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    Home > Chemicals Industry > Chemical Technology > Other methods for determining moisture in food (2)

    Other methods for determining moisture in food (2)

    • Last Update: 2021-07-25
    • Source: Internet
    • Author: User
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    3.
    Karl Fischer method to determine the moisture in food

    Karl Fischer method is a method proposed by Karl Fischer in 1935 to determine trace amounts of water in various substances
    .


    The International Organization for Standardization has set this method as an international standard for the determination of trace moisture.


    Karl Fischer method is divided into two methods: Karl Fischer volume method and Karl Fischer Coulomb method
    .


    The former is suitable for samples with moisture content greater than 1.


    (1) Principle Karl Fischer method is a non-aqueous redox titration method
    .


    When using Karl Fischer reagent (containing iodine and sulfur dioxide ) to titrate a food sample, the iodine, sulfur dioxide , pyridine and methanol in the Karl Fischer reagent react quantitatively with water in the food.


    I 2 +SO 2 +H 2 O+3C 5 H 5 N+CH 3 OH→2 pyridine hydroiodate + 2 pyridine methyl sulfate

    Theoretically, 1 mol of water needs to react with 1 mol of iodine, 1 mol of sulfur dioxide , 3 mol of pyridine and 1 mol of methanol , but in reality, the amount of SO 2 , pyridine, and CH 3 OH are all excessive
    .


    After the reaction is complete, the excess free iodine appears reddish-brown, which can be determined as the end point


    Karl Fischer moisture titration manual operation error is relatively large, now more use Karl Fischer moisture automatic titrator
    .

    (2) Reagent Karl Fischer reagent: The ratio is I 2 :SO 2 :C 5 H 5 N=1:3:10
    .


    When preparing, separate the two reagents, A and B, where the A solution is the I2 CH 3 OH solution, and the B solution is the SO 2 CH 3 OH solution


    (3) Instrument Karl Fischer moisture analyzer
    .

    (4) Safety reminder that methanol vapor is harmful to humans.
    Anhydrous methanol should be used in a fume hood , and care should be taken to protect eyes and skin during operation
    .


    Karl Fischer reagents must be treated as hazardous waste


    (5) Operation steps

    ①Equipment installation: follow the installation instructions
    .

    ② Preparation and calibration of Karl Fischer reagent
    .

    a.
    Preparation: Weigh 85gI 2 → add 670ml of anhydrous CH 3 OH to a dry brown flask with stopper → stopper → shake to dissolve all I2 → add 270 ml of pyridine → mix well → cool in an ice-water bath → pass 60g of dry SO 2 gas→plug the bottle stopper→calibrate and use after 24h in a dark place
    .

    b.
    Calibration: add 50mL of anhydrous methanol to the reactor of the moisture analyzer → turn on the power → start the electromagnetic stirrer → drop the KF reagent into the methanol to make the remaining trace water and reagent in the methanol reach the end point (ie pointer When reaching a certain scale, do not record the KF reagent dosage)→Keep for 1min→Inject 10uL of distilled water (equivalent to 0.
    01g of water) from the reactor feed port with a 10uL micro-syringe→The ammeter pointer is close to the zero point→Titrate with KF reagent to the original end point→Record
    .



    Where KFReq——The titer of water of Karl Fischer reagent, mg water/mL

    V——Volume of Karl Fischer reagent consumed by titration, mL

    m—— the mass of water in the standard solution of water - methanol , g

    ③Determination of water content: take 50mL methanol → in the reactor of the analyzer, add methanol to flood the electrode, use KF reagent to titrate the trace amount of water in 50mL methanol → drop until the pointer is equal to the calibration time and remains unchanged for 1 min → Open the feeding port → immediately add the weighed sample → plug the skin stopper → stir → drop with KF reagent until the end point remains unchanged for 1 min → record
    .

    (6) Result calculation

    Where x——moisture content,%

    KFReq——Karl Fischer reagent water titer, mg water/mL

    V——The volume of Karl Fischer reagent consumed by the titration sample, mL

    m——sample quality, mg

    (7) Tips

    ①Scope of application: Karl Fischer method is suitable for the determination of foods with low moisture content, such as dehydrated fruits and vegetables, candy and chocolate, coffee and oil, and any samples with high sugar, high protein and low moisture.


    It is an ideal method for the determination of trace moisture.


    ② When the sample contains strong reducing components such as vitamin C, the Karl Fischer method is not suitable
    .

    Any side reaction such as consuming iodine, releasing iodine, or producing water will affect the measurement result
    .
    For example, the vitamin C unsaturated fatty acid in food can react with the iodine of Karl Fischer reagent, resulting in a high water content measurement value; oxygen in the air oxidizes the iodine ions in the titration cell to iodine, thereby reducing the consumption of reagents Dosage, resulting in low measurement results; sunlight will also significantly promote the oxidation reaction of oxygen and iodide ions, so the reagents should be protected from light
    .

    ③The size of the food sample particles affects the degree of water extraction
    .
    Generally, the fineness of solid samples is 40 mesh
    .
    Use a grinder instead of a grinder to prevent water loss
    .

    ④ This method can not only measure the free water in the sample, but also its bound water, that is, the result of this method can more objectively reflect the total moisture content in the sample
    .

    Difference ⑤ Volumetric and coulometric method: Maximum difference Volumetric and coulometry is that the I 2 different sources, the volumetric method the I 2 from the titrant, and coulometry in the I 2 through electrolysis comprising the I 2 ion electrolyte Liquid produced
    .
    The speed of electrolysis is limited, so when the water content of the sample to be measured is relatively low, using the Karl Fischer Coulometric Moisture Analyzer, the detection speed is not only fast, but also the data parallelism is very good
    .
    There is a strict quantitative relationship between the amount of electricity passing through the electrolytic cell and the amount of I 2 , so the coulometric method has a higher measurement accuracy
    .
    Therefore, in terms of application, the volumetric method is more suitable for the measurement of samples with high moisture content, while the coulometric method is only suitable for the determination of trace and trace amounts of water
    .

     

     

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