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

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Ion Selective Electrode Electrode Body Ion Sensitive Area Electrical Connection

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Sensing Electrode Reference Electrode Current Flow Meter Electrochemical Measuring System

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Combination Probe Sensing Element Reference Element

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Sensing Electrode Reference Electrode Current Flow Meter Electrochemical Measuring System

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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ATC (ExceptForpH)

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

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

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

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

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NERNST EQUATION E 1 = E o + s Log C 1 E 2 = E o + s Log C 2 If C 2 = 10 x C 1 E 2 = E o + s Log 10 C 1 Then:

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NERNST EQUATION E 1 = E o + s Log C 1 E 2 = E o + s Log C 2 If C 2 = 10 x C 1 E 2 - E 1 The Slope of an Electrode is the millivolt Change that is seen for a Ten Times Change in Concentration. = s

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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 Determine Difference = Slope

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

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

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

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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 Record Slope for QA/QC Daily (or at least every two weeks) ( In mv or %)

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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 Usually ± 10 % of Ideal Check Manufacturer for Acceptable Slope Range

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

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

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