Method for determining total phosphorus, polyphosphate, orthophosphorus and polymerization rate in ammonium polyphosphate fertilizer

Abstract

The invention discloses a method for measuring total phosphorus, polyphosphate, orthophosphorus and polymerization rate in an ammonium polyphosphate fertilizer, and belongs to the technical field of fertilizer detection.The method comprises the following steps that S1, an ammonium polyphosphate sample is weighed and recorded as m, the unit is g, and the ammonium polyphosphate sample is dissolved in water; s2, adjusting the pH value of the solution to 7.5-8.0 by using alkali; s3, titrating with a Mg < 2 + > standard solution to an end point; s4, alkali is added into the titrated solution to adjust the pH value to 8.1-8.5, titration is continued with the Mg < 2 + > standard solution, titration is excessive titration, the volume consumed again is recorded as V2, and the unit is mL; s5, filtering the mixed solution obtained after titration in S4, and washing the precipitate with water; s6, drying the precipitate at 110-118 DEG C until the weight is constant, and weighing to obtain the mass of the magnesium ammonium phosphate precipitate; and S7, calculating the content of orthophosphorus, the content of polyphosphate, the content of total phosphorus and the polymerization rate. The method provided by the invention can be used for rapidly detecting polyphosphate (based on phosphorus pentoxide), orthophosphorus (based on phosphorus pentoxide), total phosphorus pentoxide content and polymerization rate in the fertilizer-grade ammonium polyphosphate.

Description

Technical Field

[0001] This invention relates to the field of fertilizer testing technology, and in particular to a method for determining total phosphorus, polyphosphate, orthophosphate, and polymerization rate in ammonium polyphosphate fertilizer. Background Technology

[0002] Fertilizer-grade polyphosphoric acid is an inorganic polymer containing phosphorus and nitrogen. Due to its high nutrient content and good solubility, it does not easily react with calcium, magnesium, iron, aluminum and other ions in the soil solution, thus deactivating phosphate ions. Therefore, it is widely used in fertilizers, and its production process is very mature.

[0003] Total phosphorus is the total content of phosphorus in all forms of fertilizer-grade polyphosphate (including orthophosphate, pyrophosphate, tripolyphosphate, and longer-chain polyphosphates) converted to phosphorus pentoxide (P₂O₅). Total phosphorus is the most basic indicator of phosphorus content in fertilizers, representing the maximum potential phosphorus that the fertilizer can theoretically provide. Orthophosphate (PO₄²⁻) is the phosphate group (PO₄²⁻). 3- Polyphosphate (PP) refers to the form of phosphorus that plants can directly absorb. It is the sum of all polyphosphate groups (including pyrophosphate, triphosphate, and longer-chain polyphosphates) formed by two or more phosphorus atoms linked by oxygen bridges. A high orthophosphate content indicates that the fertilizer is fast-acting and can be immediately absorbed by crops after application to the soil. While polyphosphate itself cannot be directly absorbed by plants, it slowly hydrolyzes in the soil, gradually releasing orthophosphate. This indicates that it has a slow-acting or long-lasting effect. The polymerization rate refers to the percentage of polyphosphate in ammonium polyphosphate. The polymerization rate is one of the most critical indicators for evaluating the production process and product quality of ammonium polyphosphate. A higher polymerization rate indicates a larger proportion of long-chain polyphosphates in the product, and more pronounced its characteristics. Therefore, testing the total phosphorus, orthophosphate, polyphosphate, and polymerization rate of fertilizer-grade polyphosphate is a prerequisite for a systematic evaluation of its comprehensive fertilizer efficiency, technological advancement, and application applicability.

[0004] Existing methods for determining phosphorus content in fertilizer-grade ammonium polyphosphate include paper chromatography, ion-exchange column chromatography, nuclear magnetic resonance (NMR), and gravimetric analysis. Paper chromatography is time-consuming and cumbersome; ion-exchange column chromatography suffers from incomplete separation and low spike recovery; NMR can only detect polyphosphates up to trimer and is expensive; gravimetric analysis is a common standard method for total phosphorus content determination, but it remains cumbersome and can only measure total phosphorus, typically using the quinomolybdate gravimetric method, which is costly and slow. Currently, there is no unified standard for detecting the polymerization rate of fertilizer-grade ammonium polyphosphate. CN108254369A discloses a method for detecting the polymerization rate of ammonium polyphosphate, which can effectively obtain the polymerization rate, but this method is spectrophotometric (or ammonium vanadate colorimetric method), which is costly; furthermore, it requires separate measurement of total phosphorus content. Summary of the Invention

[0005] To address the aforementioned deficiencies, this application provides a method for determining total phosphorus, polyphosphate, orthophosphate, and polymerization rate in ammonium polyphosphate fertilizer. This method can rapidly detect the content of polyphosphate (calculated as phosphorus pentoxide), orthophosphate (calculated as phosphorus pentoxide), total phosphorus pentoxide, and polymerization rate in fertilizer-grade ammonium polyphosphate.

[0006] A method for determining total phosphorus, polyphosphate, orthophosphate, and polymerization rate in ammonium polyphosphate fertilizer includes the following steps: S1, Weigh out the ammonium polyphosphate sample, denoted as m, in g, and dissolve the ammonium polyphosphate sample in water; S2, adjust the pH of the solution to 7.5~8.0 using alkali; S3, with Mg 2+ Titrate the standard solution to the endpoint, Mg 2+ The concentration of the standard solution is c, in mol / L, and the volume consumed is recorded as V1, in mL. S4, add alkali to the titrated solution to adjust the pH to 8.1-8.5, and continue titrating with Mg. 2+ For standard solution titration, if the titration is an excess titration, record the volume consumed again as V2, in mL; S5. Filter the mixed solution after titration in S4 and wash the precipitate with water. S6. Dry the precipitate at 108~118℃ to constant weight, weigh it to obtain the mass of magnesium ammonium phosphate precipitate, and record it as m1 in g. S7, calculate the orthophosphorus content, polyphosphorus content, total phosphorus content, and polymerization rate.

[0007] In one or more alternative embodiments of the present invention, in S3, the titration is performed using a potentiometric titrator, and the endpoint is determined by a sudden change in potential.

[0008] In one or more alternative embodiments of the present invention, in S5, filtration is performed using a pre-weighed glass core crucible; in S6, the glass core crucible is dried together with the precipitate.

[0009] In one or more alternative embodiments of the present invention, in S1, the water volume is 100 mL.

[0010] Invention principle: Fertilizer-grade ammonium polyphosphate has a chelating effect; the polyphosphate in ammonium polyphosphate first reacts with magnesium ions (Mg). 2+ The chelation reaction is carried out at pH 7.5-8.0. The generated chelated phosphoric acid dissolves in water. After the polyphosphoric acid is completely consumed, magnesium ions (Mg) are added dropwise. 2+ The solution becomes cloudy and the turbidity persists for more than 20 seconds, indicating the presence of orthophosphoric acid in the polyphosphoric acid (first step). Adjust the pH to 8.1-8.5 by adding alkali, then add excess magnesium ions (Mg²⁺).2+ The reaction of dissolving in water in the first step produces polyphosphoric acid, and the solid produced in the second step is orthophosphoric acid. The total phosphorus pentoxide content can be obtained by calculating the two: polymerization rate = polyphosphoric acid (calculated as phosphorus pentoxide) content / total phosphorus pentoxide.

[0011] In the first step: Under neutral to weakly alkaline conditions (pH 7.5~8.0), polyphosphate ions can react with magnesium ions (Mg). 2+ This forms a stable water-soluble complex, while orthophosphate reacts with Mg at this pH. 2+ and NH4 + Magnesium ammonium phosphate (MgNH4PO4) precipitate is formed. This is used as the titration endpoint, and the amount of Mg consumed... 2+ The number of moles is directly proportional to the number of moles of polyphosphate.

[0012] The reaction equation is: (Formula 1) (Formula 2) In the second step: After completing the first step, adjust the pH to 8-8.5 with alkali, and add excess magnesium ions (Mg). 2+ This process causes all the original orthophosphate ions in the sample to precipitate as magnesium ammonium phosphate. After filtration, the sample is dried and weighed, and the orthophosphate content is calculated based on the mass of the precipitate.

[0013] The reaction equation is: (Formula 3) Beneficial effects: This invention utilizes polyphosphate and magnesium ions (Mg) 2+ The polyphosphate group (Mg(OH)2) exhibits the property of forming soluble complexes at specific pH levels, while orthophosphate forms precipitates. The solubility product (Ksp) of Mg(OH)2 was determined by titration and gravimetric methods, respectively, to be 1.8 × 10⁻⁶. -11 Therefore, at pH < 8.5, magnesium hydroxide (Mg(OH)2) cannot form solid magnesium hydroxide, and the solubility product of magnesium ammonium phosphate (MgNH4PO4) is Ksp = 2.5 × 10⁻⁶. -13 Adding a slightly excessive amount of magnesium ions (magnesium ammonium phosphate has very low solubility and can be ignored; solid magnesium ammonium phosphate can be used to accurately determine the phosphorus pentoxide content) and by controlling the reaction pH, the polyphosphate and orthophosphate can be determined separately to obtain the total phosphorus content and calculate the polymerization rate.

[0014] Therefore, this invention can rapidly and accurately determine the content and polymerization rate of different forms of phosphorus (including total phosphorus, polyphosphate, and orthophosphate) in fertilizer-grade ammonium polyphosphate. The method of this invention combines magnesium salt titration for polyphosphate determination with precipitation gravimetric method for orthophosphate determination. The determination method is simple, fast, and inexpensive. Attached Figure Description

[0015] Figure 1 This is a flowchart of the present invention; Figure 2 This is a schematic diagram of the filtered magnesium ammonium phosphate of the present invention. Detailed Implementation

[0017] This application will be further explained below.

[0018] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "longitudinal," "lateral," "horizontal," "inner," "outer," "front," "rear," "top," and "bottom," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention. In addition, the terms "first," "second," and "third," etc., are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0019] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set up," "have," "install," "connect," and "connect" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0020] Please refer to Figure 1 , Figure 1 This is a flowchart of the present invention.

[0021] A method for determining total phosphorus, polyphosphate, orthophosphate, and polymerization rate in ammonium polyphosphate fertilizer includes the following steps: S1. Accurately weigh the mass m (g) of the ammonium polyphosphate sample into a 250mL beaker and dissolve it in 100mL of pure water.

[0022] S2, adjust the pH to 7.5~8.0 with an alkaline solution.

[0023] S3, using magnesium ions (Mg) at a concentration of c (mol / L). 2+ Titrate the standard solution to the endpoint and record the volume V1 (mL) consumed.

[0024] S4, adjust the pH of the titrated solution to 8.1-8.5 by adding alkali, and then add magnesium ions (Mg) at a concentration of c (mol / L). 2+ Continue titrating the standard solution until it is an excess titration, and record the volume V2 (mL) consumed.

[0025] S5, the mixed solution after titration in S4 is filtered through a weighed glass frit crucible, and the precipitate is washed several times with water. The filtered magnesium ammonium phosphate is as follows: Figure 2 .

[0026] S6. The glass frit crucible and the precipitate are dried at 108~118℃ to constant weight, and the mass of magnesium ammonium phosphate precipitate m1 (g) is obtained by weighing.

[0027] S7, calculate the orthophosphorus content (as P2O5), polyphosphorus content (as P2O5), total phosphorus content (as P2O5), and polymerization rate.

[0028] The formula for calculating polyphosphate content is: (Formula 4) The formula for calculating the phosphorus content is: (Formula 5) The formula for calculating total phosphorus content is: (Formula 6) The formula for calculating the polymerization rate is: (Formula 7) in, The molar mass of P2O5 is the molar mass of MgNH4PO4.

[0029] In S3, the endpoint is determined when the solution just becomes turbid and remains so for 20 seconds without fading, or when a potentiometric titrator is used, the endpoint is the sudden change in potential.

[0030] In the following Accuracy Experiment 1 and Accuracy Experiment 2, the total phosphorus, orthophosphorus, polyphosphorus, and polymerization rate of the ammonium polyphosphate sample are known. In Accuracy Experiment 1 and Accuracy Experiment 2, the total phosphorus, orthophosphorus, polyphosphorus, and polymerization rate of the ammonium polyphosphate sample are determined by the method of this invention and spectrophotometry (CN108254369A), respectively. To eliminate errors, blank experiments are provided for each experiment.

[0031] Accuracy Experiment 1 The ammonium polyphosphate sample was ammonium polyphosphate 18-58. The same amount of ammonium polyphosphate sample was weighed and subjected to the method of the present invention and spectrophotometry respectively. The results are shown in Table 1 below.

[0032] Table 1 Accuracy Experiment 2 The ammonium polyphosphate sample was ammonium polyphosphate 18-60. The same amount of ammonium polyphosphate sample was weighed and subjected to the method of the present invention and spectrophotometry respectively. The results are shown in Table 1 below.

[0033] Table 2 The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for measuring total phosphorus, polyphosphorus, orthophosphorus, and polymerization rate in ammonium polyphosphate fertilizer, characterized by, The method comprises the following steps: S1, weighing the sample of ammonium polyphosphate, denoted as m, unit g, dissolving the sample of ammonium polyphosphate in water; S2, adjusting the pH of the solution to 7.5-8.0 by alkali; S3, with Mg 2+ Standard solution titration to end point, Mg 2+ The standard solution concentration is c, unit: mol / L, and the volume consumed is recorded as V1, unit: mL; S4, to the titrated solution, add base to adjust to pH 8.1~8.5, continue to titrate with Mg 2+ Standard solution titration, titrate to excess, record the volume consumed again as V2, unit: mL; S5, filtering the mixed solution after titration in S4, and washing the precipitate with water; S6, drying the precipitate at 108-118℃ to constant weight, and weighing to obtain the mass of the ammonium magnesium phosphate precipitate, denoted as m1, unit g; S7, calculating the orthophosphorus content, polyphosphorus content, total phosphorus content and polymerization rate.

2. The method for measuring total phosphorus, polyphosphorus, orthophosphorus, and polymerization rate in the ammonium polyphosphate fertilizer according to claim 1, characterized by, In S3, the reaction equation is: ； And 。 3. The method for measuring total phosphorus, polyphosphorus, orthophosphorus, and polymerization rate in ammonium polyphosphate fertilizer according to claim 1, characterized by, In S4, the reaction equation is: 。 4. The method for measuring total phosphorus, polyphosphorus, orthophosphorus, and polymerization rate in the ammonium polyphosphate fertilizer according to claim 1, characterized by, In S7, the calculation formula of the polyphosphorus content is: ； The calculation formula of the orthophosphorus content is: ； The calculation formula of the total phosphorus content is: The calculation formula of the polymerization rate is: ； wherein the molar mass of P2O5, the molar mass of MgNH4PO4.

6. The method for measuring total phosphorus, polyphosphorus, orthophosphorus, and polymerization rate in the ammonium polyphosphate fertilizer according to claim 1, characterized by, In S3, the determination of the end point is that the solution just appears turbid and keeps no discoloration for 20 seconds.

7. The method for measuring total phosphorus, polyphosphorus, orthophosphorus, and polymerization rate in the ammonium polyphosphate fertilizer according to claim 1, characterized by, In S3, the titration adopts a potentiometric titrator, and the determination of the end point is the mutation of the electric potential.

8. The method for measuring total phosphorus, polyphosphorus, orthophosphorus, and polymerization rate in the ammonium polyphosphate fertilizer according to claim 6, characterized by, In S5, the filtration adopts a glass sand core crucible which has been weighed; in S6, the drying is the glass sand core crucible together with the precipitate.

9. The method for measuring total phosphorus, polyphosphorus, orthophosphorus, and polymerization rate in the ammonium polyphosphate fertilizer according to claim 1, characterized by, In S1, the water amount is 100 mL.

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