Calculated pH Based on Conductivity

This white paper discusses how power plants can derive a calculated pH value and a calculated CO2 value from measurements of specific,cation and degassed cation conductivity. These important calculated values can be transmitted via a multi-parameter transmitter for managing and validating a power plant's water cycle chemistry measurements.

The white paper covers both the underlying scientific principles that allow for the generation of a calculated pH value and a calculated CO2 value, based on the three measured conductivity values, and then explains how this is achieved quickly and directly in a multi-parameter transmitter.

The concept and practice of generating a calculated pH value, calculated carbon dioxide value and calculated anion concentrations from conductivity measurements has been available for a number of years, but in the past it required separate computer diagnostic systems. As analytical technology has continued to advance, multi-parameter transmitters offer a mix-and-match flexibility to power plants to analyze up to 11 parameters for monitoring needs at any given measurement location.

First, the white paper introduces the concept of generating calculated pH and other critical cycle chemistry measurements. It offers that in cycle chemistry samples, deriving a calculated pH measurement can sometimes be more accurate and reliable than the measurement from a standard glass pH electrode. Having both a measurement point and a calculated pH value can support validation in your water system measurements.

Before moving into the science behind calculated pH, calculated CO2 and calculated anion concentrations, the white paper covers how power plants collect and use analytical data in the modern context of cost constraints and decreasing personnel.

The white paper then provides the sound scientific background behind calculated pH and its relationship with conductivity. It also assesses the requirement to have properly specified conductivity sensors to provide accurate, reliable measurement from which calculated pH can be derived.

It then moves on to the topic of generating a calculated carbon dioxide value from specific, cation and degassed cation conductivity measurements, and anion concentration in degassed cation conductivity samples.

Finally, the white paper covers the role multi-parameter transmitters play in determining and displaying calculated pH, calculated CO2 and calculated anion concentrations.

In thermal power plants, pH measurement and control in the water-steam cycle are critical to minimizing corrosion. Cycle chemistry guidelines specifically target narrow pH ranges. However, in high-purity water and steam, there are not enough ions in the water to generate a pH response causing the pH reading to be unstable. Fortunately, conductivity has a direct relationship with pH when the sample contains only pure water with the typical alkaline treatments such as ammonia and or amines. In fact, cycle chemistry in many plants relies primarily on the related specific conductivity values. However, correction for minor contaminants by measuring the cation conductivity of the sample is needed to provide accurate results. The specific and the cation conductivity values are used in an industry-accepted calculation that provides the highest accurate conversion to pH. Under normal operating conditions, this calculation provides more accurate and reliable results than pH electrodes can provide, for two reasons. First, conductivity is linear with concentration, which provides much higher resolution than pH sensors, all of which give non-linear response to concentration. Second, conductive sensors can be directly calibrated with minimal introduction of error. All pH reference electrodes have a variable junction or diaphragm potential that can change between buffer calibration and process measurement, introducing some uncertainty to subsequent measurements. This makes calculated pH based on conductivity significantly more accurate. However, with major contamination, such as when a condensate exchange polisher's resin is exhausted and an acid leak occurs, the pH would drop below 7, which is outside the ideal range for using calculated pH and would generate errors. Thus, to avoid contamination, it is recommended that both traditional pH measurement and calculated pH based on conductivity be used for optimal pH control. Mallard Toledo Thornton's M800 multi-parameter transmitter can provide traditional pH measurement, calculated pH, as well as specific and cation conductivity to ensure accurate and reliable pH control. The M800 even has a built-in alarm to alert you if the pH measurement deviates from the calculated pH measurement for extra security and peace of mind.