Advances in pH Measurement in High-Temperature Biotech Processes

By CD Feng, Emerson Process Management, Rosemount Analytical Liquid 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. All the glass electrodes went through steam sterilization cycles of 30 minutes at 130º C. After each steam sterilization cycle, the dc resistance of the glass electrode was measured at ambient temperature. The A group of pH glass electrodes changed the least among all the electrodes tested, indicating that the new high-temperature formulation ages slower than the other pH glasses tested. One of the main symptoms of aged glass is sluggish response. To verify the finding, the pH response curve of the B-type pH sensor and the A high-temperature sensor were measured after a 50-cycle steam sterilization process. Figure 4 shows the measured pH response curves of the sensors. After 50 cycles of steam sterilization, the B-type sensors showed typical slow response to pH changes, while A showed a response speed almost equivalent to non-aged pH sensors. The 95 percent response time of the A-type sensors after 50 cycles is about 10 seconds, while that of the B-type sensors is more than one minute. Reference Junction Potential of New Steam-sterilizable pH Sensors The reference system of a pH sensor is as important as the pH glass electrode where the stability of the sensor output is concerned. Almost all pH sensor manufacturers use Ag/ AgCl electrodes as the reference, because they are well known for their stability. However, the most uncertain component in a reference system is the reference junction, which separates the reference fill solution