Determination of effective acidity and volatile acid

1.
Determination of effective acidity

1.
Principle (pH Potentiometric Method)

There is a certain quantitative relationship between the electromotive force of the battery and the H+ activity in the test solution.
According to this, the glass electrode is used as the indicator electrode, and the saturated calomel electrode is used as the reference electrode.
Read the pH of the sample solution on the meter head
.

2.
Reagents

(1) pH 3.
999 standard buffer solution (20°C): accurately weigh 10.
12g of premium-grade pure potassium hydrogen phthalate (KHC ₈ H ₄ O ₄ ) dried at (115±5)°C for 2 to 3 hours , and dissolve it Dilute to 1000 mL in water without CO ₂ and shake well
.

(2) pH 6.
878 standard buffer solution (20℃): accurately weigh 3.
387g of potassium dihydrogen phosphate (KH2PO4) dried at (115±5)℃ for 2～3h and anhydrous disodium hydrogen phosphate (Na ₂ HPO ₄₎ ) 3.
533g, dissolve in water, dilute to 1000mL, shake well
.

(3) pH 9.
227 standard buffer solution (20℃): accurately weigh 3.
80g of pure borax (Na ₂ B ₄ O ₇ ·10H ₂ O), dissolve it in water that has been heated to remove CO ₂ , dilute to 1000 mL, and shake well
.

3.
Apparatus

(1) pHS-3C acidity meter
.

(2) Raymag E-201-C or 65-1AC plastic composite electrode
.

4.
Operation steps

(1) pH meter calibration First connect the electrode of the pH meter, turn on the power, adjust the compensation temperature knob, immerse the electrode in the buffer solution, then press the reading switch, adjust the potentiometer to adjust the pointer to the pH of the buffer solution On
.

(2) Sample measurement: Draw a little juice sample into a small beaker, and measure directly with a pH meter
.

5.
Result calculation

The pH of the sample solution can be read directly from the meter head of the acidity meter
.

2.
Determination of Volatile Acid

Volatile acid can be determined by direct method or indirect method.
Direct method refers to the separation of volatile acid by steam distillation or solvent extraction, and then titration with alkali standard solution; indirect method refers to titration of non-volatile acid after evaporating the volatile acid.
, And then subtract the unused acid from the total acid to get the volatile acid content
.

1.
Principle

After the sample is processed, add an appropriate amount of phosphoric acid to free the bound volatile acid
.

2.
Reagents

(1) NaOH standard solution (0.
1mol/L)
.

(2) phenolphthalein (10g / L): Weigh 1.
0g was dissolved in 100mL 95% ethanol in
.

(3) phosphoric acid solution (100g / L): Weigh 10.
0g phosphate , with small amounts of CO.
'S No ₂ of distilled water were dissolved, diluted to 100 mL
.

3.
Apparatus

(1) Steam distillation device (see Figure 1-6 steam distillation device diagram)
.

(2) Electromagnetic stirrer
.

4.
Operation steps

(1) Sample processing

①General fruits, vegetables and beverages can be sampled directly
.

CO ② containing ₂ beverages, fermented class shall exclude CO ₂ : The method is to take samples at 80 ~ 100mL Erlenmeyer flask, while stirring with a magnetic stirrer, under a low vacuum suction 2 ~ 4min, to remove CO ₂
.

③Solid samples (such as dried and fresh fruits and vegetables and their products) and frozen and viscous samples: first take the edible part, add a certain amount of distilled water without CO ₂ (frozen products are thawed first), mash it into a slurry with a high-speed tissue masher, and then weigh Take 10g sample, add CO _{2 free} distilled water to dissolve and dilute to 250mL
.

(2) Determination

①Sample distillation: Take 25mL of the above-processed sample into a distillation flask, add 25mL CO _{2 free} distilled water and 1mL 100g/L H ₃ PO ₄ solution, connect the steam distillation device, heat and distill until the distillate is about 30mL
.

②Titration: Heat the distillate to 60～65℃, add 3 drops of phenolphthalein indicator, and titrate with 0.
1mol/L NaOH solution until the solution is reddish without fading for 30 seconds, which is the end point
.

5.
Result calculation

Where x——the content of volatile acid in the sample ( calculated as acetic acid ), g/100g

c——Concentration of standard NaOH solution, mol/L

V ₂ ——The volume of sodium hydroxide solution consumed by blank titration , mL

V ₁ ——Volume of 0.

m——sample mass or volume, g or mL

0.
06— The coefficient converted to acetic acid

6.
Tips

(1) Volatile acids in food include free volatile acids and bound volatile acids
.

(2) The water in the steam generator bottle should be boiled for 10 minutes before distillation to remove carbon dioxide in the water in the generator bottle , and the entire distillation device should be flushed with steam

(3) Each connection of the steam distillation device should be sealed, and air leakage must not be avoided to prevent the loss of volatile acid

(4) During the distillation process, attention should be paid to keeping the liquid level in the distillation flask constant, otherwise the measurement results will be affected

(5) Heat the distillate to 60-65°C during titration to make the end point obvious, speed up the reaction, shorten the titration time, reduce the contact between the solution and the air, and improve the measurement accuracy
.

Related link: Determination of total acid in fruit juice beverages