Chapter 6 Gravimetric Analysis Al modi-1 - Copy.ppt
- 2. 6.1. Principle and types of Gravimetric analysis Gravimetric Analysis: is a method in which the amount of an analyte in a sample is determined by converting the analyte to some product Mass of product can be easily measured Gravimetric analysis is potentially more accurate and precise than volumetric analysis. Gravimetric analysis avoids problems with temperature fluctuations, calibration errors, and other problems associated with volumetric analysis. Advantages - when done correctly, it is highly accurate (most accurate of all time); requires minimal equipment. 2 Gravimetry: Any method in which the signal is a mass or change in mass
- 3. Example: Determination of lead (Pb+2) in water Pb+2 + 2Cl- PbCl2(s) By adding excess Cl- to the sample, essentially all of the Pb+2 will precipitate as PbCl2. Mass of PbCl2 is then determined. - used to calculate the amount of Pb+2 in original solution Reagent Analyte Solid Product Disadvantage - requires skilled operator, slow operation. Convert analyte into a solid; filter, weigh and calculate a mole But there are potential problems with gravimetric analysis that must be avoided to get good results. Proper lab technique is critical 3
- 4. Types of Gravimetry 1. Combustion (Volatilization) Analysis Technique is accurate and usable with a wide range of compounds. In volatilization methods, the analyte or its decomposition products are volatilized at a suitable temperature. 5 Gravimetric method types are: Precipitation gravimetry: A gravimetric method in which the signal is the mass of a precipitate. Electrogravimetry: A gravimetric method in which the signal is the mass of an electrodeposit on the cathode or anode in an electrochemical cell. Volatilization gravimetry: A gravimetric method in which the loss of a volatile species gives rise to the signal. Particulate gravimetry: A gravimetric method in which the mass of a particulate analyte is determined following its separation from its matrix.
- 5. 2. 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. Example: - Determination of calcium in natural waters: In this method, an excess of oxalic acid, H2C2O4, is added to a carefully measured volume of the sample. Ammonia is then added to neutralize the solution and cause the calcium in the sample to precipitate as calcium oxalate. The reactions are: The precipitate is filtered using a weighed filtering crucible and is 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 subtraction of the known mass of the crucible. Reagent + Analyte Solid Product (collect and measure mass) 2NH3 + H2C2O4 2NH4 + + C2O4 2- Ca2+ (aq) + C2O4 (aq) CaC2O4 (s) CaC2O4(s) CaO (s) + CO (g) + CO2 (g) 7
- 6. 5.2.Properties of precipitates and precipitating agents Ideally, a gravimetric precipitation agent should react specifically, or if not, then selectively with the analyte. In addition to specificity or selectivity. the ideal precipitating reagent would react with the analyte to give a product that is: easily filtered and washed free of contaminants sufficiently low solubility so that no significance loss of the solid occurs during filtration and washing unreactive with constituents of the atmosphere known composition after it is dried, or if necessary, ignited 8 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.
- 7. 6.3. Steps in gravimetric analysis 1. Preparation of the solution 2. Precipitation 3. Digestion 4. Filtration 5. Washing 6. Drying or igniting 7. Weighing 8. Calculation 9
- 8. 1. Preparation of the Solution Gravimetric analysis usually involves precipitation of analyte from solution. This may involve: to separate potential interferences before precipitating analyte adjustment of the pH of the solution in order for the precipitate to occur quantitatively and get a precipitate of desired properties removing interferences adjusting the volume of the sample to suit the amount of precipitating agent to be added. when possible, select precipitating agents that are selective (or specific, if possible). 10
- 9. 2. Precipitation This requires addition of a precipitating agent solution to the sample solution. Upon addition of the first drops of the precipitating agent, super saturation occurs, and then nucleation starts to occur where every few molecules of precipitate aggregate together forming a nucleolus. At this point, addition of extra precipitating agent will either form new nuclei or will build up on existing nuclei to give a precipitate. 11 The precipitate should be insoluble, but not too insoluble have large crystals Easier to filter large crystals be free of contaminants The smaller the surface area the better
- 10. 3. Digestion of the Precipitate The precipitate is left hot (below boiling) for 30 min to 1 hour in order for the particles to be digested. Digestion involves dissolution of small particles and re-precipitation on larger ones resulting in particle growth and better precipitate characteristics. Digestion forces the small colloidal particles to agglomerate, which decreases their surface area and thus adsorption. Let precipitate stand in contact with solution, usually at high temperature Large crystals (small surface area) have lower free energy than small crystals (large surface area) 15 Generally, during digestion at elevated temperature: Small particles tend to dissolve and re-precipitate on larger ones. Individual particles agglomerate. Adsorbed impurities tend to go into solution.
- 11. 4. Filtering the Precipitate: Filtration should be done in an appropriate sized Goosh or ignition filter paper. Colloidal precipitates present filtering problems if particles are too small – Can plug filter paper or glass or pass right through the filter if not coagulated well – Hydrophobic colloids generally filter better than hydrophilic colloids 17
- 12. 5. Washing the Precipitate washing the precipitate very well in order to remove all adsorbed species which will add to weight of precipitate. Washing removes the mother liquor Washing may also remove some of the coprecipitated compounds Washing can cause peptization of colloids Diluting the counter-ion layer causes it to get larger, forcing coagulated colloidal particles apart And THERE THEY GO right through the filter Wash with a solution of a volatile electrolyte that will be removed in drying step 18 Peptization: The reverse of coagulation in which a coagulated precipitate reverts to smaller particles.
- 13. 6. Drying or Igniting Dry the precipitate to remove water and volatile electrolytes from wash solution Ignition (very high-temperature drying) converts precipitates to compounds more suitable for weighing Removes water of hydration Converts hygroscopic compound to non-hygroscopic compound Sometimes it even converts the filter paper to ash 7. Weighing • Properly calibrated analytical balance • Good weighing technique • Avoid static electricity 19
- 14. 6.4. Gravimetric Calculation The results of a gravimetric analysis are generally computed from two experimental measurements: the mass of a sample and the mass of a product of known composition. When the product of the analyte X, the concentration is given by the equation : Example 1: Determine the amount of iron in 2.5 g Fe2O3. Solution: To determine how many grams of iron are contained in 2.5 g Fe2O3, one could simply multiply the weight given by the fraction of it that is iron. Gram of Fe = 2.50 x 2 Fe/Fe2O3 Thus, the ratio 2Fe/Fe2O3 is called a gravimetric factor. Percent X = mass of X mass of sample x100% , where X represents the analyte 20 Gravimetric factor:It is a ratio of atomic or molecular weights that will convert the weight of some pure substance into the weight of some other substance that is stoichiometrically related to it.
- 15. Example: 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 26.6g. The mass of the crucible plus CaO (56.078 g/mol) was 26.7g. Calculate the concentration of Ca(40.078 g/mol) in water in units of gram per 100 ml of the water. Solution The mass of CaO = 26.7-26.6 = 0.1 g The number of moles Ca in the sample is equal to the number of moles of CaO or Amnt of Ca =0.1gCaO x 1molCaO/ 56.07gCaO x 1mol Ca/ molCaO = 1.78 x 10-3 mol Ca Conc.Ca = 1.78x10-3 mol Ca x 40.078g Ca/mol x 100ml 200ml sample = 0.0714 g/100ml 21