pH, ORP and ISE Theory – Troubleshooting

The most important principle in troubleshooting is isolating the components of the system and checking each component in turn. The components of the system are the meter, electrodes, buffers or standards, sample and the technique.

pH or ISE/pH Meter

The meter is the least likely problem in the system and is easily eliminated as a possible cause of error. Orion meters include an instrument check-out procedure that verifies proper meter function or locates possible malfunctions. Consult the meter instruction manual for complete instructions.

pH and ISE Electrodes

Before replacing a faulty electrode, review the electrode instruction manual and be sure to:

• Clean the electrode thoroughly
• Condition the electrode properly
• Use proper filling solutions
• Use proper technique
• Review the electrode troubleshooting checklist

pH Buffers and ISE Standards

Always use fresh buffers or standards when problems arise. The quality of analytical results depends greatly upon the quality of the standards. Using fresh buffers and standards could save hours of frustrating troubleshooting. Upon exposure to air buffers may absorb CO or other material from the air, causing a shift in the pH value. ISE standards may be contaminated or incorrectly prepared.

Sample

If the electrode functions correctly in buffers or standards but not in a sample, the electrode may be adversely affected by a substance in the sample.

Technique

Review operation procedures and instruction manuals to verify that proper measurement technique has been followed. For ISE measurements verify the method of analysis, direct or incremental technique, is appropriate for the sample.

Low Slope

• Clean and/or replace electrode
• Condition electrode properly; refer to electrode instruction manual for conditioning procedures
• Use fresh buffers or standards and clean glassware
• Prepare fresh standards using proper technique
• Use recommended ISA for the selected ISE electrode

High Slope

• Clean and/or replace electrode
• Use fresh buffers or standards and clean glassware
• Prepare fresh standards using proper technique
• Calibrate with buffers or standards at the same temperature

Drift

• Verify correct filling solution and compatibility with the sample
• Calibrate and measure at the same temperature; use an ATC probe for pH calibrations and measurements
• Clean and/or replace electrode
• Refer to the sample requirements and interferences sections in Orion electrode instruction manual

Erratic Response

• Verify correct filling solution and compatibility with the sample
• Use recommended ISA for the selected ISE electrode
• Clean and/or replace electrode
• Perform meter check-out; verify proper grounding and power

Off Scale or Over Range Reading

• Verify electrode connection; immerse electrode in solution
• Remove air bubbles by gently tapping the side of the electrode
• Clean or replace electrode
• Condition electrode properly; refer to electrode instruction manual for conditioning procedures
• Perform meter check-out

Wrong Answer

• Verify correct filling solution and compatibility with the sample
• Use fresh buffers or standards and clean glassware
• Prepare fresh standards using proper technique
• Refer to the sample requirements and interferences sections in Orion electrode instruction manual

Buffer and Sample pH Changes

Buffer and sample pH values vary with temperature because of their temperature dependent chemical equilibria. The problem of differing pH values is easily solved by calibrating the electrode with characterised standard buffers whose true pH values versus temperature are known. pH buffer values at different temperatures are shown in Table 1. Orion meters calibrate with the correct pH buffer values based on the manual, ATC or digital LogR temperature value. The problem of the sample equilibrium varying with temperature in an uncharacterisable manner will always remain. Therefore, calibration and measurement should be performed at the same temperature and pH values should be reported along with temperature. For best results, an ATC probe should be used.

Reference Element Drift

Drift can occur when the internal reference elements inside the pH and reference portions of the electrode are reaching thermal equilibrium after a temperature change. Long term drift or slow response can last until the sample and electrode are at the same temperature.

Temperature Sensor Errors

When a pH and temperature probe are placed into a sample that varies significantly in temperature, the readings can drift for two reasons. First, the temperature response of the electrode and temperature probe may not be similar which prolongs equilibration and drift. Second, a sample may not have a uniform temperature, therefore the pH electrode and temperature probe are responding to different environments.

7/10/2025