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by Chemistry Division Department of Natural Sciences College of Arts and Sciences Caraga State University A.Y. 2016-2017, 1st Semester Gravimetric Analysis • Quantitative methods that are based on determining the mass of a pure compound to which the analyte is chemically related • Gravimetric methods of analysis are based on mass measurements made with an analytical balance 2 Analytical Methods based on Mass Measurements • Precipitation gravimetry – The analyte is separated from a solution of the sample as a precipitate and is converted to a compound of known composition that can be weighed • Volatilization gravimetry – The analyte is separated from other constituents of the sample by conversion to a gas of known composition • Electrogravimetry – The analyte is separated by deposition on an electrode by an electric current • Gravimetric titrimetry – The mass of a reagent, of known concentration, required to react completely with the analyte is used to determine the analyte concentration • Atomic mass spectrometry – Uses a mass spectrometer to separate the gaseous ions formed from the elements making up a sample of matter 3 Precipitation Gravimetry • The analyte is converted to a sparingly soluble precipitate • The precipitate is then filtered, washed free of impurities, converted to a product of known composition by suitable heat treatment, and weighed 4 Determination of calcium in natural waters • Excess oxalic acid is added to an aqueous solution of the sample • Ammonia is added to neutralize the acid and causes all of the calcium in the sample to precipitate as calcium oxalate 5 Example 1 • The precipitate is filtered using a weighed filtering crucible, then dried, and ignited • This process converts the precipitate to calcium oxide • After cooling, the crucible and precipitate are weighed • The mass of calcium oxide is determined • Calcium content is calculated 6 Properties of precipitates and precipitating reagents • Gravimetric precipitating reagents should: – React specifically, or at least – React selectively with the analyte • Specific reagents (rare) react only with a single chemical species • Selective reagents (more common) react with a limited number of chemical species 7 Properties of precipitates and precipitating reagents • An ideal precipitating reagent should react with the analyte to give a product that is – Easily filtered and washed free of contaminants – Of sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing – Unreactive with constituents of the atmosphere – Of known chemical composition after it is dried or if necessary, ignited 8 Particle size and filterability of precipitates • Precipitates consisting of large particles are desirable for gravimetric work – Easy to filter and wash free of contaminants – Purer compared to precipitates made up of fine particles • Colloidal suspensions – Solid particles with diameters <10-4 cm – Little tendency to settle from solution – Difficult to filter • Crystalline suspension – Tend to settle spontaneously – Easy to filter 9 Factors that determine the particle size of precipitates • • • • • Precipitate solubility Temperature Reactant concentrations Rate of mixing reactants Relative supersaturation – Q = concentration of the solute at any instant – S = equilibrium solubility • Particle size of a precipitate varies inversely with the average relative supersaturation when reagent is being introduced – When (Q-S/S) is large the precipitate tends to be colloidal – When (Q-S/S) is small the precipitate tends to be crystalline 10 Mechanism of precipitate formation • Nucleation • Particle growth 11 Experimental control of particle size • Elevate temperatures to increase solubility of the precipitate • Dilute solutions to minimize Q • Slow addition of the precipitating reagent with good stirring • Control pH, if precipitate solubility is pH-dependent – Large, easy to filter crystals of calcium oxalate are obtained by forming the bulk of the precipitate in a mildly acidic environment in which the salt is moderately soluble 12 Many precipitates cannot be formed as crystals under practical laboratory conditions • Hydrous oxides of iron(III), aluminum, and chromium(III) • Sulfides of most heavy metal ions 13 Colloid Precipitate • So small • Retained by ordinary filter papers • Brownian motion prevents their settling out of solution under the influence of gravity 14 Coagulation of Colloid • Coagulation can be hastened by: – Heating – Stirring – Adding an electrolyte to the medium 15 Peptization of Colloids • The process by which a coagulated colloid reverts to its original dispersed state • To prevent peptization of colloids, wash the precipitate with a solution containing an electrolyte that volatilizes when the precipitate is dried or ignited • Silver chloride (AgCl) is washed with a dilute solution of nitric acid (HNO3) 16 Practical treatment of colloidal precipitates • Precipitate colloids from hot, stirred solutions, containing sufficient electrolyte to ensure coagulation • Digestion is a process in which a precipitate is heated for an hour or more in the solution from which it was formed (the mother liquor) • During digestion, weakly bound water is lost from the precipitate resulting in a denser mass that is easier to filter 17 Crystalline Particles • More easy to filter • Purer than colloidal precipitates • Size of individual crystalline particles and filterability can be controlled to a degree 18 Methods of improving particle size and filterability • Minimize Q – Use dilute solutions – Add precipitating reagent slowly and with good mixing • Maximize S – Precipitate from hot solution – Adjust the pH of the precipitation medium • Digest crystalline precipitates (without stirring) – Improves purity and filterability due to dissolution and recrystallization that occur continuously and at an enhanced rate at elevated temperature 19 Coprecipitation • A process in which normally soluble compounds are carried out solution by a precipitate • Contamination of a precipitate by a second substance whose solubility product has been exceeded does not constitute coprecipitation • Types of coprecipitation: – Surface adsorption – Mixed-crystal formation – Occlusion – Mechanical entrapment 20 Types of coprecipitation • Surface adsorption – A normally soluble compound is carried out of solution on the surface of a coagulated colloid – This compound consists of the primarily adsorbed ion and an ion of opposite charge from the counter-ion layer A coagulated colloid continues to expose a large surface area to the solution from which it was formed 21 Types of coprecipitation • Mixed-crystal formation – A contaminant ion replaces an ion in the lattice of a crystal • Occlusion – A compound/impurities is trapped within a pocket formed during rapid crystal growth • Mechanical entrapment – Occurs when crystals lie close together during growth – Several crystals grow together and in so doing trap a portion of the solution in a tiny pocket 22 Minimizing adsorbed impurities on colloids • Digestion • Wash a coagulated colloid with solution containing a volatile electrolyte • Repreciptitation – The filtered solid is redissolved and reprecipitated – Increases time required for analysis – Necessary for precipitates such as hydrous oxides or iron (III) and aluminum, which have tendencies to adsorb the hydroxides of heavy metal cations such as zinc, cadmium, and manganese 23 Coprecipitation errors • Positive error results if the contaminant is not a compound of the ion being determined – Colloidal silver chloride adsorbs silver nitrate • Either positive or negative error occurs when the contaminant does not contain the ion being determined – Determination of barium as barium sulfate – Positive error if the occluded contaminant is barium nitrate – Negative error if contaminant is barium chloride 24 Precipitation from homogeneous solution • A process in which a precipitate is formed by slow generation of a precipitating reagent homogeneously throughout a solution • No localized reagent excesses • Relative superunsaturation is kept low • Example: urea is used for homogeneous precipitation of hydroxide ion Precipitation from homogeneous solution is a technique in which a precipitating agent is generated in a solution of the analyte by a slowchemical reaction. 25 Aluminum hydroxide formed by the direct addition of ammonia (left) and the homogeneous production of hydroxide (right) 26 Methods for homogeneous generation of precipitating agents 27 Drying and ignition of precipitates • After filtration, a gravimetric precipitate is heated until its mass become constant – Removes solvent and any volatile species carried down with the precipitate – Decomposes the solid and forms a compound of known composition (the weighing form) 28 Effect of temperature on precipitate mass • Moisture is completely removed from AgCl at T>110C • Dehydration of Al2O3 is not complete until a T>1000C • For CaC2O4: – T<135C unbound H2O is removed to give CaC2O4H2O – T=225C, CaC2O4H2O is converted to anhydrous CaC2O4 – T450C, CaC2O4 decomposed to CaCO3 and CO – T>800C conversion of the carbonate to calcium oxide 29 Calculation of results from gravimetric data 30 Example 31 Sample Problems 32 Sample Problems 33 Sample Problems 34 Sample Problems 35 Sample Problems 36 Sample Problems 37 Applications of gravimetric methods • Inorganic anions and cations • Neutral species, like water, sulfur dioxide, carbon dioxide, and iodine • Organic substances – Lactose in milk products – Salicylates in drug preparations – Phenolphthalein in laxatives – Nicotine in pesticides – Cholesterol in cereals – Benzaldehyde in almond extracts 38 Inorganic precipitating agents 39 Reducing agents convert an analyte to its elemental form for weighing 40 Organic precipitating agents for the determination of inorganic species • Coordination compounds – Form slightly soluble nonionic products • Chelating agents – Forms products in which the bonding between the inorganic species and reagent is largely ionic Heme is a part of hemoglobin, the oxygencarrying molecule in human blood 41 Chelating reagents • 8-hydroxyquinoline (oxine) • Dimethylglyoxime precipitates only Ni(II) from a weakly alkaline solution 42 Organic precipitating reagent • Sodium tetraphenylborate – Near-specific precipitating agent • NH4+ • K+ 43 Gravimetric methods for organic functional groups 44 Volatilization gravimetry • Determination of water – Direct method • Water vapor is collected on any of several solid desiccants, and its mass is determined from the mass gain of the desiccant – Indirect method • The amount of water is determined by the loss of mass of the sample during heating • Volatilization of carbon dioxide – Determination of the sodium hydrogen carbonate content of antacid tablets 45 Apparatus for determining the NaHCO3 content of antacid tablets by a gravimetric volatilization procedure 46 Assignment: 1. The aluminum in a 1.200-g sample of impure ammonium aluminum sulfate was precipitated with aqueous ammonia as the hydrous Al O .xH O. The precipitate was filtered and ignited at 1000°C to give anhydrous Al O , which weighed 0.2001 g. Express the result of this analysis in terms of (a) % NH4Al(SO ) . (b) % Al2O3. (c) % Al. 2 3 2 2 4 3 2 2. A 0.6407-g sample containing chloride and iodide ions gave a silver halide precipitate weighing 0.4430 g. This precipitate was then strongly heated in a stream of Cl2 gas to convert the AgI to AgCl; on completion of this treatment, the precipitate weighed 0.3181 g. Calculate the percentage of chloride and iodide in the sample. 3. An aqueous solution contains NaNO3 and KBr. The bromide ion is precipitated as AgBr by addition of AgNO3. After an excess of the precipitating reagent has been added, (a) what is the charge on the surface of the coagulated colloidal particles? (b) what is the source of the charge? 47 (c) what ions make up the counter-ion layer?