pH/ORP Frequently Asked Questions

pH itself is unaffected by flowrate but the measurement of pH can be. In most conventional applications (not high purity water) there will be very little influence of flowrate on a pH measurement. Small effects may occur at the reference electrode junction (diaphragm) and cause variations on the order of ± 0.1 pH for flow velocities past the sensor tip from 0-2 m/sec. The volume flowrate depends on the pipe size.

Pressure may have some effect since it directly affects the reference junction, forcing trace amounts of process material into the junction. At very low or very high pH (<4 or >10 pH) this will have greater effect and could be on the order of ± 0.2 pH for large pressure changes, depending on the application.

Cycling high pressures and flowrates will cause slight compression and expansion of electrolyte contents, cause electrolyte dilution or contamination in the junction and will shorten the life of a pH sensor. Maximum sensor life and performance are achieved by locating the sensor in a side stream with moderate flowrate and discharge at atmospheric pressure.

Pure water pH measurements (samples <40 µS/cm) can be strongly affected by flowrate. It should never be attempted with a sensor directly mounted in a plastic flow chamber or fitting since the flow of high purity water past insulating surfaces generates a static charge that will make the pH reading very sensitive to flowrate. The sensor should be in an earth-grounded stainless steel flow chamber on a side stream with low flowrate - typically 50-100 mL/min. and with discharge to open drain. Low flowrates are also necessary because of the reference junction potentials that can be generated in pure waters. A flowing junction reference electrode, with refillable electrolyte reservoir is recommended for best accuracy. The Thornton pHure Sensor^TM, which uses a pre-pressurized gel reference electrode gives very similar performance.

The calibration interval needed for pH is highly application-dependent. The more uniform process conditions are (temperature, pressure, composition, freedom from coating, etc.), the more stable a pH sensor will be and the longer the calibration interval may be. Most pH installations calibrate between once a week and once a month but shorter or longer intervals may be appropriate based on experience. It is a good practice to begin by calibrating frequently and then gradually go to longer intervals, as stability in the installation and accuracy requirements allow.

A change in ORP values for a given process may be due to a number of factors. Instrument electrical stability is seldom a problem. ORP can be influenced by changing mineral composition or changing pH from alternate water sources. ORP reference electrodes will eventually drift a little with age and will ultimately have to be replaced. For more information see the Application Bulletin on ORP.

Stable ORP requires that a definite oxidizing or reducing condition be present. In pure water, ORP will drift around, responding to levels of dissolved oxygen, traces of contaminants or the last solution in which the sensor was immersed.

Thornton does not recommend "calibration" of ORP sensors in solutions. ORP measured in absolute millivolts (from an electrically calibrated instrument) will provide a firm basis for comparing readings. Standards for ORP are generally used to verify response of a sensor but their tolerance under industrial conditions is too wide to be used for calibration.

Standards are described in ASTM Practice D1498, including the most commonly used ones which are pH buffer solutions saturated with quinhydrone. Quinhydrone is an organic chemical available from laboratory supply houses. These solutions must be made up fresh for each use because of air oxidation in storage. There are some commercially available solutions using ferric/ferrous compounds claimed to be stable for longer periods. For more information see the Application Bulletin on ORP.

The pH and ORP measuring electrode (center) part of the sensors are glass and platinum respectively and can have very long life as long as they are not broken or chemically attacked. pH glass electrodes are attacked by hot caustic solutions and will gradually lose response over weeks or months, depending on the severity of conditions above 10 pH and/or above 60 degC. pH glass electrodes can also be attacked and destroyed by hydrofluoric acid over hours or days depending on the severity of fluoride concentration above 5 ppm and pH lower than 4.

General purpose pH and ORP sensors also include a reference electrode which has a finite amount of electrolyte. The electrolyte salts must maintain consistent electrical contact with the process sample though the reference junction (surrounding the measuring electrode) to allow measurement. Because of this contact, it is subject to gradual dissolving and loss in the sample, although this is retarded by a thick gelling agent.

Reference electrode life is highly application dependent, from weeks to years depending on temperature and pressure cycling which expand and contract the electrolyte, tending to "pump" it out of the sensor. In dirty samples the reference junction is also subject to coating and contamination.

Sensor life in routine, moderately clean applications at ambient temperature and pressure is typically 6 months to 2 years but this can be reduced by any number of factors, the most common of which are described above.

If a pH or ORP sensor is stored at normal room temperature with its original storage cap and solution in place on the end of the electrode, it can last well over a year with no degradation in performance. If the storage solution is allowed to dry or leaks out due to storage at high temperatures, freezing or other causes, the life may be reduced significantly.