Experiment#6
DETERMINATION OF FLUORIDE BY ION SELECTIVE ELECTRODE
Samuel Raymond
Chem 322-01
Professor: Dr. Jimenez
11/11/2020
Abstract:
The purpose of this experiment was to learn how ISE electrodes particularly fluoride ISE are
used to find the concentrations levels of this element. Through electrochemistry the study of
current in the electroactive species. Detections of fluoride throughout daily objects such as
toothpaste and foods can undergo testing to discover the constituted of Floride.
Introduction:
In the world of electrochemistry techniques to measurement of ion activity have become a staple
in monitoring electrical currents among electroactive elements. With that in mind, this
experiment utilizes an fluoride selective electrode and reference electrode equipped with unique
type of selective membrane to detect only the fluoride ion exchanges. This is possible because of
the preparation taken to ensure inference is reduced effectively by undergoing a redox reaction of
Sodium Fluoride molecule beforehand so that solely F- activity is measured.
Mechanism& Equations:
E = Eo + 2.303 (RT/F) log A ion
M1V1=M2V2
Chemicals:
Sodium fluoride( NaF)
Blue
Yellow
Red
3 - Short exposure could cause serious temporary or
moderate residual injury.
0 - Normally stable, even under fire exposure
conditions, and is not reactive with water.
0 - Materials that will not burn under typical fire
conditions , including intrinsically noncombustible
materials such as concrete, stone, and sand.
Materials that will not burn in air when exposed to
a temperature of 820 °C (1,500 °F) for a period of 5
minutes.
Lanthanum trifluoride(LaF3)
Blue
Yellow
Red
2 - Intense or continued but not chronic exposure
could cause temporary incapacitation or possible
residual injury.
0- Normally stable, even under fire exposure
conditions, and is not reactive with water.
0 - Materials that will not burn under typical fire
conditions , including intrinsically noncombustible
materials such as concrete, stone, and sand.
Materials that will not burn in air when exposed to
a temperature of 820 °C (1,500 °F) for a period of 5
minutes.
Supplies& Equipment:
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Plastic polyethylene beakers
Analytical balance
1L volumetric flask
Volumetric pipette
100-ml volumetric flasks
PH electrode
Fluoride ISE
Procedures:
To begin the preparation of 100ppm Fluoride solution several PE beakers were rinsed in DI
water wand dried. Then 0.22 grams of Sodium fluoride was weighed, and difference calculated
between the beaker and sample. Furthermore, the sample was diluted with DI water into an 1L
volumetric flask. With the stock solution prepared a series of ppm standard were made from 0.50
up to 20ppm. First a 20ppm fluoride standard was created by adding 10.0ml of TISAB together
with 20.0ml of the Fluoride stock. Additionally, a set volume of DI water was used to dilute the
mixture and then its contents were mixed thoroughly. Similarly, these measures were reproduced
for the lower range of fluoride standards. Apart from this, a mouthwash solution was prepared
with 10.0ml of sample and 10.0ml of TISAB. The mouthwash solution was diluted in 100ml
volumetric flask and had 20.0ml transferred to an PE beaker. Thereafter the process was
reproduced but with an unknown fluoride sample from the instructor. Once prepped the PH of
both solutions were then checked for assurance of the suitable working range.
Observations:
Part A
To start the prepared standards the PE breakers were labeled for the series of ppm
concentrations. 10ml of TISAB was added to all the volumetric flasks beforehand. Thereafter a
clean serological pipette was acquired to place the calculated volume of Fluoride sample into the
standards. Carefully each mixture was diluted with DI water to the same set mark. The made
solutions were then mixed thoroughly but not allowed to sit for a lengthy period inside the
volumetric flask. 20ml of each standard solution was transferred to its respective labeled PE
beaker.
Part B
The pH of the sample solution and different ppm concentration were taken beforehand. In
addition, the pH meter was calibrated in buffer solutions of with a pH 10 & 7. Through the
calibrations process the electrode was sat in the solution until the readings were confirmed ready.
Thereafter the electrode was carefully wiped with a tissue and placed into the next buffer
solution. Once pH of the samples was found the test began with the fluoride electrode. Starting
off the lowest ppm solution tested first by placing the electrode into the solution while be
cautious that the highly sensitive tip doesn’t contact any surfaces except the analyte. Meanwhile,
the electrode would sit until the device has signaled to be ready. The mV was then recorded, and
electrode removed from the solution to be carefully cleaned. This series of measures were then
done over again from the lowest ppm to the higher ppm concentrations.
Results and Discussion:
Quantitative Data & Qualitative Data:
From the series standards concentrations produced it was used in calculating the log
concentrations. In which was used to converted to find Molarity of the standards fluoride
solutions log(concentration)*(1g/1000mg) (1mol/19g) = log (-0.301) *(1g/1000mg) *(1mol/19g)
= -1.58E-05mol/L. Once calculated were conducted a calibration curve of log concentration vs
mV. Based off the graphs’ linear equation Y=mx + b= 15.9 = (-30.7) x - 9.7 served as a reference
into calculating final F- concentration of unknown. In addition, the equation was then solved for
the X variable to provide the final log concentration for both mouthwash and F- ion of unknown.
X = (4.9-15.9)/-30.7= 0.358ppm and X = (-9.7 - 15.9)/-30.7 = 0.834ppm. Finally, with the log
concentrations found a simple reverse of operations of the antilog conducted to find first ppm.
Thereafter, a simple stochiometric conversion to Molarity to discover both the concentration in
the mouthwash sample. With the mouth sample coming to be 1.20E-04 and F- ion 3.59E-04.
100
INITIAL
CONCENTRATIO
N NaF:
pH
STANDARD,
PPM (mg/L)
5
0.5
5.9
1
6.5
2
7.5
5
8.5
10
9
20
6.6 Mouthwash
7 F- ion Unknown
SLOPE
Mouthwash
F- ion Unknown
PPM
mV
log (Conc), PPM
27.7
16.4
4.3
-10.2
-13.7
-21.2
4.9
-9.7
INTERCEPT
-30.7
-30.7
15.9
15.9
MOLARITY,
moles/L
-0.301
0.000
0.301
0.699
1.000
1.301
0.358
0.834
-1.58E-05
0.00E+00
1.58E-05
3.68E-05
5.26E-05
6.85E-05
1.20E-04
3.59E-04
LOG
(CONCENTRATIO
N)
0.358
0.834
CONCENTRATION
PPM
2.28
6.82
Graph 1: Linear regression
Graph 2: Linear equation
Graph 3: Linear Equation
Sources of Error:
Miss calibration of the ISE could lead to spikes in the initial concentration reading. Thus, that
data point surrounding the slope of the trendlines greatly effects the precision of the overall
calibration curve. Without the usage of the TISAB buffer the data required to discover the
unknown samples won’t be accurate at all. Because TISAB is needed in order to detect the just
fluoride concentrations.
Conclusion:
Within the second and third graph it was evident that the pattern of these graphs followed
Nernst's’ equation. However, the desired slope was not quite achieved with the calculated slope
of coming to –30.7. Overall, the precision of the experiment was achieved but not quite on the
accuracy aspect.
Cited:
Daniel, C. Harris “Let the Titrations Begin" Quantitative Chemical Analysis,
Kate Parker,2016, pp. 365-370.