omized. Low results are the consequ

omized. Low results are the consequence. An example
is the decrease in calcium absorbance that is observed
with increasing concentrations of sulfate or phosphate.
For example, at a fixed calcium concentration, the absorbance
falls off nearly linearly with increasing sulfate
or phosphate concentrations until the anion-to-calcium
ratio is about 0.5; the absorbance then levels off at
about 30% to 50% of its original value and becomes independent
of anion concentration.
Examples of cation interference have also been
recognized. Thus, aluminum is found to cause low
results in the determination of magnesium, apparently
as a result of the formation of a heat-stable aluminum/
magnesium compound (perhaps an oxide).
Interferences due to formation of species of low
volatility can often be eliminated or moderated by use of
higher temperatures. Alternatively, releasing agents,
which are cations that react preferentially with the interferant
and prevent its interaction with the analyte, can be employed.
For example, addition of an excess of strontium or
lanthanum ion minimizes the interference of phosphate in
the determination of calcium. The same two species have
also been employed as releasing agents for the determination
of magnesium in the presence of aluminum. In both in-
stances, the s1!rontium or lanthanum replaces the analyte in
the compound formed with the interfering species.
Protective agents prevent interference by forming
stable but volatile species with the analyte. Three common
reagents for this purpose are EDTA, 8-hydroxyquinoline,
and APDC, which is the ammonium salt
of I-pyrrolidinecarbodithioic acid. The presence of
EDTA has been shown to eliminate the interference of
aluminum, silicon, phosphate, and sulfate in the determination
of calcium. Similarly, 8-hydroxyquinoline
suppresses the interference of aluminum in the determination
of calcium and magnesium.