234
RICHARD F. PORTER AND RICHARD C. SCHOONMARER
Vol. 62
GASEOUS SPECIES IN THE VAPORIZATION OF POTASSIUM HYDROXIDE1 BY RICHARD F. PORTER AND RICHARD C. SCHOONMAKER Department of Chemistry, Cornell University, Ithaca, New Yorlc Received October 6 , 2867
A mass spectrometric study of the gaseous species produced in the vaporization of potassiun hydroxide shows that in the temperature range 300 to 450" KOH vaporizes mainly as gaseous dimers. For the reaction 2KOH(s) = Ka(OH)Z(g) AHoe26 = 36 i 2 kcal. per mole of dimer. The energy of dimerization of KOH(g) a t T = 626°K. is found to be greater than 38 kcal. per mole of dimer.
Introduction Evidence for the existence of gaseous metal hydroxides in high temperature systems has recently been p~+esented.~-~ Electronic band systems observed from flame sources have been attributed to molecules such as BaOH(g) and SrOH(g).2-a Indirect spectroscopic evidence for the formation of KOH(g) and CsOH(g) in flames also has been obtained.4 The vapor pressures of liquid NaOH6 and KOH6s6have been measured, and although these compounds have volatilities comparable to the corresponding halides, the gaseous species in the vapor have not been identified. The present work was initiated to study the vaporization of metallic hydroxides with the hope of obtaining data on the thermal stabilities of the vaporization products. The results reported here are for the KOH system. Experimental
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The method of investigation was a combination of mass spectrometric and Knudsen effusion techniques. Essential details of the apparatus and procedure have been discussed in earlier work.738 Vapor effusing from the orifice of a cell containing a condensed phase in thermal equilibrium with a vapor phase is bombarded by electrons of known energy and the positive ions produced are observed in a mass spectrometer. From a systematic study of the ions produced, appearance potential measurements and ion current dependence on temperature, the neutral gaseous species are identified and thermodynamic data for these species are obtained. The mass spectrometer used in this work is a 12 inch, 60 degree, direction-focusing type instrument. Ions were collected on the first plate of a nine stage electron multiplier9 operated a t 320 volts per stage. The output from the multiplier was fed through a 100 megohm resistor and the voltage drop developed was measured with a VIbrating reed electrometer,lO model 31, which was coupled to a Leeds and Northrup 10 m.v. recorder. The ion source and furnace assembly was based on a design of Chupka and Inghram.' The effusion cell is mounted below the ion source of the mass spectrometer. Gaseous molecules leaving the orifice of the cell pass through several slits in the ion source and into the ionization chamber. Positive ions produced are then accelerated into the analyzer for mass separation by magnetic scanning and detection. By means of a shutter located between the effusion cell and (1) This research was supported by the United States Air Force through the Air Force Office of Scientific Research of the Air Research and Development Command under contract No. AF 18 (603)-1. (2) (a) M. Charton and A. G . Gaydon, Proc. Phye. S o c . , 6 9 1 , 520 (1956); (b) C. G. James and T. M. Sugden, Nature, 175, 333 (1955). (3) A. Lagerquist and C. Huldt, Naturwiaa. (London), 42, 365 ( 1955). (4) H.Smith and T. M . Sugden, Proc. Roy. Boc., 2 i l A , 58 (1952). ( 5 ) H.von Wartenberg and P. A. Albrecht, 2. Elektrochem.,2 T , 162 (1921). (6) D. D. Jackson and J. J. Morgan, J . Ind. Enp. Ckem., 13, 110 (1921). (7) W. A. Chupka and M. G . Inghram, THISJOURNAL, 69, 100 (1955). (8) M.G.Inghram, W. A. Chupka and R. F. Porter, J . Cham. Phya., 23, 2159 (1955). (9) Manufactured by Dumont Laboratories, Inc., Passaic. N. J. (IO) Manufactured by Applied Physios Cow., Pasadena, Cal.
the ion source it is possible to control the number of molecules that enter the ion source from the effusion cell. When the slit in the shutter plate is aligned with the slits in the ion source and the orifice of the effusion cell, the maximum number of molecules pass through the ion source slits. When the slit in the shutter plate is moved completely out of alignment, molecules are prevented from entering the source. The so-called "shutter effect" is the difference in an ion current observed for shutter open (i.e., aligned) and shutter closed (i.e., out of alignment). Since the shutter does not affect background currents, the "shutter effect" is a true measure of the ion intensity of a species being investigated. Radiation to heat the crucible oven was provided from an electrically heated tungsten filament surrounding the cell. The entire effusion cell assembly was surrounded by several sheets of radiation shielding for uniform heating. With this arrangement the temperature of the effusion cell could be maintained constant for several minutes. A small opening in the top of the shields allowed for passage of the vapor into the ion source. The effusion cells consisted of a metal crucible with the inqerior walls lined with a pressed layer of magnesia approximately I/a inch thick. The exterior metal crucibles were either of stainless steel or platinum. Covers for the cells were constructed in the same manner as the crucibles. The KOH sample was thus primarily in contact with magnesia. I n all cases the ratio of the effusion hold area to the total area of the evaporating material within the cell was less .than 0.02. The temperature of the cell was measured with a chromel-alumel thermocouple. The thermocouple leads were brought out of the vacuum system through kovarglass seals. A calibration of the thermocouple was made against the melting point of a sample of pure zinc obtained from the National Bureau of Standards. This point lies in the temperature range of the expzriments. The temperature correction amounted to only 1 Since purity specifications are not critical in this type of experiment, a reagent grade of commercial KOH was used. The major impurity, K&Oa, was approximately 2%. TO outgas the sample it was necessary to heat it for several hours a t 200"under vacuum in the mass spectrometer housing. The temperature range investigated was between 270 and 450". Most of the data were obtained a t cell tem eratures between 300 and 400'. The vapor pressure of K8H liquid is quite high above 400' as was indicated by a large amount of material condensed on the colder portions of the furnace assembly in experiments where temperatures in excess of 400' were reached.
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Results Several scans of the mass spectrum were made for various cell temperatures and bombarding electron energies. The major ions observed were K+, KOH+ and KzOH+. A small current of K20+ also was detected. By using 50 volt bombarding electrons, identification of ion peaks was first made by counting back-ground peaks (principally hydrocarbon) produced from residual gases in the system. To check this identification the intensities of isotopic peaks for K39 and K4I were measured and found t o agree with those calculated statistically for each species. Further confirmation of the KOH+ and K 2 0 H +masses was made by checking the deuterium isotope effect when a sample of KOD was loaded into the effusion cell. Ions
GASEOUS SPECIESIN VAPORIZATION OF POTASSIUM HYDROXIDE
Feb., 1958
of mass larger than KzOH+ produced from NOH vapor were not detected. If we take the intensities of background peaks as an upper limit, it can be stated that intensities of ions of mass higher than K 2 0 H +are less than 3% of the intensity of K20H+. K+ and KzOH+.-Rough appearance potential curves for K+and KzOH+near threshold are shown in Fig. 1. Relative intensity data are given in Table I. In Table I two sets of conditions within the effusion cell are noted. These are indicated as “slightly reducing” and “near neutral.” Slight reducing conditions prevailed within the effusion cell when the outer crucible was of stainless steel. Presumably some KOH penetrated the magnesia liner and came in contact with the outer crucible.
30
22 18
+’ 14
10 6
K +/&OH
T,‘C.
*a
0.69 346 353 3.44 Slightly reducing 376 3.13 404 5.5 457 4.6 a Measured with 10 volt electron energies.
2.2 2.8 2.5 2.5 2.3
TEMPERATURE (“C.) K O H C( T KOH+ (T
15.6
= =
457)/ 353)’”
13.8
KzOH+ (T = 457)/Kz0Ht (T = 353)a
11.5 Measured with 10 volt electron energies.
Under these conditions the K + peak was the major ion observed. When the outer crucible was platinum, the chemical reduction of the KOH was greatly diminished. The latter is referred to as a “near neutral” condition. The appearance potential curve for K + shown in Fig. 1 was obtained under the more reducing condition and the extrapolation of the curve t o zero ion current gives an appearance potential agreeing with the ionization potential of K(g). The appearance potential together with the observation that the K + intensity varied widely with the experimental conditions shows that a t least under reducing conditions I 40 kcal. per mole of KOH. Assuming the monomer to be the principal species near the boiling point of KOH (liquid), KelleyI4 has calculated from the vapor pressure data of von Wartenberg and Albrecht a value of AHozg8 = 43.9 kcal. per mole for the reaction KOH(1) = KOH(g). Combining this with a value of 1.8'' kcal. per mole for the heat of fusion of KOH(s) we obtain for the reaction KOH(s) = KOH(g) AH02S8 = 45.7 kcal. per mole with a probable uncertainty of * 5 kcal. This latter value is not in disagreement with the result obtained in this work. As is indicated by the heats of sublimation of monomer and dimer, the monomer partial pressure must rise more rapidly with temperature than that of the dimer. A rough calculation assuming the dimer to be the major species present at all temperatures gives a boiling point of liquid KOH several hunred degrees above that of 1600°K. observed by von Wartenberg and A l b r e ~ h t . ~It is entirely possible therefore that even though the monomer is unimportant relative to the dimer in the temperature range of our experiments, it may be the major species near the boiling point of KOH(1). The recent work of Miller and Kusch'8 shows a similar effect in many halide systems. By combining the AH06260K. > 37 kcal. per mole for the heat of sublimation of the monomer with the value of A H o 6 z ~ o ~=. 36 i 2 kcal. per mole for the heat of sublimation of the dimer we obtain for the reaction 2KOH(g) = Kz(OH)z(g) A H 0 6 z 6 o ~ .> 38 kcal. per mole of dimer. Using the AH0298 of sublimation of KOH(g) from von Wartenberg and Albrecht's data we obtain AH06260~.= 49 f 10 kcal. per mole of dimer for the energy of dimerization. KzO+.-An ion current of KzO+ which was approximately 10% of the KzO" peak was also ob(15) K.K. Kelley. U. S. Bureau of Mines Bulletin 477, 1948. (16) L. Brewer and D. F. Mastick, J . A m . Chem. Sac., 7 3 , 2045 (1951). (17) R. Seward and K. Martin, ibid., '71, 3564 (1949). (18) R. C. Miller and P. Kusoh, J . Chsm. Phya., 26, 860 (1956).
237
served. The ratio of K 2 0 +to K20H+ was nearly constant over the temperature range studied suggesting that KzO+ may be formed by dissociative ionization of K2(0H)2(g). The possibility arises however, that K 2 0 +could also be formed from simple ionization of KzO(g). This latter process could occur if KzO(g) had been produced by the partial decomposition of the small amount of KzC03 impurity. If we assume that all of the KzO+ was formed from KzO(g), the partial pressure of K2O(g) calculated with the aid of the sensitivity calibration is not inconsistent with the total vapor pressure data for K20(s) obtained by Morgan and Jackson.6 Calculations by Brewer and Mastick, l6 however, indicate that KzO(s) should vaporize primarily by dissociation to K(g) and 02. Experiments with solid KzO are planned in order to obtain definite information about the KzO+ion. Ion currents of OH+ and H20+ produced from vapor species were completely masked by the normally high background currents a t their masses. Thus quantitative data for these ions were not obtained. "Third Law" Check on the Heat of Sublimation of K2(OH)2(g).-From the sensitivity calibration with K(g) it is possible to obtain a rough check on the heat of vaporization of K2(0H)2 dimer. An estimate of the partial pressure of the dimer in equilibrium with the condensed KOH phase was obtained by assuming that the relative ionization cross sections to form K + from K(g) and to form KzOH+ from K2(0H)z(g)were equal. Using the ratio K39+(T= 528"K.)/K20H+(T = 626°K.) = 350/15 for 10 volt electrons and the sensitivity calibration with K(g) we obtain a partial pressure of Kz(OH)* of 3.9 X 10-6 atm. at T = 626°K. An entropy of sublimation of the dimer at T = 626°K. was obtained by combining the entropy of sublimation of the monomer and an estimate of the entropy of dimerization of the monomer. This latter quantity was approximated by the entropy of dimerization of KCl(g) a t T = 626°K. using the entyopy of K2Cll(g) as calculated by the method of Zimm and MayerIgand an entropy of KCl(g) given by Kelley.I4 We obtain for the reaction 2KOH(s) = K2(0H)z(g) at T = 626°K. ASO62.5 = 32.4 e.u. with a probable uncertainty d=8 e.u. Setting AFOT = -RT In P K ~ ( ogives H ) ~AH0626 = 36 =t 6 kcal. per mole. Although this calculation is admittedly only approximate, it adds some confidence to the value obtained from the Clausius-Clapeyron plot. (19) B. H. Zimm and J. E. Mayer, ibid., 12,362 (194.0.
