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State-of-the-art is one thing—affording state-of-the-art is another, especially in the water markets where budgets are tight. Interestingly, however, some technologies have been around so long and been developed so thoroughly that state-of-the-art can become an affordable staple through the phenomenon of standardization. Such is the case with pH measurement, where new capabilities can save water plants money and time without a premium investment.
Fundamentals of pH measurement barely have changed in decades. They are based on a pH-sensitive electrode (usually glass), a reference electrode and an element to provide a temperature signal to the analyzer because pH measurement is temperature sensitive.
The pH electrode uses a specially formulated, pH-sensitive glass tip blown onto an inert glass tube. The tip comes in contact with the solution being measured and develops a potential (voltage) proportional to the pH of the solution. The reference electrode is designed to maintain a constant potential at any given temperature and completes the pH measuring circuit. The difference in the potentials of the pH and reference electrodes provides a millivolt signal proportional to pH.
The Problems of Traditional pH
While pH measurement is time-honored technology that has never been fundamentally replaced, it is traditionally fraught with problems. The pH electrode is made of glass, which can crack through mishandling or in excessive hot or cold temperatures. As the glass ages, its slope or response rate decreases, introducing the possibility of error. Plus, hydrofluoric acid solutions can dissolve pH glass.
Problems with the pH electrode, however, generally pale compared to the effects on the reference electrode. The typical reference electrode consists of a silver wire coated with silver chloride in a fill solution of potassium chloride. For the reference electrode to maintain a reproducible potential and remain in electrical contact with the pH electrode through the process solution, the reference fill solution must remain uncontaminated.
But preventing contamination is not trivial. The reference electrode can become poisoned, meaning it is converted from a silver-silver chloride-based electrode to one based on a silver compound, generally resulting in a sudden extreme change in potential and requiring immediate reference replacement.
Additionally, plugging of the liquid junction in the reference electrode causes the pH reading to drift aimlessly. Plugging most often is caused by an ion that forms an insoluble precipitate with silver ion (e.g., sulfide ion). Undissolved solids and liquids in water or process can coat the liquid junction of the reference electrode, which may introduce errors or shut the sensor down completely.
Other problems may arise in buffer calibration. Buffers are standard solutions formulated to maintain a known pH in spite of small amounts of contamination. Buffer calibrations require two solutions, usually at least 3 pH units apart. The magnitude of the slope derived from a buffer calibration provides an indication of the condition of the glass electrode, and the zero value can indicate reference poisoning. Buffer calibration errors can result from temperature variations or from calibrations done in haste when the sensor is not given time to properly respond to the buffer solution. Such errors can happen when technicians must visit field sites in difficult weather.
These pH measurement problems result in frequent pH sensor replacement and cost and headaches for plant managers. Manufacturers have worked diligently to improve the pH workhorse to make the sensors longer lasting, easier to use and more reliable and predictable.
At the outset, these improvements generally are offered on a manufacturer’s high-end instruments because innovation usually costs money for development, engineering and retooling; consequently, it can be recovered only on higher-priced equipment. After these innovations have proven themselves in the market, however, they may become available on more mainstream instruments due to standardization.
Standardization in analytical instrumentation is the practice by manufacturers of building a common analytical instrument platform that can be changed easily from one measurement to another through the addition of circuit boards and specific sensors. Standardization benefits the manufacturer by reducing manufacturing and maintenance costs. It benefits users as they easily can switch out measurements, reduce their stock requirements and train personnel to operate and service instruments with common interfaces.
The fallout from standardization is an even bigger win for users. Often, it is easier for manufacturers to incorporate high-end capabilities into mainstream general purpose instruments to simplify production. This means users may get these capabilities largely for free. It pays to be on the lookout for these features.
One of the capabilities previously only available on premium instruments and sensors is a preamplifier that converts the high-impedance pH signal into a stable, noise-free signal. Through the preamplifier, the “smart” analyzer automatically calibrates the sensor. It automatically uploads calibration data and the associated timestamp and can be reset with user menus without power cycling. This feature is ideal for water markets, as it greatly simplifies maintenance and reduces field service requirements.
Another feature now appearing standard with the preamplifiers is sensor diagnostics. The instrument automatically records pH diagnostics, including slope, offset, reference impedance and glass impedance. This trending data allows technicians to predict maintenance requirements and estimate sensor life. This is a boon, particularly when sensors are installed in hard-to-access locations.
One of the big efforts in recent pH instrument development has been to improve sensor longevity. This is focused in two areas: improved glass formulations and enhanced reference design. The results have been dramatic. Sensors that previously lasted days now are operating for months in the same process. Today these features are entering the mainstream.
While water applications generally are not as hard on pH sensor glass as many industrial processes, having an improved long-life glass formulation is certainly a “nice to have” when there is no extra cost. Many years of research have produced glass formulations that are resistant to cracking, especially at higher temperatures at which glass failure has been a consistent problem. The new glass also reduces sodium ion error in higher-pH applications and provides faster response speed even after months of service.
Another important improvement affecting sensor life is the double junction reference, which features a specially designed porous Teflon liquid junction with a large surface area. This design provides a stable contact with the solution and resists coating in dirty applications, which minimizes junction potentials, allowing accurate measurements without additional process standardization. The reference also employs a chemically inert viscous gel electrolyte that stands up to the harshest chemicals and is unaffected by thermal or pressure cycling. The internal liquid junction is a small-diameter, low-porosity ceramic designed to minimize poisoning or the depletion of the primary reference cell to maximize sensor life. There are some designs that allow the reference junction to be refilled, further extending its usefulness with minimal cost.
Features such as easy-to-use connectors and special long-life materials also are popping up on mainstream sensors, so it pays to look again at general purpose pH sensors and analyzers. What you get for free just might save you a bundle.