. Found x 48.94 49.2 4 49.1 2
?rlz( C204)3.ZH20 Theor. Found 56.42 57.2 (140" C.)
59.67
I-
I
Terbium
(3
49.05
Ytterbium
L
'725
1
TEMPERATURE "C.
Thulium
Figure 1. Thermal d e c o m p o s i t i o n curves of rare earth metal oxalates A.
Terbium Dysprosium C. Thulium D. Ytterbium E. Lutetium
Lutetium
49.8 49.6
5
55.63
1
62.32
5
56.28
2
60.99
5
55.76
2
60.48
6
55.08
2
61.19
E.
previously described ( 1 ) . 411 other cliemicals used were of c .P. quality. Thermobalance. T h e automatic recording thermobalance has been described (4). A heating rate of 5.4' C. per minute was employed with sample sizcs ranging in weight from 70 t o 90 mg. A slow stream of air was passed through the furnace during the pyrolysis. The metal oxalates were prepared as previously described ( 5 ) . DISCUSSION
The thermal decomposition curves are given in Figure 1 and thermal decomposition data and composition data in Tables I and 11.
Table 111.
hfz(Czod3 Found 64.7 (415" C.) 64.8
64.24
65.0(435') 64.9
Decomposition Temperatures of Rare Earth Metal Oxalates
Rare Earth Metal Lant.hanum Cerium Praseodymium X'eodymium Samarium Europium Gadolinium
GENERAL OBSERVATIONS
The thermal decomposition patterns of the metal oxalates can be classified into two of the three classes previously assigned ( 5 ) . Dysprosium and terbium can be placed in class 11,with samarium, europium, and gadolinium, because the formation of intermediate hydrates was indicated but -neight leyels \\ ere not obtained. Ytterbium, thulium, and lutetium can be placed in class 111, with yttrium, holmium, and erbium, because stable hydrate weight levels were obtained. ils in the decomposition of the other metal osalates, the ?-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 t o determine the presence of a class I11 compound in a mixture n i t h a class I metal oxalate because of the stability of the class I11 metal ovalate 2-hydrate.
60.7 60.4 56.0 56.0 62.2(265') 61.9 55.6 55.9 61.0 61.4 55.2 55.2 61.4 61.3 55.1 55.5 61.8 61.1
Theor. 63.32
Terbium
Dysprosium
Holmium Erbium Thulium Ytterbium Lutehium Yttrium Scandium
Temp., a
c.
55-380 380-550 735-800 50-360 40-420 420-790 50-445 445-735 45-300 410-735 60-320 320420 45-120 120-315 375-700 45-140 140-265 265-435 435-725 4.5-140 i4G220 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
Transition 10-hydrate anhydrous Anhydrous +. La&. COZ LazOa.CO, -+ oxide 10-hydrate -+ oxide 10-hydrate +. anhydrous Snhydrous -., oxide 10-hydrate +. anhydrous Anhydrous +. oxide 10-hydrate anhydrous oxide Anhydrous 10-hydrate +. anhydrous rlnhydrous +. oxide 10-hydrate 6-hydrate 6-hydrate -+ anhydrous Anhydrous -+ oxide 10-hydrate -+ 5-hydrate 5-hydrate +. 1-hydrate 1-hydrate -+ anhvdrous Anhydrous +. oxide 10-hydrate -+ 4-hydrate 4-hydrate +. 2-hydrate 2-hydrate +. anhydrous Ahhydrous+ oxide 10-hydrate +. 2-hydrate %hydrate +. anhvdrous Anhydrous -+ oxide 6-hydrate +. 2-hydrate ?-hydrate +. anhydrous Anhydrous -t oxide 5-hydrate -+ 2-hydrate 2-hydrate +. oxide 5-hydrate +. 2-hydrate "hydrate -t oxide 6-hydrate -c 2-hydrate 2-hydrate +. oxide 9-hrdrate +. 2-hydrate 2-h;Tdrate +. anhvdrous oxide Anhydrous 6-hydrate +. 2-hydrate 2-hydrate +. oxide -f
-+ -f
-f
-f
VOL. 31, NO. 3, MARCH 1959
409
The decomposition temperatures for all the rare earth metal oxalates are given in Table 111.
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 ill.Mandle
(1) Blatt, A. H., “Organic Syntheses,” Coll. Vol. 2, p. 414, Wiley, New York,
1943, (2) Caro, P., Loriers, J., J . recherche5
centre natl. recherche sei. Labs. Bellevue j9,lo7 (1958). (3) Vickery, R. C., “Chemistry of Lanthanone,” pp. 250-1, Academic Press, Kew York, 1953. (4) Wendlandt, W. W., ANAL. CHEM. 30,56 (1958). (5) Zbid., P, 58.
RECEIVEDfor review April 28, 1958. Accepted October 7 , 1958.
Determination of Tin in inorganic and Organic Compounds and Mixtures MARIE FARNSWORTH and JOSEPH PEKOLA Research laboratory, Metal & Jhermit Corp., Rahway, N. J.
b The standardization of potassium iodate for the volumetric determination of tin and its determination in inorganic and organic compounds have been critically studied. Various rnethods 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. Copper interference is eliminated by using hypophosphorous acid as the reductant; in other cases iron powder and nickel are recommended.
I
volumetric determination of tin it is customary to titrate the reduced tin with a 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 oxTgen completely. The experience 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 N THE
410 *
ANALYTICAL CHEMISTRY
clearly impossible without a n analytical error. 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 Table I.
Total Tin in Inorganic Compounds
Total Tin, 7 0 SnEq. Sn Eq. from from hTi Fe Si Gravi- reducreducmetric tion tion
+
3 1 s SnSOd 54 74 931%’SnCl2’ 92W SnSOa 26V SnCzO4 57 41 Table II.
31N 93W 92W 26V
54.74 62.41 54 49 57.39
54 73 62 40 54 51 57.40
Stannous Tin in Inorganic Compounds
SnSO4 SnC12 SnSOl SnCaO4
Stannous Tin, % Sn Eq. SnEq. from from Fe Ni Xi reducreduction tion
+
Theoretical Sn++ Value
54.88 62.90 54.73 57.51
54.55 62.40 54.30 57.12
55.27 62.60 55.27 57.42
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 I1 shows that nickel reduction alone is not satisfactory for the standardization when stannous tin is being determined. It can be seen from Table I1 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. Kickel reduces only in a hot solution and it does not need to be removed when the solution is cooled to room temperature or loner 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 u p 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 reouidized and must be protected from the atmosphere a t 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 sill, 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 a t one time. DETERMINATION OF TOTAL AND STANNOUS TIN IN INORGANIC COMPOUNDS
Solutions. Standard potassium io-
