Preparing A Calibration Curve; %T Versus Concentration Calibration - Hach DR2400 Manual

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3.5 Preparing a Calibration Curve

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Use sensitivity when comparing different methods. For example, Hach has three

DR/2400 methods for determining iron:

Iron Analysis

Portion of Curve

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

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.

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

2. Select the proper wavelength. Standardize (zero) the instrument using an

untreated water sample or a reagent blank, whichever the procedure instructs

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.

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 (Table 9).

Entire range

Entire range

Entire range

Chemical Analysis

∆Concentration

Chemical Analysis