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pH Electrode

pH Measurement

The most common pH measurement system utilizes glass pH electrodes. The system consists of a pH sensor(whose voltage varies proportionately) to the hydrogen ion activity of the solution), a reference electrode(which provide a stable and constant reference voltage), a conductive measurement solution and a special meter to display and measure the pH.

The pH sensor incorporates a thin membrane of hydrogen sensitive glass blow on the end of an inert glass tube. This tube is filled with a buffered electrolyte, and a Ag/AgCl wire. This is called a pH half cell. A complinentary system produces a constant voltage; it also contain a Ag/AgCl wire and an electrolyte(often a KCl) solution saturated with AgCl). A small “filter”. Often a porous ceramic piece, connect this tube to the external sample. This system is called a reference half cell.

The meter measures the voltage difference between the pH half cell and the reference half cell in DC millivolts. The reading is read by the meter and displayed in either mV or pH units.

The remarkable glass pH measurement system is capable of measuring over fourteen decades of hydrogen ion concentration. The system has a proven track record for reliability and predictability. Several companies have marketed ion selective field effect transistors(ISFET technology) as a response to the need for glass free measurements(such as the use of glass in food processing). An ISFET is a chemically coated transistor that responds to hydrogen ions as well as oxidation and reduction voltages. These electrodes also require a reference electrode. ISFETS are reported to suffer from drift and instability buy may still have a niche market. The glass pH sensor is still the far better choice for the majority of applications.

Electrode Body

Until the seventies, it was a common practice to offer two half cells separately, a glass pH sensor and separate reference electrode. Today it is more common to use a single combined electrode that has both of the components. Reference electrodes still enjoy use in other electrochemical techniques and are often preferred with ion selective electrodes(ISE) half cells.

Combination pH electrodes are often made entirely of glass. The bodies of these electrodes are lead free glass, which is not pH sensitive. All glass electrodes are ideal for routine laboratory work because they respond quickly to temperature changes, are easily cleaned and are compatible with organic solvents. However, in the hands of some, glass can be very breakable.

The electrode body can be made less fragile by incorporating an outer body made from a thermoplastic the uses of PEI resin, PVDF and PP as examples of materials utilized for outer body construction.

Some industrial sensors utilize additional materials such as PVC and titanium, the space age metal. A titanium body increase immunity to electrostatic and magnetic fields and features strong corrosion resistance(even in sea water).

Matching Pin

A matching pin is a differential measurement technique used to eliminate ground loops and common mode perturbations for the measurement system. In a system without a matching pin, electrical currents in the sample can effect the reference half cell voltage that is connected via the liquid junction with the sample. In this case, the reference electrode picks up the electromagnetic field and the measurement of the pH is altered. The matching pin isolates these current/magnetic fields from the reference electrode. Many factory manufactures a number of models with the matching pin design for safe precise pH measurements.

Measurement Preparation Procedure

A coated measurement sensor or reference may yield slow responding and erroneous results. To ensure the best pH measurements possible, preventative maintenance and storage practices are strongly advised. A clean, conditioned pH electrode will provide the best measurements possible.

When using a new electrode, remove the protective bulb cap and inspect the electrode. As water may have evaporated during shipping or storage, salt crystals may be found in the cap or on the pH bulb. Rinse off with water. During transport, air bubbles may have formed inside the glass bulb. Shake down the electrode as you would an old style mercury thermometer.

Condition the sensing tip, soak the pH bulb and junction in pH storage solution for at least one hour or longer. If possible, and overnight soak is best. This will hydrate a dehydrated glass sensor, thoroughly wetting a dried reference junction(wick, ceramic etc.)

Water Conductivity and pH Measurement

pH measurement is the measurement of hydrogen ions, Ultrapure water is the perfect solvent, and readily dissolved many things. The pH glass surface can actually become dehydrated if stored or used in deionized or distilled water as ions are leached from the sensing surface. pH electrodes require ions in a solution, preferably with a conductivity of or exceeding 200 microsiemen/cm to function properly. In the case of low conductivity samples or when working below 200 microsiemens/cm. we suggest the use of specific electrodes, such as HI10538

How Temperature effects pH measurements

Samples change pH as a function of temperature due to changes in ion dissociation and increased ion activity with increases in temperature. An example of this is pH buffers whose well characterized values are published on the buffer bottles. With very pure water, a change in ~1.3 pH is observed between 0 and 100 degree celcius. This example shows that even a neutral solution can have a large temperature coefficient. All samples have a temperature coefficient that is variable for actual samples. Changes in pH due to the sample temperature coefficient are not compensated for.

This is an exception to this because buffers are well characterized, on intelligent pH meters, during calibration, the buffers are compensated for. They will display a 25 degree celcius value during calibration but will change after the calibration to read their actual pH at the temperature of measurement.

pH Compensation

It well-made pH electrode together with modern pH instrumentation can compensate for changes in temperature related pH measurements due to thermodynamic changes demonstrated by the Nernst Equation. The electrode itself is made with an isothermal point close to pH 7 and a slope that closely follows the Nernst Equation. When temperature compensation is made with a pH meter. The slope(or gain) of the meter is changed to compensate for the Nernst slope change exhibited by the electrodes.

Alkaline Error

Alkaline error exists in high pH solutions when the hydrogen ions in the gel layer are partially or completely substituted with alkali ions, and the resulting pH is lower than it actually should be. The difference between the theoretical and experimental pH is called the alkaline error. Sodium ions are typically the ions that are responsible but potassium and lithium ions can also contribute to this error, In earlier glass compositions, the alkaline error was seen to start at pH9. Newer glass formulations and ones especially formulated to minimize this error now exhibit an error starting at pH12 or 13 pH.

Half Cell/Reference Electrodes

All potentiometric measurements are taken with two electrodes, one is a sensor with a potential that varies as a function of the concentration of the species to be measured and the second is a reference electrode which keeps its potential constant. The mV reading is the difference of the two values.

Reference Half Cell Junction

The reference half cell must be constructed so as to allow contact between the fill solution inside the half cell and the sample being measured. The fill solution must meet some requirments:

* The reference fill solution should not interfere with the measurement.

* The reference fill solution should not react with the solution measured(no precipitates or complexes).

* The reference solution should be miscible with the solution measured.

* The solution measured must not react chemically with the reference half cell.

* The mobility of the ions in the fill solution should be matched(equitransferent)

* Should be non-toxic

The junction(the part in contact between the two liquids) is typically made with inert materials that will not increase a junction potential or be chemically attacked by the measured solutions. The materials most commonly used are :

Porous Ceramic. Normally used in electrodes with glass bodies, because ceramic with the correct expansion coefficient is is easily welded to glass. Ceramic is available with different porosities and diameters. Also sometimes called a diaphragm.
Porous PTFE(PolyTetraFluoro-Ethylene): is a hydrophobic material that is available with different porosities. Because of its chemical advantages, PTFE is widely used in industrial applications.
Fiber wick : This type junction is often used on plastic bodied electrodes with gelled electrolytes. The advantage of this type junction is it is renewable; as the cloth like material is pulled out from its position, the junction is renewed with an uncontaminated fresh surface.
Cone Style : This junction is also renewable. As the sleeve or collar is moved fresh fill solution cleans out the junction with fresh electrolyte. This has a higher flow rate than a ceramic type and is often specified for ISE measurements.
Open junction : This type junction is found in reference half-cell is filled with a special gel which comes into direct contact with the solution to be measure. An advantage of an open junction is low conta resistance and it is virtually impossible to clog.

Other types of juntions include :

Capillary junction : Can be made with smooth or frosted glass. The advantage of a capillary junction is a fast flow rate and a very open channel. Typically used with thickened electrolytes.