The Glass pH Electrode - Electrochemical Society

1 The Electrochemical Society Interface Summer 200419 Editor s note: We are pleased to introduce anew magazine column that aims to take the read-er back to the classroom. This series of tutorials willbe written by experts for a non-specialist audience. Pleaselet us know what you think of this new Glass Electrode is perhaps the most successful and ubiqui-tous Electrochemical sensor. It provides information aboutthe activity of hydronium ions, H3O+, in water. Becausewater, which mildly dissociates to H3O+and OH-ions, isthe most common solvent medium, and chemical reac-tions in water largely depend on H3O+activity, the abilityto measure it is essential. And conversely, because H3O+activity, or rather, its negative logarithm, the pH, is so easyto measure, pH is the most commonly monitored andrecorded parameter of liquid Glass Electrode (Fig.)

2 1) is actually a device, not an elec-trode in an Electrochemical sense of the word. It consists ofa Glass bulb membrane, which gives it its name and an elec-trically insulating tubular body, which separates an internalsolution and a silver/silver chloride Electrode from the stud-ied solution. The Ag/AgCl Electrode is connected to a leadcable terminated with some connector that can hook up toa special voltmeter, the pH meter. The pH meter measuresthe potential difference and its changes across the glassmembrane. The potential differencemust be obtained between twopoints; one is the Electrode contact-ing the internal solution. A secondpoint is obtained by connecting to areference Electrode , immersed in thestudied solution. Often, this refer-ence Electrode is built in the glasselectrode (a combination Electrode ),in a concentric double barrel body ofthe device.

3 Figure 2a shows a dia-gram of such a device. Figure 2a, thecombination Electrode and Fig. 2b, aglass Electrode and separate refer-ence Electrode , are functionally iden-tical. It is a common misconceptionthat the combination Electrode ( ) requires only one lead, fosteredbecause the round coaxial lead to theelectrode looks like a single is not so. In any potentiometricmeasurement, and pH measurementis an example of one, two inputs,one of which is a reference point, arerequired. The completed Glass Electrode with a referenceelectrode cell is represented by the Electrochemical short-handAg/AgCl | HCl | Glass || probed solution | reference Electrode (1)The potential difference relevant to pH measurementbuilds up across the outside Glass /solution interface marked| |. The key functional part, the Glass membrane, is manufac-tured by blowing molten Glass into a thin-walled bulb witha wall about mm thick.

4 The bulb is then sealed to athicker Glass or plastic tube, and filled, for example, with asolution of HCl ( mol/dm3). In this solution is immerseda silver/silver chloride Electrode with a lead to the outsidethrough a permanent hermetic seal. The filling solution hasconstant Cl-concentration, which keeps the Ag/AgCl innerelectrode at fixed pH sensing ability of the Glass Electrode stems fromthe ion exchange property of its Glass membrane. Glass ismostly amorphous silicon dioxide, with embedded oxides ofalkali metals. When the surface of Glass is exposed to water,some Si O- groups become protonatedSi-O- + H3O+ Si-O-H++ H2O(2)The exchange of hydronium (or written as proton, H+)between the solid membrane and the surrounding solution,and the equilibrium nature of this exchange, is the key prin-ciple of H3O+sensing.

5 As with any interface separating twophases between which ionic exchange equilibrium is estab-lished, the Glass membrane/solution interface becomes thesite of a potential differenceEglass wall/solution~ | a(H3O+)|(3)where Ris the molar gas constant J mol-1K-1, Tis thetemperature in kelvins, Fis the Faraday constant 96, C, is a conversion between natur-al and common logarithm, anda(H3O+) is the activity of hydroni-um, which can be at lower concen-trations equated with its concentra-tion. At 30 C the value of approximately Glass membrane has twowall/solution interfaces and there ispotential buildup on each of them,with opposite polarity. But the pHinside the bulb is constant, becausethe internal solution is , the inner surface poten-tial is constant, adding merely to anoffset to the overall potential of thedevice.

6 Additional contribution tothe offset comes from potentials ofthe inner solution Electrode , and thereference Electrode , which are alsoconstant. The changes in the devicepotential are therefore due entirelyto the pH changes of the outsidesolution and the potential of theglass Electrode /reference Electrode setup isEglass Electrode = E + a(H3O+)(4)Where E represents the sum of the constant offset poten-tials of the inner Glass surface/solution and the two Ag/AgClelectrodes. At 30 C the potential of the Glass membranechanges by about 60 mV for each one unit of pH ( , a ten-fold activity change).The possible range of hydronium activity encountered inaqueous solutions is large, as much as 10 to 10-15 Glass pH Electrodeby Petr Vany sekFIG. 1. Schematics of a Glass Electrochemical Society Interface Summer 2004To encompass such a wide dynamic range, a logarithmic scalewas established, defining the pH of a solution as pH = - log a(H3O+)(5)The negative sign assures that pH of most solutions, exceptextremely acidic ones, is always positive.

7 It is fortuitous that thepotential of the Glass Electrode (Eq. 4) is also a logarithmic func-tion of hydronium activity; therefore, the potential of the glasselectrode is a linear function of pHEglass Electrode = E (6)The Eglass electrodeis measured with a pH meter, a voltmeterwith circuitry allowing direct readout of pH. The meter sub-tracts the potential offset E . Although itcould be calculated, it is more practicalto calibrate the Glass Electrode usingbuffers. For example, if two buffers areused in sequence, one at pH 7 andanother at pH 4, it is also possible to cal-ibrate the actual slope of Eq. 6. Althoughit is mathematically given ( ),the behavior in the presence of interfer-ing ions or with deteriorating Glass elec-trode is not always that ideal. The cor-rection for temperature (T) is also madein a two-buffer calculation.

8 Increasingly, Glass electrodes and hand held pH detec-tors, have built-in temperature probes,linked to an automated electronic tem-perature correction. Because the pH ofstandard buffers is available to two deci-mal places, measured pH should bereported to no more than two decimalplaces, despite that most pH meters candisplay three places or more. Practicalreproducibility is about circuit across which the pH meter measures the poten-tial difference has high electric resistance. Even though theresistance is minimized by blowing the Glass very thin and dop-ing it with a mix of alkali metals, the circuit resistance is still107-109 , which is too high for a normal voltmeter; a specialkind with high input impedance is needed. The ubiquity ofglass Electrode pH sensors was made possible through advancesin electronics.

9 The first significant step was introduction of ahigh input impedance vacuum tube amplifier in 1928; howev-er, a key to miniaturization and low cost came through intro-duction of field-effect transistors and subsequently the knowl-edge to integrate them with the digital signal processing cir-cuitry on a single chip. Modern pH meters, combining the glassmembrane sensing element, electronics, and a display, are nowthe size of a pen, no larger than a Glass Electrode of only a fewyears , the Glass electrodes of today share with the Glass elec-trodes of yesterday both their working principle and their lim-itations. A common interference with sensing hydroniumcomes from ions of similar size, notably the alkali metals. Theinterference is somewhat alleviated by using specialty lithiumglass, with sites too small to fit Na+or K+.

10 Alkali metal interfer-ence is not the only complication. In solutions of concentratedalkali hydroxides not only the metal ion concentration is high,the activity of H3O+is small, making equilibration more com-plicated. And on top of that, strong alkaline solutions etch andpossibly permanently damage Glass surfaces. Fluoride is anoth-er ion that can physically damage Glass . Additionally, solutionswith proteins, that adsorb on surface, or solutions of ions ofmore noble metals (Ag, Cu) that could reduce and deposit onthe Glass surface, should also be lore of proper Glass Electrode handling is rich and oftenirrelevant. The Glass of the membrane has evolved, the pHmeters are more reliable, and the electrodes have become moreaffordable. The Glass membrane of the past must haveremained wet for proper hydration.