Combination ph electrode

Combination ph electrode DEFAULT

The Combination pH Electrode

Modern pH electrodes are usually of the "combination" type, meaning that a single cylinder contains both the reference electrode, and a glass membrane electrode. Schematically, the total cell may be expressed as

SCE//test solution([H3O+]=a1)/glass membrane/[H3O+]=a2, Cl-/AgCl(s)/Ag

Silver-silver chloride or SCE reference electrodes may be as used for internal and external reference electrodes.

The potential of a combination pH electrode is due to the difference in activities of H+ between the test solution and reference solution sides of the glass membrane.

Since a2 is fixed by the internal solution

where K(T) is a temperature-dependent constant. Thus the potential of the combination electrode is proportional to the pH of the test solution.

Alkaline Error

An "alkaline error" can result when cations other than H+ are present in solution. These cations can exchange for H+ in the gel layer,

In this case the electrode potential is

where Kex is the proton-ion exchange constant.

The glass can be tailored by varying the amount of alumina (Al2O3) and other common constituent to what we call glass, (Na2O, K2O, B2O3, etc.). Addition of Al2O3 increases Kex.

Using alumina, the glass becomes sensitive to alkali metal ions at moderate pH. When a1<<KexaM., typically for pH greater than 4, the glass electrode is more sensitive to alkali metals than to proton and

Solid-State Ion Selective Electrodes

Any solid that exhibits a surface activity toward binding an electro-active species can be used as the basis of an ion selective electrode if it can be made conductive.

The most famous is the fluoride ion selective electrode made from the Eu2+ doped LaF3 crystal (Eu:LaF3). In this semi-conducting crystal, F- ions are bound to the surface. The potential between the analyte and standard internal solution faces of the crystal is proportional to the logarithm of fluoride activities.

Ag/AgCl(s)/Cl-, F-/Eu:LaF3/solution//Cl-/AgCl(s)/Ag

The potential is

Here is a list of some of the ion-selective electrodes commercially-available.

  • Fluoride
  • Chloride
  • Bromide
  • Cadmium
  • Cupric
  • Cyanide
  • Iodide
  • Lead
  • Silver

Liquid-Membrane Selective Electrodes

Liquids or soluble species that exhibit affinity toward an electro-active species can be used to fabricate a selective electrode. The electrode produces a potential in much the same fashion as the solid-state selective electrodes. The main difference is that complexation takes place with a molecular compound.

The molecular compound must be one that complexes with the species of interest. In addition, the compound cannot be too soluble in water. If the compound is soluble in water, it will just dissolve into the first solution tested. The selective compound can be dissolved into an organic solvent which is not miscible with water.

There must also be a physical barrier between reference and test solutions. This is often accomplished using a membrane that the organic phase, which contains the selective molecule, can permeate. The organic-phase saturated membrane is then suspended between the reference and test solutions. Schematically, the electrode is

Ref/internal standard/organic phase/test solution//Ref

where Ref indicates one of the usual (or unusual) reference electrodes, e.g., SCE or Ag/AgCl, the internal standard has the analyte at a fixed activity, the organic phase is a liquid organic solvent in which the active molecular compound is dissolved, and the test solution is the analyte solution (sample).

Example: the Ca2+ selective electrode is made using an aliphatic diester of phosphoric acid, (RO)2PO2- where R is an aliphatic group of low aqueous solubility. The chemical equilibrium is

2(RO)2PO2- + Ca2+« [(RO2PO2]2Ca

Using an internal reference solution, the electrode half-potential can be found from

Ag/AgCl(s)/Cl-, Ca2+(int std.) /(RO)2PO2H/test solution//Ref

Since the activity of [(RO)2PO2]2Ca is related to the concentration of Ca2+ through the equilibrium constant for both reference and external solutions

Some species able to be measured with these electrodes are

  • Calcium
  • Nitrate
  • Nitrite
  • Potassium
  • Sodium

Electrodes may also be used as gas sensors.

The electrode images have been reproduced, with permission, from web pages at Orion, a leader in ion selective electrode technology. Other interested in using these images should contact [email protected] directly.

Visit Orion (manufacturer of ISEs) pages to find out much more about ion selective and pH electrodes.

Back to Chemistry 3600 Home

This page was created by Professor Stephen Bialkowski, Utah State University

Last Updated Tuesday, August 03, 2004


Glass electrode

A glass electrode is a type of ion-selective electrode made of a doped glass membrane that is sensitive to a specific ion. The most common application of ion-selective glass electrodes is for the measurement of pH. The pH electrode is an example of a glass electrode that is sensitive to hydrogen ions. Glass electrodes play an important part in the instrumentation for chemical analysis and physico-chemical studies. The voltage of the glass electrode, relative to some reference value, is sensitive to changes in the activity of certain type of ions.


The first studies of glass electrodes (GE) found different sensitivities of different glasses to change of the medium's acidity (pH), due to effects of the alkali metal ions.

In 1906, M. Cremer, the father of Erika Cremer, determined that the electric potential that arises between parts of the fluid, located on opposite sides of the glass membrane is proportional to the concentration of acid (hydrogen ion concentration).[1]

In 1909, S. P. L. Sørensen introduced the concept of pH, and in the same year F. Haber and Z. Klemensiewicz reported results of their research on the glass electrode in The Society of Chemistry in Karlsruhe. [2][3] In 1922, W. S. Hughes showed that the alkali-silicate GE are similar to hydrogen electrode, reversible with respect to H+.[4]

In 1925, P.M. Tookey Kerridge developed the first glass electrode for analysis of blood samples and highlighted some of the practical problems with the equipment such as the high resistance of glass (50–150 MΩ).[5] During her PhD, Kerridge developed the miniature glass electrode, maximizing the surface area of the tool by heat treating the platinum with platinum chloride at red heat, thus enabling a much larger signal; her design was the predecessor of many of the glass electrodes used today.[6][7]


Glass electrodes are commonly used for pH measurements. There are also specialized ion sensitive glass electrodes used for determination of the concentration of lithium, sodium, ammonium, and other ions. Glass electrodes have been utilized in a wide range of applications including pure research, control of industrial processes, analysis of foods and cosmetics, measurement of environmental indicators, and microelectrode measurements such as cell membrane electrical potential and soil acidity.


Almost all commercial electrodes respond to single-charged ions, like H+, Na+, Ag+. The most common glass electrode is the pH-electrode. Only a few chalcogenide glass electrodes are sensitive to double-charged ions, like Pb2+, Cd2+ and some others.

There are two main glass-forming systems: silicate matrix based on molecular network of silicon dioxide (SiO2) with additions of other metal oxides, such as Na, K, Li, Al, B, Ca, etc. and chalcogenide matrix based on molecular network of AsS, AsSe, AsTe.

Interfering ions[edit]

Because of the ion-exchange nature of the glass membrane, it is possible for some other ions to concurrently interact with ion-exchange centers of the glass and to distort the linear dependence of the measured electrode potential on pH or other electrode function. In some cases it is possible to change the electrode function from one ion to another. For example, some silicate pNa electrodes can be changed to pAg function by soaking in a silver salt solution.

Interference effects are commonly described by the semiempirical Nicolsky-Shultz-Eisenman equation (also known as Nikolsky-Shultz-Eisenman equation),[8][9] an extension to the Nernst equation. It is given by

E=E^{0}+{\frac  {RT}{z_{i}F}}\ln \left[a_{i}+\sum _{{j}}\left(k_{{ij}}a_{j}^{{z_{i}/z_{j}}}\right)\right]

where E is the emf, E0 the standard electrode potential, z the ionic valency including the sign, a the activity, i the ion of interest, j the interfering ions and kij is the selectivity coefficient. The smaller the selectivity coefficient, the less is the interference by j.

To see the interfering effect of Na+ to a pH-electrode:

E=E^{0}+{\frac  {RT}{F}}\ln \left(a_{{{\text{H}}^{+}}}+k_{{{\text{H}}^{+},{\text{Na}}^{+}}}a_{{{\text{Na}}^{+}}}\right)

Range of a pH glass electrode[edit]

The pH range at constant concentration can be divided into 3 parts:

{\displaystyle E=E^{0}+{\frac {2.303RT}{F}}{\text{pH}}}

where F is Faraday's constant (see Nernst equation).

  • Alkali error range - at low concentration of hydrogen ions (high values of pH) contributions of interfering alkali metals (like Li, Na, K) are comparable with the one of hydrogen ions. In this situation dependence of the potential on pH become non-linear.

The effect is usually noticeable at pH > 12, and concentrations of lithium or sodium ions of 0.1 moles per litre or more. Potassium ions usually cause less error than sodium ions.

  • Acidic error range – at very high concentration of hydrogen ions (low values of pH) the dependence of the electrode on pH becomes non-linear and the influence of the anions in the solution also becomes noticeable. These effects usually become noticeable at pH < -1.[citation needed]

Specialized electrodes exist for working in extreme pH ranges.


Scheme of typical pH glass electrode.

A typical modern pH probe is a combination electrode, which combines both the glass and reference electrodes into one body. The combination electrode consists of the following parts (see the drawing):

  1. a sensing part of electrode, a bulb made from a specific glass
  2. internal electrode, usually silver chloride electrode or calomel electrode
  3. internal solution, usually a pH=7 buffered solution of 0.1 mol/L KCl for pH electrodes or 0.1 mol/L MCl for pM electrodes
  4. when using the silver chloride electrode, a small amount of AgCl can precipitate inside the glass electrode
  5. reference electrode, usually the same type as 2
  6. reference internal solution, usually 0.1 mol/L KCl
  7. junction with studied solution, usually made from ceramics or capillary with asbestos or quartz fiber.
  8. body of electrode, made from non-conductive glass or plastics.

The bottom of a pH electrode balloons out into a round thin glass bulb. The pH electrode is best thought of as a tube within a tube. The inner tube contains an unchanging 1×10−7 mol/L HCl solution. Also inside the inner tube is the cathode terminus of the reference probe. The anodic terminus wraps itself around the outside of the inner tube and ends with the same sort of reference probe as was on the inside of the inner tube. It is filled with a reference solution of KCl and has contact with the solution on the outside of the pH probe by way of a porous plug that serves as a salt bridge.

Galvanic cell schematic representation[edit]

This section describes the functioning of two distinct types of electrodes as one unit which combines both the glass electrode and the reference electrode into one body. It deserves some explanation.

This device is essentially a galvanic cell that can be schematically represented as:

Glass electrode || Reference Solution || Test Solution || Glass electrode
Ag(s) | AgCl(s) | KCl(aq) || 1×10−7M H+ solution || glass membrane || Test Solution || junction || KCl(aq) | AgCl(s) | Ag(s)

In this schematic representation of the galvanic cell, one will note the symmetry between the left and the right members as seen from the center of the row occupied by the "Test Solution" (the solution whose pH must be measured). In other words, the glass membrane and the ceramic junction occupies both the same relative place in each respective electrode (indicative (sensing) electrode or reference electrode). The double "pipe symbol" (||) indicates a diffusive barrier that prevents (glass membrane), or slowing down (ceramic junction), the mixing of the different solutions. By using the same electrodes on the left and right, any potentials generated at the interfaces cancel each other (in principle), resulting in the system voltage being dependent only on the interaction of the glass membrane and the test solution.

The measuring part of the electrode, the glass bulb on the bottom, is coated both inside and out with a ~10 nm layer of a hydrated gel. These two layers are separated by a layer of dry glass. The silica glass structure (that is, the conformation of its atomic structure) is shaped in such a way that it allows Na+ ions some mobility. The metal cations (Na+) in the hydrated gel diffuse out of the glass and into solution while H+ from solution can diffuse into the hydrated gel. It is the hydrated gel, which makes the pH electrode an ion-selective electrode.

H+ does not cross through the glass membrane of the pH electrode, it is the Na+ which crosses and leads to a change in free energy. When an ion diffuses from a region of activity to another region of activity, there is a free energy change and this is what the pH meter actually measures. The hydrated gel membrane is connected by Na+ transport and thus the concentration of H+ on the outside of the membrane is 'relayed' to the inside of the membrane by Na+.

All glass pH electrodes have extremely high electric resistance from 50 to 500 MΩ. Therefore, the glass electrode can be used only with a high input-impedance measuring device like a pH meter, or, more generically, a high input-impedance voltmeter which is called an electrometer.


The glass electrode has some inherent limitations due to the nature of its construction. Acid and alkaline errors are discussed above. An important limitation results from the existence of asymmetry potentials that are present at glass/liquid interfaces.[10] The existence of these phenomena means that glass electrodes must always be calibrated before use; a common method of calibration involves the use of standard buffer solutions. Also, there is a slow deterioration due to diffusion into and out of the internal solution. These effects are masked when the electrode is calibrated against buffer solution but deviations from ideal response are easily observed by means of a Gran plot. Typically, the slope of the electrode response decreases over a period of months.


Between measurements any glass and membrane electrodes should be kept in a solution of its own ion. It is necessary to prevent the glass membrane from drying out because the performance is dependent on the existence of a hydrated layer, which forms slowly.

See also[edit]


Bates, Roger G. (1954). "Chapter 10, Glass Electrodes". Determination of pH. Wiley.

Bates, Roger G. (1973). Determination of pH: theory and practice. Wiley.

  1. ^Cremer, M. Über die Ursache der elektromotorischen Eigenschaften der Gewebe, zugleich ein Beitrag zur Lehre von Polyphasischen Elektrolytketten. — Z. Biol. 47: 56 (1906).
  2. ^First publication — The Journal of Physical Chemistry by W. Ostwald and J. H. van 't Hoff) — 1909).
  3. ^F. Haber und Z. Klemensiewicz. Über elektrische Phasengrenzkräft. Zeitschrift für Physikalische Chemie. Leipzig. 1909 (Vorgetragen in der Sitzung der Karlsruher chemischen Gesellschaft am 28. Jan. 1909), 67, 385.
  4. ^W. S. Hughes, J. Am. Chem. Soc., 44, 2860. 1922; J. Chem. Soc. Lond., 491, 2860. 1928
  5. ^Yartsev, Alex. "History of the Glass Electrode". Deranged Physiology. Retrieved 26 June 2016.
  6. ^Blake-Coleman, Barrie. "Phyllis Kerridge And The Miniature Ph Electrode". Inventricity. Retrieved 26 June 2016.
  7. ^Kerridge, Phyllis Margaret Tookey (1925). "The Use of the Glass Electrode in Biochemistry". Biochemical Journal. 19 (4): 611–617. doi:10.1042/bj0190611. PMC 1259230. PMID 16743549.
  8. ^D. G. Hall, Ion-Selective Membrane Electrodes: A General Limiting Treatment of Interference Effects, J. Phys. Chem 100, 7230 - 7236 (1996) article
  9. ^A. A. Belyustin. Silver ion Response as a Test for the Multilayer Model of Glass Electrodes. — Electroanalysis. Volume 11, Issue 10-11, Pages 799—803. 1999
  10. ^Bates, Roger G. (1954). "Chapter 10, Glass electrodes". Determination of pH. New York: Wiley.

E. P. Nikol'skii, M. M. Schul'tz, et al., Vestn. Leningr. Univ., Ser. Fiz. i Khim., 18, No. 4, 73-186 (1963) (This series of articles summarizes Russian work on the effect of varying the glass composition on electrode properties and chemical stability of a great variety of glasses)

External links[edit]

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  2. Tentec amplifier
  3. Sonic adventure for switch
  4. 8x10 mailers

pH Electrode Selection Guide

Electrode Components

Cole-Parmer High-Accuracy pH Electrode, 10-ft Cable
Most pH electrodes are combination electrodes. An electrode comprises two main elements. One element is a sensing half-cell and the other is a reference half-cell. Both half-cell units must be used together to complete the pH circuit in order to get a pH measurement. The sensing half-cell is the portion of the electrode that is responsible for the measurement portion in the system; think of this as the positive ( ) end of the circuit. This portion of the electrode typically contains a membrane that is sensitive to the change in the pH of the solution being measured. The reference half-cell provides a stable reference potential that is needed for pH measurement; think of this as the negative (-) end of the circuit.

Most of the pH electrodes Cole-Parmer offers are combination electrodes. Combination electrodes contain both half cells—the sensing and reference half-cell—in one single probe. Cole-Parmer also offers the traditional sensing and reference half-cells but they have become much less popular. The best option will depend on the type of samples that the user will measure. The following sections describe the different types of electrodes and explain key differences for each.

Epoxy vs Glass

Epoxy body electrodes are more durable, tend to be the more economical choice, and are ideal for environments where rough handling is expected. However, the maximum temperature limit for most epoxy body electrodes is approximately 176ºF (80ºC).

Glass body electrodes are capable of withstanding much higher temperatures, up to 230ºF (100 to 110ºC) depending on the specific electrode, and also offer chemical resistance to highly corrosive materials or solvents. Glass body electrodes are also easier to clean after use. Either electrode type will require care when handling, as even a small fracture to the bulb or body of the electrode can lead to erroneous readings.

Sealed vs Refillable Electrodes

Sealed or gel-filled electrodes are virtually maintenance free and ideal for most applications. They also tend to be a more economical choice. However, once the inner fill solution level is low (or if it dries out), the electrode will have to be replaced. This is why sealed electrodes may have a shorter life span than a refillable electrode.

Refillable electrodes have a port near the top of the electrode which allows them to be refilled once the inner fill solution is low or depleted. For many applications, this can significantly prolong the life of the electrode. Refillable electrodes allow the user to change the filling solution if it becomes contaminated. Users can also change the filling solution for specialized applications if necessary, for example when measuring pH in organic solvents.

A common misconception is that refillable electrodes are more precise than sealed electrodes. However this is not the case, as highly precise, sealed electrodes are available that offer comparable or more precise readings than refillable electrodes. One example is the Oakton® polymer-filled sealed electrodes which offer precision of 0.02 pH units. By comparison, most refillable electrodes offer a precision of 0.01 to 0.02 pH units.

Single-Junction vs Double-Junction

In combination electrodes, the reference junction allows H ions to pass freely between the reference and sensing half-cells to complete the electrical circuit. The most common junctions are made from a ceramic material and come in either single or double format. Economical single-junction electrodes are ideal for general-purpose applications and cleaner water applications. They are typically not recommended for use with samples containing proteins, organics, heavy metals, sulfides, Tris buffers, or any other biological media. These samples will react with the trace amount of silver that is present in the electrodes. Double-junction electrodes are recommended for these applications because they have an extra barrier which prevents this reaction. Double-junction electrodes also tend to last longer for many applications because of this extra barrier.

Although most reference cells feature an H -permeable glass junction, electrodes with reference junctions made of PTFE are also available. Electrodes with PTFE junctions are better suited for use with solutions that are highly viscous or those that have particulates which clog conventional glass junctions. These applications may involve measuring oils, paints, pastes, or inks.

A variety of specialized electrode junctions for specific applications are also available:

  • Flushable Junction/Sure-Flow® Electrodes

  • – ideal for viscous or dirty samples; junction is flushable and will prevent clogging and exhibit faster response times due to constant flow of fill solution into the samples being measured. It is good for all sample types even highly viscous samples though a high leak rate will require more frequent refilling.
 Thermo Scientific Orion 8172BNWP Sure-Flow pH Probe
  • Glass Capillary/Open Pore Electrodes

  • – provide a larger junction and an increased flow for a more stable junction potential.
Cole-Parmer glass pH Electrode
  • Wick Junction Electrodes

  • – typically made of glass fibers, fiber optical bundles, or Dacron®. These are used in epoxy body electrodes for aqueous samples. They exhibit a slow response time and will clog if samples are too dirty or viscous.
 Cole-Parmer Fast-Response Autoclavable Fermentation pH Electrodes
  • Ceramic Junction Electrodes

  • – Made of porous ceramic, wooden plug, or porous PTFE. This is the most common junction found in a standard laboratory. Ceramic junction electrodes will clog if samples are too dirty or viscous; they are used in glass body electrodes.
Cole-Parmer General purpose-lab, pH electrode, Refillable

Reference Type Electrodes

Silver/Silver Chloride (Ag/AgCl)

Ag/AgCl is the most common internal element in this type, suitable for almost all applications (their temperature limit is 176°F (80°C).

Another reference type is the Thermo Scientific™ ROSS™ electrodes. This reference type features a redox couple iodide/iodine (I2/I) internal reference. The iodide/iodine internal reference combined with a platinum wire creates a redox potential. This allows for a faster response and better stability over time than an electrode with a silver wire and the traditional Ag/AgCl complex fill solution. However all refillable ROSS electrodes will use a 3M KCl outer reference solution.

Temperature Measurement and Temperature Compensation

As with any pH measurement, consider the temperature measurement when choosing the correct electrode for the application. The pH of a solution can vary greatly depending on the temperature of the solution and any temperature change in the sample will also affect the readings.

Almost all meters feature either manual or automatic temperature compensation as a standard feature. Manual temperature compensation requires the user to manually enter the measured temperature value of the sample being measured. Automatic temperature compensation or ATC continually measures the temperature and corrects for changes in an electrode and readings due to a change in the temperature of the solution. It does require an additional probe to measure the temperature.

Choose from two probe options when using a meter with ATC. One option is to use a separate temperature probe from your pH electrode. The type of connection for the ATC probe is specific to the brand and model of the meter so consider this when making a selection. The major benefit of using a separate temperature probe is that it provides your meter with the flexibility to accommodate many different types of pH electrodes. This is convenient for various application changes or when accommodating different types of samples. The ATC probe also doesn’t need to be replaced when the pH electrode goes bad.

The second option is using a pH electrode with an ATC element integrated. This type of electrode is referred to as an all-in-one or three-in-one electrode. All-in-one electrodes are more convenient because only one probe enters the sample. Because these probes have both the pH and the ATC electrodes in one unit, there are usually two connectors. One connector will typically be a standard BNC connector for the pH portion of the electrode. The other connector will be the temperature connector of the electrode that is specific to the brand of pH meter. Choosing an all-in-one electrode may reduce the number of electrode options available for the meter. Custom solutions are available if requested. Many handheld pH testers incorporate both the pH and ATC electrode into one unit as well. pH testers are meters that are designed for field and more rugged applications.

Connection Type

A variety of different pH electrode connections connect to a pH meter. Most pH electrodes come with a BNC connection. BNC probes will be compatible with a wide variety of meters; however, this doesn’t apply for electrodes with built-in temperature compensation (ATC). Here is a list of the more common connection types:

    • BNC – the BNC connection is the most common and universal electrode connection type
    • DIN connector – This is still a relatively common connection and is usually for probes with a built-in ATC
    • US standard – This is an older standard that is used less frequently
    • Pin tip – This connection type was used mostly with half-cell electrodes which have been replaced with combination electrodes

In addition to the pH electrode, the ATC also has a specialized connection. ATC probes are less universal as most manufacturers use a different type of temperature sensor and connection type. Usually it is best to find the appropriate ATC electrode by looking in the accessory section of the pH meters manual. Here are a few options:

    • Phono jack (3.5 mm or other)

Specialty Electrodes

    • Standard Electrodes – approximately 12 mm in diameter; typical lab electrode
Cole-Parmer General purpose-lab refillable pH electrode
    • Narrow Electrodes – approximately 6 to 8 mm in diameter; extended length for use with bottles, vessels, and test tubes
Cole-Parmer refillable pH electrode, long and thin body
    • Semi-Micro Electrodes – approximately 6 to 8 mm in diameter; capable of measuring down to 200 µl sample size
PHR-146-BNC Waterproof phtestr 1
    • Rugged Bulb Electrodes – more robust design to prevent breakage; great for field use
Cole-Parmer pH electrode, Sealed, low maintenance
    • Spear Tip Electrodes – used for piercing solid or semisolid samples; cheeses, meats, etc.; good for small sample volumes
Cole-Parmer pH electrode, combination, spear tip, sealed, glass body, BNC
    • Flat Surface Electrodes – used for measuring the pH of surfaces, solids, or gels; good for small samples volumes
Cole-Parmer pH electrode-Flat, Sealed
    • PerpHecT™ pH Electrodes – specifically designed for use with Thermo Scientific™ PerpHecT™ pH meters; LogR temperature compensation feature allows for simultaneous pH and temperature measurement without the use of a separate ATC probe.
Thermo Scientific Orion 9102BNWP pH Probe
    • Antimony Electrodes – specifically designed for resistance to HF acid as they do not contain glass. These can generally tolerate up to 5% HF acid concentration. The major concern when using an antimony probe is that they have a different offset than standard probes. At pH of 7, the mV reading is around –400 mV ±30 mV, and the slope is around 50 mV/pH versus 59 mV. A meter that can compensate for this is required and may be more difficult to find.
Cole-Parmer General Purpose Double-Junction pH Electrode

Electrode Selection Guide for Identified Applications

    • Biological samples – Double junction or ROSS electrode
    • Pharmaceuticals – Double junction or ROSS electrode
    • Hydrofluoric acid – Antimony or HF electrode
    • Low ionic strength samples and acid rain – AccuFlow or flushable styles
    • Boiler feed water and distilled water – AccuFlow or flushable styles
    • Drinking water – Standard Ag/AgCl with single junction
    • Wastewater – Double junction or ROSS electrode
    • Solutions with heavy metals – Double junction
    • Soil samples – Soil electrode or double junction
    • pH >9 and High Na – Most single or double junction electrodes, Ag/AgCl
    • High or rapidly changing temperature – ROSS styles
    • Moist flat surfaces – Flat surface style
    • Cheese, agar, paper, and skin – Flat surface style
    • Semisolid samples – Spear tip, Ag/AgCl, ISFET
    • Fruits, cheese, and meat – Spear tip, Ag/AgCl, ISFET
    • Non-aqueous samples, solvents, and alcohols – AccuFlow styles, flushable styles, or Double junction
    • Viscous samples, slurries, suspended solids, and sludges – AccuFlow styles, flushable styles, Double junction, or ISFET
    • Emulsions and oils – AccuFlow styles, flushable styles, Double junction, or ISFET
    • Paints and inks – AccuFlow styles, flushable styles, Double junction, or ISFET

The Electrode Pair

Sensing and reference half-cell electrodes must be used together to complete the pH circuit. Most of the electrodes we offer are combination electrodes that house both half-cells in a single probe.

Sensing Half-Cells

Sensing half-cells are the measuring portion of the electrode system and contain the pH-sensitive membrane.

Glass vs ISFET Sensors

The glass membrane or bulb of an electrode is constructed for use in specific conditions. Different types of glass membranes can strengthen the electrode, expand its temperature range, or prevent sodium error at high pH values.

  • General-purpose glass: various pH ranges, temperatures to 212°F (100°C).
  • Blue glass: pH 0-13, temperatures to 230°F (110°C)
  • Amber glass: pH 0-14, temperatures to 230°F (110°C), low sodium (Na) error (In solutions with high Na concentrations, Na can be misread as H at pH 12 and higher.)

also offers solid-state ISFET (ion-specific field effect transistor) electrodes. The nonglass measuring surface won't break and wipes clean for dry storage—excellent for use in the food industry.

Epoxy vs Glass Body

Epoxy-body electrodes are impact resistant and ideal for rough handling, but should not be used at higher temperatures or for inorganics. Glass-body electrodes withstand high temperatures and highly corrosive materials or solvents.

Cole-Parmer pH electrode, single-junction, gel, BNC connector
Epoxy-body electrode
Cole-Parmer pH electrode, Sealed, low maintenance
Glass-body electrode

Reference Half-Cells

Reference half-cells provide the reference potential needed for pH measurement. Our selection of electrodes includes a variety of reference cell options:

Single- vs Double-Junction

In combination electrodes, the reference junction allows H ions to pass freely between the reference and sensing half-cells to complete the electrical circuit. Economical single-junction electrodes are ideal for general-purpose applications. Use double-junction electrodes with solutions that contain sulfides, heavy metals, or Tris buffers to prevent contamination of the reference cell.

Although most reference cells feature a H-permeable glass junction, electrodes with reference junctions made of PTFE are also available—use with solutions that may clog conventional glass junctions.

Silver/Silver Chloride (Ag/AgCl) vs Calomel (Hg/Hg2Cl2)

Ag/AgCl is the most common internal element, suitable for almost all applications [temp limit: 176°F (80°C)]. Hg/Hg2Cl2 is recommended for use in solutions containing proteins, organics, or heavy metals that could react with silver and clog the reference junction [temp limit: 158°F (70°C)].

Refillable vs Sealed

Refillable electrodes have ports that allow you to refill the reference chamber with reference solution—they are economical and long-lasting. Sealed electrodes are rugged and require virtually no maintenance; however, they must be replaced when the fill-solution level is low.

pH: How to construct pH electrode and how it works to measure pH of unknown solution

Orion™ 9102BNWP Combination pH Electrode

DescriptionWith waterproof BNC connector
Diameter (English) Body0.47 in.
Diameter (Metric) Body12 mm
Diameter (Metric) Cap16 mm
For Use With (Equipment)Standard pH Meter, BNC connector
Internal ReferenceAg/AgCl
Length (English)4.7 in.
Length (English) Cable39 in.
Length (Metric)120 mm
Length (Metric) Cable1 m
Slope92 to 102%
Storage RequirementsShort Term Storage (up to one week): Soak the electrode in pH electrode storage solution, Cat. No. 910001. Long Term Storage (more than one week): Fill the reference chamber and securely cover the fill hole. Cover the sensing bulb and reference junction with the protective cap, sleeve or storage bottle containing a few drops of storage solution. Before returning the electrode to use, prepare it as a new electrode.
Temperature (English) Operating32°F to 194°F
Connector TypesBNC Waterproof
MaterialGlass Body
Temperature (Metric) Operating0°C to 90°C
TypeCombination Ag/AgCl pH Electrode, glass body, BNC waterprood connector
pH Range0 to 14
Unit SizeEach

Electrode combination ph

pH Electrodes

The most widely used ion-selective electrode is the glass pH electrode, which utilizes a thin glass membrane that is responsive to changes in H+ activity. F. Haber, in 1901, was the first person to observe that the voltage of a glass membrane changed with the acidity of a solution. In 1906, M. Cremer observed the pH dependence of measured potential across a thin glass membrane.

Today, pH sensitive glasses are manufactured primarily from SiO2 which are connected via a tetrahedral network with oxygen atoms bridging two silicon atoms (see an interactive 3d structure at In addition, the glasses are made to contain varying amounts of other metal oxides, like Na2O and CaO. Oxygen atoms within the lattice that are not bound to two silicon atoms possess a negative charge, to which cations can ion pair. In this way, ions (primarily Na+) are able to diffuse slowly in the lattice, moving from one charge pair site to another. While the membrane resistance is very high (~100 MΩ), this movement of cations within the glass allows a potential to be measured across it.

If glasses of this type are placed in an aqueous solution containing H+, the glass surface in contact with solution becomes hydrated as water enters a short distance into the crystal lattice and causes it to swell. The “interior” of the glass remains dry. Some of the metal ions within the glass close to the solution boundary are able to diffuse into the solution, and some H+ ions are able to charge pair with oxygen near the glass surface. In this way, ion exchange equilibrium is established between the fixed negative sites on the glass surface and H+, with an increasing number of charge pairs with H+ occurring as its activity in the contacting solution increases. This equilibrium can be expressed by

\[\ce{H^{+/-}O-Si}- ⇔ \ce{H^+} + {^-\ce{O-Si}-}\]

where H+/-O-Si- and -O-Si- represent oxygen sites at the glass membrane ion paired with H+ and unpaired, respectively.

To function as a pH sensor, a layer of pH sensitive glass is placed between two solutions containing H+. As only the glass closest to solution becomes hydrated, two individual equilibria are established that are dependent upon the respective H+ activity on either side of the layer. This situation is illustrated below.

A difference in the H+ activities on either side of the glass membrane leads to a difference in the number of ion pairs that exist, and an imbalance in the surface charge between the hydrated layers. This results in a membrane potential that is pH dependent, described according to the Nernst equation

\[\mathrm{E_{membrane} = E_{inner} - E_{outer} = 0.0592 \log [(\mathcal{A}_{inner}) / (\mathcal{A}_{outer})]}\]

where Einner and Eouter are the surface potentials on either side of the membrane, and \(\mathcal{A_\mathrm{inner}}\) and \(\mathcal{A}_\mathrm{outer}\) represent the H+ activities of the inner and outer solutions, respectively.

Most commonly, pH electrodes are of a combination design, in which the glass membrane and the necessary reference electrodes are incorporated into the same electrode body. A figure of a typical combination pH electrode is shown below. (

In this design, the inner fill solution contacting the glass membrane contains a fixed activity of H+. Typically, a Ag/AgCl reference electrode is in contact with this inner solution, and the solution contains 0.1 M HCl saturated with AgCl. A second Ag/AgCl reference electrode is located within a solution compartment surrounding the inner solution compartment. This solution is in contact with the external solution of unknown H+ activity through a porous frit on the side of the electrode barrel.

The combination electrode allows the measurement of both the inner and the outer membrane surface potentials, which as we saw above, is related to the solution pH by the Nernst equation. The theoretical potential across the glass membrane changes by 59.2 mV for each unit change in solution pH. A combination electrode cell can be represented by the shorthand notation below.

Practical considerations

The potential measured by a pH indicator electrode includes not only the desired membrane or boundary potential, but also small contributions from what are known as junction potentials and asymmetry potentials. Junction potential has been described in a separate section of this module, and for the specific case of the combination pH electrode is that which develops across the porous frit separating the second reference electrode from the external measured solution. Asymmetry potentials result from physical differences between the inner and outer surfaces of the glass membrane, leading to different inner and outer potentials for the same H+ activity. Corrections for these small potential errors can be made by frequent calibration of the glass electrode in standard solutions covering the pH range in which measurements are desired.

Users of glass pH electrodes should also be aware of alkaline and acid errors that limit the pH range over which effective measurements can be made. At very high pH, generally > 10, most glass electrodes become responsive to both H+ and Na+, with the measured pH being lower than actual. At low pH, typically < 1, glass membranes are susceptible to saturation by H+, and produce pH readings that are higher than actual. Specialty glasses are available that minimize these errors if routine measurements must be made at these pH extremes.

Potentiometric pH measurement


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