Qualitative Analysis PDF

Qualitative analysis Asoc. Prof. Simona Sutkuvienė Analytical chemistry could be divided into three main parts: qualitative; quantitative; applied. Qualitative analysis – attempting to identify what materials are present in sample. What is present ? Determination of chemical ident...

Qualitative analysis Asoc. Prof. Simona Sutkuvienė Analytical chemistry could be divided into three main parts: qualitative; quantitative; applied. Qualitative analysis – attempting to identify what materials are present in sample. What is present ? Determination of chemical identity of the species in the sample.  Electrolytes are substances that solutions conduct electricity. Electrolytes If an ionic compound is dissolved in water, it dissociates into ions and Non- and the resulting solution will electrolytes conduct electricity. Acids, bases and soluble ionic solutions are electrolytes.  Non-Electrolytes are substances that don't conduct electricity. Electrolytes A nonelectrolyte does not dissociate at all in solution and and Non- therefore does not produce any electrolytes ions. Molecular compounds and insoluble ionic compounds are non-electrolytes.  Reactions in electrolyte solutions proceed between ions. Ions in electrolyte solutions can react reversibly. Ion exchange reaction is reversible, when Soluble and easy disociate compounds are formed: KCl + NaBr ↔ KBr + NaCl Molecular eq. K+ + Cl‾ + Na+ + Br‾ ↔ K+ + Br‾ + Na++ NO3‾ Ionic eq. Ions in electrolyte solutions can react irreversibly. Ion exchange reaction is irreversible, when 1) difficulty soluble compounds are formed: AgNO3 + NaCI → AgCI↓ + NaNO3 Molecular eq. Ag++NO3‾ + Na+ + CI– → AgCI↓ + Na++ NO3‾ Ionic eq. Ag+ + CI‾ → AgCI↓ Short ionic eq.  2) gases are formed: Na2S + 2HCI → 2 NaCI + H2S↑ Molecular eq. 2Na+ + S2– + 2H+ +2CI‾ → H2S↑+2Na+ +2CI‾ Ionic eq. 2H+ + S2– → H2S↑ Short ionic eq. 3) weak electrolytes (acids, bases, water), are formed: HCI + NaOH → NaCI + H2O Molecular eq. H+ + CI– + Na+ + OH– → Na+ + CI– + H2O Ionic eq. H+ + OH– → H2O Short ionic eq.  4) complex compounds are formed: Fe(CN)2 + 4 KCN → K4[Fe(CN)6] Molecular eq. Fe 2+ + 2CN– + 4K+ + 4CN ̶ → 4K+ + [Fe(CN)6]4– Ionic eq. Fe 2+ + 6 CN ̶ → [Fe(CN)6] 4– Short ionic eq.  Solubility product Solubility of compounds determines formation of precipitate. Since there are no totally insoluble substances, dynamic equilibrium gets established between the precipitate and solution. AgCl↓ ↔ Ag+ + Cl‾    + K spAgCl = Ag  Cl − Solubility product At constant temperature, multiplication of the concentrations of difficulty soluble electrolyte ions in saturated solution is constant and called solubility product, defined as Ksp. Qualitative analysis Qualitative analysis deals with detection and identification of different substances singly or in a mixture.  The aim of qualitative analysis Is to establish chemical identity of the sample species, i.e. chemical elements, atom groups, ions or molecules, forming unknown substance or the mixture of substances. Qualitative composition of materials can be determined by physical, physical-chemical and chemical methods of analysis. Qualitative analysis deals with detection and identification of different substances singly or in a mixture. If a sample contains only a single cation and anion, their identification is a fairly simple and straightforward process, although to distinguish between two cations (or anions) that have similar chemical properties is not easy and in this instance additional confirmatory tests are required. The detection of a particular ion in a sample that contains several ions is somewhat more difficult, because the presence of the other ions may interfere with the test. This problem can be circumvented by precipitating, thereby removing, the disturbing ions from solution prior to testing for the particular ion. The methods of chemical analysis are based on the observation of changes occurring in chemical reactions and rest on the statement of the theory of electrolyte dissociation about the additivity of ion’s properties, i. e. the same properties of the same ion in spite of the mixture and the properties of a solution being the sum of the properties of the ions presented. Unknown material is converted to the compound with well- known characteristic properties. Such a conversion is called analytic reaction, while the substance inducing such a change is the reagent.  Analytic reactions Are irreversible ion exchange reactions of various types in electrolyte solutions. Clearly expressed external effect of those reactions (analytic signal) evidences the presence of the ion in the solution. When the unknown solution is affected by the reagent and the precipitate is formed or dissolved, or the color of the unknown solution changes or the gas is produced, here we have the analytic signal.  Pb2+ ion analytic reactions 1. Pb(NO3)2 + 2HCl → PbCl2↓ + 2HNO3 Pb2+ + 2NO3‾ + 2H+ +2Cl‾→ PbCl2↓+ 2H+ + 2NO3‾ Pb2+ + 2Cl‾→ PbCl2↓ white precipitate PbCl2  Pb2+ ion analytic reactions 2. Pb(NO3)2 + 2KI → PbI2↓ + 2KNO3 Pb2+ + 2NO3‾ + 2K+ + 2I‾ → PbI2↓+ 2K+ + 2NO3‾ Pb2+ + 2I‾ → PbI2↓ yellow precipitate PbI2  Fe3+ ion analytic reaction FeCl3 + 6KCNS → K3[Fe(CNS)6] + 3KCl Fe3+ +3Cl‾ + 6K+ + 6CNS‾→3K++[Fe(CNS)6]3‾+3K++3Cl‾ Fe3+ + 3CNS‾→ [Fe(CNS)6] blood red complex  NH4+ ion analytic reaction NH4CI + NaOH → NH4OH↑ + NaCI Reaction mixture is heated NH3↑ + H2O NH4+ + CI‾ + Na+ +OH‾→NH3↑+ H2O + Na+ +CI‾ NH4+ + OH‾→NH3↑+ H2O odor of ammoniac  CO32- ion analytic reactions 1. K2CO3 + BaCl2 → BaCO3 + 2KCI 2K+ + CO32- + Ba2+ +2Cl‾→BaCO3↓+2K+ +2CI‾ CO32- + Ba2+ → BaCO3↓ white precipitate  CO32- ion analytic reactions 2. K2CO3 + 2HCl → H2CO3 + 2KCI CO2↑ + H2O 2K+ + CO32- + 2H+ + 2Cl‾→CO2↑ + H2O + 2K+ + 2CI‾ CO32- + 2H+ → CO2↑ + H2O gas of CO2 (bubbles)  SO42- ion analytic reaction Na2SO4 + BaCl2 → BaSO4 + 2NaCI 2Na+ + SO42- + Ba2+ + 2Cl‾→BaSO4↓+ 2Na+ +2CI‾ SO42- + Ba2+ → BaSO4↓ white precipitate Analytic reactions are characterized by: 1. Sensitivity of analytic reaction are possibility to detect minimal amounts of test material.  Sensitivity of an analytic reaction is expressed quantitatively as the minimal amount of explored substance (ion) in mg, minimal concentration of explored substance (ion) in solution or the limit of dilution (e.g. 1 : 100 000), which is still detectable by expected external effect of the analytic reaction.  Specific reaction is called a reaction, specific to one ion or molecule and not interfered by other ions in the solution. Specificity of analytic reaction E.g.: NH4+ ion analytic reaction with NaOH is specific one. When the reaction mixture is heated, only salts of ammonium produce ammonia.  Reactions, which are specific to ions of the similar properties. Selective Selective reactions, specific to a group of ions, are called group reactions selective reactions. Selective reagents, which sedimentate from solution ions of the certain group, are called group reagents.  According to the group reagents, ions are divided to analytic groups. The group reagents used for the classification of most common cations are hydrochloric acid, hydrogen sulphide, ammonium sulphide, and ammonium carbonate.  Classification of cations Classification is based on whether a cation reacts with these reagents by the formation of precipitates or not. It can therefore be said that classification of the most common cations is based on the differences of solubilities of their chlorides, sulphides, and carbonates. The five groups of cations and the characteristics of these groups are as follows: Group 1 Cations of this group form precipitates with dilute hydrochloric acid. Ions of this group are lead (II) Pb2+, mercury (I) Hg22+, and silver (I) Ag+.  Group 2 The cations of this group do not react with hydrochloric acid, but form precipitates with hydrogen sulphide in dilute mineral acid medium. Ions of this group are 2/a: mercury (II) Hg2+, copper (II) Cu2+, bismuth (III) Bi3+, cadmium (II) Cd2+, 2/b: tin (II) Sn2+, tin (IV) Sn4+, arsenic (III) As3+, arsenic (V) As5+, antimony (III) Sb3+ and antimony (V) Sb5+.  Group 2 The first four form the sub-group 2/a and the last six the sub-group 2/b. While sulphides of cations in Group 2/a are insoluble in ammonium polysulphide, those of cations in Group 2/b are soluble.  Group 3 Cations of this group do not react either with dilute hydrochloric acid, or with hydrogen sulphide in dilute mineral acid medium. However they form precipitates with ammonium sulphide in neutral or ammoniacal medium. Cations of this group are iron (II) Fe2+, iron (III) Fe3+, cobalt (II) Co2+, nickel (II) Ni2+, manganese (II) Mn2+, chromium (III) Cr3+, aluminium (III) Al3+ and zinc (II) Zn2+.  Group 4 Cations of this group do not react with the reagents of Groups 1, 2, and 3. They form precipitates with ammonium carbonate in the presence of ammonium chloride in neutral medium. Cations of this group are calcium (II) Ca2+, strontium (II) Sr2+ and barium (II) Ba2+.  Group 5 Common cations, which do not react with reagents of the previous groups, form the last group of cations, which includes magnesium (II) Mg2+, lithium (I) Li+, sodium (I) Na+, potassium (I) K+ and ammonium (I) NH4+ ions.  Classification of anions The methods available for the detection of anions are not as systematic as those which have been described above for cations.  Classification of anions Anions are divided into four The following scheme of groups on the basis of their classification of anions has reactions with dilute been found to work well in hydrochloric acid and of the practice; differences of solubilities of their barium and silver salts.  Group 1 Visible change, gas evolution and/or formation of a precipitate, with dilute hydrochloric acid. Ions of this group are carbonate CO32-, silicate SiO32-, sulphide S2-, sulphite SO32-, and thiosulphate S2O32-.  Group 2 The anions of this group do not react with hydrochloric acid, but form precipitates with barium ions in neutral medium. Ions of this group are sulphate SO42-, phosphate PO43-, fluoride Fˉ, and borate BO33-.  Group 3 Anions of this group do not react either with dilute hydrochloric acid, or with barium ions in neutral medium. However, they form precipitates with silver ions in dilute nitric acid medium. Anions of this group are chloride Clˉ, bromide Bˉ, iodide Iˉ, and thiocyanate SCNˉ.  Group 4 Common anions, which do not react with reagents of the previous groups, form the last group of anions, which includes nitrite NO2‾, nitrate NO3‾ and chlorate ClO3‾ ions.  Flame test Quick qualitative detection of some ions can be accomplished by the reaction of flame coloration. When an electron becomes excited, it will jump to a higher energy level. As the excited electron returns to its ground state, it gives off energy, often in the form of visible light. As in all cases where electrons become excited, the flame will only produce color for a few seconds at most.  Definitions Excited State - when an electron gains enough energy to temporarily leave its ground state orbital and move to an orbital of higher energy. Ground State - the level of energy that an electron will be in when it is not excited.  Flame test Flame tests are used to identify the presence of a relatively small number of metal ions in a compound. Not all metal ions give flame colours. Solutions of ions, when mixed with concentrated HCl and heated on a nickel/chromium wire in a flame, cause the flame to change to a color characteristic of the atom.  Visible colors occur with the Flame test following ions: Sodium Calcium Strontium Potassium Bright yellow Brick red (orange- Crimson Pale violet red) (medium) (intense, persistent) (slight, fleeting) (medium, fleeting)  Flame test Barium Lead Copper Light green Pale bluish Green or blue (slight) (slight, fleeting) (medium, persistent)