定量分析12重量分析法Gravimetric-s-of-Analysis课件

,*,*,Quantitative Analysis,定量分析,Quantitative Analysis定量分析,Chapter 12Gravimetric Methods of Analysis,Chapter 12Gravimetric Methods,Gravimetric methods of analysis are based on the measurement of mass.There are two major types of gravimetric methods:,Precipitation methods:,in this method the analyte is converted to a sparingly soluble precipitate.This precipitate is then filtered,washed free of impurities,and converted to a product of known composition by suitable heat treatment,and the product is weighed.,Volatilization methods:,in this the analyte or its decomposition products are volatilized at a suitable temperature.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.,Gravimetric methods of analys,定量分析12重量分析法Gravimetric-Methods-of-Analysis课件,PROPERTIES OF PRECIPITATES AND PRECIPITATING REAGENTS,A gravimetric precipitating agent should react specifically,and selectively with the analyte.The ideal precipitating reagent would react with the analyte to give a product that is,Readily filtered and washed free of contaminants,Of sufficiently low solubility so that no significant loss of the solid occurs during filtration and washing,Unreactive with constituents of the atmosphere,Of known composition after it is dried or,if necessary,ignited,PROPERTIES OF PRECIPITATES AND,Particle Size and Filterability of Precipitates,Precipitates made up of large particles are generally desirable in gravimetric work because large particles are easy to filter and wash free of impurities.In addition,such precipitates are usually purer than are precipitates made up of fine particles.,Particle Size and Filterabili,What Factors Determine Particle Size?,The particle size of solids formed by precipitation varies enormously.At one extreme are,colloidal suspension,whose tiny particles are invisible to the naked eye(10,-7,to 10,-4,cm in diameter).Colloidal particles show no tendency to settle from solution,nor are they easily filtered.At the other extreme are particles with dimensions on the order of tenths of millimeter or greater.The temporary dispersion of such particles in the liquid phase is called a,crystalline suspension,.The particles of a crystalline suspension tend to settle spontaneously and are readily filtered.,What Factors Determine Particl,The particle size of a precipitate is influenced by experimental variables as precipitate solubility,temperature,reactant concentrations,and the rate at which reactants are mixed.The particle size is related to a single property of the system called its,relative supersaturation,where,relative supersaturation=(Q S)/S,In this equation,Q is the concentration of the solute at any instant and S is its equilibrium solubility.,When(Q S)/S is large,the precipitate tends to be colloidal.,when(Q S)/S is small,a crystalline solid is more likely.,The particle size of a precip,How do Precipitates Form?,Precipitates form in two ways,by,nucleation,and by,particle growth,.The particle size of a freshly formed precipitate is determined by which way is faster.,In nucleation,a few ions,atoms,or molecules(perhaps as few as four or five)come together to form a stable solid.Often,these nuclei form on the surface of suspended solid contaminants,such as dust particles.Further precipitation then involves a competition between additional nucleation and growth on existing nuclei(particle growth).If nucleation predominates,a precipitate containing a large number of small particles results;if growth predominates,a smaller number of larger particles is produced.,How do Precipitates Form?Prec,Controlling Particle Size,Experimental variables that minimize supersaturation and thus lead to crystalline precipitates include elevated temperatures to increase the solubility of the precipitate(S in Equation),dilute solutions(to minimize Q),and slow addition of the precipitating agent with good stirring.The last two measures also minimize the concentration of the solute(Q)at any given instant.,Larger particles can also be obtained by pH control,provided the solubility of the precipitate depends on pH.,Controlling Particle Size,Colloidal Precipitates,Coagulation of Colloids:,Coagulation can be hastened by heating,stirring,and adding an electrolyte to the medium.,Colloidal suspensions are stable because all the particles present are either positively or negatively charged.This charge results from cations or anions that are bound to the surface of the particles.The process by which ions are retained on the surface of a solid is known as adsorption.We can readily demonstrate that colloidal particles are charged by observing their migration when placed in an electrical field.,Colloidal Precipitates,定量分析12重量分析法Gravimetric-Methods-of-Analysis课件,定量分析12重量分析法Gravimetric-Methods-of-Analysis课件,定量分析12重量分析法Gravimetric-Methods-of-Analysis课件,Peptization of Colloids,Peptization refers to the process by which a coagulated colloid reverts to its