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MindMap Gallery potentiometric analysis

potentiometric analysis

The method of analysis using the electrical and electrochemical properties of substances is called electrochemical analysis; ion selective electrode; the application of ion selective electrode.

Edited at 2021-11-09 18:23:05

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potentiometric analysis

Ion selective electrode

type

Original electrode

crystal membrane electrode

glass electrode

Selectivity

acid difference

Alkali difference or sodium difference

asymmetric potential

Reasons for selectivity

It is caused by subtle differences in sodium content, surface tension, and mechanical and chemical damage on the inner and outer surfaces of the glass film. It requires a long period of soaking to stabilize.

selectivity coefficient

Under the same measurement conditions, the ratio of the activity of the ion to be measured to the activity of the interfering ion when the ion to be measured and the interfering ion generate the same potential

The smaller the selectivity coefficient, the higher the electrode selectivity. It is not a constant. The measurement coefficient is different under different ion activities. It can only be used to estimate the measurement error when interfering ions exist or to determine the applicable range of the electrode.

Advantages and Disadvantages

advantage

① The generation of selective high membrane potential is not the gain or loss of electrons, and other ions cannot enter the crystal lattice to cause exchange. When the Na concentration in the solution is 1011 times higher than the H concentration, the two will produce the same potential;

② It is not affected by oxidants, reducing agents, colors, precipitations, colloids, and impurities in the solution, and is not easily poisoned;

③ By changing the composition of the glass membrane, a glass membrane electrode that responds to other cations can be made.

shortcoming

The internal resistance of the electrode is high, and the resistance changes with temperature

Fluoride ion selective electrode

Internal reference electrode Ag-AgCl electrode

Internal reference solution 0.1mol/LNaCl and 0.1mol/LNaF

Lanthanum fluoride crystal film

Features

Conductivity 2 Anti-interference 3 The size, shape and charge of ions determine whether they can enter the crystal membrane and are selective

Polycrystalline membrane electrode

Polycrystalline films are made from precipitated powders of insoluble salts under high pressure.

Amorphous membrane electrode

rigid matrix electrode

PH electrode

structure

1 internal reference electrode 2 internal reference solution 3 sensitive membrane 4 glass tube

movable carrier electrode

calcium electrode

principle

Didecyl phosphate can transfer calcium ions between the liquid film and the test solution until equilibrium is reached. Due to the difference in activity of calcium ions in the aqueous phase and in the organic phase, a phase boundary potential exists.

structure

The internal reference solution is an aqueous solution containing calcium ions. There is a solution of didecyl calcium phosphate and dioctyl phenylphosphonate between the inner and outer tubes, which is insoluble in water.

Sensitizing electrode

gas sensing electrode

Structural features: A layer of film or substance is covered on the original electrode to improve the selectivity of the electrode.

The gas of the component to be measured in the sample diffuses through the breathable membrane and enters the extremely thin liquid layer between the sensitive film and the breathable membrane, causing the ion activity in the liquid layer to change. The potential of the ion selective electrode membrane changes, so the battery electromotive force also changes. changes occur.

enzyme electrode

The special biologically active catalyst has strong selectivity for the reaction and high catalytic efficiency, allowing the reaction to proceed at normal temperature and pressure;

Common enzyme catalytic products that can be detected by existing ion-selective electrodes: CO2, NH3, NH4, CN, F, S-, I, NO2

tissue electrode

definition

Use animal and plant tissues as sensitive membranes;

Features

a. Rich in sources, many tissues contain large amounts of enzymes;

b. Stable in nature, the enzymes in tissue cells are in a natural state and can exert better effects;

c. Strong specificity;

d.Shorter life;

e. It is easy and economical to make, and the biological tissue has certain mechanical properties.

Factors affecting measurement

temperature

Measurement of electromotive force

Relative error=4%n△E

interfering ions

Solution pH

The concentration of the ion being measured

Response time

Related to the establishment of membrane potential

The stability of the reference electrode is related to

hysteresis effect

How to measure ion activity

standard curve method

Logarithm of ion concentration versus electromotive force plot

●Requirement: The ionic strength of the standard solution and the test solution should be consistent

●Simple and suitable for measurement of large batches of samples

standard addition method

Suitable for samples with complex composition and small number of measurement parts

Require

■c. ,V,,V,must be measured accurately

■General requirements c, ≥100c,, V.<50V, (after adding the standard solution, the ionic strength of the test solution has no significant change) △E is between 15~ -40mV. The measurement accuracy is high.

Features added

Small volume, high concentration

Applications of Ion Selective Electrodes

potentiometric titration

(1) E-V curve method

(2) AE/AV-V curve method

(3) △2E/AV2_ V curve method

Acid-base titration

redox titration

precipitation titration

coordination titration

Conditions for use of fluoride ion selective electrodes

Need to be used between pH 5~6

When the pH is high: OH- in the solution exchanges with F- in the lanthanum fluoride crystal film, La (OH) is formed on the crystal surface, and F is released, interfering with the measurement;

When the pH is low: F in the solution generates HF or HF2, reducing the activity of F.

n Interference and elimination: AP, Ca2, Mg2 plasma can form stable complexes (or insoluble compounds) with F-, which can be treated by adding the masking agent sodium citrate.

Total ionic strength adjustment buffer solution (TISAB)

In potentiometric analysis, the method of adding TISAB is usually used to control the total ionic strength, pH value, and mask interfering ions of the solution.

TISAB generally consists of ionic strength regulator, masking agent and buffer solution.

■For example: TISAB used to measure fluoride ions in the sample consists of sodium chloride, sodium citrate and HAc-NaAc buffer.

■Sodium chloride is used to fix the ionic strength of the solution

■Sodium citrate is used to mask interfering ions such as Fe3 and Al3

For HAc-NaAc buffer solution, the pH of the tested solution is controlled at around 5.0~6.0.

Potential

membrane potential

The ion diffusion rate or activity is different, there is no compulsion and selectivity, and the diffusion potential exists not only at the liquid-liquid interface, but also within the solid membrane

It is caused by forced diffusion and selectivity, resulting in uneven charge distribution at the two interfaces, resulting in an electric double layer structure and a potential difference.

electrode

Classification

Metal, non-metal, crystal, amorphous, glass film

Electrode reaction

reference electrode

With standard electrode potential

hydrogen standard electrode

Standard calomel electrode SCE

Silver-silver chloride electrode

Category Zero Electrode

Indicating electrode

No electrode reaction

Metal-metal refractory salt electrode

Metal-metal ion electrode

working electrode

There is an electrode reaction

Analysis principle

Nernst equation

subtopic

Classification

direct analysis

Potential analysis

conductivity analysis

Coulomb analysis

Voltammetric analysis

capacitance analysis method

Potentiometric titration

Amperometric titration

conductometric titration

electrolytic analysis

electrogravimetric analysis

definition

The method of analysis using the electrical and electrochemical properties of substances is called electrochemical analysis.