St. Andrew Presbyterian College
burinburg. North Carolina 28352
1 Determination of Hydration .. I (
An advanced undergraduate experiment
Recause uf their proximity to thr metal cation, protons i n water moleculrs orrupvinr thr first hydration sphtrr might be expected to exhibit a p r o t o n magnetic resonance peak distinguishable from the bulk water peak, for their magnetic environments differ markedly. At low temperatures (generally 5 -20°C.), the exchange of both protons azd water molecules between the hvdration where and the bulk water is slowed to the extent thai these tw; peaks may be resolved. The ratio of their areas then eives a direct measure of the ~ r i m a" r vhvdra" . tion number."" Though this method is concentuallv simnle, it has several limitations. First, incomplete resolution (oveilap between bulk and coordinated water peaks) may make accurate determination of their relative areas difficult." Second, solutions may freeze, precipitate, or become very viscous, preventing complete resolution of the peaks. Freezing may be prevented by use of solvent systems containing low freezing organic solvents or by use of solutions of high ionic strength.:l4 The former strategy may result in solubility problems, the latter in viscosity ones. Recent research has shown that the addition of a salt, such as calcium nitrate (which does not exhibit a pmr peak for coordinated water under the experimental conditions), to the solution under studv - mav. have several exnerimentallv desirahle dfects.' lonic strength may he incrcnwd without w ouslv comnoundine soluhilitv.. nroblem: rrsol~lrionofthe hulk and"cwrdinated peaks may occur a t higher temperatures than for nure solutions: a better deeree of resolution mav be obtaine&, and the anion tometaliatio may hevarwrl h i thecxmrin~enter.ThLq "inert s d t " method has been emnlo\wJ . . with some success in this investigation. Experimental Stock solutions of AI(NOs)n,CaClz, CaBrz, Cab, Ca(NOd2, NaC104, Ga(ClO&, and In(C104)s were prepared from research grade materials. used as received. Gallium and indium trichloiides, tribromides, and triiodides were prepared by oxidation of a known amount of the metal hv the a ~ n r o ~ r i a t e hydrohalic acid. The aluminum, calcium,-and peichiorate solutions were standardized by ion exchange using Rexyn 101 (Fisher Scientific) in acid form, followed by titration of the elutent with standardized sodium hydroxide solution. Densities of all solutions were obtained. samples of various concentrations were prenared from aliquots of the stock solu. . tions. All spectra were recorded on a Varian T-60 nmr spectrometer equipped with a T-6055 Permalock and a T-6080 Temperature Controller. A spectrum of the sample was first
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Permanent address: P.O. Box 147, Franklin, N.C. 28734. ISwift, T. J., and Sayre, W. G., J. Chem. Phys., 44(9), 3567 (1966). ,~~~~ ~
:' Fratiello,A,, Lee, R.
E., Nishida, V. M., and Schuster, R. E., Inorg. Chem., 8(1), 69 (1969). Fratiello, A., Lee. R. E., Nishida, V. M., and Schuster, R. E., J. Chem. Phys., 48(8). 3105 (1968). %Scweitzer,G. K.,andStephens, J. F.,Spectr.y.Letters, 3(1), 11 (1970). "Matwiyoff,N. A,, and Darley, P. E., J. Phys. Chem., 72(7),2659 (1968).
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62 / Journal of Chemical Education
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Demonstration of the effects of an inelt salt. CWrdinated water peak to the len fupfieldapproximately 124 Hz)of the bulk water peak. A. 1.75 MAICI, plus 1.65 MCa(NOd2at -3E0; 8.1.75 MAICI, at -38': C, 1.75 MAICi, at -49'.
recorded a t room temperature. The temperature was then lowered in increments of 1" to 10", and the spectrum recorded each time, until maximum resolution of the two peaks was obtained. Sample freezing is marked by a sudden decrease in resolution, and could result in sample tube breakage and probe damage. Therefore, samples were removed immediately when such a decrease was noted. Peak areas were determined with a planimeter (at least four trials), and hydration numbers computed using the formula area coord. peak X [H?O] Hydration Number = area both peaks X [M(III)] Results and Discussion At ambient temperature, a single, sharp peak is observed for all water in the solution. As the temperature decreases. this peak hrnadens greatly, and begins to split into tbe two distinct peaks, (Fig. 1) which increase in resolution until viscosity, precipitation, or freezing result in resolution losses. The results obtained for the aluminum nitrate solutions are typical of the various systems investigated. Hydration numbers were determined for solutions containing from 1.55 M tu 2.58 M aluminum nitrate and from 0.0 M to 2.60 M calcium nitrate. The studies were conducted in the temperature ranae of -20°C to -3S0C, and hydration numbers v&ying from 5 0 tu 6.1 were calculated from the experimental data. The variations in observed hydration numher may be accounted for hy resolution changes a t different temperatures, and differences in hydration resulting from &ncentration related
phenomena such as contact-ion-pairing. Several sources of error must also he considered, including incomplete resolution and errors in integration. By comparison, errors in concentration determination are small. The advantages of addition of an inert salt are shown in the figure. Resolution is gained at a higher temperature, and is more complete in the solution containing the inert salt. Use of this technique has permitted researchers to correlate hydration number with anionlmetal mole ratio.5 It was found that AI(II1) exhibits consistently lower hydration number, as
determined by this method, in the presence of excess C1- ions, which may be taken as evidence for contact-ion-pairing. Attempts to determine hydration numbers for Ga(II1) and In(II1) in this experiment were less successful because, in many cases, solutions froze or precipitated before significant resolution of the two peaks was obtained. Acknowledgment
The authors gratefully acknowledge the financial support received from the Research Corporation.
Volume 54, Number 1, January 1977 / 63
