Potentiometric Analysis: Electrodes, Methods, and Factors

Its function is to complete the measurement circuit, providing a passage from the conductivity-sensing electrode, through the solution, to the reading device. The four parts form an electrical circuit.

Types of Electrodes

1. Hydrogen Electrode
2. Calomel Electrode: Consists of a mixture of mercury and mercurous chloride in contact with a potassium chloride solution.
3. Silver Chloride Electrode
4. Glass Electrode:
   • Parts: Lead-plated, resin-filled rod, internal electrode, sensitive glass.
5. Glass Electrode and Calomel: External calomel reference, two different cells connected in series.

Analysis Methodology

To analyze a substance using potentiometric techniques (anions, cations, organic and inorganic), the substance, or some of its possible complexes, must be electroactive within the working range limited by the medium and working electrode used.

Process

1. Sample Preparation: Samples must be in solution, although some applications exist for solid and gaseous states.
2. Addition of Supporting Electrolyte: Provides electrical conductivity and can adjust the pH or form a complex with the analyte.
3. Technique Selection: Choose the technique and its parameters as a function of the substance's concentration.
4. Quantization: Compare the sample's response with respect to a standard (data vs. benchmarks).

Calibration Curves (Quantization)

Calibration curves graphically represent the electrode potential (in millivolts) versus the logarithm of known solution concentrations. Electrodes respond logarithmically to concentration. For very dilute solutions, the standard addition method is typically used. Known concentrations of the substance are added to the sample, and the potential is measured. The concentration of the original sample is determined by the x-intercept of the extrapolated calibration curve.

Analysis Parameters

• Temperature: The Nernst equation shows a direct and proportional relationship with temperature. Calibration lines at different temperatures intersect at a single point. A fundamental rule is that all analyses should be performed at the same temperature.
• Contamination: Certain substances can form insoluble deposits on the electrode surface. Oil, grease, and proteins are examples. Clean the electrode with a hydrochloric or nitric acid solution for one hour.
• Aging - Reference Electrode Potential: Its function is to provide a constant potential (E_o) against which changes in sensitive electrodes will be measured. The correct choice of internal electrolyte is very important.
• Ionic Strength and pH of the Sample: Hydroxide and hydrogen ions can react with the species of interest and decrease the amount of free ions.
• Interferences: Interference is any substance in a solution that can alter the potential measured by a selective electrode.