Analytical Chem istry - DePauw University

Chapter 8 Gravimetric Methods3958FChapter SummaryIn a gravimetric analysis, a measurement of mass or a change in mass providesquantitative information about the analyte. The most common formof gravimetry uses a precipitation reaction to generate a product whosemass is proportional to the amount of analyte. In many cases the precipitateincludes the analyte; however, an indirect analysis in which the analytecauses the precipitation of another compound also is possible. Precipitationgravimetric procedures must be carefully controlled to produce precipitatesthat are easy to filter, free from impurities, and of known stoichiometry.In volatilization gravimetry, thermal or chemical energy decomposesthe sample containing the analyte. The mass of residue remaining afterdecomposition, the mass of volatile products collected with a suitable trap,or a change in mass due to the loss of volatile material are all gravimetricmeasurements.When the analyte is already present in a particulate form that is easyto separate from its matrix, then a particulate gravimetric analysis may befeasible. Examples include the determination of dissolved solids and thedetermination of fat in foods.8GProblems1. Starting with the equilibrium constant expressions for reaction 8.1, andreactions 8.3–8.5, verify that equation 8.7 is correct.Answers, but not worked solutions, tomost end-of-chapter problems are availablehere.2. Equation 8.7 explains how the solubility of AgCl varies as a functionof the equilibrium concentration of Cl – . Derive a similar equation thatdescribes the solubility of AgCl as a function of the equilibrium concentrationof Ag + . Graph the resulting solubility function and compareit to that shown in Figure 8.1.3. Construct a solubility diagram for Zn(OH) 2 that takes into accountthe following soluble zinc-hydroxide complexes: Zn(OH) + , Zn(OH) 3 – ,and Zn(OH) 4 2– . What is the optimum pH for quantitatively precipitatingZn(OH) 2 ? For your solubility diagram, plot log(S) on the y-axisand pH on the x-axis. See the appendices for relevant equilibrium constants.4. Starting with equation 8.10, verify that equation 8.11 is correct.5. For each of the following precipitates, use a ladder diagram to identifythe pH range where the precipitates has its lowest solubility? See theappendices for relevant equilibrium constants.a. CaC 2 O 4 b. PbCrO 4 c. BaSO 4d. SrCO 3 e. ZnS