Introduction to Soil Chemistry

Concentration Optical Density
Absorbance (A)
10 0.15
30 0.3
60 0.65
90 0.95
Unk1 0.05
Unk2 0.97
1
0.8
0.6
0.4
0.2
0
regression analysis 161
y = 0.0102x + 0.0262
R 2 = 0.9957
0 20 40 60 80 100
Concentration
Figure 8.9. Standard or calibration curve for data at top.
easily measured; however, absorbences approaching or above 1 are seldom
reliable.
A blank is often used as a 0,0 point in preparing a calibration curve as in
Figure 8.10. This is an acceptable practice as long as the calibration curve is
sufficiently accurate. If it is not, or if the inclusion of the 0,0 point produces
an irregular curve, its removal from the curve is warranted. In Figure 8.10 the
inclusion of the 0,0 point gives a slightly better r 2 value (see Section 8.9) and
thus can be included in the curve. Also note that when this is done, the Unk1
value can be determined because the number is within the range of the standards
used.
8.9. REGRESSION ANALYSIS
The calibration curves in Figures 8.9 and 8.10 were prepared using Excel (xy
scatter chart), which has automatic features for adding a trend line (the
straight line), calculating an equation for the line, and calculating r 2 . The r 2 is
particularly useful in that it tells how close the line obtained is to a straight
line. If the data produced an absolutely straight line, the r 2 would be 1. When