Analytical Chem istry - DePauw University

Chapter 5 Standardizing Analytical Methods165(a)0.60S spike6.00C std0.50y-intercept = k AC A V oV f0.400.30slope = k AC stdV f0.200.100-2.00 0 2.00 4.00V stdx-intercept = -C (mL)AV oC std(b)S spike0.600.500.400.30y-intercept = k AC A V oV fslope = k A0.200.100-4.00 -2.00 0 2.00 4.00 6.00 8.00 10.00 12.00V stdx-intercept = -C AV oV f×Vf(mg/L)Figure 5.7 Shown at the top is a set ofsix standard additions for the determinationof Mn 2+ . The flask on the leftis a 25.00 mL sample diluted to 50.00mL. The remaining flasks contain 25.00mL of sample and, from left to right,1.00, 2.00, 3.00, 4.00, and 5.00 mLof an external standard of 100.6 mg/LMn 2+ . Shown below are two ways toplot the standard additions calibrationcurve. The absorbance for each standardaddition, S spike , is shown by thefilled circles.Example 5.5Beginning with equation 5.8 show that the equations in Figure 5.7a forthe slope, the y-intercept, and the x-intercept are correct.So l u t i o nWe begin by rewriting equation 5.8 asSspikekCV kCA A o A std= + × VV Vwhich is in the form of the equation for a straight-linefY = y-intercept + slope × Xfstd