6 ll. t. Ch . 22 ( 8
LATENT HEAT AND ELECTRODE POTENTIAL
hn St , Un v r t f C nn t t
In 86 , r n M r lt ( 8 0 0 br fl v n f xp r nt th t n nd th l d, b t th r d r b d th nfl n f t p r t r nd f th t t lt r l r. f th t ll l tr d n th l tr t v f r lt p nt d t th t, f nv rt bl nt l tr ( f f v lt ll ( . In th pp r z n n ll t , th l t nt h t f f n f b th h ld pr d ll h h d f nd th t h t, l tr d p t d, nd th r n f h n f . ( n ll . h n b r 2 t t p f pp r, b n d th v r t p f z n , l r r th n th b rv d t t l h n , n l d n p pr d d nt ll th f. r th l tr d thr h l d f t n. h th f f ll d n t r l d lt d d d t nv t t h t h pp n d p nd p n th t t f r t n f th t ll l h n t l l tr d p fr th l d t th l tr d , b t n h l ff t . d t t , r th r v r . In 8 , E l l C d b dr n ( 8 8 2 h b th n x pl . h t l, t d v r d ll , h h lt t 2 . 8° C nd n n ll r bl nd r d n n ntr t d O4, r n p r l d xt n v l . l Ant n n ld d nt ll th pp r n C SO 4 l t n. ( 820 8 d th pr p rt t h th r n l h t h lf ll r l tr ll j n d b n nv rt d tr l r p n ld b bt n d, t n f r t p r U t b f ll d th O4 . h l d h n t r , fr ll th l tr d f th t l, b t th t th b th h lf ll ld b h t d t 00° C. n d ff r n t t (2 . h 4 2 l tr d , l d nd h ll f f ll l l nd th b th tt d, l d r p t v l , f th th n v r r r ll , r v d n d b v l t n f h dr n. Aft r v r l pl d n l r f G 2(SO4 l t n. Wh n n h r , th f t b l z d nd n ff t d b t rr n n t d t lv n t r, d fl t n f r th n 40° th lt th th th r t r. h b th h lf ll r r p rt d, th nd t n fl f rr nt nd h n th n ll d t l nd th t p r t r b rv d fr ntl th n t f l d f t n h d b pr b n th f n l t p. h t p r t r f th pp r h lf ll r n d n h n d thr h t th xp r nt. lt f nd th t, th b th l d fr 280° t 2 0° C, th f nd r nt th l ht nd rr l r h n fr 2 . t 2 . ( f f n ll ll = 00 . r p rt d h rp h n f l d b th t l d t 264° C. ( h p. f th p r t l 2 . ° C, p bl p r l n rr d. t t d th t there is no abrupt change of this force at the moment where Figure I. Gore's fusible alloy cell the bismuth changes state. d t l r Bull. Hist. Chem. 22 (1998) 17
difference in the potentials of the electrodes. Regnauld noted that the solid electrode was the positive pole, analogous to the copper electrode in a Daniell cell. Raoult's paper was not mentioned; Regnauld was prob- ably unaware that his evidence was contrary to Raoult's conclusions. George Gore (1826-1908), Director of the Institute of Scientific Research in Birmingham, England, knew of Regnauld's findings but made no mention of Raoult. Gore continued the liquid-solid studies with alloys of low melting point (3). His apparatus is shown in Fig. 1. The electrode in glass cup A was a bar of the chosen alloy, which was immersed in the electrolyte solution. A portion of this alloy was melted in the bowl of the clay tobacco pipe B and allowed to travel well into the stem. This was to avoid thermoelectric junction effects. After the alloy had solidified and cooled, electrolyte solution was added to B. Some of this solution was drawn up into the siphon tube, thus providing electrical conti- nuity. Connections to the 100-W galvanometer were made r . W ll h M ll r by iron wires. The electrolyte solutions were (a) 1% HCl; t 60°, at a point apparently coincident with the melting in B. An experiment with solution (b v l r r lt . h nd f b h v r h n b n ll f p 0 ° C, used with solution (c). + Subsequent experiments were made with various amalgams. The sudden increase in deflection was small, probably because the amalgam melted gradually. Most successful was the amalgam: Cd 1 part, Hg 4 parts, which was more solid at 16° C than the others. This
0 amalgam was used with solution (b). Fig. 2, sketched
0 0 from Gore's diagram, shows the galvanometer response 0 as the temperature rose. A deflection maximum just be- fore the complete liquefaction of the amalgam was fol- lowed by a sudden depression, with reversal of sign. E O Then the deflection swung back as shown. Gore attrib- uted the depression to a sudden act of chemical union of , the ingredients of the amalgam. William Lash Miller (Fig. 3) re-examined the prob- lem of emf shift when a metal electrode melts or freezes r 2. G lv n t r r p n t th f n in Ostwald's laboratory in Leipzig (4). Miller (1866- f d l 1940) graduated from the University of Toronto in 1887 and obtained his Ph.D. in organic chemistry from the (b) 1% NaCl and (c) nearly saturated NaCl solution. University of Munich in 1890 but on the basis of re- Solution (a) at 16° C was used with the alloy: Bi 70 search carried out under the direction of A. W. Hofmann. parts, Pb 40, Sn 20, Cd 15; mp @ 66° C. A small flame Miller, who moved to Leipzig from Munich, later be- was applied to bowl B until the alloy melted and the came one of Canada's greatest chemists. solution above it nearly boiled. Up to the mp the alloy Miller's cell, shown in Fig. 4, was based on a large in B gradually became electrically positive to that in A. test tube. The fusible electrode, which had been melted The galvanometer reading increased suddenly from 20° into the funnel of the J-tube, and the adjacent second
18 Bull. Hist. Chem. 22 (1998)
electrode were connected tainable from a cell should be equivalent to the heat of by platinum wires to the reaction of the chemical processes involved. He found potentiometer circuit. that this was true for the Daniell cell. That Thomson's Heating or cooling theory includes all kinds of thermal effects within the controlled at approxi- cell may have become a common supposition. How- mately 1° C / min. Unless ever, the investigations of Ferdinand Braun (1850-1918) the amount of fusible showed that Thomson's theory was valid only when the metal was kept small, the temperature coefficient, dE/dT, of the emf, E, of the thermometer reading cell was negligible (6). This was the case with the Daniell showed an arrest, followed cell. The theoretical developments by Josiah Willard by sudden rise or fall. This Gibbs (1839-1903) and Hermann Helmholtz (1821- caused nonuniformity of 1894) led to the conclusion that E differed from the value temperature within the calculated from the chemical heat of the cell reaction cell. by the quantity T(dE/dT), where T is the absolute tem- Carefully purified perature. lead was used for one elec- Miller, aware of these developments, also applied trode; for the th r, silver a "thought experiment" to the solid-liquid problem. coated with AgCl. A mix- Consider a cell at the melting point X of the electrodes, ture of ZnCl2 and KCl, both of metal M. However, one electrode is liquid, the which melted at approxi- other, solid. If the cell has an emf, internal electrolysis mately 255° C, d will occur when the electrodes are connected together. a fused-salt electrolyte. Then, for example, M will dissolve from the liquid elec- Fig. 5, constructed from trode and deposition will occur on the solid electrode.
r 4. h M ll r Lash Miller's data, shows f bl t l ll the change, in potentiom- eter units, as the tempera- ture of the lead decreases. The total emf change nl 20 , with a brief rise of 1-2 mV at the solidification point, 316.5° C as read 0 on the thermometer. With tin as the fusible electrode the electrolyte was an approximately equimolar mixture of KNO , NaNO and Ca(NO )2 , melting at approxi- mately 185° C. In cooling from 268° to 200° C, there was no potentiometric indication of change of state at E 40 225° C, the thermometric solidification point of tin. Gore (3) had noted that the response at the mp of an amalgam diminished when the amalgam had been remelted several times. From his experiments with these substances, Lash Miller made a similar observation. He concluded that the processes that occur in the melting 0 of amalgams and other alloys were complicated. He fi- 2 0 00 0 nally examined the cell 10% Cd amalgam—Ag/AgCl , p r t r , °C 5% NaCl solution, over the temperature range 90° to 45° C. The emf fluctuated irregularly between 650 and r . p n f b—A A Cl2 ll (fr r f. 4 660 mV and underwent no special change at the transi- tion point. Now the deposit can be melted without change of tem- The expectation of a potential difference (pd) at the perature and taken back to the liquid electrode, so that mp between a solid electrode and its liquid form may electrolysis can continue indefinitely. Further, suppose have arisen from an extrapolation of a theory proposed that the electrodes are connected to a small motor; then by William Thomson (1824-1907), later Lord Kelvin, we will have perpetual motion. If a short wire resistor in 1851 (5). He suggested that the electrical energy ob- replaces the motor, the temperature of the wire will be- Bull. Hist. Chem. 22 (1998) 19 come greater than X. Such results are contrary both to E E E CES A O ES experience and to the principles of thermodynamics. I F. M. Raoult, "Influence de la température et de l'état Theory (7) and experiment indicate that at the mp des métaux sur la force électromotrice des éléments of M, the liquid and solid forms of M must have the voltaiques," Compt. Rend., 1869, 68, 643-645. same potential. Although this potential, or the emf, E, 2. 1. Regnauld, "Influence de l'état physique du gallium of a cell of which M forms one electrode, may change sur son role électrochimique". Compt. Rend., 1878, 86, with change of temperature, the E-T curve should be 1457-1458. smooth, . ., should show no irregularity as the tempera- 3. G. Gore, "On changes of voltaic energy of alloys during ture rises or falls through the mp of M. However, the fusion," Philos. Mag., 1891, 32, 27-31. small "kink," sometimes observed in the curve, seems 4. W. L. Miller, "Ueber die Umwandlung chemischer to be real. Ostwald suggested that the observed effect is Energie," Z. Phys. Chem. (Leipzig), 1892, 10, 459-466. 5. W. Thomson, "On the mechanical theory of electrolytes," due to a change in the temperature coefficient, dE/dT, Philos. Mag., 1851. 4, 429-444. as change of state occurs (8). This change, F, depends 6. F. Braun, "Ueber galvanische Elemente, welche upon the latent heat of fusion L and the absolute tem- angeblich nur aus Grundstoffen bestehen, and den perature, T (9). For lead, L = 1224/2 cal eqvt -1 , or 5121/ electromotorischen Nutzeffect chemischer Processe," 2 J eqvt-1 , and T= 601° K. Hence F = (2561 x 1000) / Ann. Phys. Chem., 1882, 17, 593-642. (96485 x 601) a4.4 x 10-2 mV K-1 . The apparent slope 7. Since liquid and solid are in equilibrium at the melting in the 290° to 330° C range in Fig. 5 is approximately point, DG, the free energy of fusion per equivalent, and 0.25 mV / deg., so that F is about 18% of this. the corresponding potential E must both be zero (DG = Because the change in temperature coefficient is -FÉ, where F is the Faraday constant). brief when melting or freezing is sharp, the change could 8. W. Ostwald, Chemische Energie, Engelmann, Leipzig, 2" ed., 1893, 869. be difficult to detect. Raoult and Miller made use of 9. This change, F DS/F = LIFT, where F is the change in "opposition" or "compensation" potentiometry which, dE/dT due to fusion; DS and L are, respectively, the en- involving instrument adjustment throughout measure- tropy and latent heat of fusion per equivalent. ments, does not provide continuous indication.
OU A IO S O C EMIS Y
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