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

Transcription

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

2 (Eq 2) where: Ex Eo R T z F ln aox, ared potential of the electrode standard potential of the electrode (a constant) universal gas constant absolute temperature (in Kelvin degrees) charge of the ionic species Faraday's constant natural logarithm (with base e= ) electrochemical activity of the ionic species The above equation (Eq 2) may be directly applied to the redox based electrodes, i.e. when a reductionoxidation 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 K S cx log potential of the electrode sensitive to ion x a constant slope of the electrode's characteristics concentration of ion x (mol dm3) logarithm with base 10 (common logarithm)

3 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 There are two aspects of potentiometric measurements that have to be taken into account in practice. First is that equation 5 is only valid for very dilute solutions or solutions of constant ionic strength. The second is that electrodes respond only to free ions. Therefore some measures have to be taken to make measurement meaningful and reliable. In case of fluorides (F) determination this is achieved with the help of Total Ionic Strength Adjustment Buffer (TISAB). TISAB is added to all samples to keep the ionic strength at a high and constant value. The TISAB also contains a buffer to maintain the ph of the samples at a fixed value (below ph of around 5, significant portion of F ions will be replaced by HF or HF2). Additionally, the TISAB contains a complexing agent (CDTA) which complexes multivalent cations (eg. Al3+) which otherwise can interfere with the fluorides determination. 3. Instructions 3.1. Labware potentiometer fluoride ISE (combined) 100 ml volumetric flasks 50 ml polyethylene containers pipette 10 ml pipette 25 ml magnetic stirrer with a stirring bar (6 pieces) (8 pieces) (8 pieces) 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. Calibration solution No. 1: add 10 ml of stock solution to a 100 ml volumetric flask. Make up to the

4 mark 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. Calibration solution No. 3: add 20 ml of calibration solution No. 2 to a 100 ml volumetric flask. Calibration solution No. 4: add 50 ml of calibration solution No. 3 to a 100 ml volumetric flask. Calibration solution No. 5: add 20 ml of calibration solution No. 4 to a 100 ml volumetric flask. 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).

5 Illustration 2: River water origin, Gdańsk, Sobieszewo, Martwa Wisła (North: 54 20' 29.21", East: 18 49' 54.07") Report Shortly 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 each of the samples. Exchange your result with the results obtained by other subgroups and provide the mean and 95% confidence interval for fluorides content in each sample. Provide an example of 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 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 literature source. No duplications of sources between and within subgroups 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 nopenalty 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 after more than 9 weeks will be considered as nonexistent. The only accepted form of report submission is a PDF file sent by to: wasia@pg.gda.pl