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Besides glass membranes, pH-electrodes differ in reference systems and diaphragm types.  The desired application makes the choice between pH electrode reference systems and diaphragms easier.

What is a pH electrode reference system and why do I need it?
To obtain a pH measurement you must have something to measure and the most common method is by measuring a voltage.  To measure a voltage the pH electrode must be able to measure the difference between two points with different electrical potential values.  For a pH electrode to provide a voltage measurement of a solution's ion concentration a reference electrode is necessary because its potential essentially remains constant and independent of the solution and temperature relative to the solution being measured.  The pH electrode can then use that reference electrode's potential to determine how the solution's ion concentration compares to the reference.  The voltage developed from this comparison is then turned into the pH measurement.

What kind of reference systems are there for pH electrodes and what can they provide?
The Standard Hydrogen Electrode (SHE) is used as the international reference system.  Unfortunately due to its complicated handling requirements it is not typically used for standard applications.  A common approved reference system is the Saturated calomel Electrode (SCE), however this electrode contains mercury and is toxic.  The most common reference system is the silver/silver chloride reference system (Ag/AgCl).  An alternate to the silver/silver chloride reference system is the Silamid or double junction reference system.  The Silamid double junction reference system is a special construction of the Ag/AgCl reference system.  Most electrodes having a Ag/AgCl reference system are built with an Ag wire coated with AgCl.  Silamid double junction reference systems have a glass tube with the inner part coated with Ag, then filled with AgCl, and plugged with a polyester fibre.  This double junction reference system creates greater contact surface area between Ag and AgCl compared to the standard Ag/AgCl wire system.  This results in a reference system that is long lasting and very stable.  The most recent reference system being used is the iodine/iodide system.  The iodine/iodide reference system does not precipitate silver and can be used with Tris buffers.  The advantages and disadvantages of different reference systems are displayed in table 1.  Further characteristics of the reference electrode are defined by the diaphragm.

Reference System Advantage Disadvantage
Ag/AgCl Well described, multifunctional, reproducible, wide temperature range, nontoxic Reference potential depends on temperature and could deliver a different potential, if measured at a different temperature as calibrated, precipitation of silver
Ag/AgCl (Silamid/Double Junction) More stable reference, long lifetime Precipitation of silver
Hg/Hg2Cl2 (Calomel) Stable reference potential Toxic, low temperature application range 59 to 104 °F (15 to 40 °C)
Iodine/Iodide Low polarization, low temperature dependence, free of undesired heavy metal ions Few known
Table 1: Advantages and disadvantages of different reference systems.

The most important pH electrode reference system is the Ag/AgCl system because it is well described, reproducible, and nontoxic.  In the few applications where this reference system does have problems the newer iodine/iodide reference system can be used instead.  Due to an absence of silver ions or other contaminating metal ions the iodine/iodide reference system is an excellent alternative when working with applications requiring rapidly changing temperatures.  Even with quick changing pH values such as titrations, the iodine/iodide reference system is beneficial.

Why the reference system needs a diaphragm?
The diaphragm or reference junction is the electrical connection of the measuring system to the sample.  Superior and dependable diaphragms allow the reference electrode potential to remain stable for longer periods of time.  Additionally the higher the quality of the diaphragm, the greater the accuracy of the pH measurement.  The other main purpose of the diaphragm is to act as a physical barrier between the sample solution and the reference electrolyte minimizing contamination of the reference electrolyte with the sample solution.

To generate the pH measurement voltage the diaphragm must be able to conduct current.  To enable the current flow the pH electrode's diaphragm allows some amount of reference electrolyte solution to enter the diaphragms structure.  This allows a small amount of pH electrode's reference electrolyte to contact the sample solution.  The quicker the reference electrolyte flows through the diaphragm the more effective the electrical connection is.  A greater electrical connection results in a faster response time of the pH electrode.

The disadvantage of a fast reference electrolyte flow is that the faster the electrolyte flows out of the reference electrode the more often it will need to be refilled.  Also, the reference system electrolyte can alter the sample solution if too much flows into into it.  A good alternative to a liquid electrolyte filled electrode is the gel filled pH electrode.

Type Resistance Flowout1 Applications/Properties
Ceramic ~ 1 kΩ ~ 0.2 ml/day General applications, robust, low-priced
Diaphragm has rugged cavities which are places for deposits and chemical reactions, tend to become polluted or blocked
Platinum ~ 0.5 kΩ ~ 1 ml/day Is universal with a short response time, constant, dirt resistant, clean and defined flow channels, less diffusion potential
Diaphragm can only be cleaned chemically, not mechanically
Ground Joint ~ 0.2 kΩ ~ 3 ml/day For emulsions, paste, pure water
Easy cleaning, outflow may change depending on handling, may loosen if overpressured inside, filigree
Annular Gap ~ 0.1 kΩ None2 Symmetrical annular gap, easy handling, dirt resistant
Sample can flow into reference system, no cleaning of reference system
Fiber ~ 1 kΩ None2 Quick response time, easy handling
Sample can flow into reference system, no cleaning of reference system
1) 3.3ft (1m) water column
2) Only applicable for low maintenance electrodes
Table 2: pH electrode reference system diaphragm applications and properties.

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