classic | mobile


SA Instrumentation & Control Buyers' Guide

Technews Industry Guide - IIoT 2018

Technews Industry Guide - Maintenance, Reliability & Asset Optimisation


Understanding the difference between pH and ORP measurement
August 2018, Analytical Instrumentation & Environmental Monitoring

pH measurement

The pH of an aqueous solution indicates the concentration of hydrogen ions (H+), since this concentration determines the degree of acidity or alkalinity of the solution, The pH is therefore said to be a measure of the acidity or alkalinity and is defined as the negative logarithm of the hydrogen ion concentration: pH = -log [H+]

The pH scale ranges from 0 to 14 with a pH of 7 being neutral, a pH less than 7 being acidic and a pH greater than 7 being basic (alkaline). Measurement of pH by the 871PH sensor is accomplished by immersing the probe, which consists of integral pH and reference electrodes, in the process solution. The pH electrode is sensitive to hydrogen ions in solution and develops an electrical potential proportional to pH. The reference electrode, which consists of silver/silver chloride connected to the process via a potassium chloride salt bridge through a ceramic junction, provides a stable reference potential against which the glass electrode potential is measured. These two electrodes constitute a galvanic cell having a millivolt output proportional to the pH of the solution.

ORP measurement

ORP is a measure of the electrical potential of a reaction known as an oxidation-reduction (redox) reaction. A redox reaction is one in which one component loses one or more electrons (oxidation) while another component gains one or more electrons (reduction). The oxidation-reduction potential is related to the ratio of oxidation activity to reduction activity. By convention, a solution that contains an excess of oxidising agent has a positive ORP, and a solution that contains an excess of reducing agent has a negative ORP.

Measurement of ORP is accomplished by immersing a pH sensor, configured with a gold or platinum electrode, in a solution along with its integral silver/silver chloride reference electrode. This probe develops a millivolt output (similar to pH measurement) that is proportional to the ratio of oxidising agent to reducing agent, or ORP.

Foxboro provide a solution for pH measurement in some of the harshest and most abrasive industrial applications, for instance, wood pulp bleaching in the paper and pulp industry.

Technical challenges

In Kraft pulping, close to 90% of the wood is dissolved in the cooking liquor. The removal of the remaining lignin is completed through the bleaching process. In order to produce bright coloured pulp and paper the bleaching process must use several harsh chemical processes. The majority of bleaching processes rely on molecular chlorine, chlorine dioxide, hypochlorite, and peroxide as oxidising agents to remove the residual lignin from the cellulose fibre. A typical bleach sequence has two stages, each followed by an extraction stage. The bleaching stage is most efficient when operated at low pH levels (between 3,2 and 4,2). Conversely, the extraction stage is optimal when operated at higher pH (10,0 to 11,0), to maximise colour removal. Accurate and reliable pH measurements increase process efficiency, reduce maintenance costs and minimise the excessive use of chemicals.

Process conditions inside the bleaching towers are not only harsh on the process-wetted parts of a pH sensor, but can have a progressively detrimental effect on the life of a pH probe. The highly aggressive nature of the chemicals used as bleaching agents, along with the high solids inherent to the process and elevated operating temperatures, are all factors detrimental to most pH glass membranes. These factors will poison most reference electrodes in a short period of time, thus causing error in pH measurements, delay pH response times and shorten the useful life of a pH sensor.

The Foxboro solution

Foxboro’s Dolphin series pH sensors were developed with the understanding of this type of process. The high temperature glass formulation was developed specifically to withstand exposure to higher process temperatures and aggressive chemicals. When installed at the paper mill the results spoke for themselves. The evaluation process was a couple of weeks after which the mill realised the benefit. When compared in service with several other pH sensor types, the performance of the Dolphin series was deemed superior in many respects.

The sensor’s reference half-cell is designed to have a relatively small junction, which lessens the process infusion. A double reference junction was designed and incorporates Nafion technology, which has the unique property of being permeable only to positively charged ions (cations). Therefore, it effectively holds the concentration of chloride at the electrode at a constant level and delays the migration of soluble silver chloride, which has negatively charged ions, across its boundary. This Nafion ion barrier effectively prevents silver ions from migrating through the salt bridge and silver chloride in the external diffusion barrier, thus preventing clogging of the reference junction barriers, common cause of failure in these applications.

With more than 40 years’ experience in measurement, the proven reliability and robustness of the Foxboro Dolphin sensor, along with its intelligent analysers help improve process performance, increases production yields and reduce equipment and maintenance costs.

For more information contact Johan van Jaarsveldt, EOH, +27 87 803 9783,,

Supplied By: EOH Process Automation Solutions
Tel: +27 87 803 9868
Share via email     Share via LinkedIn   Print this page

Further reading:

  • Yokogawa releases 4-wire liquid analyser
    December 2018, Yokogawa South Africa, Analytical Instrumentation & Environmental Monitoring
    New solution in the OpreX family improves maintenance efficiency and enables flexible design.
  • Temperature compensated conductivity measurement from Foxboro
    December 2018, EOH Process Automation Solutions, Analytical Instrumentation & Environmental Monitoring
    Measuring the conductivity of a liquid can reveal the concentration of salts dissolved in the solution. In fact, conductivity is often the method of choice for measuring the amount of salts dissolved ...
  • Fast and easy TOC measurement
    November 2018, Microsep , Analytical Instrumentation & Environmental Monitoring
    For power generation makeup water treatment – from reverse osmosis to demineralisers – the Mettler Toledo Thornton 5000TOC i sensor provides fast, reliable monitoring of total organic carbon (TOC) contamination ...
  • Continuous emission monitoring systems
    November 2018, Yokogawa South Africa, Analytical Instrumentation & Environmental Monitoring
    Since the 1970s, the United States Environmental Protection Agency (USEPA) has been responsible for establishing and enforcing environmental protection standards worldwide. An important element of these ...
  • Concentration monitoring for pulp and paper
    October 2018, Morton Controls, Analytical Instrumentation & Environmental Monitoring
    In the pulp and paper industry, the alkaline sulphate process, which is where fibrous material is separated from lignin, is important. These chemicals need to be recycled, due to environmental and economic ...
  • Leak detection ensures seal ­integrity for packages
    October 2018, Emerson Automation Solutions, Analytical Instrumentation & Environmental Monitoring
    In response to industry demand for increased quality in food and beverages at every level, Emerson has introduced the Rosemount CT4215 food and beverage leak detection system. The CT4215 is a quantum ...
  • Foxboro’s top performing intelligent vortex flowmeters
    September 2018, EOH Process Automation Solutions, Flow Measurement & Control
    The 84 series measures volumetric and mass flow of saturated and super-heated steam.
  • Reliable results with Zirkor oxygen analysers
    September 2018, SICK Automation Southern Africa, Analytical Instrumentation & Environmental Monitoring
    The new range of oxygen analysers in the Zirkor series from SICK Automation are exceptionally rugged and combine high quality with innovative performance. To optimise combustion, the easy to operate and ...
  • In-situ gas analyser requires ­minimal maintenance
    September 2018, Microsep , Analytical Instrumentation & Environmental Monitoring
    Tunable diode laser (TDL) spectroscopy is rapidly becoming the gas analysis technology of choice in industrial processes. Mettler Toledo Ingold offers the GPro 500 TDL Series, a versatile platform that ...
  • Packaging keeps food fresh for ­longer
    August 2018, GHM Messtechnik SA, Analytical Instrumentation & Environmental Monitoring
    Accurate residual measurement of oxygen critical for success of MAP.
  • Servotough Flue Gas Extract 700 I
    August 2018, Elemental Analytics, Analytical Instrumentation & Environmental Monitoring
    As this type of system can be installed close to the burners, the ‘lag time’ for oxygen analysis – the measurement delay due to sensor response – is minimised, giving operators a comparatively short response ...
  • Optimise chloride and sulphate monitoring in power plants
    July 2018, Microsep , Analytical Instrumentation & Environmental Monitoring
    Chlorides and sulphates cause pitting and stress corrosion in expensive power plant components, such as turbines and boilers, leading to extensive maintenance and unplanned shutdowns. Monitoring these ...

Technews Publishing (Pty) Ltd
1st Floor, Stabilitas House
265 Kent Ave, Randburg, 2194
South Africa
Publications by Technews
Dataweek Electronics & Communications Technology
Electronic Buyers Guide (EBG)

Hi-Tech Security Solutions
Hi-Tech Security Business Directory

Motion Control in Southern Africa
Motion Control Buyers’ Guide (MCBG)

South African Instrumentation & Control
South African Instrumentation & Control Buyers’ Guide (IBG)
Terms & conditions of use, including privacy policy
PAIA Manual


    classic | mobile

Copyright © Technews Publishing (Pty) Ltd. All rights reserved.