NOTES
958
aldehyde does not form a colored complex with copper(I1) at apparent pH 4.5, but that 1,2-propanediamine forms a complex having a weak absorption maximum near 675 mp. When all three reactants are present in 1: 1: 1 ratios, the absorption is much stronger and the maximum is at 620 mp. Thus, at apparent pH 4.5 the three reactants form a complex that is more stable than the simple copper(I1)-amine complex. Another mole of salicylaldehyde does not alter the nature of the complex but iiicreases its concentration through the mass action effect. Under the same conditions, the spectrum of a 1: 1 mixture of copper(I1) and A is almost identical to curve 5 of Fig. 1. The effects of raising the apparent pH of these solutions to 8.5 are shown in Fig. 2. When only copper(I1) and salicylaldehyde were present, copper (11) was precipitated. The blue 1: 1 complex of copper (11)and the aminewas converted to the 1:2 yiolet one (curve a),and the excess copper was precipitated.5 T’ery little precipitation occurred in the solution containing the three reactants i n 1: 1: 1 ratios, and only minor changes of absorption and apparent pH occurred over a period of 34 days (curve 5 ) . KOprecipitation occurred in the solution containing copper(II), l12-propanediarniiie and salicylaldehyde in ratios of 1: 1: 2 , but the absorption increased with time and the maximum shifted to 560 nip (curve 6); the apparent pH of this solution also decreased with time. The absorption of the 1: 1:-t mixture increased faster than that of the 1: 1:2 mixture, and it mas still the stronger of the two after IL month (curve 7 ) . A 1: 1 mixture of copper(I1) and A behaves in the maiiiier described for the 1: 1:2 mixture. With samples containing copper(II), 12-propanediamine and salicylaldehyde in ratios of 1: 1: 0.5, 1:1: 1, and 1 : 1:2, the salicylaldehyde in the trimethylpentane extracts was 10, 18 and 49%, respectively. of the amounts added. The salicylaldehyde extracted from a 1: 1 mixture of copper(11)and -4m-as equivalent to one mole for each mole of added. Discussion From t,he foregoing results, it is bcliewcl t h a t the blue complex has the formula6 ( 5 ) Similar behavior of the c o p ~ ~ e r ( I I ) - e t l i ~ l e n ~ ~ i system i ~ ~ ~ i i iwas ir described b y H. B. Jonassen a n d T. IT. Dexter, J . Am. Chem. Sac., 71, 1553 (1949). (6) Placing t h e salicylideneamino group on XI rather than on N2 seems reasonable for steric considerations, although there is no direct experimental evidence on this point. One of t h e referees has pointed orit t h a t t h e complex might be a dimeric species with the formula i.
CH,
Vol. 64
When the appareiit pH of the copper(II)-A system is raised above 4.1, the second mole of salicylaldehyde reacts with the free amine group of the blue complex and the product coordinates with the phenolic group to form the violet complex. The number of protons released per metal ion fails to reach the theoretical value of 4, because the formation of the violet complex is iiicomplete in the apparent pH range of 4.1 to 4.5. The results obtained upon mixing copper(I1) and A in 50% aqueous isopropyl alcohol solutions can be explained once the effects of time, pH and ratio of reactants are understood. When copper(11) is added to an unbuffered solution of A, the violet complex is the sole reaction product until the metal-to-ligand ratio exceeds 0.5 : 1 slightly. At this stage of the reaction the apparent pH has dropped to about 4.5, and a large part of the unchelated A has been hydrolyzed. The violet complex continues to be formed as more copper(I1) is added until the apparent pH drops below 4, at which point the blue complex begiiis to be formed. By the time that the ratio of copper(I1) to A reaches 1:1, however, the apparent pH is in the neighborhood of 3, and most of the blue complex has been hydrolyzed. Thus, the 1-iolet complex appears to be the only reaction product, but its concentration is considerably belov that expected for complete reaction.
T H E VOLATILITY OF ACTIN1 U M BY K. I T . F O ~ T ES R ~ ~ D L ~C;. J ‘ ~ L I I I . E ’ ~ .lionsarito Chenizml Company, M o u n d Laborntorg,“ .llianiiabuiy
Ohio
Reccwed lfurcii 9,1960 r .
l h e scarcity of actinium in natural ore5 and the exceptional radiation hazards connected with its handling have been obstacles to the tieterriiinatioii of many of the element’s chemical and physical properties. The general chemical Iiaturc of the element and the radioartive behavior of it:, various isotopes have been known for some time,3 but practically none of the physical properties have heen measured until relatively recently. As y;et no measurements of the I apor pressure or the boiling point have been found by the authors During the coiirqe of some related classified research a t this Laboratory, several 3--10 mg samples of actinium-227 met a1 m r e prepared from purified salts by the lithium reductioii of actinium (1) (a) Monsanto Chemical Co., Research a n d Engineering DIX , Dayton, Ohio; (b) Monsanto Chemical Co., South Xearny, Keu Jersey. (2) Mound Laboratory 18 operated b y Monsanto Chemioal Co., for t h e United States Atomic Energy Commission under Contrart Y o . AT-33-1-GEN-53. (3) Gmelin, “Handbuch der -4norganischen Chemir,” 8 Auflaee. Svetem-Summri 40, Actinium und Isotope (RlsThz), l - ~ r l a i :Clicniie ( r m.h.If., Rrrlin, 1942, p . 29.
NOTES
July, 1960 fl~oride.~ Thin specimens of the element were obtained by volatilization of the actinium from molybdenum crucibles onto hemispherical nickel collectors. Following completion of these volatilization experiments it was found that sufficient data had been accumulated to permit a calculation of an approximate value of the vapor pressure of the element a t the volatilization temperature. The vapor pressure was computed from the rate of evaporation by means of an equation given by La ngmuir5 log P,,
=
1.2340
+ log W + 0.5 log T/AI
where Pm, is the vapor pressure in millimeters of mercury, W is the rate of evaporation of material in g. sec.-l; M is the molecular weight of the substance; and T is the absolute temperature. The volatilizations were made a t 1600', which is well above the melting point of the element. given by Stites4 as 1050 i 50'. The rate of evaporation, W , was determined from the gain in weight of the nickel collector during the time of heating and from the average surface area of the molten metal in the crucible. The crucibles were heated by electron bombardment techniques6 and the temperatures were measured with an opticoal pyrometer. Table I shows the results of six runs during which actinium n-as volatilized a t 1600' in YUCUO of to 10-5 nim. Five of these runs mere performed primarily to achieve maximum material transfer and, therefore, were carried nearly to (7ompletion. The average surface area of the melt in these runs was assumed to be the area of the bottom of the crucible since molten metal was observed to cover the bottom during most of each run. The five-minute run was made primarily to establish a more precise value for the rate of waporation at the operating temperature. This run was kept short in order to avoid appreciable (.haages in the surface area of the molten actinium during evaporation. Since the vapor pressure \value computed for this short run agreed, within :t reasonable factor, with the results of the longer i'uns, it was decided that the data from these longer runs were equallp valid. Therefore, the arithmetic, mean of the vapor pressure values for the six ruiih, 0.006 mm., wab used to determine the boiling point of the element from Loftness' chart.' The boiling point of actinium was found to he 3200' with a i l wtimated error of *300". The error is defined by a range of vapor pressure values within a factor of ten of 0.006 mm. Since the crucible-collector configuration had I )eel1 varied significantly between runs, the ino't difficult factor to determine with any precision i n this experiment was the evaporation surface area and the various associated crucible effects. ( 4 ) J. G. Stites, XI 1 Salutsby and B. I). Stone, J A m . C h e m . Soc., 77, 237 (1955). ( 5 ) I. Langmuir, P h y s . Rev., 2, 329 (1913) (fi) (a) H. M O'Brpan. Re,, S C L I n s t r . , 6, 125 11934) (b) J \-aruood, 'High Yariiiim Tri h n i q i i f " Totln n ' i l ~ x & %,ns I n ? , V ~ Yorh N. Y. 1'446 p 87 (7) R I. L o f t n e v 4 1 apui Prebsiire Chart f o r A l r t a l s , " hA i - S K Id? JuI>, 1'152
959
However, the range of vapor pressure values in Table I indicates that the effect of these variables is not exceptionally serious. In all configurations the collection of volatilized actinium apparently was accomplished with relatively negligible loss since only minute radioactive contamination of the remainder of the vacuum system resulted. Also, the entire crucible was observed to be at approximately the volatilization temperature, whereas the temperature of the collector was never above 200". T'isual examination of the crucibles after firing indicated that the only wetting that occurred was on the crucible bottom, so it was concluded that refluxing was negligible. It was estimated that the various crucible and area effects could not affect the values for the vapor pre,wire 1)y more than a factor of two or three. TABLE I ACTINIUM T-OLATILIZ4TION EXPERIhIEh I'i IT 1600' A c in Time Collector colllyutrd criicible, of run, galn, min v p , my. min. mg. inn1
6 80 6 48 9.76 4 98 6 07 6 48
30 30 30 30 30 3
4 81 5 37 9 38 4 62 5 86 1.76
0 004 005 008 004 005 007
Subsequent neutron emissioii determinations of the vacuum deposited films of actinium8 indicated that the amount of actinium fluoride, the most likely volatile impurity, was less than 0.2 atomic % ' in any one of the samples. It was, therefore, assumed with reasonable assurance that the vapor pressurevalues in Table I represent minimum values since sources of error other than actinium fluoride contamination would tend to depress the apparent vapor pressure rather than enhance it. Refractory impurities and loss of material not deposited on the nickel collector could affect the results in this manner, but their effects are considered to be within the measuring precision of the experiment. The actinium was kept in a dry helium enviroiiinent at all times to inhibit the forniation of actinium oxide. *\lso, the material was Yolatilized a4 soon as possible after the separation of the actinium to reduce the effects of contamination by daughter products of the radioactive decay of actinium-227. It is felt that the cumulated errors in these experiments did not affect the d u e s obtained for the T-apor pressure by more than a factor of teii, which would not affect the value for thc boiling poiiit by more than 300". It is realized by the authors that thi' measurci w n t 15 a side result from relatctl experinientatioii sild i q , at hest. a cmde determination of the hiling point of the element However, became of the extensive radiation protection equipment and associated health monitoring services required for any work with significant quantities of this element, it does not appear likely that any precise M cl~aluationof its physical properties will be forthvomiiig ilk thr n ~ w ftitiirc r (SI I< W,i-oster and J C.. htitefi, T H I R J O U R ~ ~60, L ,1017 (1956).
9 GO
Tol. (3-1
COMhlUNICATION TO THE EDITOR
Acknowledgment.-The authors are indebted to R. G. Olt for their coiitributions t o this experiE. H. Daggett, A. W. Wotring, D. U. Wright and ment.
COMMUNICATION TO THE EDITOR FARADAIC RECTIFICATION AND ELECTRODE PROCESSES’
Sir : A theory of faradaic rectification extending and clarifying results of previous investigators2 mas developed. It was shown that two types of coiitrol must be considered, namely, control of the total mean cucrent density ( I ) or the me&n electrode potential ( E ) . In practice, I = 0 or E = E,, E, being the equ_ilibriumpotential. The shift of mean potential AE for control of I at zero is the same whether the alternating current or voltage is controlled as a practically harmonic-free sinusoidal function of time, Conversely, the mean current for control of E at E, is the same whether there is alternating current or voltage contyol. The time variation of AE was derived by noting that the sum of the mean faradaic and lion-faradaic (double layer charging) current densities is e-qual to zero ( I = 0). It was shown that AE = AEt4, even after as short a time as 10-3 sec. provided that reactant concentrations are large enough. Times longer than see. may be required for dilute solutions (perhap! l O - * U ) . The build-up aiid decay curves for AE = f(t) when the alternating current is switched on once and switched off are the images of each other. The inffuenc_e of the cell resiztance on the time variations of I with control of E a t E, was shown to be the same-as in the voltage-step potentiostatic method, aiid I is equivalent to the current in the latter method when a (1) I n \ estiqntion sponslred In part by the O f i r e of h‘ai a l Ilesearch and t h e National Scwnce Poundation. (2) F o r rderences see I 1 hlatsuda and P. D e l a h n ~ ,J Am. Chem. Soc., 83, 1947 (191~0)
potential step - A&m is applied while the kinetic parameters remain those a t E,. A general equation in terms of first and second partial derivatives of I_ with _respect to E , Co, and CR was derived for AE and I for any type of I-E characteristic for the electrode reaction 0 ne = R. A particular form is
+
=
n--F
RT
J’2
1k5r.L + (
4
where V is the amplitude of the alternating voltage, a the transfer coefficient, r F and XF the real and imaginary parts of the total faradaic impedance, respectively, and Ti (i = 0 or R) and 2i the corresponding components of the faradaic impedance for substances 0 or R. Values of the T’S and z’s obtained in the classical faradaic impedance theory for a variety of processes (simple discharge] discharge with preceding chemical reaction, etc.) can be- direct_ly introduced in eq. (1). Properties of AE and I are readily deduced. An instrument with application of the alternating voltage for a short d~ration-(lO-~ to lo-’ sec.) and oscilloscopic recording of AE (single pulse) mas designed for frequeiicies up to 2 megacycles per see. Heating of electrolyte near the working electrode is minimized by this method, and frequencies above 2 megacycles per sec. undoubtedly could be utilized. Theory and experimental results mill be published. GATES CHIXICAL Lumnaro~~ h U I S I A N A STATE U N I V E R S I T Y
BATON ROUGE 3, La. RECEIVED JUNE 3, lO(30
P.
L)EL.4iiAY
XI. SENDA C. H. W E I ~
