Chapter 15 Electrode Measurements

Chapter 15 Electrode MeasurementsProblems: 2, 4, 8, 11, 18In the last chapter we learned how to use oxidation-reduction reaction to makecells or batteries. In this chapter we will learn how to harness the same math andreactions to make indicator electrodesAn electrode is a cell designed specifically to measure the solution concentrationof a chemical. The whole area of using cell voltages to gain chemical information iscalled potentiometry.Electrodes and potentiometry are an important aspect and everyday applicationof chemistry. Table 15-1 from your text lists several chemical species that must bedetermined in a normal blood work. Most, if not all of these compounds are determinedusing electrodes.In the last chapter we found that there are two types of indicator electrodes,inert, where the metal of the electrode does not take part in any chemical reactions,active, where the metal of the electrode takes part in a redox reaction that helps todetermine the electrode’s potential. This chapter focuses on these types of electrodes14-1 The Silver Indicator electrodeIn the last chapter we used we used a silver/AgCl electrode as a reference electrode.Now lets learn how to use a silver electrode as an indicator electrode. In its simplestform all we have to do is to dip a silver wire into a solution, and this becomes a silverindicator electrode. Lets see how this works.+ - 0Ag + e Ag(s) E = .799Vwe need to measure this potential against something else so lets use a calomelelectrode, and get a cell that resembles the figure 15-1 on page 312The E of the calomel electrode is fixed so it gives E L = .241 V+E R = E silver ½ reactioon = .799- .059log(1/[Ag ])+E cell = ER-E L = .799- .059log(1/[Ag ])- .241 V+=.558 -.059 log(1/[Ag ])+= .588 + .059 log [Ag ]you can see that the V of this system will change by .059V each time theconcentration changes by a power of 10

2Now let’s look at the electrode you will be making and using in the lab. Figurefrom lab manual. How are these electrodes similar or different?Similar, have silver wire in solution as an indicatorDifferent, book example electrode has E l as a calomel electrode as the reference(V=.241), lab electrode has some copper wire in a solution of 0.1M CuSO 4What is going on? Copper wire in CuSO 4 solution is a simple referenceelectrode, much simpler and chemically safer than getting out mercury and making acalomel electrodeWhat is the Voltage of this reference?Reaction:2+ - 0 0Cu +2e Cu E = .339Nernst Equation:E = .339 + -.059/2 log (1/0.1)= .339-.0295= .3095Since calculating the voltage of this set of electrodes at given silverconcentrations is part of you lab exercise, I’ll leave it to you to put the pieces togetherfrom here.Titration of a Halide Ion with Ag +Now let’s link this electrode with a precipitation titration we are doing in the lab-Say we were using AgNO 3 to titrate the I in a solution+ - -17Ag + I AgI(s) K sp = 8.3x10+ - -10Ag + Cl AgCl(s) K = 1.0x10spRather than going through all the calculations, lets just cut to the chase. Twodifferent species will precipitate out in this titration, AgI, and AgCl. Which oneprecipitates first? (AgI with the smaller K sp). Before the first equivalence point the Ag+ -and the I combine and ppt out, so you calculate [Ag ] using the concentration if I and+the Ksp of AgI. After the first equivalence point Ag is now being precipitated by the- + -Cl so the [Ag ] is calculated from the Cl concentration and the K sp of AgCl. Finally,+ +after the second equivalence point you have excess Ag in the solution and the [Ag ] is+calculated from this excess and the total volume. You then plug the [Ag ] into the Eequation of the silver/copper reference cell we were just working with and come up witha voltage. The result should look like figure 6-5 from your text, although the actualvoltage may go up instead of down, depending on how you hooked up your volt meter

3between the electrodes.In the above system, the potential we measured comes directly from a redoxreaction taking place on the metal of the electrode. Now let’s explore anotherway we can get a potential in a solution that does NOT involve redox chemistry.15-2 Junction PotentialsWhenever you have two different solution in contact, a junction potential willdevelop between the liquids because the ions making the solution can’t diffuse at thesame speed. Examine the diagram below:If you go back to chapter table 12-1(Chapter 12) you will find that the ionic radius of- +Cl is 300 pM, while that of Na is 450 pM. Theslightly large size means that Na will travelthrough the solution a little more slowly. (Themobilities are given in table 15-2) What- +happens as the Cl ions pass the Na ions?You get a charge imbalance, and anytime youhave a charge imbalance, you have a potential.In this case the Junction Potential because itoccurs at the junction of two solutionsImplication of Junction potentialsI. You may have wondered why I always used KCl in my salt bridges. If you check you+ -will find that K and Cl have similar mobilities, so salt bridges made with KCl havenegligible junction potentials.II. All electrodes contain solvent /solvent junctions, and all have these built in junctionpotentials that will make a measured voltage different than its theoretical value. As aresult we ALMOST NEVER determine a concentration directly from a voltage. Weeither use an electrode in a titration where the absolute value of the potential is not asimportant as the big change that occurs in the potential at the equivalence point, or wefirst calibrate an electrode using standard of known concentrations15-3 How Ion-Selective Electrodes work.First, what and why is an ion-selective electrode.An Ion-selective electrode is one that is sensitive only to a particular ion. In thehospital setting you the most likely place you will see ion-selective electrode is indetermination of blood concentration of different ions. For instance you can makeelectrode to measure blood pH, O , CO , glucose, etc.2 2

The ion selective electrodes we will discuss here all work on the same principle.We will have some king of barrier between two solutions. If there is a difference inconcentration of an ion across the two sides of the barrier we will get a potential,analogous to the junction potential we just discussed. The only trick will be go makeour concentration dependent electrode sensitive to just one ion in the solution.Let’s say we have two solutions, one that Is 1M HCl (On the left) and one that iswater (on the right). Let’s separate these two solution by an impermeable membrane,is nothing can cross. Because nothing can pass between the cell there is a 0 potentialdifference.+No let’s manipulate the membrane so only H can pass. What happens? The+ -7 -1M H from the left side wants to move to the 1x10 M H concentration on the rightside. Since the + ions are moving and the negative ions are staying behind, youquickly build up a + charge and a junction potential. If you stick electrodes into thesesolution you can now see this potential and, by the Nernst Equation E= .059/n log-71M/1x10 = 6x.059 = .354 volts(Another way to look at it. Initially both solution are electrically neutral. If you+allow H to move, then you have a charge imbalance on the solution on both sides ofthe membrane, and the junction potential represents this imbalance)The whole trick of ion selective electrodes is to use clever chemistry tomanipulate the membrane so that only 1 kind of ion can pass through it.Thus we now have two types of indicator electrodesMetal electrodes : where the metal of the electrode is involved in redox reactionthat creates a potentialIon-selective electrodes: No redox chemistry is involved, the potential comesfrom creating a barrier between two solutions that allows a single type of ion to pass (asemi-permeable barrier)415-4 pH measurement with a glass electrodePerhaps the most familiar example of an ion selective electrode is the pHelectrode. Let’s see how this worksWell just as I have outlined above. The tricky part is the membrane. In reality itis a glass bulb about 0.1 mm thick (This is the part that I warn you about breaking in thelab)+In reality the membrane does not actually let H ions across the glass, but whatseem to happen is that the glass is a polymeric mass of Si and O (See figure 14-9) in+ +which + sodium atoms are caught. As you have H concentration differences, the Nawithin the electrode glass moves in response and has creates the same junction+potential you would have if H has moved.

The actual construction of a pH electrode is actually fairly complicated becauseit contains not only the special glass electrode, but it has two reference electrodes builtinto it, one for each side of the glass membrane (See overhead)5

6The line diagram of this electrode is:- +Ag(s)| AgCl(s)|Cl (ag)|| H (aq, outside) :Special glass:+ -H (aq,inside),Cl (aq,inside)|AgCl(s)|Ag(s)IN theory the response of an electrode should be+ +E= constant + (.059) log (H outside)/(H inside)The actual response of each electrode is different anc varies from day to day. Thusthey have to be calibrated at least daily. This is done by using buffer solutions of twoknown pH/s and tuning the electronics of the pH meter to match those values.Errors in pH MeasurementspH measurement can have errors here are some of the common ones1. The pH standards are only good to ± .01 pH units, so your absolute pHmeasurement is only good to that level as well2. There is a salt bridge in the pH electrode, that will have a junction potential sothis add an uncertainty of ± .01+ + +3. The electrode glass works by passing Na Thus if [Na ] is high and [H ] is lowerrors will occur because the electrode can sense Na +as well.4. In strong acids the measured pH is high, but we don’t know why5. You need to let the electrode equilibrate. This can take a few second inbuffered solutions or it can take minutes in unbuffered solutions (ie the end point of atitration)6. Dry electrodes must be soaked for several hours before they are ready to use.7. Measurements and calibration should take place at the same temperature.2 2Errors 1 and 2 make the total absolute error in a pH measurement ± sqrt(.01 + .01 ).014 pH units. Relative accuracy between two measurements (say in a titration curveis about .002 pH unitsSolid State pH sensors Figure 15-11New - High techUse a field effect transistor+transistor that binds H and that varies current flow through transistorlook at pictures in text15-5 Ion-Selective ElectrodesThere are 4 major types of ion selective electrodes1. Glass membrane2. Solid-state

73. Liquid-based electrodes4. Combination electrodes1. Glass membrane we have already talked about in reference to measuring pH.+ +By manipulating the formula of the glass you can make electrodes for Li , Na , K, and+Ag . None of the electrodes is perfectly selective, and the amount that an electroderesponds to another ion can be quantitated in the selectivity coefficient, but we won’tget into that. Other tah to say that your standard pH electrode tends to give bad results+ -12 +when [H ] .012. Solid-State electrodes (fig 15-12)In a solid state electrode an inorganic crystal is used at the interface betweenthe reference and test solutions. In this case the ion to be sensed is part of the crystal,and actually migrates across the crystal during the measurement.Electrodes for- - - - - -2F , Cl , Br , I , SCN ,CN and S have been made in this manner3.Liquid-based Ion-Selective Electrodes (fig 15-14)Here the barrier between liquids is a hydrophobic membrane that is saturatedwith a metal ion chelator. Because the membrane is hydrophobic, no ions can passthough it. However metals that bind to the chelator can enter the membrane andestablish a junction potential just as in out other electrodes+2 - - - -Electrode for Ca , NO , ClO BF and Cl have been made in this way.3 4 44. Compound electrodesDiagram Figure 15-17In compound electrodes you have one of the above electrodes surrounded by amembrane that will let a different analyte ion through, and then somehow covert it intoan ion that the electrode sensesThe simplest example of this is a pH electrode surrounded by a membrane thatonly allows CO 2 to pass though it. As CO 2 passes the outer membrane, you have thereaction CO 2 + H2O H2CO 3 an acid, and this lowers the pH of the solution which youthen detect with the inner electrode. Electrodes of this kind are used for the detectionof acidic and basic gasses both in the gas phase and in solutionAnother trick that can be done is to make electrodes for various enzymatic+reactions that generate H . There are lots of biochemical reaction that either generate+or utilize H . You can sense for only one of these compounds by surrounding a pHtype electrode with a semipermeable membrane the enzyme that runs the reaction inthe liquid around the membrane. As one of the compounds that run the reaction enters+the space, the enzyme run the reaction, and H is used or generated, which the pHelectrode then senses