【Weekly Topic】(39) August 20, 2015: What are the necessary conditions for a volumetric analysis method?

1) There should be an analytical balance for accurately measuring the weight of substances, as well as containers for measuring the volume of solutions； 2) There must be a standard solution available for titration ； 3) There must be an indicator that can accurately determine the equivalence point.

Answer: (1) The reaction is completed quantitatively, that is, it proceeds according to a specific reaction equation without any side reactions, and it is carried out to completion (99.9%), which forms the basis for quantitative calculations. (2) The reaction rate must be fast. For reactions with slow rates, appropriate measures should be taken to increase their reaction rate. (3) The titration endpoint can be determined in a relatively simple manner. Any reaction that meets the above requirements can be used in the direct titration method, that is, directly titrating the substance under examination with a standard solution.

I. Conditions for titration reactions There are many chemical reactions, but those suitable for titrimetric analysis must meet the following conditions: (1) The reaction must proceed quantitatively, that is, it must occur according to a specific reaction equation without any side reactions, and it must be complete (usually requiring over 99.9% completion), as this is the basis for quantitative calculations. 　　(2) The reaction speed must be fast, so that the titration reaction can be completed in an instant. For slow reactions, their reaction rate should be increased by heating, changing the acidity of the solution, or altering the titration procedure. 　　(3) The titration endpoint can be determined in a relatively simple manner, such as by using an indicator or physical-chemical methods. 　　Any reaction that meets the above requirements can be applied to the direct titration method, that is, using a standard titrant solution to directly titrate the component being analyzed. 　　II. Titration Methods (1) Direct Titration Method This is the method in which a standard titrant solution is used to directly titrate the solution of the substance being analyzed; it is the most basic and commonly used titration method. Reactions that meet the conditions of the aforementioned titration reactions can all be applied to the direct titration method; for example, titrating sodium hydroxide solution with a salt-acid standard titrant, or titrating ferrous ion solution with a potassium dichromate standard titrant. 　　When the reaction does not fully meet the aforementioned requirements, direct titration is difficult. At this point, titration can be carried out using the following methods. 　　(II) Back-titration method First, the substance to be analyzed, X, is reacted with an excess amount of standard titrant B1. After the reaction is complete, another standard titrant B2 is used to titrate the remaining amount of B1. The concentration of substance X is determined based on the actual amount of standard titrant B1 that was consumed. 　　The back-titration method is suitable for reactions in which the reactants are solids, or where the rate of direct titration is slow, or when there is no appropriate indicator for direct titration. For example, when using EDTA complexation titration to determine the aluminum oxide content in cement samples, the complexation reaction between Al and EDTA is slow; therefore, the back-titration method is often employed. An excess of EDTA standard titrant solution is added first, and heating is applied to ensure thorough complexation of Al3+ with EDTA ； Then, using PAN as an indicator, a standard copper sulfate titrant was used to back-titrate the remaining EDTA. 　　(III) Displacement titration method (1g substitution titration method) For reactions that do not proceed according to a definite reaction equation (with side reactions occurring), it is possible not to directly titrate the substance of interest. Instead, an appropriate reagent is first used to react with this substance, causing another product to be displaced, and then a standard titrant is used to titrate this resulting product. This method is known as displacement titration. For example, when using EDTA complexometric titration to determine titanium dioxide in cement samples, Al interferes with the determination. First, an EDTA standard titration solution can be used to quantitatively coordinate with TiO and Al ions; thereafter, amygdalic acid is added to react quantitatively with the TiO ions in the TiO2-EDTA complex, displacing EDTA in an equimolar amount to TiO. Finally, a copper sulfate standard titration solution is used for titration. 　　(IV) Indirect titration method Sometimes, the substance to be analyzed cannot react directly with the standard titrant, but it can undergo another chemical reaction to produce another substance that can react directly with the standard titrant; in such cases, the indirect titration method can be used for titration. For example, silica in cement can be titrated using the indirect titration method. First, the silicate ions react in a strongly acidic solution with excess potassium ions and fluoride ions to form a potassium fluosilicate precipitate. Then, potassium fluosilicate free of free acid is precipitated in boiling water, where it hydrolyzes to produce hydrofluoric acid, which can be titrated using a sodium hydroxide standard titrant, thereby indirectly calculating the silica content in the sample.

Answer: 1) There needs to be an analytical balance for accurately measuring the weight of substances, as well as containers for measuring the volume of solutions； 2) There must be a standard solution available for titration ； 3) There must be an indicator that can accurately determine the equivalence point.

What are the necessary conditions for describing volumetric analysis? 1) There must be an analytical balance capable of accurate weighing to calculate the polymerization conversion rate of vinyl chloride, as well as containers for measuring the volume of solutions ； 2) There must be a standard solution available for titration ； 3) There must be an indicator that can accurately determine the equivalence point.

How is there a question like “how to calculate the conversion rate of vinyl chloride polymerization”?

First, the purity (mass fraction) should be ≥99.9%； Secondly, its composition must be completely consistent with its chemical formula; for example, if it contains crystalline water, the amount of such water should correspond to that specified in the chemical formula ； Third, it has stable properties; under normal conditions it does not easily lose water, absorb water, or deteriorate, and it does not react with oxygen and carbon dioxide in the air ； Fourth, when participating in the reaction, it should be carried out quantitatively according to the reaction formula, with no side reactions ； Fifth, it should have a high molar mass in order to reduce the relative error during weighing.

1. An analytical balance for accurately measuring the weight of substances, and containers for measuring the volume of solutions； 2. There must be a standard solution available for titration ； 3. There must be an indicator that can accurately determine the equivalence point.

What are the necessary conditions for describing volumetric analysis? Answer: 1) There needs to be an analytical balance for accurately measuring the weight of substances, as well as containers for measuring the volume of solutions ； 2) There must be a standard solution available for titration ； 3) There must be an indicator that can accurately determine the equivalence point.

What are the necessary conditions for describing volumetric analysis? Answer: 1) There needs to be an analytical balance for accurately measuring the weight of substances, as well as containers for measuring the volume of solutions ； 2) There must be a standard solution available for titration ； 3) There must be an indicator that can accurately determine the equivalence point.