Thomas Graham.

Elements of chemistry, including the applications of the science in the arts online

. (page 43 of 60)
Online LibraryThomas GrahamElements of chemistry, including the applications of the science in the arts → online text (page 43 of 60)
Font size
QR-code for this ebook

tion are, to give a yellow precipitate with nitrate of silver, to give a
granular crystalline precipitate with ammonia and sulphate of mag-
nesia the phosphate of magnesia and ammonia, to yield the common
phosphate of soda when neutralized with carbonate of soda, to
form salts which have invariably 3 eq. of base to 1 of phosphoric
acid, and to be unalterable by boiling its solution or keeping it for
any length of time. The class of salts which this hydrate forms are
the old phosphates, which have long been known, and it is convenient
to allow them to be particularly distinguished as the phosphates or
the common phosphates.

Deuto-hydrate of phosphoric acid, or bilasic phosphate of
water, 2HO + P0 5 . Dr. Clark first discovered that when the phos-
phate of soda is heated to redness, it is completely changed, and after
being dissolved in water affords crystals of a new salt, which he
named the pyrophosphate of soda, an observation which led to inte-
resting results'*. If a solution of this salt, which it is not necessary
to crystallize, be precipitated by acetate of lead, the insoluble salt of
lead washed and decomposed by hydrosulphuric acid, as before, an
acid liquor is obtained which contains the deuto-hydrate of phosphoric
acid. It must not be warmed to expel the excess of hydrosulphuric
acid, but be left in a shallow bason for twenty-four hours to permit
the escape of that gas. This acid, when neutralized with carbonate
of soda, gives Dr. Clark's pyrophosphate of soda. It also gives
a white precipitate with nitrate of silver ; all the salts which it forms
have uniformly two eq. of base. They were named the pijrophos-
phates, and since that term has come into use, it is not likely to be
superseded by the systematic, but rather inconvenient designation of
bibasic phosphates. A dilute solution of the deuto-hydrate of phos-
phoric acid may be preserved for a month without sensible change, but

* Edinburgh Journal of Science, vol. vii. p. 298, (1820) ; or Annales dc Chim. ct de
Phys. xli. 270.


when the solution is exposed for some time to a high temperature, it
passes entirely into the terhydrate.

Protohydrate of phosphoric acid. If the biphosphate of soda be
heated to redness, a salt is formed, which treated in a similar manner
with the last, gives an acid liquid, containing the protohydrate of
phosphoric acid. To prepare the biphosphate itself, a solution of the
terhydrate of phosphoric acid is added to a solution of common phos-
phate of soda, till it is found that a drop of the latter is no longer
precipitated by chloride of barium. The biphosphate of soda, which
is now in solution, can only be crystallized in cold weather. The
glacial phosphoric acid also is in general almost entirely the protohy-
drate. This hydrate is characterized by producing a white precipitate
in solution of albumen, which is not disturbed by the other hydrates,
and in solutions of the salts of earths and metallic oxides, pre-
cipitates which are remarkable semifluid bodies, or soft solids,
without crystallization. All these salts contain only one eq. of
base to one of acid, like the protohydrate of the acid itself. The
name metaphosphates was applied to the class by myself, to mark
the cause of the retention of peculiar properties by their acid, when
free and in solution ; namely, that it was not then simply phosphoric
acid, but phosphoric acid together with water.* This is the least
stable of the hydrates of phosphoric acid, being converted rapidly, by
the ebullition of its solution, into the terhydrate. If the terms me-
taphosphoric acid and pyrophosplwric acid are employed at all, it
is to be remembered that they are applicable to the proto and deuto-
hydrates, and not to the acid itself, which is the same in all the hy-
drates. But to prevent the chance of misconception, metaphosphate
of water and pyrophosphate of water might be substituted for the
former terms.

A solution of the terhydrate of phosphoric acid, evaporated in
vacuo over sulphuric acid, crystallizes in thin plates, which are ex-
tremely deliquescent. The deutohydrate has also been obtained in
crystals. When heated to 400, the terhydrate loses a portion of
water, and becomes a mixture of the deuto and protohydrates ; and
by heating it to redness for some time, the proportion of water may
be reduced to 1 equivalent, or perhaps even less than this ; and such
is the composition of glacial phosphoric acid. But at that high tem-
perature much of the hydrated phosphoric acid passes off in vapour.
The solution of phosphoric acid is not poisonous, nor when concen-

* Researches on the Arseniates, Phosphates, and Modifications of Phosphoric Acid'
Phil. Trans. 1833, p. 253 ; or Phil. Mag. 3rd series, vol. iv. p. 401.


trated does it act as a cautery, but it injures the teeth from its pro-
perty of dissolving phosphate of lime. The soluble phosphates,
which are not acid, give a precipitate with chloride of barium, which
is the phosphate of baryta. This phosphate, in common with all the
insoluble phosphates, is dissolved by nitric acid, hydrochloric acid,
and even acetic acid, a property by which it is distinguished from
sulphate of baryta. A solution of phosphate of lime in phosphoric
acid has been prescribed in rickets, a disease which indicates a defi-
ciency of earthy phosphates in the system. The phosphate of soda,
also, is given as a mild aperient ; its taste is saline, but not disa-
greeably bitter.

Phosphates. The formation of three classes of phosphates from
the three basic hydrates of phosphoric acid, affords an excellent illus-
tration of the formation of compounds by substitution ; the quantity
of fixed base, such as soda, with which phosphoric acid combines in
the humid way, being entirely regulated by the proportion of water
previously in union with the acid, which is simply replaced by the
fixed base. Thus, the protohydrate of phosphoric acid combines
with no more than one, and the deutohydrate with no more than two
equivalents of soda, although a larger quantity of alkali be added to
it. The excess of alkali remains free. Again, supposing an equiva-
lent quantity of the terhydrate of phosphoric acid in solution, and
one equivalent of soda added to it, one equivalent only of water is
displaced, and two retained, and a phosphate formed, containing one
of soda and two of water as bases; the salt already adverted to
under its old name of biphosphate of soda. Let a second equiva-
lent of soda be added to this salt, and a second basic equivalent of
water is displaced, and a tribasic salt produced, containing two
of soda and one of water as bases, which is the common phosphate of
soda of pharmacy. A third equivalent of soda added to the last salt
displaces the last remaining equivalent of basic water, and a tribasic
phosphate is formed, of whicli the whole three equivalents of base
a,re soda, and which has the name of subphosphate of soda. But
this last salt can unite with no more soda. The same three salts may
be formed by means of the tribasic phosphate of water, in another
manner. That acid hydrate decomposes chloride of sodium, but only
to a certain extent, expelling hydrochloric acid, so as to acquire one
of soda, and becoming 2HO.NaO + PO 5 , or the biphosphate of soda
already referred to ; the same acid hydrate applied to the carbonate or
the acetate of soda, can assume two proportions of soda, displacing


twice as much of the weaker carbonic and acetic acids, as of the hy-
drochloric acid, and so becomes H0.2NaO+P0 5 , or the common
phosphate of soda ; and the same acid hydrate applied to the hy-
drate of soda (caustic soda), assumes three of soda, and becomes
3 NaO + P0 5 , or the subphosphate of soda.

Prom soluble tribasic phosphates, such as those mentioned, inso-
luble salts may be precipitated, which are likewise tribasic, by adding
solutions of most metallic salts. Thus 1 equivalent of the common
phosphate of soda, added to the nitrate of silver in excess, decomposes
3 equivalents of it, and produces the yellow tribasic phosphate of
silver, as explained in the following diagram, in which the name of a
substance is understood to express one equivalent of it, and the
figures, numbers of equivalents :

Before decomposition. After decomposition.

-DI i, 4 (% Soda /2 Nitrate of soda

PhosphateS Water A Nitrate of water

(. Phosphoric acid

1 Nitrate (2 Nitric acid . .
3 Nitrate \ Nitric add f ^

(3 Oxide of silver . - -^Phosphate of silver

(Tribasic phosp. silv.)

Here, then, is exact mutual decomposition, but it is attended with a
phenomenon which does not occur when other neutral salts decom-
pose each other. The liquid does not remain neutral, but becomes
highly acid after precipitation ; the reason is, that one of the new pro-
ducts is the nitrate of water, or hydrated nitric acid ; and consequently
the products, although neutral in composition, are not neutral to test

The pyrophosphate of soda, which is bibasic, decomposes, on the
other hand, two proportions of nitrate of silver, and gives a pyrophos-
phate or bibasic phosphate of silver, which is a white precipitate; thus

Before decomposition. After decomposition.

Pyrophosphate r2 Soda - ^2 Nitrate of soda

of soda [ Phosphoric acid . -.^^
2 Nitrate of c 2 Nitric acid . . . < \.

silver 1 2 Oxide of silver _\,Pyrophos. of silv.

(Bibasic phos. sil.)

Here there is no salt of water among the products, and consequently
the liquid is neutral after precipitation.

The metaphosphate of soda, which is monobasic, like the sulphates,


nitrates, and other familiar salts, decomposes like these but one pro-
portion of nitrate of silver, and forms a white precipitate; thus

Before decomposition. After decomposition.

Metaphosph. f Soda - -^Nitrate of soda

of soda I Phosphoric acid^^^-"
Nitrate of f Mtric acid . .^

silver I Oxide of silver ^-Metaphosphate of silv.

(Monobasic phos. silv.)

If acetate or nitrate of lead be substituted for nitrate of silver in
these decompositions, a tribasic, bibasic, or monobasic salt of lead is
obtained in the same manner ; and these salts, again, decomposed by
hydrosulphuric acid gas, afford respectively the terhydrate, deutohy-
drate, and protohydrate of phosphoric acid. The statement of the
decomposition of the metaphosphate of lead by hydrosulphuric acid
will be sufficient to explain how a hydrate of phosphoric acid comes to
be formed in all these cases :

Before decomposition. After decomposition.

TVT L i, u ("Phosphoric acid 7 Metaphosph. of water

Metaphosph. \ Q ^ / (Protohydr. of phos. ac.)

oflead (Lead ^/

Hydrosulph. < Hydrogen . . ./

acid ^Sulphur . , . Sulphide of lead.

It will be observed that the hydrosulphuric acid forms 1 equivalent
of water, at the same time that it throws down the sulphide of lead. In
this phosphate of lead, there is only 1 equivalent of oxide of lead,
and consequently only 1 equivalent of water is formed; but if there were
or 3 equivalents of oxide, there would be 2 or 3 equivalents
of water formed and conveyed to the acid ; or the phosphoric acid is
always left in combination with as many equivalents of water as it
previously possessed of oxide of lead. Thus the different hydrates of
phosphoric acid are obtained from the decomposition of the corres-
ponding phosphates of lead.

In no decomposition of this kind is there any transition from one
class of phosphates into another, because the decompositions are
always mutual, and the products of a neutral character. Hence an
argument for retaining the trivial names, common phosphates, pyro-
phosphates, and metaphosphates, for there is no changing, in decom-
positions by the humid way, from one to the other, and the salts
comport themselves so far quite as if they had different acids. The
circumstances may now be noticed in which a transition from the one
class to the other does occur :


1st. Changes without the intervention of a high temperature.
When solutions of the metaphosphate and pyrophosphate of water are
warmed., they pass gradually into the state of common phosphate,
combining with an additional quantity of water ; and the metaphos-
phate of water appears then to become at once common phosphate,
without passing through the intermediate state of hydration of the
pyrophosphate. The metaphosphate of baryta also, which is an in-
soluble salt, is gradually dissolved in boiling water, and becomes
common phosphate by assuming 2 eq. of basic water. The easy
transition from the one class of phosphates to the other, then wit-
nessed, forbids the supposition that they contain different acids, or
different isomeric modifications of phosphoric acid. Indeed, it might
as well be supposed that in the protoxide and sexqui-oxide of iron,
the metal exists in different isomeric conditions, because these oxides
possess peculiar properties, and combine in different proportions with
the same acid. Iron in its two oxides gives rise to different com-
pounds, because they are formed by substitution ; and phosphoric
acid in its three hydrates gives rise to different compounds, from the
same cause. The degree of oxidation of the iron and the degree of
hydration of the acid are anterior conditions, due to the special unex-
plained affinities with which each element or compound is invested,
It is remarkable that pyrophosphates of potash and of ammonia exist
in solution, and perfectly stable, but not in the dry state. These
salts do not crystallize. The pyrophosphate of ammonia, indeed, when
allowed to evaporate spontaneously, appears to crystallize, but in the
act of becoming solid, it passes into common phosphate (the biphos-
phate of ammonia, 2H0.1N T H 4 0+P0 5 ).

2d. Changes with the intervention of a high temperature. If a
single equivalent of phosphoric acid, anhydrous, or in any state of
hydration, be calcined at a temperature which may fall short of a red
heat (1), with 1 equivalent of soda or its carbonate, the metaphos-
phate of soda will be formed ; (2) witli 2 equivalents of soda or its
carbonate, the pyrophosphate of soda will be formed ; and (3) with
3 equivalents of soda or its carbonate, a common phosphate of soda
will be formed. Hence, the formation of none of these classes is
peculiarly the effect of a high temperature. Again, a tribasic phos-
phate, containing one or two equivalents of a volatile base, such as
water or ammonia, loses the volatile base, when ignited, and the
acid remains in combination with the fixed base. Hence, common
phosphate of soda (H0.2~NaO + P0 5 ) is converted by heat into pyro-
phosphate (2NaO + P0 5 ,) the original observation of Dr. Clark; and


the biphosphate of soda (2HO.NaO + P0 5 ) into metapliosphate of
soda (NaO + P0 5 ). The acid remains in combination with the fixed
base, and the salt produced may be dissolved in water without as-
suming basic water.

The metapliosphate of soda is susceptible of a remarkable conver-
sion, by the agency of a certain temperature., and exhibits a change
of nature, without a change of composition, such as often occurs in
organic compounds, but rarely admits of so satisfactory an explana-
tion. This particular salt, in common with all the other phosphates,
combines with water, which becomes attached to the salt, in the state of
constitutional water, or water of crystallization. The metaphosphate
of soda, so hydrated, when dried at 212, retains 1 equivalent of water,
but that water is not basic, for, on dissolving the salt again, it is
found still to be a metaphosphate. But let this hydrated metaphos-
phate be heated to 300, and without losing anything, it changes
completely, and becomes a pyrophosphate, the water which was con-
stitutional before, being now basic. The formulae of the salt in its
two states exhibit to the eye the nature of the internal change which
occurs in it :

1. Hydrated metaphosphate of soda . NaO.P0 5 + HO,
2. Pyrophosphate of soda and water . NaO.HO + P0 5 .

Phosphates of the form 3MO + 2P0 5 . The recent investiga-
tions of Meitmann and Henneberg establish the existence of two
new classes of phosphates, intermediate between the monobasic and
bibasic classes. The soda-salt of the preceding formula is produced
by fusing together, in a platinum crucible, 100 parts of anhydrous
pyrophosphate of soda and 76.87 parts of metaphosphate of soda:
the white crystalline mass which results is reduced to powder, and
quickly exhausted with water ; for, on long digestion, the ordinary
phosphates are obtained. The soda-salt is soluble in about twice its
weight of cold water, and has a faint alkaline reaction. It gives,
by precipitation with nitrate of silver and with phosphate of magnesia,
salts corresponding with the soda-salt, and which have not the pro-
perties of a mixture of pyrophosphate and metaphosphate.

Phosphates of the form 6MO + 5P0 5 . The soda-salt was ob-
tained by fusing together 100 parts by weight of pyrophosphate of
soda and 307.5 of metaphosphate. The solution is by no means
stable, but gives, when freshly prepared, a precipitate in nitrate of
silver, which is readily soluble in excess of the soda-salt, and pos-


sesses the composition, when fused, of 6AgO + 5P0 5 . (Liebig's
Annalen, Ixv. 304.)

Modifications of metaphosphoric acid. The metaphosphates
already described are prepared from the monobasic phosphate of soda
in the vitreous condition ; this phosphate, when cooled immediately
from a state of fusion, remaining a transparent, colourless glass. But
if this glassy phosphate be cooled very slowly, a beautiful crystalline
mass is obtained. On dissolving it in a small quantity of hot water,
the liquid divides into two strata, the more considerable one containing
the crystalline salt, and the other a portion of unaltered metaphosphate
of soda. The vitreous metaphosphate, and all the salts derived
from it, are remarkable for not crystallizing, but form, liquid or semi-
liquid viscid hydrates. But the crystalline metaphosphate of soda
is described as giving beautiful crystals of the triclinometric system,
containing water of crystallization. Its solution is neutral, and has
a cooling, pure, saline taste, while the vitreous metaphosphate of soda
is insipid. It is rapidly converted into the acid common phosphate
by boiling. The corresponding silver-salt is obtained by adding
nitrate of .silver to a tolerably concentrated solution of the soda-
salt. It is white, crystalline, and is represented by the formula
3(AgO.P0 5 ) + 2HO.

Phosphates were obtained by Mr. Maddrell, by adding the solution
of sulphates of magnesia, nickel, copper, soda, lime, baryta, alumina,
to an excess of phosphoric acid, evaporating, to expel the sulphuric
acid, and heating to upwards of 600 ; in the form of a crystalline
granular substance, which were all monobasic. They are all anhy-
drous, insoluble in water and diluted acids, but generally decom-
posed by concentrated sulphuric acid, and appear to form a class of
metaphosphates different from the preceding two. The magnesian
metaphosphates of this class have a disposition to combine with the
corresponding soda- salt, when any of that base is present in the
phosphoric acid with which they are ignited. The double salt of
magnesia and soda is represented by 3(MgO.P0 5 ) +NaO.PO 5 ;
that of nickel and soda, by 6(NiO.P0 6 )+ NaO.P0 5 ). (Mem.
Chem. Soc. iii. 273.)

The only explanation which can be offered of these modifica-
tions of the metaphosphoric acid, is, that they are of a polymeric
character; such as MO.P0 5 ; 2M0.2PO 5 ; 3M0.3PO 5 , or perhaps
even higher multiples of MO.P0 5 . No data, however, appear to exist
by which a place in this polymeric series can be ascribed to the re-


spective modifications with any degree of certainty. MM. Eleitmann
and Henneberg, who have lately investigated the subject with much
ability, are disposed to represent metaphosphoric acid by 6M0.6P0 5 ;
and certainly with this proportion of base constant and the phosphoric
acid variable, the other classes may be consistently represented :

Common Phosphate - 6MO + 2P0 5

Pyrophosphate - 6MO + 3P0 5

Eleitmann and Henneberg's newj 6MO + 4P0 5
phosphates - -1 6MO + 5PO 5

Metaphosphate 6MO + 6PO 5 .

The different classes of phosphates are thus represented as all
sex-basic salts, with a different polymeric acid in each, P 2 10 , P 3 O 15 ,
&c. But this theory does not embrace the modifications of meta-
phosphoric acid, nor will it serve to represent several known double
phosphates; such, for instance, as the double pyrophosphate of
copper and soda, 3 (2NaO.PO 5 ) + 2CuO.P0 5 .

Analysis of phosphoric acid and of the phosphates. Phos-
phoric acid is produced when the pentachloride of phosphorus is
thrown into water :

PCl 5 and 5HO = PO 5 and 5HC1.

It may be inferred with certainty from this decomposition, that
phosphoric acid contains 5 equivalents of oxygen, in the same manner
as the composition of phosphorous acid is deduced from the decom-
position of the terchloride of phosphorus by water (page 438). The
affiinity of phosphoric acid for water is very intense, the anhydrous
phosphoric acid taking water even from oil of vitriol and eliminating
anhydrous sulphuric acid, at a high temperature. As hydrated phos-
phoric acid cannot be made anhydrous by heat, the proportion of dry
acid in a solution of the free acid is determined by adding a known
weight of oxide of lead, evaporating to dryness, and heating the resi-
due, as in the case of sulphuric acid. The phosphate of lead formed
being anhydrous, the increase of weight which the oxide of lead sus-
tains represents exactly the weight of dry phosphoric acid.

In determining the proportion of phosphoric acid in a salt of an
alkaline or earthy base, the acid, if not already in the tribasic form, is
first brought to that condition by boiling with a little nitric acid.
1. The excess of nitric acid being then neutralized by ammonia, the

2 G


phosphate is again dissolved iii acetic acid. If the solution contains
no sulphuric acid nor chlorine, the phosphoric acid may be entirely
separated by the addition of nitrate of lead, in the form of an inso-
luble phosphate of lead, 2PbO.HO.P0 5 , which washes easily, and
loses water and becomes pyrophosphate 2PbO.P0 5 , when calcined
(Heintz). This method is based upon the insolubility of phosphate
of lead in acetic acid. 2. Phosphoric acid may also be thrown down
from the solution of an alkaline phosphate, by adding first carbonate
or hydrochlorate of ammonia and then sulphate of magnesia, when,
upon stirring the phosphate of magnesia and ammonia,

2MgO.NH 4 O.P0 5 + 12HO,

falls as a granular precipitate. This phosphate must be precipitated
in an alkaline solution, and washed with water containing hydrochlo-
rate of ammonia, as it is very soluble in acids, and even soluble in a
sensible degree in pure water. When ignited it loses its volatile con-
stituents, and remains pyrophosphate of magnesia, 2MgO.P0 5 . 3.
The phosphoric acid not being in combination with a base which
yields a phosphate insoluble in acetic acid, an addition is made to the
liquid, which may be acid, of an excess of the acetate of the sesqui-
oxide of iron. The phosphate of sesqui-oxide of iron, Fe 2 3 .P0 5 ,
immediately separates as a slightly reddish yellow flaky precipitate,
which is collected and washed upon a filter. This phosphate is dis-
solved off the filter by a few drops of hydrochloric acid, then the salt
of iron reduced to the state of protoxide by boiling it with sulphite of
soda, and afterwards the quantity of iron ascertained by finding how
much of a solution of permanganate of potash of known composition
is required to peroxidize the iron. The phosphate of iron being of
known composition, the quantity of phosphoric acid is calculated
from the iron, 2 eqs. of that metal being present in the phosphate for
1 eq. of phosphoric acid or of phosphorus ; that is, 700 parts iron re-
presenting 900 parts phosphoric acid (Eaewsky and Marguerite).
The acetate of sesqui-oxide of iron, which is not permanent, is best
prepared extemporaneously from solutions of 100 parts of iron-alum
and of 98 parts of acetate of soda in equal quantities of water, of
which equal volumes are mixed at the moment the acetate of iron is


In describing the various classes of phosphates, with their rela-
tions to each other, I have been thus minute, partly because consi-
derable explanatory detail was required, from the extent of the

Online LibraryThomas GrahamElements of chemistry, including the applications of the science in the arts → online text (page 43 of 60)