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5.5 Two-Vial Methods
Some colorimetric methods require using two vials. The water sample is added to two
identical vials. One vial is being used to zero the colorimeter, referred as to the
prepared blank. A reagent is added to the other vial, referred as to the prepared
sample. The absorbance value is determined from the prepared sample.
If the method requires two or more reagents, the prepared blank could be the resulting
solution after one or more reagents have been added to the sample.
The following procedure is typical for two-vial methods:
1. Place the prepared blank into the SP-910 sample vial compartment and press the
ZERO button to zero the instrument.
2. Place the prepared sample into the SP-910 sample vial compartment and press
the TMR1 button to start the method timer.
3. When the timing step is completed, the measured concentration will be displayed
on the top of the page. The timing step could be terminated earlier by pressing
STOP button.
4. Optionally, The SP-910 can be re-zeroed using the prepared blank after the timing
step is completed or terminated. The blank reading will be subtracted from the
measured concentration value, and the displayed concentration value on the top-
right corner will be updated. This step is optional. It is only necessary if the
prepared blank changes its color during the timing period.
5. Optionally, the prepared sample vial can be put back and read again by pressing
the READ button if the blank is re-zeroed after the timing step is completed or
terminated. A new concentration value based on the last absorbance value
measured will be calculated and displayed.
5.6 Multiple Timing Steps Method
Some colorimetric methods have two or three timing steps. The SP-910 shows a
count-down timer for the timing steps before the last timing step (Figure 16). During
these timing steps, one or more reagents are added to the sample, or operations such
as shaking the vial to mix the reagent and the sample are being performed. These
methods usually use one vial for the prepared blank and the other for the prepared
sample.
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