Bsc Environmental Science analytical chemistry 2506ENV
Laboratory report 4: Group 8, Experiment 5.
Image 1.
EC-PC6500-40S - Color Touch Screen CyberScan PC 6500 - 3 channel pH-pH FETmv-Ion-Conductivity - Resistivity- TDS-Salinity RS232, USB, IRDA Meter with electrode holder and universal power adapter. http://www.instrumentchoice.com.au/shop-by-brand/eutechinstruments/ec-pc6500-40s-color-touch-screen-cyberscan-pc-6500-3-channel-ph-ph-fet-mvion-conductivity-resistivi?gclid=COjMrZmG67oCFbBapgodbRAAuQ
Name:
Student Number:
Partners:
Date of Experiment: 2013
Due Date: 2013
Demonstrator:
A solid-state Fluoride Ion Electrode consists of a single crystal of lanthanum fluoride which acts as an insoluble membrane
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Bsc Environmental Science analytical chemistry 2506ENV
: Introduction
Fluoride Ion Selective Electrodes are used to simply and quickly, economically and accurately measure fluoride ions in aqueous solutions. Fluoride ion electrodes are used to monitor and control the fluoridation of water supplies where fluoride is added to help prevent tooth decay. Fluoride is listed as an inorganic toxicant with a short term trigger value (STV) of 2 ppm for irrigation water in the Australian New Zealand Environmental
Conservation Council (ANZECC) 2000 Guidelines (ANZECC, 2000). The EPA lists a maximum contaminant level (MCL), at 4.0 mg/L or 4.0 ppm for potable water. MCLs are set as close to the health goals as possible (epa.gov.au).
The operation of ion selective electrodes is based on a linear relationship between the electrical potential developed between an Ion Selective Electrode (ISE) and a reference electrode immersed in the same solution, and the Logarithm of the activity (or "effective concentration") of the ions in the solution (Patnaik, 2004).
The active membrane of the Fluoride Ion Electrode consists of a single crystal of lanthanum fluoride, (LaF₃), which acts as an insoluble membrane. The inside surface is exposed to a standard solution with a known and constant concentration of fluoride ions (Patnaik, 2004).
The outside surface is exposed to a solution with an unknown concentration of fluoride ions.
Doping the lanthanum fluoride with europium (II) fluoride generates anion spaces that allow fluoride ions to migrate through the crystal. Only fluoride ions are mobile in the ionic conductor crystal. A potential develops when the membrane comes in contact with a solution containing fluoride ions. The movement of ions across the membrane creates a potential difference that is related to the difference in concentration on either side of the membrane (Harris, 2007).
The response of the Fluoride Ion Electrode to the fluoride concentration in solution is given by:
Ev = constant – β (0.05916) log [F-]outside (Eq1)
Where Ev is the electrode potential, which is close to 59mV per decade over a fluoride concentration range from M to 1M , β is close to 1.00 and [F-]outside is the concentration of fluoride ions in the unknown analyte solution (Harris, 2007).
In this experiment, Ev (electrode potential) will be measured in millivolts (mV). 3 standard solutions with the same chemical makeup as sample solutions and containing a known concentration of analyte will be used to construct a calibration curve. This will show how the analytically determined property (electrode potential) depends on the known concentrations of the standard solutions. Each point on the graph represents a known concentration of fluoride ion with an electrode potential value that has been measured using a fluoride ion selective electrode (Harris, 2007). When all points are plotted, a line of
A solid-state Fluoride Ion Electrode consists of a single crystal of lanthanum fluoride which acts as an insoluble membrane
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Bsc Environmental Science analytical chemistry 2506ENV best fit is drawn to show the linear relationship between –log concentration and electrode potential. This relationship is represented mathematically by the regression model:
y = mx +b
Where y is the electrode potential (mV), m is the slope of the graph , x is the – log concentration and b is the y-intercept of the graph. The coefficient of determination, , indicates the degree of fit for the straight line between the plotted points, with a value between 0 and 1. A value close to 1 indicates a good degree of fit.
Any small deviations of points from the straight line are assumed to arise from errors in measurement (Skoog et al , 2003). The calibration curve will be used to determine the concentration of a sample solution made from fluoride solution. By using a sample solution of tea and an unknown sample solution, the electrode potential can be determined by applying the above equation, rearranged as:
(Eq2)
In this experiment, the electrode potential of duplicate sample solutions will be measured three times and the average value used to determine the concentration from the calibration curve. (Harris, 2007).
The electrode has a selectivity to fluoride ions that is greater by a factor of 1000 over most other ions. The only ion that interferes directly with fluoride measurements is hydroxide ion; this interference becomes serious at pH greater than eight. At a pH less than five, fluoride ions are converted to hydrogen fluoride forms to which the electrode is not responsive
(Skoog et al , 2003).
To ensure the maintenance and operation of solid-state ion selective electrodes a number of steps are necessary:
1.
Before use after storage, the electrode must be preconditioned by immersing in
1000ppm standard solution for 10minutes - 2 hours depending on storage time, until the millivolt reading stabilises. Rinse with de-ionised water and dry with paper towel.
2.
After use, the electrode must be rinsed with de-ionised water and dried with paper towel. Replace the caps to prevent drying out of the external filling solution of the reference electrode and to protect the ISE from mechanical damage or atmospheric oxidation/corrosion.
3.
Recalibrate after every 10 samples or if the temperature changes by one degree, to ensure precision and accuracy.
4.
Crystal membranes can also be rejuvenated by gently polishing with very fine, wet emery paper, using a circular motion, to remove all stains and blemishes, followed by washing in deionised water and drying with paper towel. They must then be preconditioned again for several hours before use ( www.nico2000.net
, 2013)
A solid-state Fluoride Ion Electrode consists of a single crystal of lanthanum fluoride which acts as an insoluble membrane
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Bsc Environmental Science analytical chemistry 2506ENV
AIM
1.
To use a solid-state fluoride ion selective electrode to become familiar with the ion selective electrode (ISE) technique.
2.
To use a solid-state fluoride ion selective electrode to determine the concentration of F- ion in various samples.
METHODS
Reagents
As per lab book – page 19
Apparatus
As per lab book – page 20
Procedure
As per lab book – page 20 – changes:
1.
Approximately 2g of tea is placed into 100ml beakers.
2.
Sample of mouthwash is used to replace fluoride tablets. These are in duplicate 20ml samples to 100ml volumetric flasks filled with Milli-Q water.
RESULTS
The data was used to construct a calibration curve for fluoride ions. Electrode potential was measured using a TPS solid-state fluoride ion selective electrode and recorded in mV. The concentration of 3 standard solutions was given in ppm and converted, according to the
Nernst equation, to obtain values necessary for a linear graph from Table 1.
Using: = mol/L and molar mass for fluoride = 18.99
Then for the first calculation:
= 0.0000526
(Eq 3)
-log [0.0000526] = 4.279
Table 1. Electrode potential of F- ions for standard solutions. ppm
1
10
100 mg/L
1
10
100 g/L
0.001
0.01
0.1
[F-](mol/L)
0.0000526
0.000526
0.00526 pF(-log [F-])
4.279
3.279
2.279 mV
199
140
80
A solid-state Fluoride Ion Electrode consists of a single crystal of lanthanum fluoride which acts as an insoluble membrane
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Bsc Environmental Science analytical chemistry 2506ENV
Figure 1 shows the calibration curve for electrode potential in mV vs –log concentration for the fluoride ions of 3 standard solutions. was 1 and therefore the straight line was considered to have a good degree of fit for the current values. The equation given by the straight line was used to determine the concentration of the fluoride solution and tea in solution and an unknown ‘C’sample solutions.
250
200
150
100 y = 59.5x - 55.434
R² = 1
Ev millivolts
Linear (Ev millivolts)
50
0
0 1 2 3 pF (-log [F-])
4 5
Figure 1. Standard calibration curve for Fluoride ion.
Table 2 shows the measured electrode potential in mV of fluoride ions for the duplicate samples of fluoride in solution, tea in solution and the single sample of unknown ‘B’ solution.
The mean and relative standard deviation was calculated and the mean was used for calculations to determine the concentration of each of the samples.
Table 2. Electrode potential of F- ions for solutions with unknown concentrations
Sample
Fluoride F1 mouthwash.
Colgate Neutraflor
220
Fluoride F2
Tea 2.04g T1
Tea 2.01g T2
Unknown
Duplicate No.
1
1
1
2
2
2
1
1
1
2
2
2
B
B
B mV
105
105
105
104
104
104
154
155
156
158
158
158
087
087
087
105.0
104.0
155.0
158
087
RSD
0.00%
0.00%
0.65%
0.00%
0.00%
A solid-state Fluoride Ion Electrode consists of a single crystal of lanthanum fluoride which acts as an insoluble membrane
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Bsc Environmental Science analytical chemistry 2506ENV
Using the rearranged regression model equation:
Where x is the –log concentration of fluoride ions (pF), y is the mean electrode potential from table 1, m is the slope of the graph and b is the y-intercept of the graph.
The concentration of F- ions in the first duplicate of the fluoride solution (F 1): m = = 59.5 y = 105.0 b = -55.434
Therefore, x (pF) = = 2.696369
To obtain values in ppm:
pF = -log [F-] mol/L =
= g/L = mol/L x molar mass (g/mol)
= 0.001995262315 x 18.99
= 0.03789
mg/L = 0.0379 x 1000
= 37.89
mg/L = ppm
The concentration of F- ions in the first duplicate of the fluoride solution (F 1) = 37.89 ppm.
Table 3 shows the results of the above calculation for each of the fluoride and tea samples; and with unknown fluoride concentration.
The dry weight of the sample substance tea (T1, T2), was measured in grams and recorded in table 3. This was used according to the following calculation to determine the mg/kg. The results of the following calculations were recorded in table 3.
A solid-state Fluoride Ion Electrode consists of a single crystal of lanthanum fluoride which acts as an insoluble membrane
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Bsc Environmental Science analytical chemistry 2506ENV mg/kg = where volume (L) was the dilution factor of the solution.
For the first duplicate of the filtered Tea solution (T1). mg/kg =
= 268.627 mg/kg
Table 3. Determined concentration for unknown sample solutions
Sample
Fluoride
Tea
X 1000 = Kg
Unknown B
Mass (g)
2.04
2.01
Duplicate
1
2
1
2
B mV( )
105.0
104.0
155
158
87 pF
2.70
2.68
3.54
3.59
2.39 ppm
37.89
39.68
5.48
4.92
77.36 mg/kg
268.627
244.776
DISCUSSION
Using the standard calibration curve, the fluoride ion concentration of unknown B was found to be 77.36 ppm. This was extremely high in comparison to the other samples. The
EPA lists a maximum contaminant level (MCL) at 4.0 mg/L or 4.0 ppm. MCLs are set as close to the health goals as possible (epa.gov.au). Fluoride is listed as an inorganic toxicant with a short term trigger value (STV) of 2 ppm for irrigation water in the ANZECC 2000 Guidelines
(ANZECC, 2000). The Short Term Exposure Limit (STEL) for Neutraflor 220 average airborne concentration is 15 minutes over a normal 8 hour day (re: attached MSDS); this was not exceeded during the analysis.
The MSDS listed sodium fluoride (<1% Conc) as below the states reporting threshold for
Crest Vivid White Fluoride Toothpaste (productsafety/msds/health).
By testing the 3 standard fluoride solutions the unknown fluoride concentrations were able to be calculated. This was also helpful in determining whether the electrode was functioning correctly. The standard addition method rather than the calibration curve method could have also been used to determine these.
As the cell potential is linearly related to the logarithm of the fluoride ion, an ideal fluoride
ISE response will have an increase of 59.16mV for every 10 fold increase in fluoride concentration. Our readings were very close with a 59.5mV decrease between the 1mg/L and 10mg/L concentrations and a 60mV decrease between the 10mg/L and 100mg/L concentrations. The samples were prepared by performing a number of dilutions which challenged the ability of the analyst in a team environment. As the ISE and millivolt TSP meter are provided, it is assumed that the care, storage and calibrations of the equipment comply with the manufacturer’s procedures; however some instrumental error may have occurred (Skoog et al.
2004). This experiment was successful in achieving a R
2 value of 1.
A solid-state Fluoride Ion Electrode consists of a single crystal of lanthanum fluoride which acts as an insoluble membrane
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Bsc Environmental Science analytical chemistry 2506ENV
Image 2.
Ion Euro scan 6. (http://www.eutechinst.com/pdt-para-ion-ecoscanion6.html)
QUESTIONS
1.
Why do we need a TISAB buffer for detection?
A Total Ionic Strength Adjusting Buffer (TISAB) is necessary for detection as it is used to adjust the pH of the solution and decrease the interferences from other ions. This will serve to decomplex the fluoride and provide a constant background ionic strength for an accurate analysis (Skoog et al , 2003).
2.
What is the appropriate axis in construction of a calibration curve?
To achieve linearity on a calibration curve and thus determine the concentration of the unknown sample, a plot of electrode potential: E (mV) along the y axis and vs concentration Log C (mg/L or ppm) along the x axis is constructed (Harris, 2007).
A solid-state Fluoride Ion Electrode consists of a single crystal of lanthanum fluoride which acts as an insoluble membrane
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Bsc Environmental Science analytical chemistry 2506ENV
References:
Australian New Zealand Environmental Conservation Council (ANZECC) 2000 Guidelines http://www.environment.gov.au/water/publications/quality/nwqms-guidelines-4-vol1.html
acc 5/9/13. http://water.epa.gov/drink/contaminants/basicinformation/fluoride.cfm
acc 21/9/13.
Harris, D.C; Quantitative Chemical Analysis, 7 th
Ed.; W.H.Freeman & Co, USA.;2007.
Patnaik, P.; Deans Analytical Chemistry Handbook, 2 nd Ed.; McGraw-Hill, USA.; 2004.
Skoog, D.A.; West, D.M.; Holler, F.J.; and Crouch, S.R.; Fundamentals of Analytical Chemistry,
8 th
Ed., Brooks/Cole-Thomson Learning, Victoria, Australia; 2003. http://www.nico2000.net/datasheets/OPinstr4electrodes.htm
acc 21/9/13. http://www.pg.com/productsafety/msds/health_care/oral_care/Crest_Vivid_White_Toothp aste.pdf
acc 21/9/13. http://www.colgate.com.au/Colgate/AU/Corp_v2/ContactUs/MSDS/pdf/neutrafluor_220_2
012.pdf
A solid-state Fluoride Ion Electrode consists of a single crystal of lanthanum fluoride which acts as an insoluble membrane
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