Instruments: 50 ml basic burette, 25.00 ml pipette, 250 ml flat volume, 250 ml conical flask, analytical balance and tray balance.

Reagent: potassium hydrogen phthalate (), 0.1mol/L.

Sodium hydroxide solution, 0.2% phenolphthalein indicator.

experimental procedure

Calibration of 1.NaOH solution

(1) On the electronic balance, weigh three copies of 0.4-0.6g potassium hydrogen phthalate reference substance by subtraction, put them in three 250mL conical flasks respectively, add 30-40mL deionized water to dissolve, and then drop 1-2 drops of 0.2% phenolphthalein indicator.

(2) Titrate with NaOH solution to be calibrated until it is colorless and reddish, and keep it for half a minute.

(3) Record the volume of NaOH solution in the burette before and after titration. Calculate the relative deviation between the concentration of NaOH solution and the calibration result.

2.

Determination of acetic acid content in vinegar

(1) Use a 25.00mL pipette to absorb vinegar samples, put them in a 250mL volumetric flask, dilute them with water to scale and shake them well.

(2) Absorb 25.00mL diluted test solution with a pipette, put it in a 250mL conical flask, add 0.2% phenolphthalein indicator 1-2 drops, and titrate with NaOH standard solution until half a drop of NaOH standard solution is added to make the test solution red, and the end point is to keep it from fading for half a minute.

(3) Repeat the operation, measure the other two samples, and record the volume of NaOH solution in the burette before and after titration. The relative average deviation of the determination result should be less than 0.2 ml.

(4) Calculate the content of acetic acid in the sample according to the determination result, expressed in grams/liter. ..

Experimental discussion

1.

Acetic acid is a weak organic acid with a dissociation constant Ka.

=

1.76×, which can be directly titrated with standard alkali solution. The reaction is as follows:

At the stoichiometric point, the reaction product is NaAc, which is a salt of strong base and weak acid. The pH value of its solution is about 8.7, and the color change range of phenolphthalein is 8- 10. At the end of titration, the pH value of the solution is in it, so phenolphthalein is used instead of methyl orange and methyl red as indicators.

2.

The main component of vinegar is acetic acid, but it also contains a small amount of other weak acids, such as lactic acid. Where is CKa?

& gt All weak acids can be titrated accurately with strong base. Therefore, in this experiment, the total acid content is determined by titration of vinegar with NaOH, and the results are commonly used:

3.

Vinegar contains about 3%-5% acetic acid, which can be diluted properly before titration. White vinegar can be titrated directly, and ordinary vinegar can be decolorized with neutral activated carbon and titrated again because of its dark color.

4.

It is the reference for calibrating NaOH, so the electronic balance should be used when weighing, and the difference subtraction should be used to make the weighing result as accurate as possible. However, it is not necessary to accurately weigh NaOH, and a tray balance can be used.

5.

When phenolphthalein indicator changes from colorless to reddish, the pH value of the solution is about 8.7. After the red solution is put in the air, it becomes colorless because it absorbs CO2 in the air.

6.

If CO2 in the air is absorbed by the calibrated NaOH standard solution during storage, and the concentration of acetic acid in vinegar is determined by it, the determination result will be high with phenolphthalein as indicator. In order to make the determination results accurate, NaOH solution should be avoided in the air for a long time.

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