Drastic Dependence of the pH Sensitivity of Fe2O3-Bi2O3-B2O3 Hydrophobic Glasses with Composition

Fe2O3-Bi2O3-B2O3 (FeBiB) glasses were developed as novel pH responsive hydrophobic glasses. The influence of the glass composition on the pH sensitivity of FeBiB glasses was investigated. The pH sensitivity drastically decreased with decreasing B2O3 content. A moderate amount of Fe2O3 and a small amount of B2O3 respectively produces bulk electronic conduction and a pH response on glass surfaces. Because the remaining components of the glass can be selected freely, this discovery could prove very useful in developing novel pH glass electrodes that are self-cleaning and resist fouling.


Introduction
Important customer issues in pH measurement are a decrease in pH sensitivity and an increase in pH response time, which mainly arise from the fouling due to contamination of the responsive glass membrane and liquid junction, and from the change in concentration of the internal liquid [1]. To avoid these issues, customers have to maintain their pH glass electrodes. This is troublesome, especially in industrial uses, because it is not easy to remove the accumulated stain from pH glass electrodes. For this reason, we have developed novel pH glass electrodes, such as TiO 2 -P 2 O 5 (TP) glasses [2][3][4][5], with a self-cleaning property based on photocatalytic activity and photo-induced hydrophilicity [6]. TP glasses with low electrical resistivity gave a high pH sensitivity and short pH response time.
On the other hand, we have also reported that Bi 2 O 3 -B 2 O 3 (BiB) glasses show hydrophobicity (contact angle of 90 ¥ ) so far [7]. Materials with hydrophobicity have been used for anti-fouling and anti-fogging [8]. Glasses with hydrophobicity such as BiB glasses may be candidates of pH glass electrodes with an anti-fouling property based on their hydrophobicity, because pH measurement is basically carried out for aqueous solutions. The anti-fouling effect based on hydrophobicity may become remarkable, when pH of products and waste is monitored in fluid system as in industrial uses. Electric resistivity lower than 10 10 Ω¤cm, which is a representative value for commercially available pH glass electrode, is desirable in practical use. However, BiB glasses are undesirable for pH glass electrodes because of the relatively high electric resistivity (>10 11 Ω¤cm) [9]. It is well known that addition of transition metal oxides into glass composition causes the electronic conduction to the Potentiometric measurements for the 20FeyBiB glasses was carried out at 25 ¥ C, at time intervals of 3 s and 0.5 s using a pH meter F-73 (HORIBA, Ltd., Kyoto, Japan) and a portable multi logger ZR-RX20 (OMRON Corp., Kyoto, Japan) equipped with a handmade cell with a glass membrane of 1mm thickness, respectively. The details of the characterization of the pH responsivity (pH sensitivity and pH response time) were described in [8].
The direct current (DC) electrical resistivity of 20FeyBiB glasses with~1 mm thickness and an Ag electrode of 6 mm ϕ on both sides was measured at 25 ¥ C using a super megohm meter SM-8215 (HIOKI E.E. Corp., Ueda, Japan). The contact angle for~2 µL of water on 20FeyBiB glasses was measured at 25 ¥ C using a mobile contact angle meter PG-3 (Matsubo Corp., Tokyo, Japan) as a measure of hydrophobicity. The density was measured at 25 ¥ C by Archimedes' method in order to conveniently estimate the distance between Fe ions linked by oxygen ions [11,12]. Figure 1 indicates the change in potential with measurement time for 20FeyBiB glasses in pH7, pH4 and pH9 buffer solutions. When the Bi 2 O 3 content increases to 79.7 mol %, the B 2 O 3 content decreases to 0.3 mol %, the change in potential related to pH sensitivity drastically decreased. The dependence of the pH4-9 sensitivity (left axis) between pH4 and pH9, and the pH4-7/pH7-9 sensitivity ratio (right axis) on the Bi 2 O 3 and B 2 O 3 contents for the 20FeyBiB glasses is shown in Figure 2. It can be seen from the left axis of Figure 2 that the pH4-9 sensitivity decreases to almost zero when the Bi 2 O 3 content increases to 80 mol %. At this time, the pH4-7/pH7-9 ratio decreases with increasing Bi 2 O 3 content. This corresponds to the decrease in H + ion-selectivity for acid solutions (right axis of Figure 2).  Dependence of pH4-9 sensitivity (left axis) between pH4 and pH9, and pH4-7/pH7-9 sensitivity ratio (right axis) on Bi2O3 and B2O3 contents for 20FeyBiB glasses. Figure 3 presents the relationship between the pH4-9 sensitivity and DC electrical resistivity for the 20FeyBiB glasses. The pH4-9 sensitivity decreased with increases in the DC electrical resistivity. In our previous work [10], the pH4-9 sensitivity of xFeyBiB glasses was not so strongly affected by the glass compositions. However, a shortage of B2O3 seems to affect the pH4-9 sensitivity strongly in the present case. This suggests that a small amount of the B2O3 component (B-OH sites) may play an important role in the pH4-9 sensitivity. The reason for the decrease in pH4-9 sensitivity is complicated, because it decreases both with decreasing B2O3 content and with increasing DC electrical resistivity. Moreover, it should be noted that the decrease in pH4-7/pH7-9 ratio as a measure of H + ion-selectivity for acid solutions is observed as the pH4-9 sensitivity decreases. Therefore, the pH4-9 sensitivity may be related to the specific glass compositions, such as the amount of B2O3, rather than the DC electrical resistivity.  Dependence of pH4-9 sensitivity (left axis) between pH4 and pH9, and pH4-7/pH7-9 sensitivity ratio (right axis) on Bi2O3 and B2O3 contents for 20FeyBiB glasses. Figure 3 presents the relationship between the pH4-9 sensitivity and DC electrical resistivity for the 20FeyBiB glasses. The pH4-9 sensitivity decreased with increases in the DC electrical resistivity. In our previous work [10], the pH4-9 sensitivity of xFeyBiB glasses was not so strongly affected by the glass compositions. However, a shortage of B2O3 seems to affect the pH4-9 sensitivity strongly in the present case. This suggests that a small amount of the B2O3 component (B-OH sites) may play an important role in the pH4-9 sensitivity. The reason for the decrease in pH4-9 sensitivity is complicated, because it decreases both with decreasing B2O3 content and with increasing DC electrical resistivity. Moreover, it should be noted that the decrease in pH4-7/pH7-9 ratio as a measure of H + ion-selectivity for acid solutions is observed as the pH4-9 sensitivity decreases. Therefore, the pH4-9 sensitivity may be related to the specific glass compositions, such as the amount of B2O3, rather than the DC electrical resistivity.  Figure 3 presents the relationship between the pH4-9 sensitivity and DC electrical resistivity for the 20FeyBiB glasses. The pH4-9 sensitivity decreased with increases in the DC electrical resistivity. In our previous work [10], the pH4-9 sensitivity of xFeyBiB glasses was not so strongly affected by the glass compositions. However, a shortage of B 2 O 3 seems to affect the pH4-9 sensitivity strongly in the present case. This suggests that a small amount of the B 2 O 3 component (B-OH sites) may play an important role in the pH4-9 sensitivity. The reason for the decrease in pH4-9 sensitivity is complicated, because it decreases both with decreasing B 2 O 3 content and with increasing DC electrical resistivity. Moreover, it should be noted that the decrease in pH4-7/pH7-9 ratio as a measure of H + ion-selectivity for acid solutions is observed as the pH4-9 sensitivity decreases. Therefore, the pH4-9 sensitivity may be related to the specific glass compositions, such as the amount of B 2 O 3 , rather than the DC electrical resistivity.      Table 1 lists the DC electrical resistivity, pH4-9 sensitivity between pH4 and pH9, pH response time and contact angles of xFeyBiB and related glasses [10] along with TP glass and a reference glass (HORIBA, Ltd., Kyoto, Japan). The DC electrical resistivity of 20FeyBiB glasses changed from 9.67 ¢ 10 7 Ω¤cm for 20Fe20BiB glass to 3.10 ¢ 10 10 Ω¤cm for 20Fe80Bi glass (the second column in Table 1). Figure 4 shows the dependence of the DC electrical resistivity on the Fe-Fe distance for the 20FeyBiB glasses. It is seen from Figure 4 that the DC electrical resistivity increases with increasing Fe-Fe distance (increasing Bi 2 O 3 content). This is because electron hopping of from Fe 2+ to Fe 3+ via O 2 ion becomes more difficult. This result is consistent with the data in [9,10].  We have previously reported that TP glasses with low DC electrical resistivity show a high pH4-9 sensitivity and short pH response time [6]. In the present work, the third column of Table 1 shows that xFeyBiB glasses with a low DC electrical resistivity did not always show a high pH4-9 sensitivity. This suggests that the pH responsive sites (B-OH and or Bi-OH) may differ from the conductive sites (Fe-OH) in xFeyBiB glasses, whereas the pH responsive and conductive sites are both Ti-OH in TP glasses. Taking the results of Figures 2 and 3 into consideration, the most important pH responsive sites are B-OH in xFeyBiB glasses.

Results and Discussion
On the other hand, the pH response time of 20FeyBiB glasses ( Table 1, column 4) tended to increase from 10 to 20 s on increasing the Bi2O3 content from 50 to 80 mol % (decreasing B2O3 content). We conclude that the pH response time of xFeyBiB glasses is mainly determined by both (a) the dissociation rate of pH responsive sites, such as B-OH, at glass surfaces, and (b) the conduction rate of carriers (e) though the bulk glass. The latter is the predominant rate-determining process for the pH response of xFeyBiB glasses, as well as in TP glasses [6]. However, the 20Fe20BiB glass with the lowest DC electrical resistivity did not show the shortest pH response time among xFeyBiB glasses. This may suggest that the rate-determining process changes from a conduction process (bulk) to a pH response process (surface) with decreasing DC electrical resistivity in xFeyBiB glasses.
Thus, a moderate amount of Fe2O3 and a small amount of B2O3 results in electronic conduction We have previously reported that TP glasses with low DC electrical resistivity show a high pH4-9 sensitivity and short pH response time [6]. In the present work, the third column of Table 1 shows that xFeyBiB glasses with a low DC electrical resistivity did not always show a high pH4-9 sensitivity. This suggests that the pH responsive sites (B-OH and or Bi-OH) may differ from the conductive sites (Fe-OH) in xFeyBiB glasses, whereas the pH responsive and conductive sites are both Ti-OH in TP glasses. Taking the results of Figures 2 and 3 into consideration, the most important pH responsive sites are B-OH in xFeyBiB glasses.
On the other hand, the pH response time of 20FeyBiB glasses (Table 1, column 4) tended to increase from 10 to 20 s on increasing the Bi 2 O 3 content from 50 to 80 mol % (decreasing B 2 O 3 content). We conclude that the pH response time of xFeyBiB glasses is mainly determined by both (a) the dissociation rate of pH responsive sites, such as B-OH, at glass surfaces, and (b) the conduction rate of carriers (e) though the bulk glass. The latter is the predominant rate-determining process for the pH response of xFeyBiB glasses, as well as in TP glasses [6]. However, the 20Fe20BiB glass with the lowest DC electrical resistivity did not show the shortest pH response time among xFeyBiB glasses. This may suggest that the rate-determining process changes from a conduction process (bulk) to a pH response process (surface) with decreasing DC electrical resistivity in xFeyBiB glasses.
Thus, a moderate amount of Fe 2 O 3 and a small amount of B 2 O 3 results in electronic conduction through the bulk glass and a pH response on glass surfaces, respectively. Because the remaining components can be selected freely, this result is a very useful in order to develop novel pH glass electrodes with functionalities such as self-cleaning [6], an anti-fouling ability [10], and so on.
So far, xFeyBiB glasses with a Bi 2 O 3 content larger than 60 mol % have shown contact angles higher than 90 ¥ [10]. Moreover, the fifth column in Table 1 reveals that a B 2 O 3 concentration larger than 10 mol % is necessary for hydrophobicity. Thus, the present results suggest that using B 2 O 3 as a glass former may play an important role in hydrophobicity as well as in pH4-9 sensitivity. Based on our results [7], Fe 2 O 3 -ZnO-B 2 O 3 and Fe 2 O 3 -ZnO-Bi 2 O 3 -B 2 O 3 glasses are candidate for pH glass electrodes with an anti-fouling property based on their hydrophobicity.

Conclusions
In the present study, the influence of Bi 2 O 3 (or B 2 O 3 ) content on the pH responsivity, electrical resistivity, and hydrophobicity was investigated for 20FeyBiB glasses in order to reveal the role of each glass component. The following results were obtained.
The pH4-9 sensitivity drastically decreased with increasing the DC electrical resistivity or with decreasing the B 2 O 3 content. In this case, a decrease in H + ion-selectivity for acid solutions was also observed.
The 20FeyBiB glasses showed contact angles higher than 90 ¥ when they contain both more than 60 mol % Bi 2 O 3 and more than 10 mol % B 2 O 3 . These results suggest that the use of B 2 O 3 as a glass former plays an important role in pH4-9 sensitivity as well as in hydrophobicity.
A moderate amount of Fe 2 O 3 and a small amount of B 2 O 3 respectively produces bulk electronic conduction and a pH response on glass surfaces. Because the remaining components of the glass can be selected freely, this discovery could prove very useful in developing novel pH glass electrodes that are self-cleaning and resist fouling.