Model 615-26

615-26 Ultra-Pure Water pH Measurement

Close spacing between sensor components sharply reduces high impedance and ion effects for stable pH readings. Small internals allow low-flow with fast pH response. Constant atmospheric head establishes steady pH sensor flow even as process pressure varies. Low ionic strength pH buffers minimize junction potential effects on calibration and speed return to on line sample pH.

Benefits

• Stainless steel total-surround shielding defeats antenna effect
• Large concentric reference assures stable ionic contact and stable readings
• Cell is driven by analyzer to produce electron-rich environment for stable readings to 18 megohm.cm
• Design passes any CO2 bubbles
• Handles 100 °C at up to 100 psi
• High-purity temperature compensation with the IC Controls 210-P analyzer

Ultra-Pure-Water pH Measurement

Due to the lack of conductive ions in Ultra high purity water, a number of interferences occur. These interferences show up as a drifting and spiked pH measurement that deviate from the correct value. Without ions providing a conductive circuit, the pH sensor, coupled to a modern ultra-high-impedance preamp, acts as a high-gain, high-efficiency antenna. Chloride ions escaping from the reference cell tend to partly short the antenna effect, causing the pH sensor to try and establish a new balance. When any changes in sample flow occur, the rate at which these ions are swept away is upset, resulting in a compensating adjustment in the pH sensor. This is often described as sweeping away the “ion cloud”. A related effect is the junction potential at the reference junction which is also influenced by the number of ions and the rate of sweeping away. As sample flow increases, the actual conductivity produced by the ions drops due to fewer ions left in the junction area. The junction, like a thermocouple, puts out a mV relative to the two dissimilar conductors it is composed of. This, of course, changes if one of the conductors (ion concentration) is changing. In calibration pH Buffers exhibit typically about 400,000 microsiemens/cm causing huge mV effects that often take hours to days to dissipate as the Ultra-pure sample is returned to the pH sensor. The 615-26 has IC Controls state of the art solutions to these problems incorporated to provide much faster response and more stable pH readings.