The phenolphthalein indicator was selected, and why was the amino group titrated

It is pink when it is near the end of the titration, and it is correct to become colorless when it is completely neutralized!

Because phenolphthalein does not change color when it encounters acid, it turns red when it encounters alkali! Phenolphthalein is selected as an indicator, and when the base is completely neutralized by the acid, it means that the basic OH- ion no longer exists! So it should be colorless.

However, it is not excluded that there is excess acid, generally in the titration, the amount of acid should be strictly controlled, because once the acid is excessive, the solution is also colorless (because phenolphthalein does not change color when it encounters acid), so when doing this kind of experiment, we should pay attention to the pink moment, which is a sign that the alkali is about to be completely neutralized, and it is also a feature that is about to reach the end point of titration! It does not mean that the acid and base are completely neutralized at this time!

To add: when the neutralization of strong and weak bases, strong bases and weak acids reaches the titration endpoint, the former solution is acidic, the latter solution is alkaline, for the latter the alkaline color change indicator (phenolphthalein) should be selected, for the former acid color change indicator (methyl orange) should be selected.

Phenolphthalein: when the acid titrates the base: the color fades from red; When alkali titrates acid: the color ranges from colorless to light red.

Methyl orange: when the acid titrates the base: the color is from yellow to orange; When alkali titrates acids: the color ranges from red to orange.

Note that litmus cannot be used as an indicator for neutralizing titrations. The reason for this is the range of discoloration of litmus.

It is too wide, and the color change is not obvious when the titration end is reached, and it is not easy to observe.

In order to reduce method errors and to coincide the titration endpoint and isoequivalence point, it is necessary to select the appropriate indicator. The narrower the discoloration range of the indicator, the better, and the indicator can change color with a slight change in pH.

When strong acids and weak bases are titrated with each other, methyl orange should be used.

When strong bases and weak acids are titrated with each other, phenolphthalein should be used.

When strong acids and strong bases are titrated with each other, either methyl orange or phenolphthalein can be used as an indicator.

I hope you can be satisfied with the occasional reply!

Because amino groups are acidic components, they can discolor phenolphthalein.

Preparation method of phenolphthalein indicator (ethanol solution of phenolphthalein): take phenolphthalein, dissolve it in 95% ethanol, without adding water, and dilute it to 100ml. According to GB T603-2002, phenolphthalein indicator (10 g L) is prepared by weighing 1 g, dissolving in ethanol (95%), and diluting to 100 ml with ethanol (95%).

Phenolphthalein is a commonly used acid-base indicator and is widely used in acid-base titration processes. Under normal circumstances, phenolphthalein does not change color when exposed to acid solution, does not change color when exposed to neutral solution, and turns red when exposed to alkali solution.

Uses of phenolphthalein indicators:Phenolphthalein solution is an acid-base indicator. Phenolphthalein is a weak organic acid that has a colorless lactone structure in a solution of pH and a quinone structure in a solution of pH.

The discoloration range of phenolphthalein is that phenolphthalein can only detect alkalis and not acids. As a commonly used indicator, phenolphthalein is widely used in acid-base titration processes. Under normal circumstances, phenolphthalein does not change color when exposed to acid solution, does not change color when exposed to neutral solution, and turns red when exposed to alkali solution.

However, phenolphthalein quickly fades from red to colorless in strong alkali, and will also change color in concentrated acid, and excessive addition of phenolphthalein dropwise in dilute acid solution causes precipitation, making the solution white and turbid. The quinone or quinone type of phenolphthalein, which is very unstable in alkaline media, will slowly convert to the colorless carboxylate form; When encountering a more concentrated lye, it will immediately transform into a colorless carboxylate formula. Therefore, when the phenolphthalein reagent is dropped into a concentrated lye, the phenolphthalein begins to turn red, and soon the red color fades and becomes colorless.

Phenolphthalein turns orange when it encounters concentrated sulfuric acid.

The above content reference: Encyclopedia - phenolphthalein test solution.

Because titrated with sodium hydroxide, phenolphthalein is in the alkaline range (

Internal discoloration, the reaction product of HAC and NaOH is weak acid and strong alkali salt NaAC, pH at the stoichiometric point, and the titration jumps in the alkaline range (such as:

The titration jump range is pH:, where the use of indicators that change color in the acidic range, such as methyl orange, will cause a large titration error (the solution is weakly alkaline at the stoichiometric point of the reaction, and the solution is weakly acidic when the indicator that changes color in the acidic range changes, and the titration is incomplete). Therefore, phenolphthalein, an indicator that changes color in the alkaline range, should be chosen here.

The selection of the indicator is mainly based on the titration burst range, and the discoloration range of the indicator should be all or part within the titration burst range, so the endpoint error is less than that of methyl orange (ph "red, pH > yellow, orange) or methyl red is in the acidic range (within the discoloration, if the titration endpoint is not reached when using methyl orange or methyl red discoloration, it will affect the experimental results.

The objectives of the experiment were as follows: 1) to learn the preparation and calibration methods of NaOH standard solutions;

2) Learn the use of volumetric flasks and pipettes, and further proficient in the titration operation of alkaline burettes;

3) Learn the principle and method of determining the nitrogen content in some ammonium nitrogen fertilizers by formaldehyde method.

Experimental principle: Because the NH4+ acid in ammonium salt is too weak in airtravel, it cannot be directly and correctly titrated with NaOH solution. Formaldehyde method is often used to strengthen weak acid, and the hair should be carried out according to the following formula.

4 nh4+ +6hcho===ch2)6nh+ +3h+ +6h2o

The acid can be calibrated with a NaOH standard solution with phenolphthalein as an indicator. NaOH solids are highly corrosive and prone to deliquescent and absorption of CO2 in the air, resulting in impurity. Therefore, the concentration of the NaOH standard solution needs to be calibrated.

Commonly used reference reagents are potassium hydrogen phthalate and oxalic acid, but because the former is easy to store and has a large molar mass, it is more widely used. It should be as follows: hp— +oh— =p2— +h2o

Since the titration to the metering point is the alkaline nature of the solution, phenolphthalein is used as an indicator.

Understand the principles and methods of formaldehyde method for determining nitrogen content in some ammonium nitrogen fertilizers.

Learn how to prepare and calibrate NaOH standard solutions.

Basic operations: Preparation of NAOH standard solutions, manipulation of basic burettes.

Experimental steps: 1) Prepare 1000ml of solution

Quickly weigh the solid in a beaker, add about 50ml of water and stir to dissolve, transfer it to a glass reagent bottle with a rubber stopper, add water to 1000ml, cover the stopper tightly, shake well, and label it.

2) (NH4)2SO4 nitrogen content.

Weigh in a beaker, stir with water to dissolve, and transfer to a 250ml volumetric flask to determine the volume. Take a sample, add 10 ml HCO and 1 2 drops of phenolphthalein to an Erlenmeyer flask, shake well and stand for 2 minutes, and titrate with NaOH standard solution.

Precautions: Formaldehyde contains a small amount of formic acid, which should be removed in advance;

Pay attention to the alkaline burette before titration to drive away air bubbles, and do not form bubbles during the titration process.

Topic analysis: the endpoint is incorrect during calibration, and the indicator should be reddish;

There is a difference between the concentration of the prepared solution and the required concentration.