J. Leland Hollenberg University of Redlands Redlands, California 92373
Determination of Hydration Numbers by Near Infrared An advanced undergraduate experiment
Recently K a h n a n d Stephens1 described a method using n m r which is suitable for student determinations of hydration numhers of inorganic salts. T h e basis of t h e improvement they descrihe over previous n m r methods is based on t h e addition of a n "inert salt," calcium nitrate. Possible hindrances to t h e u s e of t h i s m e t h o d a r e t h a t it requires t h e use of variable low t e m p e r a t u r e e q u i p m e n t a n d gives hydration numbers of nearly frozen solutions. An alternative method of determining hydration numbers, which may he more convenient for certain purposes or in some laboratories, has heen developed by Rnnner and Woolsey2. Their approach is hased on studies of differential near infrared spectra over the 600-1800 nm region of aqueous sohttions compared to pure water. They found several ahsorption hands which could be assigned to vibrational overtones and combinations of water molecules, and in single beam spectra these hands were seen to shift toward shorter wavelengths as the temperature of pure water was increased or the concentration of solute decreased. By contrast, when douhle beam spectra were run, no wavelength shifts due to temperature or concentration changes were observed. The single beam shifts were shown to be due to two overlapping water absorptions, one helonging to hydrogen-bonded water (about 94% of the water molecules a t 25°C) and the other to monomeric water (fi%). Ronner and Woolseychose the 2.1 +
