Is coulometric titration the same as amperometric titration?

This article describes the differences between coulometric titration and volumetric titration and gives some advice on when to use each titration method, the principle of measurement is based on the Bunsen reaction between iodine (I2) and sulfur dioxide (SO2), using primary alcohols as solvents and alkalis as buffers, in principle, general chemical reactions occur: RoH + SO2 + 3 RN + I2 + H2O (RNH) SO4 R + 2 (RNH) I In the above reaction, the alcohol reacts with SO2 and forms intermediate salts, It is then oxidized by the present iodine and the resulting water is consumed during this oxidation reaction.

How is moisture measured?

In alcoholic solvents, the ratio of water to iodine is 1:1 and in non-alcoholic solvents is 2:1, once the water has been completely consumed, the excess iodine is calculated by the indicator electrode in the Karl Fischer titrator (also known as the micromoisture analyzer), indicating the end point of the titration, and then, based on the concentration of iodine in the titrant and the amount of reagent consumed in the titration, the amount of water present in the sample is calculated.

In general, the recommended pH range for Karl Fischer titration is to 8 to achieve the maximum reaction rate, and for pH values outside of this range, the titration rate is either significantly slower (values less than 4) or results in the disappearance of the endpoint (values greater than 8), so samples outside the optimal range should be buffered to reach the total pH within the recommended range.

Types of Karl Fischer assays: Coulometric vs. volumetric titration.

There are two techniques for determining water content using the Karl Fischer titrator's method:

1.Volume Karl Fischer titration.

Water concentration: 100ppm to 100

Iodine is added using a burette during the titration process.

2.Coulomb Karl Fischer analysis.

Water concentration: 1 ppm to 5

Iodine is produced electrochemically.

Karl Fischer titration.

Iodine is added to the sample with a solvent in which the water is measured by consuming the Karl Fischer reagent, and there are two types of volumetric Karl Fischer systems:

One ingredient reagent.

**Cheap and easy to handle, but the titer is less stable and the titration speed is slower.

Two-component reagents.

It has long stability and faster titration times. However, there is a solvent capacity limit.

Coulometric Karl Fischer titration.

During titration, iodine is produced electrochemically. And by measuring the total charge transferred (1 mg H2O = coulombs). The advantages of coulombs include:

Reduce reagent costs.

Reduce downtime and maintenance costs.

The titer is more stable.

More suitable for automation.

Respondent: Huatian Power.

The titrant of the coulometric titration method is not added by the burette but is produced by an electrolysis reaction. Compared with the common volumetric analysis method, the coulometric titration method is easy to operate, especially suitable for some volatile and unstable titrants (such as halogens, ti( )cu( ), etc.), and has high accuracy and precision.

Coulometric titration can be used to measure both constant and trace substances.

The titrant used in the coulometric titration method is generated by electrolysis, and the titration is generated at the same time, and there is no problem of instability of the test (and) agent in the volume titration process, which expands the application range of volume analysis.

Coulometric titration generally does not require a reference material and is an absolute analytical method.

To put it simply, coulometric titration is reversed, and classical titration is forward. Take water titration as an example.

The titrant (mainly iodine, the active substance), can be added directly to the sample to be measured by a burette (volumetric method) or produced by electrochemical electrolysis in a titration cup (electrometric coulometric method).

Karl Fischer coulometric titrators are mainly used for the determination of water content with very low water content, such as less than 50-100 ppm according to the Karl Fischer method.

The biggest difference between the volumetric method and the coulometric method is that i2 in the volumetric method comes from the titrant, while i2 in the coulometric method is generated by electrolysis of an electrolyte containing i-ions. The speed of electrolysis is limited, so when the water content of the sample to be measured is relatively low, the Karl Fischer coulometric titrator is not only fast, but also the data are well paralleled. The amount of electricity passing through the cell is strictly quantified with the amount of I (iodine), so the Coulomb method has a higher measurement accuracy.

Therefore, in terms of application, the volumetric method is more suitable for the measurement of samples with high water content, while the Coulomb rule is only suitable for the determination of trace and trace amounts of water.

The titrant (mainly iodine, the active substance), can be added directly to the sample to be measured by a burette (volumetric method) or produced by electrochemical electrolysis in a titration cup (electrometric coulometric method).

Karl Fischer coulometric titrators are mainly used for the determination of water content with very low water content, such as less than 50-100 ppm according to the Karl Fischer method.

Coulometric titration is suitable for micro and trace analysis;

The titrant for coulometric titration is electrogenic and can be an unstable substance;

Coulometric titration is calculated with the required amount of electricity and does not require the titrant to be calibrated with standards.

Respondent: Huatian Power.

To put it simply, coulometric titration is reversed, and classical titration is forward. Taking water titration as an example, Karl Fischer titration is to titrate water with iodine-containing Karl Fischer titration solution, and determine the end point of the sudden jump in the electrode potential generated after the water is titrated. Coulometric Karl Fischer titration is to put the titration solution into the solution, establish an electrolytic cell, connect the inner pool with catholyte solution, and connect the outer pool with anode solution, and electrolyze through the coulomb electrode to produce iodine that reacts with water, and after the reaction is completed, excess iodine is generated, which is detected by the fuel detector to determine the end point.

There are four main types of titration methods: Direct titration. i.e., direct titration of metal ions with a standard EDTA solution, with an appropriate indicator to determine the endpoint; Since H+ is released during the reaction, a buffer solution must be used to maintain a constant pH.

For metals that are easy to hydrolyze under controlled pH, auxiliary complexing agents must also be added to inhibit them. Drip back method. For some metals, where hydrolysis is susceptible or complexes with EDTA slowly, or where there is no indicator suitable for direct titration, excess EDTA can be added and then dropped.

Displacement titration. If the metal to be measured m can participate in the displacement reaction.

And the value of each stability constant is in accordance with 108, then M can be quantitatively replaced, and then titrated with EDTA, so that M can be indirectly obtained. Another displacement titration method is to replace EDTA in the M-EDTA complex with a complexing agent HNZ that is stronger than EDTA, and titrate with M. Indirect titration.

It is mainly used to titrate those cations that are weakly complexed with EDTA, or anions that are not complexed with EDTA. For example, under suitable conditions, a certain amount of bismuth salt is added to precipitate phosphate as phosphate, and then the excess bismuth is titrated with EDTA to indirectly measure phosphorus.

Most of the indicators used in complexation titration are metal indicators. These metal indicators must be complexed with the metal to be quite dark in color, and the LGK>5 of the complex is sufficiently sensitive; The stability of the indicator complex is about 3 LGK units lower than that of the EDTA complex, and the endpoint is keen.

In addition, at the same pH, the free indicator and the indicator complex should have a significantly different color. Among the metal indicators, those that can produce fluorescence are called metal fluorescent indicators. Endpoints can also be determined using instruments such as potentiometric analysis, coulometric titration, amperometric titration, spectrophotometric titration, etc., which can improve sensitivity and accuracy, and can also be used for the determination of trace elements.

It is a class of organic compounds containing aminodiacetic acid-N(CH2Cooh)2 group, and its molecule contains ammonia nitrogen and carboxyoxygen, two coordination atoms with strong coordination ability. The following conditions must be present for a reaction to be used for complexation titration:

First, the formed complex should be quite stable, k-shaped 108, otherwise it is not easy to obtain an obvious titration endpoint.

2. Under certain reaction conditions, the complex number must be fixed (that is, only one coordination number complex is formed).

3. Respond quickly.

Fourth, there should be an appropriate method to determine the measurement point of the titration.

Properties of EDTA and its disodium salts:

EDTA) is a chelating agent containing carboxyl and amino groups that can form stable chelates with many metal ions. In chemical analysis, in addition to being used for complexation titration, it is also widely used as a masking agent in various separation and measurement methods.

Ethylenediaminetetraacetic acid is abbreviated as EDTA or EDTA acid, which is often represented by H4Y. White crystals, non-toxic, non-moisture-absorbing. Insoluble in water.

At 22, grams can be dissolved in every 100 ml of water, insoluble in ether and general organic solvents, soluble in ammonia and NaOH solution, to generate the corresponding salt solution. Structure of EDTA: In an aqueous solution, it exists as a double dipole ion.

In a complexation reaction, the substance that donates the coordination atom is called a ligand, i.e., a complexing agent. Inorganic complexing agent: Most of the molecules or ions of inorganic complexing agent are single-tooth ligands containing only one coordination atom, and their complexation reaction with metal ions is carried out step by step;

The stability of most complexes is not high, so the complexation reactions at all levels are not carried out completely.

Because the formation constants of the various levels are not much different from each other, it is easy to obtain a series of complexes with different complexation ratios, and the products have no fixed composition, so it is difficult to determine the metrological relationship of the reaction and the titration endpoint.

Organic complexing agent molecules often contain two or more coordination atoms, which are called polydentate ligands.

When complexed with metal ions, chelates with a cyclic structure can be formed, and the complexation ratio is fixed under certain conditions.

The resulting chelate is stable, the complexation reaction is highly complete, and a clear titration endpoint can be obtained. <>