**4.2.2 Window slice for testing aqueous solutions**

The most commonly used Window slice materials for IR spectroscopy of aqueous solution samples is BaF2 wafer, followed by CaF2 wafer. Though BaF2 wafer is non-dissolvable in water, it can be dissolved in acid and ammonium chloride and can react with phosphate and sulfate to generate barium phosphate or barium sulfate respectively, thus erode the surface of the wafer. When testing metal salt solution, the metal ion will exchange with barium ion and thus erode wafer surface.
