Alfred Isaac Cohn.

Indicators and test-papers; their source, preparation, application, and test for sensitiveness . . online

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acetic acid. The residue, azolitmin, is finally dried
and powdered.

Stolba impregnates washed linen with a litmus
extract, and then plunges it into a 5- or lo-per-cent.
sulphuric acid, whereby the azolitmin is precipitat.ed
on the fibers. The material is then washed with pure
water, and finally treated with weak ammonia, and
the pure azolitmin finally obtained by neutralizing the
ammoniacal solution with sulphuric acid, collecting,
washing, and drying.

The quantity of azolitmin present in litmus varies
considerably. D. R. Brown examined 7 samples of
litmus, and obtained on an average 4.6 <f of azolit-
min ; two of the samples, however, were found to
contain over 13 <f c of azolitmin.

Properties : Azolitmin occurs as a dark, reddish-
brown, amorphous powder or as scales. It is but
slightly soluble in water, and is insoluble in alco-
hol or ether. With alkalies it yields blue com-
pounds readily soluble in watei.


Tests : The sensitiveness of azolitmin is determined
by adding 0.5 Cc. of a i-per-cent. aqueous solution
of the pigment prepared by means of a few drops of
centinormal alkali, to 100 Cc. of distilled water,
and observing how much centinormal hydrochloric
acid is required to change the color to red, and
then testing again to find how much centinormal
potassium-hydrate solution is required to restore the
blue color. With good azolitmin, 1.2 Cc. of centi-
normal acid, and 3 Cc. of centinormal potassa,
should more than suffice.

Application : Azolitmin is used in the form of a
solution made by dissolving i Gm. of the pigment
in 100 Cc. of water, just sufficient centinormal
potassa-solution being added to effect a distinct
blue color. (For details of application see Litmus).
Azolitmin Test-Paper is also prepared, and is
exceedingly sensitive. (See under Test-Papers).


C 8 H B .C.2(C B H).OH.O

ALKALIES = Violet ACIDS = Yellow

Synonym : Phenolbenzein.

Preparation : Benzaurin is prepared by heating 2
molecular weights of phenol with i molecular
weight of benzotrichloride on a water-bath. After
the reaction is over, the excess of phenol is removed
by a current of steam, and the benzaurin purified


by repeatedly treating it with warm solutions of
sodium or potassium sulphite, precipitating by
adding hydrochloric acid, and then collecting and
drying the precipitate. The reaction is as follows:

CH 5 .CC1 3 -f 2CH 6 .OH = C.H..C.2(C.H 4 ).OH.O -f 3HC1
benzotricliloride benzaurin

Properties : Benzaurin occurs as hard, red crusts,
presenting a metallic reflection. It is only slightly
soluble in water, but readily soluble in alcohol,
ether, and in glacial acetic acid, yielding yellow
solutions. It is insoluble in benzene, and melts at
100 C. With alkalies, benzaurin yields a violet
color, changed by acids to yellow.

By reducing-agents it is converted into dioxytri-

Application : Like other phenol derivatives of ben-


(C.H,0 8 .(CH s N) a .N,.(C,oIV a .(NH,.SO,Na),
ALKALIES = Brownish-red ACIDS Bluish-violet

Synonym : Sodium Orthotolidine-disazobinaphtyla-

Source : Benzopurpurine B. is prepared by the ac-
tion of I molecular weight of ortho-tolidine on 2
molecular weights of betanaphtylamine-sulphonic
acid, and the conversion of the resulting product
into the sodium salt.


Properties : Benzopurpurine B. was discovered in
1885 by Duisberg. Its formula is (C 6 H 3 ) 5 (CH 3 .N 2 ) 2 :
(C 10 H B ) 2 (NH,.SO 3 Na) 2 , and it occurs as a brown
powder yielding an orange-red to brownish-red
solution with water. The color of the solution is
unchanged by alkalies, but is changed to a blue or
violet by acids.

Application : Benzopurpurine B. was recommended
by Storch as the most useful of the tetrazo dyes,
and as being superior to Congo red as an indicator.
Its aqueous solution affords an excellent reagent for
free acids; and the solution, made violet by means
of an acid, is, according to Storch, one of the most
sensitive of indicators for alkalies, known. Even
the slightest traces of free ammonia in the atmos-
phere suffice to change the color of paper impreg-
nated with a solution of Benzopurpurine B. from
bluish-violet to red. The indicator may be used
for estimating ammonia in the presence of pyridine,
as titration with hydrochloric acid will indicate the
total base ; a second titration with litmus or tro-
paeolin OOO, on which pyridine has no effect, will
show the ammonia present, and the difference will,
hence, equal the pyridine. If very much pyridine is
present, however, the indications are apt to be un-

In general the indicator may be used like phenol-
phtalein ; and it is more sensitive than litmus.



ALKALIES Purplish-red ACIDS = Yellow

Synonyms: Fernambuco ; Pernambuco ; Red-Wood.

Source : Brazil wood is a red dye-wood obtained
from several species of Ccesalpinia found in South
America and in the West Indies. The chief species
yielding the commercial wood are Cczsalpinia
echinata Lam. (Gmlandina echinata Sprengel), C.
crista, C. brasiliensis Lin., and C. vesicaria. A
species of red-wood closely allied to the Brazil wood
is also obtained from the Ccesalpinia sapan L.,
Sapan (or Japan) wood.

Brazil wood has been long used for dyeing pur-
poses, and has of late years been recommended as
an indicator, in the form of a decoction, or in the
form of a solution of the crystalline principle,
brazilin, present in the wood, and upon which the
value of the wood as a dye depends (see Brazilin).

Preparation : Brazil-wood test solution may be pre-
pared by boiling 50 Gm. of the comminuted heart-
wood with 2 50 Cc. of water for half an hour, replacing
the loss by evaporation from time to time. After
the mixture has cooled, it is strained, the residue
washed with water until 100 Cc. of liquid are
obtained, and 25 Cc. of alcohol then added.

Properties : The color of the solution is changed to
an orange-yellow by acids, and almost disappears


on dilution; caustic alkalies and their carbonates
produce a purplish-red color.

Stannic chloride gives with the decoction a red
precipitate ; iron oxide a dirty-red to brown com-
pound ; and chromium oxide yields a brown com-
pound which, on the addition of potassium chro-
mate to the decoction, is deposited as a dark
precipitate. On adding alum to the decoction,
the latter is colored red, but yields no precipitate
until a basic alkali carbonate is added, when a red
lake is deposited. A similar lake is obtained on
boiling Brazil-wood decoction with aluminum

Application : Brazil-wood test solution is used like
Brazilin (which see).


C, H 14 6 (C,H 14 5 ?)

ALKALIES = Purplish-red AciDS = Yellow

Synonym: Brasilin.

Source : Brazilin is obtained from several species of
Caesalpinia, principally C. brasiliensis Lin. and C.
echinata Lam., but is also found in Sapan (or Japan)
wood, C. sapan L.

Brazilin was first isolated by Chevreul in an im-
pure condition, and was considered by Gerhardt to
be identical with hematoxylin, a view not generally


accepted because of the difference in the colors ex-
hibited by the compounds of the two substances.

Preisser, however, in 1844, succeeded in isolat-
ing, from the inner and less colored portions of the
wood, the brazilin, in the form of colorless acicular
crystals, the solution of which became yellow on
standing and exhibited bright-red margins at the
surface, and which, on boiling, became a handsome
carmine-red and deposited on evaporation handsome
bright-red needles, which he named brazilein ; this
he believed was formed from brazilin by oxidation
of the latter in the presence of an alkali, as fol-
lows :

C JO H 1 A+0 = C, H,A + H 1

brazilin brazilein

Later investigations also seemed to make it ap-
pear probable that brazilin is formed from a gluco-
side contained in the wood, but this has not as yet
been shown to be a fact.

Preparation: Brazilin is generally obtained during
the preparation and storage of the commercial ex-
tract of Brazil wood used in dyeing, in the form of
a deposit of dark, brownish-red crusts consisting of
brazilin and its lime compound. These crusts are
either dissolved in boiling 5- to lO-per-cent. alcohol
with the addition of some hydrochloric acid and zinc
dust, or they are carefully washed with a 5-per-cent.
hydrochloric acid and then extracted with 12-per-
cent, alcohol and the brazilin crystallized.


Properties : The appearance of brazilin varies accord-
ing as it is crystallized from dilute or concentrated
solutions. From the former it is obtained as com-
pact, amber-yellow, rhombic crystals containing
I molecule of water of crystallization ; from the
latter it crystallizes as white, silky, felted needles
containing ij- molecules of water of crystallization,
which is driven off, however, at a temperature of
80 C.

Brazilin is soluble in alcohol, water, and in
ether, has a sweetish taste, and on heating sub-
limes partly unchanged. On dry distillation it
yields resorcin. Nitric acid converts it into trini-
troresorcin. On exposure to air and light it? solu-
tion rapidly becomes orange-colored.

Brazilin possesses the properties of a weak acid,
and yields with alkalies soluble, purplish-red com-
pounds affording solutions which are decolorized
by zinc dust, sunlight, sulphurous acid, and by hy-
drogen sulphide; the color of the solutions is
changed by acids to yellow.

With the earths and oxides of the heavy metals,
brazilin forms insoluble compounds. Caustic alkalies
and their carbonates yield a purplish- red color with
brazilin solutions, and with alum a red lake is

Brazilin is extremely sensitive to light and air,
by which it is rapidly darkened, becoming orange-


colored, or even red. It must be preserved, hence,
in carefully stoppered, opaque vials.
Application : Brazilin and decoction of Brazil wood
have been found to be among the most sensitive of
indicators, and especially serviceable for titrating
alkaloids, with which other indicators yield indis-
tinct colors, and particularly for quinine, atro-
pine, brucine, and cocaine.

The indicator is inapplicable where sulphurous
acid or hydrogen sulphide are liberated, as these
destroy the color of the solution.

A test-paper is also used, and is described under
under Test-papers.



Source : The red cabbage is a variety of the Brassica
oleracea, which has been brought under cultivation
from stock originally found wild on the coast of
England and the Mediterranean as well as in Con-
tinental Europe, and known as Wild Cabbage, or
Sea Cabbage.

Preparation : A tincture of the leaves of the red
cabbage is made by comminuting the leaves, and
exhausting them with water or diluted alcohol.

Properties : The tincture of the leaves is violet in
color. The color is changed to green by alkalies,
and to red by acids,


Application: Tincture of red cabbage may be ap-
plied for all titrimetric processes, and is quite sensi-
tive, even to Ammonia. A paper impregnated
with the tincture is also quite sensitive. It is but
seldom used.

ALKALIES Purplish-red ACIDS = Yellowish-red

Source : Carmine is the pigment obtained from the
dried female insect, Coccus cacti, Lin. It was first
isolated (and named) by Pelletier and Caventou in
1818, by treating cochineal with boiling ether to
remove the fat, then exhausting with alcohol so
long as any color was extracted, distilling off the
greater bulk of the alcohol, and allowing the resi-
due to evaporate spontaneously. The impure car-
mine so obtained was then treated with cold alco-
hol, which dissolved the carmine but left the fatty
matter. From its solution the carmine was then
precipitated by adding an equal volume of ether.
The carmine so obtained is, however, impure, and
it was only in 1847 that Warren de la Rue suc-
ceeded in obtaining the pure carmine in the form of
carminic acid (which see).

Preparation : Commerci-al carmine is generally pre-
pared by a process similar to the following: Pow-
dered cochineal is boiled with 40 times its weight
of water for 15 minutes, and of its weight of


powdered alurn is added. The mixture is next
boiled for 3 minutes longer, and then set aside,
when the carmine precipitates, and is removed by
filtration. On allowing the mother liquor to stand
for several days, a second deposit of carmine is
obtained, which, however, is not of as good a color
as that first obtained.

The celebrated process of Madame Cenette is as
follows: Powdered cochineal is boiled with 75
times its weight of water for two hours ; then -^ T of
its weight of potassium nitrate is added, and the
whole boiled for 3- minutes longer, and finally of
its weight of oxalic acid added, and the mixture
boiled for another 10 minutes. The liquid is then
allowed to clarify by depositing for 4 hours,
when the clear liquid is decanted into shallow glass
vessels and set aside for three weeks; the deposited
carmine is then separated from the supernatant
liquid and dried in the dark.

Properties : Carmine occurs in the form of small,
bright-red lumps, soluble in ammonia water, and
yielding a deep-red solution, the color of which is
changed to a yellowish-red by acids. It consists
chiefly of carminic acid, but contains also alumina,
lime, other organic acids, and a protein compound,
according to Liebermann.

Carmine solution is not decolorized by sodium
sulphite ; on boiling with lime-water it is colored



Tests : Carmine is frequently adulterated, according
to Donath, not only with starch, alumina, and
brick-dust, but also with baryta, zinc, and lead
lakes, and certain azoclyes. The impurities are left
behind on dissolving the carmine in ammonia.

Equal quantities of the sample and of a known
carmine are heated in porcelain crucibles, and the
odors compared : That from pure carmine resem-
bles the odor evolved on heating proteins, while
that from eosine-lake will have a distinct bromine
odor, and that from paeonin-lake will smell like
phenol. On incineration the adulterated prepara-
tions will also yield more ash.

Colorimetric comparisons may also be made be-
tween the sample and a known carmine, solutions
of both being similarly made by the aid of a small
quantity of ammonia, and compared in two similar

Application : Carmine is used in a manner similar to
that of cochineal (which see).

Alkaline Carmine Solution is also used in tit-
rimetry. It is prepared by dissolving carmine in
soda lye. For use as an indicator I Cc. of this
solution, previously decolorized by the addition of
sufficient hypobromite solution, is added to the
liquid to be titrated.

Alkaline carmine solution is very sensitive to at-
mospheric oxygen. With it, carbonates of the
alkalies are best titrated in warm solutions; and


carbonic acid gas is not so disturbing as when litmus
is used. The change is very sharp and bright, par-
ticularly with pure alkalies or acids.


Ci7 H i (jOj

ALKALIES = Purplish-red ACIDS = Yellowish-red

Source : Carminic acid is the true coloring-matter of
the dried female insect, Coccus cacti L., cochineal
(which see).

Preparation : Carminic acid was first obtained by
Warren de la Rue in 1847, by exhausting powdered
cochineal with boiling water, and precipitating the
filtered solution with lead-acetate solution acidulated
with acetic acid. The precipitate was washed with
water, and then decomposed by a current of hydro-
gen sulphide, while suspended in water. After filter-
ing, the filtrate was again precipitated by lead-
acetate solution and the precipitate decomposed by
hydrogen sulphide. The filtrate now obtained was
evaporated in vacuo, the residue dissolved in boil-
ing alcohol, and a little ether added to precipitate
nitrogenous matter, the solution again filtered, and
finally evaporated.

Ordinarily carminic acid is obtained by first treat-
ing cochineal with ether, and then exhausting the
residue with successive portions of boiling alcohol,


which, on cooling, deposits a part of the acid, and
yields the remainder on spontaneous evaporation.
It is then freed from adhering fatty matter by dis-
solving in alcohol and adding an equal quantity of
ether. The pure carminic acid is deposited in the
course of a few days.

Properties : Carminic acid occurs as a red or brownish-
red amorphous powder, or crystals, readily soluble
in .water and in alcohol, but difficultly soluble in

Its formula is C n H 18 O 10 . It is soluble in con-
centrated sulphuric acid and in hydrochloric acid
without decomposition, but, on boiling with diluted
sulphuric acid, it yields a coloring-matter, carmine
red, and a sugar, according to the following reac-

C,,H,.0 10 + 2H,0 = C,,H,,0, + C.H..O.

carminic acid carmine red

Carminic acid is hence considered to be a gluco-
side. It has the properties of a weak, dibasic acid,
and forms easily soluble salts with the alkalies, but
forms insoluble violet or purple salts with the alkali
earths, earthy metals, and metals generally.

Chlorine, bromine, and iodine decompose it, and
nitric acid converts it into oxalic and nitro-coccic
acids. Fused with potassium hydrate it yields
oxalic and succinic acids and coccicin.

The aqueous solution is faintly acid, and of a


yellowish-red, the color being changed by alkalies
to a purplish-red. Nascent hydrogen decolorizes
the solution, but the color is restored on contact
with the air.

Schutzenberger believes carminic acid to be a
mixture of at least two acids, one being the true
carminic acid, having the formula C 9 H e O 6 , and in-
soluble in a mixture of alcohol and ether; the
other, oxycarminic acid, C 9 H B O 7 , and soluble in a
mixture of alcohol and ether.

Tests: i Gm. of carminic acid should completely dis-
solve in 2 Cc. of water; the addition of 20 Cc. of 95-
per-cent. alcohol to the solution should cause no
noticeable precipitation.

A simple method of estimating the value of car-
minic acid is by colorimetric comparison. This is
accomplished by dissolving equal quantities of a
preparation of known quality and the one to be
tested, in equal volumes of water, solution being
facilitated by adding a few drops of ammonia.

The sensitiveness of carminic acid is tested by
adding 0.5 Cc. of a i-per-cent. aqueous solution of
carminic acid to 100 Cc. of water, and observing
how much centinormal ammonia, or centinormal
hydrochloric acid, is required to effect changes in
color. A good preparation will not require more
than 0.6 Cc. of the acid, 0.8 Cc. of the ammonia,
or 2.8 Cc. of centinormal sodium hydrate.


Application: Carminic acid is used in titrimetric oper-
ations like cochineal (which see). It is now being
frequently used in microscopy as a stain, although
carmine is more generally used.

Carminic acid is far more sensitive to bicarbonates
of alkali earths than is litmus. According to H. N.
Draper it yields a reaction with alkalies in solutions
of i: 1,400,000; and the reaction is only slightly,
influenced by carbonic-acid gas.

Carminic acid has also been lately recommended
as a precipitant of albumins and proteoses. It is
claimed to be exceedingly sensitive, I part of albumin
in 90,000 parts of solution being readily detected.


Co(NO 3 ),.6H 2 O

Synonyms : Normal Cobalt Nitrate; Neutral Cobalt

Preparation : Cobaltous nitrate is prepared by dis-
solving metallic cobalt, cobalt oxide, CoO ; cobalt
hydrate, Co 2 (OH) 6 ; or cobalt carbonate, CoCO 3 ; in
nitric acid, evaporating the solution, and crystal-
lizing from the concentrated solution on cooling.

Properties : Cobaltous nitrate occurs in the form of
red prisms or monoclinic plates, > which are deli-
quescent in moist air, but permanent in dry air,
and which melt at a temperature below 100 C.
At a higher temperature, cobaltous nitrate melts to


form a viscid, green liquid, and on strongly heating,
nitrogen monoxide, NO, is evolved, cobaltic
oxide, Co 2 O 3 , remaining.

Application : Cobalt nitrate has been recommended
by G. C. Stone as an indicator in the volumetric
determination of zinc by titration with potassium
ferrocyanide. The cobalt solution should be quite
dilute, and a drop of it placed on a white porcelain
plate, just touching a drop of the solution to be
tested. If the drops touch but do not mix, the end-
reaction is marked immediately by a greenish line
at the juncture of the two drops. The best strength
of the potassium-ferrocyanide solution was found to
be about 3 per cent.


ALKALIES = Violet ACIDS = Yellowish-red

Source: Cochineal is the dried female insect,
Coccus cacti Lin., one of the Hemiptera first found
living on various species of cactus in Southern
Mexico, Guatemala, and Honduras, and later on in-
troduced into the West Indies, East Indies, Spain,
Algiers, Teneriffe, and Malta.

The insect is gathered some time after being
fecundated, and is killed by immersion in boiling
water, exposure to steam, or by drying in a hot oven
or on heated plates. The last-mentioned treatment
is believed to yield the finest product, known as


(l silver cochineal" in the trade; but that obtained
by the immersion process, and known as "black
cochineal," may be frequently found covered with
powdered talcum, calcium sulphate, lead carbonate,
and even stearin, in order to simulate the grayish
color of the " silver cochineal."

Besides the above impurities, barium sulphate,
particles of lead, vermilion, chrome red, insects
already exhausted of their coloring-matter, and
grains of substances artificially prepared to imitate
the dried insect, have been met with as adulterants.

The pigment upon which the value of cochineal
depends, and to which is due its color, was first
isolated by Pelletier and Caventou in 1818, and
named by them " carmine." This was found to be
impure, however, and it was only in 1847 tnat
Warren de la Rue succeeded in obtaining the pure
pigment, which he named carminic acid (q. v.).

Properties: Dried cochineal is about 2 to 5 Mm.
long, nearly hemispherical, flat or slightly concave
beneath, convex above, transversely wrinkled, pur-
plish-gray or purplish-black in color, and easily
pulverizable, yielding a dark-red powder. It has a
faint, heavy, peculiar odor, and a slightly bitterish,
acidulous taste, and. swells up in water, to which it
yields its coloring-matter, which is soluble also in
alcohol and in ammonia, slightly soluble in ether,
and insoluble in fixed and volatile oils.

According to researches by Clark, it would


appear that the coloring-matter, carmine, is not
present in the living insect in solution, but consists
of minute, red granules surrounding a larger, color-
less granule suspended in a colorless liquid.

The aqueous infusion of cochineal is of a crimson-
violet color, which is deepened by alkalies and
brightened by acids. With bismuth, nickel, and
zinc salts, it affords lilac-colored precipitates; with
iron salts a dark-purple, almost black, precipitate is
had ; stannic salts give a brilliant scarlet and stan-
nous salts a red precipitate, and with alum it forms

Tests : The value of cochineal as an indicator de-
pends on its carminic-acid content. The process
followed by Penny for testing its value is as fol-
lows : i Gm. of the cochineal is digested with 5
Gm. of caustic potassa in 20 Cc. of distilled water
for an hour, and the mixture then diluted so as to
measure 100 Cc. Sufficient of a i-per-cent. potas-
sium-ferricyanide solution is then added to 10 Cc.
of the mixture to convert the purple into a yellow-
ish-brown color. The quantity required for this
purpose is then compared with that required for a
sample of cochineal known to be good.

0.5 Cc. of a i : 80 aqueous infusion of cochineal
added to 100 Cc. of distilled water, requires 3 Cc.
of centinormal hydrochloric acid, and 2.8 Cc. of
centinormal sodium hydrate, to effect changes of


Application : Cochineal is an exceedingly valuable
indicator. It is used in the form of solution gen-
erally, although a paper is also prepared (see Test-

A cochineal tincture recommended by Luckow,
and which is very sensitive, is made by macerating
3 Gm. of whole cochineal in 250 Cc. of. a mixture

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Online LibraryAlfred Isaac CohnIndicators and test-papers; their source, preparation, application, and test for sensitiveness . . → online text (page 3 of 14)