Method for measuring sodium alginate content and M / G ratio

Abstract

The invention discloses a method for measuring the content of sodium alginate and the M / G ratio, belongs to the technical field of quantitative analysis of sodium alginate, and solves the technical problems of complex detection method operation, long reaction time, high cost and large detection result error of the method for measuring the content of sodium alginate in the prior art. The method for determining the sodium alginate content and the M / G ratio provided by the invention comprises the following steps: acidolysis of sodium alginate, titration of a sodium alginate acidolysis solution by using a potentiometric titrator to obtain a titration curve, differentiation of the titration curve to obtain a first-order derivative curve, and calculation of the volume of a corresponding titrant to obtain the sodium alginate content. According to the detection method, a plurality of results can be obtained through one-time titration, the efficiency is high, the method is simple, and rapid detection of the sodium alginate content and the M / G ratio is realized.

Description

Technical Field

[0001] The invention relates to the technical field of sodium alginate quantitative analysis, in particular to a method for determining the sodium alginate content and the M / G ratio. Background Art

[0002] Sodium alginate is a natural polysaccharide extracted from brown algae. It is a copolymer composed of β-D-mannuronic acid (M unit) and α-L-guluronic acid (G unit) connected by β-1,4-glycosidic bonds, and composed of different proportions of MG alternating segments, MM homopolymer segments, and GG homopolymer segments. The G-block residues form a folded and rigid structure, and the M-block forms a linear and flexible structure. Due to the different molecular conformations of the M and G segments, the various properties of the M, G, and MG alternating segments differ, making the properties of sodium alginate different depending on the segment composition. For sodium alginate with similar M / G ratios, the G segment content may differ greatly from the MG alternating segment content, and the properties they exhibit are also very different. Therefore, determining the proportion of the three segments is as important as determining the M / G ratio.

[0003] The current method for determining the content of sodium alginate is mainly to measure the carbon dioxide produced by the reaction of the carboxyl group of sodium alginate. These methods have problems such as long reaction time, complex detection method, and great influence of impurities. The M / G ratio of sodium alginate is often determined by infrared spectroscopy and nuclear magnetic resonance spectroscopy. The principle is to perform peak fitting based on the characteristic peaks of the M segment and the G segment, and obtain the M / G ratio by calculating the peak area ratio. This method has the problems of high instrument cost, complex operation and poor anti-interference ability. Moreover, these methods can only calculate the M / G ratio, and cannot analyze the respective proportions of the MG alternating segment, the MM segment and the GG segment, so it may be misleading for the application of sodium alginate. Summary of the invention

[0004] In view of the shortcomings and deficiencies in the prior art, the present invention provides a detection method that can obtain multiple data results by one titration, has high efficiency and simple method, and can realize a determination method for quickly detecting the sodium alginate content and the M / G ratio.

[0005] To achieve the above objectives, the present invention is implemented by the following technical scheme: The method for determining the sodium alginate content and the M / G ratio provided by the present invention comprises:

[0006] Step 1: Acid hydrolysis of sodium alginate: dissolve sodium alginate in an acidic solution, and perform acid hydrolysis at a constant temperature until the reaction is sufficient, which is recorded as solution A;

[0007] Step 2: titrating the sodium alginate acid hydrolyzate with a potentiometric titrator to obtain a titration curve: titrating the solution A prepared in step 1 with an alkaline titrant with a potentiometric titrator to obtain a pH-titrant dosage curve A;

[0008] Step 3: Calculation of the first derivative of the titration dosage curve: Differentiate the pH-titrant dosage curve A obtained in step 2 to obtain a first derivative curve B;

[0009] Step 4 Calculate the M / G ratio in sodium alginate based on the titrant dosage: The alkaline titrant dosage corresponding to the maximum point between pH 2 and 3 is numbered as V A , the amount of alkaline titrant corresponding to pH 2.85 is numbered as V B , the amount of alkaline titrant corresponding to the maximum value between pH 3-7 is numbered as V C , the amount of alkaline titrant corresponding to the first maximum point of pH>7 is numbered as V D ;

[0010] The content of sodium alginate is obtained by calculating the corresponding titrant volume. The amount of alkaline titrant is used to determine the sodium alginate content (ω1) using formula 1, where M OH is the molar number of hydroxide in the alkaline titrant, W SA is the mass of sodium alginate tested, the constant 222 is the molecular weight of sodium alginate monomer, and the specific calculation formula is as follows:

[0011]

[0012] Preferably, in step 4, by calculating the corresponding titrant volume, the proportions of the sodium alginate MG alternating segment, the MM homopolymer segment and the GG homopolymer segment can be calculated using formulas 2-4, which are respectively recorded as ω2, ω3 and ω4;

[0013]

[0014] Preferably, the M / G ratio in sodium alginate is calculated using formula (5) according to the amount of titrant used. The specific calculation formula is as follows:

[0015]

[0016] Among them, ω2 is the proportion of sodium alginate MG alternating chain segments, ω3 is the proportion of MM homopolymer segments, and ω4 is the proportion of GG homopolymer segments.

[0017] Preferably, in step 1, the sodium alginate is dissolved in an acidic solution to a concentration of 0.1% to 20%; the acidic solution is one of hydrochloric acid, sulfuric acid, and oxalic acid, and its concentration is 0.1 to 1.0 mol / L;

[0018] The constant temperature acid hydrolysis reaction temperature is 25-110°C, and the reaction time is 5min-12h.

[0019] Preferably, in step 1, the acidolysis reaction is carried out using a heating device such as a magnetic stirring water bath, a constant temperature water bath, or an oven.

[0020] Preferably, the stirring speed of the magnetic stirring water bath is 0-500 r / min.

[0021] Preferably, in step 1, the acid hydrolysis reaction process needs to be carried out in a sealed container, and the sealed container is specifically a sealed glass bottle or a polytetrafluoroethylene-lined reactor.

[0022] Preferably, in step 2, the potentiometric titrator parameter settings are: equal titration mode, the alkaline titrant is added dropwise to the solution A prepared in step 1, and the amount added each time is 0.01 to 1 mL.

[0023] Preferably, in step 2, the alkaline titrant is one of sodium hydroxide and potassium hydroxide, and the concentration of the titrant is 0.01 to 2 mol / L.

[0024] The present invention provides a method for determining the content of sodium alginate and the M / G ratio. Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) The present invention obtains the content of sodium alginate, the proportion of MG alternating segment, MM homopolymer segment and GG homopolymer segment and the M / G ratio by calculating the corresponding titrant volume. This detection method can obtain multiple results in one titration, has high efficiency and simple method, and can realize rapid detection of sodium alginate content and M / G ratio.

[0026] By acid-hydrolyzing sodium alginate into small molecule oligosaccharides and monosaccharides, the carboxyl groups can be fully exposed to react completely with the hydroxide. The acid hydrolysis method used, in addition to converting sodium carboxylate into carboxylic acid, will not produce other side reactions on the carboxyl groups on the sugar ring, ensuring a one-to-one reaction relationship between hydroxide and carboxylate, so the repeatability is high.

[0027] (2) The present invention is easy to operate, simple to calculate, can be implemented, has low detection cost and high detection efficiency; it has universal applicability for promotion, has practical economic benefits in production and scientific research, and can guide the detection and application direction of sodium alginate. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a flow chart of the determination method of the present invention;

[0029] Figure 2 is the titration curve of sodium alginate of sample A in Example 1;

[0030] Figure 3 for Figure 2 The first derivative curve of ;

[0031] Figure 4 is the titration curve of sodium alginate of sample C in Example 4;

[0032] Figure 5 for Figure 4 The first derivative curve of . DETAILED DESCRIPTION

[0033] The endpoints and any values ​​of the scope disclosed in the present invention are not limited to the precise scope or value, and these scopes or values ​​should be understood to include values ​​close to these scopes or values. For numerical ranges, the endpoint values ​​of each scope, the endpoint values ​​of each scope and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in this article. Hereinafter, in principle, each technical solution can be combined with each other to obtain a new technical solution, which should also be regarded as specifically disclosed in this article.

[0034] Example 1

[0035] The method for determining the sodium alginate content and the M / G ratio provided by the present invention includes the following experimental and calculation steps: Figure 1 As shown, the specific operations are as follows:

[0036] Preparation of reagents:

[0037] Acid hydrolysis of sodium alginate: Add 40 mL of 0.45 M concentrated hydrochloric acid into a glass bottle, then add 2.0000 g (accurate to 0.0001 g) of sodium alginate (sample A, derived from giant kelp) while stirring, screw on the bottle stopper, place the glass bottle in a 100°C water bath, and react with constant temperature acid hydrolysis for 1 hour. After taking out, cool to room temperature and record it as solution A.

[0038] Prepare 0.25 M sodium hydroxide solution as an alkaline titrant.

[0039] pH titration uses a potentiometric titrator to titrate the sodium alginate acid solution to obtain a titration curve: first, calibrate the pH of the reference electrode and pH glass electrode of the automatic potentiometric titrator, then place the glass bottle on the stirrer of the automatic potentiometric titrator, start stirring, and insert the reference electrode and pH glass electrode. Select pH titration, enter the equal titration mode, select 0.25M as the titrant concentration, and titrate 0.2mL each time with an interval of 5s to obtain the pH-titrant dosage curve A—pH-sodium hydroxide dosage curve, such as Figure 2 shown.

[0040] Calculation of the first derivative of the pH-sodium hydroxide dosage curve: Use data processing software to Figure 2 Perform a first-order derivative operation and number the amount of alkaline titrant corresponding to the maximum point between pH 2 and 3 as V A, the amount of alkaline titrant corresponding to pH 2.85 is numbered as V B The amount of alkaline titrant corresponding to the maximum value between pH 3 and 7 is numbered as V C , the amount of alkaline titrant corresponding to the first maximum point of pH>7 is numbered as V D ,like Figure 2 and Figure 3 shown.

[0041] Numerical calculation:

[0042] The content of sodium alginate is obtained by calculating the corresponding titrant volume. The amount of the alkaline titrant is used to determine the sodium alginate content (ω1) using formula 1, where M OH is the molar number of hydroxide in the alkaline titrant, W SA is the mass of sodium alginate tested, the constant 222 is the molecular weight of sodium alginate monomer, and the specific calculation formula is as follows:

[0043]

[0044] By calculating the corresponding titrant volume, the proportions of the MG alternating segment, the MM homopolymer segment, and the GG homopolymer segment of sodium alginate can be calculated using formula (2)-formula (4), which are denoted as ω2, ω3, and ω4, respectively;

[0045]

[0046] The M / G ratio in sodium alginate is calculated using formula (5) according to the titrant dosage. The specific calculation formula is as follows:

[0047]

[0048] Among them, ω2 is the proportion of sodium alginate MG alternating segment, ω3 is the proportion of MM homopolymer segment, and ω4 is the proportion of GG homopolymer segment.

[0049] According to formula (1), the sodium alginate content is 83.65%. The moisture content is 14.98% determined by a moisture meter. It can be calculated that the sodium alginate content on a dry basis is 83.65% / (1-14.98%)=98.39%. The remaining 1.61% solids include chloride ions and calcium ions introduced during the production process, heavy metals adsorbed in the raw materials, and reasonable errors in the experimental results. According to formula (2)-formula (4), the MG alternating segment accounts for 23.83%, the MM homopolymer segment accounts for 40.37%, and the GG homopolymer segment accounts for 35.80%. According to formula (5), the M / G ratio is 1.10.

[0050] Example 2

[0051] Preparation of reagents: Same as Example 1, except that: (1) the concentration of hydrochloric acid is 1.0 M; (2) a polytetrafluoroethylene liner is used and placed in an oven and heated at 90° C., and the acid hydrolysis time is 4 h; (3) the concentration of sodium hydroxide is 0.5 M.

[0052] pH titration: Same as Example 1.

[0053] The first derivative calculation of pH-sodium hydroxide usage curve is the same as in Example 1.

[0054] Numerical calculation: According to formula 1, the content of sodium alginate on a wet basis is 84.20%. The moisture content measured by a moisture meter is 14.98%. It can be calculated that the content of sodium alginate on a dry basis is 99.04%. According to formula 2-4, the proportion of MG alternating segment is 24.00%, the proportion of MM homopolymer segment is 40.10%, and the proportion of GG homopolymer segment is 35.90%. According to formula 5, the M / G ratio is 1.09.

[0055] Example 3

[0056] Preparation of reagents: Same as Example 1, except that: (1) the sample is derived from giant kelp (sample B); (2) the concentration of hydrochloric acid is 0.3 M; (3) the acid hydrolysis time is 4 h; (4) the concentration of sodium hydroxide is 0.5 M.

[0057] pH titration: Same as Example 1.

[0058] The first derivative calculation of pH-sodium hydroxide usage curve is the same as in Example 1.

[0059] Numerical calculation: According to formula 1, the content of sodium alginate on a wet basis is 85.31%. The moisture content measured by a moisture meter is 13.98%. It can be calculated that the content of sodium alginate on a dry basis is 99.18%. According to formula 2-4, the proportion of MG alternating segment is 23.04%, the proportion of MM homopolymer segment is 35.35%, and the proportion of GG homopolymer segment is 41.61%. According to formula 5, the M / G ratio is 0.88.

[0060] Example 4

[0061] Preparation of reagents: Same as Example 1, except that: (1) the sodium alginate is derived from T algae (sample C); and (2) the acid hydrolysis time is 4 h.

[0062] pH titration: Same as Example 1. The difference is that: (1) the pH-sodium hydroxide dosage curve obtained is as follows: Figure 4 shown.

[0063] Calculation of the first derivative of the pH-sodium hydroxide dosage curve: Same as Example 1, except that: (1) Data processing software was used to calculate the Figure 4Perform the first-order derivative operation to obtain the first-order derivative curve Figure 5 , the numbering results are as follows Figure 5 shown.

[0064] Numerical calculation: According to formula 1, the content of sodium alginate on a wet basis is 89.74%. The moisture content measured by a moisture meter is 9.84%. It can be calculated that the content of sodium alginate on a dry basis is 99.53%. According to formula 2-4, the proportion of MG alternating segment is 12.95%, the proportion of MM homopolymer segment is 11.72%, and the proportion of GG homopolymer segment is 75.32%. According to formula 5, the M / G ratio is 0.22.

[0065] Example 5

[0066] Preparation of reagents: Same as Example 1, except that: (1) the source of sodium alginate is T algae (sample D); (2) the amount of sodium alginate is 1.0000 g; (3) the acid hydrolysis time is 2 h; (4) the concentration of potassium hydroxide is 0.4 M.

[0067] pH titration: Same as Example 1.

[0068] The first derivative calculation of pH-sodium hydroxide usage curve is the same as in Example 1.

[0069] Numerical calculation: According to formula 1, the content of sodium alginate on a wet basis is 86.96%. The moisture content measured by a moisture meter is 12.14%. It can be calculated that the content of sodium alginate on a dry basis is 98.98%. According to formula 2-4, the proportion of MG alternating segment is 11.13%, the proportion of MM homopolymer segment is 23.27%, and the proportion of GG homopolymer segment is 65.60%. According to formula 5, the M / G ratio is 0.41.

[0070] Comparative Example 1

[0071] The M / G ratios of sample A, sample B, sample C and sample D were determined using infrared spectroscopy.

[0072] Comparative Example 2

[0073] The M / G ratios of sample A, sample B, sample C and sample D were determined using nuclear magnetic resonance, with reference to industry standard FZ / T51018-2020.

[0074] Comparative Example 3

[0075] The sodium alginate content was calculated by measuring the carbon dioxide content produced by sodium alginate sample A, sample B and sample C after heating using thermal analysis.

[0076] The data of Examples 1 to 5 and Comparative Examples 1-3 are summarized as shown in Table 1. In Example 1 of the present invention and Example 2, different acid hydrolysis methods and titration concentrations are used for Sample A to obtain almost no difference in sodium alginate content, MG alternating segment, MM homopolymer segment, GG homopolymer segment, and M / G ratio, which proves that the determination method of the present invention has good stability and repeatability.

[0077] At the same time, compared with the detection data results of the traditional infrared spectroscopy method (Comparative Example 1), nuclear magnetic resonance method (Comparative Example 2) and thermal analysis method (Comparative Example 3), the M / G ratio determined by the potentiometric titration method has good consistency with the nuclear magnetic method. Comprehensive evaluation of the potentiometric titration method for determining the M / G ratio has simpler operation, higher efficiency, and multiple data measurement results can be obtained in one measurement, which is conducive to reducing production costs and improving efficiency.

[0078] Table 1 Data summary of Examples 1 to 5 and Comparative Examples 1-3

[0079]

[0080]

[0081] In summary, the method for determining the content of sodium alginate and the M / G ratio of the present invention is to hydrolyze sodium alginate into small molecule oligosaccharides and monosaccharides so that the carboxyl group can be fully exposed to react completely with the hydroxide. The acid hydrolysis method adopted, in addition to converting sodium carboxylate into carboxylic acid, will not produce other side reactions to the carboxyl group on the sugar ring, ensuring the corresponding relationship of the one-to-one reaction of the hydroxide and the carboxylate, so the repeatability is high. The content of sodium alginate, the proportion of the MG alternating segment, the MM homopolymer segment, the GG homopolymer segment and the M / G ratio are obtained by calculating the corresponding titrant volume. This detection method can obtain multiple results in one titration, with high efficiency and simple method, and realizes rapid detection of the content of sodium alginate and the M / G ratio. It is easy to operate, simple to calculate, can be realized, has low detection cost and high detection efficiency; it has the universality of promotion, has practical economic benefits in production and scientific research, and can guide the detection and application direction of sodium alginate.

[0082] The above are only embodiments of the present invention. For example, the constant temperature acidolysis reaction temperature is 25-110° C., and the reaction time is 5 min-12 h. The acidolysis reaction uses a magnetic stirring water bath, or a constant temperature water bath, or an oven. The acidolysis reaction process needs to be carried out in a sealed container, which is specifically a sealed glass bottle or a polytetrafluoroethylene reactor liner to prevent the solution from volatilizing. All of these can realize the method for determining the sodium alginate content and the M / G ratio of the present invention.

[0083] The present invention has been described in detail above in conjunction with specific implementations and exemplary examples, but these descriptions cannot be understood as limiting the present invention. Those skilled in the art understand that, without departing from the spirit and scope of the present invention, a variety of equivalent substitutions, modifications or improvements may be made to the technical solution of the present invention and its implementation methods, all of which fall within the scope of the present invention. The scope of protection of the present invention shall be subject to the attached claims.

Claims

1. A method for determining the content of sodium alginate and the M / G ratio, characterized in that: include: Step 1: Acid hydrolysis of sodium alginate: dissolve sodium alginate in an acidic solution, and perform acid hydrolysis at a constant temperature until the reaction is sufficient, which is recorded as solution A; Step 2: titrating the sodium alginate acid hydrolyzate with a potentiometric titrator to obtain a titration curve: titrating the solution A prepared in step 1 with an alkaline titrant with a potentiometric titrator to obtain a pH-titrant dosage curve A; Step 3: Calculation of the first derivative of the titration dosage curve: Differentiate the pH-titrant dosage curve A obtained in step 2 to obtain a first derivative curve B; Step 4 Calculate the M / G ratio in sodium alginate based on the titrant dosage: The alkaline titrant dosage corresponding to the maximum point between pH 2 and 3 is numbered as V A , the amount of alkaline titrant corresponding to pH 2.85 is numbered as V B , the amount of alkaline titrant corresponding to the maximum value between pH 3 and 7 is numbered as V C , the amount of alkaline titrant corresponding to the first maximum point of pH>7 is numbered as V D ; The content of sodium alginate is obtained by calculating the corresponding titrant volume; The content of sodium alginate is obtained by calculating the corresponding titrant volume. The amount of the alkaline titrant is used to determine the sodium alginate content (ω1) using formula 1, where M OH is the molar number of hydroxide in the alkaline titrant, W SA is the mass of sodium alginate tested, the constant 222 is the molecular weight of sodium alginate monomer, and the specific calculation formula is as follows:

2. The method for determining the sodium alginate content and the M / G ratio according to claim 1, characterized in that: In step 4, by calculating the corresponding titrant volume, the proportions of the sodium alginate MG alternating segment, MM homopolymer segment and GG homopolymer segment can be calculated using formulas (2)-(4), which are denoted as ω2, ω3 and ω4 respectively; 3. A method for determining sodium alginate content and M / G ratio according to claim 2, characterized in that: The M / G ratio in sodium alginate is calculated using formula (5) according to the titrant dosage. The specific calculation formula is as follows: Among them, ω2 is the proportion of sodium alginate MG alternating segment, ω3 is the proportion of MM homopolymer segment, and ω4 is the proportion of GG homopolymer segment.

4. The method for determining the sodium alginate content and the M / G ratio according to claim 1, characterized in that: In step 1, the sodium alginate is dissolved in an acidic solution to a concentration of 0.1% to 20%; the acidic solution is one of hydrochloric acid, sulfuric acid, and oxalic acid, and its concentration is 0.1 to 1.0 mol / L; The constant temperature acid hydrolysis reaction temperature is 25-110° C., and the reaction time is 5 min-12 h.

5. The method for determining the sodium alginate content and the M / G ratio according to claim 1, characterized in that: In step 1, the acidolysis reaction is carried out using a magnetic stirring water bath, a constant temperature water bath, or an oven.

6. A method for determining sodium alginate content and M / G ratio according to claim 5, characterized in that: The stirring speed of the magnetic stirring water bath is 0-500r / min.

7. The method for determining the sodium alginate content and the M / G ratio according to claim 1, characterized in that: In step 1, the acid hydrolysis reaction process needs to be carried out in a sealed container, specifically a sealed glass bottle or a polytetrafluoroethylene-lined reactor to prevent the solution from volatilizing.

8. The method for determining the sodium alginate content and the M / G ratio according to claim 1, characterized in that: In step 2, the potentiometric titrator parameters are set as follows: in equal titration mode, the alkaline titrant is added dropwise to the solution A prepared in step 1, with each addition amount of 0.01 to 1 mL.

9. A method for determining sodium alginate content and M / G ratio according to claim 8, characterized in that: In step 2, the alkaline titrant is one of sodium hydroxide and potassium hydroxide, and the concentration of the titrant is 0.01 to 2 mol / L.

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