ammonium hydroxide, p H 8.1, copper and cadmium were found to suppress the reduction wave of tellurium(V1) in a manner analcgous to that observed by Lingane, while nickel ion was found to have no. effect, possibly because the nickel-animonia complex is strong enough to prevent the precipitation of nickel telluride in this medium. Of the anions investigated, bromide was without effect in concentrations up to 0.1 X ; fluoride a t this same concentration depressed the apparent wave height by 20%; while tellurite a t a concentration of 0.1 millimolar depressed the apparent wave height by 10%. X o explanation is offered for the depressive effect of fluoride and tellurite ions on the reduction wave of tellurium(V1). Polarography of Selenium(VI).--A solution of selenic acid was prepared by the method of Gil-
[CONTRIBUTION FROM THE CRPOCESIC
LABORATORY AND THE
bertson and King.I5 Polarograms of this solution were obt:riried in the following media: 1 JI NH4CIS H 4 0 H ,fiH 7.7; 0.1 -11 SH4Cl-NH40H, fiH 9.0; 1 AI NH4 tartrate-n’H40H, PH 8.2; 1 M KCl; 0.1 J I S a O H ; 0.5 -11 NHI citrate-NH40H, p H tj.2; saturated NH4 oxalate-NHIOH, PH 4.9. I n no case was a selenium(V1) wave observed prior to the hydrogen wave, thus bearing out the statement of Lingane and NiedrachZbthat the +ri state of selenium is not reduced a t the dropping mercury electrode. Acknowledgment.-We are grateful to Professor Lingane and Dr. Kiedrach for consultation during the course of the present work. 15) I
I Gilbertson and C, R Kxng, THISTOURYAL,
68, 180 11’4.16)
VPTOY,L o w ISLAND, S Y
1)EPARTDIEXT O F CHEMISTRY,
THEOHIO
STATE UNIVERSITY]
Vapor Pressures of Inorganic Substances. X. Dissociation Pressures of Lithium Hydroxide between 650’ and $OO°K.l BY WALTERE. DITMARS AND HERRICK L.
JOHNSTON
RECEIVED SEPTEMBER 29, 1952
By means of the dynamic effusion method the dissociation pressure of LiOH has been measured in the temperature range 5 6786, was derived The procedure is described, and an 648-766°84 The empirical equation, log P = (-7635.1/T) evaluation of results given.
+
Introduction The dissociation pressure of LiOH has been measured by the dynamic &usion method in the temperature range 848-795’K. The study was made with the w e of nickel Knudsen cells in a simple high vacuum apparatus. The only earlier work fourid in the literature was the research by J. Johns t m 2 whose data were obtained in a completely diff eront temperature range (823- 1176’ K. ) than that of this research. Due to the experimental difficulties of hahdling the material a t higher temperatures, these earlier values are believed to be somewhat uncertain. Apparatus and Procedure The procedure employed, to measure the dissociation pressure of UOH, resembles that of the Knudsen method of vapor preswte mea~urements.~Details of the special apparatus are shown in Fig. 1 , the heater and control circuit? are shown in Fig. 2 . The cell was pressed, in two sections, from sheet nickel 0.025 cm. in thickness. The main section is itl the f o m of a cup 2.84 cm. in diameter by 2.51 cm. in height. This WILE closed by a cap pressed into position, and electfieally welded in qn atmosphere of hydrogen. The orifice was formed by drilling a hole through the center of the bottom of the cup, and producing a sharp edge by means of a lathe. Tho orifice diameter was measured by means of a travelling microscope and, since the hole was not quite round, the area was computed from the average of eight readinSs. The LiOH, in the form of a fine powder, was introduced into the cell through the orifice, in a dry nitrogen atmosp W a , aLhB the filled cell was kept in a vacuum desiccator preliminary to measurements. (1) T h i s work was supported in pdrt b y t h e Office of Naval Research under Coatrsct with The Ohio S t a t e Uhiversity Research Foundation (2) John Johnston, L p h y s t k C h e w , 62, 339 (1908) (3) R . Speiser and H L Johnston, A m S o r M f f n l s 11, 3 (1949)
The cell was mounted, in an inverted position, on the end of a monel tube, as shown in the figure. Previous to heating, the system was evacuated from 3 to 18 hours to a pressure of 3 X 10-‘? mm. Additional evacuation after this pressure was reached had no effect on the experimental results. A preliminary degassing run was then made to ensure decomposition of any LiOH.H20. The pressure maintained in the cell during LiOH decomposition runs, was approximately 1 X lo-‘ mm. Precautions were taken to avoid contact nith air during cell weighings, and during introduction in or removal from the apparatus. The iron-constantan thermocouple was calibrated by directly comparing a couple made from this same wire with a staadardized Pt-lO% Pt, Rh thermocouple. Preliminary runs were made on relatively pure LiOH obtained from the Metalloy Corporation of Minneapolis. The experimental runs were made on material (approximately 99.9% pure) prepared by Dr. Thomas W. B a ~ e r . ~ The Knudsen cell on which the preliminary runs were made was subsequently cut open. No apparent reaction had occurred between the nickel and lithium compounds, nor between the nickel and the water vapor.
Experimental Results Dissociation pressures were calculated from the formula
where A W , the weight loss was measured on a n analytical balance; T , the absolute temperature was read directly from the calibrated pyrometer controller dial; R is the universal gas constant = 8.3156 X lop7 dyne/cm.*; C = 9.8692 X lo-’ is the conversion factor to atm., A o r i f i c e was directly measured a t 2 5 O , and coefficient of thermal expansion data from “The Metals Handbook, 1949” were (4)
XI L Tohnston a n d T R‘ Rauer THIS TOI;RKAL,
(1 9.5 1).
73, 1119
DISSOCIATION PREWJRESOF LITHIUMHYDROXIDE
April 20, 195.7
TEN LAYERS W ALUMINUM raL AS INSULATOR AND RADIATION SHIELD
IICKEL KNWSEN CELL WITH LlDH IP Of IRON-CONSTANTAN THERM0 ILVER-SOLDERED I N MONEL TUBE OLIO COPPER JACKET F I d R THE YDNEL TUBE
MONEL INNER TUBE
NIZATON GAGE
PYREX GRDUHD GLASS JOINT
KOVAR-TO-QLASS SEAL HARD SOLDERED ADAPTER
SOCT BOLDERED SEAL TO W M L INNER TUdE
K%3%+%?:AD CIRCUITS
~
32.82 =CI 0.11 kcal. computed by Johnston and Bauer4 for the reaction 2LiOH(s)
~~
Fig. 1.-LiOH
dissociation pressure apparatus.
LizO(s)
+ HaO(g)
(3)
TABLEI VAPORPRESSURE DATAFOR LiOH
PYREX GLASS INSULATED HEATER COIL
PYREX GLASS INSULATED THERMOCWPLE LEAUS
1831
Total wt. loss,
OK.
Effective time, sec.
g.
Evap. rate g . cm.-2 sec. - 1 x 100
648 663 677 692 707 707 722 722 736 736 736 752 752 766 766 781 781 795
50045 48813 37907 39155 a6523 17644 24698 24705 18803 19262 18692 18574 18130 8720 8692 5872 6581 5632
0,00193 ,00706 ,00557 .01768 ,01095 .00960 .a 4 4 6 .02686 ,01969 ,01650 ,04358 ,05051 ,04329 ,04731 .Om45 ,02477 ,02319 ,04795
4,064 15,233 15.474 47,585 43.458 39.695 104.215 114.409 110.147 90.103 245,238 285.83 250.972 570.50 247.395 443.053 370.104 894.028
Run No.
Temp.,
10 13 3 17 7 19 11 15 2 6 18
1 8 12 14 4 9 16
Pressure, atm.
x
10-0
0.550 a. 084 2.140 6.645 6.141 5.609 14.%QO 16.340 15.88 12.99 35.36 41.65 36.58
83.91 36.39 65.80 54.97 134.00
The ratio of the mean free path of the gaseous molecules inside the Knudsen cell to the orifice diameter, a t several temperatures and pressures is shown in Table 11. Measurements were undertaken with a second Knudsen cell €or testihg the
LiOH CELL HEATER COIL
4C VOLT METER
ROT 8JWCTlo)(
b
IRON-CCUSTANTAN THERYOCWKE
CDLl ANTAN W A
IRON WIRE
L~ADS
BV V4Rl4ELE RESISTOR
COLD JUNCTION (ICE UTHl OF I.C ~HEWWWPLE
FOXBORO
PVROMETLR-CONTROLLER
Fig. 2.-LiOH
heater-controller circuits.
used to obtain correct values a t higher temperatures. Correction for small variations of the temperature, while the samples were coming u p to temperature and cooling a t the end of the runs, were made by a method similar to that used by Johnstoa and MarshalL6 The data are given in Table I and illustrated in Fig. 3. The straight line curve of Fig. 3 fits the equation log P (atm.) = - (7635.1/T) 5.6786. (2)
+
This corresponds to a AHof 34,94 kcal., and agrees, within experimental error, with the value A H g g g = (4) H L. Johnston and A. L Marshall, i b i d . , 6Z, 1882 (1940).
+%a
I
'
124
la
J-
132 I
Fig. 3.-Dissociation
1s
I
1 I. 0I
' '
IU-
k,*d
' -'
I,e
1 9
pressure of lithium hydroxide,
condition for the equilibrium determination cf the the standard deviation of the experimental log P’s Idpor pressure Unfortunately, the system bro’-.e diould be 0.02. The actually observed standard down at the beginning of these mCasLirtinciiti L 1 ~ i t l c:cvi,rt;on from the least squares curve is 0.14. only one datum of v d u e WAS ohtru:lcc! 1 t ft1t t h i t the dzscrepancy between the calcul,,tctl crr
