Fall Semester 2012
September 19, 2012
Gravimetric analysis is based on the quantitative isolation of the desired constituent the analyte of interest from the sample in highly pure form or in some combined form and weighing the isolated constituent. The desired constituent is usually isolated or separated by precipitation. From the weights of sample and precipitate, the percentage of the constituent in the original sample can be calculated. Precipitation is effected by inorganic or organic precipitating agents. Two common inorganic precipitating agents are silver nitrate, which is used to precipitate halide ions such as chloride, and barium chloride, which is used to precipitate sulfate ion. In all of these precipitation reactions, the product is a salt because it is formed by reactions between cations and anions.Thus the bonding is ionic or electrovalent.
The three major requirements for a good gravimetric analysis are that (a) the reagent will react only with the analyte of interest to form a precipitate, (b) it forms one and only one product with the analyte and (c) that the analyte precipitates quantitatively from solution, that is, >99.99% In aqueous solution silver ion undergoes the following reaction with chloride: Ag+(aq) + Cl-(aq) ?? AgCl(s)
Silver chloride is a relatively insoluble compound with a solubility product Ksp =[Ag+][Cl-] = 1.8 x 10-10.
Gravimetric analysis, which by definition is based upon the measurement of mass, can be generalized into two types; precipitation and volatilization. The quantitative determination of a substance by the precipitation method of gravimetric analysis involves isolation of an ion in solution by a precipitation reaction, filtering, washing the precipitate free of contaminants, conversion of the precipitate to a product of known composition, and finally weighing the precipitate and determining its mass by difference. From the mass and known composition of the precipitate, the amount of the original ion can be determined. For successful determinations the following criteria must be met: The desired substance must be completely precipitated. In most determinations the precipitate is of such low solubility that losses from dissolution are negligible. An additional factor is the "common ion" effect, this further reduces the solubility of the precipitate. When Cl- is precipitated out by addition of Ag+ Ag+ + Cl- AgCl(s)
The (low) solubility of AgCl is reduced still further by the excess of Ag+ which is added, pushing the equilibrium to the right. We can further decrease the solubility by decreasing the temperature of the solution by using an ice bath. The weighed form of the product should be of known composition. The product should be "pure" and easily filtered. It is usually difficult to obtain a product which is "pure", i.e. one which is free from impurities but careful precipitation and sufficient washing helps reduce the level of impurity.
Medium porosity sintered glass funnel| suction filtration set-up| oven set reliably to 110 degree Celsius| clean small beaker for storage of funnel| stirring rod with rubber policeman| wash bottle|
hot plate| thermometer|
crucible and beaker tongs| chloride unknown|
6 M nitric acid| 5% silver nitrate solution|
Mass of Cl- present in the original sample =
(mass of AgCl precipitate)x(molar mass of Cl- / molar mass of AgCl)
% of Cl- present in original sample = [(mass of Cl-)/(mass of the unknown sample)]x100
Step 1: Suction filtration apparatus was set up to clean the provided porosity sintered glass funnel then placed to a small beaker and kept at the oven for four days. Step 2: weight bottle with chloride sample was massed and placed in the beaker to re-mass the combined weight Step 3: 150-250 ml of distilled water was added followed by 5 ml 6M nitric...