Industrial pH/ORP Electrodes

The heart of a pH measuring loop is the electrode system. Yokogawa has designed a wide range of electrodes to ensure this heart keeps beating under the most severe conditions. The dimensions and design meet the requirements of DIN 19263 (excluding the refillable types). A high degree of standardization makes it possible to mount any electrode in the standard program of fittings. Color coded strips on electrode and cable and clear identification of sensor specifications make incorrect installation virtually impossible.

  • Wide range of electrodes to suit all process conditions.
  • Gold plated spring connector parts, for good electrical contact under the most severe conditions.
  • Coaxial plug and socket with watertight sealing that meets the requirements of IP65.
  • Color coded strips for easy identification of electrodes and cables.
  • High degree of standardization for mounting in flow- and immersion fittings



Wet scrubbers are used in utilities, paper mills, and chemical plants to remove sulfur dioxide (SO2) and other pollutants from gas streams. Undesirable pollutants are removed by contacting the gases with an aqueous solution or slurry containing a sorbent. The most common sorbents are lime, Ca(OH)2, and limestone, CaCO3

Application Note

For control of batch neutralization, a pH measurement coupled with a timer-controlled chemical feed scheme provides very satisfactory results.

This system can be adapted for either acid waste or alkaline waste neutralization.

Application Note

The term "cooling tower" is used to describe both direct (open circuit) and indirect (closed circuit) heat rejection equipment. Cooling towers are heat-transfer units, used to remove heat from any water-cooled system. The cooled water is then re-circulated (and thus, recycled) back into the system. Since the process water is re-circulated, the mineral concentration increases as a result of the evaporation.

Industry:Refining, Food and Beverage, Power, Oil and Gas, Pulp and Paper, Chemical


The proliferation of microorganisms and the resultant formation of slime is a problem which commonly occurs in aqueous systems. Problematic slime producing microbes may include bacteria, fungi and/or algae. Slime deposits typically occur in many industrial aqueous systems including cooling water systems, pulp and paper mill systems, petroleum operations, clay and pigment slurries, recreational water systems, air washer systems, decorative fountains, food, beverage, and industrial process pasteurizers, sweetwater systems, gas scrubber systems, latex systems, industrial lubricants, cutting fluids, etc.


One of the primary applications for high purity water is for boiler feed water. The measurement of pure water pH can be one of the quickest indicators of process contamination in the production or distribution of pure water. Effective chemical treatment of the feed water is vital in maintaining the useful operating life and minimizing maintenance costs of the boiler.


Process liquid analyzers such as pH meters, conductivity meters, ORP meters, and density meters play an important role at electrolysis plants in the control of concentrations of various process solutions. This requires both precision and stability under harsh conditions that include highly corrosive substances, high temperatures, and many impurities.


The control of the world's water resource is arguably one of the most important issues. Water demand from industry and domestic users is set to rise throughout the industrialized world. Yokogawa has been applying minimized maintenance measurement systems.

Application Note


The Combined Effects of pH and Percent Methanol on the HPLC Separation of Benzoic Acid and Phenol:

Many mobile-phase variables can affect an HPLC (High Performance Liquid Chromatograph) separation. Among these are pH and the percent and type of organic modifier. The pKa of a weak acid is the pH at which the acid is equally distributed between its protonated (uncharged) and unprotonated (charged) forms. This is illustrated by the Henderson–Hasselbalch equation:

pH = pKa + log ([A_]/[HA]

where [A_] is the concentration of the weak acid in its unprotonated form
and [HA] is the concentration of the weak acid in its protonated form.

If the weak acid is equally distributed between its two forms, ([A_]/[HA]) = 1, log ([A_]/[HA]) = 0, and pH = pKa. If the weak acid is not equally distributed between its two forms, then the pH will be either less or greater than the pKa of the weak acid.

For example, if [A_] < [HA], ([A_]/[HA]) < 1, log ([A_]/[HA]) < 0, and pH < pKa. Thus, a weak acid exists primarily in its protonated form at a pH below the pKa and therefore has a greater affinity for the nonpolar stationary phase. If [A_] > [HA], ([A_]/[HA]) > 1, log ([A_]/[HA]) > 0, and pH > pKa. Thus, a weak acid exists primarily in its unprotonated form at a pH above the pKa and therefore has a greater affinity for the polar mobile phase.

Fig 1 - full-factorial experimental design

Fig. 1 - A three-level, two-factor
full-factorial experimental design

Organic modifiers also have an effect on the retention of solutes in HPLC. In the reversed-phase mode (polar mobile phase, nonpolar stationary phase), the most polar solute component will elute first. This is because the most polar component interacts least with the nonpolar stationary phase.

As the polarity of the mobile phase is increased, those solute components that were previously highly retained (nonpolar components) will be retained even more.

Two species that are of public interest because of their classification as moderate environmental and health hazards are benzoic acid (pKa = 4.202) and phenol (pKa = 9.98). The purpose of this study is to investigate the combined effects of pH and percent methanol on the reversed-phase HPLC separation of these compounds.

A three-level, two-factor fullfactorial experimental design will be used to specify nine mobile phases for consideration in this study. The levels of pH were chosen to bracket the pKa value of benzoic acid (below, near,
and above 4.202). It was not possible to study a mobile phase with a pH > 7.5 owing to the pH range limit of the column. A methanol/water mobile phase was selected for this study because methanol is readily available in most undergraduate labs and relatively inexpensive. In addition, both solutes elute in a relatively short time, making completion of this lab during one or two lab periods possible.

Table 1. Mobile Phases Specified by the Experimental Design
Phase No. Methanol % pH
1 25 3.0
2 25 4.5
3 25 6.0
4 50 3.0
5 50 4.5
6 50 6.0
7 75 3.0
8 75 4.5
9 75 6.0

Major Observation

At low mobile-phase methanol concentration (25%), as pH increases, the retention time of phenol appears to be unaffected, whereas the retention time of benzoic acid decreases significantly. Over the pH range investigated, the mobile-phase pH is below the pKa of phenol. Thus, phenol will remain in its protonated form and should be unaffected by these mobile-phase changes. However, as pH increases, benzoic acid shifts from its protonated to its unprotonated form, decreasing its affinity for the nonpolar stationary phase and decreasing its retention time.

At intermediate (50%) and high (75%) mobile-phase methanol concentrations, as pH increases, the retention time of phenol remains unaffected by increases in pH while the retention time of benzoic acid decreases. This is consistent with the behaviour at low methanol concentration.

At pH 3.0, as percent methanol increases, the retention times of both phenol and benzoic acid decrease significantly. Because both solutes are polar, increasing mobile-phase polarity causes both to be retained less tightly. At pH 4.5 (slightly above the pKa of benzoic acid) and pH 6.0 (well above the pKa of benzoic acid) as percent methanol increases, the retention times of phenol and benzoic acid decrease. This is consistent with the retention behaviour at pH 3.0.

Typical Process Details

  • Customer plant: Bulk drug plant
  • Application: This is 4 cycle application. There will 
be a pipe connected to inlet which allows process to flow through the column and the same will be sent out from another pipe at outlet.
  • pH measurement is typically required at both the inlet and outlet. Temp: 30-40°C. pH range shall be 7 to 7.5. Between this range the customer can take necessary action to control his process.
  • Conductivity max. 300 micro siemens/cm.
  • Cycle 1: Process contains 95% liquid methanol, 
2% liquid ammonia, 3% water.
  • Cycle 2: Process contains 30% liquid methanol, 
70% water.
  • Cycle 3: Process contains 90% liquid methanol, 
5% liquid ammonia, 3% water, 2% sugar content.
  • Cycle 4: The column will be cleaned by flushing 
with DM water.


cobb1Paper is used in a broad array of products essential for everyday life, from newspapers, books, magazines, printing, writing papers to cardboard boxes and bags, paper napkins, sanitary tissues etc. We are daily surrounded by paper products.

The most important use of paper is writing. The quality of paper used for printing or writing should be good and it depends on many parameters. One of the parameters is Cobb, which needs to be controlled.

What is Cobb Control?

Cobb control is nothing but the control of quality and binding of pulp in such a fashion that whatever is written by any source such as ink, etc on paper it should not spread as well as leave its impression on back side of the paper.

How is Cobb Variation Minimized?

Cobb variation is minimized by maintaining pH of the pulp in the range of 5-6 pH. Before processing, the raw pulp pH is in the range of 7-8. This should be controlled and brought down to acidic range i.e. 5 to 6 pH. This is very essential for following reasons.

  1. This ensures proper binding of the fiber
  2. Cobb is controlled
  3. Further it also helps in maintaining paper 
machine run ability at head box.

How is Cobb Control Done?

Cobb control is done by addition of Alum (which is in the range of 2-3 pH) and rosin to pulp. When alum and rosin are mixed with pulp after a certain distance pH of the mixture is measured and if it is not in the desired range the transmitter will control the Alum dosing via controller so that pH of the pulp is maintained. Rosin on the other side has no such controlled action. It will be getting dosed to the pulp continuously in a specific quantity. It is the Alum whose dosing is controlled depending upon pH variations.

Application Overview

By installing pH loop in paper industry not only the Cobb control is done but also customer can achieve the following:

  1. 20% saving in Alum consumption
  2. Eliminate corrosion problem in approach flow 
pipelines, pumps and other equipment. This is because if alum dosing is not controlled, there are chances that more Alum is dosed and it can lead to corrosion of pipes and components.


Measurement points:

After Rosin tank:


Typical problems: Cleaning of sensor in such installations

Remedies: Using retractable fitting this optimum level of pH can be achieved by using Yokogawa's 4-wire pH Analyser with suitable retractable fitting and sensor. Our manual retractable fitting PR10 with Titanium ball valve is designed for use in pulp industry where frequent
cleaning of the sensor is required. For cleaning one has to take out pH sensor, close the ball valve so that the sample will not come out, clean the sensor, open the ball valve and then put back the cleaned sensor. The pH sensor will be a combination electrode having double junction electrodes, which provide long time stability and a prolonged lifetime. In our 4-wire pH analyser on 2nd mA output we have PI control facility. The speed of the alum dosing pumps can be controlled as per pH variations with the help of this feature.


Sour Water is the wastewater that is produced from atmospheric and vacuum crude columns at refineries. Hydrogen sulfide and ammonia are typical components in sour water that need to be removed before the water can be reused elsewhere in the plant. Removal of these components is done by sending the sour water from the process to a stripping tower where heat, in the form of steam, is applied.


Power plant boiler houses designed to burn coal or high sulfur oil are required by Federal and State pollution regulations to "scrub" (remove) sulfur dioxide from flue gasses to meet emission limits. SO2 in flue gasses is known to be harmful to the environment, as it is one contributor to the formation of acid rain. pH control is critical for the proper functioning of the scrubber system.

Application Note

Wastes have been considered to be a serious worldwide environmental problem in recent years. Because of increasing pollution, these wastes should be treated. However, industrial wastes can contain a number of valuable organic components. Recovery of these components is important economically. Using conventional distillation techniques, the separation of acetic acid and water is both impractical and uneconomical, because it often requires large number of trays and a high reflux ratio. In practice special techniques are used depending on the concentration of acetic acid. 


Ultrapure water, UPW, high-purity water and deionized (DI) are all terms describing basically the same property. They refer to water which has been purified to the highest standards by removing all contaminants such as, organic and inorganic compounds; dissolved and particulate matter; volatile and non-volatile, reactive and inert; hydrophilic and hydrophobic; and dissolved gases. The purified water has very low conductivity which means it is high in resistance because all the conductive components have been removed. In low conductive solutions combined with its susceptibility to contamination and temperature effects makes accurate pH measurement very difficult.


Many Ethanol plants running today are using a combination style pH electrode with a non-flowing reference to measure pH in the Mash Slurry transfer line from the Mash slurry mix tank to cook. The Mash is being pumped out of the Mash Slurry tank is at approximately 180 °F and 40 to 60 psig.


Current trend for increasing mercury awareness throughout the public sector has caused the government to take action. Recently, the Environmental Protection Agency (EPA) has focused their efforts on controlling mercury levels produced in various coal fired power plants. Based on information from several case studies, the EPA developed the Mercury and Air Toxics Standards to cut back mercury emissions. The most popular technology utilized by coal plants to meet the new standards is a scrubber which cleans the off gas from the combustion process. ORP sensors can further monitor the effluent from these scrubbers to ensure optimal mercury emission levels are achieved. By closely monitoring the mercury concentrations in the effluent, plant managers will be able to easily confirm their plants are meeting the EPA's standards.


By understanding what causes the difficulties in pH measurements and having the proper equipment, stable and accurate pure water pH measurement can be accomplished.


The lifetime of a pH sensor has a significant impact on the overall annual costs of a pH measuring loop. Optimizing four key factors will decrease these costs and optimize process control and overall plant efficiency.

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