Chemical Analysis

Chemical Analysis, the collection of research methods used for determining the composition of chemical compounds or their mixtures. Chemical analysis is divided into qualitative analysis, which makes it possible to determine what elements or groups of elements are present in a given substance, and quantitative analysis, which determines the weight ratios in which these elements are found. To achieve its goals, qualitative analysis uses various reagents, i.e., substances that with certain elements or groups of elements either form insoluble or volatile compounds (formation of a precipitate or gas) or produce characteristic coloring. Based on the objects of research and the methods used, chemical analysis is divided into: 1. Analysis of inorganic substances. 2. Analysis of organic substances. 3. Special methods of chemical analysis used in technical analysis, medical analysis, pharmaceutical analysis, forensic-medical analysis, sanitary-industrial analysis, sanitary-food analysis, etc. Qualitative analysis of inorganic substances, dealing with salts, acids, and bases, i.e., compounds that dissociate in aqueous solutions into ions, breaks down into the detection of positively charged ions - cations (metals) and negatively charged ions - anions (metalloids, acid residues). The systematic course of cation analysis is based on separating them into groups according to the different solubility of their chlorides, sulfides, and carbonates. The precipitate contains

The filtrate contains group IV

contains:

V group: Ca, Sr, Ba

Mg, Na, K * To the solution being tested, hydrochloric acid is added until a precipitate forms. Precipitate Filtrate Contains group I Precipitated with hydrogen sulfide in an acidic medium Ag, Hg^-, Pb Precipitate contains group II: Hg", Pb, Bi, Cu, Cd, As, Sb, Sn, Au, Pt, Mo, Se, Te Filtrate is freed from phosphoric acid, concentrated in the presence of nitric acid, diluted with water and precipitated with ammonium sulfide in the presence of ammonia Precipitate contains III group: Fe, Ni, Co, Filtrate, after freeing from sulfur, is precipitated with ammonium carbonate in the presence of NH, for the separation and division of cation groups. Precipitate contains III group: Fe, Ni, Co, Mn, Zn, Al, Cr, U, Ti Separation and detection of metals of group I. The precipitate separated with hydrochloric acid is boiled in water and filtered hot. The residue is washed with hot water and treated with NH₄OH. The residue contains Hg*. which will be indicated by the blackening of the precipitate. The solution contains Ag, precipitating upon acidification with nitric acid as a white precipitate. The solution contains Pb. With K₂CrO₄ solution gives a yellow precipitate of PbCrO₄. Separation and detection of metals of group II. The precipitate separated with hydrogen sulfide is treated with yellow ammonium polysulfide. Residue Solution As, Sb, Sn Pb, Hg", Bi, Cd, Cu are precipitated with HCl, the precipitate is heated with concentrated nitric acid Residue Solution Residue As. Solution Sb,Sn Hg Pb, Bi,Cu, Cd Treated with | concentrated, add NH₃. concentrated, add NH₃. In part of the solution, H₂SO₄ is added, after adding excess MgCl₂-white precipitate-black spot on platinum indicates Bi. 2. In another part of the solution, Sn is separated with the help of metallic zinc. The blue color indicates the presence of Cu. Gray flakes are dissolved in concentrated HCl and HgCl₂ is added - white or yellow precipitate indicates Sn. The gray precipitate indicates the presence of Cd. Separation and detection of metals of group IV. The precipitate separated with ammonium carbonate is dissolved in a small amount of acetic acid. To the resulting solution, diluted with water and heated to boiling, (NH₄)₂CrO₄ is added to a slight excess. Precipitate BaCrO₄. Part is dissolved in HCl and tested in flame - yellow-green color. Solution. Part of the solution is tested with calcium sulfate - precipitate indicates the presence of Sr. Sr is removed by boiling with excess dilute (NH₄)₂SO₄. The precipitate, washed with (NH₄)₂SO₄, is ignited with charcoal - the ash is dissolved in HCl and examined in the flame - carmine-red color. Ca is detected in the filtrate by precipitating (NH₄)₂CaO₄ in the presence of ammonia. Separation and detection of metals of group V. The filtrate after separation of groups I, II, III and IV is evaporated to dryness and lightly calcined. Dissolved in dilute HCl. Filtered and divided into two parts. To the first part, NH₄Cl, NH₄OH, Na₂HPO₄ are added - white crystalline precipitate-Mg. From the second part, Mg is removed by boiling with Ba(OH)₂, filtered. Ba is precipitated with (NH₄)₂CO₃. Upon heating, filtered. The filtrate is evaporated to dryness. The residue is examined in the flame: yellow color-Na. Violet color, visible through indigo solution-K. Dissolved in a small amount of water. To one part of the solution, potassium pyroantimonate K₂H₂SbO₇ is added - crystalline precipitate indicates Na. To another part, tartaric acid C₄H₆O₆ is added - crystalline precipitate indicates K. Separation and detection of metals of group III. The washed precipitate, separated with ammonium sulfide and ammonia, is treated with cold hydrochloric acid. Residue Co, Ni. Part of the residue is heated with borax. Blue bead indicates Co. Another part is dissolved in aqua regia, evaporated to dryness, dissolved in water, and after adding NaOH, bromine water is added - black precipitate indicates the presence of Ni. Solution Fe, Al, Cr, Mn, Zn is concentrated by adding HNO₃, after which excess NaOH is added, boiled and filtered. Precipitate Fe, Cr, Mn is dissolved in HCl, boiled. After adding NH₄Cl, precipitated with the smallest possible amount of ammonia. Precipitate Fe, Cr. Detection of Fe: dissolved in HCl and K₄Fe(CN)₆ is added - Prussian blue coloration. Detection of Cr: fused with soda, the fusion is dissolved in water, acidified with acetic acid, AgNO₃ is added - red precipitate. Solution Mn. When boiled with (NH₄)₂S, a flesh-colored precipitate forms - Mn; a small part of the precipitate is dissolved in concentrated HNO₃, PbO₂ is added on the tip of a knife, boiled and slightly diluted with water: in the case of Mn, the settled liquid is colored red. For the detection of anions (acid residues), specific reactions are used. Often, the detection of certain metals already excludes the possibility of certain acids being present, e.g., in the presence of Ba in the solution, sulfuric acid cannot be present, in the presence of Ag, halogen acids are excluded, etc. Quantitative analysis breaks down into gravimetric analysis (see), volumetric analysis (see) and gas analysis (see). Analysis of organic substances is divided into: 1) elementary analysis, i.e., the detection and quantitative determination of elements present in the compound being studied. For qualitative detection of carbon and hydrogen, the substance is burned and CO₂ and H₂O are detected in the combustion products. For the detection of nitrogen and sulfur, the substance is destroyed by ignition with metallic sodium. Nitrogen forms sodium cyanide, detected by conversion to Prussian blue; sulfur gives sodium sulfide, which with lead salts gives a black precipitate. Halogens can be detected by reduction to halogen acids (sodium amalgam + water, sodium + alcohol), which with silver give precipitates insoluble in nitric acid. Other elements are detected after oxidation of the substance by ordinary methods of inorganic analysis. Quantitative determination of C and H is carried out according to Liebig's principle by burning a weighed amount of substance with copper oxide, absorbing the resulting CO₂ and H₂O and weighing the absorption apparatus. For nitrogen determination, either Duma's method is used, based on measuring gaseous nitrogen obtained by reduction of nitrogen oxides formed by burning the substance in a CO₂ stream in the presence of copper oxide, or Kjeldahl's method (see), based on converting nitrogen to ammonia and titrating the latter with acid. 2) Detection and quantitative determination of individual groups in the organic compound, such as hydroxyl, carboxyl, aldehyde groups, amino groups, etc. 3) Detection and quantitative determination of individual compounds based on characteristic reactions or study of their derivatives. When small amounts of the substance being studied are available, microanalysis is used, based on the study of crystal forms under a microscope and conducting reactions on drops applied to filter paper (drop analysis). In quantitative microanalysis, special apparatus is used that allows working with small amounts of substance: microbalances, microburettes, micropipettes, etc.