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

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Presentation on theme: "ISE Ion Selective Electrodes Prepared By Michigan Department of Environmental Quality Operator Training and Certification Unit."— Presentation transcript:

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

2 Ion Selective Electrode Electrode Body Ion Sensitive Area Electrical Connection

3 Sensing Electrode Reference Electrode Current Flow Meter Electrochemical Measuring System

4 Combination Probe Sensing Element Reference Element

5 Sensing Electrode Reference Electrode Current Flow Meter Electrochemical Measuring System

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

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

8 NERNST EQUATION E = E o 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

9 NERNST EQUATION E = E o 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

10 NERNST EQUATION E = E o 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

11 NERNST EQUATION E = E o 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 -

12 NERNST EQUATION E = E o 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

13 NERNST EQUATION E = E o 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 +

14 NERNST EQUATION E = E o 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!

15 NERNST EQUATION E = E o 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

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

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

18 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

19 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

20 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

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

22 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

23 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

24 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

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

26 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

27 ATC (ExceptForpH)

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

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

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

31 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

32 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:

33 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

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 Determine Difference = Slope

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

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

37 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 T Log  C n 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 Record Slope for QA/QC Daily (or at least every two weeks) ( In mv or %)

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

40 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

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


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