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USING WATER AS AN INFRARED SOLVENT* HELENE STERNGLANZ The Perkin-Elmer Corporation, Norwalk, Connecticut To many infrared spectroscopists it may sound foolish to talk about using water as an infrared solvent. How- ever, as early as 1949, R. C. Gore (1) described the very excellent results he obtained when using water and deu- terium oxide as infrared solvents. Since then, little atten- tion has been paid to this fact. Some spectroscopists, con- fronted with an infrared analysis of a water solution, will say: Impossible! Water dissolves the rock salt cell windows and is so strongly absorbing in the region from 2/~ to 15~ that it obscures most of the absorption bands of any ma- terial dissolved in it. In spite of this, an attempt will be made to indicate the useful information that can be ob- tained from water solution spectra. Great progress has been made in the manufacture of optical crystals that resist water. Today there are at least eleven window materials available that can be used with water solutions. Some are better than others and their re- spective advantages and disadvantages will be mentioned. The following figures show the true transmittance of single windows. These are chosen, rather than empty cells, be- cause interference fringes of thin cells would distort the true picture, and the reflection loss of an empty cell would be greater than that of a cell filled with a water solution, where in most cases the reflection loss of the two inner window surfaces is eliminated. Fig. 1 shows glass, fused quartz, crystal quartz and sap- phire. Glass and crystal quartz are excellent to about 2.6~, fused quartz to nearly 4/~ and sapphire to almost 6~. These materials have the advantage that they can be fused to pyrex cell bodies and can be filled with acids, without fear of corrosion of spacers. Lithium fluoride and calcium fluoride cover a wider wavelength range, LiF to 7/z and CaF2 to 10~ (Fig. 2). Arsenic trisulfide has the disadvantage of a high re- flection loss. It can be used to 12½~ and barium fluoride to 13/~ (Fig. 3). Germanium and thallium bromide-iodide (KRS 5) transmit infrared radiation even further; germanium to 21~ and TI(Br,I) to about 35~ (Fig. 4). It should be mentioned here that large crystals of germanium are readily available today. In this laboratory, germanium was found very useful when a thin layer, less than one micron thick, was evaporated on a rocksalt window. Films from water solutions could be deposited on these windows and their spectra recorded. Silver chloride was not available in this laboratory but it too has been used successfully for work with water solutions, although it has several disadvantages. It is very soft; cells do not keep their original thickness; they have to be protected from ultraviolet and visible light; and the reflection loss is nearly as great as that of TI(Br,I). However, AgC1 transmits infrared radiation up to 25~. *Presented before the Society for Applied Spectroscopy m New York, The most useful window materials are probably sap- phire, barium fluoride and Tl(Br,I) in the different wave- length regions. Spectroscopists of the American Cyanamid Company have pointed out that it is quite possible to use rock salt windows for water-solution work, especially when the sample is dissolved in a concentrated sodium chloride solution recognizing that some repolishing of the windows may be required. Over the years spectroscopists have not been able to change the fact that water is a strong absorber in the infrared. Fig. 5 shows water spectra of 0.017 mm. and 0.057 mm. thickness. 0.05 ram. is the thickest cell one can possibly use in the 7-11~ region without losing most of the instrumental sensitivity. However, since one can usual- ly prepare concentrated water solutions, spectra of suffi- cient intensity can be recorded even in thin cells. Of course, even in a double beam spectrophotometer, when compensating with water in the reference beam, one loses all information about the sample in the 3~ and 6~ region where water is totally absorbing. Here additional reforma- tion about the sample can be gained by recording the spectrum also in D20, where the strong 3~ OH absorp- tion is shifted to the 4/z OD band and the 6/x band to 8.25/z. The question might arise, why should we try to work with water solutions, now that we have the potassium bromide pressing technique? There is definitely room for both. Fig. 6 shows 100% glacial acetic acid and a 10% water solution (compensated). There are several bands available which can be used for quantitative analysis.- Anyone who has handled any sulfonates, used so extensive- ly in detergents, knows that it is extremely difficult to obtain a homogeneous mixture of this waxy material with KBr. Here is a case where water solution work is pref- erable. (Fig. 7). Figs. 8, 9, and 10 show samples recorded as water solu- tions and as KBr disks. Notice the large differences be- tween the spectra of ascorbic acid in the liquid and solid phases! They are just as striking in sodium citrate and in sodium phosphate. Very interesting studies on crys- tallinity are possible here. Although even today any spectroscopist prefers to per- form his analysis with carbon disulfide and carbon tetra- chloride solutions, or by pressing potassium bromide disks, we should not discard water solutions as impossible to work with in the infrared, without first making an attempt to run their spectra in a thin cell equipped with appropriate windows. Literature Cited (1) R. C. Gore, R. B. Barnes, and E. Peterson, Anal. Chem. 2I, 382 (1949). February 7, 1956. NUMBER 2, 1956 77
