sufficiently pure to yield a crystalline product on evaporation.
The other process depends on the formation of a nearly
insoluble compound with lime, having the composition
C ll ,H. ) .,O n . 3CaO. This is precipitated from the molasses
by addition of powdered quick-lime, and after being purified
by washing, is decomposed by passing a stream of CO 2 through
water with which the " lime saccharate " is mixed. The lime
is separated as calcium carbonate, and a weak syrup of pure
sugar is obtained, which can readily be concentrated by evapora-
tion.
Cane-sugar crystallises in large monoclinic prisms (sugar-
candy). It is very soluble in water, but is easily crystallised
from its solutions by evaporation unless the presence of im-
purities interferes. Solutions rotate the plane of polarisation
of light to the right. On boiling with a dilute acid cane-sugar
is converted into a mixture of dextrose and levulose :
C 12 H 22 O n + H 2 = C H 1S 8 + C fl H 13 ;
Cane-sugar. Dextrose. Levulose.
and as levulose has a higher rotatory power than dextrose, the
124 MILK-SUGAR
mixture of the two thus obtained rotates to the left ; this in-
version is the origin of the name invert-sugar, which is applied
to the product thus obtained.
When heated, cane-sugar melts at 160 C., and if then
allowed to cool solidifies to a semi-transparent mass (" barley-
sugar"), which is devoid of crystalline structure. On long
standing this gradually becomes crystalline again. If heated
to about 200 C., cane-sugar is changed into a brown sub-
stance known as "caramel," or " burnt-sugar," which is used
as colouring matter by cooks.
Cane-sugar when subjected to the influence of the growing
yeast-plant is first changed into a mixture of dextrose and
levulose. As soon as any considerable quantity of these
glucoses has been formed alcoholic fermentation sets in, fol-
lowing chiefly the equation
C 6 H 12 6 = 2C 2 H 6 + 2C0 2 .
Glucose. Ethyl alcohol.
Milk-sugar, C 19 H 20 O n , is present in milk, and remains
dissolved in the whey after the casein has been separated in
the manufacture of cheese. It is less soluble in water than
cane-sugar, and much less sweet. Its solutions rotate the
plane of polarisation to the right.
Milk-sugar does not easily ferment with yeast, but by the
action of certain bacteria it readily ferments with production
of lactic acid :
Milk-sugar. Lactic acid.
This is the change which occurs when milk turns sour.
As has been already mentioned, the hydrolysis of milk-
sugar yields dextrose and galactose :
C 12 H 2211 + H 2 = C 6 H 126 + C 6 H 12 O 6'
Milk-sugar. Dextrose. Galactose.
Maltose, C 12 H 92 O 11 , is contained in malt, having been
produced by the action of a certain ferment diastase upon
the starch present in the barley or other grain which has been
malted.
xviii STARCH 125
Upon hydrolysis boiling with a dilute acid maltose yields
dextrose only :
C 12 H 2 2 ll + H 2 = 2C H lA-
Maltose. Dextrose.
It resembles the glucoses much more closely than do cane-
and milk-sugar ; thus it ferments quickly (i.e. without previous
conversion into glucoses) with yeast, and reduces Fehling's
solution readily when warmed with it.
THE CELLULOSE GROUP
In this group we include a number of carbohydrates whose
constitution is less understood even than that of the glucoses
and bioses. It is very probable that their molecular weights
are very high, but it has not yet been found possible to deter-
mine their real values, and we can only give the empirical
formulae. The most important members of the group are
starch, dextrin, and cellulose all C 6 H 10 O 5 and the gums,
whose probable formula is C^H 10 O r/
Starch, C 6 H 10 O 5 , is the form in which very many plants
store up their reserves of food. It is largely present in many
roots and seeds, as the following table will show :
Per cent
of starch.
Potatoes . . . 20
Wheat, maize . . 60
Rice .... 70
Starch is also a very important food for animals ; and arrow-
root, sago, and tapioca are nearly pure starch extracted from
certain plants. The separation of starch from the other con-
stituents of the plants is effected by beating them with water
into a thin pulp, which is then filtered through fine sieves.
The fibrous matter is kept back, and the milky liquid which
runs through deposits the starch on standing. This is then
collected and dried.
Starch is really insoluble in water, but when boiled with it
yields a liquid which can be filtered without separating the
starch. This is, however, merely present in a very fine state
126 CELLULOSE
of subdivision, forming what is called a " colloidal " solution.
The starch in it is unable to pass through a membrane of
parchment paper, whereas substances in real solution are able
slowly to diffuse through such a membrane. Neither has the
starch any effect on the freezing-point of the water containing
it (see p. 1 6).
When heated to about 200 C., starch is changed into dex-
trin.
Very characteristic of starch is the intensely blue compound
which it forms with iodine. This furnishes a very sensitive
test either for starch or for free iodine. The blue colour dis-
appears when sufficient heat is applied, but reappears on cool-
ing.
Dextrin, C 6 H 10 O 5 , is obtained by simply heating starch to
about 200 C., or by boiling it with dilute acids. Dextrin is
used as a substitute for gum. It is not coloured blue by
iodine.
Cellulose, C 6 H 1Q O 5 , is the chief constituent of the cell-
walls of plants ; wood is chiefly cellulose, while cotton-wool
and filter-paper are nearly pure cellulose. This is insoluble
in all ordinary solvents, but concentrated sulphuric acid
dissolves it, and the solution when diluted and boiled yields
first dextrin and then dextrose.
The exact chemical constitution of cellulose is matter for
future investigation. It appears, however, to contain three-
fifths of its oxygen in the form of hydroxyl groups OH, as \\e
find that by the action of acids ethereal salts of cellulose may
be prepared in which three acid groups are introduced into
the formula C 6 H 10 O 5 ; the real molecular formula of cellulose
is unknown, but it is more convenient to regard these ethereal
salts as derived from the doubled formula C 12 H 20 O 10 , in which,
of course, there are six hydroxyls.
The most important of these salts are the nitrates ; these
are prepared by treating cellulose (cotton-wool) with strong
nitric acid, the action being aided by the addition of concen-
trated sulphuric acid. When the strongest acids are employed
the product obtained is gun-cotton, which is found to be
cellulose hexa-nitrate :
C 12 H 14 4 (OH) 6 + 6HN0 3 = C 1: ,H 14 O 4 (NO 3 ) + 6H,O.
Cellulose. Gun-cotton.
GUN-COTTON 127
This material is a violent explosive, and is prepared by
steeping cotton-wool for a few minutes in a cold mixture of the
strongest nitric acid with two or three times its weight of
concentrated sulphuric acid. When thoroughly freed from
acid by washing, gun-cotton is comparatively quite safe to
handle, and may even be set fire to without anything more
violent than a rather quick combustion taking place ; but
when subjected to the shock set up by exploding a small
charge of fulminate embedded in the gun-cotton, the molecules
of the latter break down suddenly, and a powerful explosion
results ; the rearrangement of atoms which then occurs may
be roughly represented by the following equation :
C 12 H u 4 (N0 3 ) e = 3 N 2 + 7H,0 + 9 CO + 3 CO 2 .
Gun-cotton.
Pyroxylin is a less highly nitrated cellulose, chiefly the
tetra-nitrate ; it is prepared with a somewhat weaker nitric
acid. Its solution in a mixture of alcohol and ether is the
collodion which is largely used in photography (wet -plate
process), and in surgery for covering wounds with a thin
flexible film which prevents access of air.
QUESTIONS ON CHAPTER XVIII
1. What are the chief members of the group of "glucoses" ; what is
their formula, and what explanation of their isomerism may be advanced ?
2. What is the action upon dextrose of (a) Fehling's solution, (/>)
yeast, (c ) acetic anhydride ?
3. What two substances are present in largest quantity in honey ?
How do they differ from one another ?
4. What products are obtained by the action of boiling dilute acids
upon (a) cane-sugar, (/>) milk-sugar, (c) maltose?
5. Describe the preparation of gun-cotton, and give its chemical
constitution.
CHAPTER XIX
UREA AND URIC ACID
Urea, CO(NHA 2 , is one of the most important waste-
products of the animal economy ; the food which animals
consume is converted during its passage through the blood
and tissues of the body chiefly into urea, carbon dioxide, and
water. The urea is secreted along with a considerable
proportion of the water by the kidneys, and it was from urine
that this substance was first obtained in 1773.
Urea thus obtained and afterwards carefully purified was
found by analysis to have the composition CON 2 H 4 . This is
also the composition of ammonium cyanate, (NH 4 )NCO, and
though that body is itself quite distinct from, and isomeric
with, urea, in 1828 Wohler made the very important discovery
that a solution of ammonium cyanate in water yields urea on
evaporation. The great readiness with which this change
occurs while indicating that urea is the more stable of the two
isomers also seems to show that they are not very different in
constitution. Several synthetical methods which have since
been discovered for the preparation of urea show that it may
be regarded as the amide of carbonic acid, CO< N ,, 2 . Thus
just as the amide of acetic acid (acetamide) can be got by the
action of ammonia on acetyl chloride :
CH 3 .coci + NH.j = CH 3 . CONH L) + HCI,
Acetyl chloride. Acetamide.
so urea can be obtained by the action of ammonia on carbonyl
chloride COC1 2 :
PREPARATION OF UREA 129
ljj + 2N H 3 = CO(N H,) 2 + 2 HC1.
Carbonyl Carbamide
chloride. or urea.
The most convenient way of preparing urea is by
evaporating a solution in water of potassium cyanate and
ammonium sulphate mixed in equivalent proportions ; the
potassium cyanate is easily obtained by heating potassium
ferrocyanide with manganese dioxide.
EXPT. 19. Heat four parts potassium ferrocyanide with two parts of
MnOa in a clay crucible, extract the cooled melt with water, add three parts
of ammonium sulphate, and evaporate to dryness. Potassium sulphate
and urea are left, and may be separated by extraction with alcohol, in
which the urea only is soluble.
Urea crystallises in rhombic prisms which are easily
soluble in water. It is a mon-acid base, and forms salts of
which the nitrate CON 2 H 4 . HNO 3 is very sparingly soluble
in water containing nitric acid, and may therefore be used as
a means of detecting urea in solutions not too dilute.
Like other amides urea is decomposed on boiling with
dilute alkalies, and ammonia is given off :
CO(NH 2 ) 2 + H 2 O = CO, + 2NH 3 .
Urea." "
Another important reaction of urea is its behaviour when
treated with bromine and caustic soda (sodium hypobromite) ;
it is then oxidised to CO., and water while the nitrogen is
given off as such :
CON 2 H 4 + 3NaOBr = CO., + 2H 2 O + N 2 + 3NaBr.
EXPT. 20. Put some solution of urea in a boiling tube, add caustic
soda and bromine water ; notice that a gas is given off in bubbles, and by
testing with a match show that it puts out the flame. (The gas cannot
be CO.,, because the solution contains excess of alkali. ) The experiment
can be so arrafiged that the nitrogen may be collected and measured ;
from its amount that of the urea can be calculated, and on this a method
for estimating urea is based. It must, however, be remembered that
many other nitrogen compounds also give off their nitrogen when treated
with a hypobromite.
Uric Acid, C r) H 4 N 4 O 3 (5443), may be regarded as a less
completely oxidised result of the digestive and absorptive
130 URIC ACID
processes than urea. Uric acid is present only in small
quantity in the urine of man, but in certain abnormal con-
ditions of the body it is more largely produced, usually with
very unpleasant consequences. Both uric acid and its salts are
soluble only with difficulty in water, hence they are difficult to
remove when produced in the body in any considerable
quantity, and either gout, in which accumulations of urates
occur in various parts of the body, or other disturbances of
the healthy procedure occur.
In some animals, on the other hand, especially birds and
reptiles, uric acid is largely secreted, and both guano (which
is produced by sea-birds) and the excreta of serpents contain
considerable quantities, and from either of these sources the
acid may readily be prepared.
If guano is used it is best boiled with a solution of borax (i to 100 of
water), in which uric acid is fairly soluble. Addition of hydrochloric acid
to the filtered solution precipitates the bulk of the uric acid present.
Uric acid is a white powder, soluble only very slightly in
pure water, but more readily in water containing certain salts
in solution. It is a weak di-basic acid, but the best
characterised salts are these with only one equivalent of metal,
such as C 5 H 3 KN 4 O 3 , potassium urate ; they are all very
slightly soluble in water.
To test a substance for the presence 01 uric acid a few
drops of dilute nitric acid are added to it, and then evaporated
on the water bath ; if a yellow residue is left which is coloured
purple by addition of ammonia, we may conclude that uric
acid was contained in the substance examined.
QUESTIONS ON CHAPTER XIX
1. How can urea be prepared from potassium ferrocyanide ?
2. What is the action upon urea of (a) sodium hypobromite, (6)
boiling caustic soda solution ?
3. What products are formed by the action of ammonia upon (a)
carbonyl chloride, (i>) acetyl chloride?
4. From what sources can uric acid be obtained ? Write down the
formulae of uric acid and of potassium urate.
CHAPTER XX
THE CYANOGEN COMPOUNDS
THE cyanogen compounds include a large number of sub-
stances which are alike in containing the monovalent radicle
cyanogen CN, made up, as its formula shows, of one atom
each of tetravalent carbon and trivalent nitrogen : - C = N.
Sometimes the special symbol Cy is used to denote the
cyanogen radicle.
The starting-point in the preparation of the various cyanogen
compounds is potassium ferrocyanide, or " yellow prussiate of
potash," but as the composition of this substance is somewhat
complex it is better to begin with other and simpler bodies.
Cyanogen, C 2 N 2 , a compound whose molecule is formed
of two cyanogen radicles united together (just as free chlorine
or hydrogen is C1 2 or H 2 ), is made by heating mercuric
cyanide to a red heat, when it decomposes into mercury and
cyanogen :
It is a poisonous gas with a characteristic smell, and burns in
air with a peculiar ("peach-blossom colour") flame; its mix-
ture with oxygen explodes violently on application of a flame :
Cyanogen is readily soluble in water, and must therefore be
collected over mercury.
Chemically cyanogen behaves as the " nitrile" of oxalic acid. It can
be obtained from the amide of oxalic acid oxamide (CONH 2 ).2 by with-
drawing water (action of PoOj) :
132 HYDROCYANIC ACID CHAP.
CONH 2
I - 2 H,0 = C 2 N 2 ;
CONH 2
and the inverse reaction can be brought about by allowing a solution of
cyanogen in water or dilute acid to stand for several days :
C 2 N 2 + 2H 2 O = C 2 O 2 (NH 2 ) 2 .
Compare the relation of methyl cyanide (acetonitrile) CH0CN to accta-
mide, pp. 89 and 134.
Hydrocyanic Acid, HCN, or " Prussic Acid," is now
most largely prepared by the action of boiling dilute sulphuric
acid upon potassium ferrocyanide :
2K 4 FeCy 6 + 3H,,SO 4 = 3K 2 SO 4 + K 2 Fe 2 Cy 6 + 6HCN.
By this method a solution of hydrocyanic acid in water is
obtained, from which the anhydrous acid can be prepared by
passing the vapours through tubes containing calcium chloride
or other suitable dehydrating agent.
An older method of preparing the dilute acid is from
amygdalin, a compound present in bitter almonds, laurel
leaves, and parts of various other plants. The amygdalin,
when the leaves, etc., steeped in water, are exposed to the air,
usually undergoes a fermentation which results in the forma-
tion of hydrocyanic acid, oil of bitter almonds (benzaldehyde,
see Part II.), and sugar. The hydrocyanic acid is then easily
obtained by distillation.
The salts of hydrocyanic acid the cyanides are formed
whenever carbon and nitrogen come in contact with a strong
base at a high temperature. The nitrogen may be supplied
in the free state, or may be present in combination with other
elements. Thus potassium cyanide is formed when nitrogen
is passed over a heated mixture of potash and powdered coal,
and cyanides are always formed in distilling coal for the manu-
facture of coal-gas from the joint interaction of ammonia with
the nitrogen and carbon present in the coal. The two chief
sources of the cyanides (which are largely manufactured for
use in electro-plating, for making Prussian blue, and other pur-
poses) are to be associated with this method of formation.
They are
XX SOURCES OF THE CYANIDES 133
i . Potassium ferrocyanide, yellow prussiate of potash, which
is made by carbonising nitrogenous animal refuse (horn, leather
scraps, etc.) and heating the residue, which, though chiefly
carbon, still contains a considerable proportion of nitrogen,
with caustic potash and iron filings.
2. An important source of cyanides is now found in the
by-products of the manufacture of coal-gas. The cyanides
formed during the destructive distillation of the coal are re-
tained chiefly in the lime-purifiers, and are extracted from the
spent lime by treatment with quicklime at steam heat. This
decomposes the insoluble cyanogen compounds present in the
spent lime, and converts them into soluble calcium ferro-
cyanide.
Cyanides are now also recovered from the by-products of
other manufactures blast-furnaces, coke-ovens, etc.
Other reactions, of theoretical interest only, by which hydrocyanic acid
or its salts can be obtained, are
i. The action of the electric discharge upon a mixture of acetylene and
nitrogen :
2. The action of ammonia upon chloroform (in the presence of caustic
potash) :
NH 3 + CHCl.,=:3HCl-f-HCN.
Pure hydrocyanic acid, free from water, is a colourless volatile
liquid, with a strong smell and intensely poisonous properties.
It is a well-marked acid, but its salts with the alkaline metals
are easily decomposed, even carbon dioxide being sufficiently
powerful to liberate the acid from potassium or ammonium
cyanides ; hence it is that these substances always smell of
hydrocyanic acid when exposed to the air. The cyanides of
the heavy metals are, on the other hand, much more stable,
silver cyanide being unattacked even by the strong acids.
The solution of hydrocyanic acid in water readily decom-
poses with formation of ammonium formate and other sub-
stances. A similar change occurs more readily by the action
of dilute mineral acids, showing that hydrocyanic acid may be
regarded as the nitrile of formic acid :
HCN + 2H.,O = HCO 2 H + NHg,
Hydrogen cyanide. Formic acid.
with which compare
134 THIi CYANIDES
CH 3 CN + 2H.,O = CH 3 . CO 2 H + NH 3 .
Methyl cyanide Acetic acid,
or " acetonitrile."
Potassium Cyanide, KCN, is manufactured by heating
potassium ferrocyanide in iron vessels until decomposition
occurs according to the equation :
K 4 FeC N a = 4KCN + FeC 2 + N 2 .
The potassium cyanide is separated from the iron-carbide by
extracting the mass with water ; the solution is evaporated,
and the residue, after being fused, is brought into market in
lumps or sticks.
Perfectly pure potassium cyanide is best obtained by passing vapours
of HCN into a solution of KOH in alcohol.
Potassium cyanide is very soluble in water, and is extremely
poisonous. It is largely used in electro-plating for preparing
the solutions of gold or silver, and in the gold-fields for dis-
solving the gold from the quartz containing it. It is used in
the laboratory as a reducing agent in blow-pipe work.
Mercuric Cyanide, Hg(CN) 2 , is prepared by boiling
Prussian blue with water and mercuric oxide, or by dissolving
mercuric oxide in hydrocyanic acid. It is fairly soluble in
water, is very poisonous, and forms good crystals. It does
not evolve any perceptible amount of hydrocyanic acid when
treated with cold dilute sulphuric acid, but gives it off slowly
on boiling.
Silver Cyanide, AgCN, is obtained as a white precipitate
when a solution of potassium cyanide is added to one of silver
nitrate. It is insoluble in acids, but dissolves readily in excess
of the solution of KCN owing to the formation of a soluble
double salt AgCN . KCN.
On this is based a method for the quantitative estimation of soluble
cyanides ; standard solution of silver nitrate is added to the solution of the
cyanide until a permanent white precipitate just begins to form. When
this occurs, one molecule of AgNO 3 has been added for every two mole-
cules of the cyanide present :
AgNO 3 + aKCN = AgCN . KCN + KNO 3 .
XX THE FERROCYANIDES 135
Double Cyanides. In the double salt just referred to
AgCN . KCN we have an example of the marked tendency
shown by various cyanides of different metals to combine to
form double cyanides. In some of these double salts the com-
bination is only loose and is readily broken, while their pro-
perties are not fundamentally different from those of simple
cyanides. But in another important class of double cyanides
the combination is so complete that the essential properties of
the constituent salts entirely disappear in the double cyanide
formed by their union.
Potassium Ferrocyanide, K 4 FeCy , is such a double
cyanide. Its formula may be regarded as showing it to be
made up of 4KCy + FeCy 2 , four molecules of potassium
cyanide with one of ferrous cyanide, but in reality neither the
cyanogen group, nor the iron contained in the ferrocyanide
can be detected by their ordinary reactions. The ferrocyanide
is almost non- poisonous in comparison with the intensely
poisonous nature of the soluble simple cyanides, and the iron
in it is not precipitated by ammonium sulphide.
Potassium ferrocyanide is largely manufactured to serve as
a starting-point for the preparation of Prussian blue and other
cyanogen compounds. It is known commercially as yellow
prussiate of potash, and is made by heating in shallow iron
pans a mixture of charred nitrogenous refuse (horn, skin, etc.)
with potash and iron filings. The ferrocyanide is extracted
with water from the fused residue and purified by recrystallisa-
tion. It forms large tabular crystals of an amber colour. It
dissolves easily in water, and the solution gives characteristic
precipitates with solutions of several metallic salts, e.g. with
copper sulphate solution a brown precipitate of copper ferro-
cyanide is obtained :
K 4 FeCy 6 + 2CuSO 4 = 2K.jSO 4 + Cu >2 FeCy .
Potassium Copper
ferrocyanide. ferrocyanide.
When a strong acid, (HC1), is added to the concentrated
solution of potassium ferrocyanide, a white precipitate of
ferrocyanlc acid, H 4 FeCy ( ., is produced :
K 4 FeCy + 4 IIC1 = 4KC1 + H 4 FeCy e .
PRUSSIAN BLUE
The complex radicle, Fe(CN) ( , or FeCy 6 , which is present
in ferrocyanic acid and the ferrocyanides, carries the name
" ferrocyanogen."
Potassium Ferricyanide, K. { FeCy , is formed by oxidis-
ing a solution of the ferrocyanide by means of chlorine :
2 K 4 FeCy,. + C1 2 = 2 KC1 + 2 K 3 FeCy 6 .
It may be regarded as built up from three molecules of KCN
with one of ferric cyanide FeCy 3 , but just as is the case with
the ferrocyanide the properties of the compound are essentially
different from those of the simple cyanides.
Potassium ferricyanides is the commercial " red prussiate
of potash," and forms deep-red crystals.
Iron Salts and the Perro- and Ferricyanides.-
The reactions between solutions of iron salts and ferro- or ferri-
cyanides are of importance in analytical chemistry, and for the
thorough understanding of the composition of Prussian blue.
They are best shown in a tabulated form :
Solution used.
Potassium Ferrocyanide.
K 4 FeC y6 .
Potassium Ferricyanide.
K 3 FeCy 6 .
Ferrous salt
Light-blue pp. of ferrous
ferrocyanide, which
gradually darkens in
the air
Dark blue pp. of ferrous
ferricyanide ; Turn-
bull's blue
Ferric salt
Dark blue pp. of ferric
ferrocyanide ; Prus-
sian blue