2338
J . Phys. Chem. 1980, 84,2338
COMMUNICATIONS TO THE EDITOR Ca(N0,),*4H20 as an Inert Solvent for Proton Acidity Studies in Molten Salt Hydrates
Sir: Molten salt hydrates possess remarkable chemical -1 0 1 properties. Thus Sare et al.’ showed that mixtures of A1(N03)3.10H20and A1Cl3.1OH2Odissolve noble metals more rapidly than boiling aqua regia, and there is evidence from a survey of ‘H NMR shifts’ and a more restricted study by “Hammett” indicators in the ZnC1,/HzO ~ y s t e m ~ , ~ - O 05 I that some of these melts are highly acidic. However, more extensive use of organic indicators in hydrate melts is Ho t o 5 hampered by decomposition reactions, especially in nitrate systems. We report that this problem can be solved by mixing Ca(N03),.4H20 with A1(N03)3-9Hz0and other tl 0“acidic” hydrates to form supercooled melts which, with care, can be kept liquid for long periods at room temperINCREASING +1 5ACIDITY ature. j Three classical Hammett indicators, p-nitroaniline (pK t 2 0= 0.99), o-nitroaniline (pK = -0.29), and 4-chloro-2nitroaniline (pK = 1-03),were used to determine Ho values, where Ho = pK + log ([B]/ [BH+]). Determination of the 1 I I i I i I -3 5 -30 -25 -20 -1 5 -; 0 protonated indicator is prevented by the presence of n--H* log (mole fraction solute) transition below 360 nm;4 the concentration of the protonated form was therefore calculated indirectly by comFigure 1. Hammett acidity trends in molten Ca(N03),.4H20 at 25 OC; paring the concentration of unprotonated base with that semilogarithmic plots of H, vs. log (mole fraction solute). of the total base, found by diluting with excess water.5 Molten Ca(N03)z.4H20itself is “neutral” in that none information is entirely lacking. However, in the sense that of the indicators is protonated to any measurable extent. these measurements were made under conditions of nearly However, addition of even small amounts of HN03 causes constant (low) water activity, we do seem to have obtained a sharp rise in acidity. Thus for 0.047 mol %‘ HN03 (3.5 a “true” orderg of decreasing acid strength, namely, HN03 X M), Ho = -0.62, comparable with 2 M HN03 in > A1(H20),3+> Cd(Hz0),2+,which would apply in hydrate aqueous solution. This enhanced acidity is in line with the melts generally. well-known effect of low water activity on H0.6-9 Results for added HNO,, A1(N03)3-9H20,and Cd(NReferences and Notes 03)2.4Hz0are summarized in Figure 1 in the form of semilogarithmic plots of Ho vs. log (mole fraction solute). E. J. Sare, C. T. Moynihan, and C . A. Angell, J . Phys. Chem., 77, Data for A1C13-6H,0 (not shown) lie directly on the Al(N1869 (1973). J. A. Duffy and M. D. Ingram, Inorg. Chem., 16, 2968 (1977); 17, 03)-9Hz0line. The high acidity of these melts is clearly 2798 (1978). a property of the aquo metal complexes’-3 but the actual J. A. Duffy and M. D. Ingram in “Ionic Liquids”, D. Inman and D. degree of hydration is not known. Inspection of the data Lovering, Ed., Plenum Press, New York, to be published. C. R. Boston, D. W. James, and G. P. Smith, J . Phys. Chem., 72, for [A1(H20),3+]shows that when log (mole fraction) = -2, 293 (1968). Le., for a 1 mol ’YO solution, Ho = -0.55, again similar to The absorption band of each indicator was red shifted by about 15 that of 2 M HNO, in aqueous solution. Extrapolation of nm In comparison to its position In H20. Additionally, the molar extinction coefficient of the unprotonated base appears to decrease the data for [A1(H20),3+]to log (mole fraction) = 0, Le., by about 10% in Ca(N03),.4H,0 melt regardless of indicator. to the “pure” hydrate, gives Ho = -2.1. This figure agrees F. E. Critchfield and J. B. Johnson, Anal. Chem., 31, 570 (1959). remarkably well with a value of -2.3 predicted on the basis K. N. Bascombe and R. P. Bell, Discuss. Faraday SOC.,24, 158 (1957). of a linear correlation between H o values and ‘H NMR P. A. H. Wyatt, Discuss. Faraday Soc., 24, 164 (1957). shifts in aqueous HC1 and HN03 solution^^^^^ and Sare’s D. H. McDaniel, Inorg. Chem., 18, 1412 (1979). value’ for the ’H NMR shift in A1(N03)3.10H20. This N. G. Zarakhani and M. I. Vinnik, Russ. J . Phys. Chem., 36,483 result appears t o confirm the suggestion made ear lie^-^,^ (1962). J. Braunstein, A. L. Baceralla, B. M. Benjamin, L. L. Brown, and C. that lH NMR shifts and protonation of organic indicators A. Glrard, J . Electrochem. Soc., 124, 844 (1977). respond very similarly to changes in proton acidity. We intend to continue the use of the Ca(N03)z.4Hz0 Department of Chemistry Raymond D. Dyer, I11 solvent system for exploring correlations between ‘H NMR College of William and Mary Rlchard M. Fronko shifts, acidity, and chemical reactivity and for studying the Williamsburg, Virginia 23 185 M. D. Schiavelll” kinetics of proton transfer pr0cesses.l’ Preliminary experiments show that Ca(N03)z.4H20“acidified” with AlMalcolm D. Ingram” Department of Chemistry (N0J3.9HZOis indeed a powerful nitrating agent, capable University of Aberdeen of rapidly converting phenol into picric acid. A knowledge Old Aberdeen AB9 2UE, Scotland of the nature of the active species (H904+,NOz+, or Received: June 6, 1980 whatever) in these solvents is needed, but at present this
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0022-3654/80/2084-2338$01 .OO/O
0 1980 American
Chemical Society
