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Interferences
Interfering Substance
Acidity
Alkalinity
Fluoride
Iron
Phosphate
Polyphosphate
Aluminum_PP_Other_ALN_Eng_Ody.fm
Greater than 300 mg/L as CaCO
treated as follows:
a) Add one drop of m-Nitrophenol Indicator Solution (Cat. No. 2476-32) to the sample taken in
step 3.
b) Add one drop of 5.0 N Sodium Hydroxide Standard Solution (Cat. No. 2450-26). Stopper the
cylinder. Invert to mix. Repeat as often as necessary until the color changes from colorless to
yellow.
c) Add one drop of 5.25 N Sulfuric Acid Standard Solution (Cat. No. 2449-32) to change the
solution from yellow back to colorless. Continue with the test.
1000 mg/L as CaCO
. Interferences from higher alkalinity concentrations can be eliminated by
3
the following pretreatment:
a) Add one drop of m-Nitrophenol Indicator Solution (Cat. No. 2476-32) to the sample taken in
step 2. A yellow color indicates excessive alkalinity.
b) Add one drop of 5.25 N Sulfuric Acid Standard Solution (Cat. No. 2449-32). Stopper the
cylinder. Invert to mix. If the yellow color persists, repeat until the sample becomes colorless.
Continue with the test.
Interferes at all levels. See Figure 1 below.
Greater than 20 mg/L
Greater than 50 mg/L
Polyphosphate interferes at all levels by causing negative errors and must not be present. Before
running the test, polyphosphate must be converted to orthophosphate by acid hydrolysis as
described under the phosphorus procedures.
Fluoride interferes at all levels by complexing with aluminum. The actual
aluminum concentration can be determined using the Fluoride Interference
Graph when the fluoride concentration is known.
To use the fluoride interference graph:
1. Select the vertical grid line along the top of the graph that represents the
aluminum reading obtained in step 14.
• Locate the point on the line where it intersects with the horizontal grid line
that indicates how much fluoride is present in the sample.
• Extrapolate the true aluminum concentration by following the curved lines
on either side of the intersect point down to the true aluminum
concentration.
For example, if the aluminum test result was 0.7 mg/L Al and the fluoride
present in the sample was 1 mg/L F
–
with the 1 mg/L F
grid line falls between the 1.2 and 1.3 mg/L Al curves. In this
case, the true aluminum content would be 1.27 mg/L.
Interference Levels and Treatments
. Samples with greater than 300 mg/L acidity as CaCO
3
–
, the point where the 0.7 grid line intersects
Aluminum
must be
3
Aluminum
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