Monroe L. Weber-Shirk S chool of Civil and Environmental Engineering pH measurements The probe, from chemistry to voltage to a number + + V.
Post on 21-Dec-2015
218 Views
Preview:
Transcript
Monroe L. Weber-Shirk
School of Civil and
Environmental Engineering
pH measurementspH measurements
The probe, from chemistry to voltage to a number
The probe, from chemistry to voltage to a number
+
+
V
pH probepH probe
++
+
+++
++
+++
++ +
+
++
++
++
+
++
+
++ +
++
++
++
+
+
+++ +
+ ++
+
+ +
+++
+++
+
++
+
++++
++
+++
++
+
+
+++++
++
Silver/silver chloride electrode
Porous reference junction
4 M KCl solution
Glass bulb (insulator) with anionic sites
+
+
V
Sensing electrode
Reference electrode
Buffered KCl solution
++
+
+
+
+
+
+ +
+
+
+
+
+
+
+
++
+
+
+
+
+
++
+
+
+
+
+
+
+
++ +++++
+
+
+
Glass membrane (Insulator)Glass membrane (Insulator)
pH 4 solution (high H+) Voltage across glass membrane Solution voltage is 180 mV higher than reference! We need a way to measure the solution voltage
pH 4 solution (high H+) Voltage across glass membrane Solution voltage is 180 mV higher than reference! We need a way to measure the solution voltage
++++
++++
++++++++++++++++++++++++++++++++++++++++++
++++++++++++++++++++++++++++
++++++++++++++++
++++
++++
Reference VoltageCall it zero!Reference VoltageCall it zero!
+
+ +
+
+
+++
+
++
+
++
++
+
++
++
+
++
++
+
++
++
+
++
++
+
++
++
+
++++
+
++++
+
++
++
+
++
++
+
++
++
+
++
++
+
++
++
+
++++
+
++
++
+
++
++
+
++
++
+
++
++
+
++ ++
+
++
++
+
++
++
+
++
++
+
++
pH MeasurementspH Measurements
The porous frit provides electrical contact between the solution and the electrolyte Must be in contact with the sample solution Probe won’t work well if frit is clogged (fouled)
The voltage measurement requires a very high __________ circuit (high resistance) because a pH probe can’t produce much current
Gentle stirring keeps the solution next to the glass bulb from being depleted of protons
The porous frit provides electrical contact between the solution and the electrolyte Must be in contact with the sample solution Probe won’t work well if frit is clogged (fouled)
The voltage measurement requires a very high __________ circuit (high resistance) because a pH probe can’t produce much current
Gentle stirring keeps the solution next to the glass bulb from being depleted of protons
++
impedance
Difficult Measurements?Difficult Measurements?
pH is difficult to measure in poorly buffered solutions Distilled water Rain Between pKs of dilute buffers
pH is difficult to measure in poorly buffered solutions Distilled water Rain Between pKs of dilute buffers
Nernst Equation: Voltage = f(pH)Nernst Equation: Voltage = f(pH)
E ERTnF
H
H
FHGG
IKJJ
0 100
ln( )log
E ERTnF
H
H
FHGG
IKJJ
00ln
0
H +é ùë û
0E
R
F
Reference (known) [H+]
Voltage at 0
H +é ùë û
Faraday constant
Gas constant
0 0ln(10)RTE E pH pH
nFé ù= + -ë û
( )00
ln(10)
E E nFpH pH
RT
-= -
RJ
mol K
8 314.
FCoulombsmol e
96500
n mol e 1
Nernst EquationNernst Equation
RJ
mol K
8 314.
FCoulombsmol e
96500
n mol e 1
( )00
ln(10)
E E nFpH pH
RT
-= -
00
0 ln(10)E E nF
pH pHT T R
æ ö= + -ç ÷è ø
-250
-200
-150
-100
-50
0
50
100
150
200
250
4 5 6 7 8 9 10
pH
E (
mV
)
E at 0 C
E at 25 C
E at 35 C
E at 100 C
Slope vs. TemperatureSlope vs. Temperature
Temperature compensation is important! Temperature compensation is important!
50
55
60
65
70
75
0 50 100
Temperature (C)
mV
/pH
E T pH 0 000198 7.
pH CalibrationpH Calibration
It would be possible to make a pH measurement without any calibration Based on theoretical values This is how the software recognizes buffers!
Calibration accounts for non-ideal probe behavior (fouling) as well as electronic measurement errors
It is important that buffers be used covering the range of pH measurements
It would be possible to make a pH measurement without any calibration Based on theoretical values This is how the software recognizes buffers!
Calibration accounts for non-ideal probe behavior (fouling) as well as electronic measurement errors
It is important that buffers be used covering the range of pH measurements
The ChallengeThe Challenge
E T Buffer pH
Current measurement (E, T)
pH
0 to 3 known buffers used as standardsTemperature compensation optional
Calibration Cases0 or 1 standards w/ Temperature
Calibration Cases0 or 1 standards w/ Temperature
00
0ln(10)nF E E
pH pHR T T
æ ö= - +ç ÷è ø
7pH
ideal EpH kT
=- + No standards (assume ideal slope and intercept)
ln(10)pH
ideal nFk
R=
Single standard (assume ideal slope)
11
1pH
ideal E EpH k pH
T T
æ ö= - +ç ÷è ø
00
0pH
ideal E EpH k pH
T T
æ ö= - +ç ÷è ø
5040.8pH
ideal Kk
Volt=
Multiple Point Calibrations w/ Temperature Compensation
Multiple Point Calibrations w/ Temperature Compensation
How would you use this information to calculate pH?
How would you use this information to calculate pH?
E (mV) T (C)Buffer
pH
8 23 7
185 24 4
-170 20 10
Current measurement (E, T)
pH
Piecewise linear fit
Calculate E/T
top related