Analytical Chem istry - DePauw University

Chapter 8 Gravimetric Methods381of 1/2 in equation 8.14, which accounts for the stoichiometry betweenthe analyte and precipitate. We can also improve sensitivity by forming aprecipitate that contains fewer units of the analyte.Practice Exercise 8.5Suppose you wish to determine the amount of iron in a sample. Which ofthe following compounds—FeO, Fe 2 O 3 , or Fe 3 O 4 —provides the greatestsensitivity?Click here to review your answer to this exercise.Se l e c t i v i t yDue to the chemical nature of the precipitation process, precipitants areusually not selective for a single analyte. For example, silver is not a selectiveprecipitant for chloride because it also forms precipitates with bromide andiodide. Interferents are often a serious problem and must be considered ifaccurate results are to be obtained.Tim e , Co s t, a n d Eq u i pm e n tPrecipitation gravimetry is time intensive and rarely practical if you havea large number of samples to analyze. However, because much of the timeinvested in precipitation gravimetry does not require an analyst’s immediatesupervision, it may be a practical alternative when working with onlya few samples. Equipment needs are few—beakers, filtering devices, ovensor burners, and balances—inexpensive, routinely available in most laboratories,and easy to maintain.8CVolatilization GravimetryA second approach to gravimetry is to thermally or chemically decomposethe sample and measure the resulting change in its mass. Alternatively, wecan trap and weigh a volatile decomposition product. Because the releaseof a volatile species is an essential part of these methods, we classify themcollectively as volatilization gravimetric methods of analysis.8C.1 Theory and PracticeWhether an analysis is direct or indirect, volatilization gravimetry usuallyrequires that we know the products of the decomposition reaction. Thisis rarely a problem for organic compounds, which typically decompose toform simple gases such as CO 2 , H 2 O, and N 2 . For an inorganic compound,however, the products often depend on the decomposition temperature.ThermogravimetryOne method for determining the products of a thermal decomposition isto monitor the sample’s mass as a function of temperature, a process called