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Solvents, Acidimetry and Alkalimetry Titration

Solvents, Non-aqueous titration using non-aqueous solvents, Acidimetry titration, Alkalimetry titration, Direct titration, Residual titration method.
Solvents
A weak acid or base can be formed by water. There is a possibility that, when titrating very weak acids or weak bases in aqueous solutions, those weak acids or bases can compete with weak ones regarding proton donation or acceptance.

Non-aqueous titration using non-aqueous solvents
In non-aqueous titrations, the analytes are dissolved with the following four types of solvents:

1. Aprotic solvents
The ionizing substance is added to the chemically inert substance such as benzene, chloroform, etc. to depress the solvolysis process, and the endpoint is sharpened as a result. There are many examples of these substances, including benzene, toluene, carbon tetrachloride, etc.

2. Protophilic solvents
Basic in nature, they form solvated protons when they interact with acids. Weak acids are changed into strong acids through the use of strong protophilic solvents. There are many types of liquid ammonia such as amine, ether, and so on.

3. Protogenic solvents
The proton readily donates to these substances because they are acidic. Their primary purpose is to increase the basicity of weak acids. They act as a leveler for bases. Acids such as sulfuric and formic acids are examples.

4. Amphiprotic solvents
It appears that these solvents possess both Protophilic and Protogenic properties. Examples include alcohol, acetic acid, and so on.

Acidimetry titration
A method of determining alkaline substances (bases) using titration involving an aliquot of acid is known as acidimetry. This technique is used for a variety of substances included in various official compendia to provide a means of controlling their quality.

Examples -
1. Inorganic substances - A variety of inorganic substances such as sodium bicarbonate, milk of magnesia, ammonium chloride, calcium oxide, lithium carbonate, zinc oxide can also be used.

2. Several organic materials can be used, including urea, sodium salicylate, diphenhydramine, emetine hydrochloride, meprobamate, paramethadione, pyrazinamide.

The following two methods are explored in this section: direct titration and residual titration:

1. Direct titration method
In titrations, it is customary to weigh and dissolve an aliquot quantity of a solid substance with insufficient water to provide a solution with a concentration relatively close to that of the acid used. The indicator of choice is methyl orange (pH range = 0 to 4.4) because most other indicators are immediately affected by the carbonic acid (H2CO3) generated during the reaction, causing the color of the product to change even before the reaction has completed.

In the following two situations, residual titration or back titration is typically used:

Case I - Whenever a chemical reaction is rather slow or sluggish.

Case II - A substance cannot be directly titrated with an indicator when no sharp and distinctly visible endpoint can be reliably obtained.

Alkalimetry titration
Analyses of acids are typically used to determine their quantitative composition like assessments of bases. There are two generally accepted methods for the assay of acidic substances, namely:

1. Direct titration method
The direct titration method involves mixing a precise quantity of a base or acid salt with a standard alkali solution and directly titrating the product.

2. Residual titration method
In response to a radioactive source, an excess of standard alkali solution is added and residue titration is performed with a standard acid solution to determine the amount of excess.

Following are some general guidelines that should be observed:
  • To determine the normality of a dissolved acidic substance, it is required that the normality of the dissolved acidic substance be similar to that of the titrant,
  • Since many indicators offer different values at different temperatures, it is important to bring liquid acidic substances to room temperature (25°C) before titration.
  • Calculate the amount of acid to be taken such that approximately 30 to 40 ml of the previously standardized base should be used in the test.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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