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Vol. 62
TABLE I THEEFFECT OF VARIOUS HEATTREATMENTS ON THE PROPERTIES OF @-ALUMINA TRIHYDRATE Sample
Atm.
Heat treatmenTemp., OC.
Duration, hr.
X-Ray diffraution pattern
Surface area, m Vg.
.
Acidity, meq./g.
Acidity/ Area x 108
a H 8-AlzOa.3HzO 358 0.128 0.358 D Air 450 4 ?-AI208 .148 -379 390 J Steam 400 2 v-AleOa a-AlzOa.Hz0 326 .094 ' .288 G Air 650 4 q-MzOa 300 .074 .247 C Steam 450 4 258 . .048 .186 v-AlzOs c~Al2Oa.HzO E Steam 178 .036 .202 525 4 q--&Oa I Air 800 6 133 .029 .218 q-AltOa 4-O-AlzOs B Steam 132 .028 ,212 600 4 q-AlzOa A Steam 750 . 4 BAIzOa 71 .023 .324 F Steam 850 6 B-A1208 trace of a-AlZO3 48 .019 .396 p The sufface area determination followed out-gassing at 300" and the same temperature was used for heating this sample prior to acidity titration.
+ +
+
and the other in 100% steam. Furthermore, the acidity of the steam-treated sample J is in line with those of air-treated samples of approximately the same surface area.
Discussion The data show that the acidity of the alumina is not a simple function of surface area since the acidity per unit area goes through a minimum as surface area decreases. One explanation for the rapid change in acidity for small initial decreases in surface area might be the loss of volatile acidic impurities, but this is not consistent with the low figure for volatile acids found by analysis. Another explanation might be the combination of protons and bound OH groups upon heating with loss of them as water. If this were so, sample J heated in steam at 400" should have been more acidic than sample D heated in air at 450". Additional evidence against this explanation is the fact that the alumina shows no protonic acid either before or after heat treatment (samples H and A) when tested by exchange with ammonium acetate solution.8 Still another explanation is based on the concept that the acid centers consist of aluminum atoms so situated in the catalyst surface that they act as Lewis acids, i.e., they can Coordinate an electron pair.' It appears reasonable that aluminum atoms located on the edges and corners of the catalyst surface would be in a better position to form coordinate bonds by electron-pair acquisition than atoms in a plane surface. The words "edges" and "corners" are used to visualize defects which could act as acid sites. In a high surface area material, a reduction in area would occur with a larger reduction in the number of corners and edges, and a consequent larger reduction in acidity. This explanation is not inconsistent with any of the data, and probably deserves attention in considering the general phenomenon of catalytic activity which is often associated with high surface area of a catalyst. It may be noted that for the samples of highest acidity only about 1% of the aluminum atoms posses acid character. (3) Holm, Bailey and Clark unpublished procedure; presented at ACS meeting, April, 1957. (4) T. H. Milliken, Jr.. G. H. Mills and A. G . Oblad, Disc. Faraday Soc., 8, 278 (1950).
Such a simplified explanation certainly is not complete because it cannot account for the increase in acid per unit of surface area as the area is decreased below about 250 m.Z/g. One possible explanation for this change is that the acid centers are non-volatile impurities which become more concentrated on the surface as the area shrinks. This appears unlikely, however, because of the low concentration of non-volatile impurities and. because the analysis indicates that basic impurities probably predominate over acidic ones. It seems more reasonable that the dehydration with additional heat treatments creates new acid sites by exposing suitably oriented aluminum atoms, The acidity of each sample depends therefore upon the relative rates of crystallization which destroys acid sites and dehydration which creates new ones. OBSERVATIONS ON MEMBRANES USED FOR TRANSFERENCE NUMBER MEASUREMENTS IN MOLTEN SALTS BY GEORGE HARFUNGTON AND BENSON R. SUNDHEIM Department of Chemistry, New York Uniueraity, Washington Square, New York I, New York Received March 99, 1068
In a recently published theory of transference numbers in molten salts' the nature of a porous plug immersed in a molten salt undergoing electrolysis plays a central role. Sundheim presented the conjecture that such a plug will assume the mass average velocity of the liquid. If this is the cam the transference numbers measured in a Hittorf cell which restrains flows due to differences in the hydrostatic head can be predicted. Specifically, in the case of an electrolysis with electrodes reversible to one of the ionic species in a one component molten salt the relation derived on that basis indicated that there will be no net weight change in either the anode or cathode compartment (salt plus electrode). I n order to test this theory a cell was devised in which the weight change of one of the electrode compartments could be determined in situ. The apparatus, which is shown in Fig. 1, consists of an half cell suspended from a balance so that it dips into a vessel containing the molten salt and a second (1) B. R.Sundheim, Tale JOURNAL. 60, 1381 (1956).
L
Nov., 1958
1455
NOTES
electrode. The half cell contains an electrode, e.g., Ag, the molten salt, e.g., AgN03, and a porous diaphragm.2 It can be shown readily that with appropriate precautions weight changes of 0.5 mg. can be detected. The experiment was attempted first with silver nitrate and silver electrodes using a Pyrex U F gintered glass disk for the membrane. Before current was passed through the cell it was noted that the weight of the half cell changes steadily indicating that leakage of the molten salt through the membrane was occurring due to the hydrostatic head. Since this meant that the membrane is unsuitable for such a measurement a search was instituted for a membrane material which would have no detectable leakage but would also have a sufficiently low resistance (less than 10,000 ohms/cm.2) to permit use in a transference experiment. We report here on the results of this search. Five types of membrane were evaluated by determining their leakage rates and electrical resistances under a hydrostatic head of 1 cm. of the appropriate molten salt. The list includes all of the types of membrane used by researchers in this field.3 The results are summarized in Table I. TABLE I Membrane
Salt
Temp., OC.
Leakage
Resistance, ohms
Pyrex UF 2,000 PbBrz 420 >50 mg./hr. AgCl 485 10 mg./hr. 10,000 Vycor UF PbCla 550 30 4,000 500 5 10,000 (0.9-1.4mp) AgCl No. 7930" PbClz 550 0 > 100,000 510 0 > 100,000 (4 m m d AgCl "Porcelain"* AgNOa 220 20 6,000 (0.15 mp) AgCl 500 Erratic 10,000 Alundum' AgN03 220 Rapid AgCl 500 Rapid 5 A Vycor-like glass which has been leached but not shrunk. Corning Glass Co. b Selas Corp. A consolidated membrane composed of Norton Co. RA 1055 Alundum cement supported on a sintered glass disk. (0.9-1.4 mp)
The following points should be emphasized. All of the membranes leaked substantially except Corning no. 7930 which showed unmistakable signs of reaction and had too large an impedance. In all cases the passage of small currents (1 ma./cm.2) caused a substantial increase in leakage rate, probably because of local heating. This means that estimates of leakage rates which have sometimes been reported in terms of the no-current condition3 are too small. The increase in electrical resistance with decrease in leakage rate reflects the close parallel between viscosity and electrical resistivity in a molten salt. From an inspection of the magnitude of the entries in Table I it seems likely that a further search for a membrane with both low enough resistivity and low enough leakage rate will not be successful. (2) By providing a side pocket to the cell the method has been extended t o liquid electrode materials such as Pb. (3) F. R. Duke and R. W. Laity, THIS JOURNAL, 68, 549 (1955): J . A m . Chem. SOC.,7 6 , 4046 (1955); H.Bloom and N . J. Doull, THIS 60, 620 (1956); G.Jans and M.Lorena, AFOSR-TN 57-240. JOURNAL. A D No. 126537, 1957; F. R. Duke and R. W. Laity, J . Electrochem., 101, 97 (1958).
Fig. I.-Cell for evaluation of membrane leakage.
This work has been assisted by the United States Atomic Energy Commission, Contract AT (30-1) 1938 with New York University. (4) C/. M. R. Lorenz and G. J. Janz, THISJOURNAL,61, 1683 (1957).
INTERPRETATION O F THE PROPERTIES OF PROTEINS IN CONCENTRATED SALT SOLUTIONS BY J. R. MCPHEE W o o l Textile fiesearch Laboratories, Commonwealth Scienli$c Industrial Research Organization, Geelong, Auslralza Received April 26, 1968
and
The presence of high concentrations of electrolyte considerably modifies the behavior of proteins in aqueous solution affecting such properties as swelling,' solubilit8y,2denat~ration,~ meltingpoint of gelatin gels,4 thermal shrinkage of collageii,b strength6 and supercoiitraction of keratin fibers.' ( 1 ) F. Hofmeister, Arch. Bxp. Pathol. Pharmakol., Naunyn-Schniedeberg's, 24, 247 (1888); 27, 395 (1890); 23, 210 (1891). 43, 513 (2) A. R. Docking and E. Heymann, THISJOURNAL, (1039). (3) H.Chick and C. J. Martin, J . Physiol. (London), 40, 404 (1910); 46, 61 (1912): A. E. Mirsky and M. L. Anson, J . Gen. Physiol., 13, 307 (1938); A. E.Mirsky, ibid., 19, 559, 571 (1936); J. P . Greenstein, J . Biol. Chem., 130, 519 (1930); N. F. Burk, THISJOURNAL, 47, 104 (1943); R. B. Sinipson and W. Kauzmann, J . Am. Chsm. SOC.,71, 5139 (1953). (4) W. Pauli, Aich. ges. Physiol., Pjluger's, 71, 333 (1898); J. Bello. H.C. A. Riese and J. R. Vinograd, THISJOURNAL, 60, 1299 (1956). (5) J. R. Rats and A. Weidinger, Biochem. Z . , 269, 191 (1933); F. G. Lennox, Biochim. et Biophys. Acta, 3, 170 (1949). (6) A. h.1. Sookne and h l . Harris, J . Research Natl. Bur. Standards, 19, 535 (1937); J. B . Speaklnan and E. Whewell, J . Testile Inst.. 41, T329 (1950); P. Alexander, Kolloid-Z., 122, 8 (1951). (7) W. G. Crewther and L. M. D o d i n g , Nature, 178, 544 (1956).
