Henry SuFebruary 20, 2003
Lab Experiment #2CHE 331
2-5-IIDr. Rahni
Evaluation of a Specific Ion Electrode
Purpose
The purpose of this laboratory experiment is to describe the analytical properties of a specific-ion electrode and illustrate its analytical utility.
Theory
The development of ion-selective electrodes has resulted in the recent revitalization of analytical potentiometry. Over two-dozen ion-selective electrodes are now currently available and are used widely in science and industry. Almost all of the electrodes fall into three categories: glass electrodes, solid-state or precipitate electrodes, and liquid-liquid membrane electrodes. Electrodes in these categories respond as a result of an ion exchange process. However, the ion exchange process is not fully understood. These electrodes also yield potentials that can be adequately described by the classical Nernst equation or one of its expanded modifications.
A solid state electrode usually contains a silver-silver chloride reference electrode, an internal filling solution, and a synthetic single crystal membrane (see fig.1). Because the following experiment deals with the fluoride-ion electrode of the solid state, the theory discussed will be based on this electrode.
The selectivity of an electrode is determined by the composition of the membrane. The fluoride-ion electrode has a crystalline lanthanum fluoride membrane, doped with Eu+2 to lower its electrical resistance (50,000Ω vs. 100,000Ω resistance for glass membranes), thus improving conductivity. The electrode has excellent selectivity, OH- being the main interference(selectivity constant = 0.1). Pure rare earth fluorides have a high electrical conductance because fluoride ions are mobile in the crystalline lattice. He crystal membrane is permeable only to fluoride ions, so the cell potential(E) can be determined by the Nernst equation:
E = E’ + RT/F ln a F-
where the value of E’ depends on the choice of internal and external reference electrodes and the fluoride activity in the internal electrode solution. E’ is constant and independent of the composition of the sample except for a small liquid-liquid junction potential contribution.
Ideally, the electrode will be selective for fluoride ions over ranges in sample composition where the crystalline lanthanum fluoride membrane is stable. Because crystalline lanthanum fluoride has a low solubility(Ksp = 10-29), the electrode can be used over a wide pH range. It can also be used in the presence of many complexing agents.
There are many problems that can occur to produce errors in measurement when using ISE. These include clogged junctions, breakage, gel layer, organic solvents, protein solutions, errors in wiping electrodes, paints, dyes, and suspended solids, large potassium chloride crystals, low slope, and having a dried electrode. Solutions of these problems are listed as follows:
Clogged Junctions
Increasing response time and drifting are symptomatic of a clogged junction, often as a result of precipitated silver chloride (AgCl). Eighty percent of pH/ISE measurements are in error due to this one problem, especially when using an Ag/AgCl reference system. Soak these reference electrodes in saturated potassium chloride (KCl; 4.07 M) in which any precipitated AgCl is 500X more soluble than in distilled water. Heating a clogged junction in saturated KCl at 60°C for 30 minutes may help. Calomel electrodes are designed to use saturated KCl (without AgCl) and hence do not clog from within. Corning has designed a replaceable Junction (477269) for cases where blocked junctions cannot be cleared.
Breakage
Nine of the new Corning electrodes have a see-through plastic body to minimize breakage. Furthermore, the new protective guard feature protects the glass pH bulb and can be removed for easy rinsing. Replace wetting caps between measurements. DO NOT place electrodes in boiling water - they will crack.
Gel Layer
Longer response times will occur when frequent measurements are made in alkaline and phosphate samples that cause the gel layer to slowly deteriorate. Storage in 0.1 M KCl will avoid this problem. Avoid water absorbing solvents that dehydrate the gel layer (see No. 4 below).
Organic Solvents
Non-aqueous solvents have a tendency to dry out the glass of the pH electrode surface. Storage or soaking in a pH 7.00 buffer overnight will rehydrate the glass surface. Test solutions should be more than 50% water. Remove the electrode once the reading is stable.
Proteins Solutions
Longer response times will also occur when frequent measurements of made of proteinaceous solutions. Other symptoms include low slopes. A soaking in solution of 10% pepsin diluted dropwise with a 1.0 M hydrochloric acid (HCl) solution to a pH of 1-2 will remove this coating from an electrode in a few hours. Apply soaking solution as needed.
Wiping Electrodes
Wiping electrodes with paper will result in static charges building up on the surface of their glass bodies. This will require time to remove these charges when placed in samples and will result in fluctuating or erroneous readings. This problem is especially common in dry areas during winter.
Paint, Dyes, Suspended Solids
Rinse a dirty electrode alternately with a solvent for the contaminating material, then distilled water. A reverse sleeve electrode will produce a faster flowing junction and prevent clogging by syrupy liquids.
Large Potassium Chloride (KCl) Crystals
Build-up of KCl crystals in a calomel electrode indicates the fill solution needs to be changed. These crystals can block the flow through the ceramic junction. Eliminate the crystals by replacing the fill solution. Draw off old fill solution with a syringe or capillary pipette. Rinse twice with warm deionized water and once with saturated KCl, then fill with fresh saturated KCl.
Low Slope
Etch the electrode with a solution composed of 2.0 g potassium fluoride (KF) in 98 mL H20 to which 2.0 mL concentrated H2SO4 has been added. Store this solution in a plastic bottle and mark "hazardous". Wear gloves when etching. To etch, place the electrode in the etching solution for only 20-30 seconds. Rinse electrode with deionized water and soak in 7.0 buffer overnight.
Dried Electrode
A dry electrode is a dead electrode. Place the electrode tip into a wetting cap that is full of 4 M KCl. Rehydrate glass electrodes for 24 to 48 hours in buffer 4.01 after prolonged dry storage.
Apparatus
The following materials will be needed in order to perform this experiment:
Material / QuantitypH Meter / 2 (one must be suitable for use with ISE)
Glass-calomel pH electrode / 1
Fluoride-ion electrode / 1
Beakers (150 ml) / 14
Beakers (250 ml) / 4
Volumetric Flask (100 ml) / 14
Volumetric Pipet (25 ml) / 8
Pasteur Pipet / 2
Magnetic Stirrer with Teflon-coated Stirring Bar / 1
Reagents
Solution / Concentration(s)Sodium Fluoride / 100ppm; 50ppm; 25ppm; 15ppm; 10ppm; 5ppm; and 1ppm
Deionized Water / --
Water from Tap / --
NaBr / 0.1M
NaCl / 0.1M
Standard Operating Procedure for ISE Electrodes
Electrode preconditioning - The ion selective electrodes (ISE) are ready for use immediately upon removal from the shelf. Soaking prior to use is unnecessary and possibly harmful. The electrodes may be stored dry. The reference electrodes, on the other hand, should be soaked prior to use.
Stirring - The response time of an electrode is generally enhanced by moderate stirring of the sample solution. However, streaming potentials at the reference electrode's liquid junction become apparent when vigorous stirring is employed. The potential observed for stirred samples usually differs slightly from that for unstirred samples.
Electrode handling between samples - The electrode pair (the ISE and the reference electrode) may be rinsed with distilled water and blotted gently to prevent cross contamination between samples. Rinsing the electrode pair with the next solution is preferable, since it avoids the necessity of wiping the electrode, thereby eliminating the possibility of scratching the sensing element surface.
Storage - The halide electrodes can be stored dry without affecting their response. The electrodes should be cleaned before storage to insure that residues of dilute solutions containing interfering ions do not become concentrated by evaporation on the electrode sensing element.
Procedure
1-Transfer about 25 ml of 1.00e-5M fluoride stock solution into a 150 ml beaker
2-Carefully insert the pH electrode system and the F- ion electrode system into the solution and adjust the pH to within 0.2 pH units of the desired value by adding a small amount of HNO3 or NaOH from a Pasteur pipet.
3-Measure the potential of the F- ion electrode
4-Repeat the procedure at approximately 1 pH intervals from 3 to 11
5-Repeat the operation with the 1.00e-3 M and 1.00e-1M fluoride stock solutions
6-To 8 125 ml beakers, transfer about 25 ml of the fluoride stock solutions ranging from 1.00e-1M to about 1.00e-8 M F- in decade units
7-Measure the fluoride activity of these solutions
8-Prepare ~1.0e-2 M lanthanum nitrate solution by weighing out 8.7 g lanthanum nitrate and dissolving in about 200 ml water
9-Standardize this solution using a 25 ml aliquot of the 1.00e-2 M fluoride stock solution diluted with 25 ml of water in a 125 ml beaker
10-Take readings at 1 ml intervals prior to the end point region and 0.1-0.2 ml intervals at the end point region.
11-Titrate unknown fluoride solution using the same procedure.
Data and Calculations
See accompanying Excel data sheets.
Conclusion:
Upon completion of the outlined procedure, E˚ measured was calculated for a series of fluoride ion stock solutions and dilutions. This data was then used to generate graphs describing E˚ measured vs. log of fluoride ion concentration. Unknowns for the laboratory experiment were also investigated, although impurities were found in solution.