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There are many different pH glass electrodes on the market.  Each pH glass electrode has particular qualities so they should be chosen carefully to suit the measurement application.

What different kinds of pH glass electrodes are available?  What are the main characteristics of pH glass electrodes and which membrane glass is recommended for which measurement application?
Over time the glass membrane of a pH glass electrode changes due to the process of taking pH measurements.  Because of exposure to water, alkali ions dissolve from the glass surface and oxide groups of the silicate become OH groups which causes a source layer.  This source layer appears to hydrogen ions as an ion exchanger.  Using a special pH glass electrode membrane there is a reproducible balance between the sample solution and glass surface, which is only dependent on the hydrogen ion concentration in the solution and the source layer.

Because pH glass electrodes have numerous different capabilities many different kinds of membrane glasses are needed to make accurate and reliable pH measurements for all applications.  SI Analytics offers five different types of pH glass electrodes, L-, H-, S-, A- and N-.  The main characteristics of these pH glass electrodes are:
  • L type pH glass electrode: Wide application range, very low impedance resulting in accurate and rapid response times over a large temperature range
  • H type pH glass electrode: Optimized for higher temperatures up to 275 °F (135 °C) and extreme pH-values, high accuracy in stronger alkaline range (Na+)
  • S type pH glass electrode: Tolerates sudden temperature changes, provides constant measurement values with fast response time in hot alkali solutions
  • A type pH glass electrode: Fast response time in drinking water, surface water, sewage, and general applications
  • N type pH glass electrode: At normal temperatures usable for the full pH-range and almost all kinds of samples.
The following examples illustrate the use of different pH glass electrodes:
  • With a strong alkaline media the so called "alkaline measuring error" appears.  This error is triggered by the confusion of sodium with hydrogen ions (cross sensitivity) and causes a measurement inaccuracy beginning at a pH value of 12 in presence of sodium ions.  Under extreme conditions this inaccuracy could mean a difference up to 1 pH unit.  In those cases the H type pH glass electrode should be used.
  • Applications with hot alkaline treatments or sterilization by superheated steam impose great demands on the consistency of the pH glass electrode's membrane glass.  Under these conditions a pH glass electrode usually ages faster and corrodes.  In this case the right choice would be a S type pH glass electrode.
  • In common applications or drinking water measurements the challenge is the variety of applications and the low conductivity of the pH glass electrodes.  This could lead to slow response times and unstable or unreliable data.  For these demands the A type pH glass electrodes were developed.  These pH glass electrodes feature rapid response times and extended use.
What different kinds of pH Glass Electrode Membrane Shapes are available?
Glass pH electrodes also have many different membrane shapes.  Each pH glass electrode shape is designed for particular pH measurement applications.
  • Sphere pH glass electrode membrane: Very good consistent quality, low resistance because of the large surface area, for common applications
  • Dome pH glass electrode membrane: Higher resistance, shock-proof, easy to clean so they are good for measurements with automated cleaning systems
  • Flat pH glass electrode membrane: High resistance, shock-proof, easy to clean, and designed for pH measurements on surfaces
  • Cylinder pH glass electrode membrane: Medium resistance, shock-proof, for general applications especially fermentation
  • Spear pH glass electrode membrane: High resistance, suitable for penetrating semi-solid media and for measuring systems with automated ultrasonic cleaning
  • Cone pH glass electrode membrane: Robust, smooth, easy to clean, universally applicable
The pH glass electrode membrane glass type and shape are very important to your ability to make accurate pH measurements.  The glass composition, the manufacturing technique, and the shape influence the results and the characteristics of the pH glass electrodes.  Only the right choice of pH glass electrode will provide you with the best accuracy and reliability.

For general information on calibrating a pH glass electrode please click here.
For information on different pH glass electrode calibration methods and the difference between pH solutions please click here.
For specific information on pH maintenance please click here.
For information on pH glass electrode measurements please click here.
For information on pH electrode reference systems and diaphragms click here.
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