Litmus indicator solution is a weak organic acid with the characteristic of a blue purple powder. It is a blue pigment extracted from lichen plants and can partially dissolve in water to appear purple. It is a commonly used acid-base indicator with a color change range of pH=4.5-8.3. Under different effects of acid-base solutions, the conjugated structure changes and the color changes. It is a weak organic acid that undergoes a change in conjugated structure and color under different effects of acidic and alkaline solutions. That is to say, in a solution, as the acidity or alkalinity of the solution changes, its molecular structure changes and presents different color changes: in acidic solutions, molecules are the main form of its existence, making the solution red; Due to the increase in [H+], the balance shifts to the left. In alkaline solution, the ionization equilibrium of litmus shifts to the right, and the acid ions produced by ionization are its main form of existence, resulting in a blue color of the solution; Due to the increase in [OH -], the balance shifts to the right. For example, in conducting chemical experiments, if you want to know whether a solution is acidic or alkaline, you can add litmus reagent. If the solution turns red, then it is acidic; If the solution turns blue, then it is alkaline. This characteristic makes pistil one of the important tools in the laboratory. In addition to its use in the laboratory, litmus is also used in some daily lives. For example, in some diapers, litmus is added to indicate the humidity of the diaper. When the diaper becomes damp, the color change of litmus can be seen by people, thus reminding them to change the diaper.
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Extracting natural blue pigments from litmus lichens to prepare Litmus indicator solution is a delicate process involving multiple steps. Litsea lichen is a special plant that contains pigment components that can exhibit different colors under different pH environments, making it a commonly used acid-base indicator in chemical experiments.
Preparation stage
1. Material collection
Litmus lichens: Choose fresh and pollution-free lichens as raw materials. Litchi lichens usually grow on rocks, bark, or soil surfaces, and their growth environment should be avoided during collection.
Solvent: Ethanol (usually 95% concentration) and water, used for extracting and purifying pigments.
Experimental equipment: beaker, measuring cylinder, glass rod, filter paper, funnel, distillation apparatus, pH test paper, electronic balance, magnetic stirrer, etc.
2. Safety measures
Before conducting any experiment, it is necessary to wear laboratory clothing, gloves, and goggles to ensure personal safety.
Good ventilation conditions should be maintained in the laboratory to avoid the accumulation of harmful gases.
Extraction process
1. Preliminary processing
Clean the collected stone moss to remove surface soil, impurities, etc. Be careful to avoid using too much water to avoid diluting the pigment.
Dry the cleaned stone moss or gently wipe it dry with a tissue to remove excess moisture.
2. Crushing and soaking
Use a mortar or grinder to grind the lichen into small particles for better pigment release.
Transfer the crushed lithological lichen powder to a beaker and add an appropriate amount of 95% ethanol solution (such as 50mL ethanol per gram of lithological powder) to completely immerse the powder.
Use a magnetic stirrer or manual stirring to thoroughly mix the stone powder with ethanol, and let it stand for a period of time (such as 24 hours) to allow the pigment to fully dissolve in ethanol.
3. Filtration and Purification
Filter the soaking solution using filter paper and funnel to remove insoluble solid impurities.
The filtered ethanol solution may contain some impurities and partially dissolved pigment particles, which require further purification. The purity of pigments can be improved by repeatedly soaking and filtering.
In some cases, in order to remove alkaline impurities from ethanol solution (which may interfere with the color change reaction of litmus), an appropriate amount of dilute acetic acid can be added to the filtered solution to adjust the pH of the solution to neutral or weakly acidic.
Preparation of Litmus Indicator
1. Solution preparation
Dilute the purified ethanol solution (which already contains litmus pigment), usually by mixing the ethanol solution with water in a certain ratio (such as ethanol: water=1:1 or adjusted as needed), to obtain a concentration suitable as an indicator.
Pay attention to maintaining stirring during the dilution process to ensure the uniformity of the solution.
2. Acid base regulation
In order for the indicator to accurately change color in acidic and alkaline environments, it is necessary to fine tune its pH value. This is usually achieved by adding an appropriate amount of acid or base. However, due to the sensitivity of litmus itself to changes in pH, this step requires great caution.
PH test strips or pH meters can be used to monitor the pH value of the solution, and dilute acid or alkali can be gradually added as needed for adjustment.
3. Stability testing
The prepared litmus indicator needs to undergo stability testing to ensure that it can maintain stable color changing performance under different conditions.
The indicator can be placed in acidic, neutral, and alkaline environments to observe whether its color change is accurate and long-lasting.
Litmus indicator solution, as a widely used acid-base indicator, involves complex chemical and physical processes in its color changing principle. It is an indispensable part of chemistry teaching and an important tool for understanding the changes in acid-base properties of solutions.
It is a natural organic pigment extracted from lichen plants, and is used as an acid-base indicator because it can change color with the acidity or alkalinity of the solution. In nature, it mainly exists in two forms: blue and red, corresponding to its acidic and alkaline forms, respectively. When dissolved in solvents such as water or alcohol to make an indicator, it can exhibit different colors in different pH environments, thus becoming an intuitive means of determining the acidity or alkalinity of a solution.
The molecular structure of litmus is complex, containing multiple conjugated systems and functional groups, which determine its unique chemical properties. Under acidic conditions, certain functional groups (such as phenolic hydroxyl groups) in litmus molecules undergo protonation, forming positively charged ions. In this state, litmus molecules tend to absorb longer wavelengths of light (such as red light), resulting in a red solution. On the contrary, under alkaline conditions, certain functional groups (such as carboxyl groups) in litmus molecules lose their protons and form negatively charged ions. At this time, the absorption of shorter wavelengths of light (such as blue light) by litmus molecules is enhanced, and the solution appears blue.
The discoloration mechanism is mainly based on the ionization equilibrium changes of its molecules in different pH environments. Specifically, when the solution is acidic (pH<7), the acidic groups (such as phenolic hydroxyl groups) in the litmus molecule accept hydrogen ions (H+) from the solution, undergo protonation reactions, and form positively charged ions. This ionic structure enhances the absorption of red light by litmus molecules, resulting in the solution appearing red. As the pH value of the solution increases, the concentration of hydrogen ions gradually decreases, and the acidic groups in the litmus molecules begin to release hydrogen ions, returning to neutral or alkaline forms. When the solution reaches the alkaline range (pH>7), the alkaline groups (such as carboxyl groups) in the litmus molecules lose their protons and form negatively charged ions. This ion structure enhances the absorption of blue light, resulting in the solution appearing blue.
It is worth noting that the color change is not instantaneous, but there is a transition zone, known as the "color change range". Within this range, the color of the solution will gradually change with small changes in pH, transitioning from red to purple, and then to blue. This color change range is usually used to roughly estimate the pH value of a solution.
The color changing effect is influenced by various factors, mainly including the following aspects:
(1) Solution temperature:
Changes in temperature can affect the ionization equilibrium of litmus molecules, thereby affecting their color changing effect. Generally speaking, as the temperature increases, the ionization equilibrium shifts in the positive direction, which may cause a shift in the color change point (i.e. the pH value at which the color changes significantly).
(2) Solvent type:
Different solvents have different effects on the solubility and ionization degree of litmus molecules. For example, when water is used as a solvent, the discoloration effect of litmus is most pronounced; In some organic solvents, the discoloration of litmus may become less noticeable or completely disappear.
(3) Solution concentration:
The concentration of litmus indicator can also affect its color changing effect. Excessive concentration may result in colors that are too dark and difficult to accurately determine; However, a low concentration may result in less noticeable discoloration.
(4) Coexisting ions:
Other ions in the solution, especially those that can interact with litmus molecules (such as metal ions, strong acid ions, etc.), may interfere with the color changing process of litmus, causing a shift in the color changing point or a weakening of the color changing effect.
Litmus indicator has a wide range of applications in chemistry teaching, laboratory analysis, industrial production, and other fields due to its simple and intuitive characteristics. With the development of technology, people have also developed various new acid-base indicators, such as phenolphthalein, methyl orange, bromophenol blue, etc. They each have different color changing ranges and sensitivities, which can meet the needs of different fields. However, as one of the earliest discovered acid-base indicators, the classical status of the litmus indicator remains unshakable.
The color changing principle of litmus indicator is a complex process involving multiple aspects such as molecular structure, ionization equilibrium, and optical properties. By gaining a deeper understanding of its color changing mechanism, we can better master the application techniques of acid-base indicators and improve the accuracy and reliability of experimental analysis. At the same time, as an important tool in chemical education, the litmus indicator also reveals the mysteries of material property changes in nature, inspiring people's interest and exploration desire in chemical science.
Extracting natural blue pigments from litmus lichens to prepare Litmus indicator solution is a delicate process involving multiple steps. Litsea lichen is a special plant that contains pigment components that can exhibit different colors under different pH environments, making it a commonly used acid-base indicator in chemical experiments.
Preparation stage
We offer a variety of transmission components
1. Material collection
Litmus lichens: Choose fresh and pollution-free lichens as raw materials. Litchi lichens usually grow on rocks, bark, or soil surfaces, and their growth environment should be avoided during collection.
Solvent: Ethanol (usually 95% concentration) and water, used for extracting and purifying pigments.
Experimental equipment: beaker, measuring cylinder, glass rod, filter paper, funnel, distillation apparatus, pH test paper, electronic balance, magnetic stirrer, etc.
2. Safety measures
Before conducting any experiment, it is necessary to wear laboratory clothing, gloves, and goggles to ensure personal safety.
Good ventilation conditions should be maintained in the laboratory to avoid the accumulation of harmful gases.
Extraction process
1. Preliminary processing
Clean the collected stone moss to remove surface soil, impurities, etc. Be careful to avoid using too much water to avoid diluting the pigment.
Dry the cleaned stone moss or gently wipe it dry with a tissue to remove excess moisture.
2. Crushing and soaking
Use a mortar or grinder to grind the lichen into small particles for better pigment release.
Transfer the crushed lithological lichen powder to a beaker and add an appropriate amount of 95% ethanol solution (such as 50mL ethanol per gram of lithological powder) to completely immerse the powder.
3. Filtration and Purification
Filter the soaking solution using filter paper and funnel to remove insoluble solid impurities.
The filtered ethanol solution may contain some impurities and partially dissolved pigment particles, which require further purification. The purity of pigments can be improved by repeatedly soaking and filtering.
Use a magnetic stirrer or manual stirring to thoroughly mix the stone powder with ethanol, and let it stand for a period of time (such as 24 hours) to allow the pigment to fully dissolve in ethanol.
In some cases, in order to remove alkaline impurities from ethanol solution (which may interfere with the color change reaction of litmus), an appropriate amount of dilute acetic acid can be added to the filtered solution to adjust the pH of the solution to neutral or weakly acidic.
Preparation of Litmus Indicator
1. Solution preparation
Dilute the purified ethanol solution (which already contains litmus pigment), usually by mixing the ethanol solution with water in a certain ratio (such as ethanol: water=1:1 or adjusted as needed), to obtain a concentration suitable as an indicator.
Pay attention to maintaining stirring during the dilution process to ensure the uniformity of the solution.
2. Acid base regulation
In order for the indicator to accurately change color in acidic and alkaline environments, it is necessary to fine tune its pH value. This is usually achieved by adding an appropriate amount of acid or base. However, due to the sensitivity of litmus itself to changes in pH, this step requires great caution.
3. Stability testing
The prepared litmus indicator needs to undergo stability testing to ensure that it can maintain stable color changing performance under different conditions.
The indicator can be placed in acidic, neutral, and alkaline environments to observe whether its color change is accurate and long-lasting.
PH test strips or pH meters can be used to monitor the pH value of the solution, and dilute acid or alkali can be gradually added as needed for adjustment.
The use of litmus as a chemical indicator to test the acidity or alkalinity of a solution was first discovered and promoted by British chemist and physicist Robert Boyle (1627-1691). How to easily measure the acidity or alkalinity of a solution has been a headache and helplessness for Boyle and other scientists. But one day, a turning point appeared in front of Boyle. On this day, Boyle inserted a beautiful bouquet of violets he had just picked into a vase in the laboratory and began conducting experiments. But he accidentally dropped a few drops of hydrochloric acid onto the violet flowers. Boyle, who loves flowers, quickly rinsed with clean water. At this moment, Boyle saw that the violet flowers had turned into red flowers! Why do violets turn red? Boyle felt very novel and excited at the same time, and he was determined to investigate and uncover the truth. Boyle conducted experiments using HNO3, H2SO4, and CH3COOH, and the results were exactly the same - all petals turned red. After repeated experiments, Boyle determined that the extract of violet flowers can be used to test whether the solution is acidic. The initial victory was achieved, but Boyle was not satisfied and tried to find another substance to test alkalinity. He made extracts from the flowers, herbs, bark, tubers, roots, mosses, lichens, and other materials that could be found, and tested their color changing reactions in alkaline solutions one by one. Finally, it was discovered that alkaline solutions can turn the purple liquid extracted from lichens blue. Nevertheless, Boyle did not stop there. He wondered: Can a reagent be used to measure both acidity and alkalinity? He tried to drop the extract of the litmus into a hydrochloric acid solution, and the result was the same phenomenon as testing acidity with violets - the litmus extract also turned red! The problem has been completely resolved. Litmus reagent turns blue when exposed to alkali and red when exposed to acid, which is exactly the bidirectional indicator that Boyle has been searching for! From then on, Litmus indicator solution was widely used to test the acidity and alkalinity of solutions. Boyle's significant invention was made in 1646 and is still widely adopted today. So, we can easily detect the acidity or alkalinity of the solution today, thanks to the great Boyle!
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