Moisture Meter - Determination of Moisture by Distillation
Distillation was invented in the early twentieth century when it used boiling organic liquids to separate water from a sample and is still used today.
1. Principle: Put the water-insoluble organic solvent and the sample into the distillation moisture measuring device to heat, the moisture in the sample evaporates together with the solvent vapor, condense such steam in the condensation tube, and obtain it from the capacity of the moisture The moisture content of the sample.
2. Steps
Accurately weigh 2.00-5.00g of sample → put in a 250ml moisture determination retort → add about 50-75ml of organic solvent → connect to the distillation device → heat and distill slowly → until most of the water is evaporated → speed up the distillation → until the water volume in the scale tube is no longer increase → reading
calculate:
Moisture = V/W
V——The capacity of the water layer in the graduated tube ml
W - the weight of the sample (g)
3. Commonly used organic solvents and selection basis
Commonly used organic solvents are clearer than water and heavier than water.
Benzene xylene CCl4
Density 0.88 0.86 0.86 1.59
Boiling point 80°C 80°C 140°C 76.8°C
Selection basis: Foods that are unstable to heat generally do not use xylene, because it has a high boiling point, and an organic solvent with a low boiling point, such as benzene, is often used. For some samples containing sugar, which can be decomposed to release water, such as dehydrated onion and dehydrated garlic, benzene can be used, and the organic solvent should be selected according to the nature of the sample.
4. Advantages and disadvantages of distillation
excellent
point:
⑴ Adequate heat exchange
⑵The chemical reaction after heating is less than the gravimetric method
⑶Simple equipment and convenient management
shortcoming:
⑴Water and organic solvents are prone to emulsification
(2) The moisture in the sample may not evaporate at all
(3) Moisture sometimes attaches to the wall of the condenser tube, causing reading errors
It is not ideal for stratification, which will cause reading errors. A small amount of amyl alcohol or isobutanol can be added to prevent the emergence of emulsion.
This method is used to determine a large number of volatile substances in the sample besides moisture, for example, ethers, aromatic oils, volatile acids, CO2, etc. At present, AOAC stipulates that the distillation method is used for the moisture determination of feed, hops, and condiments, especially spices. Distillation is the only and recognized moisture testing and analysis method.
4. Karl Fischer method
*, the Karl Fischer method is a method for measuring trace moisture in various substances. Since it was proposed by Karl Fischer in 1935, this method has been using I2, SO2, pyridine, anhydrous CH3OH (water content below 0.05%) ), and the standardization organization has set this method as the standard for measuring trace moisture, and our country has also set this method as the 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 a redox reaction.
I2 + SO2 + 2H2O → 2HI + H2SO4
But this reaction is a reversible reaction, when the concentration of sulfuric acid reaches above 0.05%, the reverse reaction can occur. If we let the reaction proceed in a positive direction, we need to add an appropriate alkaline substance to neutralize the acid generated during the reaction. It has been proved by experiments that adding pyridine to the system can make the reaction proceed to the right.
3 C5H5N+H2O+I2+SO2 → 2 hydroiodic acid pyridinium + sulfuric anhydride pyridinium
The sulfuric anhydride produced by pyridine is unstable and can react with water, consuming part of the water and interfering with the determination. In order to make it stable, we can add anhydrous methanol.
Pyridine sulfuric anhydride + CH3OH (anhydrous) → pyridinium methylsulfate
We write the above three-step reaction as the total reaction formula:
I2+SO2+H2O+3 pyridine+CH3OH2 pyridinium hydroiodide+pyridine methylsulfate
It can be seen from the reaction formula that 1 mol of water needs 1 mol of iodine, 1 mol of sulfur dioxide, 3 mol of pyridine and 1 mol of methanol to produce 2 mol of pyridinium hydriodate and 1 mol of pyridinium methylsulfate. This is theoretical data, but in fact, the amount of SO2, pyridine, and CH3OH is excessive, and the excess free iodine after the reaction is reddish-brown, which can be determined as 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 mole ratio of I2, SO2, and 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 effective concentration of the newly prepared reagent is constantly decreasing. The reason is that each component in the reagent itself also contains some water, but the main reason for the decrease in the concentration of the reagent is caused by some side reactions, which consume a part of iodine.
This also shows that the preparation of this reagent should be prepared separately, divided into two reagents, A and B, and stored separately, and then mixed before use, and must be calibrated.
A solution of I2 in CH3OH
Liquid B CH3OH pyridine solution of SO2
This method has strict requirements on reagents, requiring methanol and pyridine to be 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 bottle → shake to dissolve all the I2 → add 270ml of pyridine → mix well → cool in an ice-water bath → pass 60g of dry SO2 gas → stopper Put on the cork → calibrate 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 → drop the KF reagent into the methanol so that the remaining trace of water in the methanol and the reagent reach the end point (that is, the pointer reaches a certain scale, do not record Amount of KF reagent)→keep for one minute→inject 10μl of distilled water (equivalent to 0.01g water) from the feed port of the reactor with a 10μl syringe→the pointer of the ammeter 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——The volume of reagent consumed by KF titration (ml)
G - the weight of water (g)
3. Steps
For solid samples, such as candy, which must be crushed in advance, weigh 0.30~0.50g in a weighing bottle
Take 50 ml of methanol → put it in the reactor, the added methanol should be able to submerge the electrode, titrate the trace water in 50 ml of methanol with KF reagent → drop until the pointer is equal to the calibration and remain unchanged for 1 minute → open the feeding port → put the scale Add a good sample immediately → put on the skin plug → stir → drop KF reagent to the end point and keep it unchanged for 1min → record
calculate:
Moisture = FV/W
F——Water equivalent of KF reagent (mg/ml)
V——Karl Fischer reagent consumed for titration (ml)
W - sample weight (g)
Note: ① This method is applicable to samples of candy, chocolate, fat, lactose and dehydrated fruits and vegetables in food;
② There are strong reducing materials in the sample, including vitamin C, which cannot be measured;
③ The Karl Fischer method can not only measure the free water in the sample, but also detect the bound water, that is, the result of this method can more objectively reflect the total water content in the sample.
④ The fineness of the solid sample should be 40 mesh, and a pulverizer should be used instead of grinding to prevent water loss.
