Analytical Chem istry - DePauw University

Chapter 5 Standardizing Analytical Methods167Practice Exercise 5.3Figure 5.7 shows a standard additions calibration curve for the quantitativeanalysis of Mn 2+ . Each solution contains 25.00 mL of the originalsample and either 0, 1.00, 2.00, 3.00, 4.00, or 5.00 mL of a 100.6 mg/Lexternal standard of Mn 2+ . All standard addition samples were diluted to50.00 mL before reading the absorbance. The equation for the calibrationcurve in Figure 5.7a isSince we construct a standard additions calibration curve in the sample,we can not use the calibration equation for other samples. Each sample,therefore, requires its own standard additions calibration curve. This is aserious drawback if you have many samples. For example, suppose you needto analyze 10 samples using a three-point calibration curve. For a normalcalibration curve you need to analyze only 13 solutions (three standardsand ten samples). If you use the method of standard additions, however,you must analyze 30 solutions (each of the ten samples must be analyzedthree times, once before spiking and after each of two spikes).Us i n g a St a n d a r d Ad d i t i o n t o Id e n t i f y Ma t r i x Ef f e c t sWe can use the method of standard additions to validate an external standardizationwhen matrix matching is not feasible. First, we prepare a normalcalibration curve of S std versus C std and determine the value of k A fromits slope. Next, we prepare a standard additions calibration curve usingequation 5.8, plotting the data as shown in Figure 5.7b. The slope of thisstandard additions calibration curve provides an independent determinationof k A . If there is no significant difference between the two values ofk A , then we can ignore the difference between the sample’s matrix and thatof the external standards. When the values of k A are significantly different,then using a normal calibration curve introduces a proportional determinateerror.5C.4 Internal StandardsS std = 0.0854 × V std + 0.1478What is the concentration of Mn 2+ in this sample? Compare your answerto the data in Figure 5.7b, for which the calibration curve isS std = 0.0425 × C std (V std /V f ) + 0.1478Click here to review your answer to this exercise.To successfully use an external standardization or the method of standardadditions, we must be able to treat identically all samples and standards.When this is not possible, the accuracy and precision of our standardizationmay suffer. For example, if our analyte is in a volatile solvent, then itsconcentration increases when we lose solvent to evaporation. Suppose we