As we all know, the Karl Fischer method is a method for determining trace water in various substances. Since this method was proposed by Karl Fischer in 1935, I2, SO2, pyridine, and anhydrous CH3OH (water content below 0.05%) have been used. ) Formulated, and the International Organization for Standardization has set this method as an international standard for measuring trace moisture, and our country has also established this method as a national standard for measuring trace moisture.
1. Principle:
In the presence of water, that is, the water in the sample and the SO2 and I2 in the Karl Fischer reagent produce an oxidation-reduction reaction.
I2 SO2 2H2O → 2HI H2SO4
But this reaction is a reversible reaction. When the sulfuric acid concentration reaches more than 0.05%, a reverse reaction can occur. If we let the reaction proceed in a positive direction, we need to add appropriate alkaline substances to neutralize the acid generated during the reaction. It is proved by experiments that pyridine is added to the system, so that the reaction can proceed to the right.
3 C5H5N H2O I2 SO2 → 2Pyridinic acid pyridine sulfuric anhydride pyridine
The pyridine generated sulfuric anhydride is unstable, can react with water, consume a part of water and interfere with the measurement, in order to make it stable, we can add anhydrous methanol.
Sulfuric anhydride pyridine CH3OH (anhydrous) → pyridine methylsulfate
We write this three-step reaction as the total reaction formula:
I2 SO2 H2O 3 pyridine CH3OH 2 pyridinium hydroiodide methyl pyridine sulfate
It can be seen from the reaction formula 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 iodate and 1 mol of pyridine methyl sulfate. This is theoretical data, but in fact, the amount of SO2, pyridine, and CH3OH are all excessive. After the reaction, the excess free iodine appears reddish brown, which can be determined as reaching the end point.
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, C5H5N (water content below 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 newly prepared reagent's effective concentration continues to decrease, because the components in the reagent itself also contain some water, but the main reason for the decrease in the reagent concentration is caused by some side reactions, which consumes a part of iodine.
This also shows that the preparation of this reagent must be separately prepared, divided into two reagents A and B and stored separately, mixed before use, and must be calibrated.
A solution of I2 in CH3OH
A solution of SO2 in CH3OH pyridine
This method requires strict reagents, requires that methanol and pyridine are anhydrous, and requires a KF moisture analyzer (manufactured by Shanghai Institute of Chemical Industry)
Preparation:
Weigh 85g I2 → put in a dry brown flask with a stopper → add 670ml of anhydrous CH3OH → stopper the stopper → shake to dissolve all I2 → add 270ml of pyridine → mix well → cool in ice water bath → pass dry 60g of SO2 gas → stop Stopper on the bottle → Calibration and use after 24 hours in the dark
Calibration:
First add 50ml of anhydrous methanol → in the reactor → turn on the power → start the electromagnetic stirrer → use KF reagent to drip into the methanol to make the trace water remaining in the methanol and the reagent reach the end (that is, the pointer reaches a certain scale, no record (KF reagent dosage) → hold for one minute → use a 10 μl syringe to inject 10 μl of distilled water (equivalent to 0.01 g of water) from the reactor feed port → the ammeter pointer is close to zero → titrate to the original end point with KF reagent → record
F = G * 100 / V
F —— Water equivalent of KF reagent (mg / ml)
V-Volume of reagent consumed by KF titration (ml)
G-weight of water (g)
3. Steps
For solid samples, such as candy must be crushed in advance, weigh 0.30 ~ 0.50g sample in the weighing bottle
Take 50 ml of methanol → in the reactor, the added methanol should be able to submerge the electrode, and titrate the trace water in 50 ml of methanol with KF reagent → when the pointer is equal to the calibration time and remains unchanged for 1 min → open the feeding port → weigh Add a good sample immediately → plug the skin plug → stir → drop to the end with KF reagent and keep it for 1min unchanged → record
Calculation:
Moisture = FV / W
F —— Water equivalent of KF reagent (mg / ml)
V-Karl Fischer reagent consumed in titration (ml)
W-sample weight (g)
Note: â‘ This method is applicable to samples of confectionery, chocolate, oil, lactose and de-fruited vegetables in food;
â‘¡ The samples with strong reducing materials, including vitamin C, cannot be determined;
â‘¢ Karl Fischer method can not only measure the free water in the sample, but also the bound water, that is, the result of this method more objectively reflects the total moisture content in the sample.
â‘£ The fineness of the solid sample is preferably 40 mesh. It is best to use a grinder instead of grinding to prevent water loss.
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