181
Interferences
This test is sensitive to small amounts of interference. Glassware
must be very clean. Repeating the test with the same glassware is
recommended to ensure that results are accurate.
The following substances interfere to the extent shown:
SPADNS 2 Reagent contains enough non-toxic reducing agent to
eliminate interference up to 5 mg/L chlorine. For higher chlorine
levels, dilute sample with deionized water by a factor that will
lower chlorine concentration to below 5 mg/L. Perform the
procedure, and multiply results by this factor to obtain
mg/L Fluoride.
To check for interferences from aluminum, read the concentration
one minute after reagent addition, then again after 15 minutes.
An appreciable increase in concentration suggests aluminum
interference. Waiting two hours before making the final reading
will eliminate the effect of up to 3.0 mg/L aluminum.
Most interferences can be eliminated by distilling the sample
from an acid solution as described below:
a) Set up the distillation apparatus for the general purpose
distillation. See the Hach Distillation Apparatus Manual.
Turn on the water and make certain it is flowing through
the condenser.
b) Measure 100 mL of sample into the distillation flask.
Add a magnetic stirring bar and turn on the heater power
switch.
Turn the stir control to 5.
c) Cautiously measure 150 mL of StillVer Distillation
Solution (2:1 Sulfuric Acid) into the flask. If high levels
of chloride are present, add 5 mg silver sulfate for each
Substance Concentration Error
Alkalinity (as CaCO
3
) 5000 mg/L -0.1 mg/L F
-
Aluminum 0.1 mg/L -0.1 mg/L F
-
Chloride 7000 mg/L +0.1 mg/L F
-
Iron, ferric 10 mg/L -0.1 mg/L F
-
Phosphate, ortho 16 mg/L +0.1 mg/L F
-
Sodium Hexametaphosphate 1.0 mg/L +0.1 mg/L F
-
Sulfate 200 mg/L +0.1 mg/L F
-
FLUORIDE, continued
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