A pH probe consists of two sensing electrodes. One electrode is encased inside a glass bulb composed of a specially formulated, extremely thin glass membrane, which reacts to pH changes. The other is called a reference electrode. It is filled with a conductive salt solution of a particular strength. It also has a porous plug of densely woven material, so that although only a very small amount of liquid passes through it, it can still pass electric current. These two electrodes are currently combined into a single electrode unit.

The pH probe, when immersed in a solution to be measured, develops a voltage differential between the two sensing electrodes. The voltage of the electrode inside the glass changes with the amount of acid in the solution. The voltage of the other electrode is constant, which is why it is called a reference electrode. The pH meter is a specialized voltmeter that draws an extremely small amount of current. It measures the voltage difference between the electrodes and then performs some arithmetic on the voltage that converts it into the pH value displayed on the meter.

Try an Acid Soak

The most problematic probe is the one in the final adjustment tank—the tank directly before flocculent addition. The glass bulb on this probe has a tendency to pick up a coating of iron- or lime-based scale. This slows the response of the probe and makes its performance less reliable, even if regularly calibrated. It can be removed by soaking the probe in a solution of two parts of muriatic acid in 100 parts tap water overnight, rinsing it thoroughly with tap water, and placing the probe in a buffer of a pH of 4 or 7 to recover for a few days. Then you can reinstall the probe and recalibrate it. That is the labor component of maintaining your probes.

There is also an investment component, as well. Of course, your system cannot be run without the probe in place. You can purchase at least one, and possibly as many as three spare probes, that will be held in the pH 7 buffer "bullpen," and simply switch to one of them. Don't forget that you have to recalibrate the meter to the fresh probe before putting it in service. Speaking of calibration, from my own experience, I make it a habit to calibrate the pH system weekly.

Following this simple protocol optimizes system performance. It also lengthens the life of each of the probes—possibly to the point where the investment in the extra probes pays for itself.

Cyanide Destruct, Chrome Reduction, and pH Probe Maintenance

If you use pH probes as a part of a pH or ORP feedback control scheme on chrome reduction and/or cyanide oxidation, you face additional challenges. Because of the extreme pH conditions required to carry out these waste treatment operations, the sensors used to regulate them have special maintenance problems. Frequent inspection and calibration are needed to avoid system upsets.

A problem common to both of these subsystem sensors is the probes can become coated with oil. In extreme cases, oily matter can completely blind the sensor, causing costly chemical overdosage and possible permit violations. If a thorough rinse with water fails to remove oil, swish the sensor in 70% isopropyl alcohol for 10 to 15 seconds, rinse very well with water, and soak the probe in pH 4 buffer for several hours before reinstalling.
Cyanide destruct chamber probes suffer from two chronic troubles. First, alkaline conditions are not kind to the glass bulb on the probe. Second, alkaline conditions tend to cause a buildup of carbonate-based scale on the bulb. To lengthen the useful life of these sensors, demount them during weekends and other times when the system is idled, and pop them into a container of pH 4 buffer. This will recondition the bulb surface and loosen any scale that may have formed. Heavy scale accumulations may require the acid-soaking procedure described earlier.

The probe in the chrome reduction chamber, because of the acidity of the process stream, is less apt to pick up scale. However, the precision of pH measurement in this process is more critical than in cyanide destruct. Alkaline chlorination of cyanides can be carried out effectively anywhere in the pH 10 to 11.5 range. But the rate of the reaction between hexavalent chromium and metabisulfite is sharply dependent on pH; a variation of as little as 0.2 pH units can affect it enough that significant unreduced hex chrome will pass through. So, check your calibration frequently!

Some Notes on Calibration

Although it is customary to calibrate a pH meter to pH 4, 7, and 10 buffers, two buffer calibrations may be preferable for these sensors. This is especially true of older meters and controllers that have no computer-assisted calibration routines. It is quite common for pH probes that have been in service for a long time to show some irregularity in slope response. This may make it impossible to set the slope adjustment so that both 4 and 10 buffers will yield readings of 4.00 and 10.00. It does not make sense to sacrifice accuracy in the range that you are measuring so you can get a reasonable reading for the buffer in the range you are never measuring in.

Therefore, I suggest calibrating to 7 and 10 buffers for the cyanide destruct sensor, and 7 and 4 for the chrome reduction sensor. For optimum conditions, the chrome reduction can be calibrated using a pH 2 buffer (this is commercially available). This gives a bit better pH value precision, which is important for chrome reduction.

ORP Probes and Setpoints for Process Regulation

ORP, which stands for oxidation reduction potential, is a measure of the voltage between a strip of metal, usually platinum, and a reference electrode. The reference electrode's voltage is independent of the composition of the liquid in which the sensor is immersed. The voltage on the metal strip, however, will change depending on the presence of oxidizing or reducing substances. This enables us to use an ORP sensor to regulate waste treatment processes in which we use such substances as sodium hypochlorite or sodium metabisulfite.

We start with raw waste to which we add a treatment chemical. The ORP will change in small (5 to 10 mV) increments as the amount of toxic material in the waste decreases. When enough treatment chemistry has been added so that the waste has been completely neutralized and the excess treatment chemical begins to persist in the mixture, a radical shift of 200 to 300 mV will be observed. After this point is passed, the ORP again changes in small increments as excess treatment chemical begins to build up in the treatment mixture. The optimal ORP set point will be in this last region—a few millivolts past the radical shift. What we want is to have a substantial (though not overwhelming) excess of treatment chemical in the waste mixture, so that the reaction takes place rapidly and completely. A solid excess of treatment chemical in the mixture also ensures that incoming raw waste will not pass through the process untreated.

The optimal set point for each system will be different depending on waste composition, the degree of agitation, the pH at which the process is run, and other factors. Don't rely on a "canned" setting from a manual or a result from a laboratory experiment. Good starting values for the ORP setpoints are +150 to 200 mV for chrome reduction, and +550 to 600 mV for cyanide destruction. However, you must still determine your optimal set point by test runs with the system. With the system idled, fill the cyanide destruct or chrome reduction tank with raw production waste. Now, batch treat it such that a strong excess of treatment chemical is present. Excess sodium hypochlorite (bleach) may be detected by a strong blue color on a strip of potassium iodide/starch test paper. Excess metabisulfite may be detected with a mixture of dilute iodine and starch indicator solutions - the intense blue will turn clear in the presence of excess sulfite. Some people use the odor of sulfur dioxide coming off the waste as a guide, but sulfur dioxide is a powerful respiratory irritant, making this an unwise practice. Finally, adjust the pH to the value you will be using. Suggested values are 2.0 for chrome reduction and 10.5 for cyanide oxidation.

Now, activate the ORP controller and associated feed pump. The pump should not turn on. If it does, back off the set point on the controller until it stops. Now, feed into the tank a slug of raw waste equal to 10 to 15% of the tank volume. This should consume the excess treatment chemical in the waste mixture. It should also cause the pump to come on until enough fresh treatment chemical is fed to the mixture to leave a strong residual in the waste mixture. The pump should then turn off. If you don't observe this behavior, make a small adjustment to the set point setting in the proper direction to correct it. It may be necessary to repeat this process several times, using several fresh slugs of raw waste. This process may be time consuming, but it only needs be done when a new system is started up, or if the waste composition changes radically.

Can ORP Sensors Be Calibrated?

Yes, ORP sensors can be calibrated, but unlike pH calibration buffers, the buffer solutions must be freshly prepared. There are various commercially available mixtures that may be used. Or, you can use quinhydrone. This is a mixture of two chemicals that are interconvertible by oxidation and reduction. The degree to which this interconversion takes place is highly dependent on pH. The calibrating mixtures are prepared by adding about 0.1 grams of quinhydrone to about 50 ml each of pH 4 and 7 buffers. The solution made with the pH 4 buffer will have an ORP of 264 mV; that made with the pH 7 buffer will have an ORP of 84 mV. With those controllers equipped with a 'zero' and a 'slope' control, use the zero adjustment to calibrate to the 84 mV mixture, and the slope adjustment to calibrate to the 264 mV mixture. You will have to work back and forth between the two to get reasonable values for both. Great exactness is not necessary.

This article is adapted from a series of essays that appeared in the "Pollution Control Corner" column of The New York American Electroplaters and Surface Finishers Society Newsletter and is used with their kind permission.