Principles and Techniques of Gravimetric Analysis

GRAVIMETRIC ANALYSIS INTRODUCTION • Gravimetric methods of analysis involve processes whereby the amount of the constituent to be determined is isolated either as an element or as a compound of definite composition and weighed in some pure form; involves the most accurate and precise measurements. APPLICATION OF GRAVIMETRIC METHOD • • • • • • Determination of Lactose in Milk Products Determination of Salicylates in Drug Preparations Determination of Phenolphthalein in Laxatives Determination of Nicotine in Pesticides Determination of Cholesterol in Cereals Determination of Benzaldehyde in Almond Extracts • • • • TWO MAJOR CONDITIONS OF A SUCCESFUL GRAVIMETRIC ANALYSIS • • The separation of the desired constituent must be sufficiently complete so that the amount left in the solution cannot produce a detectable error. The final substance that is weighed should be relatively pure and have a definite composition since the analysis and resultant calculations are based on these assumptions Gelatinous precipitates – these precipitates are flocculates colloids. Particle size is to smaller and difficult to filter. Nucleation – is a process in which a minimum number of atoms, ions, or molecules join together to produce a stable solid. Digestion – is a process in which a precipitate is heated for an hour or more in the solution from which it was formed (mother liquor); improves the purity and filterability of both colloidal and crystalline precipitates. Mother Liquor – is the solution from which a precipitate forms. Gravimetric Analysis o A chemically equivalent amount of the substance/product can be obtained from a sample/reactant by the following: ▪ Precipitation from solution ▪ Decomposition from ignition ▪ Deposition on an electrode by electrolysis o ANALYTICAL CHEMISTRY LABORATORY APPARATUS FOR GRAVIMETRIC METHODS • • • • • Analytical Balance Weighing bottles Desiccators and Desiccants Crucibles Muffle furnace CRUCIBLE ISOLATION OF THE SOUGHT-FOR CONSTITUTENT MAY BE CARRIED OUT BY: • • • • • • Precipitation from the solution of the sample Direct ignition of the sample, whereby by decomposition of the constituents, another substance is left as a residue Electrolytic decomposition on a tared electrode Extraction with an appropriate solvent and subsequent evaporation of the latter. Volatilization of the substance to be determined Absorption of the gas liberated from a sample • • TWO MAJOR TYPES OF GRAVIMETRIC METHODS • • Precipitation Methods o The analyte is converted to a sparingly soluble precipitate. o This precipitate is filtered, washed free of impurities, and converted to a product of known composition by suitable heat treatment, and the product is weighed. Volatilization Methods o The analyte or its decomposition products are volatilized at a suitable temperature. o The volatile product is then collected and weighed, or, alternatively, the mass of the product is determined indirectly from the loss in mass of the sample. TERMS IN GRAVIMETRIC ANALYSIS • Precipitation – is the process by which a solid phase forms and is carried out of solution when the solubility product of a species is exceeded. o Crystalline precipitates - these precipitates are relatively pure and easily filterable. o Curdy precipitates - these precipitate are of colloidal particle size but are filterable. Porcelain crucible and its cover are used for the ignition of precipitates in ashless filter paper. o The crucible is resistant to the attack of many reagents like that of alkaline substances and hydrofluoric acid. o The crucible with its cover can be heated to temperatures as high as 1200 °C. Gooch crucible is designed for the separation of precipitates by suction filtration. o It is also made of porcelain but has a perforated bottom. o A mat of asbestos or ceramic fibers is used to cover the perforation before its use. o It is suited for precipitates that are easily reduced by the carbon of filter paper. o Successive filtrations in the same crucible can be made when the precipitate already present is insoluble. OPERATIONS • • • • • Weighing process Filtration Process Washing Process Drying and Igniting Cooling Process GRAVIMETRIC ANALYSIS • • • • • % = W X E X 100% S W - weight of the product E - gravimetric factor = MW analyte / W ppt S - weight of the sample product = ppt; sample = analyte Analyte Chlorine Iodine Sulfate Mercury Phosphate Precipitating Reagent Silver nitrate Silver nitrate Barium chloride Hydrogen sulfide Magnesia mixture Precipitate • Silver chloride Silver iodide Barium sulfate Mercuric sulfide Magnesium ammonium phosphate ignited to Magnesium pyrophosphate SAMPLE PROBLEMS 1. A 0.3056 g sample of soluble chloride was analyzed gravimetrically for chlorine and 0.7256 g of silver chloride was obtained. Calculate the % chlorine content of the sample. Analyte = Cl2 = MW = 70 Precipitate = AgCL = MW = 143 % = W x (MWa/MWp) x 100% S % = 0.7256g x (70/143) x 100% 0.3056g % = 116.23% 2. EXAMPLE PROBLEM • If a 0.4600g sample of sodium iodide yielded 0.7200g of silver iodide precipitate, what would be the % purity of the sodium iodide? 1. The difference between the hydrate mass and anhydrate mass is the mass of water lost. 1.000 g - 0.6390 g = 0.3610 g 2. Divide the mass of the water lost by the mass of hydrate and multiply by 100. What weight of Mercury bichloride (HgCl2) should be taken as sample, assuming 100% purity so that 0.5000 g of mercuric sulfide will be obtained in gravimetric analysis? Analyte = HgCl2 = MW = 271 Precipitate = HgS = MW = 233 S = W x (MWa/MWp) x 100% % S = 0.500g x (271/233) x 100% 100% S= 0.5815 g (0.3610 g /1.000 g)(100) = 36.10% ADDITIONAL NOTES LESSON 1 - APPARATUS AND UNIT OPERATIONS OF GRAVIMETRIC ANALYSIS • DETERMINATION OF WATER CONTENT OF A HYDRATE • • • • The sample is heated in an oven at a temperature enough to drive off water. The loss in weight is taken as the amount of water present in the sample. Generally, oven-drying is carried out at the lowest practical temperature in order to minimize decomposition. Disadvantages: (1) volatilization of components other than water such as other solvents, and oils; (2) loss of water of constitution (especially with organic compounds); or (3) oxidation or decomposition of the sample. When a 1.000 g sample of CuSO4 • 5 H2O(s) was heated so that the waters of hydration were driven off, the mass of the anhydrous salt remaining was found to be 0.6390 g. What is the experimental value of the percent water of hydration? CuSO4 • 5 H2O(s) + HEAT ----> CuSO4 (s) + 5 H2O (g) 1.000 g 0.6390 g Analyte = NaI = MW=150 Precipitate AgI = MW = 235 % = W x (MWa/MWp) x 100% S % = 0.7200g x (150/235) x 100% 0.4600g % = 99.91% 3. Hydrates o This are compounds that incorporate water molecules into their fundamental solid structure. o In a hydrate (which usually has a specific crystalline form), a defined number of water molecules are associated with each formula unit of the primary material. o Gypsum is a hydrate with two water molecules present for every formula unit of CaSO4 . ▪ The chemical formula for gypsum is CaSO4 • 2H2O and the chemical name is calcium sulfate dihydrate. ▪ Note that the dot in the formula (or multiplication sign) indicates that the waters are there. o The water in the hydrate (referred to as "water of hydration") can be removed by heating the hydrate. When all hydrating water is removed, the material is said to be anhydrous and is referred to as an anhydrate. o CuSO4.5H2O + HEAT → CuSO4 (s) + 5 H2O (g) hydrate anhydrate • • The Weighing Process o Since measurement of the mass is the primary concern in gravimetric analysis, the use of the analytical balance is of primary importance. Along with the analytical balance, the apparatus, equipment and instruments shown below are used in gravimetric analysis. The Filtration Process o After precipitation process, the precipitate has to be isolated by filtration. The usual gravity filtration is through neatly folded ashless filter paper on a funnel. Filtration can be made faster through a Buchner funnel with suction applied The Washing Process o The isolated precipitate is then washed to remove excess ions of the precipitating agents and impurities. Usually, an electrolyte containing ions of the • • precipitating solution is added to the wash water to avoid loss of precipitate. The Drying and Igniting Process o To convert the precipitate to a weighable form, it can be dried in an oven, ignited over a Bunsen flame using plain crucible and clay triangle or can be placed in a muffle furnace. The Cooling Process o You have learned from the previous lessons that objects cannot be weighed when hot. It has to be cooled in a desiccator. A desiccator is a two-chamber glass apparatus where cooling is at relatively low and constant humidity. • Gelatinous Precipitate o Flocculated colloids; jelly-like, hydrous masses o Trap a great deal of water and many impurities; o Very difficult to work with; o The cation & anion react to form a soluble colloid and then coagulate to filterable-sized particles; o Examples are metal hydroxides such as ferric hydroxide; • Curdy Precipitate o Aggregates of small porous particles; o The cation & anion react to form a soluble colloid and then coagulate to filterable-sized particles; o When reasonable in size, they are filtered rapidly, easily and completely; o Examples are the silver halides compounds; GRAVIMETRIC ANALYSIS • • • • • • • • • • • • • • • • • • Gravimetric - weight of sample Volatilization - simplest o Collection of humidity Vacuum filtration o Buchner - has numerous holes Crystalline ppt - use filter paper Curdy ppt - use Buchner Digestion is successful when mother liquor is colorless Electrolysis - speed up formation of globules, Desiccator - hermitically sealed Desiccant - used to absorb moisture Ashless filter paper - use to not add to weight Weighing process - use analytical balance Filtration o Buchner funnel - faster filtration Drying - via oven Ignition o Muffle furnace - for high temperature o Direct ignition, using ethyl alcohol Cooling process o Use desiccator o Absorb moisture % = (wt. product x MW analyte/MW ppt)/wt. sample x 100 Hydrates - under volatilization Volatilization o Wt. initial before drying o Wt. final after drying o % = (wt.1 - wt.2 / wt. 1) x 100 o *remove weight of containers if necessary o *use raw wt. PRECIPITATION • Crystalline o Well-defined / regularly shaped crystals whose shapes are a function of the composition of the salt; o Relatively pure; o Ready to filter; o Are filtered rapidly, easily and completely; o Since these crystals are not pure and are too small to filter effectively, they are heated to achieve larger and purer crystals (digestion); o Examples are calcium oxalate and also the alkaline earth sulfates such as barium sulfate.