Determination of Amino Acid Nitrogen by Double Indicator Formaldehyde Titration

1.
Principle

Amino acids are amphiphilic because they contain —COOH and —NH ₂
.

2.
Reagents

(1) Neutral formaldehyde solution (40%): Using thymolphthalein as an indicator, use sodium hydroxide to neutralize 40% formaldehyde to light blue
.

(2) Thymolphthalein ethanol solution (1g/L)
.

(3) Neutral Red 50% ethanol solution (1g/L)
.

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

3.
Apparatus

Commonly used instruments in laboratories
.

4.
Operation method

Pipette 2 aliquots of sample solution containing 20-30mg of amino acid and place them in 250mL Erlenmeyer flasks, add 50mL distilled water to each, add 3 drops of neutral red indicator to 1 aliquot, and titrate with 0.
1mol/L NaOH standard solution to Red to amber is the end point; add 3 drops of thymolphthalein indicator and 20 mL of neutral formaldehyde to the other portion, shake well, let stand for 1 min, titrate with 0.
1mol/L NaOH standard solution to light blue as the end point
.

5.
Result calculation

Where x-the mass fraction of amino acid nitrogen,%

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

V ₁ ——The volume of sodium hydroxide standard solution consumed when titrating with neutral red as indicator, mL

V ₂ ——Use thymolphthalein as indicator to consume sodium hydroxide standard solution volume when titrating , mL

m——The sample solution for determination is equivalent to the mass of the sample, g

0.

6.
Tips

(1) This method is suitable for the determination of free amino acids in food
.

(2) The solid sample should be crushed first, accurately weighed, and then extracted with water, and then the extract is determined
.

(3) If the color of the sample is darker, you can add a proper amount of activated carbon to decolorize and then measure, or use potentiometric titration to measure

(4) Similar to this method, there is also the single indicator ( thymolphthalein ) formaldehyde titration method.