75
1. Select the vertical grid line along the top of the graph that represents
the aluminum reading obtained in Step 15.
2. Locate the point of the vertical line (instrument reading) where it
intersects with the horizontal grid line that indicates how much
fluoride is present in the sample.
3. 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
3+
and the fluoride
present in the sample was 1.0 mg/L F
–
, the point where the 0.7 grid line
intersects with the 1.0 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.
Fluoride Interference Graph
Summary of Method
Aluminon indicator combines with aluminum in the sample to form a
red-orange color. The intensity of color is proportional to the aluminum
concentration. Ascorbic acid is added to remove iron interference. The
AluVer 3 Aluminum Reagent, packaged in powder form shows
exceptional stability and is applicable for fresh water samples.
mg/L Al
3+
(Instrument Reading)
True Aluminum concentration (mg/L Al
3+
)
mg/L F
–
ALUMINUM, continued
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