CN106769941B - Method for detecting phosphorus content in biodiesel - Google Patents
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Abstract
A method for detecting phosphorus content in biodiesel comprises the steps of firstly generating soap through saponification reaction of KOH and the biodiesel, then carrying out temperature programming on the soap for direct ashing, and finally determining the phosphorus content through a classical phosphomolybdic blue colorimetric method. The generation of the solid soap overcomes potential safety hazards such as splashing and burning of samples in the pretreatment process of the liquid biodiesel, and improves the precision and accuracy of the method. The ashing product of the soap is soluble phosphate, and the subsequent complicated operation is avoided. The RSD of the method established by the invention is 2.84%, the recovery rate is 96.69-104.85%, the detection limit of the method is 0.36mg/kg, and the quantification limit is 1.22 mg/kg. The result shows that the method established by the invention does not depend on large-scale instruments, has lower cost and higher accuracy and precision, and can meet the requirement of daily detection of the phosphorus content in the biodiesel.
Description
Technical Field
The invention relates to a method for measuring phosphorus content, in particular to a method for detecting phosphorus content in biodiesel.
Background
Compared with the traditional petroleum diesel, the biodiesel has the advantages of being renewable, clean, safe, full in combustion and the like, and is more and more valued by people. As phosphorus in the biodiesel can poison a diesel engine exhaust catalyst and increase the emission of pollutants, the phosphorus content in the biodiesel is not more than 10mg/kg according to China Standard GB/T20828 2015, American Standard ASTM D6751, Brazilian Standard ANP 07/2008 and the like.
The above standards all specify that inductively coupled plasma optical emission spectrometry (ICP) is used for phosphorus detection. The ICP method has high detection efficiency, but the instrument is expensive, the detection cost is high, the used organic solvent such as dimethylbenzene has high toxicity, and the plasma sample has the characteristic of instability in the transmission process.
Therefore, methods for determining the content of phosphorus in biodiesel based on classical phosphomolybdic blue reaction are established, the methods convert organic phosphorus in a sample into inorganic phosphate through a sample pretreatment process, then react phosphate with a color developing agent to form a blue complex, and determine the content of phosphorus in biodiesel according to the absorbance of the blue complex.
Wu\28156Xin et al (university of Changzhou, 2004,16(2):23-25) add ZnO into biodiesel, perform sample pretreatment by carbonizing first and then ashing, add MgO into biodiesel by Silveira (Fuel,2011,90(11): 3485-.
However, the operation process of the biodiesel sample pretreatment has the following problems: firstly, the sample is easy to splash and burn in the carbonization or ashing process, potential safety hazards exist, and the precision and accuracy of sample detection are poor. Secondly, the generated ashing product is insoluble in water and needs to be dissolved by strong acid under the heating condition, and the pH value of the subsequent steps is repeatedly adjusted by acid and alkali solution to meet the requirement of color reaction. The whole detection process is tedious and tedious, and the measurement result is inferior to the ICP method specified in the standard. In addition, the biodiesel serving as an automotive fuel has a very low flash point, Wu\28156Xin adopts an electric furnace for direct heating and carbonization, and has great potential safety hazard.
Disclosure of Invention
The invention aims to overcome the defects of the method and provide the method for detecting the phosphorus content in the biodiesel, which is independent of large instruments specified in the national standard, has low cost and simple detection process, and can meet the requirements of on-site batch detection of the biodiesel.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a method for detecting phosphorus content in biodiesel comprises the steps of carrying out saponification reaction on the biodiesel and KOH to generate soap, ashing the soap by adopting a temperature programming mode, and measuring the phosphorus content in the biodiesel by a phosphorus-molybdenum blue colorimetric method; wherein the mass ratio of the biodiesel to the KOH is 6.0: 1.0 to 2.0.
The further improvement of the invention is that the temperature of saponification reaction is 20-30 ℃, and the reaction time is 30-80 min; or the temperature of the saponification reaction is 40-100 ℃, and the reaction time is 10-30 min.
The invention is further improved in that the specific process of ashing the soap by adopting a temperature programming mode comprises the following steps: the temperature is preserved for 40-80 min at 100-140 ℃, then preserved for 150-210 min at 230-270 ℃, preserved for 30-90 min at 280-320 ℃, and finally preserved for 90-150 min at 650-700 ℃.
The method is further improved in that the ash is cooled to room temperature after ashing, ash is dissolved in boiling water, the pH value of filtrate is adjusted to 2-4 by hydrochloric acid solution after filtering, and the volume is constant.
The invention has the further improvement that the specific process for determining the content of phosphorus in the biodiesel by the phosphomolybdic blue colorimetric method comprises the following steps:
① standard curve is drawn, wherein 0mL, 1 mL, 2mL, 4 mL, 6 mL and 8mL of solution are respectively transferred from the phosphate standard solution to a 50mL colorimetric tube, 8mL of hydrazine sulfate solution and 2mL of sodium molybdate dilute sulfuric acid solution are respectively added, the volume is fixed to 50mL, the solution is shaken up, the reaction is carried out for 10min at 100 ℃, the solution is cooled to the room temperature, the volume is fixed to 50mL, the absorbance of the solution is measured at 650nm, and a standard curve of phosphorus content and absorbance is drawn;
② phosphomolybdic blue colorimetry, wherein 40mL of sample solution is transferred from a volumetric flask to a colorimetric tube of 50mL, 8mL of hydrazine sulfate solution and 2mL of sodium molybdate dilute sulfuric acid solution are added, the volume is fixed to 50mL, the mixture is shaken up, the reaction is carried out for 10min at 100 ℃, the mixture is cooled to room temperature, the volume is fixed to 50mL, the absorbance of the solution is measured at 650nm, the phosphorus content in the sample solution is obtained through a standard curve, and the phosphorus content in the biodiesel is calculated by a formula (1);
in the formula, X: the phosphorus content in the biodiesel is mg/kg; p: phosphorus content in sample solution, mg; v1: volume of sample diluted after ashing, mL; v2: the volume of the measured liquid taken during the color comparison is mL; m: the quality of the biodiesel is determined by the quality of the biodiesel,g。
the invention further improves the preparation of phosphate stock solution: 0.4387g of potassium dihydrogen phosphate which is dried for 24 hours is weighed, dissolved and diluted by water to be 1000mL, and the solution contains 0.1mg/mL of phosphorus;
preparation of a phosphate standard solution: sucking 10mL to 100mL of phosphate stock solution into a volumetric flask by a pipette, adding water for dilution and fixing the volume, wherein the solution contains 0.01mg/mL of phosphorus;
preparing a hydrazine sulfate solution: dissolving 0.15g of hydrazine sulfate in 1000mL of water to obtain 0.015% hydrazine sulfate solution;
preparing a sodium molybdate dilute sulfuric acid solution: measuring 140mL of concentrated sulfuric acid with the mass fraction of 98% into 300mL of water, cooling to room temperature, adding 12.5g of sodium molybdate, dissolving, metering the volume with water into a 500mL volumetric flask, shaking up, and standing for 24h for later use;
preparing a hydrochloric acid solution: concentrated hydrochloric acid with a mass fraction of 37% was dissolved in an equal volume of water.
Compared with the prior art, the invention has the following beneficial effects: according to the invention, firstly, soap is generated through saponification reaction of KOH and biodiesel, then the soap is directly ashed after temperature programming, and finally the phosphorus content is measured through a classical phosphomolybdic blue colorimetric method. The generation of the solid soap overcomes potential safety hazards such as splashing and burning of samples in the pretreatment process of the liquid biodiesel, and improves the precision and accuracy of the method. The ashing product of the soap is soluble phosphate, so that subsequent complicated operation is avoided, and the efficiency of the whole detection process is improved. The RSD of the method established by the invention is 2.84%, the recovery rate is 96.69-104.85%, the detection limit of the method is 0.36mg/kg, and the quantification limit is 1.22 mg/kg. The method disclosed by the invention is independent of large instruments, is low in cost and high in accuracy and precision, and can meet the daily detection requirements of the phosphorus content in the biodiesel from different raw material sources.
Furthermore, saponification reaction is carried out at 40-100 ℃, so that the reaction speed can be increased.
Further, specific conditions of ashing in the present invention are: the temperature is preserved for 40-80 min at 100-140 ℃, then preserved for 150-210 min at 230-270 ℃, preserved for 30-90 min at 280-320 ℃, and finally preserved for 90-150 min at 650-700 ℃. With the ashing conditions of the present invention, ashing can be completely performed.
Drawings
FIG. 1 is a TG curve of the ashing process with different additives added to biodiesel.
FIG. 2 is a DTG curve of the ashing process with different additives of biodiesel.
FIG. 3 is a standard curve of the phosphorus content of the solution.
Detailed Description
The present invention will be described in detail with reference to the accompanying drawings.
The principle of the invention is that KOH and biodiesel are utilized to generate soap through saponification reaction, so that the ashing process of the biodiesel is converted into the ashing process of the soap, and organic phosphorus in the soap is converted into inorganic phosphorus through temperature programming and ashing. And finally, reacting the water-soluble phosphate radical with a color developing agent to form a phosphorus-molybdenum blue product, and measuring the phosphorus content in the biodiesel according to the absorbance of the phosphorus-molybdenum blue product.
The invention is realized as follows:
preparing a phosphate stock solution: 0.4387g of potassium dihydrogen phosphate dried for 24 hours were weighed, dissolved in water and diluted to 1000mL, and the solution contained 0.1mg/mL of phosphorus.
Preparation of a phosphate standard solution: and (4) sucking 10mL to 100mL of the phosphate stock solution into a volumetric flask by using a pipette, adding water for dilution and fixing the volume. This solution contained 0.01mg/mL of phosphorus.
Preparing a hydrazine sulfate solution: 0.15g of hydrazine sulfate was dissolved in 1000mL of water to obtain a 0.015% hydrazine sulfate solution.
Preparing a sodium molybdate dilute sulfuric acid solution: 140mL of concentrated sulfuric acid (mass fraction 98%) is measured and injected into 300mL of water, the mixture is cooled to room temperature, 12.5g of sodium molybdate is added, the volume is determined to be 500mL of volumetric flask by water after the sodium molybdate is dissolved, the mixture is fully shaken up and stands for 24h for standby.
Preparing a hydrochloric acid solution: concentrated hydrochloric acid with a mass fraction of 37% was dissolved in an equal volume of water.
The reagents used were all analytically pure, and the water used was distilled water.
1) Pretreatment of biodiesel samples by saponification
① saponification reaction, weighing biodiesel sample (about 6.0g) and solid KOH (1.0-2.0 g) in a crucible, and saponifying biodiesel with KOH under room temperature or heating to accelerate the reaction.
The reaction conditions at room temperature are as follows: the reaction time is 30-80 min under the condition of room temperature (20-30 ℃).
The heating may be carried out in a water bath or in a muffle furnace. The temperature is controlled to be 40-100 ℃, and the reaction time is 10-30 min. After saponification, the biodiesel is converted from a liquid state to a solid state soap.
②, ashing the soap by a temperature programming method, wherein the ashing condition is that the temperature is kept at 100-140 ℃ for 40-80 min, the temperature is kept at 230-270 ℃ for 150-210 min, then the temperature is kept at 280-320 ℃ for 30-90 min, and finally the temperature is kept at 650-700 ℃ for 90-150 min.
And cooling the crucible to room temperature after ashing, dissolving ash in the crucible by boiling water, adjusting the pH value of the filtrate to 2-4 by using a hydrochloric acid solution after filtering, and metering the volume to a 50mL volumetric flask to obtain a sample solution.
2) Determination of phosphorus content in biodiesel by phosphomolybdic blue colorimetry
① standard curve is drawn by transferring 0, 1,2, 4, 6 and 8mL of solution from phosphate standard solution into 50mL of colorimetric tube, adding 8mL of hydrazine sulfate solution and 2mL of sodium molybdate dilute sulfuric acid solution, fixing volume to 50mL, shaking up, reacting at 100 deg.C for 10min, cooling to room temperature, fixing volume to 50mL, measuring absorbance of solution at 650nm, and drawing standard curve of phosphorus content and absorbance.
② phosphomolybdic blue colorimetry, wherein 40mL of sample solution is moved from a measuring flask to carry out a color reaction, the specific process is that 40mL of sample solution is moved from the measuring flask to a colorimetric tube of 50mL, 8mL of hydrazine sulfate solution and 2mL of sodium molybdate dilute sulfuric acid solution are added, the constant volume is 50mL, the mixture is shaken up, the reaction is carried out for 10min at 100 ℃, the mixture is cooled to the room temperature, the constant volume is 50mL, the absorbance of the solution is measured at 650nm, the phosphorus content in the sample solution is obtained through a standard curve, and the phosphorus content in the biodiesel is calculated by a formula (1).
In the formula, X: the phosphorus content in the biodiesel is mg/kg; p: phosphorus content in sample solution, mg; v1: volume of sample diluted after ashing, mL; v2: the volume of the measured liquid taken during the color comparison is mL; m: mass of biodiesel, g.
The invention adds KOH and biodiesel to generate soap through saponification reaction, and then directly heats up by program to ash the soap, which has the following advantages:
1) the prior art is an ashing process of liquid biodiesel, oil drops are easy to splash and even burn in a sample pretreatment process, and potential safety hazards exist. The generation of the solid soap overcomes the splashing and burning in the pretreatment process of the sample, so that the potential safety hazard is reduced, and the precision and the accuracy of the method are improved; 2) the thermal stability of the generated soap is better than that of biodiesel, and higher ashing final temperature and faster heating rate can be adopted, so that the ashing time is shortened; 3) the product after ashing is water-soluble phosphate, so that the subsequent operation steps are simplified, and the efficiency of the whole detection process is improved.
The following details illustrate the invention and the effects of adding different additives on the ashing process of biodiesel, the effects of different additives on the detection of the phosphorus content of rapeseed crude oil and biodiesel, the effects of different alkaline additives on the determination of the phosphorus content of biodiesel, the determination of the KOH dosage, the detection limit, the precision, the accuracy and other aspects.
1. Thermogravimetric analysis of influence of different additives on biodiesel ashing process
The incineration process of the biodiesel is essentially the pyrolysis process of the biodiesel. The incineration process of the biodiesel and the influence of adding different additives such as MgO, ZnO and KOH in the biodiesel on the incineration of the biodiesel can be known through thermogravimetric analysis.
The conditions for thermogravimetric analysis were: the biodiesel pyrolysis is carried out on an STA 449F 3 synchronous thermal analyzer (Chilo-resistant company, Germany), the sample mass is 5mg, the protective gas and the reaction gas are both nitrogen (purity is 99.999%), the protective gas flow is 30mL/min, the reaction gas flow is 50mL/min, the heating rate is 5K/min, and the heating interval is 40-750 ℃. The test results are shown in fig. 1 and fig. 2, and table 1 shows characteristic parameters of thermogravimetric analysis.
TABLE 1 characteristic parameters of the pyrolysis curves of biodiesel
As can be seen from Table 1, the pyrolysis weight loss interval of the biodiesel is between 85.0 and 451.5 ℃, the peak temperature of the weight loss is 233.9 ℃, the maximum weight loss rate is 9.9465%/min, and the pyrolysis results are basically consistent with the pyrolysis results of the biodiesel reported by Chien (Energy Fuels,2008,23(1): 202-.
After the MgO is added, the thermal decomposition peak temperature in the biodiesel is reduced to 218.3 ℃, and the maximum weight loss rate is increased to 10.8773%/min, which shows that the MgO promotes the pyrolysis of the biodiesel. This is similar to the results reported for the fast pyrolysis of MgO-catalyzed organic materials by Fermoso (Catalysis Today,2016,34(04): 1022-. Due to the violent pyrolysis, various gases generated are easy to cause splashing and even burning of liquid samples in ashing.
After ZnO is added, the thermal decomposition peak temperature of the biodiesel is basically unchanged, but the maximum weight loss rate is slightly lower than that of the biodiesel, and the action mechanism of biodiesel ashing cannot be directly read from thermogravimetric data. After the addition of ZnO, although the thermal decomposition rate is reduced, the sample still has splashing and even burning phenomena because the sample is liquid.
After KOH is added, the thermal decomposition behavior of the biodiesel is different from the pyrolysis behavior of the added MgO and ZnO, the pyrolysis peak temperature is increased to 236.7 ℃, the maximum weight loss rate is obviously reduced, and a DTG curve of the biodiesel has one more pyrolysis peak at 454.8 ℃, which is consistent with the pyrolysis peak of soap at the temperature, and the reaction of the biodiesel and the KOH generates the soap.
It can be seen that the mechanism of action of metal oxides (MgO or ZnO) and metal hydroxides KOH in biodiesel ashing processes is different. The action mechanism of ZnO on the ashing process is unknown, and MgO catalyzes the ashing process of biodiesel, which is represented by the reduction of the thermal decomposition peak temperature and the improvement of the maximum weight loss rate. KOH reacts with biodiesel to produce soap, and the thermal decomposition parameters of the soap are basically consistent with those of the soap.
On one hand, the viscous semi-solid soap overcomes potential safety hazards of splashing, burning and the like of a liquid biodiesel sample, and improves the accuracy and precision of the method. On the other hand, the soap has better thermal stability, and the whole can adopt faster temperature rising rate and higher ashing temperature, thereby shortening the time of the ashing process compared with the prior art. In addition, the ashing product after adding KOH is easily soluble potassium phosphate, strong acid dissolution and a subsequent pH adjusting step are not needed, the operation steps are further reduced, and the efficiency of the method is improved.
Saponification is a type of hydrolysis of fats and oils, which is carried out in the grease industry using an aqueous solution of fats and oils and an alkali. While, as can be seen from the foregoing thermogravimetric analysis, biodiesel can react with solid base to form soap, although the reaction mechanism is unknown.
2. Influence of different auxiliary agents on detection of phosphorus content of rapeseed crude oil and biodiesel-comparison of different methods
1) Detection of phosphorus content in rapeseed crude oil
ZnO, MgO and KOH are respectively added into the rapeseed crude oil, and one part is blank.
The pretreatment process of the rapeseed crude oil sample added with ZnO and the blank sample is carried out according to the GB/T5537-2008 method; the pretreatment process of the MgO-added rapeseed crude oil sample is carried out according to the standard ISO 10540-1; the pretreatment process of the rapeseed crude oil sample added with KOH is carried out according to the method of the invention. Each set of 5 replicates was tested and the results are shown in table 2.
TABLE 2 comparison of the results of measuring the phosphorus content in crude rapeseed oil by different methods
2) Detection of phosphorus content in biodiesel
Similarly, five samples of biodiesel were taken, one blank and one phosphorus was detected by ICP method specified in GB/T20828-. The remaining 3 parts are added with ZnO, MgO and KOH respectively.
The pretreatment process of the biodiesel sample added with ZnO is carried out according to a method of Wu\28156Xin (university of Changzhou, 2004,16(2): 23-25); the pretreatment process of the biodiesel sample added with the MgO and the blank sample is carried out according to the method of Silverer (Fuel,2011,90(11): 3485-3488); the pretreatment process of the biodiesel sample with KOH added is carried out according to the method of the invention. Each set tested 5 replicates and the results are shown in table 3.
TABLE 3 comparison of the results of the phosphorus content in biodiesel measured by different methods
As can be seen from tables 2 and 3, the phosphorus content measured in the blank test without any additive is very low and almost negligible in both the vegetable oil and the biodiesel, and the phosphorus content measured by adding any additive is far greater than that of the blank test, which indicates that the additive is indispensable for measuring phosphorus and has a capturing effect on phosphorus. From the viewpoint of dissolution of ash, adding ZnO, MgO and KOH, respectively, results in the formation of insoluble zinc phosphate, magnesium phosphate and soluble potassium phosphate.
For measuring the phosphorus content in the oil, although the precision (measured by RSD) of the KOH method is obviously better than that of GB/T5537-2008 and ISO 10540-1, the measured values have no obvious difference, and the three methods can be applied to the detection of the phosphorus content in the vegetable oil.
For measuring the phosphorus content in the biodiesel, the KOH method is comparable to the ICP method specified by the standard, and the difference between the measured value and the standard ICP method is large in ZnO and MgO methods and between the RSD method and the standard ICP method, so that the requirement for measuring the phosphorus content in the biodiesel is difficult to meet.
3. Effect of different alkaline adjuvants on biodiesel phosphorus content determination
The main improvement of the invention is that alkali and biodiesel generate soap through saponification reaction, and KHCO is selected3KOAc (potassium acetate), K2CO3Common alkaline substances such as KOH and NaOH, and the influence of these alkaline auxiliaries on the measurement of phosphorus in biodiesel was examined.
The specific process is as follows: six biodiesel samples (about 6.0g) were taken and about 1.5g of solid KHCO was added separately3、KOAc、K2CO3KOH and NaOH are used for measuring the phosphorus content in the biodiesel by adopting the same pretreatment method as the method, and the other part is used for detecting the phosphorus by adopting the ICP method specified in the national standard GB/T20828-. Each set of 5 replicates of the above experiment were tested and the results are shown in table 4.
TABLE 4 influence of different basic auxiliary Agents on the determination of the phosphorus content of biodiesel
In the strong alkali weak acid salt, the alkalinity is KHCO from weak to strong in sequence3、KOAc、K2CO3KOH and NaOH are strong bases, and the generation of solid soap after the alkaline substances are added into the biodiesel is closely related to the alkalinity of the biodiesel. Adding KHCO3、KOAc、K2CO3Meanwhile, the detection result is also difficult to be compared with the national standard method, and the detection requirement cannot be met. In the experiment that the auxiliary agent is NaOH, the solution is turbid after the color reaction, and the auxiliary agent cannot be applied.
Only when the auxiliary agent is KOH, the measured result is equivalent to the national standard ICP method, so KOH is finally selected as the auxiliary agent for detecting the phosphorus content of the biodiesel.
4. Determination of the amount of auxiliary KOH
Four biodiesel samples (about 6.0g) were taken and added with about 1.0, 1.5, 2.0, 2.5g of solid KOH, respectively, and the phosphorus content in the biodiesel was measured by the pretreatment method of the present invention. Each set of 5 replicates of the above experiment were tested and the results are shown in table 5.
TABLE 5 Effect of the amount of KOH on the phosphorus content results
As can be seen from the data in Table 5, the amounts of KOH were 1.0, 1.5, and 2.0g, and the measured phosphorus contents were comparable to the national standard ICP method and all satisfied the requirements, but the addition amount of 1.5g gave the lowest RSD and the highest precision. When the KOH content is 2.5g, flocculent precipitate appears after the pH is adjusted by hydrochloric acid, and the solution after the color reaction is turbid liquid, so that the real phosphorus content can not be measured.
Therefore, the amount of KOH used was finally determined to be 1.0 to 2.0g, and 1.5g was the most preferable. Namely, the mass ratio of the biodiesel to the solid KOH is 6: 1-3: 1, and the optimal ratio is 4: 1.
5. Standard curve and limit of detection
Under the acidic condition, potassium dihydrogen phosphate reacts with sodium molybdate and hydrazine sulfate at 100 ℃ to generate a phosphomolybdic blue product, and the amount of the phosphomolybdic blue product is in direct proportion to the absorbance at the wavelength of 650 nm. And obtaining the corresponding absorbance through the known phosphorus content, and drawing a standard curve. The formula of the standard curve measured in the invention is that y is 5.7740x-0.0071, and R2 is 0.9999. y is the absorbance at 650nm, x is the phosphorus content (mg) in 50mL of solution, and FIG. 3 is a standard curve of phosphorus content in solution. In the spectrophotometry, the concentration corresponding to the absorbance of 0.01 after blank value deduction is taken as the detection limit, the detection limit of the method is obtained by calculation by taking the detected concentration of 3.3 times as the quantification limit, and the detection limit is 0.37mg/kg and the quantification limit is 1.22 mg/kg.
6. Precision of the method
To evaluate the precision of the method, two identical sets of biodiesel, 5 samples per set, were prepared. One group of samples is subjected to pretreatment, color development, detection and calculation according to the method of the invention to obtain the phosphorus content of the samples, and the other group of samples is subjected to detection by adopting a national standard ICP method. Table 6 shows the measurement results of the present invention, and the phosphorus content in biodiesel was found to be 6.1267mg/kg, and RSD was found to be 2.84%. The phosphorus content of the biodiesel measured by an ICP method is 5.9801mg/kg, and the RSD is 3.75%. Compared with the measurement result of the national standard ICP method, the precision of the measurement result of the method is slightly better.
TABLE 6 precision of phosphorus content determination in biodiesel
7. Accuracy of the method
The accuracy of the process is generally measured by a spiking recovery test, and therefore it is first necessary to determine what phosphorous species to use as the spiking species. Silvereir (Fuel,2011,90(11): 3485-. The phosphorus in the biodiesel is derived from the phospholipids in the raw materials, and although some reaction may occur during the preparation of the biodiesel, the phospholipids should be present in the form of some organophosphorus with unknown structure. As described above, ashing is a process for converting organic phosphorus into inorganic phosphorus, and therefore if the conversion of phosphorus is not complete, there is a possibility that some negative deviation may be caused in subsequent measurement values. Therefore, it is worth investigating whether inorganic phosphorus is suitable as an addition standard in the determination of the addition standard recovery rate.
1) Standard substance for determining recovery rate of added standard
In order to reduce the error of the test, the phosphorus-free methyl oleate was selected to simulate biodiesel, and about 8mg/kg of inorganic phosphorus and organic phosphorus were added to two groups of methyl oleate, respectively, wherein the inorganic phosphorus was potassium dihydrogen phosphate standard solution, the organic phosphorus was dioleoyl-sn-glycerol-3-phosphocholine (DOPC), and the ashing end temperatures were set to 400 ℃ and 680 ℃ respectively, to obtain the phosphorus content and the recovery rate, respectively, and the results are shown in Table 7.
TABLE 7 comparison of recovery rates for organic and inorganic phosphorus additions
As can be seen from Table 7, the recovery rates of inorganic phosphorus were substantially the same at different temperatures, while the recovery rates of organic phosphorus were significantly different. When the ashing final temperature is increased from 400 ℃ to 680 ℃, the detected recovery rate of the biodiesel sample added with DOPC is increased from 71.37% to 99.65%. As mentioned above, the ashing process of biodiesel is a process of converting organic phosphorus into inorganic phosphorus, and the lower ashing final temperature cannot completely convert the organic phosphorus into the inorganic phosphorus, and the part of unconverted organic phosphorus cannot participate in the subsequent color reaction, so that the detection value of the sample is relatively low. Therefore, the recovery rate of the method for measuring the recovery rate by using the organic phosphorus is more reliable, and the recovery rate test of the biodiesel needs to use an organic phosphorus standard-adding test for measuring.
2) Recovery rate of the process
The recovery using the DOPC spiking test is as follows. DOPC is dissolved in methyl oleate, is added into biodiesel to prepare samples with high, medium and low concentrations, and 3 groups of parallel measurement are carried out according to the invention and the national standard ICP method, and the results are shown in Table 8. Under three concentrations of high, medium and low, the recovery rate of the invention is 96.69-104.85%, the recovery rate measured by the national standard ICP method is 97.41-105.98%, and the accuracy of the two methods is equivalent.
TABLE 8 recovery rates for phosphorus content detection in biodiesel
8. Application of the method
Biodiesel samples prepared from different raw materials were measured by the method established by the present invention and the national standard ICP method, respectively, and table 9 shows the measurement results. Using paired t-test, confidence interval was 95%, critical t value was 0.088, degree of freedom was 3, and P value was 0.935 for the two-sided test. The P value is greater than 0.05, which indicates that no significant difference exists between the two groups of data, so that the detection result of the method has no significant difference with the detection result of the national standard ICP method, and the method is suitable for detecting the phosphorus content of the biodiesel from different raw material sources.
TABLE 9 measurement of phosphorus content of biodiesel of different feedstocks
Claims (1)
1. A method for detecting phosphorus content in biodiesel is characterized in that the biodiesel and KOH are subjected to saponification reaction to generate soap, then the soap is subjected to ashing in a mode of temperature programming, and then the phosphorus content in the biodiesel is measured by a phosphorus-molybdenum blue colorimetric method; wherein the mass ratio of the biodiesel to the solid KOH is 6.0: 1.0 to 2.0;
the temperature of the saponification reaction is 20-30 ℃, and the reaction time is 30-80 min; or the temperature of the saponification reaction is 40-100 ℃, and the reaction time is 10-30 min;
the specific process of ashing the soap by adopting a temperature programming mode comprises the following steps: firstly, preserving heat for 40-80 min at 100-140 ℃, then preserving heat for 150-210 min at 230-270 ℃, then preserving heat for 30-90 min at 280-320 ℃, and finally preserving heat for 90-150 min at 650-700 ℃;
cooling to room temperature after ashing, dissolving ash in boiling water, filtering, adjusting the pH value of the filtrate to 2-4 with hydrochloric acid solution, and fixing the volume;
the specific process for determining the content of phosphorus in the biodiesel by the phosphomolybdic blue colorimetric method comprises the following steps:
① standard curve is drawn, wherein 0mL, 1 mL, 2mL, 4 mL, 6 mL and 8mL of solution are respectively transferred from the phosphate standard solution to a 50mL colorimetric tube, 8mL of hydrazine sulfate solution and 2mL of sodium molybdate dilute sulfuric acid solution are respectively added, the volume is fixed to 50mL, the solution is shaken up, the reaction is carried out for 10min at 100 ℃, the solution is cooled to the room temperature, the volume is fixed to 50mL, the absorbance of the solution is measured at 650nm, and a standard curve of phosphorus content and absorbance is drawn;
② phosphomolybdic blue colorimetry, wherein 40mL of sample solution is moved from a measuring flask to carry out a color reaction, the specific process comprises the steps of moving 40mL of sample solution from the measuring flask to a colorimetric tube of 50mL, adding 8mL of hydrazine sulfate solution and 2mL of sodium molybdate dilute sulfuric acid solution, fixing the volume to 50mL, shaking up, reacting for 10min at 100 ℃, cooling to room temperature, fixing the volume to 50mL, measuring the absorbance of the solution at 650nm, obtaining the phosphorus content in the sample solution through a standard curve, and calculating the phosphorus content in the biodiesel according to a formula (1);
in the formula, X: the phosphorus content in the biodiesel is mg/kg; p: phosphorus content in sample solution, mg; v1: volume of sample diluted after ashing, mL; v2: the volume of the measured liquid taken during the color comparison is mL; m: mass of biodiesel, g;
preparing a phosphate stock solution: 0.4387g of potassium dihydrogen phosphate which is dried for 24 hours is weighed, dissolved and diluted by water to be 1000mL, and the solution contains 0.1mg/mL of phosphorus;
preparation of a phosphate standard solution: sucking 10mL to 100mL of phosphate stock solution into a volumetric flask by a pipette, adding water for dilution and fixing the volume, wherein the solution contains 0.01mg/mL of phosphorus;
preparing a hydrazine sulfate solution: dissolving 0.15g of hydrazine sulfate in 1000mL of water to obtain 0.015% hydrazine sulfate solution;
preparing a sodium molybdate dilute sulfuric acid solution: measuring 140mL of concentrated sulfuric acid with the mass fraction of 98% into 300mL of water, cooling to room temperature, adding 12.5g of sodium molybdate, dissolving, metering the volume with water into a 500mL volumetric flask, shaking up, and standing for 24h for later use;
preparing a hydrochloric acid solution: concentrated hydrochloric acid with a mass fraction of 37% was dissolved in an equal volume of water.
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