N. ATCHUTA RAMAIAH
218
STUDY OF THE JOSH1 EFFECT I N
H 2 O
Vol. 56
AND D20 VAPOR
BYN. ATCHUTAWMAIAH Department of Chemistry, Benares Hindu University, Benures, India Recsived December 17, I060
Occurrence of large negative Joshi effect ( -Ai), uiz., a photodecrease of the discharge currerIdi and its H.F. ulses in escited water vapor was investigated over a wide range of experimental conditions and attributed on Joshi’s t eory which contemplates that negative ion formation (i) due to capture by excited particles of photoelectrons (ii) released from an adsorption-like electrode layer (iii) reduces i, to the large electron affiity of OH. Conditions favoring (i), viz.? 1arge.pressures (up to a limit) and low potentials V , RS also those leading to enhanced (ii) ~ i z .large , intensities and frequencies of llght, augment -Ai. Formation of (iii) due to aging under discharge increased numerically -Ai; its time development obeyed a relationship suggested for sorption. -Ai was ronounced in vessels coated with KCI which was known to enhance sorptioll of OH. Rise of temperature by 40” increase$ -Ai, as also H.F. part of i in which -Ai predominates. Positive effect (+Ai), a photoincrease of i and H.Fs., was observed at very low and large V which disfavor (i). Temperature inhibited +Ai. Vanation of +Ai with other parameters was similar to that of -Ai. Rcversals +Ai e -Ai w t h V , intensity and circuital capacitance suggested the simultaneous occurrence of +Ai and - A i 8s actually revealed by current oscillograms. These studies were extended to D20 vapor; the results were similar.
K
Joshi’s theory’ for the phenomenon railzan instantaneous and (under unexceptional conditionsa) reversible photovariation of the discharge current i due to an exciting potential V (expressed in kilovolts kv., r.m.s.), postulates: Formation of an adsorption-like layer (i) of ions, electrons and excited particles derived from the discharge space; emission from (i) of electrons by external light; current decrease -Ai, as a space charge effect, due to negative ion formation (ii) by electron capture by excited particles; conditions disfavoring (ii) give +Ai. It follows from (ii) that the magnitude and range of occurrence of - Ai should depend upon the nature, chiefly electron afFinity (E, Cal.) of the medium, as illustrated by the observed order,4 chlorine (Ecl = 885a)>bromine ( E B r = 845a)> iodine (EI = 756a)>oxygen (Eo = 87,* Eo, = 625b),etc. Production of - A i in metallic vapors due to their enhanced E under excitation, was predicted and actually observed by Joshi.Ia Under = 18’) dissociates discharge, water vapor (EH~o into H (EH = 165a) and OH (Eon = 865b).8 The large EOHvalue suggested the present work. Water vapor at various pressures ~ H (3-32 ~ O mm.) was excited by transformer discharge in a Siemens-type full- (A), wire-in-cylinder like semi(B) ozonizers and tubes with external sleeves (C). As in other systems,2 the Joshi effect, -Ai, was undetected below the “threshold potential,” V , where i increases rapidly with V , following the onset of discharge. In a typical series (A, P H ~ O =. 15 mm., 30°), at 0.66 kv., i.e., just above V,,, %Ai (100 Aii/iDsrk) in the positive and negative halves of i as observed with a diode was, respectively, 33.3 and 19.6% current suppression due to light from a 200-watt (glass) bulb; and 21.3 in total i measured by a thermojunction. Increas in V decreased -%Ai, a general Ai characteristic. At p ~ , 0 < 3mm., -%Ai was negligible; progressively, increased from 2.3 at 4.5 mm. to 48.3 (11 8.8. Joshi, a. Proc. Ind. Sei. Cong., Phys. See., Abat. 26 (1946); ’
b. ibid.. Abst. 27 (1947); c. Curr. Sei.,16, 19 (1947). (2) 5. 8. Joshi. Presidential Address, Proc. I n d . Sci. Cong., Chem. Sac. (1943); Nature, 161, 561 (1943). (3) B. R. Marathe and S. 8.Bommannavar, Nature. 166,890 (1950). (4) G. S. Deshmukh, J . Ind. Chem. Soc., 14, 211 (1947). (5) (a) C. D. Weat, Tars JOURNAL, 89, 503 (1935); (b) J. Weiss, Tranu. Faradou SOC.,81, 968 (1934); 86, 856 (1940). (6) G . Glookler, Phva. Rev.. 46, 111 (1934). (7) A. Farkas and L. Farkas, Trans. Faraday SOC.,84, 111 (1938). (8) A. A. Frost and 0. Oldenberg, J . Chem. Phvr., 4 642 (1936).
at 13.6 mm. and then decreased, e.g., to 23.0 at 31.8 mm. (A, 0.66 kv.). At identical corresponding potentials (V - V,)/V, increased p affects -%Ai; this suggeststhat the homogeneous reaction is not unsignificant to Ai. At constant PH*O, rise of temperature from 300-340” K. increased - A i and -%Ai, as also the H.F. part of the current ~ H . F . , which latter has been established by Joshi9 to be the chief seat of -Ai. As in other systems,Z the Joshi effect increased with light intensity; at large values of the latter - A i showed saturation. At constant intensity the magnitude of -Ai was in the order: unfiltered white (7800-3700 8.) > violet (4064-51G6 A.) > green (5180-5860 %..) > red (6390-6600 A.). Absorption by HzO or OH in violet and green is negligible; as suggested by Joshi2 Ai is, therefore, not entirely a consequence of selective light, absorption in the vapor phase. The appreciable positive Joshi effect, +Ai, occurred just near V , in A and B and at large! V in C. Light intensity and frequency affected + A i similarly t o - Ai. Increased temperature inhibited +Ai. It increased with I)H*O up to a limit, and then decreased. + A i decreased precipitoudy and inverted to - A i in A and B with but a small rise and in C with R small decrease of V . A t the transition potential (in C), suggestively enough, a - Ai occurred by inreversal either way + A i tensity alteration. Using frequency filters, it was found that a t a given V , whilst ~B.F. showed - A i , the total i gave +Ai. This finding (as also the reversals + A i $ Ai) follows from Joshi’s suggestionlb that depending upon fhe conditions, + A i and - Ai cooccur; the detector shows the balance. This was further confirmed by oscillographic studies. Figure 1 is one typical result. Generally, + A i and - A i are characterized by amplitude increase and decrease, respectively, of H.F. pulses constituting ~ H . F . Cooccurrence of & A i is revealed by an increase of some H.F. amplitudes and simultaneously decrease of others (see Fig. 1). The adsorption-like layer (i) postulated by Joshi elucidates the observed influence of “aging” and electrode-filming on Ai.ly1Ob The former was (9) 8. 9. Joshi, Nature, 164, 147 (1944); Cum. Sci., 18, 253 (1944) Proc. I d . Acad. Sci., PO, 389 (1945). (10) (a) 6. R. Mohanty, Ph. D. Thesis, 1049, p. 209; (b) M. F‘. Ramanamurty, i6id.. 26, 255 (1948).
.
STUDYOF THE JOSHIEFFECT IN H20 AND D20 VAPOR
Feb., 1952
219
1.8
c I
-c-
u M
-
lS6 1.4 1.2
Q
I 1.0
- 0.8 M
In dark
Fig. 1.
Under light
studied in freshly prepared vessels. In all of them, initially Ai was undetectable; it developed only after a definite interval and increased prog‘ressively to a maximum within, say, 2 hours of aging. The necessary period of aging for the inception of Ai was shortened appreciably under conditions favoring adsorption (vide infra). This time development of Ai is attributable to that of adsorption contemplated in (i) of Joshi’s theory. The following relation holds for the initial phase of sorption on glass St at time t of a number of gases”: S t m = kt; m and k are constants. Since according to Joshi, sorption determines inter alia the corresponding Aillobone would expect the influence of aging to follow Ai: = k’t. It is instructive to see in Fig. 2 that as required by this relation, the plots of log Ai, log % Ai versus log t are sensibly linear for water vapor studied by the author; and also for chlorine and oxygen from published datalo for the aging influence on Ai. (11) D. H. Bangham, P h d . Mag., (7) 6,735 (1928); D. H. Bangham and F. P. Burt, I‘roc. Roy. Soe. (London), AlOS, 481 (1924); r/. also E. Swan and A . R. Urquhart,, THIE JOURNAL, Si, 251 (1927).
1.o 1.5 2.0 log 1 (min.). Fig. 2.-A-A and A- -A, chlorine; X-X and X- - X , oxygen; -, IOg ( - A i ) HZO vapor; 0-0 and @--e, log 1; ---, IOg ( -%Ai) - log 1.
0
0.5
That the intercept on the y-axis may be a measure of predischarge sorption, is being investigated. The results of Taylor and LavinI2and Frost and Oldenberg* show that adsorption of OH on glass is augmented by KC1. Series of observations of Ai were, therefore, made using discharge tubes with and without an electrode-film of KCl. With this film, it was remarkable that not only was the magnitube of Ai about four times larger, but the initial aging period was negligible compared with the 20-30 minutes found with blank vessels. A Ai in DzO vapor resembled closely that in H20 for all above mentioned parameters. The author expresses his grateful thanks to Professor S. S. Joshi, for suggesting the problem, and for his kind interest and valuable guidance during the work; and to the Ministry of Education, Government of India, for a fellowship. (12) H. (1930).
S. Taylor and G. I . Lavin. J . A m .
Chem. Soc.. SO, 1910
