Unit I: Basic and Inorganic Chemistry

Transcription

1 Unit I: Basic and Inorganic Chemistry Module 1. Introduction to analytical chemistry. Identifying unknowns: Qualitative analysis and X-ray analysis Purpose: The objective for this lab assignment is to use chemical knowledge to identify unknown substances. Identification of popular metals ions. Identification of unknown substances using X-ray diffraction. Introduction I Analytical chemistry Analytical chemistry is the science that seeks ever improved means of measuring the chemical composition of natural and artificial materials. Chemical composition is the entire picture (composition) of the material at the chemical scale and includes geometric features such as molecular morphologies and distributions of species within a sample as well as single dimensional features such as percent composition and species identity. The analytical results enabled by analytical chemistry have played critical roles in science from the understanding of basic science to a variety of practical applications, such as biomedical applications, environmental monitoring, quality control of industrial manufacturing and fornsic science. Analytical chemistry has significant overlap with other branches of chemistry, especially those that are focused on a certain broad class of chemicals, such as organic chemistry, inorganic chemistry or biochemistry, as opposed to a particular way of understanding chemistry, such as theoretical chemistry. For example the field of bioanalytical chemistry is a growing area of analytical chemistry that addresses all analytical questions in biochemistry, (the chemistry of life). Analytical chemistry and experimental physical chemistry, however, have a unique relationship in that they are very unrelated in their mission but often share the most in common in the tools used in experiments. Traditionally, analytical chemistry has been split into two main types: 1. Qualitative Qualitative inorganic analysis seeks to establish the presence of a given element or inorganic compound in a sample. Qualitative organic analysis seeks to establish the presence of a given functional group or organic compound in a sample. 2. Quantitative Quantitative analysis seeks to establish the amount of a given element or compound in a sample. Most modern analytical chemistry is categorized by two different approaches: 1

2 1. By Analytical Targets Bioanalytical chemistry Material analysis Chemical analysis Environmental analysis Forensics 2. By Analytical Methods Spectroscopy Mass Spectrometry Spectrophotometry and Colorimetry Chromatography and Electrophoresis Crystallography Microscopy Electrochemistry II. Qualitative analysis Inorganic qualitative analysis generally refers to a systematic scheme to confirm the presence of certain, usually aqueous, ions or elements by performing a series of reactions that eliminate ranges of possibilities and then confirms suspected ions with a confirming test. Sometimes small carbon containing ions are included in such schemes. With modern instrumentation these tests are rarely used but can be useful for educational purposes and in field work or other situations where access to state-of-the-art instruments are not available or expedient. In the qualitative analysis procedure, the chemical properties of an unknown substance are determined by systematically reacting the unknown with a number of different reagents. By predetermining what the particular reaction will produce if a specific ion is present, the ions that actually are in the solution can be identified. For example, if a reaction is known to produce a precipitate if ion A is present and a precipitate is formed when the reaction is run, then ion A may be present in solution (there may be, and usually are, other ions that will also precipitate with a particular reagent). If no precipitate is formed when the reaction is run, then ion A is clearly not present in the unknown solution and a different reaction will have to be run to determine what ions are present.there are two general situations in which qualitative analysis is used - in the identification of a simple salt, or the identification of multiple cations in a solution. The procedures of selected substances identyfication are described in experimental section. III X-ray crystallography Crystallography is the experimental science of determining the arrangement of atoms in solids. In older usage, it is the scientific study of crystals. Crystallographic methods depend on the analysis of the diffraction patterns that emerge from a sample that is targeted by a beam of some type. The beam is not always electromagnetic radiation, even though X-rays are the most common choice. For some purposes electrons or neutrons are used, which is possible due to the wave properties of the particles. Crystallographers often explicitly state the type of 2

3 illumination used when referring to a method, as with the terms X-ray diffraction, neutron diffraction and electron diffraction. These three types of radiation interact with the specimen in different ways. X-rays interact with the spatial distribution of the valence electrons, while electrons are charged particles and therefore feel the total charge distribution of both the atomic nuclei and the surrounding electrons. Neutrons are scattered by the atomic nuclei through the strong nuclear forces, but in addition, the magnetic moment of neutrons is nonzero. They are therefore also scattered by magnetic fields. Because of these different forms of interaction, the three types of radiation are suitable for different crystallographic studies. Some materials studied using crystallography, proteins for example, do not occur naturally as crystals. Typically, such molecules are placed in solution and allowed to crystallize over days, weeks, or months through vapor diffusion. A drop of solution containing the molecule, buffer, and precipitants is sealed in a container with a reservoir containing a hygroscopic solution. Water in the drop diffuses to the reservoir, slowly increasing the concentration and allowing a crystal to form. If the concentration were to rise more quickly, the molecule would simply precipitate out of solution, resulting in disorderly granules rather than an orderly and hence usable crystal. Once a crystal is obtained, data can be collected using a beam of radiation. Although many universities that engage in crystallographic research have their own X-ray producing equipment, synchrotrons are often used as X-ray sources, because of the purer and more complete patterns such sources can generate. Synchrotron sources also have a much higher intensity of X-ray beams, so data collection takes a fraction of the time normally necessary at weaker sources. Producing an image from a diffraction pattern requires sophisticated mathematics and often an iterative process of modelling and refinement. In this process, the mathematically predicted diffraction patterns of an hypothesized or "model" structure are compared to the actual pattern generated by the crystalline sample. Ideally, researchers make several initial guesses, which through refinement all converge on the same answer. Models are refined until their predicted patterns match to as great a degree as can be achieved without radical revision of the model. This is a painstaking process, made much easier today by computers. The mathematical methods for the analysis of diffraction data only apply to patterns, which in turn result only when waves diffract from orderly arrays. Hence crystallography applies for the most part only to crystals, or to molecules which can be coaxed to crystalize for the sake of measurement. In spite of this, a certain amount of molecular information can be deduced from the patterns that are generated by fibers and powders, which while not as perfect as a solid crystal, may exhibit a degree of order. This level of order can be sufficient to deduce the structure of simple molecules, or to determine the coarse features of more complicated molecules (the double-helical structure of DNA, for example, was deduced from an X-ray diffraction pattern that had been generated by a fibrous sample). Crystallography is a tool that is often employed by materials scientists. In single crystals, the effects of the crystalline arrangement of atoms is often easy to see macroscopically, because the natural shapes of crystals reflect the atomic structure. In addition, physical properties are often controlled by crystalline defects. The understanding of crystal structures is an important prerequisite for understanding crystallographic defects. Mostly, materials do not occur in a single crystalline, but poly-crystalline form, such that the powder diffraction method plays a most important role in structural determination. 3

4 A number of other physical properties are linked to crystallography. For example, the minerals in clay form small, flat, platelike structures. Clay can be easily deformed because the platelike particles can slip along each other in the plane of the plates, yet remain strongly connected in the direction perpendicular to the plates. Such mechanisms can be studied by crystallographic texture measurements. Crystallography is useful in phase identification: That is, when performing some kind of processing on a material, it is often desired to find out what compounds and what phases are present in the material. Each phase has a characteristic arrangement of atoms. Techniques like X-ray diffraction can be used to identify which patterns are present in the material, and thus which compounds are present. X-ray crystallography is the primary method for determining the molecular conformations of biological macromolecules, particularly protein and nucleic acids such as DNA and RNA. In fact, the double-helical structure of DNA was deduced from crystallographic data.. 4

5 Experimantal I. Classical qualitative analysis Procedure: 1. Specific reaction of Fe 3+ ions Place 1 drop of solution containing Fe 3+ ions in test tube and add 2-3 cm 3 of distilled water. Then add few drops of dilute HCl and 0.5 cm 3 solution of NH 4 NCS. 2. Specific reaction of Ni 2+ ions Place 1 drop of solution containing Ni 2+ ions in test tube, add 2-3 cm 3 of distilled water and few drops of dimethylglioksym alcohol solution. Then add NH 4 OH, drop by drop, as long as the smell of ammonium is perceptible. Heat the tube until solution is boiled. 3. Specific reaction of Mn 2+ ions Place 1 drop of MnSO 4 solution in the test tube and add distilled water up to the brim. Then pour out the tube the little amount of solution which remains on the tube s wall is sufficient to analysis. Add 1 cm 3 of water, few drops of concentrated HNO 3 (6 mol/dm 3 ), 1-2 drops of AgNO 3 and few crystals of (NH 4 ) 2 S 2 O 8. Heat the tube until solution is boiled. 4. Specific reaction of Cr 3+ ions Place few drops of solution containing Cr 3+ ions in the test tube and add 2 cm 3 of distilled water, few drops of HNO 3, 1-2 drops of AgNO 3 and few crystals of (NH 4 ) 2 S 2 O 8. Heat the tube until solution is boiled. 5. Specific reaction of Co 2+ ions Place few drops of solution containing Co 2+ ions in the test tube and add 1 cm 3 of distilled water, few drops of HCl and 1 cm 3 of NH 4 NCS. Then add 1 cm 3 of ether-alcohol mixture and extract rising compound by mixture vigorous shaking. 6. Specific reaction of Cu 2+ ions Place 1 drop of solution containing Cu 2+ ions in the test tube and add 3 cm 3 of distilled water, then add few drops of concentrated NH 4 OH. 7. Specific reaction of Al 3+ ions Place 2 drops of solution containing Al 3+ ions in the test tube, add 3 cm 3 of distilled water and few drops of concentrated NH 4 OH. Then heat the tube until solution is boiled. The colorless, jelly-like precipitate of Al(OH) 3 come into being. Then add of 2 drops of Alizarin-S solution, shake the mixture and leave the tube for few minutes. The pink-colored precipitate should settle in the bottom of a tube. 8. Specific reaction of Zn 2+ ions Place 1 drop of solution containing Zn 2+ ions in the test tube, add 3 cm 3 of distilled water and few drops of H 2 SO 4. Then add 2-3 drops of K 3 [Fe(CN) 6 ]. 5

6 9. Specific reaction of Pb 2+ ions Place 2 drops of solution containing Pb 2+ ions in the test tube, add 3 cm 3 of distilled water and few drops of H 2 SO 4. Then heat the tube until solution is boiled. The white precipitate of PbSO 4 appears. Decant the liquid carefully and add to remaining precipitate 1 cm 3 of concentrated CH 3 COONH 4. Then heat the tube until the precipitate dissolves. Add few drops of K 2 Cr 2 O The flame test 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. Clean a straight platinum wire by alternately dipping into a small volume of 12M hydrochloric acid solution and holding in the oxidizing flame until no flame color results. Dip the clean wire into the 0,1M solution to be tested (KCl, CaCl 2, BaCl 2, SrCl 2, NaCl, CuCl 2 ) and heat in the hottest part of the oxidizing flame. Note the color and intensity of flame. Notice: Students should observe reactions with concentrated attention and write down observation in a laboratory notebook. II Qualitative analysis by XRD method The identification of single or multiple crystal phases (compounds) in an unknown sample is the main application of X-ray powder diffractometry. The students will see the demonstration how to identify contents of an unknown powder on the X'Pert PRO MPD multi-purpose X-ray diffraction system. 6