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BST 152-1 Analytical Techniques for Biosystems Technology Precipitation gravimetry Several analytical methods are based on mass measurements In precipitation gravimetry, the analyte is converted to a sparingly soluble precipitate. This precipitate is then filtered, washed...

BST 152-1 Analytical Techniques for Biosystems Technology Precipitation gravimetry Several analytical methods are based on mass measurements In precipitation gravimetry, the analyte is converted to a sparingly soluble precipitate. This precipitate is then filtered, washed free of impurities, converted to a product of known composition by suitable heat treatment, and weighed. For example, a precipitation method for determining calcium in water. In this technique, an excess of oxalic acid, H2C2O4, is added to an aqueous solution of the sample. Ammonia is then added, which neutralizes the acid and causes essentially all of the calcium in the sample to precipitate as calcium oxalate. The reactions are Precipitation gravimetry The CaC2O4 precipitate is filtered using a weighed filtering crucible, then dried and ignited. This process converts the precipitate entirely to calcium oxide. The reaction is After cooling, the crucible and precipitate are weighed, and the mass of calcium oxide is determined by subtracting the known mass of the crucible. The calcium content of the sample is then computed as shown below. Precipitation gravimetry E.g- The calcium in a 200.0-mL sample of a natural water was determined by precipitating the cation as CaC2O4. The precipitate was filtered, washed, and ignited in a crucible with an empty mass of 26.6002 g. The mass of the crucible plus CaO (56.077 g/mol) was 26.7134 g. Calculate the concentration of Ca (40.078 g/mol) in water in units of grams per 100 mL of the water Properties of Precipitates and Precipitating Reagents Ideally, a gravimetric precipitating agent should react specifically or at least selectively with the analyte. Specific reagents, which are rare, react only with a single chemical species. Selective reagents, which are more common, react with a limited number of species. In addition to specificity and selectivity, the ideal precipitating reagent would react with the analyte to give a product that is 1. easily filtered and washed free of contaminants; 2. of sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing; 3. unreactive with constituents of the atmosphere; 4. of known chemical composition after it is dried or, if necessary, ignited Filterable precipitates Precipitates consisting of large particles are generally desirable for gravimetric work because these particles are easy to filter and wash free of impurities. In addition, precipitates should be usually purer than are precipitates made up of fine particles. Crystalline Precipitates Crystalline precipitates are generally more easily filtered and purified than are coagulated colloids. In addition, the size of individual crystalline particles, and thus their filterability, can be controlled to some extent. Colloidal Precipitates Individual colloidal particles are so small that they are not retained by ordinary filters. Moreover, Brownian motion prevents their settling out of solution under the influence of gravity. Fortunately, however, we can coagulate, or agglomerate, the individual particles of most colloids to give a filterable, amorphous mass that will settle out of solution. Applications of Gravimetric methods Gravimetric methods have been developed for most inorganic anions and cations, as well as for such neutral species as water, sulfur dioxide, carbon dioxide, and iodine. A variety of organic substances can also be determined gravimetrically. Examples include lactose in milk products, salicylates in drug preparations, phenolphthalein in laxatives, nicotine in pesticides, cholesterol in cereals, and benzaldehyde in almond extracts. Indeed, gravimetric methods are among the most widely applicable of all analytical procedures. Inorganic Precipitating agents Organic Precipitating Agents Numerous organic reagents have been developed for the gravimetric determination of inorganic species. Some of these reagents are significantly more selective in their reactions than are most of the inorganic reagents. E.g- Least count and error Least count depends on the capabilities of each instrument. Depending on the least count and the type of instrument, the way we report data will be different. E.g. Buret Least Count- 0.1 mm Since this is an analog instrument, the uncertain digit can be 0.01 mm (1/10th of the least count) Reading error= ± 0.01 mm Therefore, any measurement from this instrument should be reported up to two decimal places.  Beaker Least Count- 10 mm We can read the value up to 1/10th of the least count, 1 mm (1/10th of the least count) Reading error = ± 1 mm Therefore, any measurement from this instrument should be reported without any decimal places. Graduated cylinder Least Count- 1 mm We can read the value up to 1/10th of the least count, 0.1 mm (1/10th of the least count) Reading error = ± 0.1 mm Therefore, any measurement from this instrument should be reported upto 1st decimal place. Least count = 0.0001 g Since this is a digital instrument, we cannot measure anything smaller than the least count. Therefore, each measurement should be recorded up to the 4th decimal place from this instrument. Least count = 0.01 g Since this is a digital instrument, we cannot measure anything smaller than the least count. Therefore, each measurement should be recorded up to the 2nd decimal place from this instrument. Analytical separations The goals of an analytical separation are usually to eliminate or reduce interferences so that quantitative analytical information can be obtained from complex mixtures. Separations can also allow identification of the separated constituents if appropriate correlations are made or a structurally sensitive measurement technique. Several separation methods are in common use including (1) chemical or electrolytic precipitation, (2) distillation, (3) solvent extraction, (4) ion exchange, (5) chromatography, (6) electrophoresis, (7) field-flow fractionation. Analytical separations Precipitation Distillation Solvent extraction Ion exchange chromatography Analytical separations Electrophoresis Field flow fractionation Chromatography Introduction to Chromatographic techniques Chromatography is a widely used method for the separation, identification, and determination of the chemical components in complex mixtures. No other separation method is as powerful and generally applicable as is chromatography. Chromatography is a technique in which the components of a mixture are separated based on differences in the rates at which they are carried through a fixed or stationary phase by a gaseous or liquid mobile phase. Stationary phase -phase that is fixed in place either in a column or on a planar surface. Mobile phase -phase that moves over or through the stationary phase carrying with it the analyte mixture. The mobile phase may be a gas, a liquid, or a supercritical fluid. Liquid Chromatography Stationary phase Mobile phase Stationary phase – Paper (Solid) Liquid-solid Chromatography/ Mobile phase – Solvent (Liquid) Liquid Chromatography Gas Chromatography Stationary phase – packed column (Solid) Gas-solid Chromatography / Mobile phase – (Gas) Gas Chromatography Chromatography- Classification General Classification Specific Stationary phase method Gas chromatography (GC) Gas-liquid Liquid adsorbed or bonded to a solid surface Gas-solid Solid Liquid Chromatography Liquid-liquid Liquid adsorbed or (LC) bonded to a solid surface Liquid-solid Solid Ion exchange Ion exchange resin References 1. All the materials, notes, descriptions and figures are taken from Fundamentals of Analytical Chemistry, Ninth Edition Douglas A. Skoog, Donald M. West, F. James Holler, Stanley R. Crouch 2. https://www.sciencephoto.com/media/860110/view/paper-chromatography 3. https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Map%3A_Principles_of_Instrume ntal_Analysis_(Skoog_et_al.)/28%3A_High- Performance_Liquid_Chromatography/28.06%3A_Ion_Chromatography 4. https://www.shutterstock.com/search/solvent+extraction