Electrochemistry

• Electrochemistry

  Measurement of the current or voltage.
• Electrochemistry

  Many types of electrochemical analyses are used in the clinical laboratory, including potentiometry, amperometry, coulometry, and polarography.
• Electrochemical cells consists of two half-cells and a salt bridge, which can be a piece of filter paper saturated.
• Two types of Electrochemical cells are Anode and Cathode.
• Galvanic or Voltaic

  cells store electrical energy
• Electrolytic cells

  requires an external source of electrical for operation.
• The two basic electrochemical cells involved in these analyses are Galvanic and Electrolytic cells.
• In a galvanic cell, as the electrodes are connected, there is spontaneously flow of electrons from the electrode with the lower electron affinity (oxidation; e.g., silver).
• These electrons pass through the external meter to the cathode (reduction), where OH- ions are liberated. This reaction continues until one of the chemical components is depleted, at which point, the cell is "dead" and cannot produce electrical energy to the external meter.
• Current may be forced to flow through the dead cell only by applying an external electromotive force E. This is called an electrolytic cell.
• In short, a galvanic cell can be built from an electrolytic cell. When the external E is turned off, accumulated products at the electrodes will spontaneously produce current in the opposite direction of the electrolytic cell.
• Oxidation - is the loss of electrons by a species

  Reduction - is the gain of electrons.
• Redox reaction - are reactions that transfers electrons from one reactant to another.
• Reducing - agent is an electron donor.

  Oxidizing - agent is an electron acceptor.
• Oxidation-Reduction Titrants - These includes all reactions involving change in oxidation number or transfer of electrons among the reacting substances.
• The standard solutions are either Oxidizing or Reducing agents.
• Reactant Concentration - titration curves for oxidation/reduction reactions are usually independent of analyte and reagent concentrations.
• Completeness of the Reaction - the change in the equivalence-point region of an oxidation/reduct titration becomes larger as the reaction becomes more complete.
• General Redox - indicators are substances that change color upon being oxidized or reduced.
• Starch/Iodine - solution containing a little triiodide or iodide ion can also function as a true redox indicator.
• the best-known specific indicator is starch, which forms a dark-blue complex with triiodide ion. This complex signals the end point in titrations in which iodine is either produced or consumed.
• The analyte in an oxidation /reduction titration must be in a single oxidation state at the outset.

  when an iron-containing sample is dissolved usually contains a mixture of iron(II)and iron(III) ions.
• We must first treat the sample solution with an Auxiliary reducing agent to convert all the iron to iron (II).

  To be useful as a preoxidant or a prereductant, a reagent must react quantitatively with the analyte. In addition, any reagent excess must be readily removable because the excess reagent usually interferes by reacting with the standard solution.
• Auxiliary reducing agent - number of metals are good reducing agents and have been used for the prereduction of analytes. Included among these are zinc, aluminum, cadmium, lead, nickel, copper, and silver.
• Sodium Bismuthate - is a powerful oxidizing agent; it is capable, for example, of converting manganese(II) quantitatively to permanganate ion.

  The half-reaction for the reduction of sodium bismuthate can be written as

  Aluminum Peroxydisulfate

  Sodium Peroxide and Hydrogen Peroxide
• In an electrochemical cell in which the oxidizing agent and the reducing agent are physically separated from one another.

  Cell potential , is a measure of the tendency of the cell reaction to proceed toward equilibrium.
• Cathode - in an electrochemical cell is the electrode at which a reduction reaction occurs.

  Anode - is the electrode at which an oxidation takes place.
• The Standard Hydrogen Electrode (SHE), has been used a universal reference electrode. It is a typical gas electrode.
• 5 types of electrochemistry

  1. Potentiometry
  2. Polarography
  3. Coulometry
  4. Amperometry
  5. Conductometry
• Potentiometry - Is the measurement of differences in voltage at a constant current.

  The measurement of potential or voltage between two electrodes in solution by a null-balanced technique.

  The unknown voltage introduced into the potentiometer circuit opposes a known reference voltage.
• ^The voltage of the unknown is measured by comparison to determine the voltage required to exactly opposed the flow of current in the test circuit.

  ^The relationship between the measured voltage and the sought - for concentration is shown by the nearst equation.
• ISE: Ion Selective Electrode
• pH Electrode - an ISE universally used in the clinical laboratory

  Parts:

  1. Indicator Electrode
  2. Reference Electrode
  3. Liquid Junction
  4. Readout Meter
• Indicator Electrode - pH electrodes are highly selective for H+; however, other cations in high concentration interfere, the most common of which is sodium.
• reference electrode - commonly used is the calomel electrode.
• Calomel, a paste of predominantly mercurous chloride, is in direct contact with metallic mercury in an electrolyte solution of potassium chloride.
• Ag/AgCl - another common reference electrode. It can be used at high temperatures, up to 275°C, and the AgCl-coated Ag wire makes a more compact electrode than that of mercury.
• Mercury sulfate and Potassium sulfate - to avoid chloride contamination
• Liquid Junction- electrical connection between the indicator and reference electrodes is achieved by allowing a slow flow of electrolyte from the tip of the reference electrode. KCl is a commonly used filling solution because K+ and Cl− have nearly the same mobilities.
• Readout Meter

  Zero potential for the cell indicates that each electrode half-cell is generating the same voltage, assuming there is no liquid junction potential.

  The isopotential is that potential at which a temperature change has no effect on the response of the electrical cell