Quantitative analysis is a large section of analytical chemistry that allows you to determine the quantitative (molecular or elemental) composition of an object. Quantitative analysis has become widespread. It is used to determine the composition of ores (to assess the degree of their purification), the composition of soils, plant objects. In ecology, quantitative analysis methods determine the content of toxins in water, air, and soil. In medicine, it is used to detect fake drugs.
Problems and methods of quantitative analysis
The main task of quantitative analysis is to establish the quantitative (percentage or molecular) composition of substances.
Depending on how this problem is solved, there are several methods of quantitative analysis. There are three groups of them:
- Physical.
- Physico-chemical.
- Chemical.
The first are based on measuring the physical properties of substances - radioactivity, viscosity, density, etc. The most common physical methods of quantitative analysis are refractometry, X-ray spectral and radioactive analysis.
The second is based on the measurement of the physicochemical properties of the analyte. These include:
- Optical - spectrophotometry, spectral analysis, colorimetry.
- Chromatographic - gas-liquid chromatography, ion exchange, distribution.
- Electrochemical - conductometric titration, potentiometric, coulometric, electroweight analysis, polarography.
The third methods in the list of methods are based on the chemical properties of the test substance, chemical reactions. Chemical methods are divided into:
- Weight analysis (gravimetry) - based on accurate weighing.
- Volume analysis (titration) - based on accurate measurement of volumes.
Methods of quantitative chemical analysis
The most important are gravimetric and titrimetric. They are called classical methods of chemical quantitative analysis.
Gradually classical methods give way to instrumental ones. However, they remain the most accurate. The relative error of these methods is only 0.1-0.2%, while for instrumental methods it is 2-5%.
Gravimetry
The essence of gravimetric quantitative analysis is the isolation of the substance of interest in its pure form and its weighing. Excretion more frequentlyall carried out by precipitation. Sometimes the component to be determined must be obtained in the form of a volatile substance (distillation method). This way it is possible to determine, for example, the content of water of crystallization in crystalline hydrates. The precipitation method determines silicic acid in the processing of rocks, iron and aluminum in the analysis of rocks, potassium and sodium, organic compounds.
Analytical signal in gravimetry - mass.
The method of quantitative analysis by gravimetry includes the following steps:
- Precipitation of a compound that contains the substance of interest.
- Filtration of the resulting mixture to extract the precipitate from the supernatant.
- Washing the precipitate to eliminate the supernatant and remove impurities from its surface.
- Drying at low temperatures to remove water or at high temperatures to convert the sediment into a form suitable for weighing.
- Weighing the resulting sediment.
Disadvantages of gravimetric quantification are the duration of the determination and non-selectivity (precipitating reagents are rarely specific). Therefore, a preliminary separation is necessary.
Calculation with gravimetric method
The results of the quantitative analysis carried out by gravimetry are expressed in mass fractions (%). To calculate, you need to know the weight of the test substance - G, the mass of the resulting sediment - m and its formula for determining the conversion factor F. The formulas for calculating the mass fraction and the conversion factor are presented below.
You can calculate the mass of a substance in the sediment, for this, the conversion factor F is used.
The gravimetric factor is a constant value for a given test component and gravimetric shape.
Titrimetric (volumetric) analysis
Titrimetric quantitative analysis is an accurate measurement of the volume of a reagent solution that is consumed in an equivalent interaction with a substance of interest. In this case, the concentration of the reagent used is pre-set. Given the volume and concentration of the reagent solution, the content of the component of interest is calculated.
The name "titrimetric" comes from the word "titer", which refers to one way of expressing the concentration of a solution. The titer shows how many grams of the substance are dissolved in 1 ml of solution.
Titration is the process of gradually adding a solution with a known concentration to a specific volume of another solution. It is continued until the moment when the substances react with each other completely. This moment is called the equivalence point and is determined by the change in color of the indicator.
Titrimetric analysis methods:
- Acid-base.
- Redox.
- Precipitation.
- Complexometric.
Basic concepts of titrimetric analysis
The following terms and concepts are used in titrimetric analysis:
- Titrant - solution,which is poured. Its concentration is known.
- Titrated solution is a liquid to which a titrant is added. Its concentration must be determined. The titrated solution is usually placed in the flask, and the titrant is placed in the burette.
- The equivalence point is the moment of titration when the number of equivalents of the titrant becomes equal to the number of equivalents of the substance of interest.
- Indicators - substances used to establish the equivalence point.
Standard and working solutions
Titrants are standard and working.
Standard ones are obtained by dissolving an exact sample of a substance in a certain (usually 100 ml or 1 l) volume of water or another solvent. So you can prepare solutions:
- Sodium Chloride NaCl.
- Potassium dichromate K2Cr2O7.
- Sodium tetraborate Na2B4O7∙10H2 O.
- Oxalic acid H2C2O4∙2H2 O.
- Sodium oxalate Na2C2O4.
- Succinic acid H2C4H4O4.
In laboratory practice, standard solutions are prepared using fixanals. This is a certain amount of a substance (or its solution) in a sealed ampoule. This amount is calculated for the preparation of 1 liter of solution. Fixanal can be stored for a long time, because it is without air access, with the exception of alkalis that react with the glass of the ampoule.
Some solutionsimpossible to cook with precise concentration. For example, the concentration of potassium permanganate and sodium thiosulfate changes already during dissolution due to their interaction with water vapor. As a rule, it is these solutions that are needed to determine the amount of the desired substance. Since their concentration is unknown, it must be determined before titration. This process is called standardization. This is the determination of the concentration of working solutions by their preliminary titration with standard solutions.
Standardization required for solutions:
- Acids - sulfuric, hydrochloric, nitric.
- Alkalis.
- Potassium permanganate.
- Silver nitrate.
Indicator selection
To accurately determine the equivalence point, that is, the end of the titration, you need the right choice of indicator. These are substances that change their color depending on the pH value. Each indicator changes the color of its solution at a different pH value, called the transition interval. For a properly selected indicator, the transition interval coincides with the change in pH in the region of the equivalence point, called the titration jump. To determine it, it is necessary to construct titration curves, for which theoretical calculations are carried out. Depending on the strength of the acid and base, there are four types of titration curves.
Calculations in titrimetric analysis
If the equivalence point is correctly defined, the titrant and the titrated substance will react in an equivalent amount, that is, the amount of the titrant substance(ne1) will be equal to the amount of the titrated substance (ne2): ne1=n e2. Since the amount of the equivalent substance is equal to the product of the molar concentration of the equivalent and the volume of the solution, then the equality
Ce1∙V1=Ce2∙V2, where:
-Ce1 – normal titrant concentration, known value;
-V1 – volume of titrant solution, known value;
-Ce2 – normal concentration of the titratable substance, to be determined;
-V2 – the volume of the solution of the titrated substance, determined during the titration.
After titration, you can calculate the concentration of the substance of interest using the formula:
Ce2=Ce1∙V1/ V2
Performing Titrimetric Analysis
The method of quantitative chemical analysis by titration includes the following steps:
- Preparation of 0, 1 n standard solution from a sample of the substance.
- Preparation of approximately 0.1 N working solution.
- Standardization of the working solution according to the standard solution.
- Titration of the test solution with the working solution.
- Make necessary calculations.
