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MindMap Gallery Potentiometric method and permanent stop titration method

Potentiometric method and permanent stop titration method

This is a mind map about potentiometric method and permanent stop titration method. Based on the pharmaceutical "Analytical Chemistry", the chief editors are Chai Yifeng and Di Xinlai. This mind map is suitable for postgraduate entrance examinations, final review, etc.

Edited at 2021-08-05 21:45:53

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Potentiometric method and permanent stop titration method

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Outline

Chapter 9 Potentiometric Method and Permanent Stop Titration Method

Overview of electrochemical analysis P124

potentiometric method

An analytical method based on measuring the electromotive force of the primary battery to determine the content of the substance to be tested

electrolysis

An analysis method to determine the content of the analyte based on the quantitative action of the analyte on the battery electrode when power is supplied.

conductivity method

An analytical method based on measuring the conductance or change in conductivity of a solution to determine the content to be measured

Voltammetry

Based on the current and potential change curve (voltammetry curve) during the electrolysis process, a method for qualitative and quantitative analysis of the analyte

Basic principles of electrochemistry P125

chemical battery

Primary cells and electrolytic cells

Phase boundary potential and metal electrode potential

Dynamic equilibrium is reached when the speed of metal ions entering the solution is equal to the speed of metal ions deposited on the metal surface. A stable double electric layer is formed at the interface between the metal and the solution to generate a potential difference, that is, the phase boundary potential or the metal electrode potential.

Junction potential and salt bridge

The potential difference produced at the contact interface between two electrolyte solutions with different compositions or the same composition but different concentrations is called the liquid junction potential.

Indicating and reference electrodes

Indicating electrode

metal-based electrode

Metal–Metal Ion Electrode

Metal – metal refractory salt electrode

Inert metal electrode

membrane electrode

An electrode that uses a solid membrane or a liquid membrane as a sensor to produce a selective response to a specific ion in the solution.

reference electrode

Saturated calomel electrode

Composed of metallic mercury, calomel (Hg₂Cl₂) and saturated KCl solution

Silver–silver chloride electrode

Direct potential method P128

Determination of solution pH

pH glass electrode

Electrode structure

It consists of reference electrode, internal reference electrode, glass membrane, highly insulated wires and electrode plugs, etc.

pH glass electrode response mechanism

Performance of pH glass electrode

Conversion factor(S)

Refers to the change in potential of the glass electrode caused by changing one pH unit

Alkali difference and acid difference

Alkali difference: When using a pH glass electrode to measure a solution with pH > 9, the measured pH will be low, resulting in a negative error.

Acid difference: When a pH glass electrode is used to measure a strong acid with pH <1 or a large salt concentration, the measured pH will be higher, resulting in a positive error.

asymmetric potential

When a outside = a inside, the φ film should be equal to zero, but in fact the φ film is not equal to zero, and there is still a potential difference of 1 to 3 mV.

internal resistance of electrode

Operating temperature

Glass electrodes should generally be used in the range of 0 to 50°C. If the temperature is too low, the internal resistance of the glass electrode increases; if the temperature is too high, the service life of the electrode is shortened.

Measurement principles and methods

Things to note when using a pH glass electrode to measure the pH of a solution

Ordinary pH glass electrodes are suitable for measuring pH in the range of 1 to 9

The pHs of the standard buffer solution should be as close as possible to the pHx of the solution to be tested

Glass electrodes need to be soaked in distilled water for more than 24 hours before use. After measurement, they should be washed thoroughly with distilled water. When not in use, they should be immersed in buffer solution or distilled water and stored.

The preparation, use and storage of standard buffer solutions should be carried out in strict accordance with regulations.

Since F⁻ corrodes the glass film, the glass electrode cannot be used for pH measurement of acidic solutions containing fluoride.

Composite pH electrode

The glass electrode and the reference electrode are combined to form a single electrode body, which is composed of two concentric tubes inside and outside.

It has the advantages of easy use, small size, sturdy and durable, and is conducive to pH measurement of small volume solutions.

Determination of other ion concentrations

Response mechanism and performance of ion-selective electrodes

Basic structure and response mechanism

Electrode performance

linear range

The curve obtained by plotting the electrode potential of the ion selective electrode against the negative logarithm of the response ion activity (concentration) is called the working curve.

Test line

Refers to the lowest concentration of ions to be measured that an ion-selective electrode can detect. The detection limit is one of the main performance indicators of the ion-selective electrode. The detection limit can be determined by the working curve.

selectivity coefficient

Refers to the ratio of the response capabilities of the same electrode to Y (interfering ions) and X (measured ions) ions under the same conditions, that is, the activity ratio of X ions and Y ions that provide the same potential response

Effective pH range

Refers to the pH range of the ion selective electrode. Large errors will occur if it exceeds this range.

Response time

Refers to the time required for the battery electromotive force to stabilize after the ion-selective electrode and the reference electrode are immersed in the ion solution to be measured.

Classification of ion selective electrodes and common electrodes

Original electrode

crystal electrode

Amorphous electrode

rigid matrix electrode

flow carrier electrode

gas sensing electrode

A gas sensor is a gas sensor that covers the original electrode sensitive film with a breathable film (which is hydrophobic and only allows gas to pass through, but does not allow ions in the solution to pass through) to separate the original electrode from the liquid to be tested. A solution of a certain composition (intermediate liquid) is filled between the original electrode and the original electrode.

enzyme electrode

The highly selective catalytic effect of enzymes in biochemical reactions is used to rapidly decompose or oxidize biological macromolecules. The products of the catalytic reaction can be detected by corresponding ion-selective electrodes.

How to measure ion concentration

The relationship between battery electromotive force and ion concentration

quantitative analysis

direct comparison method

Also known as double measurement method or standard control measurement method

working curve method

Use the reference substance of the ion to be measured to prepare a standard series of solutions (the matrix should be the same as the sample), and then, under the same measurement conditions, use the selected indicator electrode and reference electrode to measure the battery electromotive force from low to high concentrations.

standard addition method

Also known as the addition method or incremental method, the standard solution is added to the sample solution for measurement.

Measurement error of ion selective electrode

Electrode selectivity error

Electromotive force measurement error

Potentiometric titration P136

Principles and characteristics of potentiometric titration

Compared with the classic titration method, potentiometric titration has the following characteristics:

There is no subjectivity in determining the end point, there is no observation error, and the results are more accurate.

Can titrate colored liquids, turbid liquids and sample solutions without suitable indicators

Easy to achieve continuous, automatic and micro titration

It can be used for the determination of thermodynamic constants such as dissociation constants of weak acids or weak bases and stability constants of complexes.

Troublesome operation and time-consuming data processing

Determination of titration end point

Graphical method

E–V curve method

Arithmetic mean curve method of ΔE/ΔV–V

Δ²E/ΔV²–V curve method

Second-order derivative interpolation method

Types of Potentiometric Titration

Acid-base titration

precipitation titration

coordination titration

redox titration

Permanent stop titration method P138

Also known as double current or double ampere titration, it is a method of determining the titration end point based on changes in current during the titration process.

Judgment of I–V titration curve and end point

reversible couple titration irreversible couple

Irreversible Electric Couple Titration Reversible Electric Couple

Reversible Electron Couple Titration Reversible Electron Couple