1
Ken Yokoyama
CHE 331
February 5, 2003
Dr. Rahni
EXPERIMENT 2: Evaluation of an Ion Selective Electrode (2-5 II)
Objectives
In this experiment a fluoride sensitive electrode and an external reference electrode
will be used to measure the fluoride-ion content of a solution. The [F-] of unknowns then
will be determined from a calibration curve.
Theory
A large number of indicator electrodes with good selectivity for specific ions are
based on the measurement of the potential generated across a membrane. Electrodes of
this type are referred to as ion-selective electrodes or ISE. This incorporates a special ionsensitive membrane, which may be glass, a crystalline inorganic material or an organic
ion-exchanger. The membrane interacts specifically with the ion of choice, in our case
fluoride, allowing the electrical potential of the half-cell to be controlled predominantly
by the F- concentration. The potential of the ISE is measured against a suitable reference
electrode using an electrometer or pH meter. The electrode potential is related to the
logarithm of the concentration of the measured ion by the Nernst equation.
RT
log  M 
nF
0.059

log aF 
zF 
E  E   2.303
25°C

Ecell  Econstant
at
where n is the ion charge (negative for anions). The factor 2.303 RT/F
has a theoretical value of 59 mV at 25 °C. The equation is only valid
for very dilute solutions or for solutions where the ionic strength is
constant. z F  and aF  are the charge and activity, respectively, of the
fluoride ion in the sample. The level of fluoride, is the “effective concentration”
of free fluoride ions in solution. The total fluoride concentration, Ctotal, may include some
2
bound or complexed ions as well as free ions. The electrode responds only to free ions,
whose concentration is:
Cfree = Ctotal - Cbound
where Cbound is the concentration of fluoride ions in all bound or complex forms.
The ISE responds to the activity of the free ion in solution. In order to calculate
concentration from activity, the ionic strength of the solution must be known. This
information is not usually available for unknown samples. Therefore, the usual practice is
to treat both standards and samples with an ionic strength adjustor (ISA) to make the total
ionic strength very high. The ionic strength is then constant for all solutions and becomes
Ecell  Econstant 
0.059
log  F  
zF 
When interfering species are a factor, the general equation is
Ecell  Econstant 
0.059
log(ai  Kij a nj z )
zF 
where K ij is the selectivity coefficient and ai and a j refer to the activity and charge,
respectively, of the interfering ions. The selectivity coefficient is a measure of the extent
of the interference posed by a particular ion that might be present in the sample. A small
selectivity coefficient for a given ion indicates less interference from that ion.
A calibration curve is prepared for the response of a particular electrochemical cell to a
series of standard fluoride solutions. The measured potentials are plotted versus the
logarithm of the concentrations or activities of the fluoride ion. The result is a straight
line with a slope of 59.16 mV if a Nernstian response is obtained. In neutral solutions,
fluoride concentration can be measured down to 10-6 M (0.02 ppm) fluoride. The upper
limit of detection is a saturated fluoride solution.
3
Procedure
A. Preparation of Solutions
1. Prepare each of the following fluoride solutions in 25 ml volumetric flasks
by serial dilution of the 100 ppm fluoride standard, 50, 25, 15, 10, 5, and 1
ppm.
2. The unknowns are samples of sodium fluoride.
B. Making the Measurement
1. Set the pH meter to read millivolts (mV). A glass beaker will be used for
each fluoride measurements.
2. Place the reference and fluoride ISE into solution, allow the pH meter
reading to stabilize, and record the voltage in millivolts (mV).
3. When measuring the voltage of the standard solutions, begin with the most
dilute solution and measure the fluoride standards in order of increasing
concentration.
4. Rinse the electrodes well with distilled water and blot dry between each
measurement with kimwipes.
C. Calculations
1. A calibration graph is obtained by plotting the cell potential in millivolts
against the -log [F-]. Determine the fluoride ion concentration of your
unknown from the calibration graph. Calculate the slope of the calibration
curve over the linear region. The plot may deviate from the linearity at low
fluoride concentrations.
2. By comparison of the potentials measured for the unknown and standards,
estimate the concentration of your unknown.
4
DATA
Analyte
Standards
DI water
Tap water
0.01 M NaBr
0.01 M NaCl
Unknown #1
(NaF)
Unknown #2
(NaF)
Unknown #3
(NaF)
Instrument F- (ppm)
A
100
B
100
A
50
B
50
A
25
B
25
A
15
B
15
A
10
B
10
A
5
B
5
A
1
B
1
A
B
A
B
A
B
A
B
A
15?
B
A
100?
B
A
100?
B
[F-] (Molarity) -logF0.0053
2.28
0.0053
2.28
0.0026
2.58
0.0026
2.58
0.00132
2.88
0.00132
2.88
0.00079
3.10
0.00079
3.10
0.00053
3.28
0.00053
3.28
0.00026
3.58
0.00026
3.58
0.000053
4.28
0.000053
4.28
4.89
4.68
5.28
5.28
0.000676
0.000387
0.00417
0.00179102
0.00427
0.001941
3.17
3.08
2.38
2.40
2.37
2.37
Potential (mv)
1
2
163
162
162
163
176
176
179
179
188
188
194
196
202
201
210
212
209
210
218
219
226
227
237
238
276
277
299
300
281
315
349
360
291
296
301
307
334
325
332
341
325
328
344
349
208
209
203
202
163
163
164
164
162
162
162
162
3
162
162
176
179
187
195
201
210
210
221
231
239
273
297
323
368
295
314
326
345
332
350
208
201
163
164
162
162
Avg.
162
162
176
179
188
195
201
211
210
219
228
238
275
299
306
359
294
307
328
339
328
348
208
202
163
164
162
162
STDEV
0.577
0.577
0.000
0.000
0.577
1.000
0.577
1.155
0.577
1.528
2.646
1.000
2.082
1.528
22.301
9.539
2.646
6.506
4.933
6.658
3.512
3.215
0.577
1.000
0.000
0.000
0.000
0.000
CV%
0.36%
0.36%
0.00%
0.00%
0.31%
0.51%
0.29%
0.55%
0.28%
0.70%
1.16%
0.42%
0.76%
0.51%
7.28%
2.66%
0.90%
2.12%
1.50%
1.96%
1.07%
0.92%
0.28%
0.50%
0.00%
0.00%
0.00%
0.00%
5
Standard Curve:
y = 56.368x + 28.768
R2 = 0.9887
290
y = 67.142x + 3.8875
2
R = 0.987
Instrument Standard Curve A
300
Instrument Standard Curve B
Emeas (mV)
Emeas (mV)
270
250
230
210
190
250
200
170
150
2.25
150
2.75
3.25
3.75
4.25
4.75
2.25
2.75
3.25
3.75
4.25
4.75
-Log F-
-log F-
-The Standard curve for Instrument A showed better precision and had a correlation of
determination of 0.9887 and instrument B had 0.9870. Instrument A has a 0.17% more of
a shared variance than Instrument B does. Instrument A was more accurate with a slope
of 56.368 that was a 4.72% error compared to the expected 59.16. Instrument B had a
slope of 67.142 that was a 13.49% error.
Inter-Instrument Assay:
Analyte
Instrument F- (ppm)
Standards
A
100
B
100
A
50
B
50
A
25
B
25
A
15
B
15
A
10
B
10
A
5
B
5
A
1
B
1
t-test
1
0.035099
0.008163
0.008811
0.004723
0.009852
0.000204
-The inter-instrument assay shows by the
t-test statistic that there at a high Fconcentration the means between the
machines are the same. By 50 ppm, the
probability is below the α=0.05 error that
there is a difference between the
machines.
6
Unknowns
Unknown #1
Unknown #2
Unknown #3
Actual(M) Actual (ppm) Expected(M) Expected(ppm) Percent Error (%)
0.000676
12.844
0.00079
15.00
14.43%
0.00417
79.23
0.0053
100.00
21.321%
0.00427
81.13
0.0053
100.00
19.434%
-Unknown #1 had an activity of 208 mV and a concentration of 12.84 ppm. Unknown #2
had an activity of 163 mV and a concentration of 79.23 ppm. Unknown #3 had an activity
of 163 and a concentration of 81.13 ppm.
Conclusion:
The experiment went very well. Flouride ISEs only respond to free ionized F- in
solution, therefore the electrode showed response to F- activity. The precision of the
measurements was good with a range of 0.00% to 7.28% CV% for both instrument A and
B for the serially diluted standards. I used the calibration curve from instrument A,
because the straight line had a little better fit of 0.9887 compared to 0.9870 of instrument
B. Instrument A also had better accuracy than instrument B with a slope of 56.368 that
was 4.70% off to the expected 59.16 mV. T-test statistics on the inter-instrument assay
showed that as the concentration of F- decreased, the variability between the
measurements of A and B increased after 50 ppm. This may be do to the decrease of
linearity as concentration decreases.
I was not able to determine the selectivity coefficients of NaBr and NaCl, because
we had picked concentrations that we thought would be in the mid-range of the standard,
instead they were above the higher limits of the curve. Unknown #1 had an activity of
208 mV and a concentration of 12.8 ppm. I assumed that it might be at the expected
concentration of 10 ppm. If this is so, then there is a 14.4% error. Unknown #2 had an
activity of 163 mV and a concentration of 78.314 ppm. Unknown #3 had an activity of
163 and a concentration of 81.52 ppm. I had assumed that they are both expected to be at
concentrations of 100 ppm. There was a 21.3% and 19.4% error respectively.