Hach DR/850 Procedures Manual page 121

CHLORINE, FREE
Accuracy Check
continued
A common error in testing for chlorine is introduced when a
representative sample is not obtained. If sampling from a tap, let
the water flow for at least 5 minutes to ensure a representative
sample. Let the container overflow with the sample several times,
then cap the sample container so there is no headspace (air) above
the sample. If sampling with a sample cell, rinse the cell several
times with the sample, then carefully fill to the 10-mL mark.
Perform the analysis immediately.
Standard Additions Method (using powder pillows)
a) Snap the top off a LR Chlorine PourRite Ampule
Standard Solution.
b) Use a TenSette Pipet to add 0.1 mL of the standard to the
reacted sample (this is the spiked sample). Swirl to mix.
c) Re-zero the instrument using the original sample (the
blank).
d) Place the spiked sample in the cell holder and press
Record the results.
e) Calculate the concentration of mg/L chlorine added to the
sample:
mg/L Chlorine added
f) The spiked sample result (step d) should reflect the
analyzed sample result + the calculated mg/L Cl
(step e).
g) If this increase does not occur, see Standard Additions in
Section 1 for more information.
Standard Additions Method (using AccuVac Ampuls)
a) Snap the top off a LR Chlorine PourRite Ampule
Standard Solution.
b) Use a graduated cylinder to measure 25 mL of sample into
each of two beakers.
c) Use a TenSette Pipet to add 0.2 mL of the standard to one
of the beakers (this is the spiked sample). Swirl to mix.
d) Fill a DPD Free Chlorine AccuVac completely from each
121
( ) Label value (mg/L Cl
0.1 vol. standard added
= -----------------------------------------------------------------------------------------------------------------------------
( standard volume )
10.1 sample +
.
READ
× )
2
added
2