Maintaining proper, slightly alkaline pH levels in boilers and Heat Recovery Steam Generators (HRSGs) is critical in preventing boiler tube corrosion that can be costly to an operation and potentially harmful to employees.

The measurement of pure water pH can provide indication of process contamination in systems that rely upon pure water. However, there are things you must know to accurately obtain this measurement:

Low conductivity

Accurately measuring pH in high-purity water requires a low-resistance current path between the measuring and reference electrodes. However, high purity water has inherently low conductivity and limited buffering capacity that often impedes or prohibits the establishment of the necessary current path (unless specially made reference electrodes are used). The resulting electrode drift, unacceptable flow sensitivity and poor temperature compensation causes non-reproducible and inaccurate results.

Temperature

Ideally, the sample temperature should fall between 20°C and 30°C, and remain constant to minimize two potential temperature effects – the Nernstian (or electrode) correction and the solution equilibrium (or dissociation constant) correction.

The magnitude of the Nernstian correction is determined directly, using the Nernst Equation that posits that as a glass electrode increases in temperature, its output voltage increases when the actual pH in the measured solution remains the same. The effect is minimal at or near a pH of 7 and increases linearly above and below it.

The effect of solution equilibrium (or dissociation constant) correction could also be significant. Different solutions respond to changes in temperature in their own specific way. The dissociation constant of pure water is 0.172 pH/10°C. This means at 50°C, pure water would have a pH of 6.61 and a pH of 7.47 at 0°C. In acidic solutions (below pH 6) the effect of the water dissociation is masked by the high concentration of acidic H⁺ ions already present in the solution, no correction is needed. However, in strong basic solutions (above pH 8), the dissociation effect is magnified, and corrections on the order of 0.323 pH/10°C may be needed. The amount of temperature change involved and the critical nature of the measurement dictate whether or not compensation must occur.

Temperature Compensation

There are two types of temperature compensation related to pH measurements:

Automatic Temperature Compensation (ATC) compensates for the varying milli-volt output from the electrode due to temperature changes of the process solution.

Solution Temperature Compensation (STC) corrects for changes in the solution chemistry that occur as the temperature of the solution changes (i.e., in ultra-pure water, the pH of the solution will typically increase as the solution cools). Solution temperature compensation is primarily used in power plants and other pure water applications less than 30µS/cm conductivity.

Flow

For high-purity water, stagnant samples translate to errors. Constant flow rates between 50 ml and 150 ml give the best results.

Similarly, it is beneficial to measure pH in the smallest sample volume possible. Direct pH measurement in large volume drums or tanks and other samples with flowing or moving water tend to fluctuate and require excessive stabilization time.

Air

Air must not be allowed into the sample stream. The buffering capacity of pure water is very low, and when it is exposed to air the absorption of carbon dioxide (CO₂) causes the pH reading to decrease. Taking grab samples and moving over to a lab bench meter should be avoided because atmospheric CO₂ will contaminate the sample.

Online analyzers

For many years it was believed that these pure water properties could not be satisfactorily modeled mathematically in order to achieve the desired measurement accuracy and reliability.

However, many contemporary online analyzers have reference systems designed for ultra pure, low conductivity water and have proven effective and accurate in:

• Measuring samples without CO₂ interference
• Accounting for solution temperature compensation
• Introducing the online pH electrode into the process water for an extended period of time in order to equilibrate to both the temperature and chemistry of the process sample
  
  

Overcoming theses complexities to accurately measure the pH of high-purity water is possible when you partner with Sentry ProShield service experts. Contact us today to learn more on how an online analyzer could work for you.