Method for measuring total aldehyde content of white spirit by pH meter

1. Introduction 1.1 Principles.

Addition of trace aldehydes and sodium sulfite in the wine releases sodium hydroxide, causing pH changes in the water. The amount of total aldehyde in the wine can be quantitatively determined by the change of pH as the response indicator of the measurement method.

HCHONa2SO3H2®H2C(OH)SO3NaNaOH

1.2 Main reagents and instruments Laboratory pH meter (accuracy 0.01) (to be calibrated before use). Ethanol-free (Preparation): Take 1500mL of ethanol in a 2000mL distillation flask, add 15g of 2,4-dinitrophenylhydrazine, 0.75mL of concentrated hydrochloric acid, and a few zeolites, reflux for 4h and let stand for 4h, then change the condenser to be fine Distillate column, slowly distillation, discard about 100mL of the initial effluent and about 200mL of the remaining solution of yellow solution, collect the middle distillate, sealed with nitrogen in a brown bottle. The distillate should be clear, transparent and colorless, otherwise it should be redistilled. Sodium sulfite solution (0.1000 mol·L-1): 6.320 g of sodium sulfite solution was dissolved in 50 mL of water, transferred to a 500 mL volumetric flask, and diluted to the mark with deionized water. Potassium hydroxide-ethanol solution (100 g·L-1): Dissolve 10.0 g of potassium hydroxide in 20.0 mL of water, transfer to a 100 mL volumetric flask, dilute to volume with aldehyde-free ethanol, and mix. Formaldehyde standard solution: 10g·L-1 formaldehyde stock solution was prepared and calibrated with iodometric method. When used, it was diluted to a working solution of 1.000 g·L-1. The reagents are all analytically pure, and water is distilled twice.

1.3 Experimental method Accurately draw 1.000g·L-1 formaldehyde standard solution 2.0mL, transfer to 100mL volumetric flask, add 50.0mL aldehyde-free ethanol, add 10.0mL sodium sulfite solution (0.1000mol · L-1), dilute to the mark with water, Mix well and let stand for 10 minutes. The pH of the solution was then measured with a pHS-3C precision pH meter in a 50 mL beaker, referenced to the reagent blank. The indicator value is the pH of the assay solution minus the pH of the reagent blank.

2. Results and discussion 2.1 The amount of sodium sulfite solution The upper limit of the total amount of formaldehyde in this standard curve is 10.0 mg (0.3330 mmol). To make the reaction more complete, let sodium sulfite excess, and the amount of sodium sulfite (the amount of substance) to take total formaldehyde. The amount (amount of material) is 3 times, so the amount of 0.1000 mol·L-1 sodium sulfite solution is 10 mL.

2.2 Effect of Reagent Blanks When Sodium Sulfite Dosage Is Not Used at the Same Time Although the amount of sodium sulfite is always more than 3 times higher, when the standard curve is used, the amount of different formaldehyde is different from the amount of sodium sulfite consumed, so it is necessary to examine the concentration of sodium sulfite in the reagent blank. The effect of changes on pH. The method was investigated by adding 50 mL of aldehyde-free alcohol to a 100 mL volumetric flask, followed by adding a volume (5, 6, 7, 8, 9, 10 mL) of 0.1000 mol·L-1 sodium sulfite solution, and then diluting it to the mark with distilled water. , After shaking for 10 minutes, determine the pH. When the amount of 0.1000 mol·L-1 sodium sulfite solution was between 5 and 10 mL, the pH of the 50% aldehyde-free alcohol solution did not change.

2.3 Effect of reaction time The reaction time of this law was examined. The method of investigation was to add 50 mL of aldehyde-free alcohol to a 100 mL volumetric flask, followed by 1 mL of a standard solution of 1.000 g·L-1 formaldehyde, and 10 mL of a 0.1000 mol·L-1 sodium sulfite solution. Then dilute to the mark with distilled water and place it in shakes. The pH was measured after (5, 10, 15, 20, 25, 30 min). The reaction time was between 5 and 30 minutes at room temperature, and the pH of the reaction system did not change, indicating that the reaction had become complete.

Draw a standard curve of 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, 2.0 mg, 4.0 mg, 6.0 mg, 8.0 mg, 10.0 mg). Determine the pH value of each solution according to the experimental method, and calculate the corresponding hydroxide ion concentration (C), total aldehyde (measured by formaldehyde) content (mg) as the abscissa, ΔC (ΔC=C measured value-C Blank) is the ordinate. The working curve is drawn as shown in Fig. 1. The linear equation and parameters corresponding to this figure are: C=-0.00940.0333 ́X, R=0.9994, SD=0.04456. The linear range of the method was 10 to 100 mg·L-1 and the detection limit was 4.0 mg·L-1.

2.5 Determination of Samples Select the four types of commercial white wine No. 1, No. 2, No. 3, and No. 4. Accurately remove 50.0 mL of the sample, adjust the pH of the liquor with potassium hydroxide-ethanol solution to 7, and then press the test. Methods The pH of the sample was measured, and the concentration difference (ΔC) between the sample and the blank hydroxide ion was calculated. The aldehyde content (equivalent to the formaldehyde content) was determined from the standard curve and the total aldehyde amount was 18, 23, respectively. 30,39 mg·L-1. At the same time recovery test. It can be known from the measurement results that this method has high sensitivity and simple operation and is a simple, quick and feasible method for determining the content of aldehydes in liquor.