An undergraduate inorganic or spectroscopic experiment

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Preparation and Characterization of Deuterated Crystalohydrates An Undergraduate Inorganic or Spectroscopic Experiment Marina Ivanovska, Kiro Stojanoski, and Zoran Zdravkovski Institute of Chemistry, Cyril and Methodius University, PO6 162, Skopje, Macedonia Isotopically labelled compounds play an important role in the structural characterization of substances as well as in various mechanistic studies. However, due to the high cost of the labelled compounds, even experiments with heaw water are not commonlv ~erformedin underaaduate laboratories. We propose a very simple and inexpensive nrocedure for deuteratine crvstalohvdrates that can be im" plemented either as an inorganic or spectroscopic experiment.

salt and the gaseous heavy water. For this purpose, the heavy water can be added to a small bottle (1-2 mL) inside the weighing vessel. We recommend that you leave the reaction vessel without heating overnight.

The Vapor-Phase Exchange Reaction The usual method of obtaining deuterated crystalohydrates is repeated recrystalization of the protonated analogue from heavy water. Even in the best case, when the compound is readily recrystalized, at least several milliliters of D20 are consumed. Amore rational approach proposed by Eriksson et al. (I, 2),is the vapor-phase exchange reaction of the water molecules with the solid dehydrated salt.

WW4.HzO good results were obtained with, for example, the following crystalohydrates: Ba(C10,)z.H20 CuS0,.5Hz0 NH4Fe(S04)2.7H20 Good results were also obtained with some complex crystalohydrates with amino acids. Examples include

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crystalohydrate(s)+ dehydrated salffs) + nHzO(g) dehydrated salt(s) + nDzO(g)+ crystalodeuterate(s)

Discussion Assignments

In class, it is useful to assign a salt to each pair of students. Besides the

The greatest advantage of this approach is the very high degree of deuteratiou achieved despite using little more than the stoichiometric amount of heavy water. Experimental In our experiments all compounds were commercially available, analytical-grade, substances that were used without further purification. The degree of isotopic enrichment of the original D20 was 99.9%. Due to the sensitivity of the dehydrated salts, the experiments were carried out in an AtmosBag (Aldrich)in a dry nitrogen atmosphere, in which the relative humidity could be lowered to less than 25%.' The IR spectra were scanned in hexachlorobutadiene between cesium bromide plates. Any IR spectrophotometer will work. In a tvoical ex~eriment70 me of wwdered ~otassium oxalate monohydrate was heated at least 1 h in a weighing vessel at 100 'C. The dehvdrated salt was weiehed. and the mass of the water loss was compared to the-theoretically calculated value. In this case, the water loss was 7 mg, as expected. The presence of water can also be followed by IR spectroscopy to confirm the results obtained by gravimetric methods. In the next step, the amount of heavy water was 7 mg increased by 10-15%. This was added to the same closed vessel as the dehydrated salt. It is important to keep water from the substance to be studied so that the reaction occurs between the solid dry

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Presented in part at the Sixth Yugoslav Natural Sciences Student Meeting, Tvat. 1991. 'The dry bag can be easily constructed from a transparent plastic bag, plastic gloves, and tape.

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wavenumber 1 cm-' Room temperature infrared spectra in the OHIOD stretching region of (b)K2C204;and (c) K2C204.D20. (a) K2C204.H20; Volume 70 Number 7 July 1993

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The students are first asked to consult the literature data for the dehydration temperatureofthe given salt, and then to determine its validity experimentally. The reversibility of the reaction can be tested with plain water first. Then, only if the result is positive, heavy water can be used. The amount of salt used for the experiment was determined from the crystal water content in the salt. A 10-pL syringe was used, so it was suitable to start with an amount of the crystalohydrate that contained &8 mg of crystal water. Although this amount can be scaled up, for the purpose of this experiment it was more than enough. IR Spectra

In the absence of an integrator or computer-controlled IR spedrophotometer, the areas can be determined by count.ing the squares under the bands or by the cutting and weighing method. In the case of deuterated potassium oxalate monohydrate the calculated value was very high, around 90%. ~~The extent of exchange depends on the nature of the salt, but other factors play an important role. Some salts can be dehydrated without difficulty, but they require very careful manipulation because the reversible reaction is so fast that some hydration, even in the dry box, is possible. Also, D90. which is hiehlv can be another source of . hvmosco~ic, -w%r in the fin2 product (51.~

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Advanced Spectroscopy Courses

The discussion of IR spectra can be conducted a t different degrees of sophistication, depending on the material to be stressed. Simple comparisons of the spectra in the 3500-2300 ern-' region show marked differences.

For more-advanced spectroscopy courses other structural considerations can be discussed:

Calculating the Isotopic Ratio For example, the infrared spectrum of the potassium oxalate monohydrate shows a very strong and broad band around 3250 em-' due to the v(OH) stretching vibration from the water molecules (Fig. la). On dehydration this band disappears, which is another indication of the absence of crystal water (Fig. lb). On deuteration this band is shifted to the region around 2450 cm-' (Fig. lc). The calculated isotopic ratio

These can be correlated with data obtained from X-ray and neutron diffractionmeasurements. For such purposes it is necessary to scan the spectra a t liquid nitrogen temperature and to prepare samples with different deuterium content.

is a typical value for the crystal water vibrations.

The Extent of Isotope Exchange The extent of isotope exchange, that is, the deuterated fraction, can be calculated from the absorption spectra using the following formula:'

hydrogen bond strength symmetry of the water molecules the anhamonieity of the vibrations

Conclusion An obvious advantage of this method is the negligible amount of heavy water used: 1mL is enough for almost a hundred exneriments. This iustifies the use of an even more expekive isotopically modified water: T20, H2I80, etc. Another important consideration is the extent of exchange. With recrystalization procedures, degrees of deuteration of 70% are considered verv high. hut this method yields products with greater than 90%exchange. The drawback. however. is that the method is not applicable to all salts: Some crystalohydrates decompose heating, while others exist in different types of hydrates.

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Literature Cited

where A(H) andA(D) are the areas of the v(0H) and v(OD) hands, respectively. %ome authors use 2 as the weighting factor instead of p2 (4.

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Journal of Chemical Education

4. Fifer, R. A.; ScWer, J. J C h m . Phys. 1910,52(5),266G2670 5. Harp. W R.;Eifiert, R. C.Anol. Chpm. lPB0,3217),79P796.