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Potentiometric analysis and permanent stop titration
This is a mind map about potentiometric analysis and permanent-stop titration, including electrochemical analysis, potentiometric analysis, potentiometric titration, permanent-stop titration, etc.
Bearbeitet um 2022-11-04 00:59:36
Potentiometric analysis and permanent stop titration
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Potentiometric analysis and permanent stop titration
electrochemical analysis
Definition: A method of analyzing substances using electrochemical principles and techniques
Advantages: good selectivity, fast analysis speed, less sample consumption, suitable for micro-analysis, and has relatively good sensitivity, accuracy, and repeatability.
Depending on the measured electrical signal
potentiometric method
Based on measuring the electromotive force between two electrodes, quantitative analysis of the object to be measured is carried out
Classification
direct potentiometry
Calculate the content of the object to be measured directly based on the measured value of electromotive force
potentiometric titration
Determine the titration end point based on the sudden change in electromotive force during the titration process
Voltammetry
Definition: Analysis based on the current-voltage curve obtained during the electrolysis process
Classification
Polarography
The microelectrode is a mercury dropping electrode
dissolution method
Under a certain constant voltage, the substance to be measured is first precipitated from the electrode, and then the precipitate is dissolved, and the analysis is carried out according to the current-voltage or current-time curve during dissolution.
Amperometric titration
Determine the titration end point based on the change in current during the titration process at a fixed voltage
conductivity method
Definition: An analytical method based on measuring the conductance of a solution to determine the content of the substance to be measured
Classification
direct conductivity analysis
Directly determine the content of the analyte based on the measured conductivity data
conductometric titration
Determine the titration endpoint based on the change in solution conductance during the titration process
electrogravimetric analysis
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
Coulomb method
The amount of electricity consumed during electrolysis of the object to be tested
coulometric titration
The solution formed by the product of the electrode reaction is used as the titrant, which interacts with the substance to be measured in the solution, and the content of the substance to be measured is determined based on the power consumed at the titration end point.
potentiometric analysis
chemical battery
An electrochemical reactor usually consists of two electrodes and an electrolyte solution. The electrochemical reaction is an oxidation-reduction reaction that occurs at the interface between the electrodes and the electrolyte solution.
According to electrolyte type
Aneroid junction battery
Two electrodes inserted into the same solution
Battery with liquid junction
It consists of two electrodes inserted in two solutions with different compositions but that can be connected to each other.
The two solutions are usually separated by some kind of porous material membrane, or a salt bridge device is used to connect the two solutions.
Purpose: To prevent the two solutions from mixing and provide necessary channels for ion migration when electricity is applied.
Potential analysis method mainly uses liquid junction cells
Depending on whether the electrode reaction can proceed spontaneously
Primary battery
Electrode reaction proceeds spontaneously
chemical energy converted into electrical energy
Cell
The electrode reaction cannot proceed spontaneously, and a certain external voltage needs to be applied to the two electrodes.
electrical energy converted into chemical energy
battery electromotive force
The algebraic sum of the potential differences at the phase boundaries that make up a chemical battery
The electromotive force is positive: primary battery
Electromotive force is negative: electrolytic cell
E battery = φ positive - φ negative
Potential
phase boundary potential
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.
liquid junction potential
The slight potential difference that exists at the contact interface between two electrolyte solutions with different compositions or the same composition and different concentrations.
electrode
subtopic
Classification according to the function of electrodes
Indicating electrode
During the electrochemical measurement process, the electrode potential changes with the activity of the ions to be measured in the solution, and can reflect the activity of the ions to be measured.
glass electrode, ion selective electrode
reference electrode
During the measurement process, the potential of the electrode is not affected by changes in the composition of the solution, and the potential value of the electrode is fixed.
Saturated calomel electrode
Composed of metallic mercury, calomel, and potassium chloride solutions
Silver-silver chloride electrode
It consists of a silver wire coated with a layer of silver chloride inserted into a potassium chloride solution of a certain concentration.
It has a simple structure and can be made into a small volume. It is often used as an internal reference electrode for glass electrodes and other ion-selective electrodes.
Reversible electrodes and batteries
Reversible electrode reaction: When an infinitesimal electric flux passes through the electrode in the opposite direction, the electrode reaction proceeds at the electrode equilibrium potential, and the electrode reactions that occur are reverse reactions of each other.
Reversible electrode: The electrode reaction of the electrode is reversible and the reaction speed is very fast.
Reversible battery: The two electrodes that make up the battery are both reversible electrodes
potentiometric titration
Concept: During the titration process of titrating a solution to be measured with a standard solution, a titration analysis method is used to determine the end point of the titration by monitoring the potential change of the indicating electrode of the subject to be measured.
Determination of end point of potentiometric titration
E-V curve method
S-curve
Titration end point: turning point of the curve (inflection point)
Requirement: The potential jump at the titration stoichiometric point is obvious
△E/△V-V curve method
peak curve
Titration end point: the highest point of the peak curve
△²E/△V²-V curve method
Curve with two extreme values
Titration end point: the intersection point of the curve and the zero line of the ordinate
Compared to indicator titration
It is objective and reliable, has high accuracy, is easy to automate, and is not limited by the color and turbidity of the solution.
Applications and Examples
Acid-base titration
precipitation titration
complex titration
permanent stop titration
Concept: Also known as double current or double ampere titration, during measurement, two identical indicator electrodes (usually platinum electrodes) are inserted into the solution to be titrated to form an electrolytic cell, and a small voltage (about a few seconds) is applied between the two electrodes. Ten millivolts and then titrate.
End point determination: During the titration process, determine the end point of the titration based on recording the i-V relationship curve of the current change versus the volume of the titrant, or observing the mutation point of the current change.
This method is an analytical method in amperometric titration.
Advantages: It has the advantages of simple device, accurate analysis results and easy operation.
Principle: The permanent stop titration method uses the characteristics of the reversibility of the electrical couple between the substance to be measured and the titrant to act on the current to determine the arrival of the titration end point.
Glossary
reversible electric pair
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
Irreversible electric pair
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.
The i-V relationship curve mainly has the following three different situations:
The titrant is a reversible couple, and the measured object is an irreversible couple.
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.
The titrant is an irreversible couple and the analyte is a reversible couple.
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.
Both the titrant and the titrant are reversible pairs
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.
Applications and Examples
NaNO2 titration method
Determination of trace moisture by Karl Fischer titration