J . Phys. Chem. 1986, 90, 6590-6594
6590
as compared to the interpenetration distance. In conclusion, we have demonstrated that the reactivity of e, with Ag' is governed by the long-range interactions. Further, when the radius of e, is estimated from L = L,, and the DebyeSmoluchowski estimation of the specific rate is made, the agreement of k , with kald is remarkable. A further improvement in this agreement results when one assumes that L = Leffand the Leffis given by a tunneling formalism. In addition, the distances at the moment of electron transfer are larger than the corresponding contact distances and unlike neutral scavengers no interpenetration of Ag' into the outer sphere of e, is required. Registry No. Ag, 7440-22-4; methanol, 67-56-1; ethanol, 64-17-5; propanol, 71-23-8; butanol, 71-36-3; pentanol, 71-41-0; hexanol, 11127-3; heptanol, 1 1 1-70-6;octanol, 11 1-87-5; nonanol, 143-08-8;decanol.
kanols revealed that (see Figure 2).
k, = where A = 4.6 X lo9 and x = 0.45.2 This departure of x from unity has previously been attributed to the interpenetration of the reaction partnew2 On the contrary, the mechanism in eq 7 and 8 precludes the interpenetration of the reaction partners and hence indicates the inapplicability of eq 12 to the reaction of e, and a charged species such as Ag'. For such a mechanism one expects an empirical equation of the type given by eq 13 to hold. Inspection of Figure 2 reveals that
k , = 1.4
X
lolog-'
'
(13)
which, in our view, confirm the possibility that the reaction between e, and Ag' takes place while the reactants are far apart
I 12-30-1.
Vapor-Phase Composition above the Cd1,-NaI
and the Th1,-NaI Systems
Timothy D. Russell General Electric Company, Lighting Research Laboratory, Nela Park, Cleoeland, Ohio 441 I 2 (Received: May 9, 1986)
The activities of Cd12 in NaI and of ThI, in NaI have been determined by using vapor-phase UV absorption spectroscopy. The activity coefficient, y, for the Cd1,-NaI system was found to be constant below 0.05 mole fraction of CdI, and equal to 0.23. For the Th1,-NaI system, y was found to be constant below 0.06 mole fraction of ThII and equal to 0.02. No vapor-phase hetero complexes of CdI, or ThI, with NaI were observed. The effects of vapor-phase dimerization of NaI on its absorption spectrum are discussed and temperature-dependent spectra are presented.
Introduction Thorium and cadmium are important metallic additives which affect the color temperature and the life of metal halide ( M H ) discharge lamps containing N a I and ScI,.'s2 The cadmium is added as a Hg-Cd alloy while the thorium is added as ThI,. Cadmium acts as a getter by reducing the partial pressure of iodine during lamp operation and thereby promoting the deposition of thorium onto the tip of the electrode where it functions as an activator by lowering the work function of the tungsten surface. The operation of the thorium-tungsten electrode in metal halide arc tubes has been previously described by W a y m ~ u t h . , , ~Basically, the thorium concentration at the electrode tip is governed by the equilibrium expressions for the dissociation of thorium iodide and the vaporization of condensed thorium: ThI,(g) = T U ) + xUg)
(1)
Th(g) = Th(c)
(2)
If the partial pressure of Th(g) due to the dissociation of thorium iodide exceeds the equilibrium vapor pressure of Th(c) a t the electrode tip temperature (-2500 K), then thorium deposits onto the electrode tip. The Th(g) pressure can be seen to be dependent on both the thorium iodide partial pressure, which is determined by the activity of ThI, in the molten iodide dose, and the iodine partial pressure. The iodine partial pressure itself is dependent upon the partial pressures and the intrinsic dissociation constants of the metal iodides that are present in the arc tube. (1) Spencer, J. E.; Bhattacharya, A. K.
US.Patent N o . 4360756, 1982. (2) Khoury, F.; Waymouth, J. F. U.S. Patent N o . 3313974, 1967. (3) Waymouth, J. F. Electric Discharge Lamps: M.I.T. Press: Cambridge, MA, 1978. (4) Gilliard, R. P.; Russell, T. D. Presented at the 37th Annual Gaseous Electronics Conference, October 1984; paper HB-2.
0022-3654/86/2090-6590$01.50/0
During manufacturing, arc tubes are dosed with Hg, NaI, Sci,, and ThI, plus various impurities such as 0, and H 2 0 which are inadvertently introduced. Once the lamp is operated, the ScI, reacts with 0, and H 2 0 producing HgI, according to the following overall reactions: 2ScI,
+ 3H20 + 2Si0, + 3Hg = Sc,Si207 + 3Hg1, + 3H2 (3)
4Sc13
+ 3 0 , + 4SiO2 + 6Hg = 2Sc2Si207+ 6Hg12
(4)
(The individual steps of the above reactions may involve the formation of Sc,O, and ScOI.) Analogous reactions between ThI, and 0, and H,O producing HgI, are also believed to occur. The Hgl, that is present decomposes in the high-temperature arc of an operating lamp into H g and I atoms. According to eq 1 and 2 above, if the iodine pressure due to the dissociation of HgI, is sufficiently high, thorium is prevented from depositing on the electrode, which results in a high work function electrode and poor lumen m a i n t e n a n ~ e . ' - ~ The addition of cadmium to the arc tube results in the following overall reactions: Cd HgI, = CdI, Hg (5)
+
6Cd
+
+ 7Si0,
= 6Cd1,
+ + 2Sc,Si207 + 3Si
(6)
In reaction 5, cadmium getters the iodine from Hg12 and forms CdI, which dissolves in the molten iodide dose. In reaction 6. cadmium reacts with ScI, and the silica wall of the arc tube again producing CdI,. Within a short time (minutes), all of the cadmium added to the arc tube reacts to form CdI,.' By reacting with HgI,, which would vaporize and subsequently dissociate during lamp operation, and forming CdI,, which dissolves in the molten dose, cadmium effectively lowers the partial pressure of iodine in the arc. The extent to which the pressure of iodine is lowered is dependent upon the activity of CdIz in the
0 1986 American Chemical Society
Cd12-NaI and Th1,-NaI Systems
Choppar
*e3
L
I I I
Ratiomatar
Ah
A
1b
I SampIa Furnaca
I
m S t a p p a r Motor
r
A
7
I
HP-87
Computer
molten dose. Lamp experiments have shown that the lowering of the iodine pressure due to the addition of cadmium is sufficient to enhance the deposition of thorium onto the electrode tip.' This paper presents the results of an experimental study utilizing UV absorption spectroscopy to determine the composition of the vapor phase above the systems Cd12-NaI and ThI,-NaI.
Experimental Section The Cd12, NaI, and Th14 were purchased commercially and resublimed several times before use. Spark source mass spectrographic analysis found metallic impurities to be in the 100 ppm range for each iodide. The major contaminants in each sample were chlorine and fluorine which, combined, were in the 200 ppm range. Material handling and the dosing of the optical cells were carried out in an argon-filled glovebox with oxygen and water concentrations of less than 1 ppm. Fused silica optical cells with a 1.0-cm path length and a 4.5-cm height were purchased from N S G Precision Cells Inc. and modified by fusing a reservoir and an extension arm to the bottom of the cells. In order to clean and outgas the cells, they were attached to a vacuum system, evacuated to 10" Torr, heated to incandescence, and then allowed to cool while under dynamic vacuum. They were then backfilled with argon, capped with Cajon Ultra-Torr fittings, and loaded into the glovebox. After the addition of metal iodides, the cells were recapped, removed from the glovebox, and reevacuated to about 10" Torr before vacuum sealing with an oxygen-hydrogen torch. Each cell was positioned vertically in a three-zone furnace. The reservoir at the bottom was maintained about 5 K cooler than the upper part of the cell so as to prevent condensation of the iodides on the optical surfaces. Temperatures were measured with a calibrated platinum-platinum-10% rhodium thermocouple connected to an Omega Model 660 electronic thermometer. The absolute cold spot temperature could be measured to about k3
K. A schematic of the absorption spectrometer system is shown in Figure I . The light source was a Hamamatsu deuterium lamp driven by an Oriel regulated power supply. Front-surface mirrors focused the light through the cells, with quartz plates being used as beamsplitters to direct a portion of light through a reference cell. EG&G PARC choppers were used to modulate the light
5
W
lll~l'tlbllll~ll'l~tll'
~ m 4 0 0 ' 1 1IL1
WAVELENGTH (NM) Figure 2. Saturated vapor-phase absorption spectrum of NaI. The spectrum was taken at a temperature of 1068 K, using a I-cm cell.
through the sample and reference cells at different frequencies. Spectra were taken with a Spex 1/4-mscanning monochromator with the slits set for a resolution of 1 nm. The two modulated signals were detected with a single Hamamatsu photomultiplier tube (R331 or R943-02) and separated with two PARC Model 128A lock-in amplifiers. A P A R C Model 188 ratiometer transmitted the log of the signal ratio to a Hewlett-Packard Model 87 microcomputer for signal processing. The HP-87 also controlled the scanning of the Spex monochromator in I-nm increments. The data were stored on 5'/4-in. floppy disks. Hard copies of the data were obtained by using an HP-7470A plotter. In order to correct for unmatched transmission through the sample and the reference cells, base-line spectra were taken at temperatures of 100-200 K below the experimental temperatures and then digitally subtracted from the experimental spectra.
Results and Discussion The saturated vapor-phase absorption spectra of NaI, Cd12, and Th14 are shown in Figures 2, 3, and 4, respectively. The absorption cross section, u, for each metal iodide species was
The Journal of Physical Chemistry, Vol. 90, No. 24, 1986
6592
Russell
.m W
u
z
