Fe2O3-Bi2O3-B2O3 glasses as lithium-free nonsilicate pH responsive glasses – Compatibility between pH responsivity and hydrophobicity

• Fe2O3-rich FeBiB glasses show high pH sensitivity and short pH response time.
• Bi2O3-rich FeBiB glasses show relatively high contact angle for water.
• FeBiB glasses are lithium-free nonsilicate pH responsive ones.
• pH responsivity and hydrophobicity are obtained for optimum glass compositions.

Lithium silicate-based glasses have widely been used as commercially available pH glass electrodes. It was revealed that Ti3+-containing titanophosphate (TiO2-P2O5, TP) glasses are pH responsive as lithium-free nonsilicate glasses for the first time. TP glasses with the compatibility between pH responsivity and self-cleaning property were obtained by the sequential post-annealing (oxidation and reduction) of as-prepared glasses. Bi2O3-B2O3 (BiB) glasses are relatively hydrophobic and are expected to show anti-fouling effect. They are unsuitable for pH responsive glasses, because they have high electrical resistivity. In the present study, xFe2O3·yBi2O3·(100 − x − y)B2O3 glasses (xFeyBiB, x = 0–20 mol%, y = 20–80 mol%) glasses were selected as new pH responsive glasses with hydrophobicity, because Fe2O3 is a representative component for causing hopping conduction to the glasses. BiB glass did not show pH responsivity, whereas xFeyBiB glasses showed good pH responsivity. xFeyBiB glasses are lithium-free nonsilicate pH responsive ones as well as TP glasses. The electrical resistivity and pH response time decreased with increasing Fe2O3 content. The pH repeatability for standard solutions increased with increasing Bi2O3 content. Silicate glass (20Fe70BiSi) showed better pH responsivity but lower contact angle than those of borate glass (20Fe70BiB). pH sensitivity increased in order of TP glasses (about 80%), xFeyBiB glasses (about 90%) and commercial pH responsive glass (about 100%). xFeyBiB glasses showed short pH response time compared to commercial pH responsive glass. The contact angle for water of xFeyBiB glasses was relatively high (about 90°) as well as BiB glasses, and increased slightly with increasing Bi2O3 content regardless Fe2O3 content. The high contact angle was related to low OH content determined by FT-IR measurement. Thus, 20Fe2O3·70Bi2O3·10B2O3 glass was the most suitable one as pH responsive glasses with hydrophobicity. TP glasses are pH responsive ones with self-cleaning property, whereas xFeyBiB glasses are expected as pH responsive ones with anti-fouling property based on hydrophobicity.

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