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In the presence of water, the water in the sample undergoes redox reactions with SO2 and I2 in the Karl Fischer reagent.

I2 + SO2 + 2H2O → 2HI + H2SO4

But this reaction is reversible, and when the concentration of sulfuric acid reaches 0.05% or more, a reverse reaction can occur. If we want the reaction to proceed in a positive direction, we need to add an appropriate alkaline substance to neutralize the acid generated during the reaction process. It has been experimentally proven that adding pyridine to the system can cause the reaction to proceed to the right.

3 C5H5N+H2O+I2+SO2 →+Pyridine sulfate

Pyridine sulfate is unstable and can react with water, consuming a portion of the water and interfering with the measurement. To stabilize it, we can add anhydrous methanol.

Pyridine sulfate+CH3OH (anhydrous) → Pyridine methylsulfate

We write the above three steps of the reaction as the overall reaction equation:

I2+SO2+H2O+3 pyridine+CH3OH → 2 pyridine+methyl sulfate pyridine

From the reaction equation, it can be seen that 1 mol of water requires 1 mol of iodine, 1 mol of sulfur dioxide, 3 mol of pyridine, and 1 mol of methanol to produce 2 mol of pyridine and 1 mol of methyl sulfate pyridine. This is theoretical data, but in reality, the amounts of SO2, pyridine, and CH3OH are all excessive. After the reaction is complete, the excess free iodine appears reddish brown, indicating that the endpoint has been reached.

I2︰SO2︰C5H5N = 1︰3︰10

2. Preparation and Calibration of Karl Fischer Reagent

If methanol is used as the solvent, the molar ratio of I2, SO2, and C5H5N (with a water content of less than 0.05%) in the reagent is: I2: SO2: C5H5N=1:3:10

The effective concentration of this reagent depends on the concentration of iodine. The effective concentration of newly prepared reagents continues to decrease due to the fact that each component in the reagent also contains some moisture. However, the main reason for the decrease in reagent concentration is caused by some side reactions, which consume a relatively high amount of iodine.

This also indicates that this reagent needs to be prepared separately, divided into two types of reagents, A and B, and stored separately. It should be mixed before use and calibrated. However, the Karl Fischer water content determination reagents used in the Chinese market, whether single component or two-component, are generally prepared by manufacturers and can be directly used as products. However, due to the unstable nature of Karl Fischer reagents, users must calibrate them when using them to determine their true water equivalent data.

The Karl Fischer method for determining moisture should pay attention to the following points:

① This method is applicable to most organic samples, including candy, chocolate, oil, lactose, and dried fruits and vegetables in food;

② Samples containing strong reducing materials, including vitamin C, cannot be measured; If the sample contains ketones and aldehydes, they will undergo ketal and acetal reactions with the reagent, and aldehyde ketone reagents must be used for testing. For some samples that do not dissolve in methanol, it is necessary to find a suitable solvent to dissolve them before testing, or use a Karl Fischer furnace to vaporize the water and determine.

③ The Karl Fischer method can not only measure the free water in the sample, but also objectively reflect the total moisture content in the sample.

④ A fineness of 40 mesh is recommended for solid samples, and a pulverizer should be used instead of grinding to prevent moisture loss.