Unit II- Student Notes.docx

UNIT II Electrochemistry- electrons, mainly for blood gas analysis The activity of a wide variety of ions plays essential roles in our physiological wellbeing. Thus, measurement of specific ions is an important diagnostic procedure Analysis is accomplished though the measurement of a potential Pot...

UNIT II Electrochemistry- electrons, mainly for blood gas analysis The activity of a wide variety of ions plays essential roles in our physiological wellbeing. Thus, measurement of specific ions is an important diagnostic procedure Analysis is accomplished though the measurement of a potential Potentials(two different metals that are reacting potential is the force it takes to move those electrons, the voltage) can and do develop as a result of chemical reactions This is known as electrochemical activity Electrochemistry applies to the movement of electrons from one compound to another The donor of electrons is oxidized The recipient of electrons is reduced Leo the lion says ger The direction of flow of electrons from one compound to another is determined by the electrochemical potential Copper is more electronegative than Zinc When the two metals are connected electrically, current (electrons) will flow spontaneously from Zinc to Copper—zinc wants to give up electrons and copper wants to gain Zinc is oxidized; Copper is reduced Zinc is the anode; Copper is the cathode Zinc and copper are just one example of a redox couple, but the principle of how the two electrodes react is the basis for electrochemistry and potentiometry in the Chemistry lab Every redox couple has its own electrode potential, which is measured against a standard reference electrode in the lab Potentials can also be calculated using the Nernst equation Connected by a salt bridge, sodium chloride to help keep everything at equilibrium As we lose elctrons it will slowly become positive, the chloride will helo make it equal again Zn0 → Zn2+ + 2e- Cu2+ + 2e- → Cu0 The Nernst Equation Where E = Potential at temperature T E0 = Standard electrode potential (25ºC, 1.0M) R = Ideal gas constant F = Faraday’s constant n = number of electrons transferred don’t need to know just need to know that the Nernst equation deals with electrochemistry Potentiometry Is the measurement of an electrical potential difference between two electrodes (half-cells) in an electrochemical cell The method is based on the measurement under zero-current conditions The difference in voltage between the two electrodes in measured on a pH or voltage meter Applications include measurement of pH, pCO2, electrolytes (Na+, K+, Cl- etc.) in whole blood, serum, plasma, and urine One electrode is called the indicator electrode; the other is the reference electrode The reference electrode is a half-cell that is used as a fixed reference (known voltage) for the cell potential measurements The indicator electrode is the main component of potentiometric techniques that measures the desired analyte(s) The two electrodes are connected by an electrolyte solution e.g. NaCl which can conduct an electric current because the charged ions are mobile in solution We always have a measuring and reference electrode attached by a wire. We measure voltage, we can use for ph electrolytes Indicator electrode It’s important that the indicator electrode be able to respond selectively to an analyte species The most commonly used in clinical chemistry is the ion-selective electrode (ISE) The use of ISEs is based on the measurement of a potential from a select cation or anion that develops across a selective membrane The response is based on an interaction between the membrane and the analyte being measured that alters the potential across the membrane The potential produced is proportional to the concentration of the ion in question Types of ISE Glass- only for measuring PH specialized only for hydrogen Various combinations of SiO2 with metal oxides Polymer membrane Mechanism of response falls into 3 categories: Charged, associated ion-exchanger (Ca2+)- testing calcium or ionized calcium Charged, dissociated ion-exchanger (Cl-)- based on lipophilic so how it reacts to fats Neutral ion carrier a.k.a. ionophore (K+) These types of membranes are cast into plasticized poly (vinyl chloride) (PVC), regardless of which ion specific material is used Other techniques Amperometry- measuring current, the movement of electrons Potentiometric methods measure electrochemical potential, whereas amperometric methods measure the flow of electrical current Potential (or voltage) is the driving force behind current flow Current is the amount of electrical flow (electrons) produced in response to an electrical potential Ex. pO2 electrode the little p stands for partial pressure The pO2 electrode a.k.a. Clark electrode(know both names) is an electrochemical cell with a small platinum cathode and a silver/silver chloride anode in a phosphate buffer with potassium chloride. It uses a polypropylene membrane to prevent proteins and other oxidants from reaching the cathode surface. How does it work? Oxygen in the sample diffuses across the polypropylene membrane. The oxygen combines with hydrogen ions in the sample (4 electrons) and the end result is ionization to produce hydroxyl ions. These ions react with the silver/silver chloride anode, which results in the reduction of the silver and production of an electron current. The current flows to the platinum cathode producing a result on a meter. The current flow is proportional to the rate of diffusion, which is proportional to pO2 in the sample. Per oxygen molecule we will get 4 hydroxyl ions, the hydroxyl is what react with the anode (silver silver chloride) when the oh reacts with the anode they will then give of the electron and it will pass over the platinum wire that is attached to a meter, how many electrons that flow depends on how many oxygen molecules were in our patient sample larger current means larger concentration of oxygen in patient sample Reaction The amount of current (e-) is proportional to the concentration of O2 Coulometry Based on Amperometry An electrochemical technique used to measure the amount of charge (in coulombs) passing between 2 electrodes. The amount of charge is proportional to the amount produced or consumed by the re-dox process at the electrodes. Waiting for end of redox reaction to then take reading At the endpoint is when we take our measurement Example: Chloride determination in blood, plasma, or serum