Ion Selective Electrode

One of the Method to measure electrolytes

1. Ion Selective Electrode Dr. Ritu Bashyal MD Biochemistry

2. History Credit for the first glass sensing pH electrode is given to Cremer, who first described it in his 1906 paper In 1949, George Perley published an article on the relationship of glass composition to pH function The commercial development of ISE began when John Riseman ( an engineer ) thought he could develop a useful blood-gas analyzer . He teamed up with Dr. James Ross, an electrochemist from MIT . Together they formed Orion Research By the mid 1960s, the newly formed Orion Research was producing Calcium electrodes for use in blood gas analyzers Since then numerous probes have been developed for the analysis of samples containing many different ions.

3. I ntroduction to ion analysis Electrochemical Analysis I s the group of chemical analytical methods in which a signal is generated by an electrochemical reaction and is used for quantitative determination of a substance Electrodes I s an  Electronic Conductor  in contact with an  Ionic Conductor , the Electrolyte Electrode Reaction   - in which charge transfer takes place at the interface between the electrode and the electrolyte.

4. An  Indicator Electrode  shows by means of its potential the  Activity  of an ion in a solution Electrochemical Cells An electrode in contact with an electrolyte represents a  Half-Cell and t wo combined half-cells form an  Electrochemical Cell  - i.e. one solution in contact with an ISE and a reference electrode The  Electrochemical Potential (E)  can be measured across the two terminals of the cell.

5. E lectrochemical and optical sensors are established in clinical analysis system for measurement of blood gases, electrolytes, and metabolites When integrated into chromatographic systems , they provide a very sensitive and selective means to detect a variety of other analytes , such as therapeutic drugs, neurotransmitters, glutathione, and homocysteine Also, electrochemical detection has been applied successfully fo r monitoring coagulation reactions, detecting toxic lead in blood samples, and developing novel ultrasensitive immunoassay schemes.

6. BASIC CONCEPTS Potentiometry is the measurement of an electrical potential difference between two electrodes (half-cells) i n an electrochemical cell when the cell current is zero (galvanic cell) Such a cell consists of two electrodes that are connected by an electrolyte solution (ion conductor) An electrode, or half-cell, consists of a single metallic conductor that is in contact with an electrolyte solution

7. The ion conductors can be composed of one or more phases that are either in direct contact with each other or separated by membranes permeable only to specific cations or anions One of the electrolyte solutions is the unknown or test solution; this solution may be replaced by an appropriate reference solution for calibration purposes By convention, the cell notation is - left electrode is the reference electrode; the right electrode is the indicator (measuring) electrode

8. TYPES OF ELECTRODES Many different types of electrodes are used for potentiometric applications They include Redox electrode I on-selective membrane (glass and polymer) PCO 2 electrodes

9.   Ion Selective Electrodes (ISE)   A ka a  specific ion electrode  ( SIE ), is a transducer (or sensor) that converts the activity of a specific ion dissolved in a solution into an electrical potential, which can be measured by a voltmeter or pH meter How ISE work ?? Work s on the theory that an electrode develops a potential due to ion-exchange occurring between the sample and the inorganic membrane This potential is measured against a stable reference electrode of constant potential The potential difference between the two electrodes will depend upon the activity of the specific ion in solution. This activity is related to the concentration of that specific ion

10. ISEs all work on the basic principle of the Galvanic Cell. The Galvanic cell consists of two half cells that are made up of two different metals dipped into two separate solutions of salt of the corresponding metal. For e.g. Copper electrode in Copper Sulfate solution or Zinc electrode in Zinc Sulfate solution. One of these metals is able to reduce the cation of the other metal while the other cation will oxidize the first metal.

11. When one of the metals from the half cel l is oxidized, anions are transferred into that half cell. This is done to keep a balance of electrical charge from the cation that is produced. The solutions cannot mix for this to work. To accomplish this, the t w o solutions are connected via a salt bridge that is porous Thi s allows the solutions to be in separate beakers while still allowing the charge to flow throughout the entire system.

12. As the current flows, it is attached to a volt meter that record and displays the charge This is accomplished by measuring the electric potential that will be generated by the selected ions From here it is compa i red to the reference electrode where the volt meter displays the difference in the two It is extremely important that the reference electrode remains constant. The general equation for the Galvanic cell is E cell = E ISE - E ref Where E ISE = the potential of the ISE E ref = The potential of the reference electrode

13. How Does the mV Reading Correspond to the Concentration? C alibration cuve helps in this situation Standard solutions of known concentrations must be accurately prepared ; These solutions are then measured with the pH/mV meter The mV reading of each solution is noted and a graph of concentration vs. mV reading must be plotted Now the unknown solution is measured ; The mV value of the unknown solution is then located on the graph and the corresponding solution concentration is determined.

14. C omponents of ISE I on selective electrode with membrane at the end – allows ions of interest to pass, but excludes the passage of the other ions I nternal reference electrode – present w ithin th e ion selective electrode which is made of silver wire coated with solid silver chloride, embedded in concentrated potassium chloride solution (filling solution) saturated with silver chloride . This solution also contains the same ions as that to be measured

15. R eference electrode similar to ion selective electrode, but there is no to-be-measured ion in the internal electrolyte C ommonly used – calomel electrode The lower end of the reference electrode is sealed with a porous ceramic frit which allows the slow passage of the internal filling solution and forms the liquid junction with the external test solution Dipping into the filling solution is a silver wire coated with a layer of silver chloride (it is chloridised ) which is joined to a low-noise cable which connects to the measuring system.

16. Reference Electrode  is needed as t he p otential between an electrode and a solution cannot be directly measured I t is necessary to include in the circuit a stable reference voltage which acts as a half-cell from which to measure the relative deviations. The reference electrode should have a known, or at least, a constant Potential value under the prevailing experimental conditions. One problem with reference electrodes is that, in order to ensure a stable voltage, it is necessary to maintain a steady flow of electrolyte through the porous frit

17. Thus there is a gradual contamination of the test solution with electrolyte ions This can cause problems when trying to measure low levels of K, Cl , or Ag, or when using other ISEs with which these elements may cause interference problems In order to overcome this difficulty the double junction reference electrode was developed In this case the silver / silver chloride cell described above forms the inner element and this is inserted into an outer tube containing a different electrolyte which is then in contact with the outer test solution through a second porous frit

18. The outer filling solution is said to form a " salt bridge " between the inner reference system and the test solution and is chosen so that it does not contaminate the test solution with any ions which would effect the analysis. Commonly used outer filling solutions are: potassium nitrate - for Br, Cd , Cl , Cu, CN, I, Pb , Hg, Ag, S, SCN. sodium chloride - for K, ammonium sulphate - for N0 3 , magnesium sulphate - for NH 4 ,

19. One disadvantage with double junction reference electrodes is that they introduce an extra interface between two electrolytes and thus give the opportunity for an extra liquid junction potential to develop. It must be noted that the E  factor in the Nernst equation is the sum of all the liquid junction potentials present in the system and any variation in this during analyses can be a major source of potential drift and error in measurements. The two solutions in a double junction reference electrode are connected by a so called Liquid Junction A liquid junction is a semi-permeable membrane separating two solutions, which prevents wholesale mixing but allows the passage of ions by diffusion.

20. C ircuit O ther end of both the electrode is fitted with a low noise cable or gold plated pin for connection to the millivolt measuring device. Since the potentials of the two reference electrodes are constant, any change in cell potential is due to change in potential across the membrane.

21. I ons that can be measured The most commonly used ISE is the pH probe. CATIONS:  Ammonium (NH 4 + ), Barium ( Ba ++ ), Calcium (Ca ++ ), Cadmium ( Cd ++ ), Copper (Cu ++ ), Lead ( Pb ++ ), Mercury (Hg ++ ), Potassium (K + ), Sodium (Na + ), Silver (Ag + ) ANIONS:  Bromide (Br - ), Chloride ( Cl - ), Cyanide (CN - ), Fluoride (F - ), Iodide (I - ), Nitrate (NO 3 - ), Nitrite (NO 2 - ), Perchlorate (ClO 4 - ) Sulphide (S - ), Thiocyanate (SCN - ).

22. The pH Electrode The pH electrode is the most well known and simplest one and can be used to illustrate the basic principles of ISE This is a device for measuring the concentration of hydrogen ions and hence the degree of acidity of a solution - since pH is defined as the negative logarithm of the hydrogen ion concentration; i.e. pH=7 means a concentration of 1x10 -7  moles per litre .

23. The most essential component of a pH electrode is a special, sensitive glass membrane which permits the passage of hydrogen ions, but no other ionic species When the electrode is immersed in a test solution containing hydrogen ions the external ions diffuse through the membrane until an equilibrium is reached between the external and internal concentrations Thus there is a build up of charge on the inside of the membrane which is proportional to the number of hydrogen ions in the external solution.

24. Because of the need for equilibrium conditions there is very little current flow and so this potential difference can only be measured relative to a separate and stable reference system which is also in contact with the test solution, but is unaffected by it A sensitive, high impedance millivolt meter or digital measuring system must be used to measure this potential difference accurately The potential difference developed across the membrane is in fact directly proportional to the Logarithm of the ionic concentration in the external solution

25. The relationship between the ionic concentration (activity) and the electrode potential is given by the Nernst equation: E = E 0  + (2.303RT/ nF ) x Log(A) Where E = the total potential (in mV) developed between the sensing and reference electrodes. E  = is a constant which is characteristic of the particular ISE/reference pair.(It is the sum of all the liquid junction potentials in the electrochemical cell) 2.303 = the conversion factor from natural to base10 logarithm. R = the Gas Constant (8.314 joules/degree/mole). T = the Absolute Temperature. n = the charge on the ion (with sign). F = the Faraday Constant (96,500 coulombs). Log(A) = the logarithm of the activity of the measured ion.  

26.   Differences Between pH and Other Ion-Selective Electrodes In contrast to the pH membrane, other ion-selective membranes are not entirely ion-specific and can permit the passage of some of the other ions which may be present in the test solution, thus causing the problem of ionic interference The influence of the presence of interfering species in a sample solution on the measured potential difference is taken into consideration in the Nikolski-Eisenman equation   z   charge including the sign a  the activity i  the ion of interest j  the interfering ions k ij  is the selectivity coefficient The smaller the selectivity coefficient, the less is the interference by  j .

27. The calculation of ionic concentration is far more dependent on a precise measurement of the potential difference than is the pH, because the pH depends on the order of magnitude of the concentration rather than the precise value For example it would take an error of more than 5 millivolts to cause a change of 0.1 pH units, but only a 1 millivolt error will cause at least a 4% error in the calculated concentration of a mono- valent ion and more than 8% for a di-valent ion.

28. T ypes of membrane A membrane is considered to be any material that separates two solutions It is across this membrane that the charge develops. Several types of sensing electrodes are commercially available They are classified by the nature of the membrane material used to construct the electrode It is this difference in membrane construction that makes an electrode selective for a particular ion.

29. G lass membrane Glass membranes are made from an ion-exchange type of glass (silicate or  chalcogenide ) This type of ISE has good selectivity, but only for several single-charged  cations ; mainly H + , Na + , and Ag + Chalcogenide glass also has selectivity for double-charged metal ions, such as Pb 2+ , and Cd 2+ The glass membrane has excellent chemical durability and can work in very aggressive media A very common example of this type of electrode is the pH glass electrode

30. pH electrode Glass membrane manufactured from SiO2 with negatively charged oxygen atom Inside the glass bulb, a dilute HCl solution and silver wire coated with a layer of silver chloride is kept The electrode is immersed in the solution and pH is measured.

31. Crystal-Membrane Electrodes M ade from mono- or polycrystallites of a single substance They have good selectivity, because only ions which can introduce themselves into the crystal structure can interfere with the electrode response Selectivity of crystalline membranes can be for both  cation  and anion of the membrane-forming substance An example is the fluoride selective electrode based on L anthanum fluoride  crystals

32. T he membrane consists of a single lanthanum fluoride crystal which has been doped with europium fluoride to reduce the bulk resistivity of the crystal It is 100% selective for F -  ions and is only interfered with by OH - which reacts with the lanthanum to form lanthanum hydroxide, with the consequent release of extra F -  ions This interference can be eliminated by adding a pH buffer to the samples to keep the pH in the range 4 to 8 and hence ensure a low OH -  concentration in the solutions.

33. Gas-sensing electrodes A re available for the measurement of dissolved gas such as ammonia, carbon dioxide, nitrogen oxide, and sulfur dioxide These electrodes have a gas permeable membrane and an internal buffer solution Gas molecules diffuse across the membrane and react with a buffer solution, changing the pH of the buffer The change is detected by a combination pH sensor within the housing Due to their construction, gas sensing electrodes do not require an external reference electrode.

34. Ion-exchange resin or polymer membrane electrode A re based on special organic polymer membranes which contain a specific ion-exchange substance (resin) A re employed for monitoring pH and for measuring electrolytes) including K+ , Na+, CI-, Ca2+ , Li+, Mg‘+ They are the predominant class of potentiometric electrodes used in modern clinical analysis instruments The mechanism of response of these ISEs falls into three categories: charged, dissociated ion-exchanger charged associated carrier the neutral ion carrier ( ionophore )

35. An early charged associated ion-exchanger type ISE for Ca'+ was developed and commercialized for clinical application in the 1960s This electrode was based on the Ca2+-selective ion exchange/ complexation properties of 2-ethylhexyl phosphoric acid dissolved in dioctyl phenyl phosphonate (charged associated carrier) A porous membrane was impregnated -with this cocktail and mounted at the end of an electrode body This type sensor was referred to as the «liquid membrane" ISE

36. Later a method was devised where these ingredients could be cast into a plasticized poly(vinyl chloride) (PVC) membrane, that was more rugged and convenient to use than its wet liquid predecessor. This same approach is still used today to formulate PVC-based ISEs for clinical use." A major breakthrough in the development and routine application of PVC type ISEs was the discovery by Simon and co-workers that the neutral antibiotic valinomycin could be incorporated into organic liquid membranes (and later plasticized PVC membranes), resulting in a sensor with high selectivity for K+ over Na+

37. The membrane is usually in the form of a thin disc of PVC impregnated with the macrocyclic antibiotic valinomycin This compound has a hexagonal ring structure with an internal cavity which is almost exactly the same size as the diameter of the K +   io n. Thus it can form complexes with this ion and preferentially conducts it across the membrane Unfortunately it is not 100% selective and can also conduct small numbers of sodium and ammonium ions. Thus these can cause errors in the potassium determination if they are present in high concentrations

38. Enzyme electrodes Enzyme electrodes definitely are not true  ion -selective electrodes but usually are considered within the ion-specific electrode topic Such an electrode has a "double reaction" mechanism - an enzyme reacts with a specific substance, and the product of this reaction (usually H +  or OH - ) is detected by a true ion-selective electrode, such as a pH-selective electrodes All these reactions occur inside a special membrane which covers the true ion-selective electrode, which is why enzyme electrodes sometimes are considered as ion-selective An example is glucose selective electrodes.

39. Sources of Error Diffusion  - differences in the rates of diffusion of ions based on size can lead to some error. In the example of Sodium Iodide, sodium diffuses across the junction at a given rate. Iodide moves much slower due to its larger size. This difference creates an additional potential resulting in error To compensate for this type of error it is important that a positive flow of filling solution move through the junction and that the junction not become clogged or fouled. Sample Ionic Strength - the total ionic strength of a sample affects the activity coefficient and that it is important that this factor stay constant In order accomplish this, the addition of an ionic strength adjuster is used. This adjustment is large, compared to the ionic strength of the sample, such that variation between samples becomes small and the potential for error is reduced.

40. Temperature  - It is important that temperature be controlled as variation in this parameter can lead to significant measurement errors. A single degree (C) change in sample temperature can lead to measurement errors greater than 4%. pH  - Some samples may require conversion of the analyte to one form by adjusting the pH of the solution ( e.g . ammonia). Failure to adjust the pH in these instances can lead to significant measurement errors. Interferences  - The background matrix can effect the accuracy of measurements taken using ISE's T here may interference from other , undesired, ions ; A ll are sensitive to other ions having similar physical properties, to an extent which depends on the degree of similarity Covington points out that some interferences may be eliminated by reacting the interfering ions prior to analysis.

41. Advantages of (ISE) Technique R elatively inexpensive (as compaired to Atomic Adsorption Spectrophotometry or Ion Chromatography ), simple , have an extremely wide range of applications and wide concentration range. It is a real-time measurement, which means it can monitor the change of activity of ion with time. As it measure the activity, instead of concentration, it is particularly useful in biological/medical application. It can determine both positively and negatively charged ions.

42. N ot interfere d by color or turbidity in the sample ISEs can be used in aqueous solutions over a wide temperature range. Crystal membranes can operate in the range 0 C to 80 C and plastic membranes from 0 C to 50 C. Ideal for monitoring environmental pollution or water quality etc. - where the operator simply wants to be sure that a certain ion is below a particular threshold value, or where only the order of magnitude may be required

43. O ther applications of ISE Pollution Monitoring: C yanide , F lourine , S ulphur , C h l orine , N itrate Agriculture: NO 3 , Cl , NH 4 , K, Ca, I, CN in soils, plant material, fertilisers and feedstuffs. Food Processing: NO 3 , NO 2  in meat preservatives. Salt content of meat, fish, dairy products, fruit juices, brewing solutions. F in drinking water and other drinks. Ca in dairy products and beer. K in fruit juices and wine making. Corrosive effect of NO 3  in canned foods. Detergent Manufacture: Ca, Ba , F for studying effects on water quality. Paper Manufacture: S and Cl in pulping and recovery-cycle liquors. Explosives: F, Cl , NO 3  in explosive materials and combustion products. Electroplating: F and Cl in etching baths; S in anodising baths. Biomedical Laboratories: Ca, K, Cl in body fluids (blood, plasma, serum, sweat). F in skeletal and dental studies. Education and Research

44. Limit of ISE T hough the membrane is ion selective, the truth is there is no such membrane that only permits the passage of one ion, and so a s a result, the measured potential are affected by the passage of the “unwanted” ions Also, because of its dependence on ion selective membrane, one ISE is only suitable for one ion and this may be inconvenient sometimes Another problem worth noticing is that ion selective measures the concentration of ions in equilibrium at the surface of the membrane surface This does matter much if the solution is dilute but at higher concentrations, the inter-ionic interactions between the ions in the solution tend to decrease the mobility of ions and thus the concentration near the membrane would be lower than that in the bulk. To better analyze the results of ISE, we have to be aware of these inherent limitations of it.

45. Conditioning Rinsing It is necessary to rinse the ISE between measurements to insure accurate readings Use a steady stream of deionized or distilled water Take care not to rub the electrode with a cloth to dry the probe It is usually best to "shake off" any excess water Take care not to hit the probe against anything while shaking the electrode.

46. The ISE needs to remain moist at all times even when not in use. Electrodes with a polymer membrane must not come in contact with organic solvents Do not store in water for extended periods—dry before storing  Store Combined Ion Selective Electrodes in dilute ISA (ionic strength adjuster) solution—for long term storage, remove reference solution and store dry Clean crystal membranes with a mild abrasive, then rinse with water. Toothpaste is an excellent cleaning agent, for fluoride electrodes use a fluoride toothpaste

47. THANK YOU

48. R eferences Tietz, Clinical chemistry and molecular diagnostic Harvey, Modern analytical Chemistry C linical chemistry, Techniques, Principles and Correlations Beginners Guide to ISE Measurement , Chris C Rundle