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gen:

H 2 SO 3 + 6 H = 3 H 2 O + H 2 S.

In nature hydrogen sulphide occurs in sulphur springs, vol-
canic gases, and wherever organic matter is decomposing, as in
sewer gas, in the intestinal gases, and in some pathological
cases in urine.

Hydrogen sulphide is a colorless gas which is 1.19 times as
heavy as air. It boils at 62 and melts at 86. The gas
has a very disagreeable odor, being that of rotten eggs, in
which it is contained. Hydrogen sulphide is a very poisonous
gas and overcomes persons and animals suddenly, in which respect
it resembles hydrocyanic acid. Inhaled in small amounts,
hydrogen sulphide produces headache and at times vomiting.
The gas is combustible, burning with a blue flame to water
and sulphur dioxide :

2 H 2 S + 3 O 2 = 2 H 2 O + 2 SO 2 .

In an insufficient amount of oxygen, the products are, in part,
water and sulphur :

2 H 2 S + 2 = 2 H 2 + 2 S.

In water, hydrogen sulphide is but slightly soluble, about
3 volumes being absorbed by 1 volume of water at ordinary
temperature and pressure. On boiling this solution, all the
gas escapes. On standing exposed to the air, the gas in the



196 OUTLINES OF CHEMISTRY

solution is gradually oxidized to water and sulphur which
separates out in the form of a precipitate.

When chlorine, bromine, or iodine act on hydrogen sulphide,
the latter is decomposed, sulphur being liberated and hydro-
halogen being formed, so for instance :

H 2 S + I 2 = 2 HI + S.

The aqueous solution of hydrogen sulphide is feebly acid
toward litmus, and in many ways it deports itself like a weak
acid. So it will react with metals even at room temperature,
forming sulphides and hydrogen, thus :

2 Ag + H 2 S = Ag 2 S + H 2 .
Pb + H 2 S = PbS + H 2 .

Furthermore it reacts with many basic oxides and hydroxides,
thus :

PbO + H 2 S = PbS + H 2 0.

2 NH 4 OH + H 2 S = (NH 4 ) 2 S + 2 H 2 O.

KOH + H 2 S = KSH + H 2 O.
2 KOH + H 2 S = K 2 S + 2 H 2 O.

The sulphides of sodium and potassium show a strong alkaline
reaction toward indicators. They are salts of a very weak
acid with a strong base, and hence are decomposed by water
by hydrolysis. The reaction, which is reversible, may be
written thus :



When passed through a red-hot tube, hydrogen sulphide is
decomposed to hydrogen and sulphur. It thus parts readily
with its hydrogen, and is consequently a good reducing agent,
as is evident, for instance, from the fact that it will reduce
sulphuric or nitric acid, thus :

H 2 SO 4 + H 2 S = 2 H 2 O + SO 2 + S.
2 HN0 3 + 3 H 2 S = 4 H 2 O + 2 NO + 3 S.

Hydrogen sulphide is a very important reagent in chemical
analysis, for while the sulphides which it forms with metals
like sodium, potassium, calcium, and magnesium are soluble in
water, other sulphides like those of iron, zinc, and nickel are
not soluble in water, but soluble in dilute acids, and still other
sulphides like those of arsenic, copper, and lead are insoluble



SULPHUR, SELENIUM, AND TELLURIUM 197

both in water and dilute acids. A very careful study of these
and similar properties of the sulphides of the metals has led to
a system by means of which the metals can be detected and
separated when they occur together.

Polysulphides and Hydrogen Persulphide. When sulphur
is added to a solution of sulphide of potassium, sodium, calcium,
ammonium, etc., it dissolves, forming polysulphides. Thus,
with K 2 S sulphur may form compounds varying in composition
from K 2 S to K 2 S 5 according to the amount of sulphur dissolved.
When such a persulphide is gradually added to a very dilute
solution of hydrochloric acid, a thick, yellow oil of disagree-
able odor separates out which has the composition H 2 S 5 , no
matter what the sulphur content of the poly sulphide was,
thus:

2 K 2 S 3 + 4 HC1 = 4 KC1 + H 2 S + H 2 S 6 .
4 Na 2 S 2 + 8 HC1 = 8 NaCl + 3 H 2 S + H 2 S 6 .

Hydrogen persulphide bleaches organic dyestuffs. It reacts
with iodine, forming hydriodic acid and sulphur. It gradually
decomposes into hydrogen sulphide and sulphur on standing.

Comparison of Hydrogen Sulphide with Water. It is evident
that hydrogen sulphide and water possess many points of
analogy. Thus the one is H-S-H and the other H-O-H. With
the univalent metals they form hydrosulphides MSH and
hydroxides MOH, respectively; furthermore, the corresponding
sulphides M 2 S, and oxides M 2 O, are also known. With ele-
ments of higher valence, analogous sulphides and oxides are
formed. Thus we have FeS and FeO, P 2 O 6 and P 2 S 6 , Sb 2 O g
and Sb 2 S 3 , etc. Again, just as oxygen and hydrogen form a
peroxide H 2 O 2 , so sulphur and hydrogen form a persulphide,
which, to be sure, has the composition H 2 S 5 . We shall later
see further points of resemblance between oxygen and sulphur
in their chemical behavior. The two elements indeed belong
to the same family group.

Compounds of Sulphur with the Halogens. Fluorine unites
directly with sulphur to form sulphur hexafluoride SF 6 , which
consists of white crystals that melt at 55. The substance
boils but slightly above its melting point. The gas is colorless,
odorless, tasteless, and practically as indifferent toward othei
reagents as nitrogen.



198 OUTLINES OF CHEMISTRY

When dry chlorine is passed over molten sulphur in a tubu-
lated retort, sulphur monochloride S 2 C1 2 , boiling at 138, is
formed. It is a fuming yellowish red liquid of suffocating
odor. Its specific gravity is 1.7. It dissolves sulphur readily^
solutions containing over 60 per cent sulphur being obtainable.
For this reason sulphur monochloride is used in preparing
vulcanized rubber. Water decomposes sulphur monochloride,
thus :

2 S 2 C1 2 + 2 H 2 O = 4 HC1 + SO 2 + 3 S.

Sulphur dichloride SC1 2 is formed when sulphur monochlo-
ride is saturated with chlorine in the cold. It is an oil of reddish
brown color and specific gravity 1.6. It readily decomposes at
64, yielding sulphur and sulphur monochloride. It is also de-
composed by water, thus:

2 SC1 2 + 2 H 2 O = 4 HC1 + SO 2 + S.

Sulphur fetrachloride SC1 4 is formed by saturating sulphur
dichloride with chlorine at temperatures below 25. The
substance forms crystals which melt at 30. It readily
dissociates above 22, the decomposition being practically
complete at +6. With water it reacts violently, thus:

SC1 4 + 2 H 2 = S0 2 + 4 HC1.

With bromine, sulphur forms sulphur monobromide S 2 Br 2 , a
brownish red liquid which congeals at 46 and boils at about
200, accompanied by partial decomposition.

With iodine, sulphur forms sulphur monoiodide S 2 I 2 , consist-
ing of dark grayish crystals melting at 60, and also sulphur
hexaiodide SI 6 , which forms dark crystals that readily decom-
pose on standing, yielding free iodine.

Sulphur Dioxide and Sulphurous Acid. When sulphur is
burned in the air or in oxygen, the following reaction takes
place :

S + 2 = S0 2 .

The resulting sulphur dioxide occupies the same volume as the
oxygen, which may be demonstrated by means of the apparatus
of Victor Meyer shown in Fig. 77. The sulphur is burned in
oxygen, with which the flask has been filled. On cooling, the
manometer indicates that the volume of the gas in the appara-
tus has not changed.



SULPHUR, SELENIUM, AND TELLURIUM



199



Sulphur dioxide is a colorless gas of suffocating odor. It is
2.21 times heavier than air. It may readily be condensed to a

liquid at ordinary pressure by
cooling to 10. Under a pres-
sure of about two atmospheres
it may be liquefied at room tem-
peratures. The liquid boils at

8, and the solid melts at

76. Sulphur dioxide will-
not support combustion ; never-
theless, at higher temperatures
many metallic oxides unite
vigorously with it with evolu-
tion of light, thus :

Pb0 2 + S0 2 = PbS0 4 .




FIG. 77.



Besides being produced by
the burning of sulphur, sulphur
dioxide is formed by heating sulphides of certain metals in the
air ; thus, pyrite acts as follows :

2 FeS 2 + 11 O = Fe 2 O 3 + 4 SO 2 .

In the laboratory, sulphur dioxide is commonly made by
heating copper turnings with concentrated sulphuric acid :



2 H 2 SO 4



Cu = 2 H 2 O



CuSO



SO 2 .



It may also be formed by heating concentrated sulphuric acid
with carbon or sulphur :

2 H 2 S0 4 + C = 2 H 2 + C0 2 + 2 SO 2 .
2 H 2 S0 4 + S = 2 H 2 + 3 SO 2 .

When dilute sulphuric acid acts on sulphites, sulphur dioxide
is formed ; also when metallic oxides are heated with sulphur:



NaHSO 3 + H 2 SO 4 = NaHSO 4 + SO 2
2 MnO 2 + 48=2 MnS + 2 SO 2 .



H 2 O.



2 CuO + 2 S = Cu 2 S + SO 2 .

In the presence of water, sulphur dioxide bleaches many organic
coloring matters. Figure 78 shows the bleaching of flowers by
sulphur dioxide evolved by burning sulphur. This bleaching



OUTLINES OF CHEMISTRY



does not depend upon the oxidation of the dyes, but rathei
upon their union with the sulphur dioxide, for on warming
some of the articles thus bleached their color may be restored.
In other cases, the bleaching action depends upon the subtrac-
tion of oxygen from the sub-
stances. Sulphur dioxide is used
to bleach silk, wool, straw, and
other fibers that would be de-
stroyed by means of chlorine. It
is also used as an antiseptic and
disinfectant, for it is a powerful
germicide. For these purposes it
may now be obtained in liquid
form in tin cans.

About 50 volumes of sulphur
dioxide are dissolved by 1 volume
of water at 15, while at 40 but
18.8 volumes are thus absorbed.
From the solution all of the sul- FlG - 78 '

phur dioxide may be expelled by boiling. The solution reacts
acid and behaves as though it contained sulphurous acid H 2 SO 3 ,
but this substance has never been isolated, thus :




SO



H 2 O = H 2 SO 3 .



With bases, sulphurous acid forms salts called sulphites, thus : -



H 2 SO 3



NaOH = NaHSO



H 2 O.



H 2 SO 3 + 2 NaOH = Na 2 SO 3 + 2 H 2 O.
H 2 SO 3 + Ca(OH) 2 = CaSO 3 + 2 H 2 O.

Sulphurous acid is dibasic in character. Both the acid and
the normal sulphites of the alkali metals are soluble in water,
but other normal sulphites are sparingly soluble. From sul-
phites, sulphur dioxide may readily be regenerated by addition
of sulphuric or hydrochloric acid. This fact is used in the
detection of sulphites in chemical analysis.

Sulphur dioxide is a reducing agent, which property is pos-
sessed in a still greater degree by its aqueous solutions. This
is because sulphurous acid is able to take up additional oxy-
gen readily, thus passing over into sulphuric acid. Even the



SULPHUR, SELENIUM, AND TELLURIUM 201

oxygen from the air slowly converts sulphurous acid into sul-
phuric acid in solution, thus :

2H 2 SO 3 +O 2 =2H 2 SO 4 .

Chlorine, bromine, or iodine rapidly change sulphurous acid
into sulphuric acid, thus :

H 2 SO 3 + H 2 O + C1 2 = H 2 SO 4 + 2 HCL
H 2 S0 3 + H 2 + I 2 = H 2 S0 4 + 2 HI.

Sulphur Sesquioxide. This compound has the composition
S 2 O 3 . It may be prepared by treating molten sulphur trioxide
SO 3 with pulverized sulphur. The product consists of bluish
green crystals. With fuming sulphuric acid it forms a blue
solution. Water decomposes the sesquioxide into sulphuric
acid and sulphur.

Sulphur Trioxide and the Contact Process of making Sulphuric
Acid. Sulphur trioxide SO 3 is formed by heating sulphates
of many of the heavy metals, thus :

Fe 2 (S0 4 ) 8 = Fe 2 3 +3S0 3 .

Oxygen unites but very slowly with SO 2 to form SO 3 , in spite
of the fact that the union is accompanied with considerable
evolution of heat. However, when a mixture of sulphur diox-
ide and oxygen is passed over finely divided platinum, the
union readily takes place, the action being practically complete
at 450. In this process, the platinum remains unchanged. It
acts as a contact or catalytic agent. In place of finely divided
platinum, ferric oxide or chromic oxide will also serve. The
residues of the oxides obtained by roasting pyrites are some-
times used for this purpose. The sulphur dioxide obtained by
burning sulphur or roasting native sulphides, generally pyrites,
is mixed with air in such proportion that there is present a
large excess of oxygen beyond what is needed to produce sul-
phur trioxide according to the equation :

2.S0 2 +0 2 ^2S0 3 ;

for this, reaction is a reversible one and the presence of the
excess of oxygen, according to the law of mass action, displaces
the equilibrium toward the right. The temperature should be
held at about 400 to 450, for at higher temperatures the sul-
phur trioxide dissociates, that is, the action reverses. The

CALIFORNIA COLLE6E

of PHARMACY



202



OUTLINES OF CHEMISTRY



gases should be purified. It is especially necessary that they
be freed from dust and from arsenic. The latter is generally
present in the gases and is removed by means of steam. Both
the residues from roasting pyrites, and platinized asbestus are
used at present in thus preparing sulphur trioxide by what is
known as the "contact process." The bulk of this sulphur
trioxide formed is used in making sulphuric acid, and to this
end it is absorbed in sulphuric acid of 97 to 98 per cent
strength. The strength of the acid is regulated by addition
of water. Enormous quantities of sulphuric acid are now pre-
pared annually by the contact process, both in Europe and




FIG. 79.

America; and this method, the success of which on a commer-
cial scale is due to the labors of Knietsch (1901), has to a large
extent displaced the lead chamber process for making sulphuric
acid, at least so far as making concentrated sulphuric acid is
concerned. On a small scale, in the laboratory, sulphur trioxide
can readily be made by means of the apparatus shown in Fig. 79.
Sulphur dioxide from a generator and oxygen from a tank
are passed into the wash-bottle JB; the mixed gases then pass
through the drying tube T, filled with pumice soaked in sul-
phuric acid, and finally enter the tube containing the asbestus,
which contains finely divided platinum heated to 400. The
SO 3 formed is condensed in the receiver.

Sulphur trioxide is also formed by heating fuming sulphuric



SULPHUR, SELENIUM, AND TELLURIUM 203

acid or warming concentrated sulphuric acid with phosphorus
pentoxide, or by heating sodium or potassium pyrosulphate,
thus :

H 2 S 2 O 7 = H 2 SO 4 + SO 3 .
H 2 S0 4 + P 2 5 = S0 3 + 2 HP0 3 .
K 2 S 2 7 = K 2 S0 4 4-S0 3 .

Sulphur trioxide forms long, colorless, prismatic crystals that
melt at 14.8, forming a colorless, mobile liquid that boils at
46. At 20 the specific gravity is 1.97. Below 27 sulphur
trioxide forms sulphur hexoxide S 2 O 6 , the crystals of which
look like long-fiber asbestus and melt at 50. On further heat-
ing, it passes over into vapors that are identical with those of
SO 3 , i.e. it dissociates into SO 3 , which on cooling yields a liquid
boiling at 46. Sulphur trioxide has great affinity for water.
It fumes strongly in the air, and unites with water with great
avidity and liberation of much heat which forms steam, causing
a hissing noise as the substance is brought into contact with
water. It is dangerous to bring large quantities of sulphur
trioxide into contact with water at once, for the heat liberated
causes explosions. At temperatures above 600 sulphur triox-
ide dissociates into sulphur dioxide and oxygen, the reaction
being practically complete at 1000.

Sulphuric Acid and the Lead Chamber Process. Sulphuric
acid H 2 SO 4 has been known for a long time. The alche-
mists prepared it by heating ferrous sulphate, green vitriol
FeSO 4 7 H 2 O, hence the name oil of vitriol. This process was
described by Basil Valentine in 1450, who also prepared the
acid by burning sulphur in presence of saltpeter. In 1746
Roebuck, in England, made use of the principle of the latter,
method by burning sulphur mixed with saltpeter in closed
leaden chambers in presence of moisture which absorbed the
gases, forming sulphuric acid. By admitting more air to the
chambers, and burning more sulphur in them, additional sul-
phuric acid was formed, and so on. This process was the
beginning of what is to the present day known as the lead
chamber process of the manufacture of sulphuric acid. In its
essence the method consists of oxidizing sulphurous acid
H 2 SO 3 to sulphuric acid H 2 SO 4 , by means of nitric a< id and
its decomposition products.



204 OUTLINES OF CHEMISTRY

In practice, the manufacture of sulphuric acid by the lead
chamber process involves: (1) The burning of sulphur to sul-
phur dioxide, either by using sulphur or commonly by roasting
native sulphides like pyrite FeS 2 , copper pyrite, CuFeS 2 , gale-
nite PbS, zinc blende ZnS ; (2) the oxidation of the sulphur
dioxide in presence of water by means of nitric acid and nitro-
gen dioxide, one of its decomposition products ; (3) the oxi-
dation of the nitric oxide NO formed by the reduction of the
nitric acid and NO 2 ; and (4) the concentration of the sul-
phuric acid obtained. In the roasting of the native sulphides
mentioned, the latter are heated in a current of air, whereby
sulphur dioxide and the oxides of the metals result. The nitric
oxide is oxidized to NO 2 by means of oxygen of the air. We
may write the chemical changes involved as follows :

(1) S + 2 =S0 2 .

(2) 3 SO 2 + 2 H 2 O + 2 HNO 3 = 3 H 2 SO 4 + 2 NO.

(3) 2 NO + H 2 + 3 O = 2 HNO 3 , and

(4) NO + O = NO 2 .

(5) S0 2 + H 2 + N0 2 = H 2 S0 4 + NO.

Thus it will be seen that when nitric acid acts on sulphur diox-
ide in presence of moisture (equation 2), sulphuric acid and
nitric oxide result. The latter is then oxidized by oxygen
from the air, in part to nitric acid (equation 3), and in part
to nitrogen dioxide (equation 4). The nitric acid so formed
then reacts with more sulphur dioxide, according to equa-
tion (2), and the nitrogen dioxide oxidizes sulphurous acid
according to equation (5), the nitric oxide NO formed in both
cases being again oxidized by oxygen, and. in turn reduced by
sulphurous acid with concomitant formation of sulphuric acid,
and so on.

While the above equations may be used to represent what
occurs in the manufacture of sulphuric acid, the actual process
is no doubt of more complicated character. It has been studied
by various investigators, among whom George Lunge holds that
a compound HO -SO 2 -O(NO), nitrosyl sulphuric acid, is formed
in the chambers during the process, and that this compound is
then decomposed by water with resulting formation of sulphuric



SULPHUR, SELENIUM, AND TELLURIUM 205

acid HO SO 2 OH. The reactions involved in this explanation

are :

(1) S0 2 + HN0 8 = HO - S0 2 0(NO).

(2)



The nitrosyl sulphuric acid is then again decomposed by water.,
according to equation (2), and so on. In nitrosyl sulphuric
acid we have the univalent -N=O group, which takes the
place of one of the hydrogen atoms in sulphuric acid. Now,
in the ordinary manufacture of sulphuric acid, when things
are running properly, the formation of nitrosyl sulphuric acid,
which consists of colorless crystals known as "chamber crys
tals," is not observed. It is only when the supply of water is
deficient that these crystals are actually formed, for they are
decomposed by water, as stated above. Although there is dif-
ference of opinion as to what actually occurs in the details of
the sulphuric acid manufacture, the changes in which process
are undoubtedly rather. complicated, it nevertheless is certain
that by this process sulphurous acid is completely and economi-
cally converted into the end product, sulphuric acid. The oxides
of nitrogen can be used over and over again, though of course
there is always some loss of the latter that must be replenished.
The accompanying Fig. 80 shows in diagrammatic form the
essentials of a lead chamber sulphuric acid factory. In the
furnaces F, the pyrites and other native sulphides are roasted in
a current of air. The sulphur dioxide thus produced contains
dust carried along mechanically, which deposits in a special
long dust flue in which the gas is also mixed with air in proper
proportion. The gases, which are at a temperature of about
300, then pass into the Glover tower 6r. This is a structure
about 10 meters high and 3 meters in diameter, lined inside with
sheet lead and filled with acid proof stones, over which dilute
sulphuric acid containing oxides of nitrogen in solution contin-
ually trickles from the reservoir on top of the tower. This acid
is derived from the Gay-Lussac tower and from the chambers,
and contains also some nitric acid, which has been added to
replace the oxides of nitrogen that are inevitably lost during
the process of manufacture. As the hot gases from the furnaces
come into contact with this sulphuric acid of the Glover tower,



206



OUTLINES OF CHEMISTRY




SULPHUR, SELENIUM, AND TELLURIUM 207

they are gradually cooled till they attain a temperature of about
70 when they reach the top. At the same time, the acid is
heated up and thus concentrated, water being lost which is car-
ried off with the gases in form of steam. Again, practically all
of the oxides of nitrogen are carried off by the gases, which when
they leave the tower pass into the first lead chamber laden with
oxides of nitrogen and water vapor. The acid which flows from
the bottom of the Glover tower contains only traces of oxides
of nitrogen and is about 80 per cent strong. There are com-
monly three lead chambers, so connected that the gases enter
at the top of each and pass out at the bottom. In these cham-
bers, which often have a volume of 1000 cubic meters each, the
reactions above' mentioned take place. In the first and second
chambers, water vapor is added to the gases. This is done
either by blowing in steam from the boiler, or by forcing water
into the chambers in form of a spray.- In the third chamber the
gases are cooled, and they then pass (charged with oxides of
nitrogen regenerated during the formation of sulphuric acid in
the chambers) into the bottom of the Gay-Lussac tower. This
is lined with lead and filled with coke over which 80 per cent
sulphuric acid continually trickles from the tank at the top of
the tower L. This 80 per cent acid is obtained from the reser-
voir at the bottom of the Glover tower, from which place it is
forced through a lead pipe P to the top of the Gay-Lussac tower.
In the latter the 80 per cent acid dissolves practically all the
oxides of nitrogen, and the residual gases, consisting mainly of
nitrogen, leave the top of the tower and pass into a large chimney
which keeps up a sufficient draught. The acid drawn from the
bottom of the Gay-Lussac tower is thus strongly charged with
oxides of nitrogen. It is the purpose of this tower to preserve
these oxides. This acid, together with some of the chamber
acid, is used again in the Glover tower as already explained.

The acid produced in the chambers is known as "chamber
acid." It is about 60 to 70 per cent strong, i.e. of specific gravity
of about 1.5 to 1.6. The acid may be further concentrated by
evaporation in leaden pans to 78 per cent. Stronger acid than
this attacks lead too much, and so the 78 per cent acid must be
further concentrated by evaporation either in cast-iron, glass, or
platinum vessels. The chamber acid is commonly used directly
in the manufacture of so-called " superphosphate " fertilizers,



VVti OUTLINES OF CHEMISTRY

and the acid from the bottom of the Glover tower is employed
in the Le Blanc soda process.

The concentrated sulphuric acid on the market has a specific
gravity of from 1.83 to 1.84, and hence contains from 93 to 98
per cent of H 2 SO 4 . In making concentrated sulphuric acid, the
contact process already described obviously has distinct advan-
tages, and it is fast taking the place of the lead chamber method.
The latter will, however, very likely continue to serve to pre-
pare the more dilute acid, for which purpose it is well adapted.

The amount of sulphuric acid produced in the world annually
is over four million tons. The material is used in making
soda, aniline dyes, fertilizers, and explosives like gun cotton,
nitro-powder, and dynamite. Again, it is used in storage
batteries, in converting starch to sugar in the glucose industries,
in refining petroleum, in making alum, copper sulphate, and
many other sulphates that are used in medicine and in the arts.

Properties of Sulphuric Acid. Sulphuric acid is a colorless,
odorless, heavy, oily liquid of specific gravity 1.8384 at 15.
It has a very great affinity for water, with which it unites with
great evolution of heat. For this reason the acid, when it is to
be diluted with water, must always be poured gradually into an
excess of water. It is dangerous to proceed in the reverse



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