Quantification of H2O contents in silicate glasses using IR spectrocopy - a calibration based on hydrous glasses analyzed ba Karl-Fischer titration

Abstract

Water-related IR absorption bands were investigated in hydrous soda-lime-silica glass (SLS) and float glass (FG). Glasses containing between 0.5 and 6 wt% H2O (corresponding to 0.7 and 8 mol/1) were synthesized in platinum capsules by re-melting glass powder to which water had been added. Synthesis were perfomed in an internally heated gas pressure vessel at temperatures of 1200 to 1250 °C and pressures 100 to 500 MPa. The total water content (Cwater) of glasses was analyzed by pyrolysis and subsequent Karl- Fischer titration. Five bands at 3550, 2850, 2350, 1730 and 1635 cm-1 in the IR spectrum of an SLS glass containing 4 wt% H2O were identified as vibration modes of water-related species by comparison with a D2O-bearing glass. With increasing Cwater the absorbance band at 3550 cm-1 (OH stretch of weakly H-bonded hydrous species) grows in intensity relative to the band at 2850 cm-1 (OH Stretch of strongly H-bonded hydrous species). As a consequence, the practical absorption coefficient for the band at 3550 cm-1 (ε3550pract, defined by assigning the total water to this band) is 20 % lower at 0.1 wt% water than at 4 wt% water. The dependence of ε3550pract on Cwater is most pronounced above 0.5 wt% but appears to be present also in water-poor glasses. In contrast, ε2850pract does not noticeably depend on water content (FG: ε2850pract = (40.2 ± 2.4) l∙mol-1∙cm-1; SLS: ε2850pract = (50.8 ± 2.0) l∙mol-1∙cm-1) and, hence, it is preferred for water determination in float glass and soda-lime-silica glass. Scholze's two-band method [1] using the peak height of both bands at 3550 and 2850 cm-1 to quantify Cwater in glasses is a suitable first approach also for hydrous glasses, predicting the water content within 20 % relative. However, for a more precise determination the dependence of the ε-values on Cwater has to be considered. A preliminary evaluation of the H2O bending vibration band at 1635 cm-1 shows that molecular H2O is a minor hydrous species in FG and SLS glasses with a maximum concentration of 2.0 wt% in FG glass containing 5.7 wt% total water. On the basis of the new spectroscopic results and data from literature the assignment of the so-called ABC triplet (bands 2850, 2350 and 1750 cm-1) is discussed.