How to measure the linear range of ion selective electrode performance

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1 Response range: The range of the straight part of the standard curve is the Nernst response range (generally 10-1 10-6 mol l), in this range, the straight slope of the monovalent ion should be: 57 61mV Pai;

2 The selectivity coefficient is similar to that of glass electrodes.

3 Response time The time it takes from electrode insertion to when the potential value stabilizes at 1mV.

4 Stability is expressed as a value of change in the potential of extension over time.

5 Internal resistance The internal resistance of the selective electrode is higher, generally 104 109. Use a potentiometer with high impedance.

6 Accuracy is expressed as the relative error of the analysis result and the error of the electromotive force measurement. When the measurement error is 1mV, the relative error of concentration that may be caused by monovalent ions is about 4%; Divalent ions are about 8%.

The basic structure of the ion-selective electrode (see color map) is shown in the figure. The sensitive membrane of the electrode is fixed at the top of the electrode tube, which is filled with an internal reference electrode (usually an AG AGCL electrode), and the function of the internal filling solution is to maintain the stability of the inner surface of the membrane and the potential of the internal reference electrode.

The ion-selective electrode is a half-cell (with the exception of the gas-sensitive electrode), and its potential cannot be measured alone, but must be combined with a timely external reference electrode to form a complete electrochemical cell, and then the electromotive force of the battery is measured, which includes the following:

e=em+en+e1-ew, where en is the potential of the internal reference electrode;

Ew and E1 are the potentials of the external electrode and the wetted potentials of the wetted parts.

In general measurements, all three of the above requirements are required to remain constant, so there is only one constant term between the electromotive force e and EM, and its change fully reflects the change of EM.

Ion-selective electrodes: (electrode structure, crystal electrodes (homogeneous crystal film electrodes (F, Ag2S), multiphase crystal film electrodes), flow carrier electrodes (K+, Ca2+), sensitized ion-selective electrodes).

The basic characteristics of ion-selective electrodes should be included in the main brigade (sensitivity, response range, selectivity coefficient, response time, stability, internal resistance, accuracy).

The construction of the ion-selective electrode mainly includes:

The electrode chamber is made of glass or high-polymer material.

The internal reference electrode is usually an AG agCl electrode.

The internal reference solution consists of a strong electrolyte solution of chloride and response ions.

Sensitive membrane Membrane with high selectivity in response to ions.

Classification of ion-selective electrodes In 1906, the phenomenon of glass film potential was discovered, and in 1929, a practical glass pH electrode was made; At the end of the 50th century of the last century, alkali metal glass electrodes were made; In 1965, the halide ion electrode was made; Subsequently, various electrodes with selective responses were rapidly developed. In 1976, IUPAC proposed that these electrodes be referred to as ion-selective ballaster electrodes (SIEs) and that they be classified in detail.

Ion-selective electrodes are also known as ion-selective electrodes. A class of electrochemical sensors that use membrane potential to determine ion activity or concentration in a solution. 1906 by R

Kremer was the first ion-selective electrode to measure the pH of the solution, which was first studied by Kremer and then made by Haber et al. in Germany, and by the end of the 60s, there were more than 20 kinds of ion-selective electrodes. Ion-selective electrodes have the ability to convert the activity of a particular ion in a solution into a potential that is linearly related to the logarithm of a given ion activity in a solution. The ion-selective electrode is a membrane electrode, the core component of which is the sensing membrane at the tip of the electrode.

According to the structure, it can be divided into solid membrane electrode, liquid membrane electrode and diaphragm electrode. The ion-selective electrode method is a branch of potentiometric analysis that is generally used for direct potentiometric methods and can also be used for potentiometric titration. The law is characterized by the grip of suspicion:

The activity of a particular ion in the solution is determined rather than the total concentration; It is easy and fast to use, and has a wide range of applications, especially suitable for the determination of alkali metals, nitrate ions, etc.; It is not affected by the color and turbidity of the test solution, and is especially suitable for continuous automatic monitoring and on-site analysis of water quality. At present, the late-burning ion-selective electrode method used for the determination of pH and fluoride ions has been set as the standard method, and more than 10 items in the automatic continuous monitoring system of water quality have adopted the ion-selective electrode method.

Ion-selective electrodes are a class of electrochemical sensors that use membrane potential to determine the activity or concentration of ions in a solution, and when it comes into contact with a solution containing the ions to be measured, a membrane potential directly related to the activity of the ions is generated at the phase interface between its sensitive membrane and the solution. Ion-selective electrodes, also known as membrane electrodes, have a special electrode membrane with a selective response to specific ions, and the relationship between the potential of the electrode membrane and the content of the ions to be measured conforms to the Nernst formula. This type of electrode is the most widely used indicator electrode for potentiometric analysis due to its good selectivity and short equilibration time.