Determination of total acid in fruit juice beverages

This practical training adopts the indicator method (according to GB/T12456-2008 Determination of Total Acid in Food).

1. Purpose requirements

(1) Further proficiency and standardization in titration operations.

(2) Familiar with the determination of total acid in fruit juice beverages.

(3) Master the determination method, operational skills, and result calculation of total acid in fruit juice beverages.

2. Principles of Practical Training

According to the principle of acid-base neutralization, use sodium hydroxide standard solution as the titrant and phenolphthalein as the indicator to titrate until the solution turns pale red (pH=8.2) and does not disappear within 0.5 minutes as the endpoint. Calculate the total acid content of the sample based on the concentration and volume of the consumed standard alkaline solution.

3. Training supplies

(1) Commonly used laboratory instruments.

(2) 0.1mol/L NaOH standard solution: prepared and calibrated according to GB601-2002.

(3) 0.01mol/L or 0.05mol/L NaOH standard solution.

(4) 1% phenolphthalein indicator solution: 1g of phenolphthalein is dissolved in 60mL of 95% ethanol (GB679-2002) and diluted with water to 100mL.

All reagents are analytical pure, and water is distilled water or water of equivalent purity. Before use, it must be boiled and cooled.

4. Safety Reminder

Sodium hydroxide solution is corrosive and should be avoided from contact with the skin. During the practical training process, there are issues related to safe electricity use, such as not touching electrical appliances with damp hands to prevent electric shock.

5. Operation steps

Take 25.00-50.00mL of the test solution to contain 0.035-0.070g of acid, and place it in a 250mL triangular flask. Add 40-60mL of water and 0.2mL of 1% phenolphthalein indicator, and titrate with 0.1mol/L NaOH standard solution until a slight red color does not fade for 30 seconds. Record the volume (V1) of 0.1mol/L NaOH standard solution consumed. The same tested sample needs to be tested twice.

Perform a blank test using water instead of the test solution. Record the volume (%) of 0.1mol/L NaOH standard solution consumed.

6. Result calculation

(1) Data recording

(2) Result calculation

In the formula, x represents the total acid content in the juice sample, g/kg

V1- Volume of sodium hydroxide standard titration solution consumed during titration of test solution, ml

V2- Volume of sodium hydroxide standard solution consumed during blank titration, mL

C - Concentration of sodium hydroxide standard solution, mol/L

K - The conversion coefficient of acid, i.e. 1mol NaOH is equivalent to the mass of the main acid (g). The conversion coefficients for several acids are: malic acid, 0.067; Acetic acid, 0.060; Tartaric acid, 0.075; Citric acid, 0.064; Citric acid containing one molecule of crystalline water, 0.070; Lactic acid, 0.090; Hydrochloric acid, 0.036; Phosphoric acid, 0.049

V - Sampling amount of the sample, mL

F - Dilution ratio of the test solution