What Is Capacity Analysis?

Also called titration analysis. It is an important quantitative analysis method. This method adds a known concentration of a reagent solution to the test solution of the test substance, and determines the amount of the test substance according to the amount of reagents consumed to complete the chemical reaction. The instrument used for volume analysis is simple, but also has the advantages of convenience, speed, and accuracy. It is especially suitable for constant component determination and routine analysis of large batches of samples. ^[1]

Volume analysis is an ancient method of analysis. In 1729, French CJ Ziffrois first used volume analysis to determine the concentration of acetic acid with pure potassium carbonate. He added acetic acid dropwise to a certain amount of potassium carbonate solution until bubbles no longer occurred. until. In the 19th century, capacity analysis was widely used due to the successful synthesis of various indicators.

This method adds a known concentration of a reagent solution to the solution of the test substance, and determines the amount of the test substance based on the amount of reagents consumed to complete the chemical reaction. The chemical reactions that can be used for volume analysis must meet certain conditions, the most important of which is the established stoichiometric relationship between the reactants. The tools used for volume analysis are very simple. Generally there are burettes, volumetric flasks, pipettes, graduated pipettes, etc .; at the same time, they have the advantages of convenience, speed, and accuracy. They are especially suitable for constant component determination and routine analysis of large batches of samples . Volumetric analysis is usually divided into visual titration methods such as acid-base titration, complexometric titration, redox titration, precipitation titration, and non-aqueous titration; in addition, there are currently widely used instrument titration methods, such as potentiometric titration, conductivity titration, and photometric titration . According to the titration method, there are direct titration, displacement titration, indirect titration and back titration.

The capacity analysis method usually puts a certain volume of the test component solution X in an Erlenmeyer flask, and then a standard solution of a certain reagent R is added dropwise to the Erlenmeyer via a burette. Let the chemical reaction between X and R be: aX + bR = cP

When the ratio of the amount of the added R and X substances is exactly equal to the Fa / b leaf, the dropwise addition is stopped. This process is called titration. This reaction is called a titration reaction. According to the concentration _{R of R} , the volume VR consumed, and the volume V _{X of X} , and according to the measurement relationship a / b between X and R, the concentration of X can be obtained:

When the relationship between the added reagent and the amount of the substance of the component to be measured exactly meets the metrological relationship between the two represented by the titration reaction formula, the titration is said to have reached the equivalence point, also called the stoichiometric point. Theoretically, the titration should stop at the equivalence point, but the system often does not have any external characteristics to be detected at the equivalence point, so the actual titration end point must usually be determined by the color change of the added indicator.

If the chemical reaction used is highly complete and the reaction proceeds according to a certain reaction formula, that is, there is a certain metering relationship between the reactants, the reaction speed is fast, and there is an appropriate method to determine the end point, the titration can be performed by direct titration. Titration analysis generally refers to the direct titration method. Such as hydrochloric acid titration of sodium hydroxide, sodium hydroxide titration of acetic acid and so on. If the chemical reaction used does not meet the above conditions, or there is no suitable chemical reaction, the back titration method or indirect titration method can often be used. For example, when titrating calcium carbonate with hydrochloric acid:

2HCl + CaCO ₃ = CaCl ₂ + CO ₂ + H ₂ O

Because calcium carbonate needs to be dissolved first and reacts slowly, it is not suitable for direct titration with hydrochloric acid. However, a back-titration method can be used, that is, first adding a certain excess of a standard hydrochloric acid solution and heating to completely dissolve and react the calcium carbonate. After cooling to room temperature, the remaining hydrochloric acid is titrated with a sodium hydroxide standard solution. Based on the difference between the amounts of the added hydrochloric acid and sodium hydroxide, the content of calcium carbonate can be measured. Another example is that the Ca in the solution cannot be titrated directly by the redox titration method, but the indirect titration method can be used, that is, the Ca is first precipitated into calcium oxalate:

Ca + C ₂ O ₄ = CaC ₂ O ₄

The precipitate was separated and washed, and then dissolved in hydrochloric acid. The oxalic acid formed was titrated with a potassium permanganate standard solution:

2MnO ₄ + 5H ₂ C ₂ O ₄ + 6H = 2Mn + 10CO ₂ + 8H ₂ O

From this, the content of Ca can be measured indirectly. ^[2]

The titration analysis method is further divided into acid-base titration method, complex titration method, precipitation titration method and redox titration method according to the different reactions based on it. According to the different standard solution measurement methods, titration analysis can be divided into volume titration and weight titration. The volumetric titration method uses a volumetric burette to measure the volume of the standard solution used for the reaction, while the weight titration method uses a weight burette and an analytical balance to measure the weight of the standard solution used. Titration analysis is usually used for constant analysis and sometimes microanalysis. The titration analysis method has high accuracy, and the relative error of the determination is about 0.2% in general. Compared with the gravimetric method, it is simple and fast. Therefore, it has great practical value in production practice and scientific research.

In volume analysis, the weight of the sample to be analyzed must be accurately weighed, or the volume of the sample solution must be accurately measured. Dissolve the sample in a suitable solvent, perform some pretreatment, separate if necessary, and then titrate the resulting solution. During the titration, a component in the titrant solution chemically reacts with the measured substance. In order to meet the requirements of volume analysis, these reactions must be rapid, and it is necessary to know exactly how many molecules or ions of the measured substance have reacted with the titrant molecules or ions.

In volume analysis, it is necessary to measure when the number of reagent molecules or ions added is equivalent to the substance being tested. This point in the titration is called the equivalence point. Determining equivalence points can be performed visually with colored indicators or reagents. Or use some physical and chemical methods to measure. When the indicator changes color, the end of the titration is reached, and the volume of the titrant solution added at that point is measured. But the end point and equivalence point of the titration are sometimes not consistent.

For example, before the color changes, the indicator itself may consume a small amount of the titrant, or the concentration of the titrant in the solution significantly exceeds the concentration at the isochronous point before the indicator changes the color. In the titration, in order to reduce the error between the end point of judgment and the real equivalent point, it is necessary to carefully select the indicator and the method of determining the end point. In order to calculate the amount of substance equivalent to the volume or weight of the titrant used, the concentration of the titrant must be accurately known. Methods to determine the exact concentration of a titrant include: weighing a certain amount of reagent and a certain volume of the titrant solution to react; or using a titrant to titrate a certain weight of a sample of known composition.