1.1: Classification of Analytical Methods

Analytical chemistry has a long history. On the bookshelf of my office, for example, there is a copy of the first American edition of Fresenius's A System of Instruction in Quantitative Chemical Analysis, which was published by John Wiley & Sons in 1886. Nearby are many newer texts, such as Bard and Faulkner's Electrochemical Methods: Fundamentals and Applications, the most recent edition of which was published by Wiley in 2000. In 883 pages, Fresnius's text covers essentially all that was known in the 1880s about analytical chemistry and what we now call classical methods of analysis. Bard and Faulkner's text, which is 864 pages, covers just one category of what we now call modern instrumental methods of analysis. Whether a classical method of analysis or a modern instrumental method analysis, the species of interest, which we call the analyte, is probed in a way that provides qualitative or quantitative information.

Classical Methods of Analysis

The distinguishing feature of a classical method of analysis is that the principal measurements are observations of reactions (Did a precipitate form? Did the solution change color?) or the measurement of one of a small number of physical properties, such as mass or volume. Because these measurements are not selective for a single analyte, a classical method of analysis usually required extensive work to isolate the analyte of interest from other species that would interfere in the analysis. As we see in Figure \(\PageIndex{1}\), Fresenius's method for determining the amount of nickel in ores required 58 hours, most of which was spent bringing the ore into solution and then isolating the analyte from interferents by a sequence of precipitations and filtrations. The final determination of the amount of nickel in the ore was derived from two measurements of mass: the combined mass of Co and Ni, and the mass of Co. Although of historic interest, we will not consider further classical methods of analysis in this text.

Figure \(\PageIndex{1}\): Fresenius’ analytical scheme for the gravimetric analysis of Ni in ores. After each step, the solid and the solution are separated by gravity filtration. Note that the mass of nickel is not determined directly. Instead, Co and Ni first are isolated and weighed together (mass A), and then Co is isolated and weighed separately (mass B). The timeline shows that it takes approximately 58 hours to analyze a single sample; although several sample could be prepared in parallel, the throughput of samples was limited to just a few per day.

Modern Instrumental Methods of Analysis

The distinguishing feature of modern instrumental methods of analysis is that it extends measurements to many more physical properties, such as current, potential, the absorption or emission of light, and mass-to-charge ratios, to name a few. Instrumental methods for separating analytes, such as chromatographic separations, and instrumental methods that allow for the simultaneous analysis of multiple analytes make for a much more rapid analysis. By the 1970s, flame atomic absorption spectrometry (FAAS) replaced gravimetry as the standard method for analyzing nickel in ores [see, for example, Van Loon, J. C. Analytical Atomic Absorption Spectroscopy, Academic Press: New York, 1980]. Because FAAS is much more selective than precipitation, there is less need to chemically isolate the analyte; as a result, the time to analyze a single sample decreased to a few hours and the throughput of samples increased to hundreds per day.