At present, the main methods for measuring the sugar content of cotton fiber include the Japanese colorimetric method, the German Bremen melanin test method, and the American chromatographic method and infrared spectrometric method. In China, there are visual colorimetric measurements based on the Japanese colorimetric method and quantitative method tests based on the German Bremen melanin test method. Our country has also explored and researched chromatography and infrared spectroscopy.
1. Reducing sugar separation and detection method
Put cotton fiber in boiling water or soak it to extract the sugar. The solution uses Behedict, Fehing2Mas2sat and other reagents to mainly detect exogenous sugars in cotton fiber. The reducing sugar content in the sum of endogenous sugars.
(1) Bébert’s liquid determination method
In China, the Bébert’s solution determination method is called visual colorimetric determination method. It is a determination method based on the Japanese colorimetric method and can be directly called the colorimetric method. According to the GB/T13785-1992 standard, this method is a test method that uses colorimetry to quickly evaluate the sugar content of cotton fiber.
1. Determination principle
The main sugar components of cotton fiber are glucose, fructose and sucrose. These sugars are of two types, reducing and non-reducing. The aldehyde and ketone groups contained in the molecular structure of reducing sugars are reducing and can react with Bayer’s reagent. The molecular structure of non-reducing sugars does not contain aldehyde and ketone groups and cannot react with Bayer’s reagent.
Bell’s reagent consists of trisodium citrate, anhydrous sodium carbonate and copper sulfate. After the two liquids are mixed, trisodium citrate forms a blue complex in the alkaline solution. Add sugar-containing cotton fiber to blue Bayer’s reagent and heat it to azeotrope, and the divalent copper ions in the solution are gradually reduced to red cuprous oxide. The composition ratio of the sky blue copper complex and the red cuprous oxide produced by the reduction of divalent copper ions in the solution constantly changes with the chemical reaction, and the solution presents various colors. Due to the different sugar content in cotton fiber, the solution shows five colors: blue, green, grass green, orange and tea red. Then visually compare the color of the solution with the standard color card or Munsell chromatographic color scale to qualitatively determine the sugar content of the cotton fiber, see Table 5-5.
2. Instruments and reagents
(1) 1 set of 3 to 4 parallel temperature-adjustable electric furnaces.
(2) One balance with a graduation value of 0.01g.
(3) Standard color comparison sample card or Munsell chromatographic color scale.
(4) 1 each of 1000mL and 250mL beakers; 20 150mL beakers; 1 each of 1000mL and 50mL graduated cylinders; 2 1000mL volumetric flasks; 1 10mL quantitative liquid adding tube; 20 50mL colorimetric tubes; colorimetric 2 racks.
(5) Reagents: analytically pure or chemically pure anhydrous sodium carbonate, analytically pure or chemically pure sodium citrate and analytically pure or chemically pure crystalline copper sulfate.
3. Preparation of Bell’s reagent
Liquid A: 193g sodium citrate and 100g sodium carbonate were dissolved in 800mL distilled water.
Liquid B: 17.3g copper sulfate was dissolved in 100mL distilled water.
Mix liquid A and liquid B and dilute it to 1000mL, which is Beihe’s reagent. If the prepared reagent is not in use, the bottle cap should be capped and stored in a shaded place at room temperature to prevent failure. Shake a few times before use to thoroughly mix the solution before use.
4. Preparation of test samples
Laboratory samples shall be drawn from batch samples in accordance with the provisions of GB6097-1985. At least 32 clumps of approximately 300 mg per clump were taken to constitute a test sample of 8 to 10 g; remove cotton seeds, seed shavings, leaf shavings, dust and other impurities from the test sample, mix thoroughly, and take out 5 test samples. Each test sample weighs 1.00g±0.01g. Keep the rest for later use.
5. Determination steps
(1) Blank determination: In a 150mL beaker, add 40mL distilled water and 10mL Bayer’s reagent, heat and boil for 1 to 2 minutes, and pour into a colorimetric tube as a standard for “no” sugar. sample (this standard sample must be prepared once a day).
(2) Test sample measurement: Put the five accurately weighed test samples into five 150mL beakers, add 40mL distilled water and 10mL Bell’s reagent to each, heat on the electric stove, and boil for 1 to 2 minutes , and stir continuously. Remove the beaker, squeeze out the cotton sample with tweezers, and pour the remaining 30mL solution into the colorimetric tube.
(3) Visual color comparison: Paste a piece of white paper behind the colorimetric tube holder and use natural light for visual color comparison.
6. Judgment of sugar content
According to the color of the solution, compare the standard color card or Munsell chromatography color standard and the blank sample to determine the sugar content of each test sample (Table 5-5 ).
Among the five measurement results, if there are two or more lower gears in the upper and lower gears, it is regarded as the lower gear, otherwise it is regarded as the upper gear. The maximum and minimum in the measurement results exceed two levels. If there is no, trace, small amount or slightly more, 1 to 2 test samples should be added or deleted. If there are two or more minimum levels of all measurement results, it will be determined as the smallest level, otherwise it will be determined as the previous level.
The characteristics of the Bell’s liquid determination method are that it is simple to operate and easy to master. However, because cuprous oxide precipitates quickly, the color stability of this method is poor, and quantitative colorimetry cannot be performed with a spectrophotometer.
In order to solve the problem of poor color stability, some people studied the characteristics of using Bayesian reagent to react with reducing sugar and then react with phosphomolybdic acid to form a color complex, and developed an instrument colorimetric method to determine the content of cotton fibers. Method of reducing sugar. Although this method provides the possibility for quantitative determination of reducing sugars in cotton fiber, it has not been popularized yet.
(2) Fehling’s solution determination method
Fehling’s solution is an oxidant, divided into A liquid and B liquid. Liquid A contains copper sulfate and methylene blue; liquid B contains potassium sodium tartrate and yellow blood salt. In an alkaline solution, copper sulfate reacts with sodium hydroxide to form a sky blue copper hydroxide precipitate, which immediately reacts with potassium sodium tartrate to form a dark blue complex of copper oxide and potassium sodium tartrate – tartaric acid. Potassium, sodium and copper, the reaction formula is as shown below.��
The red cuprous oxide precipitate reacts with the yellow blood salt in Fehling’s solution to form a soluble compound. Using blue methylene blue as an indicator, when all the copper in Fehling’s solution is reduced by sugar, the reducing sugar immediately reduces the methylene blue to colorless, indicating that it has been neutralized. The sugar content can be obtained by calculation.
Fehling’s solution assay can not only measure the total sugar content in cotton fiber, but also the amount of reducing sugar. Compared with other methods, Fehling’s solution method is the most accurate method for determining the sugar content of cotton fiber. However, the experimental operation requirements are strict and the measurement time is long.
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