Analytical Chem istry - DePauw University

Chapter 5 Standardizing Analytical Methods159So l u t i o nEquation 5.5 allows us to calculate the value of k A for this method usingthe data for the standard.kASstd0.474= = = 0.2709 ppbC 175 . ppbstdHaving determined the value of k A , the concentration of Pb 2+ in the sampleof blood is calculated using equation 5.6.CASsamp0.361= = = 133 . ppb-1k 0.2709 ppbA-1Mu l t i p l e Ex t e r n a l St a n d a r d sFigure 5.3 shows a typical multiple-point external standardization. Thevolumetric flask on the left is a reagent blank and the remaining volumetricflasks contain increasing concentrations of Cu 2+ . Shown below thevolumetric flasks is the resulting calibration curve. Because this is the mostcommon method of standardization the resulting relationship is called anormal calibration curve.When a calibration curve is a straight-line, as it is in Figure 5.3, theslope of the line gives the value of k A . This is the most desirable situationsince the method’s sensitivity remains constant throughout the analyte’sconcentration range. When the calibration curve is not a straight-line, the0.250.20S std0.150.100.0500 0.0020 0.0040 0.0060 0.0080C std(M)Figure 5.3 Shown at the top is a reagentblank (far left) and a set of five externalstandards for Cu 2+ with concentrationsincreasing from left to right.Shown below the external standards isthe resulting normal calibration curve.The absorbance of each standard, S std ,is shown by the filled circles.