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MLAB 2401: C LINICAL C HEMISTRY K ERI B ROPHY -M ARTINEZ Designs in Instrumentation Electrochemistry.

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Presentation on theme: "MLAB 2401: C LINICAL C HEMISTRY K ERI B ROPHY -M ARTINEZ Designs in Instrumentation Electrochemistry."— Presentation transcript:

1 MLAB 2401: C LINICAL C HEMISTRY K ERI B ROPHY -M ARTINEZ Designs in Instrumentation Electrochemistry

2 E LECTROCHEMISTRY Basic principle Electrodes are used to selectively measure particular ions Instruments utilizing electrodes measure the potential difference (charge) that builds up at an interface when two different concentrations of the same ion are in contact with each other The electrodes are sensitive to this potential difference 2

3 E LECTROCHEMISTRY : T ERMS Oxidation loss of electrons (leaves electrode positive) Anode the electrode where oxidation occurs (where electrons are lost) Reduction gain of electrons (electrode becomes negative) Cathode the electrode where reduction occurs (where the electron gain occurs) Polarography the measurement of the gain or loss of electrons in a chemical reaction by detecting a change in the potential 3

4 E LECTROCHEMISTRY Galvanic cell – two half cells used to demonstrate the flow of electrons during the processes of oxidation and reduction 4 reduction oxidation

5 E LECTROCHEMISTRY Electrolytic cell differs from the galvanic cell in that The cathode and anode are immersed in the same solution There is an electromotive force used to drive the chemical reaction 5

6 E LECTROCHEMISTRY Testing Procedures involving electrochemical principles Potentiometry pH meters Ion-selective electrodes Blood gas analysis Chloridometers

7 P OTENTIOMETRY Measurement of potential or voltage between two electrodes in a solution Nerst equation Predicts amount of change in millivolts due to a temperature change As the temperature rises, so does hydrogen ion activity

8 P OTENTIOMETRY System Components Reference electrode Indicator or measuring electrode Liquid junction Readout device

9 P OTENTIOMETRY Reference electrodes The electrode against which the potential charge created in the indicator electrode is measured. Consists of a metal and its salt in contact with a solution containing the same anion. These are half-cell potentials extremely stable and easy to reproduce 9

10 P OTENTIOMETRY Reference Electrodes Calomel electrode composed of mercury/mercurous chloride is dependable but large, bulky, and affected by temperature Silver/silver chloride reference electrodes are more compact and handle temperature fluctuations better - overall better & faster 10

11 P OTENTIOMETRY Indicator (measuring) electrodes The electrode, whose half-cell potential responds to changes in the activity or concentration of the substance in the solution that is being measured. The response of the electrode to the substance (species) being measured and not to other substances present is defined as the selectivity of the electrode. The selectivity of an electrode for a particular species is dependent on the type of membrane that separates the electrode and the solution containing the species to be measured. 11

12 E LECTROCHEMISTRY Liquid junction – also known as a salt bridge are required to complete the circuit (between the reference and without contaminating anything. KCl (potassium chloride) is the most common 12

13 P OTENTIOMETRY Ion Selective Electrodes Also called Ion Specific Electrodes Indicator electrode Modified electrochemical half cells Advantages No reagent preparation No standard curve preparation Cost effective Fast analysis times Very sensitive and selective for an analyte Easy to maintain 13

14 I ON S ELECTIVE E LECTRODES Different membranes that are selectively sensitive to the electrical effects of different electrolytes can be placed over the tips of the measuring electrodes, making them susceptible only to the effects of these particular electrolytes Differences in the measured potential of the circuit can be calibrated with known concentrations of electrolytes Because ISE measures electrical potential, it is an example of potentiometry 14

15 I ON S ELECTIVE E LECTRODES Common substances measured by ISE Sodium (Na) – membrane composed of selective glass Potassium (K) valinomycin ( example of liquid layer membrane) Chloride (Cl) – crystal membrane Ionized Calcium (Ca) Hydrogen ions (H) glass membrane different composition than Na’s 15

16 I ON S ELECTIVE E LECTRODES ( ISE ) Consist of two main components to an electrochemical half cell Reference Electrode ( generates a known, constant voltage ) Measuring Electrode ( generates a variable voltage ) These components must be connected together, but can not contaminate each other. The joining is done using a “salt bridge” 16

17 P OTENTIOMETER M EASUREMENT pH Electrode based on the measurement of a potential (voltage) difference between two electrodes immersed in a solution under the condition of zero current. Each cell has a half-cell reaction The potential difference between the two electrodes is usually measured using a pH/milli-volt meter. 17

18 P H ELECTRODES 18 Combination electrode Both the indicator & reference electrodes are housed in the same assembly. Internal reference Ag/AgCl type bathed in KCl The measuring or indicator electrode has a “glass membrane”

19 B LOOD G AS E LECTRODES 19 pCO2 Electrode Measurement of PCO2 in routine blood gases is done using the Severinghaus electrode A modified pH electrode with a CO2 permeable membrane covering the glass membrane surface A bicarbonate buffer separates the membranes Change in pH is proportional to the concentration of dissolved CO2 in the blood

20 B LOOD GAS ELECTRODES pO2 Electrode Clark PO2 (polarographic) electrode. Measurement of PO2 is done using a Clark electrode which works on the principles of polarography principle of polarography- involves measurement of gain or loss of electrons in a chemical reaction, by detecting a change in potential. 20

21 P O2 E LECTRODE The cathode is a platinum wire and the anode is a silver wire in AgCl both electrodes are in contact with electrolyte solution Oxygen in the sample diffuses through the semipermeable membrane and is reduced (gains electrons) at the cathode. The flow of electrons stops when no more oxygen is available. Current generated in the system is proportional to the amount of oxygen 21

22 E LECTROCHEMISTRY - C OULOMETRIC / AMPEROMETRIC CHLORIDOMETERS Coulometry a titration in which the titrant is electrochemically generated Silver (Ag) is the most common titrant. Amperometry Measurement of current flow produced by an oxidation- reduction reaction 22

23 C OULOMETRIC / AMPEROMETRIC CHLORIDOMETERS The basic components consist of two pair of electrodes immersed in an electrolyte diluent (containing acetic and nitric acids) 23

24 C OULOMETRIC / AMPEROMETRIC CHLORIDOMETERS Procedure Sample added to diluent Electrodes releases silver ions into the diluent Ions combine with chloride to form a precipitate When chloride is gone, excess silver causes a change in current flow Flow change signals the end of titration

25 R EFERENCES Bishop, M., Fody, E., & Schoeff, l. (2010). Clinical Chemistry: Techniques, principles, Correlations. Baltimore: Wolters Kluwer Lippincott Williams & Wilkins. Sunheimer, R., & Graves, L. (2010). Clinical Laboratory Chemistry. Upper Saddle River: Pearson. 25

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