pH Electrode FAQ

A: Yes. The output signal and impedance of all pH/ORP electrodes are compatible with virtually all meter and controllers. When ordering, please specify the type of connector, the temperature compensation (if using Automatic Temp Comp), and the installation configuration (if installing in a tank or pipe).

A: No. Due to the hazardous materials (mercury and mercurous compounds) used in Calomel electrodes, Sensorex has developed an alternative calomel free pH electrode using non-hazardous materials. Calomel electrodes were primarily used in applications involving proteins, sulfide, Mercaptan and silver ions. These materials react with the silver ions in the typical reference solution/gel and precipitate at the reference junction, plugging the junction, creating a reference junction potential, and/or contaminating the reference solution/gel. Sensorex offers a calomel free design or a conventional double junction design with no silver or chloride ions in contact with the sample. The classical problems (hysteresis, instability, and short life at elevated temperatures) characteristic of Calomel electrodes are eliminated with the Sensorex Calomel Free or Conventional Double Reference Junction design.

A: Yes. However, if your application involves proteins, a Sensorex double junction electrode is recommended. Proteins will react with the silver ions in the typical reference solution/gel and precipitate at the reference junction, plugging the junction, creating a reference junction potential, and/or contaminating the reference solution/gel. Sensorex offers a calomel free or a double junction design with no silver or chloride ions in contact with the sample. If your application involves low ionic (less than 100 microsiemens) samples, Sensorex also has a low ionic electrode which will provide greater stability and longer life in these applications. Contact the factory for these specific applications.

A: Each Sensorex pH electrode is supplied with a soaker bottle or storage cap containing soaker/storage solution. If you no longer have the solution, you can purchase it by pint bottles from Sensorex (part number S16). If soaker solution is not available, you can use the following in order of preference: 4 pH Buffer 7 pH Buffer Tap Water Note: Do not store any pH electrode in DI water. DI water is for rinsing, not storing.

A: Standard pH electrodes contain 3.5M KCl as a reference gel. Samples like DI water and other low ionic solution contain very small amounts of salt. These solutions try to leach ions from the reference gel to raise their conductivity levels. This results in very slow, sluggish, and unstable readings from a conventional pH electrode. Sensorex offers any model in double junction electrode in a special low ionic configuration which addresses these measurements difficulties. Contact Sensorex for more details.

A: All pH electrodes provide a high impedance millivolt signal. This signal is amplified by a pre-amp. The pre-amp is typically built into the instrument or controller, but can be a separate component or even built into the electrode. Normally, cable runs under 25 feet are not a problem for the millivolt pH signal. If you want to ensure a strong signal or need long cable runs, the Sensorex pHAMP-1 battery powered amplifier can ensure quality signal transmission. If you are using an S8000 platform solution, there is a simple EM800 add on module that can be used. Contact Sensorex for more details.

A: Using a flat measuring surface in agitated turbulent flow will minimize coating problems on the measuring surface of the electrode. The coating tends to move off the measuring surface since the entire surface is exposed to flowing sample. Normal spherical bulb measuring surface have a dead flow area which promotes coating buildup. See our Operating Principles section or contact Sensorex for details.

A: Sensorex and all manufacturers' pH electrodes conform to the Nernst Equation for pH output as a function of temperature. At 25 degrees C, the pH output is 59.16 millivolts/pH unit. The electrode will give zero output at a pH of 7.0 at all temperatures.

A: pH glass impedance goes up as temperature goes down by a factor of 2.5 times for each 10 degrees C change in temperature. For example, if the glass impedance is 100 Megaohms at 25 degrees C, it will be 250 Meg ohms at 15 degrees C. Higher impedance slows down the electrode's speed of response to changing pH levels. This is characteristic of all pH electrodes.

A: As a pH electrode ages, the impedance of the pH sensitive glass increases causing the output signal to become slow, sluggish, or drifting. Zero will shift beyond the adjustable range and span will shorten. In most typical applications, a pH electrode will function for 6 to 24 months. However, in fluoride applications or very caustic applications, the typical pH glass has a much shorter life. See our section on Fluoride Resistant electrodes for these applications.

A: All pH electrodes will have a small amount of offset as manufactured. The zero adjustment on your meter is designed to correct this offset. Consequently, you must calibrate the meter and electrode together, as a system. Sensorex supplies a pH Checker to calibrate or isolate a problem to your meter or electrode. But final calibration must be performed with the electrode connected to the meter. Sensorex also supplies buffers for performing pH and REDOX system calibrations.

A: The most common cause for this problem is an electrical ground loop in your system. To verify this problem, remove the electrode and calibrate it with a known buffer solution in a beaker. If the electrode measures within specification (stable and adjustable zero) when calibrated, place a copper wire in the beaker and the other end in your system. If the reading become unstable or shifts, a ground loop is your problem. The source of the ground loop could be any motor, pump, conductivity probe, or other electrically powered device in the media with the pH electrode. Do not attach the conductivity probe or any other electrically powered device to the same ground on your meter or controller as the pH electrode. You can also try placing a large (12-14 AWG) copper conductor into the media and the other end to the meter/controller ground terminal to draw the ground loop away from the pH electrode. See our section on Ground Loop Interrupt pH electrodes for other solutions.

A: There are three possibilities to explain these observations:

1. the electrode sensing surfaces may be coated
2. the electrode may be reaching its useful life (typically 6 to 24 months)
3. a low ionic (conductivity below 100 microsiemens) media is being measured.

Call Sensorex for our special low ionic pH electrodes or specify low ionic application on the order form.

A: A 2% - 5% hydrochloric acid (HCl) solution will dissolve most coatings. It is best to use a cleaning chemical that will dissolve the coating on the pH glass but not harm the materials of construction of the pH electrode. Refer to the specific product data sheet for materials of construction.

A: If your application includes proteins, heavy metals (Ni, Cd, Cu, Cr, Ag), sulfides, cyanides or iodides, or any other material which will react with silver or chloride ions, a Double Junction electrode will provide added protection and last longer.