01 October 2002

Process pH measurement continues evolution

By Eric Pfannenstiel

The acid-base relationship was important then . . .

In the sixteenth century, alchemist Leonard Thurneysser discovered that the hue of violet sap changed with the addition of either sulfurous or sulfuric acids. This early indicator was widely used through the subsequent centuries to detect acids.

With Arrhenius's introduction of ionic theory in the 1880s, the first theories concerning disassociation of acids and bases were developed. Bronsted, who postulated that acids and bases are substances capable of either donating or accepting hydrogen ions, further refined these initial theories.

By 1904, Hans Friedenthal had successfully established the first scale for classifying acids by determining the dissociation constants for weak acids, according to conductivity and correlating color changes corresponding to different hydrogen ion concentrations using 14 indicating dyes.

The hydrogen ion concentration numbers from Friedenthal's calculations were small and awkward to manipulate. Thus, Lauritz Sorensen suggested using the negative logarithm of these numbers, which he dubbed the "hydrogen exponent" or "pondus Hydrogennii."

This led to the development of the term pH and the creation of the modern pH scale.

In 1889, Walter Nernst published the law that bears his name, the Nernst Equation:

E = E₀ + (RT/nF) log[H⁺]

where

E = galvanic potential of the electrode in equilibrium with the solution
E₀ = standard potential of the electrode at pH 7
R = gas constant
T = temperature in degrees Kelvin
n = charge number of the ion
F = Faraday's constant
H⁺ = hydrogen ion activity

This is the basis for the quantitative determination of hydrogen ion concentration or pH.

The modern pH scale

PROVIDE STABLE POTENTIALS

A process pH measuring system has three fundamental requirements. The first is the pH-measuring sensor that provides a direct voltage corresponding to the concentration of hydrogen ion, [H⁺], in solution.

The second is the pH meter, which amplifies the electrode potential, compensates for the theoretical temperature effects, adjusts for the individual properties of the cell, and allows adjustment of the zero point and slope through calibration. It also generally provides a local display and often an output of the measured value.

The third is some means of interfacing the sensor with the process for in situ measurement.

Basic pH electrode sensor design consists of two independent half-cells. The first half-cell, or measuring half-cell, detects the electrical potential attributed to hydrogen ion activity.

The second half-cell, or reference cell, detects the potential of the process solution outside the sensor. It also completes the electrical circuit with the measuring half-cell and provides stable internal potentials.

Measuring and reference half-cells

In 1875, Thomson recognized that glass is a solid electrolyte in which alkali metal ion can carry current. Max Cremer first applied glass as a semipermeable membrane in electrochemical determinations in 1906.

Three years later, Haber and Klemensiewicz realized the relevance of the Nernst Equation and began to deliberately use the glass electrode to record titration curves. Completed glass electrodes did not become available until the 1920s, and commercial manufacture began in the 1930s.

Today, glass electrodes are the most prevalent measuring electrodes used.

Initially, the zinc amalgam electrode in saturated zinc sulfate served as a reference electrode. However, it had considerable shortcomings for quantitative use.

By 1893, Ostwald had replaced this with the mercury/calomel electrode. The silver/ silver chloride reference system is the predominant component in use today because of its ease of preparation and reproducibility.

Early pH sensor designs used two separate measuring and reference electrodes. In 1934, Dr. Arnold Beckman introduced the first practical application for pH measurement with his acid-o-meter.

By 1947, Cannon had introduced the first laboratory combination electrode. As sensor technology continued to increase, further refinements to glass formulations that improved selectivity and reduced alkalinity error took place.

Measurement Electrode	Reference electrode	Combination Electrode

Temperature measurement also integrated into the system to correct for temperature effects in the glass. Early reference electrodes used open, flowing junctions. However, this was unsuitable for continuous use, resulting in the introduction of restrained-flow porous junctions.

Initially, ceramic and wood junctions worked. In 1978, Gary Bukamier patented the porous Teflon reference junction, which is the choice these days.

In 1906, Cremer used an electrostatic galvanometer to measure the electric potentials generated by early hydrogen electrodes. Modern pH measurement would be unthinkable without the advances in electronic measurement techniques.

In 1966, P. Carderio developed and patented the first pH preamplifier. The first electronic pH meters were of analog design. Continued developments have led to the microprocessor-based digital pH meters used today.

The advent of the combination electrode and the analog pH meter enabled the first true opportunity to make in situ pH measurements. Initially, separate reference and measuring electrodes sat in flow cells, and process samples diverted through bypass lines to the holder. Most systems used flowing liquid reference junction sensors with external reservoirs, due to the high consumption of liquid potassium chloride electrolyte.

Upon the introduction of the combination electrode, process connections integrated into the body of the sensor, allowing direct threading, insertion, or submersion of the sensor in the process.

Sensor life was often short, however, due to the rapid depletion of the liquid electrolyte. Poisoning of the reference electrode also became an issue. The introduction of highly concentrated potassium chloride electrolyte, polymer solidified gels, and double reference junctions eventually overcame these drawbacks.

. . . and even more so with further applications now.

Old-time process pH sensors were large, cumbersome, and expensive, and laboratory technology was usually a part of the package-as in, "Run this grab sample up to the lab, and get me a number."

The need to gather representative, real-time information forced the development of in-line, process pH measuring systems. The evolution of pH measurement technology has enabled tremendous changes to occur in the performance and price of today's sensors.

Automated pH system

Fundamental advances in technology have dramatically affected the cost and performance of modern process pH measuring systems. Sensors that once cost $750 to $1,000 can now be purchased for prices ranging from $175 to $400.

With the reduction in cost, the user has also attained greater reliability and accuracy in the measurement due to improved sensor designs. Smart, two-wire transmitters not only are less expensive than they were 10 years ago but also offer advanced digital bus and sensor diagnostic capabilities.

In addition, advances in automation capabilities now enable users to reduce associated maintenance and calibration costs.

Within the pH sensor itself, several performance improvements have come about. The traditional glass electrode has dramatically evolved. Glass formulations have been refined to improve the selectivity of the glass toward hydrogen, reducing the effects of sodium ion error.

Typically, lithium oxides and heavy alkali metal additions to the glass formulation have produced this effect. Manual glass blowing of the measuring bulb is still extensively used. However, the development of automated glass-blowing machines has demonstrated improvement to the overall reproducibility of the finished glass product.

Thermal stress has long been a problem. Fusion of the glass-measuring bulb to the supporting stem glass enables thermal stress fractures to occur at the glass weld. Due to slight variations in the composition and the corresponding differences in elasticity or thermal expansion coefficients, repeated changes in temperature will induce cracking.

This is especially a problem in hygienic industries such as food and beverage or pharmaceutical, where high-temperature cleaning agents or steam are part and parcel. To overcome this problem, optimized thermally matched glass is often used.

Close comparisons of raw glass materials and slight changes in glass formulations have occurred to reduce the effects of repeated temperature changes. For certain applications, ion-selective field effect transistor sensors can serve in lieu of traditional glass electrodes.

REDUCE ELECTRODE POISONING

The reference electrode has also undergone changes. Although flowing liquid junction sensors are occasionally used, gel-filled ones are prevalent today. Initial gel-filled electrodes consisted of highly concentrated gelatinous potassium chloride (KCl) suspended in liquid KCl. This was effective in maintaining a consistent concentration of electrolyte and extending the lifetime of the reference electrode.

However it did nothing to prevent or reduce electrode poisoning or contamination. The introduction of polymer-stabilized electrolytes enabled this to occur.

Acrylamide or vinyl alcohol polymerized with ammonium persulfate or titanium alcoholates serves as the stabilizing agent. Polymerized gels do not require a restrained flow junction and have been shown to be highly temperature stable.

Sensors are also available with hybrid electrolyte solutions. Lithium chloride and potassium nitrate are examples of electrolytes occasionally used in conjunction with KCl, depending on the potential contaminants.

Reference junction design has also evolved. Initially, single junctions sufficed. Commonly, two and even three Teflon reference junctions may function in today's sensors. Ceramic and even zirconium are alternative materials used due their resistance to abrasion and uniform flow characteristics.

Historically, combination electrodes have used integral fixed cables to connect to the transmitter or pH meter. Occasionally users employed coaxial connectors. However the temperature compensation element was not integrated into the connection, necessitating either separate connection to the monitoring device or no temperature compensation of the electrode.

Additionally, concerns arose regarding the integrity of the electrical connection and its ability to maintain watertightness. Within the past five years, several manufacturers have introduced quick-disconnect electrical connections at the sensor head.

These connections generally satisfy IP 67 or NEMA 4X requirements. They also integrate the temperature compensation element into the plug-in termination. This has reduced sensor costs by eliminating the associated integral wiring costs.

In addition, replacement labor costs have significantly decreased because electricians are not spending man-hours pulling replacement sensor cable through conduits and rewiring transmitters when sensors fail.

CHANGING THE PROCESS PIPING

Unfortunately, a single U.S. standard does not exist regarding process connections on combination electrodes. Depending on manufacturer, 1/2-inch (in), 3/4-in, 1-in, and even 11/2-in process connections have been molded into the body of combination sensors.

This has often made it difficult to modernize existing sensor installations or try alternative sensor designs. The costs associated with changing the process piping are too great.

In Europe, high-performance 12-millimeter (mm) design electrodes are commonly used, and manufacturers adhere to a design standard. Rather than encasing the electrodes in molded housings, 12-mm sensor electrodes integrate into a glass stem.

The head of the sensor has an electrical connection and threads into unique electrode holders. Because these sensors are very compact in design and do not contain an integral molded process body, they can easily adapt to fit into just about any existing process connection.

Molded or machined bodies or "holders" are available that enable the 12-mm sensor to be integrated into standard 1/2-in, 3/4-in, 1-in, or 11/2-in process connections.

This allows users to easily evaluate alternative sensor technologies or upgrade their current process installation with minimal cost. Furthermore, it allows one common sensor design to operate throughout a plant. This eliminates unnecessary inventory and promotes plantwide standardization.

The development of advanced sensor diagnostics has also been a tremendous enabler. Many of today's transmitters offer the ability to assess the condition of the measuring and reference electrodes, temperature compensation element, and current calibration.

By measuring the impedance of the measuring glass, the pH meter can detect aging or cracking caused by abrasion, thermal stress, or simple glass fatigue. Monitoring the resistance between the reference junction and the reference electrode can assess material buildup or junction fouling.

Monitoring the continuity of three-wire resistance temperature detectors allows detection of temperature compensation element failure.

Lastly, evaluation of the sensor slope, asymmetry, and zero point alerts the user to potential failures and provides the user with insight into the approximate remaining useful life of the sensor.

Each of these diagnostic features allows the user to perform predictive maintenance rather than relying exclusively on schedule maintenance. This translates to savings in unneeded labor.

STARTING AS LOW AS $8,000

Automating all aspects of the process has led to dramatic savings in manufacturing costs. It has long been a goal to automate the measurement, cleaning, and calibration of pH sensors.

Integrating the pH sensor into the process was the first step. Then attempts to add spray nozzles and ultrasonic cleaning devices to sensor holders occurred. These would attempt to clean sensors at predefined time intervals. Automated retractable pH systems debuted in the 1990s.

These systems allowed the sensor to automatically retract from the process, clean itself with chemical agents or detergents, automatically recalibrate, and then to return to work, all at predefined time intervals.

Later, inputs were added to allow these actions to be activated remotely. However, the cost of the holder and associated hardware prevented most users from utilizing this technology.

Typical systems sold for $20,000 to $30,000. With continued improvements in design and manufacturing, these systems are now available at prices starting as low as $8,000. In addition, they provide expanded capabilities compared with their predecessors.

Advanced diagnostics now enable the system to predict when cleaning or calibration cycles are required. Multiple cleaning solutions and even steam are possible. Automated pH buffer recognition allows the system to automatically recalibrate without user intervention.

Onboard buffer level monitoring happens to ensure sufficient volumes are available for recalibration. System self-diagnostics monitor sensor response time, calibration slope, and the zero point offset to alert the user when sensor replacement is required.

Through use of logbook functions, the scheme can compare previous calibrations to predict when sensor replacement may be required. The logbook also stores temperature or pH excursions outside nominal operating ranges, alerting the user when possible process upset conditions have occurred.

These features afford the opportunity for dramatic savings in material replacement costs and labor. Today's pH measurement solutions, on the whole, can provide their users with a competitive advantage. IT