pH is one of the most important parameters of cell-culture and fermentation processing in the biotech industry. Yet pH measurement in various industrial processes still heavily relies upon century-old pH glass electrode technology, due to its reliable performance and robustness.
However, with growing demand for increased accuracy of pH measurement and operational life of sensors, new technologies are emerging—especially regarding sensor durability in high-temperature steam-sterilization processes.
This article presents recent research on the aging mechanism of the pH-sensing glass after hightemperature exposure by using the non-destructive complex impedance spectrum method. The aging behavior of new high-temperature glass formulations in steam-sterilizable sensors, as well as the conductivity dependence of alumina-based reference junction potential, are discussed in comparison with standard steam-sterilizable pH sensor formulations. The article also introduces a more robust, built-in temperature sensor design for steam-sterilizable pH sensors.
Glass Aging in a High-Temperature Environment
All users know that pH glass electrodes age much faster in high-temperature environments. A typical symptom of an aged pH-sensing glass is its sluggish response toward pH changes. Understanding the aging mechanism is important in slowing the aging process of a pH-sensing glass. In this study, the complex impedance analysis method is used.
The method is non-destructive and has been widely used in studying electrochemical processes of an electrode. Figure 1 shows a typical complex impedance spectrum of a pH glass electrode with the real component of the impedance assigned to the abscissa and the imaginary component to the ordinate. At a high-frequency region, a semicircle is observed; and at a low frequency region, a declined line resembling a constant phase element is observed.
Figure 1. Impedance spectrum of a pH-sensing glass with the frequency range of 1 KHz to 0.01 Hz and 100 mV excitation
Analysis and simulation of the impedance spectrum yield an equivalent circuit of the pH-sensing glass as shown in Figure 2, where C is the capacitance formed by the pH glass membrane as the dielectric; R1 is the electric resistance of the bulk layer of the pH glass membrane; R2 is the electric resistance of the gel layer; and Q is the constant phase element of the gel layer representing the transportation properties of ions through the gel layer, which are frequency dependent.
Figure 2. Equivalent circuit of the pH-sensing glass
By comparing the equivalent circuit in Figure 2 and the impedance spectrum in Figure 1, it is clear that C and R1, which are reflected by the semicircle at the high-frequency region, represent the properties of the bulk body of the sensing glass; and R2 and Q, which are reflected by the declined line at the low-frequency region, represent the properties of the surface gel layer of the sensing glass. Since the complex impedance analysis is a nondestructive method, the same pH glass electrode was exposed to an aging process at 110º C for seven days.
Figure 3. Complex impedance spectra of a glass electrode before and after an aging process at 110º C for seven days
Figure 3 compares the impedance spectra of the glass before and after the aging process. From Figure 3, it is clear that the main change of the glass after the aging process is with the declined line at the low frequency region, which means that the aging process has an impact mostly on the surface gel layer of the sensing glass. The value of all the components in the equivalent circuit can be obtained through simulation of the impedance spectra.
Table 1. Simulation value of the equivalent circuit of a glass electrode before and after the aging process
Table 1 summarizes the value of all the components in the equivalent circuit before and after the aging process. As shown in Table 1, R2, the surface gel layer resistance, had the most impact after the aging process. Further understanding of the gel layer resistance is the key to improving the durability of the pH-sensing glass against aging in a high-temperature condition.
Steam Sterilization Cycle Test
As shown above, the increase in gel layer resistance of a pH-sensing glass R1 is responsible for its aging. Since both Q and C are frequency dependent, a dc resistance measurement will reflect the change of R1, indicating the aging process of the measured pH-sensing glass. To compare the aging process, three groups (A, B and C) of pH glass electrodes were tested.
The A electrodes were made from a new high-temperature glass formulation based upon the research showing that aging occurs primarily in the gel layer of the sensor. The new formulations are designed to achieve exceptional thermal resistance at temperatures as high as 145º C. B and C sensors were supplied by manufacturers of existing steam sterilizable equipment.