Method for quickly measuring total aldehyde content in liquor with pH meter
principle
After the trace amounts of aldehydes in the wine are added to sodium sulfite, sodium hydroxide is released, causing changes in the pH of the water. Using the change in pH as the response index of the measurement method, the amount of total aldehydes in wine can be quantitatively measured.
HCHO Na2SO3 H2®H2C(OH)SO3Na NaOH
Main reagents and instruments
Laboratory acidity meter (accuracy 0.01) (must be calibrated before use). Formaldehyde-free ethanol (preparation): Place 1500mL of ethanol into a 2000mL distillation flask, add 15g of 2,4-dinitrophenylhydrazine, 0.75mL of concentrated hydrochloric acid and a few grains of zeolite, reflux for 4h and leave for 4h, then replace the condenser with Distillation column, slowly distill, discard about 100mL of the initial effluent and about 200mL of the remaining yellow solution, collect the middle fraction, fill it with nitrogen and seal it in a brown bottle. The distillate should be clear, transparent and colorless, otherwise it should be re-distilled. Sodium sulfite solution (0.1000mol·L-1): Dissolve 6.3020g sodium sulfite solution in 50mL water, transfer to a 500mL volumetric flask, and dilute to the mark with deionized water. Potassium hydroxide-ethanol solution (100g·L-1): Dissolve 10.0g potassium hydroxide in 20.0mL water, transfer to a 100mL volumetric flask, dilute to volume with aldehyde-free ethanol, and mix. Formaldehyde standard solution: Prepare a 10g·L-1 formaldehyde stock solution, calibrate it using the iodometric method, and dilute it to a working solution of 1.000g·L-1. The reagents were all of analytical grade, and the water was double distilled water.
experimental method
Accurately draw 2.0mL of 1.000g·L-1 formaldehyde standard solution, transfer it to a 100mL volumetric flask, add 50.0mL of aldehyde-free ethanol, and then add 10.0mL of sodium sulfite solution (0.1000mol·L-1), dilute to the mark with water, and mix. Let stand for 10 minutes. Then use a PHS-3C precision pH meter to measure the pH value of the solution in a 50mL beaker, using the reagent blank as a reference. The indicator value is the pH value of the measurement solution minus the pH value of the reagent blank.
Results and discussion
Dosage of sodium sulfite solution
The upper limit of the total amount of formaldehyde in the standard curve in this article is 10.0mg (0.3330mmol). In order to make the reaction complete, sodium sulfite is excessive here. The amount of sodium sulfite (amount of substance) is 3 times the total amount of formaldehyde (amount of substance). Therefore, the dosage of 0.1000mol·L-1 sodium sulfite solution is 10mL.
The influence of reagent blank when using different amounts of sodium sulfite
Although the amount of sodium sulfite is always more than 3 times excessive, when making a standard curve, the amount of sodium sulfite consumed is different for different amounts of formaldehyde. Therefore, it is necessary to examine the impact of changes in sodium sulfite concentration in the reagent blank on pH. The method of investigation is to add 50 mL of aldehyde-free alcohol to a 100 mL volumetric flask, then add a certain volume (5, 6, 7, 8, 9, 10 mL) of 0.1000 mol·L-1 sodium sulfite solution, and then dilute to the mark with distilled water. , shake well and leave it for 10 minutes to measure the pH. The results are shown in Table 1. It can be seen from Table 1 that when the dosage of 0.1000 mol·L-1 sodium sulfite solution is between 5 and 10 mL, the pH of the 50% aldehyde-free alcohol solution does not change.
Effect of reaction time
The reaction time of this method was investigated. The method of investigation is to add 50 mL of aldehyde-free alcohol into a 100 mL volumetric flask, followed by 6 mL of 1.000 g·L-1 formaldehyde standard solution, 10 mL of 0.1000 mol·L-1 sodium sulfite solution, and then dilute it with distilled water to the mark, shake well and let it sit for a certain period of time. Measure the pH after a certain period of time (5, 10, 15, 20, 25, 30 min). The results are shown in Table 2. It can be seen from Table 2 that the reaction time at room temperature is between 5 and 30 minutes, and the dissolved pH of the reaction system does not change, indicating that the reaction has become complete.
Drawing of standard curve
Pipette a series of 1.000g·L-1 formaldehyde standard solution 0.0mL, 1.0mL, 2.0mL, 4.0mL, 6.0mL, 8.0mL, 10.0mL (equivalent to the total amount of formaldehyde 0.0mg, 1.0mg, 2.0mg, 4.0mg ,6.0mg,8.0mg,10.0mg). Determine the pH value of each solution according to the experimental method, and calculate the corresponding concentration of hydroxide ions (C). Taking the total aldehyde (calculated as formaldehyde) content (mg) as the abscissa, ΔC (ΔC=C measured value-C blank) is the ordinate. Draw the working curve as shown in Figure 1. The corresponding linear equation and parameters of this figure are: C=-0.0094 0.0333´X, R=0.9994, SD=0.04456. The linear range of this method is 10~100mg·L-1, and the detection limit is 4.0mg·L-1.
Sample measurement
Select 4 commercially available liquors numbered No. 1, No. 2, No. 3, and No. 4, accurately pipette 50.0 mL of each sample, use potassium hydroxide-ethanol solution to adjust the pH of the liquor to 7, and then measure the test according to the test method. The pH value of the sample was calculated, and the concentration difference (ΔC) of the hydroxide ion between the sample and the blank was calculated. The aldehyde content (equivalent to the formaldehyde content) was found from the standard curve. The total aldehyde content was 18, 23, and 30, respectively. 39mg·L-1. A recovery test was conducted at the same time, and the results are shown in Table 2. It can be seen from the measurement results that this method has high sensitivity and simple operation. It is a simple, fast and feasible method for determining the content of aldehydes in liquor.
