By Michael Ross

How does a pH neutralization system operate? What problems may be encountered? How can an operator resolve these problems?

For many years, automatic pH control systems have been used to optimize manufacturing processes and to assure product quality. More recently, stringent new governmental regulations regarding wastewater discharge require any industry that discharges effluent into sewer systems, lakes or streams to neutralize this effluent before allowing it to be discharged. In many instances, recording the pH of the discharge is also required. A pH system consists of five basic components:

1) Monitoring, controlling and recording instrumentation
2) pH electrodes and holders
3) Effluent holding tank
4) Chemical pumps and reagent storage tanks
5) Mixers/agitators

The accompanying illustration (fig. 1) depicts a simple automatic pH control system.

In this system, effluent flows into the holding tank where a pH electrode senses the pH of the solution. The pH electrode relays this information to the pH controller which operates chemical pump(s) to inject acid or caustic as required to neutralize the effluent. The mixer serves to evenly distribute the neutralizing chemicals throughout the holding tank to ensure accurate neutralization.

After neutralization has been completed, the effluent is discharged to waste while a second pH electrode senses the discharge effluent's pH. The electrode relays this information to a recording device which provides a record of the discharge for inspection or future reference.

Both cost and trouble-free operation are important considerations when choosing a pH control system. Manufacturers of holding tanks, controllers, recorders and mixing devices are usually more than helpful in assisting with sizing and selecting equipment for specific needs. As with any purchase, don't be afraid to ask questions of potential vendors to ensure that you are obtaining the best system for your application.

Many problems encountered in a pH neutralization system are centered around the pH electrode. Incorrect choice of electrodes, holders and placement are sources of common problems.

pH Electrode selection Electrodes are available in many different configurations. Choosing the best electrode for your needs is essential to proper system operation. Some basic questions which should be considered when choosing a pH electrode are:

Reference type- sealed vs. Refillable

Historically, it had been though that stable reference electrodes required relatively high-flow liquid junctions. For that reason, all reference electrodes were refillable. During the last several years, sealed, essentially no-flow designs have been shown to be of equal stability. Being permanently sealed, these electrodes eliminate the need for refilling and can be submerged in tanks used in pressurized applications without the need for external pressurization. With a few exceptions, the sealed reference is the better choice for most process applications.

Reference electrode type- single or double junction

Many pH measurements can be made with single junction reference electrodes. However, material such as heavy metals, sulfides, proteins and other materials that interact with silver can precipitate at that junction. Double junction designs use KNO3 in the section contacting the sample and prevent these reactions, and provide additional protection against contamination.

Temperature compensation-manual or automatic (ATC)

Many neutralization applications do not require ATC since there is no temperature error at pH values close to seven. The pH error based on temperature is expressed as 0.03pH/pH unit/10(C. For example, between 15 and 35(C, with a working pH between six and eight, the error would only be 0.03 pH. If the application has wide pH and temperature excursions, the ATC can be useful.

You need to decide on how important and cost effective ATC is to the particular application. Any instrument that has ATC capabilities can be converted to manual temperature compensation by putting an inexpensive resistor across the terminal where the wires for the ATC terminate.

Contact the manufacturer of your instrument for the correct value of the resistor to insert.
Features, benefits & replacement cost

Careful consideration should be taken in evaluating the pH electrode you are selecting. Electrodes which contain built-in temperature elements and/or potted preamps can cause your replacement costs to escalate. Since most pre-amps and temperature elements will outlast several pH electrodes, it may be advisable to use preamps and temperature elements which are reusable and not built into the pH sensor or electrode.

Mounting holders

Several pH electrode mounting choices are available. From the following list choose the mounting method which best suits the specific application.

Submersion: for use in open tanks, sumps and vessels.
By-pass: for use in 3/4" by-pass flow lines.
In-line: for use in tanks with 3/4" taps
Insertable: for use on active lines up to several feet in diameter where the electrode can be removed without the need to shut down the process line.

Electrode Problems

The most common problems experienced with pH electrodes are:

Oily and solid coatings requiring frequent removal for cleaning.
pH bulb breakage or premature failure as a result of abrasives or solid materials in the solution.

Reference junction fouling or plugging requiring frequent removal for cleaning
Reference side contamination as a result of interactions between the silver ions in the electrodes and materials in the solution which interact with silver. This result is short electrode life.

These problems can be solved by careful pH electrode selection. For example, Sensorex, a manufacturer of pH and ORP electrodes, has developed a durable and economical pH electrode for today's industry. A flat surface self-cleaning electrode has been designed to solve or minimize the above mentioned difficulties. The flat surface design is used rather than the normal spherical shaped bulb for the following reasons:

In a stirred or flowing solution, the back, or downstream side of the usual spherical bulb does not see flow and encourages buildup. With flat design, the material is pushed across the face of the electrode with new material pushing away the old. In most applications, the scrubbing action of the flow across the electrode's flat surface keeps it clean.

Breakage or premature failure from abrasion is also reduced by eliminating the protruding spherical bulb: particles simply sweep the flat surface without hitting the glass sensing surface.

Normal spherical bulb type electrodes tend to have very small reference paths. The flat design utilizes a large surface area porous reference junction which has thousands of paths for reference diffusion and the possibility of all of these paths becoming plugged is quite remote.

The flat surface design incorporates a double junction reference design for maximum protection against reference side contamination which can cause premature failure.

pH electrode placement

The location of the pH electrode relative to the point where chemicals are added is crucial to achieving proper pH control. Where the chemicals are added, the pH will be dramatically different from the average effluent pH. It is essential that thorough mixing occurs so that chemicals are evenly distributed throughout the tank.

Perfect mixing throughout the tank can be difficult to obtain and this fact makes the pH electrode placement an important consideration. If the electrode is located too close to the chemical injection site, it will sense the pH before mixing can occur. If the electrode is too far away from the addition site, it may be late in sensing the change and over-adjustment and chemical waste can occur. Some experimenting may be needed to find the optimum electrode location.

Awareness of what products are available as well as potential problems is essential when purchasing or upgrading a pH neutralization system. Proper electrode selection can save time, money, and help ensure trouble free operation.