What are the benefits of mixing indicator use

The acid-base indicator is generally an organic weak acid or a weak base, and its acid color is different from the basic color. In solution, the following equilibrium is present:

in- +h+ = hin

Alkaline color) (acid color).

When [hin] [in-] 10, the predominantly acid color is presented;

When [hin] [in-] 10, the predominantly basic color is presented.

Because the discoloration of the indicator is not a mutation, but a change within a pH range, and the discoloration point of the indicator is not exactly the same as the stoichiometric point, there will be a certain error when the indicator indicates the end point, and the amount of the indicator will also affect the size of the error.

Definition: For reactions.

A A (ANALYTE) + T T (titrant) = R r + q q

et=[（a/t）nt - na] / na

In acid-base titration, mixed indicators can be used when the use of a single indicator is not sensitive to discoloration, or when it is necessary to limit the endpoint to a very narrow range.

There are two types of mixed indicators: one is to use both indicators at the same time; One is composed of an indicator and an inert dye, which complements each other in color to make the endpoint discoloration more acute.

Although the first measurement point can be determined by using a mixed table indicator of cresol red and thymol blue, the color change is not obvious from the light purple of pH to the rose color of pH. Methyl orange is used as an indicator, indicating the second measurement point of the titration, and the titration to orange is the end point, which is not sensitive. This is due to the formation of HCO3 CO3 buffer system after the first metering point, which gradually decreases the pH and changes the color of methyl orange.

In order to solve the above problems, multiple groups of mixed indicators are used.

The multi-group mixing indicator is composed of phenolphthalein, thymol blue, bromothymol blue, and methyl orange in a certain proportion.

It's easy to tell you that mixing indicators can reduce the pH range! That's the benefit

The indicator is used to indicate the reagent at the end point of the titration.

Indicators are a class of chemical reagents. Under certain medium conditions, its color can change, turbidity or precipitation, and fluorescence. It is often used to test the acidity and alkalinity of the solution; It is used in titration analysis to indicate the titration endpoint; Hazardous substances are inspected in the environmental testing of the banquet.

Generally, it is divided into acid-base indicators, redox indicators, metal indicators, adsorption indicators, etc.

In the process of various titrations, with the addition of titrant, the concentration of the titrated substance and the titrant are constantly changing, and the ion concentration will change greatly near the equivalence point, and the reagent that can display the change in the ion concentration (such as changing the color of the solution, generating precipitation, etc.) is called an indicator.

If the titrant or the substance being titrated is colored, they themselves act as an indicator, such as potassium permanganate.

The history of the development of indicators dates back to the 17th century. This substance was already used by many practical chemists in ancient times, who collected and sorted out the sap of plants (i.e., indicators) in the course of experiments and stained them on a small piece of paper, and then dropped a drop of the solution of the silver they had studied on this test paper, and used this to judge some of the properties of the chemical reaction they were studying.

According to the current records, Boyle was the first scientist to use the sap of various natural plants as indicators, some of which were used as solutions and some as test strips.

Methyl red indicator can be used for titration of strong acids, and phenolphthalein indicator can be used for titration of strong acids.

Strong acid drops weak alkali with methyl orange (yellow to orange) or phenolphthalein (red to colorless) strong alkali drops with phenolphthalein colorless to pink, note that the discoloration range is pink) 10 (red) NaOH titration HAC titration product NAAC is a strong alkali weak salt, alkaline, so the indicator of discoloration in the alkaline range should be selected.

The discoloration range of methyl orange is pH; The range of methyl red discoloration is known; The discoloration range of phenolphthalein is pH 8-10; The discoloration of litmus ranges from 5-8 pH.

If it is a strong acid titration of a weak base (e.g., hydrochloric acid titration of sodium acetate), and the end product is weakly acidic, methyl orange (pH change range) or methyl red (pH change color range indicator) should be used. If it is a strong base titration of a weak acid (e.g

Sodium hydroxide titration acetic acid), the end product is weakly alkaline, and phenolphthalein (pH discoloration range 8-10) indicator should be selected.

1. Indicators are generally divided into acid-base indicators, redox indicators, metal indicators, adsorption indicators, etc. Laugh lead.

2. Indicator is a type of chemical reagent, under certain medium conditions, its color can change, turbidity or precipitation, and fluorescence. It is often used to test the acidity and alkalinity of the solution; It is used in titration analysis to indicate the titration endpoint; Detection of hazardous substances in environmental testing.

3. The indicator is a reagent used to indicate the end point of the titration. In the process of various titrations, with the addition of titrant, the concentration of the titrated substance and the titrant is constantly changing, near the equivalence point, the concentration of ions will change greatly, and the reagent that can display the change in the concentration of this ion (such as changing the color of the solution, generating precipitation, etc.) is called an indicator. If the titrant or the substance to be titrated is colored, they themselves have the function of an indicator, such as potassium permanganate, and different indicators show different colors in different acid-base environments.

The advantage of mixing indicators is that experimental data can be obtained at one time, avoiding the need for multiple tests.

The mixing indicator uses the complementary effect between the colors to narrow the discoloration range, so that the color change is sharp at the end point. There are generally two ways to prepare it. One is a mixture of two or more indicators.

For example, bromocresol green and methyl red indicators. The other is to add an inert dye (whose color does not change with the pH of the solution) to a certain indicator, which makes the color change sensitive due to color complementarity, but the color change range does not change.

The principle of action of the indicator 1, acid-base indicator: generally some organic weak acid or weak base, or organic acid-base amphoteric 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.

2. Classification: Monochrome indicator: There is only one type of color indicator in the acid or basic type.

Such as phenolphthalein two-color indicator: acid or basic type has color indicator. Such as methyl orange.

2. The pH range of indicator discoloration: 1. The principle of discoloration: Hin indicates that the indicator Hin = H+ + + in- acid type Basic type ka is the dissociation constant of the indicator, and the color of the solution is determined by the ratio of [in-] Hin], which changes with the change of [H] of the solution.

When [in-] Hin] 1 10 ph pka-1 acid color when 10 > [in-] hin]>1 10 ph between pka 1 color gradually change mixed color when [in-] hin] 10 ph pka+1 basic color 2, discoloration range: when the pH of the solution changes from pka-1 to pka+1 (or vice versa), the indicator can be observed from acid color mixed color to basic color, the pH range of this color change, that is, ph===pka 1 The range of discoloration called the indicator. 3. Theoretical discoloration point:

When the concentration of the acid form of the indicator is equal to that of the base type, i.e., [in-] hin]==1, the pH of the solution = pka, which is called the theoretical discoloration point of the indicator.

3. Factors affecting the range of indicator discoloration: 1. Dosage of indicator: two-color indicator:

The range of discoloration of the indicator is not affected by the amount of it used. However, the discoloration of the indicator also consumes a certain amount of titrant, which introduces errors. Monochrome Indicator:

As the dosage of the monochrome indicator increases, the discoloration range shifts to the direction of decreasing pH. The dosage should be appropriate. 2. Temperature:

The change of temperature will cause the dissociation constant of the indicator and the proton self-transmission constant of water to change, so the discoloration range of the indicator will also change, and the effect on the basic indicator is more obvious than that of the acidic indicator. 3. Neutral electrolyte: 4. Solvent:

Different solvents have different dielectric constants and acid-bases, resulting in a dissociation constant and discoloration range for one line of indicators.

Four: Mixing indicator: 1. The mixing indicator makes use of the complementary effect between colors, has a very narrow range of discoloration, and changes by a keen color at the end of the titration.

2. Preparation method: [1] It is prepared by using a dye (called an inert dye) whose color does not change with the change of H concentration in the solution and an indicator; [2] Select two (or more) indicators with similar PK values and mix them in a certain proportion.