CN110726803A - Method for determining amino value by automatic potentiometric titration - Google Patents
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Abstract
The invention discloses a method for determining an amino value by an automatic potentiometric titration method, which is characterized in that the method utilizes the principle of diazotization reaction of aromatic primary amine compounds and nitrite, takes sodium nitrite standard solution as a titrant, takes a Pt electrode as an indicating electrode, and determines a titration end point by the maximum value of a first-order derivative of a titration curve to realize quantitative analysis of the amino value. The introduction of hydrobromic acid in a system and the control of titration temperature solve the problem that the potential titration end point of part of low-activity aromatic primary amine compounds has unobvious leap, so that the rate of titration reaction is controlled, and test data can be obtained more quickly and accurately.
Description
Technical Field
The invention belongs to the technical field of analytical chemistry, and particularly relates to a method for testing an amino value of aromatic primary amine by a potentiometric titrator.
Background
The aromatic primary amine compound is common pesticide and medical intermediate, such as p-aminophenol, 2, 5-dichloroaniline, 3' -dichlorobenzidine, sodium p-aminosalicylate and the like. The compounds are characterized in that the compounds all contain aromatic primary amino (Ar-NH)2) The method can be used for carrying out diazotization reaction with sodium nitrite under acidic condition to generate diazonium salt, and the amino value of the compound can be measured by utilizing the principle, so that the absolute content can be obtained. The amino value is used as an important test index of the product quality of the aromatic primary amine compound and is often used for judging the product grade of the compound. Currently, most aromatic primary amine compounds are tested according to a manual titration method in related national standards, an external indicator is adopted to judge a titration end point, namely, the titration end point is indicated by the characteristic that starch potassium iodide test paper turns blue due to slightly excessive sodium nitrite at the titration end point, the end point indicates poor sensitivity, particularly, some samples with certain colors are difficult to control the parallelism and accuracy of the result measured by the method, and the reason is mainly two points: (1) partial aromatic primary amine has low diazotization reaction activity and slow diazotization rate, so that the sensitivity of end point indication is not high; (2) the diazotization reaction temperature is generally lower, resulting in slower reaction rate of the aromatic primary amine with lower partial activity. The patent CN108375620A describes a method for measuring the content of 2,4, 5-triamino-6-hydroxypyrimidine sulfate by an automatic potentiometric titration method, a Pt electrode is used as an indicating electrode for oxidation-reduction potentiometric titration, a titration end point is judged by the maximum value of a first-order derivative ERC of a titration curve, quantitative analysis is realized, and the defect that the end point of an indicator method is not obvious is avoidedThe problem is solved.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a potentiometric titration method for measuring an amino value by using a composite platinum electrode in a mixed acid system at a titration temperature of 0-30 ℃. The innovation points are as follows: (1) a certain amount of hydrobromic acid is added into the system, so that the generation rate of nitrosyl ions can be greatly increased, and the diazotization reaction rate is further improved; (2) because diazotization titration is not used for preparing diazonium salt, the reaction rate can be effectively accelerated by raising the temperature within a certain range, and the titration accuracy is not influenced. Under the optimized test condition, the stability and the parallelism of the result of the mixed acid system method of hydrobromic acid and hydrochloric acid are superior to the titration result of a hydrochloric acid system, the automatic titration end point jump is more obvious due to the improvement of the temperature, so that higher stability and parallelism are obtained, the titration end point can be automatically judged by using an automatic potentiometric titrator and the result can be directly calculated, errors such as reading and color judgment of a manual titration method are avoided, the accuracy and the efficiency of amino value determination are effectively improved, and better precision and standard addition recovery rate are achieved compared with the manual titration method.
The method comprises the following specific steps:
(1) preparing 0.1mol/L sodium nitrite standard solution, calibrating by using an anhydrous sulfanilic acid reference reagent, and directly purchasing the certified sodium nitrite standard solution, wherein the specific concentration is marked as C;
(2) weighing 0.1-0.5 g of 2 parts of sample as a parallel sample, recording the specific mass of the sample as m, using 10mL of 20% hydrochloric acid or glacial acetic acid as a base solution for dissolving the sample, performing ultrasonic dissolution, then adding 1-20 mL of 40% hydrobromic acid, 5mL of 100g/L KBr solution, adding 10-20 mL of distilled water, taking the obtained solution as a sample solution to be detected, and weighing the sample to be accurate to 0.0001 g;
(3) blank experiment: 10mL of 20% hydrochloric acid or glacial acetic acid, and 10mL of 40% hydrobromic acid, 5mL of 100g/L KBr solution and 50mL of distilled water mixed solution are added to serve as blank test solution;
(4) instrument test parameters: titration mode: DET U; pre-volume: 0 mL; pausing: 15 s; signal drift: 20 Mv/min; minimum latency: 10 s; maximum waiting time: 120 s; minimum increment: 20 mu L of the solution; liquid adding speed: a maximum value; stopping volume: 50 mL; stop equivalence point: 3; liquid addition volume after equivalence point: 2 mL; liquid suction speed: a maximum value; equivalence point recognition criteria: 40 mV; equivalent point identification: and max.
(5) Measuring by an instrument: transferring 0.1mol/L sodium nitrite standard solution to liquid adding equipment according to the operation steps of an automatic potentiometric titrator, adopting the instrument testing parameters in the step (4), stirring the liquid to be tested and the blank test solution processed in the steps (2) and (3) respectively in constant temperature baths with the same temperature for about 5-10 min, carrying out potentiometric titration, selecting the corresponding constant temperature bath temperature according to the characteristics of the aromatic primary amine compound to be tested, wherein if the temperature is too low, the reaction activity is too low, the titration time can be prolonged, the titration end point judgment can be influenced, and if the temperature is too high, the titrant sodium nitrite can generate self-decomposition in the titration process, so that the testing result is higher, and the temperature range is generally controlled between 0-30 ℃. Taking the titration volume as the abscissa and the potential value as the ordinate, selecting the volume of the sodium nitrite standard solution consumed when the first derivative of the titration curve reaches the maximum value as the titration volume, wherein the blank solution is marked as V0The sample solution to be measured is marked as V1；
(6) Calculation of results
Amino value content of the arylamine compound:
X＝C*(V1-V0)*Mr/N/m/1000*100％
in the formula:
c: the molar concentration, mol/L, of the sodium nitrite standard solution;
V1: titrating the volume of the solution by using the sodium nitrite standard solution, wherein the volume is mL;
V0: titrating the volume of the blank test solution by using the sodium nitrite standard solution, wherein the volume is mL;
mr: the molar mass of the aromatic primary amine compound, g/mol;
n: the aromatic primary amine compound contains the number of primary aromatic amino groups;
m: sample mass weighed, g.
The relative deviation of the results of the two replicates was not more than 0.5%, and the arithmetic mean of these measurements was taken as the measurement result.
The invention has the beneficial effects that: by adopting the method, the determination of the amino value of most aromatic primary amine compounds can be carried out by a potentiometric titration method, and the titration end point is judged by the maximum value of the first derivative (ERC) of the titration curve, so that the problem that the end point is difficult to judge due to the use of an indicator is avoided, and the human error is reduced. The introduction of hydrobromic acid and the control of titration temperature solve the problem that the potential titration end point jump of part of low-activity aromatic primary amine compound is not obvious, so that the rate of titration reaction is controlled, and test data can be obtained more quickly and accurately.
Drawings
FIG. 1 is a potentiometric titration curve for the amino value of p-nitroaniline.
FIG. 22, potentiometric titration curve for amino acid of 5-dichloroaniline.
FIG. 33, 3 '-dichloro-4, 4' -diaminodiphenylmethane amino value potentiometric titration curve.
FIG. 43, 3' -dichlorobenzidine potentiometric titration curves.
FIG. 5 is a potentiometric titration curve for p-phenylenediamine.
FIG. 6 is a potentiometric titration curve of o-chloro-p-nitroaniline.
Detailed Description
The technical scheme of the invention is further explained by combining specific examples
Example 1 determination of amino value of p-nitroaniline
(1) Instrument for measuring the position of a moving object
A Wantong 888 automatic titrator and a Wantong Pt electrode;
(2) reagent
Electrolyte: 100g/L KBr solution; base solution: 20% hydrochloric acid; 40% hydrobromic acid; sodium nitrite standard titration solution: 0.1000 mol/L;
(3) instrumental test parameters
Titration mode: DET U; pre-volume: 0 mL; pausing: 15 s; signal drift: 20 Mv/min; minimum latency: 10 s; maximum waiting time: 120 s; minimum increment: 20 mu L of the solution; liquid adding speed: a maximum value; stopping volume: 50 mL; stop equivalence point: 3; liquid addition volume after equivalence point: 2 mL; liquid suction speed: a maximum value; equivalence point recognition criteria: 40 mV; equivalent point identification: and max.
(4) Operation of
Solid sample determination: accurately weighing 0.2-0.3 g (accurately to 0.0001g) of a solid sample, adding 20mL of 20% hydrochloric acid, completely dissolving the solid by ultrasonic wave, adding 10mL of water, adding 10-20 mL of hydrobromic acid and 5-20 mL of LKBr solution, placing in an oil bath kettle at 35 ℃, stirring for 10min, titrating by adopting the instrument parameters (if the room temperature is lower, the stirring time needs to be prolonged so as to preheat the base solution to the specified temperature), and simultaneously carrying out a blank experiment.
(5) Formula for automatically calculating content
P-nitroaniline amino value content X ═ C (V)1-V0)*Mr/N/M/1000*100％
In the formula:
c: the molar concentration, mol/L, of the sodium nitrite standard solution;
V1: titrating the volume of the paranitroaniline solution by using the sodium nitrite standard solution, wherein the volume is mL;
V0: titrating the volume of the blank test solution by using the sodium nitrite standard solution, wherein the volume is mL;
mr: the molar mass of p-nitroaniline, g/mol;
n: the p-nitroaniline contains the number of aromatic primary amino groups;
m: sample mass weighed, g.
EXAMPLE 2 determination of amino value of 2, 5-dichloroaniline
(1) Instrument for measuring the position of a moving object
A Wantong 888 automatic titrator and a Wantong Pt electrode;
(2) reagent
Electrolyte: 100g/L KBr solution; base solution: 20% hydrochloric acid; 40% hydrobromic acid; sodium nitrite standard titration solution: 0.1000mol/L
(3) Instrumental test parameters
Titration mode: DET U; pre-volume: 0 mL; pausing: 15 s; signal drift: 20 Mv/min; minimum latency: 10 s; maximum waiting time: 120 s; minimum increment: 20 mu L of the solution; liquid adding speed: a maximum value; stopping volume: 50 mL; stop equivalence point: 3; liquid addition volume after equivalence point: 2 mL; liquid suction speed: a maximum value; equivalence point recognition criteria: 40 mV; equivalent point identification: and max.
(4) Operation of
Solid sample determination: accurately weighing 0.2-0.3 g (accurately to 0.0001g) of a solid sample, adding 50mL of 20% hydrochloric acid, carrying out ultrasonic treatment to completely dissolve the solid, adding 5-10 mL of hydrobromic acid, 10mL of KBr solution and a little ice, placing in a constant temperature bath at 0 ℃, stirring for 5min, carrying out titration by adopting the parameters of the instrument, and simultaneously carrying out a blank experiment.
(5) Formula for automatically calculating content
2, 5-dichloroaniline amino value content X ═ C (V)1-V0)*Mr/N/M/1000*100％
In the formula:
c: the molar concentration, mol/L, of the sodium nitrite standard solution;
V1: titrating the volume of the 2, 5-dichloroaniline solution by using the sodium nitrite standard solution, wherein the volume is mL;
V0: titrating the volume of the blank test solution by using the sodium nitrite standard solution, wherein the volume is mL;
mr: molar mass of 2, 5-dichloroaniline, g/mol;
n: the number of aromatic primary amino groups contained in the 2, 5-dichloroaniline;
m: sample mass weighed, g.
Example 33, 3 '-dichloro-4, 4' -diaminodiphenylmethane amino value determination
(1) Instrument for measuring the position of a moving object
A Wantong 888 automatic titrator and a Wantong Pt electrode;
(2) reagent
Electrolyte: 100g/L KBr solution; base solution: 20% hydrochloric acid; 40% hydrobromic acid; sodium nitrite standard titration solution: 0.1000mol/L
(3) Instrumental test parameters
Titration mode: DET U; pre-volume: 0 mL; pausing: 15 s; signal drift: 20 Mv/min; minimum latency: 10 s; maximum waiting time: 120 s; minimum increment: 20 mu L of the solution; liquid adding speed: a maximum value; stopping volume: 50 mL; stop equivalence point: 3; liquid addition volume after equivalence point: 2 mL; liquid suction speed: a maximum value; equivalence point recognition criteria: 40 mV; equivalent point identification: and max.
(4) Operation of
Solid sample determination: accurately weighing 0.2-0.3 g (accurately to 0.0001g) of a solid sample, adding 30mL of 20% hydrochloric acid, carrying out ultrasonic treatment to completely dissolve the solid, adding 10-30 mL of hydrobromic acid and 10mL of LKBr solution, placing in a constant temperature bath at 20 ℃, stirring for 5min, carrying out potentiometric titration by adopting the titration parameters, and simultaneously carrying out blank experiments.
(5) Formula for automatically calculating content
3,3 '-dichloro-4, 4' -diaminodiphenylmethanamine group content X ═ C (V)1-V0)*Mr/N/M/1000*100％
In the formula:
c: the molar concentration, mol/L, of the sodium nitrite standard solution;
V1: titrating the volume of the 2, 5-dichloroaniline solution by using the sodium nitrite standard solution, wherein the volume is mL;
V0: titrating the volume of the blank test solution by using the sodium nitrite standard solution, wherein the volume is mL;
mr: the molar mass of 3,3 '-dichloro-4, 4' -diaminodiphenylmethane, g/mol;
n: the 3,3 '-dichloro-4, 4' -diaminodiphenylmethane contains the number of primary aromatic amino groups;
m: sample mass weighed, g.
Example 43, 3' -Dichlorobenzidine amino value determination
(1) Instrument for measuring the position of a moving object
A Wantong 888 automatic titrator and a Wantong Pt electrode;
(2) reagent
Electrolyte: 100g/L KBr solution; base solution: 20% hydrochloric acid; 40% hydrobromic acid; sodium nitrite standard titration solution: 0.1000mol/L
(3) Instrumental test parameters
Titration mode: DET U; pre-volume: 0 mL; pausing: 15 s; signal drift: 20 Mv/min; minimum latency: 10 s; maximum waiting time: 120 s; minimum increment: 20 mu L of the solution; liquid adding speed: a maximum value; stopping volume: 50 mL; stop equivalence point: 3; liquid addition volume after equivalence point: 2 mL; liquid suction speed: a maximum value; equivalence point recognition criteria: 40 mV; equivalent point identification: and max.
(4) Operation of
Solid sample determination: accurately weighing 0.2-0.3 g (accurately to 0.0001g) of a solid sample, adding 50mL of 20% hydrochloric acid, carrying out ultrasonic treatment to completely dissolve the solid, adding 10-30 mL of hydrobromic acid and 10mL of LKBr solution, placing in a constant temperature bath at 10 ℃, stirring for 5min, carrying out potentiometric titration by adopting the titration parameters, and simultaneously carrying out blank experiments.
(5) Formula for automatically calculating content
3, 3' -dichlorobenzidine having an amino value content of X ═ C (V)1-V0)*Mr/N/M/1000*100％
In the formula:
c: the molar concentration, mol/L, of the sodium nitrite standard solution;
V1: titrating the volume of the 2, 5-dichloroaniline solution by using the sodium nitrite standard solution, wherein the volume is mL;
V0: titrating the volume of the blank test solution by using the sodium nitrite standard solution, wherein the volume is mL;
mr: molar mass of 3, 3' -dichlorobenzidine, g/mol;
n: the 3, 3' -dichlorobenzidine contains the number of primary aromatic amino groups;
m: sample mass weighed, g.
EXAMPLE 5 amino number determination of p-phenylenediamine
(1) Instrument for measuring the position of a moving object
A Wantong 888 automatic titrator and a Wantong Pt electrode;
(2) reagent
Electrolyte: 100g/L KBr solution; base solution: 20% hydrochloric acid; 40% hydrobromic acid; sodium nitrite standard titration solution: 0.1000mol/L
(3) Instrumental test parameters
Titration mode: DET U; pre-volume: 0 mL; pausing: 15 s; signal drift: 20 Mv/min; minimum latency: 10 s; maximum waiting time: 120 s; minimum increment: 20 mu L of the solution; liquid adding speed: a maximum value; stopping volume: 50 mL; stop equivalence point: 3; liquid addition volume after equivalence point: 2 mL; liquid suction speed: a maximum value; equivalence point recognition criteria: 40 mV; equivalent point identification: and max.
(4) Operation of
Solid sample determination: accurately weighing 0.2-0.3 g (accurately to 0.0001g) of a solid sample, adding 50mL of 20% hydrochloric acid, carrying out ultrasonic treatment to completely dissolve the solid, adding 5-15 mL of hydrobromic acid, 10mL of KBr solution and a little ice, placing in a constant temperature bath at 0 ℃, stirring for 5min, carrying out potentiometric titration by adopting the titration parameters, and simultaneously carrying out a blank experiment.
(5) Formula for automatically calculating content
P-phenylenediamine amino value content X ═ C (V)1-V0)*Mr/N/M/1000*100％
In the formula:
c: the molar concentration, mol/L, of the sodium nitrite standard solution;
V1: titrating the volume of the 2, 5-dichloroaniline solution by using the sodium nitrite standard solution, wherein the volume is mL;
V0: titrating the volume of the blank test solution by using the sodium nitrite standard solution, wherein the volume is mL;
mr: the molar mass of p-phenylenediamine, g/mol;
n: the p-phenylenediamine contains the number of aromatic primary amino groups;
m: sample mass weighed, g.
EXAMPLE 6 measurement of amino value of o-chloro-p-nitroaniline
(1) Instrument for measuring the position of a moving object
A Wantong 888 automatic titrator and a Wantong Pt electrode;
(2) reagent
Electrolyte: 100g/L KBr solution; base solution: 20% glacial acetic acid; 40% hydrobromic acid; sodium nitrite standard titration solution: 0.1000mol/L
(3) Instrumental test parameters
Titration mode: DET U; pre-volume: 0 mL; pausing: 15 s; signal drift: 20 Mv/min; minimum latency: 10 s; maximum waiting time: 120 s; minimum increment: 20 mu L of the solution; liquid adding speed: a maximum value; stopping volume: 50 mL; stop equivalence point: 3; liquid addition volume after equivalence point: 2 mL; liquid suction speed: a maximum value; equivalence point recognition criteria: 40 mV; equivalent point identification: and max.
(4) Operation of
Solid sample determination: accurately weighing 0.2-0.3 g (accurately to 0.0001g) of a solid sample, adding 15mL of glacial acetic acid, carrying out ultrasonic treatment to completely dissolve the solid, adding 10-30 mL of hydrobromic acid, 10mL of KBr solution and a little of ice, placing in a constant temperature bath at 0 ℃, stirring for 5min, carrying out potentiometric titration by adopting the titration parameters, and simultaneously carrying out a blank experiment.
(5) Formula for automatically calculating content
O-chloro-p-nitroaniline amino value content X ═ C (V)1-V0)*Mr/N/M/1000*100％
In the formula:
c: the molar concentration, mol/L, of the sodium nitrite standard solution;
V1: titrating the volume of the 2, 5-dichloroaniline solution by using the sodium nitrite standard solution, wherein the volume is mL;
V0: titrating the volume of the blank test solution by using the sodium nitrite standard solution, wherein the volume is mL;
mr: the molar mass of the o-chloro-p-nitroaniline, g/mol;
n: the o-chloro-p-nitroaniline contains the number of primary aromatic amino groups;
m: sample mass weighed, g.
Precision of the method
6 parts of samples of the same batch with different masses are weighed respectively aiming at the six compounds, the content of amino value is determined by adopting respective titration conditions, and the result is shown in table 1:
TABLE 1 precision test results
(HPLC: high pressure liquid chromatography, RSD: relative standard deviation)
The data in the table show that the titration result RSD meets the requirements and has good repeatability.
Claims (2)
1. A method for determining an amino value by an automatic potentiometric titration method is characterized in that a principle that an aromatic primary amine compound and nitrite are subjected to diazotization reaction is utilized, a sodium nitrite standard solution is used as a titrant, a Pt electrode is used as an indicating electrode, and a titration end point is judged by the maximum value of a first-order derivative of a titration curve, so that the quantitative analysis of the amino value is realized;
the method comprises the following specific steps:
(1) preparing 0.1mol/L sodium nitrite standard solution, calibrating by using an anhydrous sulfanilic acid reference reagent, and directly purchasing the certified sodium nitrite standard solution, wherein the specific concentration is marked as C;
(2) weighing 0.1-0.5 g of 2 parts of sample as a parallel sample, recording the specific mass of the sample as m, using 10mL of 20% hydrochloric acid or glacial acetic acid as a base solution for dissolving the sample, performing ultrasonic dissolution, then adding 1-20 mL of 40% hydrobromic acid, 5mL of 100g/L KBr solution, adding 10-20 mL of distilled water, taking the obtained solution as a sample solution to be detected, and weighing the sample to be accurate to 0.0001 g;
(3) blank experiment: 10mL of 20% hydrochloric acid or glacial acetic acid, and 10mL of 40% hydrobromic acid, 5mL of 100g/L KBr solution and 50mL of distilled water mixed solution are added to serve as blank test solution;
(4) measuring by an instrument: according to the operation steps of an automatic potentiometric titrator, 0.1mol/L sodium nitrite standard solution is transferred to liquid adding equipment, a titration mode is set as dynamic potentiometric titration, liquid to be measured and blank test solution are respectively stirred in a constant temperature bath at the same temperature for 5-10 min and then potentiometric titration is carried out, the corresponding constant temperature bath temperature is selected according to the characteristics of the aromatic primary amine compound to be measured, the temperature range is controlled between 0-30 ℃, the titration volume is used as a horizontal coordinate, the potential value is used as a vertical coordinate, the volume of the sodium nitrite standard solution consumed when the first derivative of the titration curve reaches the maximum value is selected as the titration volume, wherein the blank test solution is marked as V0The sample solution to be measured is marked as V1；
(5) Calculation of results
Amino value content of the arylamine compound:
X＝C*(V1-V0)*Mr/N/m/1000*100％
in the formula:
c: the molar concentration, mol/L, of the sodium nitrite standard solution;
V1: titrating the volume of the solution by using the sodium nitrite standard solution, wherein the volume is mL;
V0: titrating the volume of the blank test solution by using the sodium nitrite standard solution, wherein the volume is mL;
mr: the molar mass of the aromatic primary amine compound, g/mol;
n: the aromatic primary amine compound contains the number of primary aromatic amino groups;
m: weighing the mass of the sample, g;
the relative deviation of the results of the two replicates was not more than 0.5%, and the arithmetic mean of these measurements was taken as the measurement result.
2. The automated potentiometric titration method of claim 1, wherein the instrument tests parameters: titration mode DET U; pre-volume is 0 mL; pausing for 15 s; signal drift is 20 Mv/min; minimum latency 10 s; maximum wait time 120 s; minimum increment 20 μ L; the liquid adding speed is the maximum value; stopping volume: 50 mL; stop equivalence point 3; adding 2mL of liquid after the equivalence point; liquid suction speed: a maximum value; equivalence point recognition criteria: 40 mV; equivalent point identification: and max.
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| CN113917072A (en) * | 2021-10-11 | 2022-01-11 | 聚光科技(杭州)股份有限公司 | Detection method based on reaction process control |
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