Hach DR2400 - 3.5 Preparing a Calibration Curve

Chemical Analysis
Chemical Analysis
5940022ChemAnal.fm Page 45
Use sensitivity when comparing different methods. For example, Hach has three
DR/2400 methods for determining iron:
Notice that the FerroZine method has the greatest sensitivity of the three
methods because it will measure the smallest change in concentration. The
technical definition of sensitivity comes from a calibration curve with Abs on the
x-axis and concentration on the y-axis.
1. If the calibration is a line, the sensitivity is the slope of the line multiplied
by 0.010.
2. If the calibration is a curve, the sensitivity is the slope of the tangent line to the
curve at the concentration of interest multiplied by 0.010.
The sensitivity value is also used as an estimate of the lower limit of the test. The
value may be used as a starting point in determining MDL.
3.5 Preparing a Calibration Curve
1. Prepare five or more standards of known concentration that cover the
expected range of the test. Run tests as described in the procedure on each
prepared standard. Then pour the customary volume of each known
solution into a separate clean sample cell of the type specified for your
instrument.
2. Select the proper wavelength. Standardize (zero) the instrument using an
untreated water sample or a reagent blank, whichever the procedure instructs
you to use.
3. Measure and record the absorbance of the known solutions within the time
constraints detailed in the procedure. To use absorbance vs. concentration, see
Section %T Versus Concentration Calibration.
3.5.1 %T Versus Concentration Calibration
If measuring %T, use semilogarithmic graph paper and plot %T (vertical scale)
versus concentration (horizontal scale). For Figure 12, iron standard solutions of
0.1, 0.2, 0.4, 0.8, 1.2, 1.6 and 2.0 mg/L were measured on a Spectronic 20 at 500
nm using half-inch test tubes. Results were plotted and the calibration table
values were extrapolated from the curve (Tab l e 9).
Iron Analysis
Method
Portion of Curve ∆Abs ∆Concentration
FerroVer Entire range 0.010 0.022 mg/L
FerroZine Entire range 0.010 0.009 mg/L
TPTZ Entire range 0.010 0.012 mg/L