Thursday, March 31, 2022

When and why is back titration used?

Back titration or indirect titration is a type of complexometric titration in which a known excess of standard reagent is introduced to the solution and the excess is titrated instead of the original sample. 

Back titration is a titration performs in reverse. In a back titration, two steps are commonly followed: measuring the amount consumed by the analyte and calculating the excess quantity.

Why is back titration used?

When the endpoint of the reverse titration is easier to identify than the endpoint of the regular titration, such as in precipitation reactions, back titration is useful. Back titrations are also effective when the interaction between the analyte and the titrant is very slow, or when the analyte is in a non-soluble solid.


When do we use back titration?

  • Back titration is performed when the molar concentration of an excess reactant is known, but we need to determine the concentration of an analyte.
  • When the analyte and titrant reaction is slow or when the analyte is in an insoluble solid
  • When determining the endpoint of a normal titration is difficult, such as weak acid and weak base titration.
  • It is used when the acid or base is a salt that is insoluble, such as calcium carbonate.
  • Back titration is most typically used when determining the endpoint of a regular titration is difficult.
  • When one of the reactants is volatile, such as ammonia.

Why is back titration used in EDTA?

Titrations with EDTA can be done in different ways, including direct, back (indirect), displacement, and masking. Using normal titration procedures isn't always possible. Back titration is a technique that can be used in such instances.

Generally, we used two reagents in back titration: one that reacts with the original sample and the other that reacts with the first reagent. In this method, the excess of EDTA is added to the metal ion solution, which is then titrated with a known concentration of a second metal ion.

With EDTA, the second metal ion must form a weaker complex than the solute ion. As a result, the second metal does not displace the solute ion from its complex with EDTA.

Why do we use back titration in the analysis of aspirin?

For direct titrations, several reactions are slow or have an unfavorable equilibrium. Aspirin is a weak acid with a slow hydrolysis rate; i.e., each molecule of aspirin reacts with 2 hydroxide ions. To overcome this problem, we use back titration in the analysis of aspirin. 

In which, an HCl (Hydrochloric acid) titration is used to determine the amount of unreacted base when a known excess amount of base is introduced to the sample solution. This type of reaction occurs at a rapid rate, resulting in an abrupt and easily seen end-point.

Why is back titration used to determine calcium carbonate?

Because calcium carbonate (CaCO3) is insoluble in water and cannot be directly titrated, determining its concentration requires a back titration. Back titration is done when the primary standard is not soluble in water and the direct titration is not possible.

Where is back titration used?

When the molar concentration of an excess reactant is known, but the strength or concentration of an analyte must be determined, back titration is performed. In acid-base titrations, back titration is commonly used: When the acid or base is an insoluble salt (for example calcium carbonate).

Why is back titration more accurate?

In a direct titration, a standard titrant is added to the analyte until the endpoint is reached. In a back titration, an excess of standard titrant is added to the analyte, and the excess titrant is titrated to determine how much is in excess, this makes it a more accurate method.

What is the example of back titration in analytical chemistry?

Determination of acetylsalicylic acid in aspirin and determination of calcium carbonate (CaCO3) is an example of back titration that performs in the lab.
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How does back titration work?

A volumetric analysis in which the endpoint of the analysis or titration is identified by the formation of a colored complex is known as complexometric titration. It is mostly used to determine the concentration of a mixture of metal ions in a sample solution.

In these titrations, an indicator is used to indicate the endpoint, which is based on the formation of a complex among the titrant and the solute. Types of complexometric titrations include a direct, replacement, back titration, etc.

What is back titration in complexometric titration?

A back-titration can also be known as indirect titration. A back titration is a titration method that involves reacting an analyte with a known amount of excess reagent to determine its concentration. The remaining excess reagent is then titrated with a different, second reagent.

The result of the second titration shows how much excess reagent was used in the first titration, allowing the concentration of the original analyte to be determined. An indirect titration is also known as back titration.

How is a back titration performed?

A back titration usually consists of two steps:
  1. An excess of reagent is allowed to react with the volatile analyte.
  2. Titration is performed on the remaining quantity of known solutions.
This is a method of calculating the excess quantity by measuring the amount consumed by the analyte.

It is a two-step method in which the titrand is reacted with a specific, excess amount of titrant whose concentration is known. Instead of titrating the original sample, a known excess of the standard reagent is added to the solution, and titration is performed. The standard titrant will then react with the analyte, and the excess remains in the solution. A back-titration can be used to detect how much standard reagent is left.
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Saturday, March 26, 2022

Fajan's method of precipitation titration

Learn about the definition, principle, reaction, indicator, endpoint, and advantages of Fajan’s method.

Titration is a method of determining the concentration of an unknown solute using a known concentration solution. In which a titrant is usually applied from a burette to a known compound volume until the reaction is complete, the endpoint is detected using an indicator. Acid-base titration, redox titration, precipitation titration, and complexometric titration are all types of titrations.

What is precipitation titration?

Precipitation titration is a type of titration in which the titration reaction formed a precipitate. The titrant reacts with the compound to produce an insoluble substance (precipitate). This uses silver ions to find out chloride levels and continues until all compounds have been consumed.

Precipitation titration is a volumetric process based on the formation of a slightly soluble precipitate. On the other hand, the argentometric technique is a precipitation titration method that uses silver nitrate (AgNO3) as the precipitating agent. There are three forms of precipitation titration: Mohr, Vollhard, and Fajan.

Principle involved in precipitation titrations:

The basic principle of precipitation titrations is that the amount of precipitating reagent or precipitant added is equivalent to the substance being precipitated.

Amount of added precipitating agent = the amount of a compound that is precipitated

What is Fajan’s method in chemistry?

The Fajans method is an analytical technique that uses adsorption to detect halide content. This technique was presented by Kazimierz Fajan, a chemist from the United States, for this reason, it is called Fagan's method. It is commonly used to quantitatively analysis of halide ions or thiocyanate ions.

Silver chloride is also known as indication adsorption as chloride ions are adsorbed on its surface. This method uses a dichlorofluorescein indicator. The suspension (of AgCl and indicator) has reached the endpoint when it turns pink from green.

Definition of Fajan’s method:

"Fajan's method is a precipitation titration in which silver ions are titrated with halide/thiocyanate ions in the presence of an adsorption indicator"

Example of Fajans method:

Cl– ions titration using AgN03 in the presence of an adsorption indicator, in which, AgN03 is filled in a burette, and Cl– ion solution with indicator is added in a conical flask.

Reaction involved in Fajans method:

Adsorption indicators work differently than chemical indicators and they can be used in a different precipitation titration, not simply argentometric ones. Assume we want to analyze Cl- in a sample solution by titrating with Ag+. The titration reaction would be

Ag+ + Cl− ⇌ AgCl(s)

Colloidal particles are formed by silver chloride. Because of the adsorption of excess Cl- to the surface of the precipitant particles before the equivalence point, the surface of the precipitant particles will be negatively charged.

Indicator used in Fajan’s method:

The dye dichlorofluorescein can be used as an indicator to determine chloride in Fajan's titration, by which the endpoint is detected by the green suspension turning pink.

Fajan’s method endpoint:

When the anionic indicator becomes adsorbed on the cationic particles of the precipitate, the green suspension turns pink, this is the endpoint in Fajans titration. The indicator for the titration of Cl- with Ag+ is dichlorofluorescein.

Fajan's method of precipitation titration

Frequently Asked Question (FAQ):

Which titrant is used in the Fajan method?

In the Fajans method for Cl-, Ag+ is used as a titrant. This method is used to detect the concentration of chloride is in a sample. The sample solution is titrated against a known concentration of silver nitrate (AgNO3) solution. When AgNO3 is added to a solution containing chloride ions results in the formation of a finely divided white precipitate AgCl.

What is the method used in Fagan’s method?

The indicator absorption method is used for Fajan's method.

Which titration is known as argentometric titration?

The titrations with silver nitrate are known as argentometric titration. This titration is used to determine the concentrations of chloride, bromide, and cyanide ions. As an indicator, sodium chromate is used.

What is Fajans rule give one example?

According to Fagan's rule, a compound with a low positive charge, a large cation, and a small anion form an ionic bond, whereas a compound with a high positive charge, a small cation, and a large anion form a covalent link. For example, an aluminum atom with a +3 charge, has a greater positive charge.




References:
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Sunday, March 20, 2022

Characteristics of a good primary standard

High purity, ready and cheap availability, stability, non-toxicity, low hygroscopicity, and high equivalent weight are the properties of a good primary standard.

What is a standard?

Standards are materials that contain an exactly known concentration of a substance for use in quantitative analysis. A standard provides a reference that can be used to determine unknown amounts or calibrate analytical instruments. There are two types of analytical standards: primary standards and secondary standards.

What is the primary standard in chemistry?

In chemistry, a primary standard is an exceedingly pure substance (99.9% accurate), which is easy to weigh and gives an indication of the number of moles in a compound. They do not react with the components of the air and maintain their structure for a long period.

They have special chemical and physical properties and are very pure and stable. Primary standards are frequently used in titration studies and other analytical chemistry techniques to determine an unknown concentration of solute.
"A primary standard is a measurement used to calibrate working standards. A primary standard is chosen as of its precision and stability when exposed to other compounds. Metrics such as length, time, and mass can be used to measure primary standards"

Characteristics of the primary standard:


Properties of primary standard:
It has a high degree of purity
Has a non-toxic or low toxicity
It has low reactivity and high stability
Is affordable and easily available
It has a high equivalent weight
It has a high solubility
Not hygroscopic


Let’s get a detailed explanation of the characteristics of a good primary standard. A primary standard must meet the following requirements:

  • High purity:
It should be available in the purest form and also be preserved in a pure state.
  • High stability:
A primary standard must be highly stable. Less stable compounds do not act consistently, making them reactive and unsuitable for use as standards. Even during storage, the composition of a primary standard should remain consistent.
characteristics of primary standard
  • Non-toxic:
The standard should be a non-toxic chemical. Any material that is a nuisance or aesthetically ineffective and does not pose a health hazard is considered non-toxic.
  • Cheap in cost, and readily available:
The standard should be easily accessible and economical as it may usually be employed in a large amount.
  • Low reactivity:
The reactivity of the primary standard should be minimal. The results of chemical analysis are affected by reactive substances, which are not employed as primary standards.
  • Non-Hygroscopic:
The substance should be stable in the open atmosphere and unmodified in the air, i.e. during weighing.
  • High molecular weight:
The relative molecular weight of a primary standard should be high to reduce weighing errors.
  • Solubility:
It must be highly soluble in the solvent/ water to make the standard for further process.


Frequently Asked Question (FAQ): 

What are some examples of primary standards?
Potassium dichromate (K2Cr2O7), sodium chloride (NaCl), sodium carbonate (Na2CO3), and potassium hydrogen phthalate (KHP) are some of the examples of primary standards which are used in different types of titration.

What is a secondary standard?
A secondary standard is made in the lab for a specific analysis. In chemistry, it is usually standardized against the primary standard. Sodium hydroxide (NaOH) is an example of a secondary standard.


References:
  • Wikipedia contributors. "Primary standard." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 26 Oct. 2021.
  • Helmenstine, Ph.D. Anne Marie. “Learn About Primary and Secondary Standards in Chemistry.”
  • Skoog, Douglas A., Donald M. West and F. James Holler. "Fundamentals of Analytical Chemistry 8th ed." Harcourt Brace College Publishers.


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    Saturday, March 19, 2022

    Methyl red: Frequently Asked Questions

    Methyl red (2-(N, N-dimethyl-4-aminophenyl) azobenzenecarboxylic acid), is an indicator dye commonly known as C.I. Acid Red 2. It is an azo molecule in which, unlike methyl orange, the sulfonic acid function has been replaced by the carboxylic acid function.

    Methyl red (C15H15N3O2) is a weak acid that dissociates in water to form a red neutral molecule. It's an azo dye that comes in the form of a dark red crystalline powder and is used as a pH indicator. With a pKa of 5.1, it turns red below pH 4.4, yellow above pH 6.2, and orange in the neutral condition.

    Frequently Asked Question (FAQ):


    Is methyl red a synthetic or natural indicator?

    The methyl red is a synthetic indicator, not a natural indicator. It can be prepared by the diazotization of anthranilic acid in an alcoholic solution.

    Is methyl red soluble in water?

    It is practically insoluble in water, soluble in ethanol and acetic acid.

    Why is methyl red not soluble in water?

    Because methyl red has a pKa of 4.95, it does not ionize in distilled water.

    Why does methyl red turn red?

    It shows red color below pH 4 as a structural change in methyl red causes the pH-dependent color change. When the dimethylamino group is protonated at pH 4.0-6.0, the predominant structural form is the quinone diamine form on the right.

    At what pH does methyl red turn red?

    Methyl red turns red when the pH drops below 4.4.

    Is methyl red a universal indicator?

    Methyl red is not a universal indicator, although it is a component of it. Phenolphthalein, thymol blue, methyl red, bromothymol blue, and are the major components of a universal indicator solution.

    How do you make a methyl red indicator?

    To make a methyl red indicator you will need to dissolve 50 mg of methyl red in 100 ml of ethanol, containing 04.00 ml of 0.05N NaOH solution.

    Methyl red indicator is suitable for which titration?

    Methyl red is suitable for acid titration due to its low pH range. It is also commonly used for the titration of a strong acid (Analyte) vs a strong base.

    Why is methyl red used instead of methyl orange?

    Methyl orange has a pH range of 3.1 to 4.4, while methyl red has a pH range of 4.4 to 6.2, and both will indicate changes on the pH scale.

    Methyl Red is an azo-dye indicator that indicates the endpoint/equivalence point of the reaction (changes color) when the pH of the medium it's in about 4.4 to 6.2. For this reason, it is used as an indicator in acid-base titrations.

    What does it mean when methyl red turns yellow?

    This means that the pH of the solution is at basic conditions, it shows a yellow color above pH 6.2.

    What is methyl red used for?

    It is used as an indicator in the laboratory, used as a dye in leather products, textile products, and in some food products, used in microbiology for identifying bacteria, and also used in research, and for industrial purposes.
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    Friday, March 18, 2022

    What does the bromothymol blue indicate the presence of?

    Bromothymol blue or bromothymol sulfonephthalein or BTB is a yellow-colored indicator dye that changes color when an acid is present. On the other hand, carbon dioxide, when added to a solution, produces carbonic acid, which lowers the pH. This pH change can be used to assess a variety of things and has numerous clinical applications. Bromothymol blue is a acid-base indicator commonly used to determine the presence of carbonic acid in a liquid.

    Explain what does bromothymol blue indicates the presence of?

    The BMB turns yellow when the pH is less than 6, it is green when the pH is between 6 to 7.6, and blue when the pH is greater than 7.6.

    What caused the bromothymol blue solution to change color?

    Due to the chemical interaction with carbon dioxide, the bromothymol blue solution changed color.

    What does it mean when BTB turns green?

    Bromothymol blue shows green in a neutral solution, which means the pH of the solution is around pH 07.00. So, if we are looking at the reaction to a change in pH, we can see the color change when the pH changes.

    How long does it take for bromothymol blue to turn yellow?

    Generally, 15 to 30 minutes takes for bromothymol blue to turn yellow, and in the titration process, it will depend on the concentration of titrand and titrant.

    What color does BTB turn when carbon dioxide is present?

    Bromothymol blue indicator turns yellow when carbon dioxide is present, more carbon dioxide is indicated by the color yellow. It turns blue when there is little or no carbon dioxide present.

    What color does bromothymol blue turn in water?

    Because the pH of water is neutral, bromothymol blue turns green in the water. BTB is a weak acid it shows yellow in acidic solutions, blue in basic solutions, and green in neutral solutions.

    What happens when bromothymol blue is added to a base?

    When bromothymol blue is exposed to basic conditions (pH more than 7) it turns blue because photosynthesis occurs, carbon dioxide is used up and the pH increases, making the solution bluer.
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    Thursday, March 17, 2022

    How does the bromothymol blue indicator work?

    Bromothymol blue is a dye commonly known as bromothymol sulfonephthalein or BTB that is used as a pH indicator. Bromothymol blue is a low-acid compound. Depending on the pH of the solution, it might be acidic or basic.

    It is a weak acid that can be in the form of an acid or a base depending on the pH of the solution. Bromothymol blue has a pKa of 7.1 and a pH range of 6.0 to 7.6. It is commonly used in acid-base indicators to detect the endpoint of the reaction which turns yellow in acid and blue in basic, and green in a neutral. It is also generally used to detect the presence of carbonic acid in a sample.

    How does the bromothymol blue indicator work?

    In a solution, bromothymol blue acts as a weak acid. It may appear yellow or blue depending on whether it is protonated or precipitated. In a neutral solution, it is brilliant aquamarine and greenish-blue. 

    The color difference is due to the deprotonation of the neutral form, resulting in a highly conjugated structure. The greenish color in neutral solution is due to an intermediate of the precipitation mechanism.

    The acid-base indicator is explained by two theories: Ostwald's and the quinonoid theory. An acid-base indicator is a type of indicator in which the analyte emits a visual signal that confirms the presence of an acid or a basic.
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    Wednesday, March 16, 2022

    Bromothymol blue indicator in titration

    In a titration, an indicator is a substance that is used to determine the endpoint. Organic substances (weak acids or weak bases) are commonly used as indicators in acid-base titrations. Each has its own pKa, pH range, and color change, and is employed in different types of titrations depending on the requirement. Quinonoid and Ostwald’s Theory are the two theories that have been presented to explain why acid-base indicators change color when pH changes.

    What is bromothymol blue indicator?

    Bromothymol blue, commonly known as bromothymol sulfonephthalein or BTB, is a dye that is used to determine pH. Bromothymol blue is a weak acid that can be in the form of an acid or a base depending on the pH of the solution. It is commonly employed in applications that require the measurement of chemicals at neutral pH (approximately 7.0).

    Bromothymol blue is prepared by mixing elemental bromine with thymol blue in a glacial acetic acid solution. Depending on whether it is protonated or deprotonated, it may appear yellow or blue. It is usually sold in solid (powder, crystals, or chunks) form because the acid indicator is sodium salt.

    Bromothymol blue has a pH range of 6.0 to 7.6 and a pKa of 7.1. It becomes yellow in an acidic medium, blue in basic conditions, and green in a neutral solution. Bromothymol blue is commonly used to determine the presence of carbonic acid in a liquid.

    Physical properties of bromothymol blue:


    Chemical formula C27H28Br2O5S
    Molecular Weight/ Molar Mass 624.38 g•mol−1
    Density 1.25 g/cm3
    Melting point 202 °C
    Solubility Sparingly soluble in water
    Acidity (pKa) 7.0
    pH range 6.0 to 7.6
    Color in acid Yellow
    Color in base Blue



    FAQ (Frequently Asked Questions):

    Bromothymol blue is the best indicator for which titration?

    In chemistry, BTB is a suitable choice for titrations whose equivalence point is near-neutral pH. Generally, bromothymol blue indicator is used for strong acid-strong base titration.

    Is bromothymol blue is a natural indicator?

    The bromothymol blue is not a natural indicator; it is an organic compound that is synthesized in the laboratory.

    Is bromothymol blue a weak acid or base?

    Bromothymol blue acts as a weak acid. As a result, it may appear blue or yellow, depending on whether it is protonated or deprotonated.

    What kind of indicator is bromothymol blue?

    Bromothymol blue (BTB) is a pH indicator that is also known as bromothymol sulfone phthalein. It's most commonly employed in applications that require measuring substances with a pH that's close to neutral.

    What is the pH range of the bromothymol blue indicator?

    Bromothymol blue indicator has a working pH range of 6.0 to 7.6.

    What is the color of the bromothymol blue indicator in an acidic solution?

    In an acidic medium, the color of the bromothymol blue indicator is yellow.

    What color is the bromothymol blue indicator in a basic solution?

    In a basic medium, the color of the bromothymol blue indicator is blue.

    How do you make a bromothymol blue indicator?

    To make the bromothymol blue indicator, weigh 50 mg of bromothymol blue correctly and dissolve it in 100 ml of ethanol, containing 04.00 ml of 0.05N NaOH solution.

    Is bromothymol blue a universal indicator?

    It is not a universal indicator, but it is a portion of it. A universal indicator is a collection of indicators that display color changes in a solution and can be used to determine whether a solution is acidic or basic. Phenolphthalein, bromothymol blue, thymol blue, and methyl red, are the main component of universal indicator




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    Saturday, March 12, 2022

    Why phenolphthalein is colorless in acid?

    Because the concentration of In- in the acidic solution is very low and the concentration of H+ is quite high, the phenolphthalein is colorless.

    Phenolphthalein (C20H14O4) is a chemical compound that belongs to the phthalein family generally used as an acid-base indicator. It has a pKa of 9.3 and a pH range of 8.3 to 10.0. In acidic conditions, the phenolphthalein indicator is colorless; in basic medium, it becomes pink.

    Why phenolphthalein is colorless in acid solution?

    Phenolphthalein is a weak acid that does not dissociate in an acidic environment and remains in an unionized form. Because the unionized form is colorless, phenolphthalein in an acidic solution is also colorless.

    For example,
    • Titration of 0.1N HCl versus 0.1N NaOH in the presence of phenolphthalein indicator.
    • In a conical flask, take 10.00 ml titrand (hydrochloric acid solution) and 2–3 drops of phenolphthalein were added.
    • No titrant (sodium hydroxide solution) is added to the solution at this point.
    • Therefore, phenolphthalein is colorless, because it is in an acidic environment.
    • The titrant (NaOH) in the burette is now used to titrate this mixture.
    • It is completely neutralized as soon as we add some quantity (about 8-10 ml) NaOH. (HCl + NaOH =) NaCl + H2H2O.)
    • At this stage, the phenolphthalein indicator is in a neutral state and gives the solution a very pale pink color. This is an equivalence point of the reaction.
    • Now, if we add a few drops of NaOH after the equivalence point, phenolphthalein comes under basic conditions gives the solution a pink color.
    • This is the endpoint of the titration which indicates that the reaction is completed.
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    Friday, March 11, 2022

    Why did the color of phenolphthalein change after the addition of an acid?

    Phenolphthalein (C20H14O4) is a chemical compound used as an acid-base indicator that belongs to the phthalein family. In- is the ionic form of phenolphthalein, and HIn is the molecular form. In the basic solution, the molecule loses a hydrogen ion and becomes a negative ion. When titrating weak acids with strong bases, phenolphthalein is the ideal indicator.

    It's a yellowish-white to pale orange thin crystalline powder. It has a pKa value of 9.3 and the working pH range is 8.3 to 10.0. Phenolphthalein indicator is colorless in acidic conditions and forms pink color in basic conditions

    Why did the color of phenolphthalein change after the addition of an acid in a basic solution?

    The phenolphthalein (HIn) is weakly acidic in nature and dissociates into H+H+ and In-In ions. The concentration of In-In ions in the solution gives the solution its pink color, and since the concentration of In-In is very low in solution under acidic conditions and the concentration of H+H+ is very high, the solution is colorless.

    Similarly, under basic conditions, the concentration of H+H+ ions is extremely low, while the concentration of In-In is very high, resulting in a pink-colored solution.

    What happens when acid is added to phenolphthalein?

    The ion of phenolphthalein is pink, and the molecule of phenolphthalein is colorless. However, when acid is added, the phenolphthalein ion converts to a phenolphthalein molecule, rendering the solution colorless.

    What color does phenolphthalein turn in an acid?

    Phenolphthalein is colorless in acidic solutions and purple in basic solutions. The indicator is a substance that changes color when it comes into contact with an acid or a basic.

    Why does phenolphthalein change its colour in acid and bases

    What happens when phenolphthalein is added to distilled water?

    Since pure water is neither acidic nor alkaline, there is no color change when drops of phenolphthalein are added to it. Phenolphthalein is colorless at pH 7.0, usually turning pink in alkaline solutions at pH 8.2 or higher.
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    Thursday, March 10, 2022

    Methyl yellow indicator in titration

    A compound that changes color when exposed to acidic or basic conditions is called an indicator. Color indicators are widely used to detect pH and can be added to reaction mixtures to determine titration endpoint/equivalence point.

    In chemistry, different indicators are used in several types of titration, including acid-base, precipitation, redox, and complexometric. Each has its own pH range, pKa, and produces a specific color at different pH values. The two theories that explain the acid-base indicator are Ostwald's theory and the quinonoid theory.

    What is methyl yellow?

    Methyl yellow (4-dimethylaminoazobenzene-C6H5N2C6H4N (CH3)2 is an organic compound that an azo dye derived from dimethylaniline generally used as an acid-base indicator. It comes in yellow solid powder whose solubility in water is 13.6 mg/l; it is also soluble in ethanol, acetone, chloroform, ether, etc.

    The pH range of dimethyl yellow is 2.9 to 4.0, and in an aqueous solution, it changes color from below pH 2.9 red to above pH 4.0 yellow and vice versa. Methyl orange, methyl red, are also types of indicators used in the titration.

    Methyl yellow is used as an acid-base indicator, typically employed to titrate a weak base against a strong acid. It is a part of a universal indicator and is also used as a dye for plastics.     

    Properties of Methyl yellow:
    Molecular Weight/ Molar Mass 225.295 g•mol−1
    Formula C14H15N3
    Density 1.0±0.1 g/cm3
    Appearance Yellow crystals
    Melting point 11–116 °C
    pKa 3.3
    pH range 2.9 to 4.0
    Color in acid Red
    Color in base Yellow

    FAQ (Frequently Asked Questions):


    What is the color of methyl yellow?

    The solid methyl yellow that comes in the form of crystals is yellow in color.

    What is the pH range of the methyl yellow indicator?

    The transition interval of methyl yellow is pH 2.9 (red) to pH 4.0 (yellow).

    What is the color of methyl yellow in an acidic and basic environment?

    Methyl yellow undergoes a transition between pH 2.9 and 4.0, showing red in low pH and turning yellow above pH 4.0.

    What is the color of methyl yellow in acid?

    Methyl yellow is a pH indicator that turns red when exposed to acidic conditions.

    What is the color of methyl yellow in the base?

    Methyl yellow is a pH indicator that turns yellow when exposed to basic conditions.

    How do you make a methyl yellow indicator?

    To make the methyl yellow indicator needs to weigh 100 mg of dimethyl yellow and dissolve in 100 ml of ethanol.

    Methyl yellow is used for which titration?

    It is more suitable for titration of a weak base against a strong acid.


    References:
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    Tuesday, March 8, 2022

    How do acid-base indicators work?

    An acid-base indicator is a weak acid or base that will be protonated or deprotonated at a certain pH. It can be used for both titrations and general pH control. The behavior of acid-base indicators is explained by two theories.

    Indicators are compounds that change color when exposed to acid/base. Color indicators are commonly used to measure pH and can be used to identify the titration endpoint/equivalence point by adding them to the reaction mixture.

    Several types of indicators are employed in different types of titrations, including acid-base, complexometric, redox, and precipitation. Each has its own pKa and pH range and produces a different color at different pH levels.

    In chemistry, there are three types of indicators: natural indicators, artificial indicators, and olfactory indicators, while artificial and natural indicators are the two most commonly employed forms.


    What is an acid-base indicator?

    A weak acid or weak base that changes color as the concentration of hydrogen (H+) or hydroxide (OH-) ions in an aqueous solution changes is known as an acid-base indicator. In titration, acid-base indicators are most commonly used to determine the endpoint of an acid-base reaction. 

    The acid-base indicators can be classified into three groups, phthaleins, sulphophthaleins, azo indicators, triphenylmethane indicators, etc.

    Examples of the acid-base indicator with their color change:

    • Phenolphthalein (Colorless to pink)
    • Methyl orange (Red to yellow)
    • Bromphenol blue (Yellow to blue)
    • Methyl red (Red to yellow)
    • Phenol red (Yellow to red)
    • Thymol blue (Red to yellow)
    • Bromcresol green (Yellow to blue)
    • Thymolphthalein (Colorless to blue)

    The acid-base indication is a type of indicator in which the analyte displays a visual signal confirming the presence of an acid or a basic. Below we briefly explain the principle of how acid-base indicators work.

    Theory of acid-base indicator:

    Ostwald's theory and the quinonoid theory are the two theories that explain the acid-base indicator.
    • According to Ostwald's theory, the color change is due to the ionization of the acid-base indicator. The color of the unionized form differs from that of the ionized form.
    • According to the quinonoid theory, the ionization of the indicator is greatly affected in acids and bases as it is either a weak acid or a weak base. If the indicator is a weak acid, its ionization in acids is relatively low due to the presence of common H+ ions, whereas it ionizes well in bases. Due to the common OH- ions, if the indicator is a weak base, its ionization is very high in acids and low in bases.

    How an acid-base indicator works:

    The acid and its conjugate base are distinct colors if the indicator is a weak acid. The base and its conjugate acid show distinct colors if the indicator is a weak base.

    According to the chemical equation, equilibrium is reached in a solution of a weak acid indicator with the molecular formula HIn:
    HIn(aq) + H2O(l) ↔ In-(aq) + H3O+(aq)
    HIn(aq) is an acid that’s color is different from the base In-(aq). The concentration of the hydronium ion H3O+ is higher when the pH is lower, and the equilibrium is toward the left, resulting in the color A. Since the concentration of H3O+ is lower at higher pH, the equilibrium tends to the right side of the equation, resulting in the color B.

    Phenolphthalein, a weak acid that dissociates in water to generate a magenta or red-purple anion, is an example of a weak acid indicator. Because equilibrium is to the left in an acidic solution, the solution appears colorless, however as pH rises, equilibrium shifts to the right, revealing the magenta color.
    Below equation can be used to find the equilibrium constant of a reaction:
    KIn = [H3O+][In-] / [HIn]
    In which the KIn is the indicator dissociation constant. At the moment where the acid and anion base concentrations are equal, the color changes:
    [HIn] = [In-]
    This is the point at which half of the indicator is in the form of acid and the other half is its conjugate base.


    Frequently Asked Question (FAQ):

    What is the use of acid-base indicators?


    Acid and base indicators are used to detect the pH of a sample and to determine the endpoint of the reaction. In chemistry, a substance to be studied (Titrand) is placed in a conical flask, 2-4 drops of a suitable indicator is added, then a drop by drop addition of titrant of known strength from a burette until the chemical reaction is complete.

    How acid-base indicator is selected?


    The indicator for an acid-base titration is chosen based on its pH range falling within the reaction's pH change.

    Why indicator changes color?


    The color change is caused by an electron isomerism resonance. Because different indicators have different ionization constants, they change color at different pH intervals.



    References:

    1. Anne Marie, “Definition and Examples of Acid-Base Indicator” ThoughtCo, 18 Jan. 2011 Available Here
    2. ‘PH Indicator’. Wikipedia, 22 Feb. 2022. Wikipedia, Available Here:
    3. Acid-Base Indicator - an Overview | ScienceDirect Topics. Available Here:
    4. Reactions of Acids and Bases in Analytical Chemistry. Hulanicki, A. and Masson, M.R. New York: Halsted Press, 1987.
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