The accuracy with which we determine kA and Sreag depends on how accurately we can measure the signal, Stotal. We measure signals using equipment, such as glassware and balances, and instrumentation, such as spectrophotometers and pH meters. To minimize determinate errors that might affect the signal, we first calibrate our equipment and instrumentation by measuring Stotal for a standard with a known response of Sstd, adjusting Stotal until
Stotal = Sstd
Here are two examples of how we calibrate signals; other examples are provided in later chapters that focus on specific analytical methods.
When the signal is a measurement of mass, we determine Stotal using an analytical balance. To calibrate the balance’s signal we use a reference weight that meets standards established by a governing agency, such as the National Institute for Standards and Technology or the American Society for Testing and Materials. An electronic balance often includes an internal calibration weight for routine calibrations, as well as programs for calibrating with external weights. In either case, the balance automatically adjusts Stotal to match Sstd.
See Chapter 2.4 to review how an electronic balance works. Calibrating a balance is important, but it does not eliminate all sources of determinate error when measuring mass. See Appendix 9 for a discussion of correcting for the buoyancy of air.
We also must calibrate our instruments. For example, we can evaluate a spectrophotometer’s accuracy by measuring the absorbance of a carefully prepared solution of 60.06 mg/L K2Cr2O7 in 0.0050 M H2SO4, using 0.0050 M H2SO4 as a reagent blank [Ebel, S. Fresenius J. Anal. Chem. 1992, 342, 769]. An absorbance of \(0.640 \pm 0.010\) absorbance units at a wavelength of 350.0 nm indicates that the spectrometer’s signal is calibrated properly.
Be sure to read and follow carefully the calibration instructions provided with any instrument you use.