Why can t litmus be an indicator?

Neutralization titration. The following aspects should be considered when selecting an indicator:

1) The narrower the discoloration range of the indicator, the better, and the indicator can change color with a slight change in pH. Litmus.

The solution is not used in neutralization titration because of its wide range of discoloration and the fact that the color change is not easy to observe at the equivalence point.

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. Strong acids and strong bases when neutralized, despite phenolphthalein and methyl orange.

It can be used, but when the alkali is titrated with acid, the 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 the methyl orange is chosen. 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) Strong acids and weak bases, strong bases and weak acids.

When the titration endpoint is reached, 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. If satisfied, hope.

Because the discoloration range of litmus is relatively large, and the contrast of various colors is not obvious, it is not easy to distinguish the titration endpoint, and the error is large.

Litmus is discolored due to acid and alkali, and the range of change is too small to be used.

The acid-base indicator is usually an organic weak acid or an organic weak base, and the principle of color change is due to the difference in the structure of the molecule and the ionized ion, so the color of the molecule and the ion is also different.

In different pH solutions, the color of the molecules is different due to the different concentrations. For example, litmus is an organic weak acid, which is a commonly used indicator in the laboratory, and can be used to represent litmus molecules. Because the discoloration of litmus is not obvious and the discoloration range is too wide, it is not used as an indicator for acid-base neutralization titration.

The color change range of litmus is 5-8, and the concentration of hydrogen ions is as high as 1000 times, which is very insensitive compared to methyl orange and phenolphthalein, so litmus cannot be used as an indicator for acid-base neutralization titration. Hope mine is helpful to you!

Acid-base indicator: generally some organic weak acids or weak bases, or organic acid-base two-day ignition of spinal substances, they can also participate in the proton transfer reaction in the acid-base titration process, due to the change of molecular structure and cause their own color change, and this color is reversible with the transformation of the structure. For example, phenolphthalein, methyl orange.

Classification: Monochromatic indicator: Among the acid or basic forms, only one type has a color indicator. Such as phenolphthalein.

Two-color indicator: Acidic or basic type has a color indicator. Such as methyl orange.

In fact, litmus is relatively rarely used. Commonly used are: methyl yellow, phenol red, thymol blue, etc., several indicators can also be prepared into mixed indicators according to different ratios, no less than twenty or thirty kinds.

Let's be a litmus indicator.

Purple litmus solution and phenolphthalein are indicators of the acidity and alkalinity of the solution, and their color changes depending on the pH of the solution. What we usually say about a pH<7 solution that turns purple litmus red, so that the colorless phenolphthalein does not change color, is just a rough way of saying it. In fact, the purple litmus test solution and phenolphthalein have a certain range of discoloration, see figure.

H+ and OH- exist in any aqueous solution, and the pH depends on the relationship between the H+ concentration and the OH- concentration of the solution.

H+Concentration=OH-ConcentrationpH=7 The solution is neutral.

H+ concentration》OH- concentration pH<7 The solution is acidic.

H+ concentration 7 solution is alkaline.

In the acid solution, the H+ concentration >> OH- concentration, so the pH < 7,

In the KhSO4 solution, there is a large amount of H+, pH <7. Therefore, it should be said that "the solution that can make the purple litmus solution red" must be an acidic solution. "A solution that does not discolor the phenolphthalein solution may be an acidic solution, a neutral solution, or a weakly alkaline solution.

It may also be a neutral solution or a weakly alkaline solution".

No, there are many indicators.

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 -naphthalene phenolphthalein, etc., which are all organic weak acids.

3 Sulfophenylphthaleins include phenol red, cresol red, bromophenol blue, thymol blue, etc., which are all organic weak acids.

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

There are many types of indicators.

Classification of indicators.

1. Acid-base indicator. Indicates the change in the concentration of H+ in the solution and is an organic weak acid or organic weak base that takes on different colors in acidic and alkaline solutions. Taking methyl orange as an example, the pH of the solution is red; When the pH is >, it appears yellow; In addition, a mixture of red and yellow orange appears, which is called the discoloration range of the indicator.

Different acid-base indicators have different discoloration ranges.

2. Metal indicators. Most of the indicators used in complexation titration are dyes, which can be complexed with metal ions at a certain pH to show a completely different color from the free indicator and indicate the end point.

3. Redox indicator. It is an oxidizing agent or reducing agent, and its oxidation form has a different color from the reduced form, and when it is oxidized (or reduced) in the titration, it changes color, indicating a change in the potential of the solution.

4. Precipitation titration indicator. It is mainly the titration of AG+ and halogen ions, with potassium chromate, ammonium iron or fluorescent yellow as indicators.