pH Measurement In Industrial Waters

pH Measurement In Industrial Waters

Determination of pH is a critical and ubiquitous operation whenever water is used in the chemical process industries (CPI).

This one-page reference provides a review of pH chemistry and information on pH measurement equipment in CPI applications.

CHEMISTRY OF PH

The quantity pH is a measure of the acidity or basicity of an aqueous solution. It is expressed as the negative logarithm of the activity of hydrogen ions in solution (pH = – log  aH  +; where  aH  + is the activity of hydrogen ions).

Effectively, the activity represents the concentration of hydrogen ions. However, hydrogen ions (that is, free protons) do not exist by themselves in solution. H + ions quickly bond with surrounding water molecules, making H 3 O + (hydronium ion).

Because the pH scale is logarithmic, a change of one unit of pH corresponds to a ten-fold change in hydrogen ion concentration. In a sample of pure water, it has been determined experimentally that the auto-ionization of water results in a H + concentration of 10 –7 mol/L, which is equal to the concentration of OH – ions. Because of this, neutral pH is said to be equal to 7.0.

Values below 7 are acidic, while those above 7 are basic.

PH ELECTRODES

Measurement of pH is generally accomplished with a pH-sensitive electrode. In a glass pH sensor, the glass responds to the acidity level. pH electrodes consist of an inert glass tube with a pH-sensitive glass tip, usually with a bulb-shaped end. The tip contains a fill solution with a known pH.

The electrochemical influence of the fill solution compared to the process solution generates a millivolt electrical potential. The glass electrode is coupled with a reference electrode and a temperature-sensing element (to account for the temperature dependence of pH).

Figure 1. A typical bulb of pH-sensitive glass in a pH sensor is shown at the top and a schematic representation of the “leached” layer formation on the both sides of the pH-glass is shown below. If the pH of the reference electrolyte equals that of process solution, then no potential (E) is generated across the glass. Typically, the reference electrolyte is buffered to pH 7, so no voltage is generated if the process liquid has the same acidity as the buffer. If the process liquid is acidic, then E 0, and positive potential is generated Figure 1. A typical bulb of pH-sensitive glass in a pH sensor is shown at the top and a schematic representation of the “leached” layer formation on the both sides of the pH-glass is shown below. If the pH of the reference electrolyte equals that of process solution, then no potential (E) is generated across the glass.

Typically, the reference electrolyte is buffered to pH 7, so no voltage is generated if the process liquid has the same acidity as the buffer. If the process liquid is acidic, then E 0, and positive potential is generated

pH electrodes use specially formulated glass capable of generating electrical potential (voltage) that is proportional to the pH of the solution it is measuring.

In a standard pH-glass electrode, the sensing element is a gel layer with sub-micron thickness on the glass bulb. The potential change is measured in relation to a reference electrode that is in contact with the solution, such that a closed electrical loop is created.

Specific formulations of the pH glass are trade secrets for pH-sensor manufacturers. However, it is not a secret that alkali metals render silicate glass pH-sensitive. The “leached” layer is formed on the surface of the glass membrane once it’s hydrated (Figure 1).

The glass electrode must therefore be kept in an aqueous solution — once the glass is dehydrated (if the bulb is not immersed or is exposed to non-aqueous chemicals) the leached layer disappears and the sensor stops working or develops an erratic signal. The leached layer is only about 5–10 nm, but it contributes the most toward the conductivity of the pH glass.

REFERENCE ELECTRODES

The purpose of the reference electrode is to create a stable reference potential against which the pH signal can be measured. Having a constant reference potential is essential for high-impedance measurement. The typical reference in commercial pH sensors uses a potassium chloride electrolyte.

The consistency of the electrolyte can vary from liquid to gel to solid in order to slow down the migration of process ions inside the reference that come in through the reference junction (Figure 2).

Depending on the application, the chemistry of the electrolyte can be changed to satisfy specific requirements of the chemical process. The competitive marketplace offers “rebuildable” sensors with a variety of fill solutions.

Figure 2. The glass electrode creates a potential proportional to the pH of the process liquid, while the reference electrode completes the electrical circuit and provides a small and stable potential as a reference for the pH signal. Electrode temperature compensation is highly desirable, especially at elevated temperatures

Figure 2. The glass electrode creates a potential proportional to the pH of the process liquid, while the reference electrode completes the electrical circuit and provides a small and stable potential as a reference for the pH signal. Electrode temperature compensation is highly desirable, especially at elevated temperatures

The reference junction is usually a part of a rebuild-kit and is pre-soaked in the corresponding electrolyte solution. The choice of the materials for the reference junction can vary from Teflon to ceramic, to wood.

The purpose of the reference junction is to provide electrical continuity with the process liquid. It also serves as a guard to prevent the process liquid from penetrating and contaminating or poisoning the sensor.

Source: Chemical Engineering