PHOTO-T-OLTBIC EFFECTS I S GRIGNARD SOLCTIOSS. I. P R E L I 3 I I S A R T SURVEY BY R. T. DUFFORD
The existence of a photo-voltaic effect, or “Becquerel Effect”, in cells containing ether solutions of Grignard reagents, has been noted in an earlier article,’ in which some early results on a fev cells are described. The present article summarizes the experimental results obtained in a study of over ninety such cells, so that the main facts concerning the effect can be stated more definitely than before. Theoretical considerations are reserved for discussion in a later article. I n this series of experiments, eleven typical Grignard compounds were tested, including methylmagnesium bromide, ethylmagne~imnbromide, npropylmagnesiuni bromide, n-butylmagnesium bromide, ethylmagnesium chloride, ethylmagnesium iodide, phenylmagnesium chloride, phenylmagncaium bromide, phenylmagnesium iodide, p-chlorophenylmagnesium bromide, and p-bromophenylmagntsium bromide. Tlie ethyl- and phenylniagnesium bromides were studied more extensively than the other compounds. Eight metals, including platinum, gold, magnesium, lead, zinc, copper, aluminum, and iron, were used. The cells were made up in one-inch test-tubes, a layer of solution one to two inches deep being placed in the tubes; the elecrrodts m r e supported by wires sealed in the ends of glass steins passing through holes in the rubber stoppers that closed the cells tightly. Except for a slow loss of ether through the stopper, such a cell is about as good, for several weeks at least, as if hermetically sealed in glass. Usually one electrode vas of platinum, the other of some different nietal. The platinum clcetrodes w r e pieces of bright foil about I cm. square. The other metals ww txvo or three times as large, and mere arranged so that the spot at which they were welded t o the seal-in wire did not dip below the surface of the solution. The bath uzed for storing the cells iyas made of a metal t u b in sawdust in a wood box. -1l a y r of presswood, with many holes so that the cells would be supported by the flanges at the mouths of the tubes, was set at such a height that the cells dipped into the water to a depth greater than that of the solution inside. h light-tight metal cylinder at the centre, about ten inches in diameter, divided the bath into inner and outer cornpartmtnts. -1 IOOwatt incandescent bulb was placed at the centre of the inner bath, and holes in the cover were arranged so that up to four cells could be placed in the inner bath at equal distances (about t,hree inches) from the filament of the bulb, both bulb and cells being immersed in water. Kear the lamp, three test-tubes contained resistance coils in oil, adjusted to dissipate the same number of watts as the lamp; a double-throw swit,ch permitted either the
’ Dufford, Nightingale, and Gaddum: J. .4m. Chem. SOC.,49, 1858 1,1927).
PHOTO-VOLTAIC EFFECTS I N GRIGNARD SOLUTIONS
I545
lamp or the heating coils to be turned on at will, so that one or the other was alx-ays on. A cooling-coil connected to the tap-water, and good mechanical stirring, combined to give a bath that would not change in temperature by much more than a degree C., usually less, and that slowly, no matter h o s often the illumination was turned on or off. This amount of variation in the temperature is not very serious, as will appear below. The temperature of the outer bath was even more steady. The entire bath was covered with a light-tight box, with a trap-door lid. Wires from numbered binding-posts led to corresponding points of a selector switch, so that, without changing any connections, any cells (up to four) could be put into the inner bath and the effect of illumination observed, while any number of others up to thirty could be kept undisturbed in the dark in the outer bath, under observation, and readings made on any cell a t any time without opening the bath, all the control switches being of course on the outside. The entire outfit was in a photographic dark-room, so that unintentional illumination of the cells was very improbable. h small auxiliary bath, a two-liter beaker in a light-tight box with a shutter, a stirrer, and a hand-regulated heater, was found exceedingly useful in certain experiments, such as temperature and intensity tests and studies of the effects of various wave-lengths of light. X 3z5-watt flood-lamp, either close to the shutter or, better, focussed by a lens, was used for lighting this bath. K h e n the lens was used, the effect on the temperature of the bath vias very slight. Connection to the cells in thit bath was made through the same selector switch as for the large bath. The wiring arrangements left little to be desired in the way of convenience. Since the effect of illumination is a change of electromotive force, the voltages of the cells were read by potentiometer, in order to take no current from the cells. h high-resistance Iiohlrausch spiral bridge wire proved most satisfactory for use as a potentiometer; readings could be taken to .OOOI volt, and could be obtained in 30 to 60 seconds. h Weston cell was used as standard. The resistances of the cells (D. C.) are of the order of thousands of ohms, a large part of which is probably due to the film which is known to exist on the electrodes.2 The behavior of these cells in the dark is often irregular, and hence unsatisfactory. It has not been found possible to produce two similar cells, even from the same batch of solution and with electrodes prepared as nearly alike as possible, which will have exactly the same voltage. Moreover, the voltage of any one cell is not always constant. Freshly prepared cells usually have a relatively high voltage, 0.4 to 0.8 volt or more, with the platinum the positive terminal. The volt'age will fall steadily for 1 2 to 2 4 hours, sometimes crossing zero if the solution is of a n aromatic compound, but usually remaining positive if the solution contains a n aliphatic compound. After the first sharp fall, the voltage is more steady, but not always entirely constant. It may remain for hours without changing as much as .OOI volt; Dufford: J. Opt. SOC.America, 18, 17 (1929).
I546
R . T. DUFFORD
but more likely it will be changing slowly, over a range of several hundredths of a volt, sometimes varying almost linearly with time, sometimes recrossing zero value, somet'iines oscillating about a mean value in such a way that the voltage-time curve suggests a sine curve; the period may be anything from many hours to, as seen in a few cases, a small fraction of a second, so that the galvanometer simply fluctuated so violently that no balance could be secured, Sometimes less regular changes occur. The cells seem to be trying very hard to tell what is going on inside them, and no doubt their message would be exceedingly interesting if only we understood their language. Rut the reason for the variations is not known. I t seems clear that they are not due to stray light, nor to temprrature variations; for similar and adjacent cells show no consistency in these erratic changes, such as would be expected from any but local causes within the individual cells. The writer believes the effects to be due to some sort of variation in the surface films covering the electrodes. I t is true that the cells can be polarized rather readily, and the voltage t h w affected for several minutes or more, depending on the nature of the solution used. The writer at first tended to blanie the potentiometer method for the erratic variations, thinking that t'he current drawn from the cells while obtaining balance might cause the effects. Occasionally such an effect existed to an extent sufficient to interfere with the accuracy of the readings; but it seems to be recognizable when present. The currents were very small, however; a 0.2 megohm resistance was kept in series with the sensitive enclosed-scale galvanometer used, and the cells themselves have high resistances. K i t h proper handling of the apparatus, it is not necessary to draw enough current, at least in most cases, to disturb t,he cells perceptibly for more than a few seconds. The voltage-time curves are so smooth and definite, in most cases, that t,hey seem to contradict this theory. And, as will be shown in a later paper, similar curves occur when the readings are obtained by electrometer, so that there can be no question of varying polarization by currents, unless due to insulation leakage, and the currents of this nature can be said certainly t o have been less than 1 0 - l ~ ampere, perhaps much less. A11 the evidence, from numerous tests, seems to show that the potentiometer readings are essentially accurate and entitled to confidence. Another possible source of the effects is mechanical disturbances. Yiolent shaking is frequently found to produce violent changes in the voltage of such cells, though very little effect is produced by careful handling, as when transferring a cell from one bath to another. The vibration due to the stirring motor seemed not to have much effect. Hence it seems improbable that this cause is responsible for much of the variation. The difficulty of avoiding oxidation of the solutions has so far prevented the use of cells in which the solution was continuously stirred. Since there is little chance for the electrodes to come into equilibrium with their ions, except perhaps in the case of magnesium, and since the electrode films no doubt have an important effect on the voltage, perhaps it is too much to expect steady
PHOTO-VOLTAIC EFFECTS I N GRIGNARD SOLUTIONS
I547
voltage in such cells. Yet if the cause of the fluctuations could be found, and the effect controlled, these cells would give promise of considerable usefulness in certain fields. The cells seem to be light-sensitive a t every stage of their lives. The effect is strongly marked if obtained when the voltage is otherwise fairly steady, as in Fig. I , but still distinguishable under less favorable conditions, as in Fig. 2 . I n no single case was a photo-voltaic effect completely absent; VOLTS
0
FIG.I
A typical voltage-time curve, showing effect of repeated illumination on a Pt
+
LIgBrAl cell. The upward-pointing arrows show when the light was turned on, the inverted arrows when it was turned off.
indeed, one of the important results of VOLTS this study is the establishment of the widespread existence of the effect, for ' it appears that it is probably char:2 acteristic of most if not all solutions of Grignard reagents. T h e voltage -,1 change due to illumination is rather more definite a n d regular t h a n t h e voltage itself. Since the initial voltage :I before illumination may have almost 0 any value, within limits, it is meanFIG.2 ingless to discuss percentage changes Of A photo-voltaic effect superposed on a voltage. However, i t can be s t a t e d unusuallyrapiddrift, in a P t + M g B r C e that changes of 0.2 volt are frequently cell. Readings were taken a t w-second intervals over the critical parts of this observed with aromatic compounds, curve. which as a class are more sensitive than aliphatics. The changes here discussed are produced by illuminating both electrodes, and because of the large amount of scattered light, both sides of the electrodes were illuminated, so that little or no change was found on rotating a cell to expose it to illumination from different directions. The effect changes when either the electrode materials or the kind of solution is changed. h cell containing only ether had too high a resistance for its voltage to be read on the potentiometer. Dissolving iodine helps the conductivity, but does not seem to give much light-sensitivity, though here the readings were doubtful. ~
-J
_,
1548
R. T. DUFFORD
In discussing these cells, it is convenient for brevity to say that the voltage is positive if the platinum is the positive electrode, and to say that the response to light is positive if the platinum becomes more positive (or less negative). Thus any tendency of the other electrode to become more positive will be classed as a negative effect. It, is probable that the platinum is associated with the greater part of the response; though it is by no means entirely responsible for it, for it can be replaced by gold with good results, and responses were obtained when the electrode metals were Ng-Cu, Al-lIg, Zn-Mg, and A-Zn, in phenylniagnesium bromide solution. Khile the responses are smaller, they arc more regular among the aliphatic cells. Of such cells, 2 7 had a positive initial voltage, 3 a negative voltage, and 4 changed from positive to negative, during the time they were kept under observation. I n their response to light, 2 4 gave a negative response, 3 a positive, 3 first positive, then negative, and 2 first negative, then positive. Obviously the most of these cells had a positive voltage and a negative response; actually, 2 2 of them did so, while I had a positive voltage and a positive response, 3 had a negative voltage and a positive response, 2 had a negative voltage and a negative response, and 3 a positive voltage and a response at first positive, then negat'ive. Among the cells containing solutions of aromatic compounds, less regularity was found: 14 had a positive voltage during the time they were kept under observation, 18 a negative voltage, 4 changed from positive t o negative, and 8 changed from negative to positive (probably some of these started out with positive voltages, but had changed before readings were begun). Of these cells, 1 6 gave positive responses, 2 1 gave negative, 3 gave first positive, then negative, and I gave first negative, then positive. Of the cells wit,h a positive voltage when illuminated, 6 gave a positive response, 1 2 a negative, z changed from positive to negative; and of those with a negative voltage when illuminated, 8 had a positive response, 9 a negative, I changed from positive to negative. Only those eases in xhich the changes were clearcut and unambiguous are included in the foregoing counts. One of the most important questions was to determine to what extent the observed responses were due to temperature changes. I t would have been ideal if the observations could have been carried out under absolutely constant temperature; but since the effect to be studied depends on the absorption of radiation, which necessarily warms the absorber, this is not an attainable ideal. It is possible only to say that the temperature changes were small, numerous tests showing them rarely to reach 1°C. except when deliberately made larger for purposes of experiment. A long series of tests with a cell with a thermometer between the electrodes showed that the cell temperature followed the bath temperature closely, and that the effect of temperature alone on the voltage was relatively slight, From a large number of readings, data were selected as free from irregular variation as possible, and temperature coefficients computed for ten cells. The temperature changes ranged from 1.j"C. to 2 5 O C . The coefficients, or rather rates of change of voltage with temperature, ranged from 0.004 volt per degree C. to zero, averaging about 0.002 volt per degree, except for one cell which gave a value of .0072.
PHOTO-VOLTAIC
EFFECTS IS GRIGSARD SOLUTIONS
I549
And in every case except the unusual one just mentioned, the direction of the temperature effect was opposite to the direction of the response to light. I n cells containing a platinum electrode, the temperature effect tends to make the platinum electrode more positive, or less negative. The observations just quoted were made on cells containing aromatic compounds, the aliphatics being omitted through an oversight. Some later work done a t the University of Missouri seems to show that the aliphatic cells have slightly larger coefficients, and that the effect is more frequently in the direction of the light-response. It seems fair to conclude that if temperature effects could be eliminated] the responses in the cells containing aliphatic compounds would be somewhat smaller, while in the cells containing aromatic compounds, most of the effects would be slightly larger. However, one need not assume that under illumination the electrodes have the same temperature as the solution. The temperature gradient around the electrodes might affect the result. To test this, a Pt/@MgBr/Pb cell was prepared, with the P b electrode, which would be the one more heated by the radiation] made of lead foil wrapped around but insulated from a small heating coil operated by a battery. The current from the battery would warm the electrode several degrees in air. B u t in the cell it was found that heating the electrode continuously for six minutes produced less than a sixth of the effect that light did, though the temperature change from the heater must have been many times that due to radiation. The two effects were here in the same direction, however. The general plan of this investigation called for the testing of several pairs of electrode metals in each solution, and also the use of several different concentrations of a number of different solutions with one reliable pair of electrode metals (Pt-A1 was the combination used.) It seems unnecessary to take space to describe the results for individual cells, or even series of cells. The results may be summarized in a few general statements. I n general, aryl compounds give stronger responses than the corresponding alkyl compounds; bromides are considerably more sensitive than the corresponding iodides, and the chlorides would probably prove somewhat more sensitive than the corresponding bromides, though the number of observations on chlorides was too small to make this last conclusion entirely certain. I t appears that each type of solution has a n optimum concentration for photo-sensitivity, the best value for aromatic compounds being around IM, while for aliphatic compounds the best value is nearer to 0.jM. More observations would be desirable here, too, especially to test how the concentration effect varies with the age of the cells. Figs. 3 and 4 will illustrate the results obtained for two groups of cells which are typical. As to electrodes, Pt-AI, Pt-Cu, and Pt-Pb are desirable combinations, the last being the least sensitive; Pt-Zn and Pt-Mg are likely to be unreliable, as sometimes is Pt-Fe.
As indicated in an earlier paper' the writer first studied the photo-voltaic effect as a converse effect of the luminescence associated with the electrolysis of the Grignard solutions. Roughly it seems to be true that the substances
IjjO
R. T. DUFFORD
which give the brightest light on combining iTith 0xygen~,'.5,~ are the most light-sensitive, rather than those which give the most light on electrolysis. But there are two exceptions to such a statment; p-BrQLIgBr, for esample, is less sensitive than would be expected. Probably the two sets of phenomena are only remotely related.
FIG.3
Simultaneous voltage-time curves for a group of P t MgBr A1 cells, showing the increase in response as the concentration decreased to I M . Similar tests down to O . I Mshow a decrease in response below ~ h l .
The curves given show how, in typical cases, the light causes a sharp break in the voltage-time curve, the change gradually approaching a minimum which may be passed if the illumination is continued long enough. The recovery also begins sharply, usually, though the complete recovery does not take place for many hours, if at all, and frequently the recovery weakens nyth continued illuminations. A series of tests was made on one cell, with the intensity of illumination increased each exposure by moving the lamp nearer to t'he shutter of the small bath. From the voltage-time curve, it was found Evans and Dufford: J. .4m. Chem. Soc., 45, 278 (1923). 4Dufford, Calvert, and Nightingale: J. rlm. Chern. Soc., 45, 2058 (1923); 47, 9 j (1925); J Opt. Sac. America, 9, 405 (1924:. Evans and Diepenhorst: J. Am. Chem. Soc., 48, 71j (1926). 6 Duffard: J. Am. Chem. Soc., 50, 1822 ( 1 9 2 8 ) .
PHOTO-VOLTAIC EFFECTS I N GRIGNARD SOLCTIONS
1551
that the initial rate of change of voltage was fairly accurately a linear function of the intensity of illumination, over a range of j o to 13,600 metercandles. Many experiments were made to test how the effect depended on the wave-length of the incident light. The method was to put filters of known spectral transmission in front of the shutter of the small bath, keeping other
FIG.4 Simultaneous voltage-time curves for a group of Pt C2HsMgBr A1 cells, showing best response a t near o.j R.5 concentration. The responses are smaller than those shown in Fig. 3, as is regularly the case when similar aliphatic and aromatic compounds are compared.
conditions constant so far as possible. These experiments were all somewha unsatisfactory, for the reason that the light-sensitivity seems to be spread throughout the entire spectrum (as perhaps should be expected, since the compounds have no pronounced absorption bands), so that interposing a monochromatic filter for any single region of the spectrum reduces the amount of the effect to the point where the readings require too much time, and become complicated by the irregular variations which are likely to occur. One cell appeared to show a different direction of response to red light from the response to the blue, as Svensson’ has found the oxygen electrode to do. The writer is suspicious of this result; the red response might have been a temperature effect, If the photovoltaic effect were a temperature effect, one would expect the filter transmitting the red and beyond to give the maximum response. But the maximum appears to lie in the green or blue, and to be T. Svensson: Arkiv. f . Remi, 7,
I
(1919).
1552
R. T. DCFFORD
very broad. Most of the cells showed a sensitivity extending from the extreme red through to the near ultraviolet. It appears definitely true that the sensitivity extends over a wider range of wave-lengths than the emission band in the spectrum of the chemiluminescence to which the effect appeared to be a sort of converse effect. Certain other somewhat puzzling features were found. In a few cases what appeared to be a true Minehins reversal was found, Le., the light-effect passed a maximum, decreased, and finally reversed. Along with this, are several cases where the first response was positive, but instead of recovering, the cell continued to change in the direction in which the illumination started it, until all later illuminations changed the voltage in the negative direction. h similar effect is sometimes found, usually in old 3r over-concentrated cells, in which a momentary change occurs at the beginning of the illumination, in the opposite direction to the full effect, of the light, and likewise at the end of the illumination, the voltage gives a sudden jerk beyond the value to which the light has forced it, before the normal recovery occurs. The chemistry of these Grignard cells is hardly well enough understood to decide whether the interesting theory of the Minchin reversal given by Tuckerg will apply to them, but it appears probable to the writer that such a theory may be made to explain part a t least of the phenomena. But conditions in these cells are very different from those in aqueous solutions, except perhaps for a certain similarity with the coated electrodes and the cases like aluminum and tantalum electrodes. The behavior of the Grignard cells is so erratic that it is not possible to predict, from the electropositive series, even the sign of the voltage which a given pair of metals will give if used as electrodes. The writer hopes to be able to present additional facts, and to discuss the theory of these cells further, in later articles. I.
Summary It is shown that cells containing ether solutions of Grignard com-
pounds, with various metals as electrodes, all show a photo-voltaic effect. The effect is probably common to all Grignard solutions. 2. The effect is shown not t o be due to temperature changes. 3 . The light-sensitivity is greater for aromatic than for the corresponding aliphatic compounds; greater for bromides than for iodides, and probably greater for chlorides than for bromides. I t is thus roughly proportional to the brightness of the oxy-luminescence of these compounds; though there are exceptions to this rule. 4. It is found that the optimum concentration for light-sensitivity is somewhat higher for aromatic compounds than for aliphatic. j. The initial rate of change of voltage was found nearly proportional t o the intensity of the illumination. 6. The light-sensitivity usually exists for all visible wave-lengths, with a very broad maximim in the green or blue. Minchin: Phil. Mag., ( 5 ) 31, 207 (1891). Carl W. Tucker: J. Phys. Chem. 31, 1357 (1927).
PHOTO-TOLTAIC E F F E C T S IN GRIGNARD SOLCTIONS
'553
7. The unexplained variation of the voltage of such cells while in the dark, is described. The work described above was done at the Research Laboratory, Incandescent Lamp Department of General Electric Co., Kela Park, Cleveland, Ohio. The writer takes pleasure in expressing his gratitude to Director W. E. Forsythe, Dr. Elliot Q. Adams, and the other members of the staff, who made it possible to carry out the work under unusually pleasant circumstances. Department o j Physics, l n i e e r s i t y of .14issoiirt, Columbia, M o .
