Galvanic probes as pH and oxidation reduction potential...

Details Galvanic probes as pH and oxidation reduction potential... Galvanic probes as pH and oxidation reduction potential...

2001-07-10

2003-11-25

Deb, Anjan K. (Department: 2858)

Electricity: measuring and testing

Electrolyte properties

Using a conductivity determining device

C324S438000, C204S400000, C204S433000, C205S775000, C205S787500

Reexamination Certificate

active

06653842

ABSTRACT:

BACKGROUND OF THE INVENTION
(1) Field of the invention
The present invention relates to galvanic probes and cells, and more particularly to the use of such probes and cells in the measurement of various characteristics, such as pH and oxidation reduction potential, of fluids such as water.
(2) Description of the Related Art
It is often necessary or desirable to measure the pH or oxidation reduction potential (ORP) of a fluid. The ORP measures the relative tendency of materials in the fluid to undergo oxidation or reduction (particularly the ability of the fluid to destroy bacteria in it). Moreover, it is often desirable to monitor pH and/or ORP at frequent intervals—or even continuously. Measurement of pH and ORP levels are of interest in a wide variety of industrial, commercial and domestic processes and situations. For example, many chemical processes are pH-dependent or ORP-dependent. The pH or ORP of effluents from factories is commonly of environmental concern. The pH or ORP of water is critical in many settings. A setting familiar to many laypersons is in various water storage systems, such as air conditioning systems or swimming pools. Therefore, for ease of explanation, much of the following discussion will be with reference to the swimming pool setting, although it should be borne in mind that the discussion is likewise applicable to any other situation in which the pH and/or ORP of a fluid is of interest.
In a swimming pool, the quality of the water is closely related to the pH and ORP of the water. In swimming pools, the ORP of the water is a measure of the free chlorine level in the water, which is related to the biological antiseptic quality of the water. Therefore, the pH and chlorine levels of swimming pool water must be monitored to ensure that an adequate quality level is maintained. Conventionally, this is carried out by hand and the owner or other caretaker in charge of maintaining the pool must repeatedly go to the pool with vials and chemicals, scoop out the water into the vials, shake the vials, compare the colors of the resulting solutions to those on charts coordinated with the pool volume to determine the amounts of chemicals to add to restore the proper pH and/or chlorine level, obtain those chemicals, measure them out and add them to the pool. Not only is this a cumbersome process, but if it is not carried out at frequent enough intervals, the quality of the pool water can become unacceptable very quickly. Thus, for example, if the pool caretaker is away for a several days, he or she may return to find a pool filled with murky water. Or the pH or chlorine level may fall out of acceptable range too soon before the next testing and the water may become unhealthful, and the pool caretaker may not realize that fact until it is too late. Therefore, it is desirable to have a pH monitor and/or an ORP monitor that would carry out the measurement task automatically and frequently even continuously—on a real time basis.
Unfortunately, however, current technology is ill equipped for such frequent or continuous monitoring. Conventional pH meters typically comprise a porous glass membrane enclosing a quantity of a liquid about a sensing electrode. For example, commercial pH electrodes typically employ a pair of silver/silver chloride electrodes, one in a saturated potassium chloride solution, the other in hydrochloric acid, encased separately in porous glass, and the pH is determined once the migration of hydrogen ions across the glass membrane has reached equilibrium. Reaching equilibrium can require a substantial amount of time. In fact, if the pH of the fluid being tested changes frequently, such electrodes might never reach equilibrium and so pH measurement may be impossible without extraction of a sample and testing the sample, in which case the measurement is merely historical.
Generally, such commercial pH meters suffer from several other serious drawbacks as well. They are expensive (typically about US$250), unavailable in large quantities, require soaker caps to keep the tips wet when not in use, are affected substantially by temperature, require frequent recalibration, experience substantial signal drift and, as with all electrodes that employ liquid fill solutions, must be maintained in an almost vertical position to keep the electrodes in the tested fluid. Moreover, because the glass and fill solutions required for such electrodes, they are fragile, have limited lives, are incompatible with some industrial processes, especially those in harsh environments.
Other pH meters have been noted in the scientific literature, but are likewise undesirable. Ion selective field effect transistors (ISFETs), in which the signal is based on a selective membrane attached to the gate, have been used, but they also are expensive (about US$300), are unavailable in large quantities (particularly for commercial applications) and are highly sensitive to ambient temperature. Potentiometric solid state sensors, in which measurement is based on ion specific voltage developed between a reference electrode and a measuring sensor, have been described in scientific publications, but their commercialization is extremely limited, and no large-scale production process has been developed. In any event, they also suffer from other serious disadvantages. They require frequent calibration, they suffer from significant drift, they have very limited lives, have limited applicability because some common salts damage their sensor membranes, are expensive (although at about US$20, are less expensive than the previously mentioned meters), are affected substantially by temperature, and while they can be stored dry, they require about 24 hours for stabilization.
Antimony electrodes also have been employed in some pH sensors, usually in combination with a silver/silver chloride reference electrode and glass-encased fill solution, with all the attendant disadvantages of glass membranes and fill solutions noted above. U.S. Pat. No. 5,497,091 describes a pH sensor that employs an antimony electrode in combination with a ceramic reference electrode, but provides no clear description of the ceramic reference electrode. However, known antimony-based pH sensors typically employ a polished antimony surface for enhanced sensitivity and so suffer from deteriorating sensitivity as they lose polish. In addition, known antimony based pH sensors typically also suffer from substantial drift as the quality of the polish diminishes over time, and, as with the other pH sensors dependent on ion exchange between the fluid being tested and the fill solutions, known antimony-based pH sensors frequent recalibration and limited lifetimes due to gradual dilution of the fill solution. Moreover, because they employ a liquid fill solution, they must be maintained in a vertical position. Thus, state of the art pH monitors are unsuitable for many uses, especially those in which a durable, inexpensive and readily available (or easy to manufacture) probe for frequent, accurate, real time pH measurements with low drift.
Monitors for measuring ORP also are available commercially. Their measurements are based on a voltage developed between a silver/silver chloride reference wire in an internal fill solution and a platinum wire isolated from the fill solution. Such monitors also suffer from serious drawbacks, including high cost (about US$250), unavailability in large quantities, limited life based on the fill solution, lengthy response time, the requirement of a soaker cap to keep the tip wet when not in use, and the requirement of all electrodes that employ fill solutions that that they be maintained in an almost vertical position to keep the electrodes in the tested fluid. Thus, as with the state of the art pH monitors, state of the art ORP probes are unsuitable for many uses, especially those in which a durable, inexpensive and readily available (or easy to manufacture) probe for frequent, accurate, real time ORP measurements with low signal drift.
SUMMARY OF THE INVENTION
Briefly, therefore, the present invention is directed

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