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Most of colorimetric methods have only one timing step. As an example, in the DPD
free chlorine method, it takes three minutes for the DPD powder reagent to completely
react with chlorine in the water sample. The DPD free chlorine method has a single
three-minute timing step. Figure 10 shows the main page of a method with a single
timing step.
Figure 14 Single Timer Method
5.4 Single-Vial Procedure
1. Place the sample vial filled with the water sample in the SP-910 sample vial
compartment and press the ZERO button. The SP-910 will display the page shown
in Figure 14.
2. Take the sample vial out and add the reagent to the sample vial.
3. Place the sample vial back into the sample vial compartment and press the timer
button TMR1. The SP-910 will start to monitor the reaction between the reagent
and the species you want to measure in the water sample. The concentration is
shown in the chart as a function of time (Figure15).
4. When the timer reaches the preset time and the reaction is complete, the value of
concentration will be shown on the top of the page.
5. The rate of the reaction is often faster than the standard pre-set time, which will
become apparent from the concentration-time plot. You can press the STOP
button to stop the timer and terminate the timing step. The last read concentration
value will be displayed on the top of the page after you terminate the timing step.
Figure 15 Concentration as a Function of Time
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