Potentiometry PDF

POTENTIOMETRY INTRODUCTION Electric rays have electric organs that are capable of generating strong electric shock. Such fishes produce a powerful discharge of current to stun enemies and prey. Nerve terminals rapidly emit acetylcholine that causes sodium ions to surge through the mem...

POTENTIOMETRY INTRODUCTION Electric rays have electric organs that are capable of generating strong electric shock. Such fishes produce a powerful discharge of current to stun enemies and prey. Nerve terminals rapidly emit acetylcholine that causes sodium ions to surge through the membranes, producing a rapid separation of charge and a corresponding potential difference, or voltage, across the membrane. The potential difference then generates an electric current of several amperes in the surrounding seawater. Humans have learned to separate charge mechanically, metallurgically, and chemically to create cells, batteries, and other useful charge storage devices. How do fish make electricity https://www.youtube.com/watch?v=z0M7_HPSi14 POTENTIOMETRY INTRODUCTION Electricity can be used to drive a chemical reaction. A chemical reaction can be used to produce electricity. Electric charge (q), the SI unit of electrical charge is the coulomb (C). Electroneutrality is maintained by a balance between the flow of negative charges and the flow of positive charges so there is no significant buildup of charge in any region. The presence of electric charge creates an electric potential that attracts or repels charged particles. The electric potential difference, E, between two points is the work per unit charge that is needed or can be done when charge moves from one point to the other. Potential difference is measured in volts (V). Current (I) is the rate of charge flow in a circuit or solution. One ampere of current is a charge flow rate of one coulomb per second (1 A = 1 C/s). ELETTROCHEMISTRY ELECTROCHEMICAL CELLS Electrochemistry is a branch of analytical chemistry that uses electrical measurements of chemical systems for analytical purposes. The electrochemical cell comprises two half-cells (or electrodes). The salt- bridge separates the two half-cells and maintains electroneutrality throughout the cell. Anions flow to the anode and cations flow to the cathode. The potentiometer (voltmeter) measures the electric potential difference (voltage) between the two metal electrodes. Voltage tells us how much work can be done by electrons flowing from one side to the other. The potentiometer has high electrical resistance so that little (ideally no) current flows through the meter. Conductive V Conductive element element Half-cell or Half-cell or electrode (1) electrode (2) Electrolyte Electrolyte solution solution Salt bridge ELETTROCHEMISTRY ELECTROCHEMICAL CELLS In each half-cell a half-reaction occurs Left electrode (oxidation) Cd(s) → Cd2+ (aq) + 2e− Right electrode (reduction) 2Ag+ (aq) + 2e− → 2Ag(s) Reazione complessiva Cd(s) + 2Ag+ (aq) → Cd2+ (aq) + 2Ag(s) ELETTROCHEMISTRY ELECTROCHEMICAL CELLS When an electric potential difference is present and electrodes are connected through a low resistance circuit, an electric current is generated. ELETTROCHEMISTRY STANDARD POTENTIALS The standard potential (E°) shows the driving force for the redox reaction under standard conditions: - When the electrode is connected to a standard hydrogen electrode - When all the chemical species have activity = 1. These half-cells will work as a cathode when connected to the standard hydrogen electrode (SHE). These half-cells will work as an anode when connected to the standard hydrogen electrode (SHE). ELETTROCHEMISTRY NERNST EQUATION The Nernst equation includes two terms: E° and one term that shows the dependence on reagent concentrations (better said activities). Let’s consider a generic half-reaction: aA + bB + ne−  cC + dD ( E) RT aCc  aDd RT E = E − ln a = E  − ln Q nF a A  aBb nF The logarithmic term in the Nernst equation, Q, is the reaction quotient: When Q = K the system has reached the equilibrium, therefore E=0 When Q = 1 the system is in standard conditions, therefore E = E° For any other value of Q, we can calculate the corresponding E value: If the temperature is 298,15 K (25,00 °C): 0,0592 E = E − log Q n POTENTIOMETRY INTRODUCTION Potentiometric techniques are electrochemical analytical techniques that exploit the measurement of the voltage of an electrochemical cell to obtain information on the concentration (better said on the activity) of a chemical species. Employing suitable electrodes (e.g., ion selective electrodes), measurements are easy, rapid, relatively sensitive and selective. Measurements can be performed with laboratory or with portable instrumentation. Benchtop potentiometer Portable battery-operated potentiometer POTENTIOMETRY APPLICATIONS A reference electrode is an essential component of any potentiometric system The glass electrode for pH measurement is the most commonly employed ion selective electrode Potentiometric techniques are used in clinical chemistry analyzers for the measurement of the main plasma electrolytes Portable pH-meters allow on-field pH measurements as well as measurement of other ions when equipped with suitable electrodes POTENTIOMETRY INTRODUCTION Potentiometry is the use of electrodes to measure voltages that provide chemical information. As the flowing current is very little (ideally zero), the sample is not modified in its composition during the measurement. - + V The cell potential is Reference Indicator calculated as the electrode electrode difference: Solution that 𝐸𝑐𝑒𝑙𝑙 = 𝐸𝑖𝑛𝑑 − 𝐸𝑟𝑖𝑓 contains the analyte Indicator electrode (working electrode): it responds to the analyte Reference electrode: it maintains a fixed reference potential. Ion selective electrodes http://www.youtube.com/watch?v=Z7e9w3wRvi8 POTENTIOMETRY INTRODUCTION In an electrochemical cell the movement of electrons is related to differences in electric potential (electrons spontaneously move towards positive potentials). These differences depend on the electrodes that make up the cell even if there are also other contributions to the potential, e.g., the junction potentials related to the characteristics of the salt bridge. V Flow of electrons Cd Ag Salt bridge Electrical potential CdCl2(aq) AgNO3(aq) Interphase Interphase Interphase Potenziale elettrico Electric potential all'interno inside the cell della cella POTENTIOMETRY LIQUID-JUNCTION POTENTIAL What is a liquid-junction potential? The cell potential also depends on liquid-junction potential (EJ) that usually amounts to few mV. Despite its small value, junction potential must be considered, as it cannot be accurately determined and therefore it limits the accuracy of potentiometric measurements. The correct expression for calculating the cell potential is therefore: E = Eind – Erif + ΣEJ. The junction potential results from differences in the migration rates of ions and it is produced at each interface between solutions with different composition (e.g., at the salt bridge ends). When exposed to a concentration gradient, Cl- ions migrate more rapidly than Na+ ions. This causes a charge separation and therefore a liquid- junction potential at the interface between the two solutions. Na+ Cl- Na+ Cl- Na+ Cl- NaCl solution Water NaCl solution Water POTENTIOMETRY LIQUID-JUNCTION POTENTIAL The liquid-junction potential can be reduced by employing, in electrodes, electrolyte solutions that contain high concentration of cations and anions with similar mobility. Mobility of ions in water (25°C) Liquid-junction potentials (25°C) KCl at high concentration is very often employed as an electrolyte in reference electrodes to reduce junction potentials as much as possible, since K+ and Cl- have very similar mobility. POTENTIOMETRY REFERENCE ELECTRODES The Nernst equation for Systems Involving Precipitates The most employed reference electrodes rely on a half-reaction that involves a metal and its cation in an insoluble salt, e.g.: The electrode potential is: AgCl s + e− ↔ Ag s + Cl− aq E° = 0.222V o Erif = EAgCl/Ag − 0,059 log[Cl− ] As the electrode potential only depends on [Cl-] , the internal solution contains KCl at a fixed and high concentration, to avoid [Cl-] variations over time. Saturated KCl → Erif = + 0,197 V KCl 3,5 M → Erif = + 0,205 V KCl 1,0 M → Erif = + 0,235 V POTENTIOMETRY REFERENCE ELECTRODES The potential of a reference electrode is known, constant (at constant temperature), and completely independent of the analyte concentration. Silver wire How does a reference electrode work? The most employed reference electrodes rely on a Solid AgCl half-reaction that involves a metal and its cation in an insoluble salt, e.g.: Solution containing KCl at high concentration and AgCl s + e− ↔ Ag s + Cl− aq E° = 0.222V saturated with AgCl Solid KCl (only when saturated KCl is present in The electrode potential is: the internal solution) Porous plug o − Erif = EAgCl/Ag − 0,059 log[Cl ] As the electrode potential only depends on [Cl-] , the internal solution contains KCl at a fixed and high concentration, to avoid [Cl-] variations over time. Saturated KCl → Erif = + 0,197 V KCl 3,5 M → Erif = + 0,205 V KCl 1,0 M → Erif = + 0,235 V POTENTIOMETRY SILVER-SILVER CHLORIDE REFERENCE ELECTRODE Ag|AgCl electrode: E° = + 0.222 V AgCl( s ) + e −  Ag( s ) + Cl − E(saturated KCl) = + 0.197 V POTENTIOMETRY CALOMEL REFERENCE ELECTRODE Calomel electrode: E° = + 0.268 V 1 Hg 2 Cl 2 ( s ) + e −  Hg(l ) + Cl − E(saturated KCl) = + 0.241 V 2 A calomel electrode saturated with KCl is called a saturated calomel electrode (S.C.E.). POTENTIOMETRY REFERENCE ELECTRODES Sample matrix components can interact with the reference electrode, causing damages and measurement errors. As concerns the Ag/AgCl reference electrode, all substances that react with Ag+, such as anions yielding precipitates that clog the porous plug (e.g., S2-, Br-, I-), Ag+ complexing agents and proteins are interferents. To prevent these problems, double junction reference electrodes can be used, in which a Per evitare questi rischi si usano elettrodi di riferimento a doppia giunzione con una camera intermedia contenente un elettrolita (di solito KCl). Durante l'uso l'elettrolita fluisce lentamente all'esterno impedendo l'ingresso dei costituenti del campione e la contaminazione dell'elettrodo interno. Tappo per il riempimento dell'elettrodo di riferimento Tappo per il riempimento Elettrodo di della camera riferimento intermedia Setto poroso Elettrodo di argento rivestito di AgCl Setto poroso Camera intermedia con soluzione elettrolitica POTENTIOMETRY INDICATOR ELECTRODES An indicator electrode has a potential that varies in a known way with variations in the concentration of an analyte. An indicator electrode must rapidly and reproducibly respond depending on the analyte concentration. It should also be specific or at least display a certain selectivity for the analyte even in complex matrices. Metal indicator electrodes Ion selective electrodes (ISE) The electrode response is based on a redox They do not display a redox process; they are reaction that involves the analyte. based on: They respond to electroactive substances. - a selective interaction of the analyte with an electrode Metal indicator electrodes have very few - the difference in analyte concentration between analytical applications, especially when the electrode internal solution and the sample complex samples are analyzed, as it solution in which the electrode is immersed. provides low selectivity. They respond to charged substances. Sensitive and selective for their ion. POTENTIOMETRY METAL INDICATOR ELECTRODES Intercept = E°Mn+/M The reaction at the Ag indicator electrode is: Eind Slope = -0,0592/n Ag + + e −  Ag( s ) E+o = 0.799 V The calomel reference half-cell pM reaction is: Hg 2 Cl 2 ( s ) + 2e −  2Hg(l ) + 2Cl − E− = 0.241 V The Nernst equation for the entire cell is: POTENTIOMETRY ION SELECTIVE ELECTRODES (ISE) Ion Selective Electrodes (ISE) do not involve a redox process, they respond selectively to one ion Internal reference through a thin membrane capable of binding only electrode that ion. Filling solution: analyte ainner Ion-selective membrane Analyte solution outside the electrode aouter The Figure shows a liquid membrane- based ion-selective electrode. The ion- selective membrane is a hydrophobic organic polymer containing a viscous organic solution, which contains an ion- exchanger (ionophore) that selectively binds the analyte cation, C+. POTENTIOMETRY ION SELECTIVE ELECTRODES (ISE) How do ISE work? When a selective membrane is in contact with The internal solution has a higher two different solutions containing the analyte, concentration. In this example the analyte is a cation. The membrane internal face acquires on each membrane face an equilibrium is a higher positive charge with respect to the reached between ions in solution and bound external face to the membrane. The potential difference between the + + membrane faces (boundary potential Eb) + + + + + + depends on the analyte concentration + + + Eb difference between the internal and + + external solution. + + + + + + + + Internal solution Ion selective External solution 0,059 [X]est [X]int membrane [X]est Eb = ± log n [X]int The sign depends on the charge of the analyte. As usual, activities should be considered instead of Whenever there is a charge concentrations. imbalance across any material, there is an electrical potential difference across the material. POTENTIOMETRY ION SELECTIVE ELECTRODES (ISE) ISE can be produced employing membranes made of different materials. ISE based on solid membranes ISE based on liquid membranes. The ISE based on glass membranes. The membrane membrane is an organic solvent, not miscible with is in glass. Its selectivity is obtained by using water, that contains an ion exchanger. glass with different composition. The glass electrode for pH measurement is a typical example ISE based on a crystal membrane. The membrane is a crystal (a single crystal or a polycrystalline material). ISE based on polymer membranes. The membrane is a polymer that contains an ion exchanger. Composite electrodes. They employ a second membrane that isolates the analyte from the electrode. POTENTIOMETRY pH MEASUREMENT WITH A GLASS ELECTRODE The glass electrode used to measure pH is the most common ion-selective electrode. The pH-sensitive part of the electrode is the Silver wire thin glass bulb at the bottom of the coated with electrode. AgCl(s) The electrode contains an internal solution with fixed H+ concentration (a diluted HCl Solution with solution or a buffered solution) and fixed H+ saturated with AgCl. concentration A silver wire coated with AgCl is immersed and saturated in the internal solution, thus constituting a with AgCl reference Ag-AgCl electrode (internal Glass reference electrode), which is connected to membrane one terminal of the potentiometer. A second reference electrode (external reference electrode) is connected to the other potentiometer terminal. POTENTIOMETRY pH MEASUREMENT WITH A GLASS ELECTRODE Schematic structure of silicate glass, which consists of an irregular network of SiO4 tetrahedra connected through oxygen atoms. Cations such as Li +, Na +, K +, and Ca2+ are coordinated to oxygen atoms. The silicate network is not planar. The diagram is a projection of each tetrahedron onto the plane of the page. The two surfaces swell as they absorb water. Metal ions in these hydrated gel regions of the membrane diffuse out of the glass and into solution. H+ can diffuse into the membrane to replace metal ions. A pH electrode responds selectively to H+ because H+ is the main ion that binds significantly to the hydrated gel layer. To avoid membrane drying, the elctrode is always stored in a buffer solution POTENTIOMETRY pH MEASUREMENT WITH A GLASS ELECTRODE Eb Eint Eest -OH -O- -OH -O- Acidic solution Alkaline solution -O- (high concentration -O- (low concentration -OH of H+) -OH of H+) -OH -O- -OH -O- The membrane has a The membrane has a small negative charge high negative charge The boundary potential depends on the different H+ ion concentration between the two solutions. [H + ]est The pH of the internal solution is fixed. As the pH of the Eb = 0,059 log + [H ]int external solution (the sample) changes, the electric potential difference across the glass membrane changes. POTENTIOMETRY pH MEASUREMENT WITH A GLASS ELECTRODE The potential difference between the inner and outer silver-silver chloride electrodes depends on the chloride concentration in each electrode compartment and on the boundary potential across the glass membrane. Because [Cl-] is fixed in each compartment and because [H+] is fixed on the inside of the glass membrane, the only variable is the pH of analyte solution outside the glass membrane. EAg/AgCl EAgCl/Ag Eb POTENTIOMETRY pH MEASUREMENT WITH A GLASS ELECTRODE The potential of a glass electrode (Eglass) includes contributions from the various components of the elctrode. [H + ]ext Eglass = 0,059 log + + Erif + Easymm Asymmetry potential is a contribution to [H ]int the glass electrode potential that has been experimentally determined by considering that boundary potential is not zero when [H+]int = [H+]ext.. This is due to the fact that Boundary Internal reference Asymmetry the two sides of the glass membrane are potential electrode potential not perfectly equal. potential By grouping the constant terms, we can obtain the formula that relates glass electrode potential to pH of the external solution (sample). 1 Eglass = 0,059 log + + Erif + Easymm + 0,059 log [H + ]ext [H ]int The electrode is sealed, therefore [H+]int is constant and can be included in the Eglass = L − 0,059 pHext constant term (L) POTENTIOMETRY pH COMBINATION ELECTRODE The pH combination electrode incorporates both glass and reference electrodes in one body. Potentiometric pH measurement http://www.youtube.com/watch?v=P1wRXTl2L3I POTENTIOMETRY pH MEASUREMENT WITH A GLASS ELECTRODE The response of real glass electrodes is described by the Nernst-like equation The value of β, the electromotive efficiency, is close to 1.00 (typically >0.98). We measure the constant and β when we calibrate the electrode in at least two solutions of known pH, such that the pH of the unknown lies within the range of the standards. Commercial standards in are accurate to ± 0.01 pH unit. A pH electrode must be calibrated at the same temperature as the unknown before it can be used. It should be calibrated at least every 2 h during sustained use POTENTIOMETRY pH MEASUREMENT WITH A GLASS ELECTRODE How do we calibrate a glass electrode? Glass electrodes need to be periodically calibrated (usually once a day) employing standard buffers at known pH. This is due to the fact that the electrode response (in particular the asymmetry potential) vary with time because of membrane deterioration. Calibration is usually performed with two standards (in some cases three). Standards must provide a calibration range within which the successively measured samples will fall (e.g., for acidic samples I can employ pH 4.01 and 7,00 standards). Commonly used standards for glass electrode calibration pH (at 25°C) 1,68 4,01 6,86 NIST US Standard 7,00 Standard 9,18 10,01 12,45 POTENTIOMETRY pH MEASUREMENT WITH A GLASS ELECTRODE Controlling temperature is essential for measurements accuracy, as it influences the pH value of standards and also the electrode response. For optimal accuracy, calibration and sample measurements shall be performed at the same temperature. Most pHmeters contain a thermal probe that is immersed on the solution to correct for any variation due to temperature changes How the pH of standard buffer depend on temperature How the electrode response depends on temperature POTENTIOMETRY pH MEASUREMENT WITH A GLASS ELECTRODE Benchtop pH-meter The value displayed on the screen is frozen when the measurement reaches a stable value Standards used in the calibration Thermal probe Glass electrode This alarm will pop up if The thermal probe will enable there is an abnormal automatic correction of behaviour of the electrode measured pH values or a calibration standard has according to the solution not been properly measured temperature (ATC, Automatic Temperature Compensation) POTENTIOMETRY pH MEASUREMENT WITH A GLASS ELECTRODE Before using a pH electrode, be sure that the air inlet is not capped. (This hole is capped during storage to prevent evaporation of the electrode filling solution). Wash the electrode with distilled water and blot it dry with a tissue. Do not wipe it, because this action might produce a static charge on the glass. Store a glass electrode in aqueous solution to prevent dehydration of the glass. Ideally, the solution should be similar to that inside the reference compartment of the electrode. If the electrode has dried, recondition it in dilute acid for several hours. If the electrode is to be used above pH 9, soak it in a high-pH buffer. http://www.youtube.com/watch?v=HinpfI_jMBg POTENTIOMETRY ERRORS IN pH MEASUREMENT WITH A GLASS ELECTRODE Standards. A pH measurement cannot be more accurate than standard buffers, which are typically ± 0.01 pH unit. Junction potential. A junction potential exists at the porous plug near the bottom of the electrode. If the ionic composition of the analyte solution is different from that of the standard buffer, the junction potential will change. This effect gives an uncertainty of at least 0.01 pH unit. Junction potential drift. The junction potential changes during usage, therefore the pH electrode requires frequent calibration Equilibration time. It takes time for an electrode to equilibrate with a solution. A well buffered solution requires 30 s with adequate stirring. A poorly buffered solution (such as one near the equivalence point of a titration) needs many minutes. Hydration of glass. A dry electrode requires several hours of soaking before it responds to H+ correctly. Temperature. A pH meter should be calibrated at the same temperature at which the measurement will be made. Cleaning. If an electrode has been exposed to a hydrophobic liquid, such as oil, it should be cleaned with a solvent that will dissolve the liquid and then conditioned in aqueous solution. The reading of an improperly cleaned electrode can drift for hours while the electrode re-equilibrates with aqueous solution. POTENTIOMETRY ERRORS IN pH MEASUREMENT WITH A GLASS ELECTRODE Sodium error or alkaline error. When [H+] is very low and [Na+] is high, the electrode responds to Na+ and the apparent pH is lower than the true pH. Acid error. In strong acid, the measured pH is higher than the actual pH, perhaps because the glass is saturated with H+ and cannot be further protonated. POTENTIOMETRY ERRORS IN pH MEASUREMENT WITH A GLASS ELECTRODE Errors related to standards and junction potential limit the accuracy of pH measurement with the glass electrode to ± 0.02 pH unit, at best. An uncertainty of ± 0.02 pH unit corresponds to an uncertainty of 65% in [H+] Measurement of pH differences between solutions can be accurate to about ± 0.002 pH unit, but knowledge of the true pH will still be at least an order of magnitude more uncertain. POTENTIOMETRY ION SELECTIVE ELECTRODES FOR OTHER IONS There are also other types of membrane electrodes that are selective for other ions, for example: Modified composition glass membrane electrodes for single charged cations such as Na+, K+, NH4+, Rb+, Li+, Cs+, Ag+. Crystalline membrane electrodes for ions such as F-, I-, Br-, Cl-, CN-, SCN-, S2-, Ag+, Cu2+, Cd2+ e Pb2+ Electrodes with hydrophobic liquid membrane or with polymer membrane for ions such as Ca2+, Na+, K+, NH4+, NO3-, ClO4-, BF4-, Cl- POTENTIOMETRY GLASS ELECTRODES By varying the composition of the glass membrane (e.g., a portion of SiO2 is substituted with Al2O3 or B2O3) glass electrodes with peculiar characteristics can be obtained, suitable for specific applications. Electrodes with reduced alkaline error They provide accurate pH measurement up to pH = 12 – 13. Electrodes for other monovalent cations Electrodes selective for other monovalent cations (e.g., Na+, Li+, NH4+) can be obtained. Thanks to their high selectivity, these electrodes can be used to measure such ions directly in complex samples, such as blood or urine. Glass electrodes for measuring the total content of monovalent cations are also available. POTENTIOMETRY ION SELECTIVE ELECTRODES FOR OTHER IONS ISE with crystal membrane for measuring F- The membrane is composed of a LaF3 crystal doped with Eu(II). The response of the electrode to F- is given by the equation EISE = L − 0,059 log [F − ] Wire for connecting to the potentiometer Negative sign due to the analyte negative charge Internal reference electrode The electrode is quite selective and it is Internal solution used in environmental monitoring. Alkaline containing a fixed F- solutions cannot be analysed because OH- concentration binds to the membrane and is therefore an interferent. In acidic solution the response to F- decreases, because HF is formed. LaF3 crystal doped Ion selective with Eu2+ membrane POTENTIOMETRY ION SELECTIVE ELECTRODES FOR OTHER IONS Other ion selective electrodes with crystal membrane Concentration range Membrane Working pH Analyte (M) composition range Interferents POTENTIOMETRY ION SELECTIVE ELECTRODES FOR OTHER IONS Ion selective electrodes with polymer membrane or liquid membrane rely on an ion exchanger (ionophore), while the rest of the membrane only acts as a support. The ion exchanger contains a binding site for the ion, surrounded by an hydrophobic structure that makes it soluble in an organic solvent or polymer. Nonactin (NH4+) Valinomycin (K+) Calcium ionophore I (Ca2+) Sodium ionophore III (Na+) POTENTIOMETRY ION SELECTIVE ELECTRODES FOR OTHER IONS Ion selective electrode with liquid membrane for Ca2+ detection The membrane is composed of a porous hydrophobic material soaked with an organic solvent, immiscible with water, in which the ion exchanger is dissolved. The electrode responds to Ca2+ following The electrode suffers of interference from other the equation divalent cations (especially Fe2+) and from H+ (in acidic environment the ion exchanger is 0,059 protonated and it cannot and calcium anymore). EISE = L + log [Ca2+ ] 2 Wire for connecting The ion exchanger has a binding pocket for to the potentiometer Ca2+ and relatively long aliphatic chains (C8 – C10) to provide solubility in organic solvent. Internal reference electrode Solution containing a fixed Ca2+ concentration Stock of organic solvent with dissolved ion exchanger Membrane of porous material POTENTIOMETRY ION SELECTIVE ELECTRODES FOR OTHER IONS Other ion selective electrodes with liquid membrane Limit of Interferents and selectivity Analyte detection, µM coefficient (logarithm scale) The selectivity coefficient describes the response to an interferent responseinterferent with respect to the response to the analyte. For ions with the same k= responseanalyte charge, the selectivity coefficient is defined as: POTENTIOMETRY ION SELECTIVE ELECTRODES FOR OTHER IONS Ion selective electrode with polymer membrane for NH4+ detection The membrane is composed of a plastic material (e.g., PVC or silicon rubber) in which an ion exchanger is dispersed. The response of the electrode to NH4+ The electrode suffers of interference mainly from K+ (which is given by the equation has the same charge and very similar size). The response decreases in alkaline environment, where ammonium is EISE = L + 0,059 log [NH4+ ] deprotonated to NH3. Nonactin has a cavity which size is suitable for hosting NH4+ and that has groups able to form hydrogen bonds with a tetrahedral geometry. Wire for connecting The geometry confers selectivity with respect to to the potentiometer K+ that has a spherical symmetry. Internal reference electrode Solution containing a fixed NH4+ concentration Polymer membrane POTENTIOMETRY ION SELECTIVE ELECTRODES FOR OTHER IONS Potentiometric gas-sensing probes (CO2) The potentiometric CO2–sensing probe consists in a glass electrode and a reference electrode, both immersed in an electrolyte solution (e.g., sodium bicarbonate). The solution is in contact with a membrane (rubber, Teflon o polyethylene) permeable to CO2. CO2 can cross the membrane and diffuse in the solution; other species (especially H+ or OH-) cannot cross the membrane, CO2 sample which would modify the pH of the internal solution. ↕ CO2 internal solution Wires for connecting to H2 O ↕ the potentiometer H2 CO3 ↕ External reference H + HCO3− + electrode Glass electrode CO2 that crosses the membrane causes a decrease of pH of the internal solution, which is detected by the glass electrode Electrolyte solution Selectivity is provided by the membrane. Other potentiometric gas-sensing probes are available with membranes selective for other gases that are able to Membrane solubilize in water and modify its pH (e.g., SO2, H2S, permeable to NH3). CO2 POTENTIOMETRY USE OF ION SELECTIVE ELECTRODES Ion selective electrodes offer several advantages, such as Linear response in a wide range of analyte concentrations Possibility to perform non-destructive analyses No sample contamination Rapid analysis Can be used in turbid or colored solutions Nevertheless, their main limitations must be considered to reduce errors Measurement precision is not very high, an accurate preparation of standards and samples is crucial to obtain good accuracy Electrodes get contaminated in the presence of proteins or other organic molecules, causing slow response or drift; some contaminants can damage the electrode surface Only the fraction of analyte ions is detected by the electrode: if a fraction of the analyte is present in a complex, it will not be detected: ligands must therefore be eliminated or masked Solutions with different ionic strength will give different responses: to overcome this problem, a high concentration of an inert salt can be added to all the standards and samples, to provide the same ionic strength to all of them. POTENTIOMETRY ANALYTICAL APPLICATIONS Calibration methods for analyses with ISE External calibration Calibration with standard addition Errors can arise when samples have different This method avoids errors due to ionic strength ionic strength with respect to the standards used differences and, in general, to the sample matrix. for calibrating. To avoid errors due to ionic strength differences between samples and standards, the saturation approach can be employed: all samples and standards are added of an excess electrolyte so that all of them will have a high and equal ionic strength. The ionic strength of a solution is the concentration of ionic charge in solution. POTENTIOMETRY ANALYTICAL APPLICATIONS The features of potentiometric techniques (rapid, direct, selective and non-destructive analysis) make them particularly suited for real-time monitoring of complex systems. pO2 is actually measured employing a amperometry, exploiting a membrane permeable to oxygen. Glass electrode (pH) Potentiometric gas-sensing probe (pCO2) Clark probe (pO2) Bioreactor for small scale cell cultures