What are the specific steps of the rapid titration of formaldehyde

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Formaldehyde titration for the determination of amino nitrogen.

One. Objective.

Preliminary grasp of the principle and operation points of formaldehyde titration for the determination of amino nitrogen content.

Two. Principle.

Amino acids are amphoteric electrolytes that are balanced in aqueous solution as follows:

NH3 is a weak acid, with a pH of 11 12 or higher at complete dissociation, and if amino acids are determined by alkaline titration of H released by NH3, the general indicator color change gamut is less than 10, and it is difficult to accurately indicate the titration endpoint.

At room temperature, formaldehyde can rapidly combine with the amino group of amino acids to form hydroxymethyl compounds, which shifts the above equilibrium to the right, promotes the release of NH3 H, increases the acidity of the solution, and moves the titration neutralization end point to the discoloration domain of phenolphthalein (left and right). Therefore, phenolphthalein can be used as an indicator and titrated with a standard sodium hydroxide solution.

If the sample is a known amino acid, the amino nitrogen content can be calculated from the formaldehyde titration results. If the sample is a mixture of multiple amino acids, such as a protein hydrolysate, the titration results cannot be used as a basis for the quantification of amino acids. However, this method is simple and rapid, and is commonly used to determine the degree of hydrolysis of proteins, and the titer value also increases with the increase of the degree of hydrolysis, and when the titration value no longer increases, it indicates that the hydrolysis has been complete.

Three. Instruments, reagents, materials.

1.Instrument.

25 ml Erlenmeyer flask; 3 ml microburette; Straw; Mortar.

2.Reagent.

1) 300ml of standard glycine solution.

Accurately weigh 375mg of glycine, dissolve it and set the volume to 100ml.

2) 500ml of standard sodium hydroxide solution.

3) 20ml phenolphthalein indicator.

Phenolphthalein in 50 ethanol solution.

4) 400 ml neutral formaldehyde solution.

Add 1ml of phenolphthalein ethanol aqueous solution to 50ml of 36 37 analytical pure formaldehyde solution, titrate to reddish with sodium hydroxide solution, and store in a closed glass bottle.

Four. How it works:

1.Take 3 25 ml Erlenmeyer flasks and number them. Add 2ml of standard glycine solution and 5ml of water to the bottle and mix well.

Add 7 ml of water to bottle 3. Then add 5 drops of phenolphthalein indicator to each of the three bottles, add 2 ml of formaldehyde solution to each of the three bottles, mix well, and titrate with standard sodium hydroxide solution until the solution is slightly red.

Repeat the above experiments twice, recording the number of milliliters of standard sodium hydroxide solution consumed per bottle each time. The average value was taken to calculate the ** rate of glycine amino nitrogen.

2.Take 2ml of glycine solution of unknown concentration, measure it according to the above method, make several copies in parallel, and take the average value. Calculate the number of milligrams of amino nitrogen per milliliter of glycine solution.

Five. Result processing.

1.**Rate Calculation:

Preliminary grasp of the principle and operation points of formaldehyde titration for the determination of amino acid content.

Second, the principle. The amino acids in the aqueous solution are facultative ions, so the carboxyl groups of amino acids cannot be directly titrated with alkali. Formaldehyde can be combined with -N+H3 on amino acids to form hydroxymethyl derivatives such as -NH-CH2OH, -N(CH2-OH)2, so that H+ on N+H3 can be freed, so that N+H3 can be titrated with alkali to release H+ to measure amino nitrogen, so as to calculate the content of amino acids.

If the sample contains only a single known amino acid, the amino acid content can be calculated from the results of this titration.

If the sample contains multiple amino acids (e.g., protein hydrolysate), the amount of amino acids cannot be calculated by this method.

After the interaction of proline and formaldehyde, the compounds formed are unstable, resulting in low results after titration. Tyrosine contains a phenolic structure, resulting in high titration results.

3. Materials, reagents and utensils.

a) Reagents.

1. Phenolphthalein alcohol solution.

Phenolphthalein is called dissolved in 100ml of 60% alcohol.

2. Bromothymol blue solution.

take bromodeoxin blue and dissolve it in 100ml of 20% ethanol solution.

Glycine solution.

Dissolve 1g of glycine in 100ml of distilled water.

4. Standard sodium hydroxide solution.

5. Neutral formaldehyde solution.

take 50ml of formaldehyde solution, add about 3ml of phenolphthalein indicator, add the solution dropwise, make the solution slightly pink, and neutralize it before use.

2) Experimental equipment.

1. Erlenmeyer flask 2. Basic burette.

3. Pipette ear wash ball 4. Balance.

5. a volumetric flask 6. a reagent bottle.

7. Graduated cylinder 8. Glass rod and beaker.

Fourth, the operation steps.

1. Take 3 100ml Erlenmeyer flasks and add the reagents according to the table below.

2. After mixing, titrate to purple with standard sodium hydroxide solution (

3. Result calculation.

The number of amino acids per milliliter of amino acid in the solution is.

5. Precautions.

The formaldehyde drip method can be used to determine the degree of hydrolysis of proteins. As the degree of proteolysis increases, the titer also increases, and when the proteolysis is complete, the titer no longer increases.

Formaldehyde titration, a biological term, is a method for determining the content of amino nitrogen. At room temperature, formaldehyde can rapidly combine with the amino group of amino acids to form hydroxymethyl compounds, which shifts the above equilibrium to the right, promotes the release of NH3 H, increases the acidity of the solution, and moves the titration neutralization end point to the discoloration domain of phenolphthalein (left and right). Therefore, phenolphthalein can be used as an indicator and titrated with a standard sodium hydroxide solution.

Precautions. 1.Standard sodium hydroxide solutions should be calibrated prior to use and stored in closed bottles. Do not use leftover sodium hydroxide stored in the microburette on alternate days.

2.The neutral formaldehyde solution is prepared before use, and if it has been left for a period of time, it needs to be re-neutralized before use.

3.This experiment is a quantitative experiment, the concentration of glycine and sodium hydroxide should be strictly calibrated, the dosage should be accurate, and all operations should be carried out according to the requirements of analytical chemistry.

4.Proline reacts with formaldehyde to form unstable compounds, resulting in a low titration milliliter. The titration of tyrosine in milliliters is on the high side.

Sodium thiosulfate works in an alkaline environment, but too strong alkalinity will cause iodine elements to disagree with the alkali, and formaldehyde is added to make the phenomenon more obvious (carbon dioxide gas).