position point, often originating controversy since the value found is a function of the method and apparatus. The erroneous ideas are readily suggested by any method which, like differential thermal analysis (DTA) or weight-temperature thermogravimetry, procceds by recording a physical characteristic of the product studied as a function of temperature. There is a strong tendency to apply to chemical reactions the assumption made in DTA, which is correct as far as the transition temperatures are concerned. This tendency must be discouraged. Another troublesome limitation is brought about by the solubility of gases in liquids or solids. The fundamental assumption made in thermogravimetry, a s in many other physical chemistry technics, is that a gaseous reaction product is evolved as soon as i t is produced by the reaction, or at least, a constant time after. However, i t often happens that when one of the initial or final products of the reaction is liquid, gases are soluble in it. There is then a delay before this gas leaves the crucible, and this delay varies during the experiment. During the first part the gas concentration increases, as does the rate of evaporation. At equilibrium,
the rates of formation and evaporation are equal, but the weight of the liquid, either initial or final product, is not constant. Moreover, the equilibrium concentration depends on the partial pressure of the gas in the furnace, in the neighborhood of the crucible, which is unknown and very difficult to measure. It is thus impossible to calculate a correction. Solubility may be measured in separate experiments, but i t is often difficult to have an order of magnitude of the equilibrium concentration in the actual experiment. This concentration may be decreased to a small value by using wide crucibles without covers, a thin layer of liquid, and a flow of an inert gas through the furnace, This flow is almost always necessrtry to facilitate the diflusion of gases t o and from the sample, which is another very important condition. This limitation is insidious, and it is very difficult to eliminate or even memure the error involved; it is generally unknown. An idea of the possible importance of such a phenomenon is given by the following experiment (I) :
Chevenard thermobalance. The internal diameter of the crucible is 22 mm. (the initial thickness of the li uid layer is about 12 mm.). Nitric aci! is determined in the ammonium nitrate after cooling; ,O.S% remains. Nitric acid has no catalytic effect on the decomposition of ammonium nitrate, which gives no nitric acid under thew conditions, 80 that only the slowness of its evfiporation can explain this result.
A sample of 6 grams of ammonium nitrate m t h 1% nitric acid i s heated for 3 hours at 200’ C. in *e furnace of a
GEOROES Omorno# Laborahire de Chimie Physi ue &ole Polytechnique, Paris, .&me
As Newkirk explains, there is a l w a p a difference between the actual temperature of the a m p l e and that of the furnace. This difference is impossible to calculate except in a few c a w , for a deflnite purpoae, when the kinefio constants of the reaction are known (8). Thermogravimetric measurements am esay t o carry out, but somewhat difficult to account for.. LITERATURE CITED
(1) Guioohon, Georgee, Ann. chim. (Paris) 5, 296 (1960). (2) Guioohon, Georges, Mem. p d r u 42,47 (1960). (3) Newkfrk, A. E., ANAL.CHEW32,1858 (1QW.
Dissolution of Tungsten by Hydrogen Peroxide SIR: The dissolution of tungsten by hydrogen peroxide is little known; among the more comprehensive reference works of inorganic chemistry, those of Mellor (4) and Pascal (6) fail to recognize the effect. Gmelin (9) devotes only one sentence t o the subject: “HzOl 16st unter starker positiver Wkrmetonung zu WOa.HzOt.” [H20, dissolves (tungsten) quite exothermically to WOa.H202.J Thenard ( Y ) , in his pioneering studies of hydrogen peroxide, indicates that the reagent oxidizes tungsten; Lottermoser (S), dealing with the catalytic decomposition of hydrogen peroxide by tungsten, states that tungsten . . am besten in moglichst feiner, kolloider Verteilung . . ,,’ can be dissolved in “. . . Perhydrol.” (Freely translated: Tungsten, best in finest possible colloidal form, can be dissolved in 30% hydrogen peroxide.) Thus, the literature concerning the dissolution of tungsten by hydrogen peroside is meager. Conscquently, when i t was observed recently that commonly encountercd commercial tungsten products such as powder, wire, and sheet arc completely dissolved in 30% hydrogen peroxide within minutes t o hours, it was thought that these findings should be rcported because of their intcrmting implications for both ana-
“.
lytical and inorganic chemistry, and for metallurgy. By analogy with the known behavior of molybdenum, and of tungsten trioxide, it would appear that dissolution of tungsten in hydrogen peroxide results in the formation of soluble pertungstic acid, to which Lottermoser (3) assigns the formula WOa.H& However, more recent studies of the WOrH80, system (6) indicate the formation of two pertungstic acids, one having an active oxygen t o tungsten ratio of 1 to 1, the other 4 to 1; the formulas of these are not established with certainty. EXPERIMENTAL
Experiments were performed to determine the effects of temperature, concentration of hydrogen peroxide, and state of subdivision of the metal on the rate of reaction. Tests were also made t o determine whether the addition of other common reagents would improve the rate of dissolution of tungsten. Comparison tests were made on molybdenum. The rate of dissolution was observed to increase with temperature; however, a t temperatures above 60’ C. an increasing rate of spontaneous de-
composition of hydrogen peroxide tended to slow the reaction. A p proximately 60” C. was found to give the most rapid dissolution. Samples of tungsten and molybdenum wires, 0.2 gram in weight, cut to a p proximately l/&ch length, were dissolved in 10 ml. of 30% hydrogen peroxide. The times required for complete dissolution of different sizes of wires, at 60” C. and at room temperature, are given in Table I. The dissolution of 0.2 gram of tungeten powder or molybdenum powder in 30% hydrogen peroxide proceeded more vigorously, and care was required to avoid partial loss of the sample. The dissolution times of the powders in Table I. Dissolution Times for 0.2 Gram of Various Sizes of Molybdenum and Tungsten (Wires in 10 ml. of 307’
Dia.meter, Mils
hydrogen peroxide) Dissolution Time, Hours 60” C. Room temp. Mo W Mo W
VOL. 33, NO. 8, JULY 1961
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Table II. Dissolution Times for 0.2 Gram of W and Mo Powders at Room Temperature in Various Concentrations of HsOz
Powder Grain, Microns 40-60 3 5
H101, % 30 20 10 30 20
HzOi Used, MI. 5 5
5 1
Dissolution Time, Min. Mo W Not tested 10 Not tested 30 Not tested Incomplete”
