In a long article on phosphorus, Edmond Willm wrote in 1876: "For a
century, urine was the only source from which phosphorus was obtained.
After Gahn, in 1769, recognized the presence of phosphoric acid in
bones, Scheele indicated the procedure for making phosphorus from
them."[19] Actually, Gahn used at first hartshorn (_Cornu cervi
ustum_), and Scheele doubted, until he checked it himself, that his
esteemed friend was right. A few years later, Scheele corrected Gahn's
assumption that the _sal microcosmicum_ was an ammonia salt; instead, it
is "a tertiary neutral salt, consisting of _alkali minerali fixo_ (i.e.,
sodium), _alkali volatili_, and _acido phosphori_."[20]
In the years after 1770, phosphorus was discovered in bones and many
other parts of various animals. Treatment with sulfuric acid decomposed
these materials into a solid residue and dissolved phosphoric acid. Many
salts of this acid were produced in crystalline form. Heat resistance
had been considered one of the outstanding characteristics of phosphoric
acid. Now, however, in the processes of drying and heating certain
phosphates, it became clear that three kinds of phosphoric acids could
be produced: _ortho_, _pyro_, and _meta_.
Berzelius cited these acids as examples of compounds which are ISOMERIC.
This word was intended to designate compounds which contain the same
number of atoms of the same elements but combined in different manners,
thereby explaining their different chemical properties and crystal
forms. It was in 1830 that Berzelius propounded this companion of the
concept, ISOMORPHISM, which was to collect all cases of equal crystal
form in compounds in which equal numbers of atoms of different elements
are put together in the same manner. Together, the two concepts of
isomerism and isomorphism seemed to cover all the known exceptions from
the simplest assumption as to specificity and chemical composition.
However, only a few years later Thomas Graham (1805-1869) proved that
the three phosphoric acids are not isomeric. He used the proportion of 2
P to 5 O in the oxide which Berzelius had thought justified at least
until "an example of the contrary could be sufficiently
established."[21] Refining the techniques of Gay-Lussac (1816) and
several other investigators, Graham characterized the three phosphoric
acids as "a terphosphate, a biphosphate, and phosphate of water."
Actually, this was the wrong terminology for what he meant and
formulated as trihydrate, bihydrate, and monohydrate of phosphorus
oxide. In his manner of writing the formulas, each dot over the symbol
for the element was to indicate an atom of oxygen; thus, he wrote:
... :: .. ... . .
H^{3} P H^{2} P and H P.[22]
[Illustration: Figure 5.--OVEN FOR THE CALCINATION OF BONES, about 1870.
"The operation is carried out in a rather high oven, such as shown....
The fresh bones are thrown in at the top of the oven, B. First, fuel in
chamber F is lighted, and a certain quantity of bones is burnt on the
grid D. When these bones are burning well, the oven is gradually filled
with bones, and the combustion maintains itself without addition of
other fuel. A circular gallery, C, surrounds the bottom of the oven and
carries the products of combustion into the chimney, H. The calcined
bones are taken out at the lower opening, G, by removing the bars of
grid B." (Translation of the description from FIGUIER, _Merveilles de
l'industrie_, volume 3, 1874, page 537.)]
[Illustration: Figure 6.--AN ADVERTISEMENT with view of plant for
manufacturing superphosphate about 1867. (From E. T. FREEDLEY,
_Philadelphia and its Manufacturers in 1867_, page 288.)]
Graham had come to this understanding of the phosphoric acids through
his previous studies of "Alcoates, definite compounds of Salts and
Alcohol analogous to the Hydrates" (1831). Liebig started from analogies
he saw with certain organic acids when he formulated the phosphoric
acids with a constant proportion of water (aq.) and varying proportions
of "phosphoric acid" (P) as follows:
2 P 3 aq. phosphoric acid
3 P 3 aq. pyrophosphoric acid
6 P 3 aq. metaphosphoric acid.
[Illustration: Figure 7.--FLORIDA HARD-ROCK PHOSPHATE MINING. (From
Carroll D. Wright, _The Phosphate Industry of the United States_, sixth
special report of the Commissioner of Labor, Government Printing Office,
Washington, 1893, plate facing page 43.)]
Salts are formed when a "basis," i.e., a metal oxide, replaces water.
When potassium-acid sulfate is neutralized by sodium base, the acid-salt
divides into Glauber's salt and potassium sulfate, which proves the
acid-salt to be a mixture of the neutral salt with its acid. Sodium-acid
phosphate behaves quite differently. After neutralization by a potassium
"base" (hydroxide), the salt does not split up; a uniform
sodium-potassium phosphate is obtained. Therefore, phosphoric acid is
truly three-basic![23]
This result has later been confirmed, but the analogy by means of which
it had been obtained was very weak, in certain parts quite wrong.
The acids from the two lower oxides of phosphorus were also considered
as three-basic. Adolphe Wurtz (1817-1884) formulated them in 1846,
according to the theory of chemical types:
(PO)···
O^{3} phosphoric acid
H^{3}
(PHO)··
O^{2} phosphorus acid
H^{2}
(PH^{2}O)·
O hypophosphorous acid.[24]
H
Further proof for these constitutions was sought in the study of the
esters formed when the acids react with alcohols.
Among the analogies and generalizations by which the research on
phosphoric acid was supported, and to the results of which it
contributed a full share, was the new theory of acids. Not oxygen,
Lavoisier's general acidifier, but reactive hydrogen determines the
character of acids. In this brief survey, it seems sufficient just to
mention this connection without describing it in detail.
The study of phosphoric acids led to important new concepts in
theoretical chemistry. The finding of polybasicity was extended to other
acids and formed the model that helped to recognize the
polyfunctionality in other compounds, like alcohols and amines. The
hydrogen theory of acids was fundamental for further advance. In another
dimension, it is particularly interesting to see that large-scale
applications followed almost immediately and directly from the new
theoretical insight. The first and foremost of these applications was in
agriculture.