How to choose an acid base indicator? What is the basis for the selection of acid base indicators

1) The narrower the discoloration range of the indicator, the better, and the indicator can change color with a slight change in pH. Due to the wide range of discoloration and the difficulty of observing the color change at the equivalence point, litmus solution is not used in neutralization titration.

2) The change of the color of the solution from light to dark is easy to observe, while it is not easy to observe from dark to light. Therefore, an indicator should be chosen that changes the color of the solution from light to dark at the end of the titration. When strong acid and strong base are neutralized, although phenolphthalein and methyl orange can be used, when the alkali is titrated with acid, methyl orange is added to the alkali, and when the equivalence point is reached, the color of the solution changes from yellow to red, which is easy to observe, so methyl orange is selected.

When the acid is titrated with alkali, phenolphthalein is added to the acid, and when the equivalence point is reached, the color of the solution changes from colorless to red, which is easy to observe, so phenolphthalein is selected.

3) When the neutralization of strong acid and weak alkali, strong alkali and weak acid reaches the titration endpoint, the former solution is acidic, the latter solution is alkaline, and the alkaline colorization indicator (phenolphthalein) should be selected for the latter, and the acidic colorization indicator (methyl orange) should be selected for the former.

4) In order to prevent the discoloration of the indicator from abnormally causing errors, the dosage of the indicator should not be too much during neutralization titration, the temperature should not be too high, and the concentration of strong acid or strong alkali should not be too large.

For different acid-base reactions, we should select indicators according to the actual situation, and the selection of indicators can be determined according to the neutralization titration curve. (Because many acid-base neutralization reactants are obvious, it is necessary to use an appropriate indicator to indicate the end point of the reaction during the titration process.) ）

Theoretically, the pH of the complete reaction of strong acids and bases should be 7, and the indicator that changes color at pH=7 should be selected during titration, but some reactions, such as: when sodium hydroxide is added to 20 ml of hydrochloric acid solution, (assuming the same concentration), when the volume of sodium hydroxide changes, the pH abruptly changes to Therefore, as long as the indicator in this range of discoloration is selected, there will be no great error. Here I will talk about the range of discoloration of each indicator:

Phenolphthalein; Methyl orange; Methyl red.

The selection rule of acid-base indicator: assuming that the reaction ratio between the standard solution and the test solution is m:n, the excess of the standard solution is less than one percent of the standard solution during titration, the pH of the mixed solution is only the upper and lower bound of the mutation, and the discoloration range of the indicator can be within this range.

It is also necessary to consider whether changes such as the color development of the indicator are obvious. Hope it works for you!

Acid-base indicators generally do not specify the dosage. For example, the amount of qualitative testing does not matter much. If quantitative titration is not too much.

Because acid-base indicators (e.g., phenolphthalein. Litmus. methyl orange, etc.) itself has a certain acidity and alkalinity.

Too much will inevitably lead to a large error.

It is selected according to the acidity and alkalinity of the product after neutralization. For example, strong acid titration weak base, because the neutralized salt will be partially hydrolyzed and acidic, so it is necessary to choose an indicator with a discoloration range in the acidic region.

Generally, it should be judged according to the titration curve. We generally first use the existing data, such as the PKA of weak acid, to take the amount of strong acid droplets as the abscissa, and the pH value of the system as the ordinate, and then draw the titration curve, observe the range of the curve abrupt over, and select the one with the same color change range and the abrupt range or the largest overlap.

This is the practice of analytical chemistry in college, ...... high schoolThere will not be too many requirements for the titration indicator, as long as you know that methyl orange is used for acid drops, phenolphthalein is used for alkali drops, and litmus is not used.

Strong acids and strong bases drop each other, and whoever drops methyl orange and phenolphthalein can be.

Strong acids and weak bases with methyl orange.

Phenolphthalein for strong bases and weak acids.

Acid drops alkali with methyl orange.

Alkali drops acid with phenolphthalein.

with a burette.

<>1, the discoloration of the indicator should be obvious.

2. The end point of titration is consistent with the range of discoloration of the indicator.

Strong acid and weak base: methyl orange.

Strong base and weak acid: phenolphthalein.

Strong acids and bases: phenolphthalein or methyl orange.

Commonly used acid-base indicators are methyl orange, methyl red, phenolphthalein, and litmus, which are generally used as indicators for acid-base titration.

A class of organic weak acids or organic weak bases with complex structures can be partially ionized into indicator ions and hydrogen ions in solution. When the pH of the solution changes, the indicator gets protons, which change from basic to co-acid, or loses protons, which change from acid to co-wheel base, and the color changes due to the transformation of its structure.

Acid-base indicators, also known as pH indicators and hydrogen ion concentration indicators, are chemical reagents used to test pH values. They are weak acids or bases and contain pigments, which combine with H+ hydrogen ions or OH hydroxide ions when dropped into the solution and convert into the corresponding acid or base formula, thus showing different colors. Since the pH indicator can produce reversible color changes in solutions with different pH values, it can be used in neutralization and analysis to indicate the end point of the reaction and determine the pH value of the test solution.

The quality of the pH indicator is mainly controlled by the sensitivity of the pH discoloration range.

The six easy-to-make acid-base indicators can be extracted from sugar radish, blueberry, carrot, cherry, delphinium petals, geranium petals, grapes, etc. When the pH of the solution changes, the indicator gets protons, which change from basic to conjugated acid, or loses protons, which change from acid to conjugated base, and the color changes due to the transformation of its structure.

Acid-base indicator homemade methodExtract the acid-base indicator from the carrot skin: After scraping off the red skin of the carrot, soak it in 95% alcohol for about a day, and filter out its filtrate, which is the acid-base indicator. According to the needs of the test, several standard solutions of pH 1 14 were made, 10ml of each standard solution was placed in a test tube, and then 10 drops of carrot skin soaking solution were added respectively, and plugged tightly as a colorimetric sample.

Extract acid-base indicator from comfrey: take 5g of comfrey and soak it in 50% alcohol for one day to obtain the purple alcohol solution of comfrey, that is, the acid-base indicator. The discoloration of the acid-base is the same as that of the litmus solution.

Extract acid-base indicator from purple cabbage: take about 250g of purple cabbage, wash and chop, put it in a stainless steel pot with water and boil for 10 minutes, then filter and cool it and place it in a container to be used as an acid-base indicator.

Common acid-base indicators are purple litmus solution and colorless phenolphthalein solution.

The petals or fruits of certain plants (e.g., morning glory, moonflower, purple cabbage, etc.) can also be used as acid-base indicators.

The purple litmus solution turns red when it meets the acid solution (the solution containing H+), and the alkali solution (the solution containing OH-) turns blue, and it is purple in the neutral solution.

The colorless phenolphthalein solution does not change color when it encounters acid solution, does not change color in neutral solution, and turns red when exposed to alkali solution.

Acid-base indicators commonly used in high school chemistry are methyl orange, litmus and phenolphthalein.

The discoloration of methyl oranges ranges from 3.1 to 4.4, less than 3.1 is red, between 3.1 and 4.4 is orange, and more than 4.4 is yellow.

The litmus solution has a discoloration range of 5 to 8Less than 5 is red, between 5 and 8 is purple, and greater than 8 is blue.

The discoloration of phenolphthalein test solution ranges from 8.2 to 10Less than 8 o'clock 2 is colorless, between 8 o'clock 2 and 10 is light red, and more than 10 is red.

The discoloration of the indicator is in whole or part within the titration jump range).

Acid-base indicators, also known as pH indicators and hydrogen ion concentration indicators, are chemical reagents used to test pH values.

1. Classification. There are four main types of acid-base indicators commonly used:

1) Nitrophenols: This is a class of acidic indicators, such as p-nitrophenols.

2) Phenolphthalein: There are phenolphthalein, thyme phenolphthalein and naphthalenophthalein, etc., which are all organic weak acids.

(3) Sulfophenylphthaleins: there are phenol red, cresol red, bromophenol blue, thymol blue, etc., which are all organic weak acids.

4) Azo compounds: methyl orange, neutral red, etc., they are both amphoteric indicators, which can be used for acid dissociation and basic dissociation.

2. The principle of color change.

Since the acid-base indicators are all organic weak acids or organic weak bases, they have different degrees of ionization in different acid-base solutions, so they will show different colors. The pH test paper is made of a variety of indicator mixtures, usually the pH test paper is golden yellow, pH is about 5, it can be seen that in the production, the indicator mixture has been adjusted to weak acidity, not neutral, this is to weaken the influence of CO2 in the air on the determination. In addition, when neutralizing the reaction, only 2 to 3 drops of acid-base indicators can be used, also because acid-base indicators are organic acids or organic bases, and using too much will increase the error.