Determination of fluorides in aqueous samplesusing membrane, ion selective electrode (ISE)

(dr hab. inż. Andrzej Wasik, Gdańsk 2016)

The aim of this laboratory exercise is to familiarise students with an analytical method called direct potentiometry. Students will use direct potentiometry to determine the content of fluorides in a sample of river water, black tea infusion and tap water.

1. IntroductionPotentiometry is an electroanalytical method, in which the concentration (activity) of ionic

species is measured using an electrochemical cell consisting of two special electrodes. One of these electrodes is called a reference electrode, the other is known as indicating or sensing electrode. When these two electrodes are in contact with the electrolyte (a sample to be tested) they form an electrochemical (galvanic) cell and each of electrodes develop a property called potential1. The most important feature of the cell is the difference of potentials (E) between the indicating (Eind) and reference (Eref) electrodes:

(Eq 1)

This difference of potentials is sometimes called a cell voltage or electromotive force (EMF).

The potential of a single electrode depends on number of variables and can be described by Nernst's equation:

1 It is impossible to measure the potential of a single electrode, but we can measure the difference of potentials. Therefore the galvanic cell consist of two electrodes.

Illustration 1: Instrumental setup for determination of fluorides using; (A) two separate electrodes, (B) so called "combined" electrode.

(Eq 2)

where:Ex - potential of the electrodeEo - standard potential of the electrode (a constant)R - universal gas constantT - absolute temperature (in Kelvin degrees)z - charge of the ionic speciesF - Faraday's constantln - natural logarithm (with base e=2.718281828)aox, ared - electrochemical activity of the ionic species

The above equation (Eq 2) may be directly applied to the redox based electrodes, i.e. when a reduction-oxidation reaction is the source of potential and two forms of ionic species, at different oxidation states, are present. The copper electrode may serve as an example:

(Eq 3)

In this case the Nernst's equation will look like this:

(Eq 4)

When working with membrane ion selective electrodes (ISE), no redox reactions involving ions of interest occur. ISE's potential develop due to the fact that ions to be measured have different concentrations at both sides of the membrane being the part of ISE. This membrane is the most important part of ISE. It is responsible for selectivity and sensitivity of ISE. The membrane may be manufactured in many forms and from different materials. It can be made of glass, a single crystal, many small crystals or a polymeric material. In case of fluoride – ion selective electrodes, the membrane is commonly made of single crystal of lanthanum fluoride (LaF3) doped with some europium fluoride (EuF2).

In the laboratory practice the potential of ISE's is described by somewhat simplified Nernst's equation:

(Eq 5)

where:Ex - potential of the electrode sensitive to ion xK - a constantS - slope of the electrode's characteristicscx - concentration of ion x (mol·dm-3)log - logarithm with base 10 (common logarithm)

The potential of the indicating electrode is (due to its construction), directly related to the concentration (activity) of the ions to be measured, while the reference electrode is always chosen in a such way that its potential is constant and independent on sample composition. Therefore the resulting cell voltage is dependent only on the concentration of ionic species of interest (fluorides, in case of this laboratory exercise).

2. Practical considerations

When making potentiometric measurements, three limitations should be considered.

1. First of all, keep in mind that Equation 5 is only valid for very dilute solutions, with low ionic strength, because then the ion activity coefficient is practically equal to 1, so the activity of ions corresponds to their concentration.

2. In practice, however, the test samples and calibration solutions differ in ionic strength. As a result, fluoride ions activity coefficient in these solutions will be different, and hence the potential of ISE in these solutions it can be different even though fluoride concentrations are the same.

3. Thirdly, the electrodes only react to free ions. Ions in the form of complexes are "invisible" for ISE.

Therefore, some measures must be taken to make measurements reliable. In case of fluorides (F-)it is achieved with the help of Total Ionic Strength Buffer (TISAB). TISAB is added to all samples to maintain their high and constant ionic strength. This results in a certain decrease in the sensitivityof the measurement due to the fact that the increased ionic strength decreases the activity of fluorideions. However, maintaining a constant ionic strength value eliminates the error associated with the fact that fluoride ion activity coefficient can be different for the test samples and standard solutions.In addition, TISAB contains a buffer that maintains the pH of the samples at a set value (for a pH below about 5, a significant portion of F- ions will be in the form of HF or HF2

-). In addition, TISAB contains a complexing agent (CDTA), which forms complexes with multivalent cations (eg Al3+), which otherwise would form complexes with fluorides and influence the results of their determination.

3. Instructions

3.1. Labware

- potentiometer (1 piece)- fluoride ISE (combined) (1 piece)- 100 ml volumetric flasks (6 pieces)- 50 ml polyethylene containers (8 pieces)- pipette 10 ml (1 piece)- pipette 25 ml (8 pieces)- magnetic stirrer with a stirring bar (1 piece)

3.2. Reagents

- standard solution of F- (1 mg/ml)- TISAB solution

Prepare calibration solutions:

Use a dedicated pipette for each solution.

Stock solution: add 20 ml of fluoride standard solution (1 mg/ml) to a 100 ml volumetric flask. Make up to the mark with distilled water, close with a stopper and mix well. Calculate molar concentration of this solution.

Calibration solution No. 1: add 10 ml of stock solution to a 100 ml volumetric flask. Make up to themark with distilled water, close with a stopper and mix well. Calculate molar concentration of this solution.

Calibration solution No. 2: add 50 ml of calibration solution No. 1 to a 100 ml volumetric flask. Make up to the mark with distilled water, close with a stopper and mix well. Calculate molar concentration of this solution.

Calibration solution No. 3: add 20 ml of calibration solution No. 2 to a 100 ml volumetric flask. Make up to the mark with distilled water, close with a stopper and mix well. Calculate molar concentration of this solution.

Calibration solution No. 4: add 50 ml of calibration solution No. 3 to a 100 ml volumetric flask. Make up to the mark with distilled water, close with a stopper and mix well. Calculate molar concentration of this solution.

Calibration solution No. 5: add 20 ml of calibration solution No. 4 to a 100 ml volumetric flask. Make up to the mark with distilled water, close with a stopper and mix well. Calculate molar concentration of this solution.

Prepare calibration curve:

Add exactly 20 ml of a calibration solution and 20 ml of TISAB solution to a clean and dry polyethylene container. Carefully put the stirring bar in the mixture and place the container on the stirrer. Start mixing the content of the container, then immerse the tip of fluoride ISE in the solution.

Wait for readout to stabilise and write it down. Repeat the procedure for all calibration solutions. Start from the most dilute one. Each students group will perform full calibration of their ISE.

Wash the stirring bar and the ISE with distilled water after each measurement. After washing remove excess of water from ISE and stirring bar by blotting with an absorbent tissue (e.g. Kleenex).

Draw the calibration curve for your ISE, i.e. the graph illustrating the relationship between the measured signal and log10 of molar concentration of fluorides.

Analyse your samples:

Note (or weigh) the mass of tea leaves being used (most commonly it is between 1.5 and 2 g). Prepare the tea infusion in usual way, soaking tea leaves for 5 minutes. Cool the infusion to the room temperature before taking measurements.

Analyse your samples starting with tap water. Add exactly 20 ml of tap water sample and 20 ml of TISAB solution to a clean and dry polyethylene container. Carefully put the stirring bar in the mixture and place the container on the stirrer. Start mixing the content of the container, then immerse the tip of fluoride ISE in the solution. Wait for readout to stabilise and write it down.

Wash the stirring bar and the ISE with distilled water after each measurement. After washing remove excess of water from ISE and stirring bar by blotting with an absorbent tissue (e.g. Kleenex).

Repeat the procedure for other samples (river water, tea infusion).

ReportShortly describe the laboratory experiment, present all measured data and graphs. Discuss

the impact of the fluorides on the environment and human health.Determine slope of the electrode's characteristics. How much does it differ from the theoretical (Nernstian) value? Using calibration curve of your ISE, determine the fluoride concentration in eachof the samples. Exchange your result with the results obtained by other sub-groups and provide the mean and 95% confidence interval for fluorides content in each sample. Provide an example of

Illustration 2: River water origin, Gdańsk, Sobieszewo, Martwa Wisła (North: 54° 20' 29.21", East: 18° 49' 54.07")

detailed calculations for at least one sample.Compare your results with a data from scientific literature – do not use popular web sites as

they frequently publish unproven, poor quality data. Try to use e.g. Google Scholar, Elsevier's Science Direct or http://pubs.acs.org. In most cases these search engines will provide you with full texts when used form within GUT network.

Each member of laboratory group must compare its group's results with one unique literaturesource. No duplications of sources between and within sub-groups will be allowed! The faster you prepare your report the bigger choice you have. Give the name of the person doing comparison.Quality of conclusions drawn by a given student will affect its grade.

The no-penalty period for returning and acceptance of a report is one week. After this time each started week of delay will result with 0,25 grade malus. This means that a report accepted aftermore than 9 weeks will be considered as non-existent.

The only accepted form of report submission is a PDF file sent by email to: wasia@pg.edu.pl