Lecture 2: Chromatography and Separation Techniques PDF

Lecture 2 Course: Chromatography and Separation Techniques Code: PMC 322 Dr. Ahmed Sayed Saad Dr. Ahmed Sayed Saad Instrumental Analysis (PMC 227) 1 2  Importance of Separation Separations are extrem...

Lecture 2 Course: Chromatography and Separation Techniques Code: PMC 322 Dr. Ahmed Sayed Saad Dr. Ahmed Sayed Saad Instrumental Analysis (PMC 227) 1 2  Importance of Separation Separations are extremely important in synthesis, in industrial chemistry, in the biomedical sciences, and in chemical analyses. Dr. Ahmed Sayed Saad 3  Analytical Preparative Item Separation Separation Purpose Identify and quantify Isolate and purify components of a specific compounds mixture from a mixture Scale Small scale (microliter Large scale (larger sample injections) sample volumes) Outcome Provides qualitative Produces purified and quantitative data compounds for further use or analysis Separation can be classified as either: Separation Preparative Separation Analytical Separation Dr. Ahmed Sayed Saad 4  Analytical Methods Criteria Specific Selective Non-Selective Methods that can identify and Methods that can target a particular Methods that do not discriminate Definition quantify a single analyte without group of analytes while avoiding between analytes, detecting multiple interference from others. interference from certain others. components at once. High, as it may detect multiple Interference from Other Minimal to none, highly resistant to Low, as it can distinguish between components and is sensitive to Compounds interference. analytes to some extent. interferences. Need Separation prior to Separate the analyte from the Extensive and thorough separation is Minimal if any analysis interfering species only mandatory An interferent is a chemical species that causes a systematic Analyte error in an analysis by enhancing or attenuating the analytical signal or the Analyte background. Dr. Ahmed Sayed Saad 5  Separation Methods We should note that a separation always requires energy because the reverse process, mixing at constant volume, is spontaneous. Dr. Ahmed Sayed Saad 6  Separation Methods The goals of an analytical separation are usually to eliminate or reduce interferences so that quantitative analytical information can be obtained from complex mixtures. Complete Separation Partial Separation 7 Separation Methods 8  A. Separation by precipitation Separations by precipitation require large solubility differences between the analyte and potential interferents. Challenges: 1. Coprecipitation of contaminants: May cause extensive contamination of a precipitate by an unwanted component even though the solubility product of the contaminant has not been exceeded. 2. Slow rate of precipitation Feasible precipitation may be too slow to be useful for a separation 3. Colloidal suspension (difficult to and slow coagulation) Dr. Ahmed Sayed Saad Instrumental Analysis (PMC 227) 9  A. Separation by precipitation A.1. Separations Based on Control of Acidity - Based on the enormous differences among the solubilities of the hydroxides, hydrous oxides, and acids of various elements. - The pH (H+ and OH- concentration) can be easily controlled using acids, bases or buffers. - These separations can be grouped in three categories: - (1) those made in relatively concentrated solutions of strong acids, - (2) those made in buffered solutions at intermediate pH values, and - (3) those made in concentrated solutions of sodium or potassium hydroxide. Dr. Ahmed Sayed Saad Instrumental Analysis (PMC 227) 10  A. Separation by precipitation A.1. Separations Based on Control of Acidity - Based on the enormous differences among the solubilities of the hydroxides, hydrous oxides, and acids of various elements. - The pH (H+ and OH- concentration) can be easily controlled using acids, bases or buffers. - These separations can be grouped in three categories: - (1) those made in relatively concentrated solutions of strong acids, - (2) those made in buffered solutions at intermediate pH values, and - (3) those made in concentrated solutions of sodium or potassium hydroxide. Dr. Ahmed Sayed Saad Instrumental Analysis (PMC 227) 11  A. Separation by precipitation A.2. Sulfide Separations - Extensively used since: 1. It is relatively easy to control the sulfide ion concentration of an aqueous solution of H2S by adjustment of pH. 2. With the exception of the alkali metals and alkaline-earth metals, most cations form sparingly soluble sulfides whose solubilities differ greatly from one another. 3. Sulfides can be conveniently precipitated from homogeneous solution, with the anion being generated by the hydrolysis of thioacetamide 4. These treatments, however, may not always produce realistic conclusions about the feasibility of separations because of coprecipitation and the slow rates at which some sulfides form. For these reasons, we often rely on Dr. Ahmed Sayed Saad Instrumental Analysis (PMC 227) 12  Both group II and group IV have their sulfides insoluble in water. But group IV have higher Ksp How? So to ppt gp II only we must use fewer S2- ions. By common ion effect, using dil HCl with H2S. gp. II gp. IV 𝐻2𝑆 ⇌ 2𝐻 + + 𝑆 2− 𝑃𝑎𝑟𝑡𝑖𝑎𝑙𝑙𝑦 𝑖𝑜𝑛𝑖𝑧𝑒 𝐻𝐶𝑙 → 𝐻 + + 𝐶𝑙 − 𝐶𝑜𝑚𝑝𝑙𝑒𝑡𝑒𝑙𝑦 𝑖𝑜𝑛𝑖𝑧𝑒 Common ion effect: HCl ionize → ↑H+ → ↓Ionization of H2S → ↓[S2-] conc → sufficient to exceed the low Ksp of gp II (ppt group II) but insufficient to exceed the high Ksp of gp IV (does not ppt group IV). Dr. Ahmed Sayed Saad 13  H2S 2H+ + S2- Partially ionize HCl → H+ + Cl- Completely ionize H+ S2- S2- H+ S2- H+ 0.2-0.3N HCl ↓ Acidity = more basic ↑ Acidity It was found that 0.2 - 0.3 N HCl (0.25 N) exerts a common ion effect sufficient to ppt gp II completely without interference from gp IV. Dr. Ahmed Sayed Saad 14  A. Separation by precipitation A.2. Sulfide Separations Dr. Ahmed Sayed Saad Instrumental Analysis (PMC 227) 15  A. Separation by precipitation A.3. Separations by Other Inorganic Precipitants Dr. Ahmed Sayed Saad Instrumental Analysis (PMC 227) 16  A. Separation by precipitation A.4. Separations by Organic Precipitants Examples: Dimethyl Glyoxime 8-hydroxyquinoline Advantage of Organic precipitants: 1. The selectivity of this sort of reagent is due to the wide range of solubility among its reaction products 2. They are usually anions that are the conjugate bases of weak acids. Thus, separations based on pH control can be realized just as with hydrogen sulfide. Dr. Ahmed Sayed Saad Instrumental Analysis (PMC 227) 17