The influence of moisture on potassium bromide disks used in infrared spectrometry

Abstract

When using the alkali-halide disk technique in infrared spectrometry we noticed that moisture from the atmosphere can penetrate into a KBr disk, causing recrystallization of the KBr. A critical relative humidity was observed above which moisture is taken up. This value depends on the fineness and structure of the KBr; for optical grade, handground KBr (Merck) it is about 70 per cent. “Wet” disks lose water if stored in an atmosphere of a lower humidity; replaced in the original atmosphere, they take up water again but to a lower extent.

In order to study the water sensitivity of KBr disks containing a sample, KSCN was used. K8CN has two CN stretching bands dependent on the form in which it occurs: as a solution at 4·84 μ and in the solid phase at 4·87 μ. The latter can be converted into the former by water uptake of the disk; on drying of the disk the band is shifted back to its original wavelength.

In the disk are small cracks through which condensed moisture and dissolved sample can be transported. This is demonstrated by the behaviour of KSCN. If a KBr disk, containing a single KSCN crystal completely surrounded by carrier material, is exposed to a moist atmosphere the KSCN is transported through the whole disk.

The same transport, due to the presence of water in a disk, was observed with a number of similar inorganic salts. Sometimes the accompanying bandshifts could be eliminated by drying, sometimes they persisted, probably indicating that a solid solution has been formed. In the absence of water none of these phenomena were observed. Interaction between sample and water can be demonstrated also with D2O. In a moist D2O atmosphere active H atoms of sample molecules in a disk are substituted by D atoms. This phenomenon can be applied successfully in structure determination.