【精品】光线照度计操作说明书 Title80.pptVIP

Lecture 6 Gravimetric Analysis and Precipitation Equilibria Section 10.1 Gravimetric analysis is potentially more accurate and more precise than volumetric analysis. You should get better results from part 1 of Experiment 2 than from part 2. Gravimetric analysis avoids problems with temperature fluctuations, calibration errors, and other problems associated with volumetric analysis. But there are potential problems with gravimetric analysis that must be avoided to get good results. Proper lab technique is critical Steps in a Gravimetric Analysis 1. Preparation of analyte solution Gravimetric analysis usually involves precipitation of analyte from solution. May need to separate potential interferences before precipitating analyte May require pH and/or other adjustments to maximize precipitate formation When possible, select precipitating agents that are selective (or specific, if possible). 2. Precipitation 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 The smaller the solubility, S, the greater the tendency to form lots of small crystals that trap contaminants and are difficult to filter The greater the solubility, the more analyte is left unprecipitated We need to strike a balance (but not the analytical balance, please) The von Weimarn Ratio von Weimarn ratio = (Q – S)/S A measure of relative supersaturation The lower the better If high, get excessive nucleation, lots of small crystals, large surface area If low, get larger, fewer crystals, small surface area Q = concentration of mixed reactants before precipitation Keep it low by using dilute solutions, stir mixture well, add reactants slowly S = solubility of precipitate at equilibrium Keep it high with high temperatures, adjusting pH Can lower S later by cooling mixture after crystals have formed Impurities in Precipitates Coprecipitation Precipitation of normally soluble compound al