Page 160 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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402 Elemental Analysis, Organic Compounds

to use the yellow color produced by the addition of am-
monium vanadate and ammonium molybdate to the phos-
phate solution. If the amount of phosphate in the digestion
mixture is below the milligram level, the blue color pro-
duced by the addition of ammonium molybdate followed
by a reducing agent is recommended. Because the colored
solution contains complexes of more than one species, the

FIGURE 10 Series of three distillation apparatuses for the de-
termination of fluorine. [Reprinted with permission from Ma, T. S.,
and Rittner, R. C. (1979). “Modern Organic Elemental Analysis.”
Dekker, New York. Copyright 1979 Dekker.]

spectrophotometry based on the formation of colored
complexes such as those produced by reaction with
zirconium-SPANS [sodium-2-(p-sulfophenylazo)-1, 8-
dihydroxynaphthalene-3,6-disulfonate], cerium alizarin,
and zirconium erochrome cyanine. The potentiometric
method with the fluoride ion-selective electrode can also
be used.

VII. DETERMINATION OF ARSENIC
AND PHOSPHORUS

Organic compounds containing phosphorus or arsenic are
decomposed by heating (digestion) with a mixture of nitric
and sulfuric acids, which forms phosphate and arsenate
ions, respectively:

HNO 3 ,H 2 SO 4
Organic phosphorus compound −−−−−−→ PO 3− (27)
4
and

HNO 3 ,H 2 SO 4
Organic arsenic compound −−−−−−→ AsO . (28)
3−
4
FIGURE 11 Distilling head for steam distillation from a solution at
After digestion, the phosphate obtained is usually de-
a constant temperature above 100 C. [Reprinted with Permission
◦
termined colorimetrically. When the phosphorus content from Ma, T. S., and Gwirtsman, J. (1957). Anal. Chem. 29, 141.
of the organic substance is higher than 5%, it is preferable Copyright 1957 Analytical Chemistry.]