b. Interference: Negative errors are caused by both fluoride and polyphosphates. When the
fluoride concentration is constant, the percentage error decreases with increasing amounts of aluminum. Because the fluoride concentration often is known or can be determined readily,
fairly accurate results can be obtained by adding the known amount of fluoride to a set of
standards. A simpler correction can be determined from the family of curves in Figure
3500-Al:1. A procedure is given for the removal of complex phosphate interference.
Orthophosphate in concentrations under 10 mg/L does not interfere. The interference caused by
even small amounts of alkalinity is removed by acidifying the sample just beyond the
neutralization point of methyl orange. Sulfate does not interfere up to a concentration of 2000
mg/L.