Analytical Chem istry - DePauw University

380 Analytical Chemistry 2.0of ±0.1% using an analytical balance with a sensitivity of ±0.1 mg, wemust isolate at least 100 mg of precipitate. As a consequence, precipitationgravimetry is usually limited to major or minor analytes, in macro or mesosamples (see Figure 3.5 in Chapter 3). The analysis of trace level analytes ormicro samples usually requires a microanalytical balance.Ac c u r a c yProblem 8.27 provides an example of howto determine an analyte’s concentration byestablishing an empirical relationship betweenthe analyte and the precipitate.For a macro sample containing a major analyte, a relative error of 0.1–0.2%is routinely achieved. The principle limitations are solubility losses, impuritiesin the precipitate, and the loss of precipitate during handling. Whenit is difficult to obtain a precipitate that is free from impurities, it may bepossible to determine an empirical relationship between the precipitate’smass and the mass of the analyte by an appropriate calibration.PrecisionThe relative precision of precipitation gravimetry depends on the sample’ssize and the precipitate’s mass. For a smaller amount of sample or precipitate,a relative precision of 1–2 ppt is routinely obtained. When workingwith larger amounts of sample or precipitate, the relative precision can beextended to several ppm. Few quantitative techniques can achieve this levelof precision.SensitivityEquation 8.13 assumes that we have useda suitable blank to correct the signal forany contributions of the reagent to theprecipitate’s mass.For any precipitation gravimetric method we can write the following generalequation relating the signal (grams of precipitate) to the absolute amountof analyte in the samplegramsprecipitate= k × grams analyte8.13where k, the method’s sensitivity, is determined by the stoichiometry betweenthe precipitate and the analyte. Consider, for example, the determinationof Fe as Fe 2 O 3 . Using a conservation of mass for iron, the precipitate’smass isgFeO231 molFe FW Fe O2= gFe× ×AW Fe 2 molFe3and the value of k isk = 1 ×2FW Fe O2 38.14AW FeAs we can see from equation 8.14, there are two ways to improve a method’ssensitivity. The most obvious way to improve sensitivity is to increase theratio of the precipitate’s molar mass to that of the analyte. In other words,it helps to form a precipitate with the largest possible formula weight. Aless obvious way to improve a method’s sensitivity is indicated by the term