Henry E. (Henry Enfield) Roscoe.

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and Siedentopf , Physikalische Zeitschr. , 1905, 6, 855.

5 Atti R. Accad. Lined, 1912 [v.], 21, i. 467.

Ber., 1906, 39, 2859 ; Paal and Kiihn, Ber., 1908, 41, 51.


Traces of iron render these solutions unstable, but in the
absence of iron fairly stable solutions, yielding about 30 per
cent, of available chlorine, can be prepared. When more con-
centrated solutions are cooled they deposit solid sodium
hypochlorite in fine needles belonging to the tetragonal system,
which contain 39'9 per cent, of NaCIO, 54'7 per cent, of water,
and small amounts of sodium chloride, chlorate, and hydrate.
These crystals are much more stable than the solutions from
which they crystallise. 1

Technical Preparation. Sodium hypochlorite in solution
was known about 1790, and in 1820 it was well known as
Eau de Labarraque, and subsequently as Eau de Javelle indis-
criminately with the potassium hypochlorite solution. The
solution was first made by passing chlorine into a soda solution,
but afterwards by decomposing bleaching powder by a soda
solution or by sodium sulphate ; these solutions have been made
in large quantities in South Lancashire and France for bleaching
purposes ; they contain 7 to 15 per cent, available chlorine.

Still weaker solutions are now prepared in large quantities
at a large number of paper and bleaching works by electrolysing
a solution of common salt under suitable conditions so that the
hypochlorite shall not be converted into chlorate, nor yet re-
duced by the escaping hydrogen to chloride. The solutions are
therefore made only weak, seldom exceeding 1 per cent, available
chlorine, but as they are manufactured where used, this is no
disadvantage. The first practical process was that of Hermite
in 1884, since when numerous variations have been introduced ;
a full account of these has been given by Engelhardt, 2 and the
theoretical considerations underlying the process have been
described by Abel. 3 The solutions frequently contain hypo-
chlorous acid. Probably 10,000 horse-power is used in the various

The crystals prepared by Muspratt and Smith in their
first experiments melted at about 20 C. and then rapidly
decomposed, so that there was little prospect of making this
material technically useful; but in 1903 Muspratt succeeded in
drying the crystals in a vacuum, and obtained a powder melting

1 Muspratt and Shrapnell-Smith, J. Soc. Chem. Ind., 1898, 17, 1096; 1899,
18, 210.

2 Hypochlorite und elektrische Bleiche, 1903. See also Allmand, Applifl
Electrochemistry, 1912, p. 318; and Kershaw, J. Soc. Chew. Jn</.,

31, 54.

y Ifyiwhlorit, nml Mtr'whv Hleiche, 1905.


about 45 C. and containing from 40 to 60 per cent, available

Sodium Chlorate, NaClO 3 . This salt is obtained, according to
Muspratt and Eschelmann's process, by saturating milk of
magnesia with chlorine and adding sodium carbonate to the re-
sulting solution of magnesium chloride and chlorate. The liquor,
after filtering from precipitated carbonate, is boiled down, the
sodium chloride fished out as it separates, and the sodium
chlorate purified by recrystallisation. In Pechiney's process the
solution of calcium chlorate and chloride obtained by passing
chlorine into hot milk of lime is evaporated down and as much
as possible of the calcium chloride removed by crystallisation,
the liquid being then decomposed with salt-cake :

CaCl a + Ca(C10 3 ) 2 + 2Na 2 SO 4 = 2CaS0 4 + 2NaCl + 2NaC10 3 .

According to the process of Best l it is manufactured also by
acting with chlorine on a solution of sodium carbonate or
bicarbonate at temperatures not exceeding 35, the reaction
being probably :

3Na 2 C0 3 + 3C1 2 = NaC10 3 + 5NaCl + 3CO


It is also prepared by the electrolysis of the chloride in a similar
manner to the potassium salt.

Sodium chlorate is dimorphous, crystallising in combinations
of the cube and tetrahedron showing also tetartohedral faces
(Class 28, p. 201), or in rhombohedral forms of the hexagonal
system. It has a sp. gr. of 2*29, and becomes damp when kept
exposed to air. It is much more soluble in water than the
potassium salt, 100 parts of water dissolving according to
Kremers :

At 20 40 60 80 100 120

Parts of NaC10 3 81'9 99 123'5 1471 175'6 232'6 333'3.

It dissolves readily also in warm alcohol.

Sodium chlorate melts at 302 (Carnelley), 248 (Retgers),
and is then for the most part converted into sodium chloride
and perchlorate, with evolution of only a small quantity of
oxygen (Schlossing). It is used chiefly in the manufacture
of aniline black, its greater solubility making it more suitable
for this purpose than the potassium salt.

1 J. Soc. Chem. Ind., 1895, 14,865. See also Grossman n, /. Soc. Chem.
Ind., 1896, 15, 158.


Sodium Perchlorate, NaClO 4 , is prepared by heating the
chlorate or by neutralising perchloric acid with soda. It is
very soluble in water, from which it crystallises at the ordinary
temperature in long pointed plates containing one molecule of
water, and at 50 in anhydrous rectangular prisms. 1 It occurs
in small quantities in Chili saltpetre.


138 Sodium Sulphides. These compounds are prepared in a
similar manner to the corresponding potassium salts (p. 337),
which they closely resemble. The monosulphide, Na 2 S, crys-
tallises with 9H 2 O at ordinary temperatures ; hydrates with
6H 2 O and with 5JH 2 are formed from solutions above 48. 2 The
hydrosulphide, NaHS, crystallises with 2H 2 and 3H 2 O, and
is also obtained as an anhydrous powder by heating the sulphide
at 310 in a current of hydrogen sulphide, 3 The only crystalline
polysulphide which has been obtained has the formula
Na 4 S 9 ,14H 2 O, although there is evidence that lower poly-
sulphides are formed by the direct union of sodium or sodium
sulphide with sulphur. 4

The sulphides Na 2 S and Na 2 S 5 have been obtained also by
the action of sulphur on a solution of sodium in liquid ammonia,
and the selenides, Na 2 Se, Na 2 Se 4 , and tellurides, Na 2 Te, Na 2 Te 3 ,
are formed in a similar manner. 5 Sodium sulphide is prepared
on the large scale, and used in the manufacture of soluble

Sodium Hyposulphite, Na 2 S,O 4 . This salt, the formation and
preparation of which have already been discussed (Vol. I.,
p. 448), 6 is used by the dyer and calico printer as a reducing
agent for indigo, and is also employed in the laboratory for tin-
purpose of estimating free oxygen or the quantity of that
element contained in substances from which it is readily

1 Potilitzin, J. RUM. Phys. Chem. Soc., 1889, i. 258.

2 Parravanoand Fornaini, Atti. R. Arcad. Lined, 1907 (v.), 16, ii. 464.

3 German Patent, 194881.

4 Bloxam, Jonrn. Chem. Soc., 1900, 67, 761 ; Locke and Austell, Amu:
Chem. J. t 1898, 20, 592; Blanksma, Proc K. Akad. Wetensch. Am*tcrd-uH,

6 Hugot, Compt. rend., 1899, 129, 299, 388.

6 See also />'////. Nor. ///. M,,!!,,.,,*-, 1904,74, 348; far., 1905,38, 104S :
.lellinek, Zcli. /</. Chem., 1911, 70, 93.


evolved. It has been obtained also by the direct action of
sulphur dioxide on sodium hydride l according to the equation :

2NaH + 2S0 2 = Na 2 S 2 O 4 + H 2 .

According to Binz 2 and to Reinking, Dehnel, and Labhardt, 3



x SNa
this salt has the constitutional formula 0( , which re-


presents it as an anhydro-salt of sulphurous acid, H'SO 2 'OH,
and of the hypothetical sulphoxylic acid, H'SO'OH. This
constitution is confirmed by its behaviour to organic reagents. 4

Normal Sodium Sulphite, Na 2 S0 3 . The anhydrous salt is
obtained as a crystalline precipitate by heating a cold saturated
solution of the hydrated salt, and remains unaltered in the air.
The hydrate Na 2 S0 3 ,7H 2 O is prepared by saturating a
solution of a known quantity of sodium carbonate with sulphur
dioxide, and then adding the same quantity of sodium carbonate.
The crystals which are deposited are transparent monoclinic
prisms, which lose the whole of their water at 150. The
anhydrous salt crystallises in short hexagonal prisms. 5 The
solution has an alkaline reaction and sharp taste. Crystals
of the hydrate Na 2 SO 3 ,10H 2 O are obtained, according to
Muspratt, by allowing the aqueous solution to evaporate over
sulphuric acid.

The solubility of the anhydrous salt is almost independent of
the temperature ; 100 parts of water dissolve 28'01 parts at 37
and 28-26 parts at 84. The solubility of the hydrate
Na 2 SO 3 ,7H 2 O is as follows : 100 parts of water dissolve 5

At 2 10-6 18-2 29 37 '2

Parts of Na 2 S0 3 14'82 20-01 25'31 34'99 44'08

The transition temperature of the two modifications is
about 22. 5

Sodium sulphite is used as an "antichlor" to remove any

1 Moissan, Compt. rend., 1902, 135, 647.

2 Ze.it. Farb. u. Textil. Chem., 4, 161. 3 Ber., 1905, 38, 1075.

4 Btr., 1906, 39, 3317.

5 Hartley and Barrett, Journ. Chem. Soc., 1909", 95, 1178.



excess of chlorine from fabrics which have been bleached with
chloride of lime, and sometimes also as an antiseptic.

Sodium Hydrogen Sulphite, NaHSO.. If a solution of sodium
carbonate is saturated with sulphur dioxide whilst cold, this
salt separates out in cloudy crystals, and alcohol precipitates it
from aqueous solution as a white powder. It has an acid
reaction, smells of sulphurous acid, and has an unpleasant
sulphurous taste. A saturated solution of this salt is a com-
mercial article, and is used by brewers for sterilising their
casks, etc.

Sodium Bisulphite or Sodium Metabisulphite, Na 2 S 2 5 , or
Na'S0 2 'OS0 2 'Na, is obtained by supersaturating a soda solu-
tion with sulphur dioxide, and may be prepared on the large
scale by passing sulphur dioxide into dry monohydrated sodium
carbonate, Na^OgjH.jO. 1 It is a white crystalline soluble
salt, which gradually evolves sulphur dioxide in the air and is
converted into sodium sulphate.

139 Normal Sodium Sulphate, Na 2 SO 4 . This compound is
commonly known in the anhydrous state by the commercial
name of Salt-cake, whilst the hydrate, Na 2 SO 4 ,10H 2 O, is
called Glauber's salt. We find the first mention of this salt
in Glauber's work, De naturd salitim, published in 1658. He
obtained it as the residue left in the preparation of hydro-
chloric acid by the action of oil of vitriol upon common salt,
and believed this simple aperient to be possessed of most
valuable medicinal properties, whence it came to be known as
Sal mirabile Glauberi.

The salt occurs native in the anhydrous condition as thenar-
dite, and in solution in sea- water and in the water of salt-lakes.
as well as in large quantities in certain mineral springs. Thus
the water at Friedrichshall contains large quantities of the salt,
which since 1767 has been obtained by evaporation and used
in medicine as Sal aperitivum Fridericianum. A native com-
pound of sodium sulphate with calcium sulphate, Na 2 SO 4 ,CaSO 4 ,
termed glauberite, is also found in several localities.

Sodium sulphate is prepared on an enormous scale as salt-
cake, the first step in the manufacture of carbonate of soda by
the Leblanc process, about 360,000 tons of salt-cake being
annually produced at present (see p. 288). For this purpose
common salt is decomposed either by sulphuric acid or by the

1 Carey and Hurter, Patent 4512, Nov. 1882 ; J. Soc. Chtm. Intl., 1883, 2,


combined action of sulphur dioxide, air, and aqueous vapour.
The details of these processes will be described under the head
of the alkali manufacture.

Sulphate of sodium is also obtained as a residue in many
chemical operations, especially in the preparation of nitric acid
from Chili saltpetre in the sulphuric acid manufacture.

If ordinary Glauber's salt is allowed to remain exposed to the
air. or more quickly if heated, the anhydrous salt is obtained ;
and if a solution of Glauber's salt saturated at about 35 is
slightly heated, rhombic crystals of the anhydrous salt separate
out. These are identical in form with crystals of thenardite,
and isomorphous with those of silver sulphate, Ag 2 SO 4 . It has
a specific gravity of 2'655, melts at 883 , 1 possesses a saline
bitter taste, has a neutral reaction, and does not dissolve in
alcohol. When heated on charcoal before the blowpipe in the
reducing flame, sodium sulphide is formed.

One hundred parts of water dissolve the following quantities
of the salt calculated as anhydrous sodium sulphate (Mulder 2 ) .
At 10 20 30 34 40

Na 2 S0 4 5-02 9'00 1940 40'00 55-00 48'8

At 50 60' 70 80 s 90 100 103'5

Na 2 SO 4 467 45'3 44'4 437 431 42'5 42'2

The saturated solution boils at 103'5 (Mulder), 101'9 (Berkeley).
The abnormal solubility of this salt has already been discussed
(p. 113).

Hydrated Sodium Sulphate. The decahydrate, Na 2 S0 4 ,10H 2 0,
commonly known as Glauber's salt, crystallises from aqueous
solution at the ordinary temperature in large colourless mono-
clinic prisms, which are isomorphous with chromate and
selenate of sodium. These crystals effloresce on exposure to dry
air, melt in their water of crystallisation at 32*48, and lose the
whole of it below 100.

The Hepta-hydrated Salt, Na 2 S0 4 ,7H 2 0, is deposited in
hard, clear, rhombic crystals when a supersaturated solution of
the decahydrate is allowed to cool below 12, or when such a
solution is covered with a layer of warm alcohol of specific
gravity 0'835. 3

1 Nacken, Chem. Centr., 1908, i. 1850.

2 Compare Berkeley, Phil. Trans., 1904, A. 203, 209.

3 Compare Hartley, Jones and Hutchinson, Journ. Chem. Soc., 1908, 93,
825 ; Gernez, Compt. rend., 1909, 149, 77 ; Smits and Wuite, Proc. K. Akad.
Wetensch. Amsterdam, 1909, 12, 244.



Hydrogen, Sulphate, NaHSO 4 . This salt, commonly
known as bisulphate of soda, is obtained in large triclinie
prisms when equivalent quantities of sodium sulphate and sul-
phuric acid are dissolved in water and the solution evaporated
at a temperature above 50 . Like the corresponding potassium
salt it is decomposed by alcohol at once into sulphuric acid and
the normal salt. A hydrated salt, NaHSO 4 ,H 2 O, is also
known, as well as a salt of the formula Na 3 H(SO 4 ) 2 , which crys-
tallises in lustrous needles. 1

Sodium Disulphate, Na 2 S 2 O 7 , is formed by heating sulphur
trioxide together with common salt (Rosenstiel) :

2NaCl + 3S0 3 = Na 2 S 2 O 7 + SO 2 C1 2 .

The same compound is formed by the gentle ignition of sodium
hydrogen sulphate. When more strongly heated, it yields sulphur
trioxide and normal sulphate.

Sodium Thiosulphate, Na 2 S 2 0. i . This salt, discovered in
1799 by Chaussier, and still commonly known by its old
name of hyposulphite of soda, is prepared on the large scale for
use as an antichlor in the paper manufacture, and as a solvent
for the unaltered silver bromide in photography. It is obtained
by boiling sulphur with soda-lye, and passing sulphur dioxide
into the yellow solution until it is colourless, or by boiling
sodium sulphite with sulphur. A cheaper process is to decom-
pose the soluble calcium thiosulphate obtained by the oxidation
of alkali waste, either by means of sodium carbonate or sodium
sulphate. The solution of sodium thiosulphate is drawn oft'
from the carbonate or sulphate of calcium and evaporated down
in iron pans. Anhydrous sodium thiosulphate is obtained by
passing oxygen or air over anhydrous sodium sulphide at
100-150. 2

Sodium thiosulphate forms large transparent prisms belong-
ing to the monoclinic system, which contain 5H 2 O. The salt is
odourless, possesses a cooling taste, and does not exhibit an
alkaline reaction, nor does it undergo alteration in the air.
The crystals melt in their own water at 48*5, and when heated
to 215 all the water is driven off, whilst at 220 they decom-
pose with separation of sulphur (Pape). The salt has a specific
gravity of 1'672, is very soluble in water, in which it readily
forms supersaturated solutions, but does not dissolve in alcohol.

Sodium thiosulphate forms a large number of hydrates, and the

1 D'Ans, Ber., 1906, 39, ir>:U. ' Cennan Patent,


equilibrium curve of this substance and water is very complex. 1
The ordinary pentahydrate is stable in contact with water at
all temperatures up to 48'2 ; from this point to 65 the
dihydrate is stable, and above 70 the anhydrous salt. Isomeric
hydrates with 5H 9 O and 2H 2 also exist as well as hydrates with
6H 2 O, 4H 2 O, 1JH 2 0, 4/3 H 2 O, 1H 2 O (3 isomerides), and iH 2 O.

The aqueous solution cannot be preserved for any length of
time without decomposition, as it very slowly deposits sulphur
and is partially converted into sulphite. Sodium thiosulphate

is represented by the formula 2 SO 2 -j g-^. When treated

with sodium amalgam it yields sodium sulphite and sodium
sulphide :

Na 2 S 2 3 + 2Na = Na 2 S0 3 + Na 2 S.

It is acted upon by iodine in aqueous solution at the ordinary
temperature with formation of sodium tetrathionate :

2Na 2 S 2 3 + 1 2 = 2NaI + Na 2 S 4 O 6 ,

and is largely employed in volumetric analysis in association
with the estimation of iodine. Ferric salts also convert it into
tetrathionate. 3


140 Sodamide, NaNH 2 . This compound, discovered by Gay-
Lussac and Thenard, is prepared by passing dry ammonia gas
over sodium heated to 300-400 in an iron retort, and forms a
waxy mass of crystalline structure, which is white when pure
but frequently has a greenish or olive-brown colour. 4 It softens
at 149 and melts at 155, and at a temperature of 500-600
slowly decomposes into its elements, but does not, as stated by
Gay-Lussac and Thenard, 5 yield a nitride of the formula Na 3 N.
When heated in a current of carbon dioxide sodamide glows with
formation of caustic soda and cyanamide, and on warming

1 Taylor, Proc. Roy. Soc., Edin., 1898, 22, 248; Young and Mitchell,
/. Amer. Chem. Soc., 1904, 26, 1389, 1413; Young and Burke, J. Amer.
Chem. Soc., 1906, 28, 315 ; Dawson and Jackson, Journ. Chem. Soc., 1907,
91, 552; Jones, ibid., 1909, 95, 1672.

2 Compare Vol. I. p. 455 ; also Price and Twiss, Journ. Chem. Soc., 1907,
91, 2024.

3 Hewitt and Mann, Journ, Chem. Soc., 1913, 103, 324.

4 Titherley, Journ. Chem. Soc., 1894, 65, 504.

5 See, however, Zehnder, Ann. Phys. Chem., 1894, 52, 56.


with nitrous oxide it is converted into sodium azoimide (Vol. I.,
p. 519).

Sodium Azoimide, NaN 3 , forms hexagonal crystals and is
readily soluble in water. It can be kept molten for some hours
without decomposition. 1

Sodium Nitrite, NaN0 2 , is formed when the nitrate is
heated, and is usually manufactured either by the electrical
oxidation of atmospheric nitrogen (Vol. I., p. 547), or by heat-
ing sodium nitrate with lead, or with charcoal or sulphur in
presence of caustic soda. 2 To obtain the pure salt, silver nitrite
is treated with the equivalent amount of a solution of sodium
chloride, or nitrous fumes from nitric acid and starch or arsenious
acidare passed into a solution of caustic soda or sodium carbonate. 3
The salt is faintly yellow, melts at 271, and yields a yellow-
solution in water, which is faintly alkaline to litmus (Divers), but
according to Boguski the pure dry salt is colourless. 4 It crystal-
lises in oblique four-sided prisms or transparent rhombohedra,
and is somewhat hygroscopic ; at 15, 100 parts of water dissolve
83*3 of the salt. It is largely used in the preparation of the
coal-tar colours.

141 Sodium Nitrate, NaNO 3 . This salt, commonly known as
cubic saltpetre or Chili saltpetre, is of special historical interest,
as it was by the examination of differences in crystalline form
exhibited by this compound and ordinary nitre that the distinc-
tion between the alkalis potash and soda was first observed by
Bohn in 1683. Boyle, somewhat later, noticed that cubic salt-
petre was formed in the preparation of aqua regia from common
salt and nitric acid, and Stahl first pointed out the distinct
character of the alkali-basis of common salt, and fully described
the preparation of cubic saltpetre.

Sodium nitrate occurs in nature, as a wide-spread deposit known
as " Caliche " in the rainless districts of Tarapaca, Peru,
Northern Chili, and Bolivia. 5 In these beds it is associated with
common salt, gypsum, sodium sulphate, and smaller quantities <f
sodium iodate, chlorate and perchlorate, the crude material con-
taining from 27 to 65 per cent, of the pure salt. This is
purified by solution and crystallisation. A-fter refining, the salt

1 Curtius and Rissom, J. pr. Chem., 1898 (2), 58, 261 ; Dennis and Benedict,
Ze.it. anorg. Chem., 1898, 17, 18.

2 See Morgan, /. Soc. Chem. 2nd., 1908, 27, 483.

3 Divers, Journ. Chem. Soc., 1899, 75, 86.

4 J. Puss. Phys. Chem. Soc., 1899, 31, 543.
8 See Bryce's South America, pp. 42, 208.



FIG. 83.

contains about 97'7 per cent, of pure nitrate, 1 '84 of sodium
chloride, 0'35 of sodium sulphate, and Oil of water. 1

The best mode of separating the last 2 per cent, of
sodium chloride is to add to the boiling and saturated solution
one-tenth of its weight of nitric acid, stir it until cool, and
collect the precipitated nitrate, which
rnay then be washed by a dilute acid
and afterwards dried.

Sodium nitrate crystallises in obtuse
rhombohedra, 2 whose interfacial ter-
minal edge angle is 106 36' (Fig. 83),
and is therefore isogonous with calc-

The specific gravity of the salt is

2-26. It melts without decomposition at 316-318 (Carnelley),
and when ignited undergoes decomposition with evolution of
oxygen, nitrogen, and nitrous fumes. It is very soluble in
water and deliquesces when exposed to moist air. One hundred
parts of water dissolve : 3

At 10 20 40 60 80 100 119

Parts 73 80-5 88 104'9 124'6 148 175'5 208'8

The solution containing 58*5 of the salt to 100 of water
solidifies at 18*5, and the saturated solution boils at 119.

This salt dissolves also in dilute alcohol. One hundred parts
of spirit, containing 61'4 per cent, of alcohol, dissolve at 26
21*2 parts of sodium nitrate. It does not deflagrate so
violently as nitre with charcoal or other inflammable bodies,
but it has sometimes been used for making blasting and other
powders which are not required to fire quickly.

Sodium nitrate is used in considerable quantities for the
manufacture of nitric acid and also of sulphuric acid, but much
the largest quantity is employed as a manure, since it readily
yields up its nitrogen to plants growing in soil where it is
present. This salt and ammonium sulphate are by far the
most important sources of nitrogenous manure. In 1860 the
output of the crude nitrate from South America was over 60,000

1 Compare Newton, J. Soc. Chem. Ind., 1900, 19, 408.

2 Senarmont, Oompt. rend., 1854, 38, 105; Barlow and Pope, Journ. Chem.
Soc., 1908, 93, 1538.

3 Berkeley, Phil Trans., 1904, A. 203, 211 ; Miers and Isaac, Journ.
Chem. Soc., ' 1906, 89, 428.

264 S( )DIUM

tons, and has greatly increased since that date, being about
764,000 tons in 1888, and 2,500,000 tons in 1912.


142 Sodium Phosphide, Na 3 P. Sodium dissolved in liquid
ammonia reacts with red phosphorus to form a red compound,
Na 3 H 3 P 2 , which loses phosphine when heated and is decomposed
by water and acids with evolution of phosphine. 1 Phosphine
also reacts with sodium in presence of liquid ammonia forming
the compound, NaH 2 P, which is converted by heat into the
phosphide, Na 3 P. This is decomposed by water yielding
phosphine. 2 An unstable phosphide, Na 2 P 5 , is formed when
sodium and phosphorus are heated together in a vacuum at
450 for several days. 3

Sodium Hypophosphite, NaH 2 P0 2 ,H 2 O. This salt is ob-
tained by adding sodium carbonate to a solution of calcium
hypophosphite, and allowing the solution to evaporate in a
vacuum. Pearly tabular crystals are deposited, which deliquesce
on exposure to the air, and are readily soluble in absolute alcohol.
It is now employed in medicine for the same purposes as

Sodium Orthophosphates. As orthophosphoric is a tribasic
acid, three sodium salts exist in which one, two, or three atoms
of the hydrogen in the acid are replaced by metal :

(1) Trisodium or normal sodium orthophosphate, Na 3 PO 4 ,
12H 2 0.

(2) Hydrogen disodium orthophosphate, Na 2 HPO 4 ,12H 2 0.

(3) Dihydrogen sodium orthophosphate, NaH 2 PO 4 ,4H 2 O.
These all give yellow precipitates of trisilver phosphate, Ag 3 PO 4 ,
when their solutions are brought into contact with silver
nitrate. (Vol. I., p. 662.)

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