First method, acidity meter method

1、 Principle

By utilizing the zwitterionic effect of amino acids and adding formaldehyde to fix the alkalinity of the amino group, the carboxyl group exhibits acidity. Titrate with sodium hydroxide standard solution and determine the endpoint using an acidity meter.

2、 Reagents

Unless otherwise specified, all reagents used in this method are analytical pure, and the water is Grade III water as specified in GB/T 6682.

Formaldehyde (36%~38%): should not contain polymers (without precipitation and solution without layering).

Phenolphthalein.

Ethanol.

Potassium hydrogen phthalate: reference substance.

3、 Reagent preparation

Sodium hydroxide standard titration solution [c (NaOH) -0.050mol/L]: A standard titration solution or preparation method that has been certified by the state and has been awarded a sub standard substance certificate is as follows:

1. Phenolphthalein indicator solution: Weigh 1g of phenolphthalein and dissolve it in 95% ethanol, dilute with 95% ethanol to 100 mL.

2. Sodium hydroxide solution [sodium oxychloride standard titration solution c (NaOH)=0.05mol/L]: Weigh 110g of sodium hydroxide into a 250mL beaker, add 100mL of water, shake to dissolve it into a saturated solution, cool and place in a polyethylene plastic bottle, close the stopper, clarify, and set aside for later use. Take 2.7 mL of the upper clear liquid and add an appropriate amount of freshly boiled cold distilled water to 1000 mL, shake well.

3. Calibration of sodium hydroxide standard titration solution: Accurately weigh about 0.36g of reference potassium hydrogen phthalate dried to constant weight at 105 ℃~110 ℃, add 80mL of freshly boiled water to dissolve it as much as possible, add 2 drops of phenolphthalein indicator solution (10g/L), titrate with sodium hydroxide solution until the solution turns slightly red and does not fade for 30 seconds. Record the number of milliliters of sodium hydroxide solution consumed. Simultaneously conduct a blank test.