Question

Lab Experiment: Fluoride Speciation and Analysis by Flow Injection using Ion-Selective Electrode.

Q1) what is the Nernst equation for the Fluoride Ion Selective Electrode (ISE)? How does this show that measured potential is proportional to Fluoride ion concentration?

2Q) What is a common cause of error when using a reference electrode?

3Q) IS the pH of solutions important when using the Fluoride ISE? If so, why?

Answer 1

Q1) The electrode potential is related to the logarithm of the concentration of the measured ion by the Nernst equation:

where n is the ion charge (negative for anions). The factor 2.303 RT/F has a theoretical value of 59 mV at 25 °C. The equation is only valid for very dilute solutions or for solutions where the ionic strength is constant.

Q2) This cell acts as a concentration cell with transference where the fluoride transport number is 1. As transference of charge through the crystal is almost exclusively due to fluoride, the electrode is highly specific to fluoride. The only ion which significantly interferes is hydroxide (OH⁻). Generally such "alkaline error" can be avoided by buffering the sample to a pH below 7.

Q3) Yes, when you are using this electrode you must use a buffer, for example, TISAB buffer. Remember, the only interference on the fluoride electrode is from the hydroxyl ion (OH-) with a selectivity coefficient of about 0.1. This is eliminated by ensuring that the pH is kept below 8. The other function of the TISAB buffer, apart from equalizing the ionic strengths, is as a de-complexing agent to release any fluoride ions which may be bound up in complex molecules. Note low pH range (4 to 8) for this electrode.