Why hydroxide interferes with fluoride selective electrodes

The membrane should also have low electronic conductivity. Single crystal LaF 3 is widely used to determine F -. The crystal is usually doped with europium to improve the conductivity. At each membrane-solution interface, the following equilibrium takes place:.

The equilibrium will be shifted to the right for the solution with a smaller F - concentration, and the potential will become more positive relative to the other side of the membrane. It is this potential difference across the LaF 3 crystal membrane that is measured and related to F - concentration.

The fluoride electrode is extremely selective for F - but can experience interference from OH - above pH 8. This electrode is used in one of the experiments listed at the end of this learning module. Click here to learn more. One of the most famous liquid membrane electrodes has been used for calcium determination. When this was determined to be unfeasible, liquid membranes were developed. This electrode works by an ion-exchange process.

The cation-exchanger is an aliphatic diester of phosphoric acid, RO 2 PO 2 - , where each R group is an aliphatic hydrocarbon chain containing between 8 and 16 carbons. The cation exchanger is dissolved in an organic solvent and held in a porous compartment between the analyte solution and internal reference calcium chloride solution.

Figure 6. Calcium dialkyl phosphate complex. Calcium ISEs are commonly used to measure calcium ion activity in biological fluids, as calcium ion is important in many physiological processes, such as bone formation. Ionophores, or chelating agents, that selectively complex ions include crown ethers and the antibiotic valinomycin see Figure 4. The important feature of the neutral carrier molecule is its cavity which has dimensions approximately that of a molecule or ion.

The valinomycin electrode was one of the first polymer membrane electrodes and is routinely used to determine potassium. The outer lipophilic part of the valinomycin molecule allows it to remain in the polymeric membrane.

In the United States alone, nearly million measurements are made annually of blood potassium levels using this electrode.

For all of the ISEs described above, the same equations can be used to predict the relationship between potential and analyte activity A or concentration. The potential measured, E meas , is the potential difference between the analyte E outer side of the membrane and the reference E inner side of the membrane:.

The potential of each side is related to activity A or concentration as described by the Nernst equation, where z is the charge of the ion of interest:. Since A ref and E o are both constant, E inner equation 3 is constant. If equations 2 and 3 are plugged into equation 1, they combine to give the following:. You can see that ISEs should exhibit a Nernstian response, as you previously learned for the direct indicator electrodes.

You have learned that one of the most important analytical characteristics of ISEs is their selectivity; i. In reality, ion-selective electrodes can experience interferences by responding to the presence of other ions.

Equation 4 assumes that all of the electrode response E meas is due to one ion. We will call the interfering ion j with corresponding activity A j and charge z j. This new equation is called the Nikolskii-Eisenman equation:. The selectivity coefficient is a numerical measure of how well the membrane can discriminate against the interfering ion.

As you can see from the equation, the smaller the k ij values, the less impact the interfering ion will have on the measured potential. When k ij values are less than 1, the ISE is more responsive to the analyte ion and when k ij values are greater than 1, the ISE is more responsive to the interfering ion. For example, a k ij value of 0. Selectivity coefficients can be experimentally determined. Selectivity coefficients for some of the electrodes previously discussed are listed below.

For some help with this problem, open up the following spreadsheet. Make sure that you are on the F - tab. For further reading on ISEs, refer to references 8 through 13 and the following online resources:. ISE products from GlobalSpec an engineering search engine with free registration. Ion-exchange process.

Figure 3. Development of a potential at an ISE. The Membrane By now you have learned that the identity of the membrane determines the selectivity of the electrode. There are three main types of membranes.

The electrodes are classified by the membrane material. The membrane is composed of a silicate glass. Calibrate the electrode as follows. Prepare a series of fluoride standards using the 0. Measurement procedure: Set up the pF meter by connecting the fluoride selective and reference electrodes to the plugs at the rear of the instrument. Readings can be taken in either potential Volts or pH.

Readings will be calibrated to actual solution fluoride concentrations. To prepare for measurement, place Place beaker on magnetic stirrer and mix at medium speed. Immerse electrodes in the stirred solution. When taking measurements, electrodes must remain in solution for at least 3 minutes and until the potential readings have stopped drifting.

Readings for higher concentrations take a shorter time to come to equilibrium. Calibration of potentiometer: A Measure the fluoride electrode response to the series of standard solutions prepared above. Perform these measurements in triplicate. Rinse the electrodes with distilled water and wipe with a tissue between each measurement. B Calculate the mean and standard deviation for each standard.

Using either semi-logarithm paper, or a spread sheet program, make a standard working curve by plotting fluoride concentration on the logarithm scale and the potentiometer reading on the linear scale. Draw error bars for each point. Error bars may be drawn across s , 1. Tap water measurement: Measure in five times each the meter reading for distilled water, tap water, and the standard solution that comes closest to the tap water reading.

Rotate the measurements so that one reading of each of the three solutions is taken before repeating the sequence. This procedure is facilitated by placing the three solutions blank, sample, and standard in separate mL beakers. Be sure to rinse the electrodes off with distilled water and dry with a tissue between readings. Natural water measurement: Repeat the measurement procedure for the tap water sample using the natural water sample s taken during the laboratory quarter.