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ISE. Ion Selective Electrodes. Prepared By Michigan Department of Environmental Quality Operator Training and Certification Unit. Ion Selective Electrode. Electrode Body. Electrical Connection. Ion Sensitive Area. Electrochemical Measuring System. Meter. Reference Electrode.

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ISE

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  1. ISE Ion Selective Electrodes Prepared By Michigan Department of Environmental Quality Operator Training and Certification Unit

  2. Ion Selective Electrode Electrode Body Electrical Connection Ion Sensitive Area

  3. Electrochemical Measuring System Meter Reference Electrode Sensing Electrode Current Flow

  4. Combination Probe Sensing Element Reference Element

  5. Electrochemical Measuring System Meter Reference Electrode Sensing Electrode Current Flow

  6. T Log C E = Eo + 2.3 n NERNST EQUATION E = Measured Voltage Eo = Reference Constant T = Temperature n = Charge on Ion = Ionic Strength C = Concentration

  7. T Log C E = Eo + 2.3 n E = Measured Voltage Eo = Reference Constant T = Temperature n = Charge on Ion = Ionic Strength C = Concentration NERNST EQUATION

  8. T Log C E = Eo + 2.3 n E = Measured Voltage Eo = Reference Constant T = Temperature n = Charge on Ion = Ionic Strength C = Concentration NERNST EQUATION What the Meter Tells Us } Meter Reading Also Affected By All This What We Want To Know

  9. T Log C E = Eo + 2.3 n E = Measured Voltage Eo = Reference Constant T = Temperature n = Charge on Ion = Ionic Strength C = Concentration NERNST EQUATION What the Meter Tells Us } Must Be Accounted For To Get True Concentration What We Want To Know

  10. T Log C E = Eo + 2.3 n E = Measured Voltage Eo = Reference Constant T = Temperature n = Charge on Ion = Ionic Strength C = Concentration NERNST EQUATION What the Meter Tells Us Must Maintain Reference Electrode What We Want To Know

  11. T Log C E = Eo + 2.3 n E = Measured Voltage Eo = Reference Constant T = Temperature n = Charge on Ion = Ionic Strength C = Concentration NERNST EQUATION What the Meter Tells Us Will Be Constant for Specific Ion, Whole Number, 1, 2, 3, etc. + or - What We Want To Know

  12. T Log C E = Eo + 2.3 n E = Measured Voltage Eo = Reference Constant T = Temperature n = Charge on Ion = Ionic Strength C = Concentration NERNST EQUATION What the Meter Tells Us Must Be Controlled By Making It A Very High Value What We Want To Know

  13. T Log C E = Eo + 2.3 n E = Measured Voltage Eo = Reference Constant T = Temperature n = Charge on Ion = Ionic Strength C = Concentration NERNST EQUATION What the Meter Tells Us Add Ions ISAB OH- Na+ What We Want To Know

  14. T Log C E = Eo + 2.3 n E = Measured Voltage Eo = Reference Constant T = Temperature n = Charge on Ion = Ionic Strength C = Concentration NERNST EQUATION What the Meter Tells Us Follow The Directions! What We Want To Know

  15. T Log C E = Eo + 2.3 n E = Measured Voltage Eo = Reference Constant T = Temperature n = Charge on Ion = Ionic Strength C = Concentration NERNST EQUATION What the Meter Tells Us Must Be Controlled What We Want To Know

  16. T Log C E = Eo + 2.3 n E = Measured Voltage Eo = Reference Constant T = Temperature n = Charge on Ion = Ionic Strength C = Concentration NERNST EQUATION

  17. T Log C E = Eo + 2.3 n T 2.3 n NERNST EQUATION y = mx + b = s = slope Slope is Direction of “Curve” When Plotting E vs. C

  18. Effect of Temperature s is the Direction (Angle) of the Line 100o C 50o C 0o C Electrode Potential (mv) Concentration (mg/L) Temperature Affects the Slope

  19. Effect of Temperature 100o C O 50o C O 0o C O Electrode Potential (mv) Concentration (mg/L) One Concentration will Result in Different Readings Dependent on Temperature

  20. Effect of Temperature Sample Temperature X Calibration Temperature Electrode Potential (mv) O O Concentration (mg/L) When the Sample Temperature is Different from the Calibration Temperature, an Incorrect Sample Concentration will be Obtained

  21. Effect of Temperature 100o C 50o C 0o C Electrode Potential (mv) Isopotential Point Concentration (mg/L)

  22. Effect of Temperature 100o C 50o C 0o C Electrode Potential (mv) Isopotential Point Concentration (mg/L) At the Isopotential Point, One Concentration will Result in the Same Reading Independent of Temperature

  23. Effect of Temperature 100o C 50o C 0o C Electrode Potential (mv) Normal Range of Analysis } Concentration (mg/L) The Range of Analysis is Far From the Isopotential Point for Most Samples

  24. Effect of Temperature 100o C 50o C 0o C Electrode Potential (mv) Normal Range of Analysis } Concentration (mg/L) Temperature Must be Accounted For in All Analyses

  25. Automatic Temperature Compensation ATC Adjusts Slope in Relation to Temperature Sounds Good! Does Not Work!

  26. Automatic Temperature Compensation ATC Adjusts Slope in Relation to Temperature Is Accurate OnlyWhen Analysis is Close to Isopotential Point Only For pH

  27. Automatic Temperature Compensation ATC (Except For pH)

  28. Effect of Temperature Calibrate With Standards and Samples At The SAME Temperature (Usually Room Temperature) Electrode Potential (mv) Concentration (mg/L)

  29. Calibration Control . Electrode Potential (mv) Standard Solution Concentration (mg/L) The Calibration Control Adjusts the “Curve” to the Point of Standard Used

  30. Slope Control . Standard Electrode Potential (mv) Concentration (mg/L) The Slope Control Rotates the “Curve” Around the Point Of the First Standard

  31. Slope Control Second Standard . . Standard Electrode Potential (mv) Concentration (mg/L) The Slope Control Adjust the “Curve” through the Point of the Second Standard Used

  32. NERNST EQUATION Log C1 E1 = Eo + s Log C2 E2 = Eo + s Log 10C1 E2 = Eo + s If C2 = 10 x C1 Then:

  33. NERNST EQUATION Log C1 E1 = Eo + s Log C2 E2 = Eo + s If C2 = 10 x C1 E2 - E1 = s The Slope of an Electrode is the millivolt Change that is seen for a Ten Times Change in Concentration.

  34. ISE Checking Electrode Slope Carefully Prepare Two Standards 10 X Concentration Difference Set Meter to Read in millivolts (or Relative mv) Record mv Reading for Each Standard = Slope Determine Difference

  35. T Log C E = Eo + 2.3 n Electrode Potential (mv) Concentration (mg/L) ISE Checking Electrode Slope

  36. T Log C E = Eo + 2.3 n ISE Checking Electrode Slope The Slope Value Determined is Affected By: Temperature (T) Charge on the Ion of Interest (n) Positive or Negative 1, 2, etc.

  37. T Log C E = Eo + 2.3 n ISE Checking Electrode Slope The Slope Value Determined is Affected By: Temperature (T) Charge on the Ion of Interest (n) Quality of the Standards Efficiency of the Electrode

  38. ISE Checking Electrode Slope The Slope of the Electrode Will Change {Loss of Efficiency} The Slope Should be Checked Regularly to Assure Reliable Results Most Meters Give Slope When Calibrating ( In mv or %) Record Slope for QA/QC Daily (or at least every two weeks)

  39. ISE Checking Electrode Slope Ideal Slope Depends on Ion of interest (n) Charge on the Ion of Interest (+ or -) Approximately 59 mv for n=1 or 29 mv for n=2 Check Manufacturer for Acceptable Slope Range Usually ± 10 % of Ideal

  40. ISE Checking Electrode Slope If Determined Slope is Outside of Acceptable Range May Be Due To: Poor Quality of Standards Improper Probe Maintenance Faulty Electrode The Slope Should be Checked Regularly to Assure Reliable Results

  41. ISE Ion Selective Electrodes Prepared By Michigan Department of Environmental Quality Operator Training and Certification Unit

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