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ADAPTING PROCEDURES
ADAPTING HACH PROCEDURES FOR USE WITH OTHER
PHOTOMETERS
Hach test procedures can be used with other instrumentation if
calibration curves are established to convert test results from %
transmittance or absorbance to the concentration of the constituent being
measured. Regardless of the instrument used, the sample and
standardizing solutions are prepared the same way and the optimum
wavelength specified in these procedures applies to testing with other
spectrophotometers. In the example below, a sample calibration for iron
concentrations of 0 to 2.4 mg/L is described. A series of iron standard
solutions are prepared and measured to establish the calibration curve.
The readings are plotted on semilogarithmic paper as % transmittance
vs. concentration (or absorbance vs. concentration on linear-linear
paper). Points on the graph shown (Figure 3) are connected with a
smooth curve and the curve is used to make the calibration table if
desired. The procedure follows:
1. Prepare several known concentrations with values covering the
expected range. At least five standards are recommended. Run tests on
25-mL samples as described in the procedure. Then pour the customary
volume of each known solution into separate, clean sample cells of the
type specified for your instrument.
2. Select the proper wavelength and standardize the instrument using
untreated sample water or a reagent blank as specified by the test
procedure.
3. Measure each of the known solutions and plot the % transmittance
readings on semilogarithmic graph paper as % transmittance vs.
concentration. Plot the % transmittance values on the logarithmic
(vertical) scale and the concentration values on the linear (horizontal)
scale. In the following example, iron standard solutions of 0.1, 0.2, 0.4,
0.8, 1.2, 1.6 and 2 mg/L were measured on a Spectronic 20
Spectrophotometer at 500 nm. Half-inch test tubes were used. Results
were plotted as shown on the graph (Figure 3) and the calibration table
values (Table 3) were extrapolated from the curve.
To convert transmittance readings to mg/L iron, use Table 3 and select
the appropriate line from the "% T Tens" column and the appropriate
column from the "% T Units" group of columns. For example, if the
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