George A. Vidulich College of the Holy Cross Worcester, Massachusetts 01610 and Anthony Fratiello California State University. Los Angeles Los Angeles, California 90030
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A Room Temperature Proton Magnetic Resonance Hydration Number Study
Several years ago an undergraduate physical chemistry laboratory experiment was reported in this Journal' describing the measurement of the primary hydration numher of A13+ by the direct area nuclear magnetic resonance (nmr) method. This experiment required the use of a low temperature accessory with the nmr spectrometer. Recently we have observed that the proton exchange also can he slowed even a t room temperature by using very dilute acetone solutions of Al(CI0A hvdrates. The soectra anoear similar to that ohtai'ned a t low temperature ;or more concentrated solutions in a c e t ~ n e .Here ~ . ~ we reoort a modification of the orieinal low temperature nmr hydrkion number experiment' whGh allows the measurement of the primary hydration numher of Al3+ a t the ambient temperature of the spectrometer (for most instruments this is around 34°C). We have been doine this experiment in our physical chemistry laboratory (GAV) the past few years with much success. The experiment can he completed easily in one afternoon of lahoratory.
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By measuring spectra of the saturated aqueous solution in mixtures containing lower amounts of acetone, an interesting variation of this amhient constant temperature study can he done to show the effect on the resolution from a variation in the proton exchange rate.' The series of spectra resemble those ohtained on concentrated salt solutions when the temperature is lowered gradually.'-3 They range from the single completely coalesced average signal to the two well separated peaks. We have ohtained best results with solutions having the following peaks: A13+:HzO:acetone mole ratios; 1:16:8; 1:16:10; 1:16:25; 1:16:100. Examples of these spectra are shown in Figure 2.
Materials A supply of AI(C104)~9HzO,distilled water, deuterated acetone, thin wall precision nmr sample tubes, nmr chart paper and access to a proton magnetic resonance spectrometer will he required. Regular acetone can be substituted fordsacetone, hut the larae solvent peak makes intearation of the hulk water signal difficult. Procedure Aluminum oerchlorate hvdrate is hvdrosconic and if excessively wet ihould he stored overnigh in a desiccator containine anhvdrous CaCL. The drvine neriod should not exceed - -. 24 hr i n order to the water content from dropping below 9 molecules. Anhydrous CaS04 should he placed in the ds-acetone container to lower the water content of the solvent. Prior to the lahorntory perind, a saturated aqueous solution of AI(C104):,should he prepared (H20/Alb = 1611) and the concmtration determined by titration of the effluent ohtained after passage through a cation exchange resin. The acetone solutions are best made up in small vials equipped with tight screw caps. With the aid of a syringe, increments of 1ml of ds-acetone are added rapidly to these small vials containine annroximatelv 50 me of AIfCIOd~..9H.0 .. " salt and 50 mg of saturated AI(C~O&s&tion. These solutions will he sufficientlv dilute for eood resolution of the two water signals hut concekrated enough to give signals of sufficient intensity to he well above the noise level. The latter condition is necessary for an accurate integration of the peaks. A sample of each solution can then he sealed in an nrnr tube with a pressure cap t o prevent loss of solvent by evaporation. The nmr spectrum of each sample should he recorded a t probe temperature. Examples of these spectra are shown in Figure 1. A value of six will be ohtained for the primary hydration numher of AP+ for both solutions, in agreement with that reported el~ewhere?.~
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F~gure1 The Perh +Elmer R20 poron magnetrc resonance specba of aq-us A CI0413solut 0"s m &-acetone. I seal ng I kslg~isaue to soluem. odlr walw (&*I)andnydrated water m te Altl 11 Inner shell (C#) T h e m e rat oof water to ~ i ( 1 i l )is given above each spectrum
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Fratiello, A., and Schuster, R. E., J. CHEM. EDUC., 45, 91 (19681. . . Fratiello, A,, "Inorganic Reaction Mechanisms: P& 11," (Editor: Edwards, John 0.1,Interscience Publishers, New York, 1972, p. 67-99 -.
Fratiello, A., Lee, R. E., Nishida, V. M., and Schuster, R. E., J. Chem.Phys., 48.3705 (1968). 672 1 Journal of Chemical Education
Figure 2. The Perktn-ElmerR20 proton magnetic remance s w a , at 34%. for saturated aqueous Ai(CiO&solutions (H20/AiS+ 1611)at different concentrations in acetone, showing the effect 01 the variation in proton exchange rate. The peak furthest downfieid is due to lhe hydrated water and the narrow peak is the ' C satellite of acetone. Different spectral amplitudes were used.
