Transcript pH measurements - Cornell University

pH probe
Buffered KCl solution
4 M KCl solution
Reference electrode
Silver/silver chloride electrode
Sensing electrode
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Porous reference junction
Glass bulb (insulator) with
anionic sites
Glass membrane (Insulator)
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Reference Voltage +
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Call it+ zero!
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 pH 4 solution (high H+)
Voltage across glass membrane
 Solution voltage is 180 mV higher than reference!
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Nernst Equation: Voltage = f(pH)
E E 0
E E
0
RT FH
 lnG
nF G
HH
I
JJ
K
RT ln(10) FH

logG
G
nF
HH
E 0 at H
0
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éH + ù Reference (known) [H+]
ë
û
0
0
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107
RT ln(10)
E 
pH 7
nF
F
Faraday constant
R
Gas constant
0
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0
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0
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E0
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é
H
Voltage at ë ùû
I
JJ
K
An easy reference condition!
pH 
log H 
Nernst Equation
J
T ln(10)
mol K
E 
Coulombs

1 mol e 96500
mol e 
8.314
F
G
H
E 
0.000198T pH 7
E 
S pH 7
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5040.8 E
7 pH
(T 27315
. )
pH 7
IJ
K
J
R 8.314
mol K
Coulombs
F 96500
mol e 
n 1 mol e 
For T in Celsius
Voltage vs pH
E (mV)
E 
0.000198T pH 7
250
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100
50
0
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-100
-150
-200
-250
E at 0 C
E at 25 C
E at 35 C
E at 100 C
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pH
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10
Slope vs. Temperature
mV/pH
E 
0.000198T pH 7
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55
50
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100
Temperature (C)
 Temperature
compensation is important!
pH Measurements
The voltage measurement requires a very high
impedance circuit (high resistance) because a pH
probe can’t produce much current
 The porous frit provides electrical contact between
the solution and the electrolyte
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Must be in contact with the sample solution
 Probe won’t work well if frit is clogged (fouled)
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Gentle stirring keeps the solution next to the glass
bulb from being depleted of protons
Difficult Measurements?
 pH
is difficult to measure in poorly buffered
solutions
 Distilled
water
 Rain
 Between
pKs of dilute buffers
pH Calibration
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It would be possible to make a pH measurement
without any calibration
Based on theoretical values
 This is how the Accumet meter recognizes buffers!
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Calibration accounts for non-ideal probe behavior
(fouling)
 It is important that buffers be used covering the
range of pH measurements
 Typically a piecewise linear fit is used
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Buffer pH vs Temperature
 Buffers
change pH slightly as the
temperature changes.
 Accumet meters automatically compensate
for this when calibrating