Experiment ISE: Measurements with Ion Selective Electrodes: Determination of Fluoride in Toothpaste 67 You have been hired by the government to check the fluoride concentration labelling on some major brand name toothpastes. Your job will be to determine if the label accurately reflects the content, and to quantify variation from different lot numbers. 1 Introduction: The form of the theoretical Nernst equation implies that electrochemical potential measurements could give information as to type and amount of redox species in solution. In fact, measurement of electrode potentials in selected systems can either give absolute activities of ions or can give changes in activities as the solution is altered. The most common uses of cell potential measurements are direct activity measurements and indirect titration measurements. Theoretically, direct measurement of ionic species should be possible simply by measuring a cell potential and inserting the value into the Nernst equation. Practically, however, this is not as easy as it sounds. For example, the platinum electrode is commonly used for potentiometric measurements. The potential of the Pt electrode is dependent on the concentration ratio of the oxidized and reduced species of a redox couple according to the Nernst equation, eg Fe 2+ Fe 3+ or Ce 3+ Ce 4+. The Pt electrode is a non selective electrode, however, because it responds to the potential developed by many redox couples, this lack of selectivity can lead to error if unrecognized complications are present. Ion selective electrodes respond selectively to the activity of a specific ion. The first electrode of this type was the glass electrode which responds selectively to the hydronium (H + ) ion. This was followed by glass electrodes of different glass compositions that respond selectively to Na +, K + and Li +. Renewed activity in the field of ion selective electrodes had recently resulted in a number of electrodes responding selectively to a wide variety of ions, including F, Cl, I, S 2, ClO 4, NO 3, SO 4 2, PO 4 3, Ca 2+, Pb 2+, Mg 2+, Cu 2+, Ni 2+, Zn 2+, and Fe 2+. These electrodes all consist of an internal reference electrode in contact with a standard activity of the specific ion to be measured, separated from the unknown test solution by a membrane which responds reversibly to the specific ion or group of ions. A potential proportional to the log of the specific ion activity ratio develops across the membrane. Three types of membranes are used: (l) a porous medium (porous glass, filter paper, etc) impregnated with an immiscible "liquid ion exchange solution" residing in the interstitial spaces of the porous membrane; (2) a single crystal of an insoluble salt, such as a silver halide; (3) silicone rubber impregnated with an insoluble salt, or PVC in which an organic sensor is dissolved. Ion selective electrodes measure the activity, and not the concentration, of the specific ion. The activity of a specific ion, a i, is related to its concentration, C i, by: a i = γ i C i
68 where γ i is the activity coefficient. The activity coefficient depends on the ionic strength, μ, of the solution according to: where Z i is the charge on the specific ion. This equation is reasonably accurate up to ionic strengths of about 0.l. The ionic strength depends on the concentration and charges of all the ions in the solution according to: For example, a solution containing 0.05 M NaF, 0.20 M K 2 SO 4 and 0.l5 moles NaCl has ionic strength as follows (the number under each ion is the total concentration of that ion): μ Na + K + 2 SO 4 Cl F = l/2 (l x 0.20 + l x 0.40 + 4 x 0.20 + l x 0.15 + l x 0.05) M = 0.80 M At low ionic strengths, activities approach concentrations. At high ionic strengths, activities are substantially less than concentrations, and concentrations can be determined from ion selective electrode measurements only by accounting for the ionic strength effect. This can be done by: l preparing a calibration curve of electrode potential versus ion concentration in a medium of ionic strength comparable to that of the test solution; 2 calculating the concentration from the measured activity using the above equations if the concentrations of other ionic species are known. The ion selective electrodes respond only to free ions, ie, to ions in the aquated form. Complexed ions do not affect the potential of the electrode. Thus, to determine the total concentration (activity) of a specific ion, solution conditions must be adjusted so that all of the ions are present in the free form. For the fluoride selective electrode, for example, the Nernst equation gives: where E is the measured potential of the fluoride electrode relative to a standard reference electrode, eg, a saturated calomel electrode. E B is that portion of the fluoride electrode potential due to the internal reference electrode, internal solutions, and the specific membrane. The Nernst factor is 2.3 RT/F (59.l6 mv at 25 B C); R and F are constants, T is the temperature in Kelvin, and a F is the fluoride ion activity in the sample solution. In this experiment, the response of a single crystal fluoride selective electrode will be studied over a range of fluoride concentrations after which the free fluoride in a toothpaste will be determined.
The electrode membrane, ie the electrode sensing element, is a single crystal of lanthanum fluoride. The lanthanum fluoride crystal is an ionic conductor for fluoride ion and the solution can be adjusted with TISAB (Total Ionic Strength Adjustment Buffer) to make the crystal selective for fluoride ion alone. The high ionic strength of the buffer (TISAB) is required to assure that the standard solutions and the unknown solutions have the same activity coefficients. There is an internal reference electrode in our F selective electrodes, so all measurements are made relative to this reference. ph is a solution parameter that affects measurements made with the fluoride selective electrode. Hydroxide ion can cause the response to be non Nernstian when the hydroxide ion concentration is in the order of one tenth the fluoride concentration. Also, hydrogen fluoride is a weak acid that can exist as HF and HF 2 in acidic solution. Thus, in acidic solution, the fluoride electrode responds only to fluoride present as F. The equipment you are using in this experiment is easily damaged or ruined. The fluoride electrodes can be damaged by a single scratch on the crystal surface. The electrode should be rinsed with distilled water before they are inserted in a sample or standard solution. Blot water from the sensing element of the fluoride electrode with a soft tissue. Don't wipe the sensing element because that may cause scratches. Do not store the electrode in distilled water. Store it in a solution of fluoride and TISAB when the experiment is finished. 2 Instrumental procedure for the Accumet XL 50 meter: The XL 50 is capable of making direct concentration measurements, when used in Ion Mode. Instructions for the use of the meter in this mode are provided in the instrument manual, pages 96 98, under Direct Reading with Standards. You will be required to standardize the meter with 5 standards. The following instructions will also help with your experimental setup. 3 Ion Selective Electrode Measurements Rapid equilibration of electrodes requires stirring. In the laboratory, this can be provided with a magnetic stirrer. The recommended field procedure is to shake the beaker back and forth for about l5 seconds before taking a reading. The ion selective electrode should be rinsed with distilled water and blotted with an absorbent tissue between readings to prevent carry over of solution 69 4 Experimental Procedure
70 4.1 Accurately weigh six samples (3 from each of two different lot #s) of about 200 mg (each) toothpaste into the bottom of six separate 100 ml beakers. Ensure that the sample is at the bottom of the beaker to avoid sample loss due to poor mixing. 4.2 Add a few ml of distilled water and carefully swirl the beakers and contents to dissolve the toothpaste. This will require about 5 minutes of constant swirling. The unknown is dissolved when no large lumps remain, and no lumps are stuck to the bottom of the beaker. The fluoride dissolves readily, but the abrasives and fillers do not, therefore the solution will be cloudy. 4.3 All standards and unknown solutions are made up containing 50 % TISAB. Quantitatively transfer the contents to a 50 ml volumetric flask using a total of 25 ml of TISAB and then distilled water. Dilute to the mark with distilled water. Exercise caution in handling the unknown as it readily foams (recall that there s soap in toothpaste!). Gently invert the flask to mix the diluted unknown. 4.4 To measure the fluoride potential in the unknown, pour the unknown back into the same beaker in which it was dissolved. 4.5 Use the sodium fluoride stored in the oven to prepare a 100 mg/l fluoride stock solution. Use this stock solution to prepare at least 100 ml each of five standards between 1 mg/l and 5 mg/l fluoride (containing 50 % TISAB). One of these standards should be prepared as a 200 ml, so it can be used as a check as indicated in the next step. 4.6 Connect the F ion selective electrode to the Accumet XL 50 meter, and standardize according to the instructions in the instrument manual (p.96 98). This calibration should only need to be done once, but it is a good idea to check with a fresh aliquot of a standard mid way through the experiment, and again at the end. 4.7 Measure the fluoride concentration of each sample. (Remember method validation steps as well! A QC solution is available, and must be prepared according the instructions on the bottle) 4.8 When finished, rinse and dry the electrode, and store dry with the cap in place over the tip. 5 Treatment of Data Follow normal reporting guidelines, answering the following questions in your discussion. 1. Report your fluoride concentration in each of the toothpaste lots in the same units as on the toothpaste label.
71 2. How would you describe the lot to lot variability of the toothpaste sample you were measuring? Were the results from the different lots statistically different from each other? Were either of them statistically different from the value the company reports on its label? 3. Quantify the relative variability of the instrument and the variability of different samples prepared from the same lot. Compare the relative variance of each and comment on the results. 4. Discuss the reliability of your results. Use calculated numbers to back up your claims. Remember to include discussion of appropriate method validation results. Based on your findings, do you think this method is well suited to the analysis of fluoride in toothpaste? Can you suggest another technique that would work for this analysis? 5. Think about any sources of error you observed in the lab. What effect would each of these errors have on your final results? Do you think these errors contributed to any potential bias or imprecision that was measured?