What method is used to determine the biuret content?

Determination of dicyandiamide content in urea (spectrophotometric method) 1 Main contents and scope of application 1.1 This standard specifies the determination of dicyandiamide content in urea using the copper complex spectrophotometric method. 1.2 This standard applies to the determination of the urea dimethylenediamine content in urea produced by the synthesis of ammonia and carbon dioxide for industrial and agricultural use. 2 Normative reference documents The provisions contained in the following standards become part of this standard by being referenced herein. At the time of publication of this standard, all versions shown are valid. All standards will be revised, and those using this standard should consider the possibility of using the latest versions of the following standards. HG/T2843-1997 Common Standard Titration Solutions, Standard Solutions, Reagent Solutions, and Indicator Solutions for Chemical Analysis of Fertilizer Products 3 Principle: Dicyandiamide forms a purplish-red complex in an alkaline solution containing copper sulfate and sodium potassium tartrate; its absorbance is measured at a wavelength of 550 nm. 4 Instruments 4.1 General laboratory instruments and a water bath at 30°C ± 5°C; 4.2 Spectrophotometer with a 3 cm cuvette. 5 Reagents and Solutions Unless otherwise specified, the reagents, solutions, and water used in this test method shall meet the requirements of HG/T2843. 5.1 Copper sulfate solution, 15 g/L; 5.2 Potassium sodium tartrate alkaline solution, 50 g/L; 5.3 Biuret standard solution, 2.00 g/L. 6 Analysis procedures 6.1 Preparation of the standard curve: Prepare the standard colorimetric solutions by transferring the biuret standard solution into eight 100 ml volumetric flasks, one after another, as indicated in Table 1. Volume of biuret standard solution, ml; Corresponding amount of biuret, mg: 0, 0, 2.5, 5, 5.0, 10, 10.0, 20, 15.0, 30, 20.0, 40, 25.0, 50, 30.0, 60. Each volumetric flask was diluted to 50 ml with water, after which 20.0 ml of the sodium potassium tartrate alkaline solution and 20.0 ml of copper sulfate solution were added sequentially and mixed well. The solution was then diluted to the mark, and the volumetric flask was placed in a water bath at 30±5°C for about 20 minutes, with occasional shaking. Absorbance determination: Within 30 minutes, using a solution with a biuret absorbance of zero as the reference solution, the absorbance of the standard colorimetric solution (6.1.1) was measured at a wavelength of 550 nm using a spectrophotometer. For the preparation of the standard curve, the milligrams of biuret contained in 100 ml of the standard colorimetric solution are used as the abscissa, while the corresponding absorbance is used as the ordinate to plot the graph, or a linear regression equation is determined. 6.2 Determination – Preparation of the test solution: Depending on the different levels of biuret in urea, the amount of sample to be weighed is determined according to Table 2; the weighing should be done with an accuracy of 0.002 g. The weighed sample is then carefully transferred into a 100 ml volumetric flask, and a small amount of water is added to dissolve it (the amount of water added should not exceed 50 ml). The mixture is allowed to reach room temperature, after which 20.0 ml of sodium potassium tartrate alkaline solution and 20.0 ml of copper sulfate solution are added sequentially. The mixture is stirred well, diluted to the mark, and the volumetric flask is placed in a water bath at 30±5°C for about 20 minutes, with occasional stirring. Table 2 shows the amount of sample to be weighed for different levels of biuret: Biuret (X)%: X≤0.3, 0.3<X≤0.4, 0.4<X≤1.0, >1.0; Amount of sample to be weighed in grams: 10, 7, 5, 3. Blank test: A blank test is carried out following the same procedure as described above, except that no sample is used; the procedures and reagents employed are the same as those used in the actual determination. The absorption measurement follows the same steps as those for preparing the standard curve; the absorption of the test solution and the blank test solution is determined. Note: (1) If the test solution is colored or turbid, in addition to measuring the absorbance according to section 6.2, add 20.0 ml of the sodium potassium tartrate alkaline solution to each of two 100 ml volumetric flasks; add the same volume of the test solution to one of these flasks, dilute the solutions to the mark with water, and mix well. Using the solution without the test sample as the reference solution, the absorbance of another solution is measured under the same conditions as those used for the measurement, and this value is subtracted during calculation. (2) If the test solution is only turbid, add 0.3 ml of hydrochloric acid solution, shake vigorously, filter through medium-speed filter paper, wash with a small amount of water, collect the filtrate and washings quantitatively in a volumetric flask, and then proceed as per the procedure for preparing the test solution. 7 Calculation of analysis results: Determine the mass of biuret corresponding to the measured absorbance from the coordinate curve, or calculate the mass of biuret using the curve coefficients. The content of biuret in the sample, denoted as X, is expressed as a mass percentage (%), and is calculated using the following formula: (m1 – m2)×10-3 ×100 = X = (m1 – m2)/m×10. Here, m1 represents the mass of biuret measured in the solution, in mg ； m2—the mass of biuret measured for the blank solution, in mg ； m—mass of the sample, g ； The arithmetic mean of the parallel measurement results is taken as the measurement value, which is expressed to two decimal places.