Analytical Chemistry Karl Fischer Method Very weak acids are titrated with alkalis

The principle of Karl Fischer method for the determination of trace amounts of water in various substances: in the presence of water, i.e., the water in the sample reacts with SO2 and I2 in Karl Fischer reagent.

i2 + so2 + 2h2o → 2hi + h2so4

However, this reaction is a reversible reaction, and when the concentration of sulfuric acid reaches above, the reverse reaction can occur. If we allow the reaction to proceed in a positive direction, an appropriate alkaline substance needs to be added to neutralize the acid formed during the reaction. It has been experimentally proven that the addition of pyridine to the system allows the reaction to proceed to the right.

3 C5H5N+H2O+I2+SO2 2 Hydroiodic Acid Pyridine + Sulfate Pyridine.

The production of pyridine sulfate is unstable, can react with water, consume a part of the water and interfere with the determination, in order to make it stable, we can add anhydrous methanol.

Pyridine sulfate + CH3OH (anhydrous) pyridine methylsulfate.

Let's write the above three-step reaction as the total reaction equation:

I2 + SO2 + H2O + 3 pyridine + CH3OH 2 hydroiodate pyridine + pyridine methyl sulfate.

From the reaction formula, it can be seen that 1mol of water needs 1mol of iodine, 1mol of sulfur dioxide, 3mol of pyridine and 1mol of methanol to produce 2mol of pyridine hydroiodate and 1mol of pyridine methylsulfate. This is a theoretical data, but in practice, the dosage of SO2, pyridine, and CH3OH is excessive, and the excess free iodine appears reddish-brown after the reaction, which can be determined to reach the endpoint.

i2∶so2∶c5h5n = 1∶3∶10

Don't know how to lift up.

1. Karl Fischer reagent: known accurate concentration of I2, excess SO2 than I2 and an appropriate amount of pyridine, a mixture of methanol as a solvent (I2 + SO2 + PY + CH3OH).2. Principle:

I2 Oxidation of SO2 requires a certain amount of water. In the presence of pyridine, the methanol reaction is added before quantifying to the right. i2 + so2 + ch3oh + py +

The Karl Fischer method, referred to as the Fischer method, is a volume analysis method for determining water proposed by Karl Fischer in 1935. The Fischer method is one of the various chemical methods used to determine the moisture of a substance, and it is the most specialized for water.

First, the most accurate method. Although it is a classic method, it has been improved in recent years to improve accuracy and expand the measurement range, and has been listed as a standard method for the determination of water in many substances.

There are two methods: titration method and coulometric electricity method. It is suitable for the determination of water content in many inorganic compounds and organic compounds. It is a world-recognized classic method for determining the moisture content of substances. It can quickly determine the moisture content in liquids, solids, and gases, and is the most specialized.

1. The most accurate chemical method is the world's common industry standard analytical method. It is widely used in petroleum, chemical industry, electric power, medicine, pesticide industry and scientific research in colleges and universities.

The high-precision weighing system of the Mark 3 HP moisture analyzer is also suitable for the precise analysis of volatiles and moisture content in very dry samples. This feature makes the Mark 3 HP a true alternative to Karl-Fisher titration.

Karl Fischer volumetric determination of the DU water content of BAI in a sample is calculated based on the amount of KARC's reagent consumed during the titration process.

Karl Fischer volumetric determination of the water content in a sample is calculated based on the amount of KF reagent consumed during the titration.

There are two methods of Karl Fischer volumetric method: titration method and coulometric electricity method. It is suitable for the determination of water content in many inorganic compounds and organic compounds. It is a world-recognized classic method for determining the moisture content of substances. It can quickly determine the moisture content in liquids, solids, and gases, and is the most specialized.

1. The most accurate chemical method is the world's common industry standard analytical method.

To be able to directly titrate, it is necessary to meet CSPKA 10-8, and according to this condition, it is judged that C is not possible! The answer is C!

The reason for choosing C is very simple, according to the acid-base titration requirements, CKE 10-8 is required to be accurate for direct titration of strong alkali direct strong alkali solution. Strong acid titration strong base requirements are CKB 10-8.

A acetate ionization is weakly alkaline, of course, it cannot be titrated with alkali.

Select C, weak acid roots can not be titrated with alkali.

Pick B! NaH2PO4 solution is weakly acidic, pH 1<7.

Na2HPO4 solution is weakly alkaline, pH2>7

So b right!

[H2PO4]- and [HPO4]2- can be seen as conjugated acid-base pairs:

h2po4]- h+ +hpo4]2-

H2PO4]- because it can release H+ to form [HPO4]2-, so the former is a conjugate acid, the latter is a conjugate base, and the former must be more acidic than the latter, so there is: PH1 PH2.

It can be seen that the theoretical discoloration point of the indicator is 55, then khin==

So choose A

It's very simple, the midpoint of the indicator is the midpoint of the discoloration, so as long as you look for the 4 options, which option has the closest negative logarithmic value, you will know that it is C at a glance

LS was right, but miscalculated

If analyzed theoretically, khin = [h+][in-] [hin], and at the midpoint of discoloration [in-] = [hin], so khin = [h+].