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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. - PowerPoint PPT PresentationTRANSCRIPT
ISE
Ion Selective Electrodes
Prepared ByMichigan Department of Environmental Quality
Operator Training and Certification Unit
Ion Selective ElectrodeIon Selective Electrode
ElectrodeBody
IonSensitive
Area
ElectricalConnection
SensingElectrode
ReferenceElectrode
Current Flow
Meter
ElectrochemicalMeasuring System
Combination ProbeCombination Probe
SensingElement
ReferenceElement
SensingElectrode
ReferenceElectrode
Current Flow
Meter
ElectrochemicalMeasuring System
NERNST EQUATIONNERNST EQUATION
E = Eo + 2.3 T Log C n
E = Measured VoltageEo = Reference ConstantT = Temperaturen = Charge on Ion = Ionic StrengthC = Concentration
NERNST EQUATIONNERNST EQUATION
E = Eo + 2.3 T Log C n
E = Measured VoltageEo = Reference ConstantT = Temperaturen = Charge on Ion = Ionic StrengthC = Concentration
NERNST EQUATIONNERNST EQUATION
E = Eo + 2.3 T Log C n
E = Measured VoltageEo = Reference ConstantT = Temperaturen = Charge on Ion = Ionic StrengthC = Concentration
What the Meter Tells Us
What We Want To Know
} Meter Reading Meter Reading Also Affected Also Affected By All ThisBy All This
NERNST EQUATIONNERNST EQUATION
E = Eo + 2.3 T Log C n
E = Measured VoltageEo = Reference ConstantT = Temperaturen = Charge on Ion = Ionic StrengthC = Concentration
What the Meter Tells Us
What We Want To Know
} Must Be Must Be Accounted For Accounted For To Get True To Get True ConcentrationConcentration
NERNST EQUATIONNERNST EQUATION
E = Eo + 2.3 T Log C n
E = Measured VoltageEo = Reference ConstantT = Temperaturen = Charge on Ion = Ionic StrengthC = Concentration
What the Meter Tells Us
What We Want To Know
Must Maintain Must Maintain Reference Reference ElectrodeElectrode
NERNST EQUATIONNERNST EQUATION
E = Eo + 2.3 T Log C n
E = Measured VoltageEo = Reference ConstantT = Temperaturen = Charge on Ion = Ionic StrengthC = Concentration
What the Meter Tells Us
What We Want To Know
Will Be Will Be Constant for Constant for Specific Ion,Specific Ion,
Whole Number, Whole Number, 1, 2, 3, etc.1, 2, 3, etc.
+ or -+ or -
NERNST EQUATIONNERNST EQUATION
E = Eo + 2.3 T Log C n
E = Measured VoltageEo = Reference ConstantT = Temperaturen = Charge on Ion = Ionic StrengthC = Concentration
What the Meter Tells Us
What We Want To Know
Must Be Must Be Controlled By Controlled By
Making It A Making It A Very High ValueVery High Value
NERNST EQUATIONNERNST EQUATION
E = Eo + 2.3 T Log C n
E = Measured VoltageEo = Reference ConstantT = Temperaturen = Charge on Ion = Ionic StrengthC = Concentration
What the Meter Tells Us
What We Want To Know
Add IonsAdd IonsISABISABOHOH--NaNa++
NERNST EQUATIONNERNST EQUATION
E = Eo + 2.3 T Log C n
E = Measured VoltageEo = Reference ConstantT = Temperaturen = Charge on Ion = Ionic StrengthC = Concentration
What the Meter Tells Us
What We Want To Know
FollowFollowThe The
Directions!Directions!
NERNST EQUATIONNERNST EQUATION
E = Eo + 2.3 T Log C n
E = Measured VoltageEo = Reference ConstantT = Temperaturen = Charge on Ion = Ionic StrengthC = Concentration
What the Meter Tells Us
What We Want To Know
Must Be Must Be ControlledControlled
NERNST EQUATIONNERNST EQUATION
E = Eo + 2.3 T Log C n
E = Measured VoltageEo = Reference ConstantT = Temperaturen = Charge on Ion = Ionic StrengthC = Concentration
NERNST EQUATIONNERNST EQUATION
E = Eo + 2.3 T Log C n
2.3 Tn = s = slope
Slope is Direction of “Curve”When Plotting E vs. C
y = mx + by = mx + b
Effect of TemperatureE
lec
tro
de
Po
ten
tia
l (m
v)
Concentration (mg/L)
0o C50o C
100o C
s is the Direction (Angle) of the Line
Temperature Affects the Slope
Effect of TemperatureE
lec
tro
de
Po
ten
tia
l (m
v)
Concentration (mg/L)
OO
O
0o C50o C
100o C
One Concentration will Result in Different Readings Dependent on Temperature
Effect of TemperatureE
lec
tro
de
Po
ten
tia
l (m
v)
Concentration (mg/L)O O
Calibration Temperature
Sample Temperature
XX
When the Sample Temperature is Different from the Calibration Temperature, an Incorrect Sample
Concentration will be Obtained
Effect of TemperatureE
lec
tro
de
Po
ten
tia
l (m
v)
Concentration (mg/L)
0o C50o C
100o C
IsopotentialPoint
Effect of TemperatureE
lec
tro
de
Po
ten
tia
l (m
v)
Concentration (mg/L)
0o C50o C
100o C
IsopotentialPoint
At the Isopotential Point, One Concentration will Result in the Same Reading Independent of Temperature
Effect of TemperatureE
lec
tro
de
Po
ten
tia
l (m
v)
Concentration (mg/L)
0o C50o C
100o C
}}NormalNormal
Range of AnalysisRange of Analysis
The Range of Analysis is Far Fromthe Isopotential Point for Most Samples
Effect of TemperatureE
lec
tro
de
Po
ten
tia
l (m
v)
Concentration (mg/L)
0o C50o C
100o C
}}NormalNormal
Range of AnalysisRange of Analysis
Temperature Must be Accounted For in All Analyses
Automatic Temperature CompensationAutomatic Temperature CompensationATCATC
Adjusts Slope in Relation to TemperatureAdjusts Slope in Relation to Temperature
Does Not Work!Does Not Work!
Sounds Good!Sounds Good!
Is Accurate OnlyIs Accurate Only When AnalysisWhen Analysisis Close to Isopotential Pointis Close to Isopotential Point
Only For pHOnly For pH
Adjusts Slope in Relation to TemperatureAdjusts Slope in Relation to Temperature
Automatic Temperature CompensationAutomatic Temperature CompensationATCATC
ATCATC
Automatic Temperature CompensationAutomatic Temperature Compensation
(Except(ExceptForForpH)pH)
Effect of TemperatureE
lec
tro
de
Po
ten
tia
l (m
v)
Concentration (mg/L)
Calibrate With Calibrate With Standards and Standards and
Samples At TheSamples At TheSAME TemperatureSAME Temperature
(Usually Room (Usually Room Temperature)Temperature)
Calibration ControlE
lect
rod
e P
ote
nti
al (
mv)
Concentration (mg/L)
. StandardSolution
The Calibration Control Adjusts the “Curve” to the Point of Standard Used
Slope ControlE
lect
rod
e P
ote
nti
al (
mv)
Concentration (mg/L)
. Standard
The Slope Control Rotates the “Curve”Around the Point Of the First Standard
Slope ControlE
lect
rod
e P
ote
nti
al (
mv)
Concentration (mg/L)
. Standard
.Second
Standard
The Slope Control Adjust the “Curve” through the Point of the Second Standard Used
NERNST EQUATIONNERNST EQUATION
E1 = Eo + s Log C1
E2 = Eo + s Log C2
If C2 = 10 x C1
E2 = Eo + s Log 10C1
Then:
NERNST EQUATIONNERNST EQUATION
E1 = Eo + s Log C1
E2 = Eo + s Log C2
If C2 = 10 x C1
E2 - E1
The Slope of an Electrode is the millivolt Change that is seen
for a Ten Times Change in Concentration.
= s
ISEISE
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
ISEISE
Checking Electrode Slope
E = Eo + 2.3 T Log C nE
lect
rod
e P
ote
nti
al (
mv)
Concentration (mg/L)
ISEISE
Checking Electrode Slope
The Slope Value Determined is Affected By:
Charge on the Ion of Interest (n)
Temperature (T)
E = Eo + 2.3 T Log C n
Positive or Negative1, 2, etc.
ISEISE
Checking Electrode Slope
The Slope Value Determined is Affected By:
Charge on the Ion of Interest (n)
Temperature (T)
Quality of the Standards
E = Eo + 2.3 T Log C n
Efficiency of the Electrode
ISEISE
Checking Electrode Slope
The Slope of the Electrode Will Change
{Loss of Efficiency}
The Slope Should be Checked Regularlyto Assure Reliable Results
Most Meters Give Slope When Calibrating
Record Slope for QA/QC Daily(or at least every two weeks)
( In mv or %)
ISEISE
Checking Electrode Slope
Ideal Slope Depends on Ion of interest (n)
Charge on the Ion of Interest (+ or -)Approximately 59 mv for n=1or 29 mv for n=2
Usually ± 10 % of Ideal
Check Manufacturer for Acceptable Slope Range
ISEISE
Checking Electrode SlopeIf 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 Regularlyto Assure Reliable Results
ISE
Ion Selective Electrodes
Prepared ByMichigan Department of Environmental Quality
Operator Training and Certification Unit