The Journal of Physical Chemistry, Vol. 83,
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(10 "C), 0.765 f 0.009(25 "C), and 0.62 f 0.01 (45 "C), and for Pr4NI0.824 f 0.008 (10 "C). Since the D values of an aqueous solution of inorganic ions are small,13Kaminsky's14 anion coefficients B were used for the separation of the molal B values into ionic contributions. Finally, the following values of BPra' were obtained: 0.95 (10 "C), 0.80 (25 " C ) , andl 0.64 (45 "C). These values are in good agreement with the data of Desnoyers and Perron13 a t 25 "C. Using the obtained values of B P r d N * instead of the experimental values8 for the construction of a figure such as Figure 1, 'we found, by the extrapolation of the temperature dependence of the values BPrdN+ up to the intersection wi1,h the straight line of the Einstein equation, that the dimension of bare Pr4N+ ions are 3.42 This value is only greater by 2.7 % than the real ionic dimension. Unfortunately, a similar calculation for Bu4N+ ions was impossible due to the lack of experimental data for solutions of moderate concentrations.
No. 6, 1979 765
A Na-M o Na-Y A H-Y
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a.
Acknowledgment. This research was supported by the Centre for Absorption in Science, the Ministry for Immigrant Absorption, State of Israel. References and Notes (1) E. S. Krumgalz, Faraday Discuss Chem. SOC.,No. 84, 247, 336 (1978). (2) R. H. Stokes and R. Mills in "The InternationalEncyclopedia of Physical Chemistry and Chemical Physics", Vol. 3, "Viscosity of Electrolytes and Relateij Properties", Pergarnon Press, New York, 1965. (3) A. Einstein, Ann. Phys., 19, 289 (1906). (4) G. Jones and M. Dole, J . Am. Chem. Soc., 51, 2950 (1929). (5) E. S. Krumgalz, J . Gen Chem. USSR, 44, 1585 (1974). (6) R. A. Robinson and R. H. Stokes, "Electrolyte Solutions", Butterworths, London, 1966. (7) E. S. Krumaalz. Russ. J . Phvs. Chem.. 45. 1448 119711. i8j R. L.Kay, T-Vtuccio, C.Zawiyski, and D.F. Evans, J: Pbys. Cbem., 70, 2336 (1966). E. S. Krurngalz, J . Solution Chem., in press. B. S. Krumgalz, J . Struct. Chem., 13, 727 (1972). D. F. Evans and R. L. Kay, J . Phys. Chem., 70, 366, 2325 (1966). P. R. Phillip and C. Jolicoeur, J . Phys. Chem., 77, 3071 (1973). J. E. Desnoyers and G. Perron, J. Solution Chem., 1, 199 (1972). M. Kaminsky, Z. Phys. Chem. (FrankfurfamMain), 8, 173 (1956); Discuss. Faraday Soc., 24, 171 (1957). Based on the new experimental values of the ionic equivalent conductivities of the Pr,N+ ion in organic solvents published after 1971, we found that the value of the real dimension of this ion was equal to 3.33 i 0.10 A9 in contrast to the value 3.35 A which was reported earlier.' values of The dotted lines in Figure 2 correspond to the f3R'N+Elnst unhydrated Pr,N+ and Bu,N+ ions, 0.234 and 0.360 crn3/rnol, respectively, BS calculated above. Israel Oceanographic & Limnological Research Institute P.O.6. 8030, Haifa, Israel
Borls S. Krumgalz
Received July 6, 1978
Use of Hammcatt Indicators for Acidity Measurements in Zeolites
Sir: The classiic method of acid strength measurements in catalysts by noting the color changes of adsorbed dye indicators (Hammett indicators) during titration with an organic base (amines) in a nonaqueous solvent1S2has become a well-established procedure for catalyst evaluation. It was shown that this method can be applied to acidity measurements in zeolites3 since parameters such as type of cation exchanged, degree of exchange, and cation distribution within the Si/Al framework all greatly influence the color changes of the indicators used.4 It has been suggested" that acidity measurements depend more 0022-3654/79/2083-0765$0 1.OO/O
+7
+5
+3
+I
0-1
-3
-5
-7
-9
PK,
Figure 1. n-Butylamine titration of Na-Y (100% exchange), Na-M (100% exchange), and H-Y zeolites in dry benzene using Hammett indicators.
strongly on the size of the organic base than of thc indicator molecules, in that the comparably smaller base molecules move inside the channels, whereas the large indicator molecules remain in accessible sites. The actual interaction of the indicator molecules, Le., color changes with acids or bases, should then only take place at a fraction of the acidic centers. If this is really the case, experimentally determined butylamine titers should be orders of magnitude lower than those ~ b s e r v e dand , ~ the whole titration process should proceed quite fast, its speed depending upon the movement of the small base molecules. Reaching the end point with Hammett indicators during the titration with faujasites, however, often took some days, which indicates a slow diffusion process of the dye niolecules into the channels. The process was considerably accelerated by immersing the test tubes in an ultrasonic tank.4 Indicators used were from Eastman Kodak, as follows: neutral red (pK, = +6.8), 4-(pethoxyphenylazo)-mphenylenediamine (pK, = +5.0), 4-phenylazo-1-naphthylamine (pK, = +4.0), 4-o-tolylazo-o-toluidin (pK, = +2.0), 4-phenylazodiphenylamine (pK, = +1.5), 2-nitrodiphenylamine (pK, = -2.1), dicinnamalacetone (pK, = -3.01, chalcone (pK, = -5.6), and anthraquinone (pK, = -8.2). Figure 1 describes the formulas of some of the indicators used, the atomic distances of their planar projection being calculated with the help of ref 6. In the present work the results of n-butylamine titration in ion-exchanged faujasite Y (Na-Y, H-Y) are compared with those of H mordenite (Zeolon-H) and Na mordenite (100% exchanged). The adequately pretreated (calcined, H-M a t 500 "C, H-Y a t 290-300 "C, a t torr) zeolites (0.1 g) were placed in sealed test tubes and titrated with n-butylamine in dry benzene using a microburet for this purpose. Figure 2 shows the results of these measurements. Whereas the acidity of both Na-Y and Na-M is nearly zero, H--Y exhibited a very strong acidity with about 2.0 mequiv of n-butylaminelg of zeolite for the most acidic centers (equivalent to 90% H2S04,see ref 7). "Total" acidity a t pK, = +6.8 (neutral red) was found to amount to 2.8 mequiv of n-butylaminelg of zeolite. Therefore, more than two-thirds of all the acidic centers of H-Y are very strong. On the other hand, determination of the acidity of the H mordenite was impossible, since most of t2 1979 Arnericari
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The Journal of Physical Chemistry, Vol. 83, No. 6, 1979 nil,i31
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not possible (around 2.1-2.4 mequiv of n-butylaminelg of mordenite); phenylazonaphythylamine gives only weak acidic coloration, which disappears with the first drops of butylamine; tolylazo-o-toluidin and phenylazodiphenylamine give acidic coloration, end points being a t 2.5-2.6 mequiv of n-butylamine/g of mordenite and 2.4-2.5 mequiv of n-butylaminelg of mordenite, respectively; 1,9-diphenyl-1,3,6,8-nonatetraen-5-one (dicinnamalacetone) gives acidic coloration, however, the end point cannot be determined since no color change happens during titration; anthraquinone does not give acidic coloration. The results indicate that certainly the indicator molecules must enter the zeolite channels to have access to the inner acidic centers which are considered to be the strongest. In the case of faujasite the indicator molecules obviously enter and leave the channels easily; with mordenite their mobility seems to be severely restricted, the bigger ones such as diphenylnonateraen being trapped inside. It should be mentioned further that H mordenite containing some percent of adsorbed water only gave a very weak coloration with the Hammett indicators. Water, in conclusion, seems to block effectively the pore openings of the mordenite. The use of Hammett indicators is, therefore, restricted to the pore diameter of the catalyst being investigated.
u
References and Notes 0 4
t
I
(1) (2) (3) (4) (5) (6)
Walling, Ch. J . A m . Chem. SOC. 1950, 72, 1164. Benesi, H. A. J . Am. Chem. SOC. 1956, 78, 5490. Hirschler, A. E. J . Catall968, 2 , 248; 1968, 1 1 , 274. Kladnig, W. J . Phys. Chem. 1976, 80,262. Barthomeuf, D. ACS Symp. Ser. 1977, No. 4 0 , 453. Mitchell, H. D.; Cross, L. C. "Tables of Interatomic Distances and Configurations in Molecules and Ions" Chemical Society, London, 1958. (7) Hammett, L. P.; Deyrup, A. J., J . Am. Chem. SOC.1932, 54, 2721. ( 8 ) This work was performed at the Instituto Venezolano de Investigaciones Cientificas, Centro de Petroleo y Quimica Caracas, Apartado 1827, Venezuela.
Worcester Polytechnic Institute' Department of Chemical Engineering Worcester, Massachusetts 0 1609
W. F. Kladnig
Received July 24, 1978; Revised Manuscript Received January 8, 1979
H'
'H
Zeolite Acidity Titration with Colored Indicators 11.98
19 2 %
Figure 2. Projections of molecular formulas of Hammett indicators as determined by use of ref 6. Criiical diameters of molecules are designed with: neutral red (3-amino-7-dimethyiamino-2-methylphenazin),a = 6.5 X 12.9 A; 4-p-ethoxyphenylazo-m-phenyienediamine,u = 6.5 X 16.2 A; 4-phenyiazo-l-naphthylamine, a = 7.8 X 11.9 A; 4-0-tolylazo-o-toluidin, a = 6.5 X 11.9 A; 1,9-diphenyl-l,3,6&nonatetraen5-one, CJ = 5.4 X 19.2 A. (8-Nitrodiphenylamine, not shown in Figure 2, has been measured with u = 5.9 X 9.2 A,)
the indicators (Figure 1) cannot enter the 6.7-A pores. The indicators used had the following behavior: neutral red and p-ethoxyphenylazo-m-phenylenediamine give acidic coloration, but exact determination of the end point was 0022-3654/79/2083-0766$01 .OO/O
Publication costs assisted by the Centre National de la Recherche Scientifique
Sir: Measuring the acidity of solids with n-butylamine and colored indicators is of great interest.l Two major points have to be considered. First it has been shown that only some indicators give meaningful results.2 Secondly the principle of the titration is quite similar to what happens in solution. The indicator concentration has to be maintained low enough in order to have only a small fraction of the acid sites reacting with the indicators molecules, the major part of the acid centers being neutralized by the base molecule. Besides these fundamental features some complications occur in zeolites due to the size of the pores which in some cases is close to those of the base and of the indicators. In a very recent paper Kladnig' showed that in HY zeolites, and to a lesser extent in H mordenite, strong acid centers are titrated. He concludes that the corresponding indicators must enter the cavities and disagrees with a suggestion3 that the indicator molecules interactions with acidic centers should only take place a t a fraction of these sites. We would like to explain in more detail what we 0 1979 American Chemical
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