Hach DR/850, Procedures Manual, Page: 129 / 460

129
Do not use the same sample cells for free and total chlorine. If
trace iodide from the total chlorine reagent is carried over into the
free chlorine determination, monochloramine will interfere. It is
best to use separate, dedicated sample cells for free and total
chlorine determinations.
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 containers 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 chlorine analysis immediately.
Accuracy Check
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 into the cell holder and press
READ
. Record the results.
e) Calculate the concentration of mg/L chlorine added to the
sample:
f) The spiked sample result (step d) should reflect the
analyzed sample the calculated mg/L Cl
2
added
(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
mg/L chlorine added
0.1 (vol. standard added)
i Label
value (mg/L Cl
2
)
×
10.1 sample standard volume +
( )
----------------------------------------------------------------------------------------------------------------------------- =
CHLORINE, TOTAL,
continued