Determination of total acid in fruit juice drinks

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

1.
Purpose requirements

(1) Further proficient and standardized titration operation
.

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

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

2.
Principles of training

According to the principle of acid-base neutralization, with sodium hydroxide standard solution as titrant and phenolphthalein as indicator, titration until the solution appears light red (pH=8.
2) and does not disappear within 0.
5 min as the end point
.

3.
Training supplies

(1) Commonly used instruments in laboratories
.

(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
.

4.

Sodium hydroxide solution is corrosive, so avoid contact with skin
.

5.
Operation steps

Take 25.
00～50.
00mL test solution, make it contain 0.
035～0.
070g acid, and place it in a 250mL Erlenmeyer flask
.

Use water instead of test solution for blank test
.

6.
Result calculation

(1) Data logging

(2) Result calculation

Where x——the content of total acid in the juice sample, g/kg

V ₁ ——The volume of sodium hydroxide standard titration solution consumed when titrating the test solution, ml.

V ₂ ——Volume of sodium hydroxide standard solution consumed during blank titration , mL

c——The concentration of sodium hydroxide standard solution, mol/L

k——The conversion factor of acid, that is, 1mol NaOH is equivalent to the mass of the main acid (g)
.

V——Sampling volume of the sample, mL

F-the dilution factor of the test solution

7.
Tips

(1) containing CO.
'S ₂ sample beverages and the like, before the measurement must first remove CO.
'S ₂ , to prevent interference
.

(2) The amount of water used for dilution of the test solution should be carefully selected according to the total acid content in the sample
.

(3) If the acidity of the sample is low, use 0.
01mol/L or 0.
05mol/L NaOH standard solution for titration
.

(4) If the sample solution has color (such as colored fruit juice, etc.
), use the same volume of the sample solution without CO ₂ and dilute with distilled water before titration, or use a test titration method, that is, for the colored sample solution, use an appropriate amount of non-CO ₂ Dilute with CO ₂ distilled water, and add phenolphthalein indicator according to the ratio of 0.
3mL phenolphthalein to 100mL sample solution.
When the NaOH standard solution is used to titrate near the end point, take 2～3mL of this solution and transfer it into 20mL CO _{2 free} distilled water.
The color of the solution is quite light and it is easy to observe the color of phenolphthalein).
If the experiment shows that the end point has not been reached, pour the particularly diluted sample solution back into the original sample solution and continue titration until the end point appears
.

(5) If the color of the sample solution is too dark or turbid, potentiometric titration should be used
.