Based on measuring the electromotive force between two electrodes, quantitative analysis of the object to be measured is carried out

Classification

Definition: Analysis based on the current-voltage curve obtained during the electrolysis process

Classification

Definition: An analytical method based on measuring the conductance of a solution to determine the content of the substance to be measured

Classification

Definition: During the electrolysis process, the substance to be measured is quantitatively deposited on the electrode, and the substance to be measured is measured based on the increase in the mass of the electrode.

Classification

Aneroid junction battery

Battery with liquid junction

Primary battery

Cell

The electromotive force is positive: primary battery

Electromotive force is negative: electrolytic cell

The potential difference at the interface between two different objects

Metal electrode potential: The phase boundary potential between the metal and the solution generated when a metal electrode is inserted into an electrolyte solution containing the metal.

The slight potential difference that exists at the contact interface between two electrolyte solutions with different compositions or the same composition and different concentrations.

Indicating electrode

reference electrode

S-curve

Titration end point: turning point of the curve (inflection point)

Requirement: The potential jump at the titration stoichiometric point is obvious

peak curve

Titration end point: the highest point of the peak curve

Curve with two extreme values

Titration end point: the intersection point of the curve and the zero line of the ordinate

When two electrodes react at the same time, current will pass between them, which is called a reversible electric pair.

When the titration reaches the half-titration point (half of the titration is completed), that is, when the concentrations of the oxidized and reduced forms of the substance being dropped are equal stoichiometry, the current passing through is the largest; when the concentrations of the oxidized form and the reduced form are unequal, the current Determined by the concentration of oxidized (or reduced) substances with small concentrations

If the oxidized and reduced solutions of a certain electric pair do not undergo electrolysis under the above conditions and no current passes through the battery, this material is electrically symmetrical and irreversible.

Electrolysis occurs only when a large external voltage is applied, which is due to the occurrence of other types of electrode reactions.

Before the titration end point, there are only irreversible electric pairs in the solution. Although there is an external voltage, no electrode reaction can occur on the electrode. Although there is product I- in the solution, the I2 concentration is always very low, and no obvious electrolysis reaction will occur. Therefore, the current The meter pointer has been stationary at a position close to zero current. Once the titration end point (stoichiometric point) is reached and a slight excess of iodine element is added, an obvious I2/I-reversible electric pair is established in the solution, the electrolysis reaction proceeds, and the generated electrolysis current deflects the ammeter pointer and stops it. Returning to the zero current position, with the addition of excess I2, the deflection angle of the ammeter pointer increases, and the turning point of the curve is the end point of the titration.

This is the case when titrating dilute iodine I2 solution with sodium thiosulfate From the beginning of the titration to the stoichiometric point, there is an I2/I-reversible electric pair in the solution, and electrolytic current flows through the battery.

The size of the current depends on the concentration of the titration product in the solution [I-]. [I-] changes from small to large, and the electrolysis current also changes from small to large. The current is maximum at the half-titration point. After crossing the half-titration point, the size of the current changes to Depending on the concentration of 2 remaining in the solution, [I2] gradually becomes smaller, and the electrolysis current also gradually becomes smaller. At the stoichiometric point, the concentration of I2 approaches 0, and the electrolysis current also approaches 0.

After the stoichiometric point, although there is an irreversible titrant couple in the solution, there is no obvious electrolysis reaction. Therefore, after crossing the stoichiometric point, the current will stay near zero current and remain motionless.

This type of titration method determines the end point of the titration based on the phenomenon that the current drops to 0 and stays in place during the titration process.

Cerium ion titration ferrous ion

The current drops to the lowest point at the end of the titration

After the end point, with the addition of Ce4, there is an excess of Ce4, and a Ce4/Ce3 reversible electric pair is established in the solution. A current passes through the electrolytic cell, and the current begins to rise. With the addition of excess Ce4, the angle of the ammeter pointer increases.