Joseph William Mellor.

A comprehensive treatise on inorganic and theoretical chemistry (Volume 2) online

. (page 46 of 156)
Online LibraryJoseph William MellorA comprehensive treatise on inorganic and theoretical chemistry (Volume 2) → online text (page 46 of 156)
Font size
QR-code for this ebook

or in a current of dry air free from carbon dioxide at 100 ; when further heated
to 150-180, it lost oxygen, and at a strong red heat, it lost the other mol. of water.
According to C. Opl, bleaching powder yields chlorine : Ca(OCl)Cl=CaO+Cl 2 ,
when heated in the absence of moisture, and if moisture is present, oxygen is evolved :
2Ca(OCl)Cl=2CaCl 2 +0 2 , always with the simultaneous formation of chlorate:
6Ca(OCl)Cl=Ca(C10 3 ) 2 -j-Ca 5 Cl 2 " He could not distinguish between the action of
light and heat. An aq. soln. of bleaching powder in sunlight gives a gas containing
about 95 per cent, of oxygen. The speed of decomposition in light and the amount
of oxygen evolved is proportional to the intensity of the light. 12 According to
A. Bobierre, bleaching powder loses its water arid becomes pasty when heated to
redness, forming calcium chloride and chlorate ; at a red heat H. St. C. Deville and
H. Debray obtained 50 litres of oxygen from a kilogram of the powder.

If the bleaching powder or the filtered soln. be treated with very dil. nitric,
hydrochloric, or sulphuric acid, just sufficient to neutralize the free and combined


lime, hypochlorous acid, HOC1, is formed : Ca(OCl)Cl+HCl=CaCl 2 +HOCl, and
the soln. smells of hypochlorous acid but not of chlorine. The hypochlorous acid
can be separated by distillation. If an excess of acid be present, the hypochlorous
acid is decomposed, forming water and chlorine : HOCl+HCl=H 2 0-}-Cl 2 . Dry
carbon dioxide has little or no action on dry bleaching powder, but with moist
carbon dioxide at 70, most of the chlorine is removed, although this gas has no
action on calcium chloride. According to R. L. Taylor, 13 the action of carbon
dioxide on bleaching powder solid or soln. is like that of any other acid, for
hypochlorous and hydrochloric acids are produced and these decompose one
another with the evolution of chlorine.

W. Odling 14 found that alcohol does not extract appreciable amounts of calcium
chloride from bleaching powder. When bleaching powder is dissolved in water,
it forms a strongly alkaline soln., and an insoluble residue is obtained, consisting
almost wholly of calcium hydroxide, but it was not possible to remove the last
traces of the available chlorine by washing. J. F. Persoz 15 stated that the insoluble
residue contains a special bleaching compound which acts more destructively on
vegetable fibre than does a soln. of bleaching powder, but, according to G. Lunge,
Persoz's statement is wrong ; the damage is caused by the fine particles of the
powder settling on the vegetable fibre and locally forming cone, soln., which trans-
form cellulose into oxycellulose. The solution contains calcium hypochlorite,
Ca(OCl) 2 , the chloride, CaCl 2 , and the hydroxide, Ca(OH) 2 . the amount of the
latter corresponds with its solubility. G. Lunge's analysis 16 of a soln. prepared
from a good sample of commercial bleaching powder is equivalent to

Ca(OCl) 2 CaCl 2 Ca(ClO 3 ) 2 Ca(OH) a

Grms. per litre . 129 "09 10 '54 0'38 4 -21

G. Lunge also gives a table of specific gravities of soln. of this sample of bleaching
powder for different amounts of available chlorine in grams per litre : . . 1-0025 1-01 1-02 1'04 1'06 1'08 1-10 I'll

Grms. Cl . . T40 5 -58 11 '41 23'75 35'81 49'96 61'50 68'00

Bleaching powder which has been kept some time will contain more chloride and
chlorate. The titre of available chlorine did not change appreciably when kept
for 24 days out of contact with air and in the dark, but a slight change was noticeable
in 33 days. There was no appreciable loss after keeping 12 days open to air and
in darkness, but a loss of about one-eighth of its strength in 33 days. Diffused day-
light accelerates the decomposition of the solutions. The pink colour of the soln.,
obtained by leading carbon dioxide through a warm soln. of bleaching powder or
other hypochlorite, has been attributed to manganese as permanganate and to
iron probably as calcium ferrate. G. E. Davis 17 established the presence of
manganese spectroscopically ; and T. L. Bailey and P. H. Jones obtained no pink
coloration with soln. free from manganese even when considerable amounts of
iron were present.

Whatever be the constitution of bleaching powder it is fairly certain that the
aq. extract contains the three compounds hydroxide, chloride, and hypochlorite
in soln. The existence of the hypochlorite is evidenced by the isolation of crystals
of this salt by C. T. Kingzett. L. T. O'Shea also argued that if the soln. contains
a chlorohypochlorite, Ca(OCl)Cl, it will diffuse so that the ratio of the total chlorine to
oxidizing chlorine will remain 2 : 1 both in the diffusate, and in the original soln.,
whereas if the soln. contains the two salts hypochlorite and chloride the one will
diffuse faster than the other, and the ratio in question will not be the same in the
diffusate and in the original soln. It was found that the diffused liquid was richer in
chloride and poorer in oxidizing chlorine than in the original soln.. showing that the
chloride diffused faster than the hypochlorite, and that if solid bleaching powder is a
compound of calcium hypochlorite and chloride, or of calcium dhlorohypochlorite, it
is decomposed by the action of water into a mixture of the two salts. In addition,


M. Duyk has shown that the hypochlorites are hydrolyzed in aq. soln. : Ca(OCl) 2
+2H 2 O^Ca(OH) 2 +2HOCl, so that a soln. of bleaching powder with 0*02162 grm.
of active chlorine in 10 c.c. contained the eq. of 0*002485 grm. of free hypochlorous
acid and O'OllO grm. of free lime ; on adding 100 c.c. of water the free hypochlorous
acid and lime were increased to 0'008697 grm. and 0'0137 grm. respectively ; and
a further addition of another 100 c.c., to 0'01629 grm. and 0'014:0 grm. respectively.
The hydrolysis is further confirmed by the removal of some of the hypochlorous
acid by the passage of a current of purified air through the liquid. F. Forster and
H. Bischoff, and A. Sieverts, also attribute the relatively higher bleaching action
of soln. of the alkali bleaching liquors prepared electrolytically to the relatively
large proportion of free hypochlorous acid they contain.

According to K. Thummel, 18 a mixture of equal parts of bleaching powder and
charcoal explodes when heated in a closed vessel, and, according to H. G. de Claubry,
sulphur in place of charcoal acts in the same way. T. L. Phipson found that when
exposed to hydrogen sulphide, the latter loses its smell, the odour of chlorine appears,
and sulphur is formed. Hypochlorous acid is probably set free, and this reacts
with hydrogen sulphide, forming sulphur and chlorine.

Bleaching.- In bleaching by eau de Javelle or by bleaching powder, the preliminary
treatment is often complex and is determined by the character of the material. Various
fatty or colouring agents may have to be extracted by washing in hot alkali lye.
The washed fabric is steeped in a clear dil. aq. soln. of the bleaching agent chemicking
and then soured by steeping in dil. acid. Hypochlorous acid is thus produced, and then
free chlorine. The free chlorine does its work within the fibres of the wet fabric. In
high -class goods required to be " brilliant white," the operations may have to be repeated
a number of times. The bleaching action of hypochlorous acid is generally stated to be
twice as great as that of the chlorine it contains, supposing the latter were free : 2C1 2 + 2H 2 O
=4HC1 + O 2 ; 4HOC1=4HC1 + 2O 2 ; but it must be remembered that two atoms of chlorine
are needed to form one molecule of HOC1, since an eq. amount of HC1 is formed at the
same time.

The constitution of bleaching powder. In the early days, C. L. Berthollet, 19
J. L. Gay Lussac, and J. J. Berzelius assumed that the powder prepared by
C. Tennant in 1798 by the action of chlorine on slaked lime is a compound of lime,
CaO, with chlorine, viz. CaOCl 2 ; but after his investigation of the hypochlorites in
1835, A. J. Balard 2 concluded" that bleaching powder is a compound or a mixture
of calcium chloride, CaCl 2 , and calcium hypochlorite, Ca(OCl) 2 , mixed with an excess
of calcium hydroxide. Numerous attempts have been made to explain the nature
of this product, and many contradictory statements have been made about what
appear to be questions of fact. Although it is generally agreed that calcium hypo-
chlorite plays an important role, there is no universal agreement on the constitution
of bleaching powder, probably because the attempt has been made to find a con-
stitutional formula for an indefinite mixture, on the assumption that it is a
chemical individual. The main discussions can be arranged under the following
heads :

(1) Formulae based on the assumption that bleaching powder is a mixture of calcium
hypochlorite and calcium chloride. It will be evident that bleaching powder cannot
be the calcium salt of hypochlorous acid, Ca(OCl) 2 , because the product would then
furnish twice the amount of available chlorine actually obtained when it is treated
with hydrochloric acid, in virtue of the reactions symbolized : Ca(OCl) 2 +4HCl
=CaCl 2 +2H 2 0+2Cl 2 . Hence, A. J. Balard suggested that bleaching powder is
a mixture of chloride and hypochlorite, CaCl 2 +Ca(OCl) 2 . J. L. Gay Lussac con-
firmed Balard' s conclusion that bleaching powder is a mixture of eq. proportions
of calcium chloride and hypochlorite. Against this view, it is very probable that
bleaching powder contains but little calcium chloride because : (1) Nearly all the chlorine
can be expelled from bleaching powder by the action of carbon dioxide, but not
from calcium chloride. To get over this difficulty, it has been suggested that
possibly some reaction like that assumed by A. J. Balard takes place : CO 2
+ Ca(OCl) 2 =CaC0 3 +CaCl 2 +Cl 2 0, followed by Cl 2 0+C0 2 +CaCl 2 =CaC0 3 +2Cl 2 .


(2) Calcium chloride is very deliquescent, bleaching powder is not. (3) Calcium
chloride is readily dissolved by alcohol, whereas an alcoholic soln. of bleaching
powder contains but very small quantities of calcium chloride. (4) When bleaching
powder is treated with successive small quantities of water, the first washing
contain much less calcium chloride than would be the case were calcium chloride
present as such in the powder. (5) Bleaching powder made by mixing calcium
hypochlorite with sufficient calcium chloride to furnish the same percentage amount
of bleaching chlorine as commercial bleaching powder, when treated by moist carbon
dioxide gives off an amount of chlorine corresponding with the hypochlorite only,
whereas the commercial bleaching powder parts with 80 per cent, of bleaching
chlorine. Hence it is inferred that bleaching powder is not a mere mixture of
calcium hypochlorite and chloride, but rather a compound of the two, viz. Ca(OCl)Cl.
However carefully prepared, bleaching powder always contains an excess of
lime ; at any rate, a deposit of slaked lime is always found as a residue when bleaching
powder is extracted with water. Since the available chlorine in commercial bleaching
powder usually ranges between 36 and 38 per cent., and under the very best con-
ditions a maximum of about 43'5 per cent, of available chlorine is taken up, it is
inferred that the calcium hydroxide, Ca(OH) 2 , is not completely sat. with chlorine,
and that calcium hydroxide is present either free or combined.

(2) Formulce based on the assumption that bleaching powder is a double salt of
calcium chloride and hypochlorite. As emphasised by K. Kraut, 21 the non-deliquescent
character of bleaching powder may of course mean that the calcium chloride and
hypochlorite are united to form a double salt, CaCl 2 .Ca(OCl) 2 , which has not quite
the same meaning as the simplified formula, Ca(OCl)Cl, but is rather analogous
with the non-deliquescent double salt of calcium chloride and acetate,
CaCl 2 .(CH 3 COO) 2 Ca.5H 2 0. Mol. wt. determinations are not available. Attempts
to decide between these two formulae CaCl 2 .Ca(OCl) 2 and Ca(OCl)Cl by the extrac-
tion with solvents have not been successful, because the solvent may react with the
compound e.g. W. von Tiesenholt noted that when bleaching powder is shaken up
with carbon tetrachloride, two powders of different density and with a different pro-
portion of active chlorine are formed; the same thing occurs when alcoholic chloroform
reacts with the hypochlorite. Similarly, the fact that carbon dioxide drives most of the
chlorine may mean that the hypochlorous acid hydrolytically dissociated or expelled
from its salts by carbon dioxide attacks the chlorides yielding chlorine and the
free base or a carbonate, MC1+HOC1=MOH+C1 2 . F. Forster has obtained direct
evidence of the reaction : NaCl+6HOCl=NaC10 3 +3H 2 0+3Cl 2 , and W. von Tiesen-
holt found that a soln. containing equimolecular proportions of calcium hypochlorite
and chloride gives off more chlorine than would be furnished by calcium hypo-
chlorite alone. A. Thiel, however, points out that W. von Tiesenholt's conclusion
does not follow from his experiments, for they only show that when bleaching powder
is dissolved in water, a mixture of the chloride and hypochlorite is formed.
W. von Tiesenholt later showed that chlorine is expelled from moist artificial
mixtures of the solids at a temp, between 40 and 100. Hence, the expulsion of
chlorine from bleaching powder does not decide in favour of the mixed salt, Ca(OCl)Cl,
against the double salt, CaCl 2 .Ca(OCl) 2 . H. Ditz, however, has shown that
W. von Tiesenholt's formula does not account for the loss of water which occurs
when bleaching powder is heated to 100 in a stream of dry air, free from carbon
dioxide, and the loss of oxygen on further heating to 150-180.

(3) Formulce based on the assumption that bleaching powder is a hydroxyhypo-
chlorite.In 1859, P. A. Bolley 22 suggested that the calcium hydroxide of bleaching
powder is mixed with it in a mechanical way, and it represents the cores of the
granules which escaped chlorination by being encrusted with bleaching powd<
proper. R. Fresenius and F. Rose suggested that bleaching powder is a hydrated
oxychloride and hypochlorite: Ca(OCl) 2 .CaCl 2 .2CaO.4H 2 O. This would m.'aii
that bleaching powder can contain as a maximum 32 per cent, of available chlorine,
although if, as K. Kraut suggested, the basic chloride or oxychloride be CaCl 2 .CaO,


the theoretical proportion of available chlorine would approximate closer to the
observed value, but would still be less than the maximum obtainable. J. Kolb
suggested the formula : 2CaOCl 2 .H 2 O.Ca(OH)2, and assumed that the double com-
pound is split by water into insoluble calcium hydroxide, and soluble calcium
hypochlorite, but the formula gives a maximum of 38*7 per cent, of available
chlorine less than the observed value. C. Stahlschmidt suggested the formula :
2Ca(HO)(OCl)CaCl 2 .2H 2 0. In order to make the assumption fit the facts, it is
supposed that the basic calcium hypochlorite, HO.Ca.OCl, invented by C. Stahl-
schmidt is not attacked by chlorine, and is decomposed by water into calcium
hypochlorite and hydroxide: 2Ca(OH)(OCl)=Ca(OCl) 2 +Ca(OH) 2 . C. Stahl-
schmidt's formula : 2Ca(OH)OCl.CaCl 2 .2H 2 0, required a maximum of 39 per cent,
for the available chlorine this is less than the observed maximum ; the presence
of calcium chloride, CaCl 2 , is an assumption which cannot be granted. In 1884,
E. Dreyfus based some arguments for C. Stahlschmidt' s formula on some experi-
ments which G. Lunge and R. Schoch showed to be, erroneous.

(4) Formulae based on the assumption that bleaching powder is a chloroper oxide.
In 1803, C. L. Berthollet suggested that the composition of bleaching powder
corresponds with the formula CaOCl 2 , and many chemists have accepted a similar
formula, but with different interpretations. For example, N. A. E. Millon 23
regarded bleaching powder as a kind of peroxy chloride, : Ca : C1 2 ; C. Gopner,
(CaO)Cl 2 ; E. Richters and G. Juncker, W. Wolters, and C. Opl likewise
maintain that although bleaching powder and its soln. both contain the com-
pound CaOCl 2 , this is not a mixture of hypochlorite and chloride. J. Mijers, 24
like N. A. E. Millon, assumes a peroxide formula, and represents the formula by
Cl 2 =Ca=(OH) 2 , on the assumption that the calcium atom is quadrivalent. This
formula may also be regarded as a monohydrate of W. Odling's Cl Ca OC1.
N. Tarugi also considers bleaching powder to be a chloride of calcium peroxide,
because when dehydrated lime is left exposed to the air, it exhibits a peroxide
reaction colouring tincture of guaiacum blue ; ether and chromic acid, blue ;
and ferrous sulphate and potassium thiocyanate, red. He considers that bleaching
powder is therefore Ca0 2 Cl 2 .H 2 0, corresponding with a maximum of 44'09 per cent,
of available chlorine ; with its action of mercury on bleaching powder : Ca0 2 Cl 2
+Hg=Ca0 2 -|-HgCl 2 ; and with the fact that in order to get the best quality of
bleaching powder free oxygen is necessary. H. Ditz, however, contradicts several
of N. Tarugi's statements.

(5) Formulce based on the assumption that bleaching powder is a chlorohypo-
chlorite. W. Odling 25 suggested that when chlorine is allowed to act upon a
bivalent base say, calcium hydroxide, Ca(OH) 2 a molecule of each of the two
monobasic acids hydrochloric and hypochlorous acids formed by the action of
chlorine on water, is neutralized by one molecule of the base, and what seems to be
a mixed salt is formed :

,OH , HOC1 /OC1 , OTT n
Ca <OH+HCl =Ca <01 + 2H *

W. Odling, therefore, regarded bleaching powder as a mixed salt hypochlorite and
chloride. Against this view, K. Kraut cited the formation of lithia bleaching
powder, but G. Lunge and P. Naef showed that lithia bleaching powder is funda-
mentally different from lime product.

E. Schwarz found that the bivalent bases baryta and strontia give compounds
analogous in properties with that from lime. Ban/ta bleaching powder is very
unstable, while strontia bleaching powder is more stable and is readily prepared. 26
G. Lunge has further shown that the various properties of bleaching powder are
best co-ordinated by assuming that the active agent is the mixed salt formulated
Cl.Ca.OCl by W. Odling. This view is further confirmed by L. Limpach, L. T. O'Shea,
E. Schwarz, etc. After washing samples of bleaching powder with alcohol to
remove calcium chloride, L. T. O'Shea determined the ratios indicated below, and


compared them with the values computed for other formulae. He found W. Odling's
formula alone gave results in accord with observations :

CaO : Total Cl. Available Cl : Total Cl. CaO : Available Cl

Observed ..... 1:2 1:2 1:1

Calculated Stahlschmidt Ca(OH)Cl 1:1 1:1 1:1

Gay Lussac Ca(OCl) 2 . 1:2 1:1 1-2

Odling Ca(OCl)Cl . 1:2 1:2 1:1

E. Schwarz assumes that the fundamental reaction is : HoO-fCl^HCl+HOCl.
So long as an excess of free lime is present, the reaction is merely one of neutraliza-
tion, but as soon as the lime is used up, the excess of hydrochloric acid liberates
hypochlorous acid from the bleaching powder, and this in conjunction with the
hypochlorous acid formed by the action of chlorine on water oxidizes the undecom-
posed hypochlorite to chlorate. The excess of chloride always found in com-
mercial bleaching powder is mainly derived from the hydrogen chloride present in
the chlorine used. If an excess of water is present, the hypochlorite is hydrolyzed.
When treated with carbon dioxide synthetic bleaching powder prepared by the
action of chlorine monoxide on calcium oxide in the presence of moisture gives up
less chlorine than the product obtained by the action of chlorine on slaked lime.
This is supposed to be explained by the assumption that the former is a mixture of
calcium hypochlorite and chloride, whereas the latter is a mixed salt, CaCl(OCl).
H. Ditz 2 7 considers that commercial bleaching powder is not a homogeneous
chemical individual, but rather a mixture formed by a series of reactions, the first
of which is represented : 2CafOH) 2 +Cl 2 =H 2 O.CaO.Ca(OCl)Cl+H 2 0. If a suffi-
cient excess of water be present, and the temp, be not too low, this product is hydro-
lyzed : H 2 0+H 2 O.CaO.Ca(OCl)Cl-:Ca(OH) 2 +H 2 O.Ca(OCl)Cl, and the Ca(OH) 2
so produced is available for rechlorination. The bleaching powder so formed
is a mixture of both CaO.Ca(OCl)CLH 2 and H 2 O.Ca(OCl)Cl. The compound
CaO.Ca(OCl)Cl.H 2 0, or 2Ca(OH) 2 .CaCl 2 .Ca(OCl) 2 , is formed at 10 ; and the com-
pound Ca(OCl)Cl.H 2 has also been produced at higher temp. In practice, these
reactions are modified by the amount of water in the slaked lime, and they do not
usually proceed beyond the stage represented by a final product : (2n 2)Ca(OCl)Cl
+CaO.Ca(OCl)Cl.H 2 0+(2rc 1)H 2 0. The complex H 2 O.CaO.Ca(OCl)Cl appears
to be stable at 100, but is decomposed at 130 with the evolution of oxygen ;
the molecule of water, however, is not expelled below a red heat. The formation
of bleaching powder, says H. Ditz, is not a simple process, but several reactions
succeed one another giving different products. Indeed, he was able to make
a product with 48-74 per cent, of available chlorine and 0-64 per cent, of non-
chlorinated lime, by adding a little water from time to time. According to
F. Winteler, the moisture plays the part of a catalytic agent, in the formation of
bleaching powder. Chlorine water is first formed ; this hydrolyzes : C1 2 +H 2
^HCl+HOCl the hydrochloric acid reacts most rapidly with the lime, forming
Ca(OH)Cl and CaCl 2 ; some water is formed at the same time. The hypochlorous
acid acts on the original Ca(OH) 2 , on the Ca(OH)Cl, and on the CaCl 2 , with the
result that the finished product may contain

as well as free hypochlorous acid, which then forms chloride and oxygen, or chloride
and chlorate. Hence, says F. Winteler, bleaching powder is not a homogeneous
product, and no definite constitutional formula can be assigned to it. Otherwise
expressed : Chlorinated lime is not a homogeneous individual but rather a wi.rture of
calcium chlorohypochlorite, Ca(OCl)Cl, or a double wit, CaCl 2 .Ca(OCl} 2 , infh />///</
products, the relative proportions of which depend upon tin- eodi(i<in iwAv which the

chlorination has been performed temperature, proportion of water present, rate at
which the chlorine is led over the lime, etc. It is convenient to assume that bleaching


powder is eq. to Ca(OCl)Cl with more or less free calcium hydroxide. The argu-
ments, however, are not final. As Roger Bacon once said : Argument may appear
to decide a question, but it cannot make us feel certain unless the truth be also
established by experience.

In 1832, J. J. Berzelius prepared bromine bleaching powder, or bromide of lime,
by the action of bromine on slaked lime. By heating calcium hydroxide suspended
in water with iodine, G. Lunge and R. Schock 28 obtained a colourless soln. which
bleached vegetable colouring matters logwood, litmus, and cochineal gave no
coloration with starch ; gave a separation of iodine when treated with acids ;
gave an evolution of oxygen with hydrogen peroxide ; decomposed slowly in
darkness, rapidly in sunlight ; and even on long boiling only partially decomposed.
G. Lunge and R. Schock conclude that this soln. contains an iodine compound
analogous with iodine bleaching powder, or iodide of lime, Ca(IO)I. W. A. R. Wilks
studied the action of soln. of chlorine, bromine, and iodine in carbon tetrachloride
on dry slaked lime, and found that bromine is adsorbed until a maximum of adsorp-
tion is attained. In some cases a slight " superadsorption " is noticed. The colour
of bromine bleaching powder is due to this adsorption product. The concentra-
tions of bromine in soln. (C 2 ) and in lime (C^ before the maximum of adsorption
is reached are connected by the equation C 1 /C 2 *=A;. The amount of bromine
adsorbed by slaked lime depends on the dry ness of the lime. Similar results were
observed in the case of iodine and lime. The value n in the formula (C 1 /C 2 ) W =A:
is one-third, as with bromine, but the constant k is different for bromine and iodine.
For the same specimen of lime the mol. ratio of iodine to lime at the maximum
of adsorption is less than the corresponding ratio for bromine. Chlorine behaves
quite differently from the other two halogens under the same conditions. Adsorp-
tion phenomena could not be detected, but the results indicate the formation of a

Online LibraryJoseph William MellorA comprehensive treatise on inorganic and theoretical chemistry (Volume 2) → online text (page 46 of 156)