Analytical Chemistry: Qualitative Analysis Methods

ANALYTICAL CHEMISTRY types of molecules, functional groups, and chemical bonds. QUALITATIVE ANALYSIS GENERAL METHODS OF QUALITATIVE ANALYSIS INTRODUCTION QUALITATIVE ANALYSIS • • • • You have learned previously that analytical chemistry has two main divisions: qualitative analysis and quantitative analysis. Qualitative analysis involves the identification of elements or compounds in a sample. It answers the question, “What is present?”. Quantitative analysis measures the amount of those elements or compounds precisely. It answers the question, “How much is present?”. If the general composition of the material is unknown, a preliminary qualitative examination is often necessary before a satisfactory quantitative analysis can be made. This is because the method of determining accurately the percentage of a constituent depends to a great extent upon the nature and relative amounts of the other constituent present. This module is focused only on qualitative analysis. Unlike quantitative analysis which is concerned with the amount of substance present in a sample, qualitative analysis is mainly concern with its identity. In this module, you will study how to identify compounds as well as cations and anions in solution. Qualitative analysis has two branches namely inorganic qualitative analysis and organic qualitative analysis. Inorganic analysis looks at the elemental and ionic composition of a sample, usually by examination of ions in aqueous solution. Organic analysis tends to look at types of molecules, functional groups, and chemical bonds. There are two methods that can be employed to establish the identity of a substance. One is by instrumental methods that are rapid but requires specialized equipment. Another one is by chemical methods which are applicable in laboratory settings using only test tubes and ordinary chemical reagents. LESSON 1 - GENERAL METHODS • • • Let’s say somebody gave you a solution to analyze and you have no knowledge about the chemical composition of the solution, how are you going to find out what’s your solution composed of? A qualitative test is designed to identify the presence (or absence) of species in solution. The solution is reacted with different reagents, taking advantage of the different chemical properties of ions such as solubility in a solution, complex formation, or precipitation. The first step involves the separation of ions into groups. This can be done due to the tendency of certain groups of ions to precipitate under certain conditions. The following chemical properties of ions in solution can be utilized to precipitate (or re-dissolved) them in solution. o soluble versus insoluble in water o solubility with temperature o solubility and pH o complex ion formation o amphoteric behavior o changes in solubility with a change in the oxidation state. Once the ions have been separated into groups, confirmatory tests can then be applied. It is necessary that any interfering ion is removed prior to conducting a confirmatory test. Aside from the confirmatory test, the identity of a substance can be established by comparing it with an authentic sample or literature data. The process of separating the ions into groups is called selective precipitation. Selective precipitation is a technique of separating ions in an aqueous solution by using a reagent that precipitates one or more of the ions while leaving other ions in solution. To find out how selective precipitation is used as a technique in chemical analysis of ions in solution, please watch the video below. QUALITATIVE CHEMICAL ANALYSIS • • Qualitative analysis involved finding the kind of ions present in a solution o Concerned with finding the quantity of substances. Qualitative analysis by selective precipitation, wet chemistry, has largely been replaced by less timeintensive instrumental techniques. A sample containing a mixture of metal cations is subjected to the addition of several precipitating agents. After each addition, cations that precipitate are removed • Centrifuge to further precipitate and easier decant. • • INTRODUCTION • • Before a satisfactory quantitative analysis can be performed on a sample, a preliminary qualitative analysis is necessary especially if the composition of the sample is not yet known. The two main branches of qualitative analysis are inorganic qualitative analysis and organic qualitative analysis. Inorganic analysis looks at the elemental and ionic composition of a sample, usually by examination of ions in aqueous solution. Organic analysis tends to look at ANACHEM • 1 • To confirm, add a drop ppt agent. Presence of cloudiness confirm presence of ion. Adding ppt agent should produce no cloudiness to confirm full precipitation. FLAME TEST OF METAL IONS • • • • • • • • • • Group 1 & 2 inert metal loop is used, the end is dipped into the solution and put into the flame the excitation characteristic color is like a fingerprint for an element K+ (potassium) lavender Ba2+ (barium) apple green Ca2+ (calcium) yellow-red Sr2+ (strontium) red Li+ (Lithium) red Na+ (sodium) bright yellow Cu2+ (copper) green (most vivid) GASES • • • • • • Group 1 - Insoluble Chlorides o Mixture is treated with 6 M HCl o Most chlorides are soluble o But Ag+, Pb2+, and Hg22+ form insoluble chlorides o Solids are separated from the liquid Group 2 - Acid Insoluble Sulfides o The acidic mixture is treated with H2S, a weak diprotic acid ▪ H2S ⇌ H+ + HS▪ HS- ⇌ H+ + So The concentration of S2- is pH-dependent o At low pH, the S2- is low and only the most indoluble metal sulfides precipitate: Hg2+, Cd2+, Bi3+, Cu2+, Sn4+, As3+, Pb2+, and Sb3+ o Solids are separated from the liquid Group 3 - Base-Insoluble Sulfides & Hydroxides o Additional base and H2S are added o The added base shifts the H2S equilibria and increases the S2- concentration o Ions that precipitate as sulfides: Fe2+. Co2+, Zn2+, Mn2+, and Ni2+ o Ions that precipitate as hydroxides: Cr3+, Fe3+, Al3+ o Solids are removed. Group 4 - Insoluble Phosphates o Only alkali and alkaline earth metal ions remain o Alkaline earth metals are removed by addition of (NH4)PO4: Mg2+, Ca2+, Ba2+ o Solids are removed. Group 5 - Alkali Metals and NH4+ o The only cations that can still be present in the liquid are Na+, Li+, and NH4+, which cannot be precipitated o Sodium and potassium can be identified with flame tests ▪ Sodium - bright yellow flame ▪ Potassium - purple flame TESTING FOR HYDROGEN, OXYGEN, CARBON DIOXIDE, AMMONIA, AND CHLORINE • • • • • • • • • • LESSON 2 - QUALITATIVE METHODS OF ANALYSIS ELEMENTS • Different elements in an inorganic compound can be identified by means of a flame test. It is difficult to tell the identity of because most of them are colorless. • • • We test for gases in the laboratory because it is nearly impossible to determine the identity of a gas just by its appearance. As an example, hydrogen, oxygen, and carbon dioxide are all colorless and odorless. To test for hydrogen, place a lit splint at the mouth of the reaction vessel. o You should hear a very distinctive squeaky pop, which confirms its presence. o This is due to the combustion reaction of hydrogen in the presence of oxygen, creating water as the only product. To test for oxygen, place a glowing splint at the mouth of the reaction vessel. o Keep in mind that the splint should be glowing, not lit. o A glowing splint relights in the presence of oxygen. Why does hydrogen burn with a squeaky pop? o Hydrogen is highly flammable and the pop sound that you hear is actually a mini-explosion. Why does oxygen relight the glowing splint? o The glowing splint relights in the presence of oxygen as there is a higher concentration of oxygen in the reaction vessel than compared with air, which is only 21% oxygen. Carbon dioxide will extinguish a lit splint, but the same occurs in the presence of ammonia. A more accurate test is to bubble carbon dioxide though limewater, which is an aqueous solution of calcium hydroxide, also known as slaked lime. Limewater turns milky in the presence of carbon dioxide due to the formation of calcium carbonate. Ammonia extinguishes a lit split, and turns damp red litmus paper blue. o In the presence of concentrated hydrochloric acid, a white smoke will form. o This is ammonium chloride, and confirms the presence of ammonia. Chlorine turns damp blue litmus paper red and eventually bleaches it to white. In conclusion, hydrogen burns with a squeaky pop oxygen relights a glowing splint, and ANACHEM • 2 • • • carbon dioxide turns limewater milky. Ammonia turns damp red litmus paper blue and forms a white smoke of ammonium chloride in the presence of concentrated hydrochloric acid. Chlorine turns damp blue litmus paper red and continues to bleach it white. • ORGANIC COMPOUNDS ORGANIC QUALITATIVE ANALYSIS Functional Group Alkene Test Observation Add bromine water Bromine water is decolorized (orange to colorless) Chloroalkane - white ppt (AgCl formation) Bromoalkane - cream ppt (AgBr formation) Iodoalkane - yellow ppt (AgI formation) Orange ppt *recrystallize to purify Haloalkane Add silver nitrate and ethanol Place in water bath at 50°C Carbonyl Add 2,4dinitrophenyhydrazine Aldehyde 1° and 2° alcohols and aldehyde Carboxylic acid Phenol (2,4-DNP/ Brady’s reagent) Add ammoniacal silver nitrate (Tollen’s reagent) and warm Add acidified dichromate (VI) and warm in water bath Silver mirror Orange to green (also applicable w/ aldehydes) Add aqueous sodium carbonate Test pH using a pH meter/indicator Add aqueous sodium carbonate Add Bromine water Effervescence pH < 7 No effervescence Bromine is decolorized and white ppt forms ANACHEM • 3