Alfred Isaac Cohn.

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

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alcohol is used, 3 to 5 drops of which are added to
100 Cc. of the solution to be titrated. Its sensi-
tiveness is such that 100 Cc. of distilled water to
which 0.2 Cc. of the indicator-solution has been
added will require 3 Cc. of decinormal hydrochloric
acid (methyl orange similarly tested requires only
I Cc. of centinormal acid).


ALKALIES Blue ACIDS = Carmine Red

Synonyms : Eupitton ; Pittakal ; Hexaoxymethyl-
aurin ; Hoffmann's Indicator.

Preparation : Eupittonic acid was first isolated by
Liebermann from pittakal, which was discovered in
1835 by Reichenbach during the fractional distilla-
tion of beech-tar creosote on treating certain por-
tions of the distillates with baryta and exposing to
air. Later on Hoffmann demonstrated 'that the
pittakal resulted from the oxidation of pyrogallol-
dimethyl ether and the dimethyl ether of methyl-
pyrogallol, the reaction occurring as follows :


C 8 fr o (OCH 8 ),OH + C.H a (CH 3 ).(OCH 8 ),OH + 3 O

pyrogallolmethyl ether methylpyrogallol-dimethyl ether

= C l .H.(OCH,).0, + 3H 1

eupittonic acid

Eupittonic acid is prepared by dissolving 2 mo-
lecular weights of pyrogalloldimethyl ether and I
molecular weight of methylpyrogalloldimethyl
ether in alcohol, adding an excess of caustic soda,
and heating the mixture on an oil-bath for half an
hour to a temperature of from 200 to 220 C.
The mass soon becomes greenish, then blue. It is
then treated with hydrochloric acid, and the brown-
ish-red residue crystallized by dissolving in alcohol
and precipitating with ether.

Properties Eupittonic acid occurs as orange-yellow
crystals, .insoluble in water, soluble in alcohol and
in ether, and yielding with glacial acetic acid a
brown solution. The acid is bibasic, and yields
with alkalies blue compounds soluble in water, with
a violet-blue color. The solutions are, however,
precipitated by an excess of alkali. In ammoniacal
solutions it dissolves with a reddish-blue color.
Acids change the color of alkaline solutions to a
carmine-red. With salts of lead, tin, and other
heavy metals eupittonic acid yields blue, difficultly
soluble lakes.

Application : Eupittonic acid is practically unused as
an indicator. Its sensitiveness is stated to be such,
however, that 0.6 Cc. of centinormal alkali suffices


to afford a distinct change in 50 Cc. of solution con-
taining the indicator.

ALKALIES = Intense Yellow ACIDS = Colorless

Synonym : Lux's Indicator.

Source '. Flavescin is a peculiar coloring-matter ob-
tained from oak shavings.

Preparation: It is obtained by passing hot air satu-
rated with steam through oak shavings, condensing
the vapors in a cooled receiver, filtering the liquid
thus obtained, and shaking it out two or three
times with ether. The ethereal solution, contain-
ing the flavescin and empyreumatic acids, is evap-
orated, and the vessel heated to 40 or 50 C.,
while a current of air is passed through it until the
odor of acetic acid has disappeared. The brown-
ish residue is then digested with ten or twenty
volumes of cold water; this treatment yields a con-
centrated solution of flavescin, which is preserved
by diluting it with several volumes of alcohol.

Properties : Flavescin occurs as a brown, translucent,
tenaceous mass, readily soluble in water, alcohol, and
in ether. It yields colorless solutions changed to an
intense yellow by alkalies. The reaction is very
sharp, and is clearly seen even by artificial light.
Nascent carbonic-acid gas and bicarbonates de-


colorize it, the coloration and decoloration being
more rapid in alcoholic solution. Flavescin is
decomposed by most inorganic and organic acids,
the solution being decolorized. Even the fatty-
and resin-acids act thus. The color is, however,
restored by alkalies.

Application : Flavescin was recommended by Warder
F. Lux as an indicator, and in his hands has given
results closely agreeing with those of other indi-
cators. It is not serviceable, however, for titrating
small quantities of caustic alkalies when consider-
able quantities of carbonates of the alkalies are
present, although it is useful for carbonates in the
presence of bicarbonates, and also for carbonic acid.
The reaction occurring with carbonates is due to the
decomposition of the latter into a bicarbonate and
a compound of flavescin. It is useful for inorganic
acids, as well as for organic acids.


(C 8 H 3 OH) 2 .C.O.(C 6 H4.COO)

Green No


Fluorescence Fluorescence

Synonyms : Resorcin-phtalein ; Diresorcin-phtalein ;
Tetra-oxyphtalophenone ; Uranine; Kruger's In-

Preparation : Fluorescein was discovered by Baeyer
in 1871. It is prepared by heating an intimate


mixture of 2 molecular weights of resorcin and
I molecular weight of phtalic anhydride on an
oil-bath at a temperature of from 190 to 200 C.,
until aqueous vapors are no longer evolved. It is
important that pure materials, particularly resorcin,
be used, in order to obtain a pure product. If
impure materials are employed, the final purifi-
cation of the fluorescein is rendered extremely

The mass obtained is then treated with boiling
water, and the residue is powdered, dissolved in
potassa solution, and reprecipitated by means of
an acid.

Tests : The flubrescence of a solution of o. I Cc. of
a 1:100 fluorescein-solution in 100 Cc. of distilled
water is destroyed by th^ addition of 0.5 Cc. of
decinormal hydrochloric acid.

Properties: Pure fluorescein occurs as a yellowish-
red, crystalline powder, or dark-brown crystals, in-
soluble in water. It is but difficultly soluble in
cold alcohol, more readily soluble in boiling
alcohol, and quite soluble in ether and in dilute
acids. With alkalies it yields solutions having a.
magnificent, bright-green fluorescence, permanent
even in very dilute solutions; by transmitted light
the solutions are red. Its formula is

O(C e H 3 .OH) a .C.C 8 H 4 .O.CO + H 3 O.


The sodium or potassium salt forms a yellowish-
brown powder, soluble in alcohol and in water with
intense green fluorescence.

Application: Fluorescein was recommended for use
as an indicator by Kriiger. It is very useful for
titrating in dark-colored liquids in which color-
changes are observed only with difficulty, because
the fluorescence is clearly visible so long as the
liquid is alkaline, but disappears the instant the
liquid becomes neutral or acid. When fine white
precipitates are formed in the liquid, however, the
fluorescence may be masked ; hence the precipitates
(e.g., barium sulphate) are allowed to settle before
determining the end of the reaction.

Fluorescein is serviceable in the presence of car-
bonic-acid gas, as this does not affect it. It is,
however, inapplicable in the presence of acetic acid,
as the latter dissolves the fluorescein with fluo-
rescence, and the reading would, hence, be incor-

For use, a solution may be prepared by shaking
together I Gm. of fluorescein and 100 Cc. of diluted
alcohol, and then filtering. The solution has an
orange-red color; it does not keep very well.
From S to 5 drops are used.

Fluorescein gives fairly good results in titrating
brucine, cocaine, co'deine, coniine, emetine, nicotine,
pelletierine, quinine, sparteine, strychnine, and the-
baine ; it is very good, however, with atropine.



C 30 H 18 N 3 HC1



Colorless in Excess

Synonyms: Magenta; Roseine; Aniline Red; Rubin;
Azaleine; Solferino; Fuchsiacine ; Erythrobenzeine ;
Harmaline; Rubianite; Rosaniline Hydrochloride.

Preparation: Fuchsine was discovered by Natanson,
of Warsaw, in 1856, by heating aniline with ethy-
lene chloride in sealed tubes at a temperature of
200 C. In 1858 Hoffmann also obtained it by
heating carbon bichloride and aniline for 30 hours
at a temperature of 170 to 180 C., while looking
for carbo-triphenyltriamine ; he considered the red
color due to an impurity. It was first made on a
practical scale by Verguin, in 1859, by substituting
tin bichloride for the carbon bichloride. The pro-
cesses now generally followed are modifications of
those patented by Hellman in 1859 anc * by Med-
lock in 1860, in which arsenic is used; and that
patented by Coupier in 1866, in which nitrobenzene
is employed.

Fuchsine is obtained by heating a mixture of
equal molecular weights of orthotoluidin, para-
toluidin, and aniline with a concentrated solution
of arsenic acid at a temperature of 170 to 180 C.
A part of the water distills over during the opera-
tion, which lasts from seven to eight hours. The


mass is then comminuted, after cooling, and ex-
hausted with boiling water under pressure ; the
arsenous and arsenic acids present are neutralized by
the addition of lime. Sodium chloride is now
added, and the rosaniline hydrochloride, fuchsine,
obtained by crystallization ; it is then recrystallized
several times, but cannot be obtained entirely free
from arsenic acid.

In order, however, to obtain an arsenic-free
product, a mixture of aniline, nitrobenzene, nitro-
toluene, iron, and hydrochloric acid is heated as
in the arsenic process. The fuchsine is extracted
from the fused mass, and purified as already de-

Properties : Fuchsine occurs in the form of crystals
of various size having a brilliant green, metallic
color. They are soluble in water and in alcohol,
yielding intensely crimson solutions. The com-
mercial preparation is most generally a mixture of
pararosaniline and rosaniline hydrochlorides, the
latter predominating, and is most generally found
to contain arsenic, which should always be tested
for; if found to be present, the preparation should
be rejected.

Pure fuchsine is chemically rosaniline hydro-
chloride, C 10 H 19 N 1 HC1.

Tests: Fuchine should be tested for arsenic by
Marsh's apparatus.


Mineral impurities maybedetectedbyincineration.

Pure fuchsine is decolorized by sulphurous acid.
Application : A solution of fuchsine in glacial acetic
acid constitutes a delicate reagent for nitrous acid
and nitrites, which change the color of the solution
to violet first, then through blue, green, yellow, and
finally into orange. Nitric acid does not yield the
color reactions.

The crimson color of fuchsine solutions is
changed by acids to yellow; on greatly diluting the
solution, however, or on adding an alkali, the crirr-
son color is restored. Ammonia, however, and con-
centrated caustic alkalies also destroy the color,
hence fuchsine is not satisfactory for use as an indi-
cator generally.

Fuchsine-Sulphurous Acid, prepared by passing
sulphurous acid gas into a dilute solution of fuchsine
until the crimson color has changed to a pale yel-
low, has been found to be a very delicate reagent
for aldehydes and some of their derivatives. These
all afford an intense, violet-red color with it. The
solution may be preserved in well-stoppered bottles.


(C,H a .O.HO) 3 .(CO),.O.C 8 H 4

ALKALIES = Bright Red ACIDS = Pale Brown

Synonyms'. Alizarin Violet; Anthracene Violet;
Pyrogallol Phtalein ; Dechan's Indicator.

Preparation: Gallein was discovered by Ad. Baeyer


in 1870. It is prepared by heating I molecular
weight of phtalic anhydride and 2 molecular weights
of pyrogallol at a temperature of from 190 to 200 C.
for several hours, dissolving the melted mass in alco-
hol, and precipitating by adding water. The pre-
cipitate is purified by repeated solution in alcohol,
reprecipitation by water, and finally drying.

Properties : Purified gallein has the composition
C ao H 10 O 7 , and forms dark-reddish crystals having a
greenish reflection, or a brownish-red powder. It
is almost insoluble in cold water, chloroform, or
benzene, and is difficultly soluble in hot water,
ether, acetone, or glacial acetic acid. It is but
slightly soluble in cold alcohol, but is very readily
soluble in hot alcohol, yielding a dark-red solution.
It also dissolves in cold sulphuric acid unchanged,
yielding a dark-red solution.

With minute quantities of alkalies, lime, or baryta,
it yields salts which dissolve in water with a red
color, which is changed to a blue, however, on
adding an excess of alkali. Acids, even sulphurous,
precipitate the gallein from the solution unchanged.
Gallein also dissolves in ammonia-, lime-, or baryta-
water, with a violet color; the color of the am-
moniacal solution remains unchanged by heating.

With alumina and chromium oxide gallein forms
grayish-violet, insoluble lakes. Reducing agents
convert it into hydrogallein, and finally into gallin.

Application : Gallein was proposed by Dechan as an

82 2ND 1C A TORS

indicator. It is more sensitive towards alkalies
than phenolphtalein, and may be advantageously
used in titrating many alkaloids, such as strychnine,
morphine, quinine, cinchonidine, and atropine. It is to
some extent affected by carbonic-acid gas. Am-
moniacal salts, when present, do not affect it, and
it is serviceable for ammonia. Organic acids may be
titrated by means of gallein, as the indicator is
sharply affected by them. Carbonates and alkaloidal
bases .which are neutral toward phenolphtalein yield
a rose-red color with the indicator.

For use a solution of I : 1000 in alcohol may be
made, about 10 drops being added to 100 Cc. of
liquid to be titrated.


C H M 6 -f Aq


Source: Haematoxylin was discovered in 1811 by
Chevreul, who named it Haematin, in the heart
wood of Hcematoxylon campechianum L. (logwood ;
Campeachy wood), one of the Leguminosae native
to Campeachy, Yucatan, Honduras Bay, and several
of the West Indies.

Preparation : Haematoxylin was obtained by digest-
ing ground logwood with water at a temperature
of from 50 to 55 C. for several hours, straining,
evaporating the extract to dryness, and exhausting
the residue with strong alcohol. The alcohol was


then evaporated, the residual liquid mixed with a
little water, and the whole set aside to crystallize.

Erdmann, in 1842, obtained the haematoxylin
from the commercial extract by powdering the
latter, mixing it with some sand, and exhausting
with repeated portions of ether containing a slight
proportion of water. The ethereal extract was then
distilled off, and the remaining syrupy liquid mixed
with water, loosely covered, and set aside to crys-
tallize. The crystals separated in a few days, and
were then collected, washed with cold water, and
recrystallized from hot water, or from water to
which a little ammonium- or sodium-sulphite had
been added.

Properties: Haematoxylin forms colorless, or slightly
yellowish, sweetish, efflorescent, shining prisms
containing three molecules of water of crystalliza-
tion, or occurs as rhombic crystals with one mole-
cule of water of crystallization. The crystals have
the composition C ]6 H M O 8 -j- Aq. , and are soluble in
ether, alcohol, and in hot water, but are only slightly
soluble in cold water. They melt at from 100 to
120 C. in their water of crystallization, and char at
higher temperatures. On exposure to air they are
rapidly converted into haematein.

Haematoxylin is very soluble in a solution of
borax; and with warm solutions of this a very con-
centrated, syrupy solution may be obtained, which
is no longer alkaline, but neutral, or even slightly


acid, has a bluish tinge, and does not deposit
borax on adding alcohol or alcohol and ether, or
yield haematoxylin crystals on evaporation. Acids
do, however, precipitate the haematoxylin crystals
with one molecule of water of crystallization ; so do
also certain salts, such as sodium- or ammonium-
chloride, precipitate haematoxylin, but in an
amorphous form.

Boiling hydrochloric acid or potassa lye does not
decompose haematoxylin; nitric acid converts it
into oxalic acid, and concentrated sulphuric acid
dissolves it with brownish-yellow color.

With alkalies, haematoxylin yields solutions hav-
ing a purple color changed to blue by the action of
atmospheric oxygen which converts the haematoxy-
lyn by oxidation into haematein, C 10 H 13 O, , the color
changing finally to a yellowish-brown.

On dry distillation haematoxylin yields pyrogallol
and resorcin ; on exposure to light it is colored

With baryta and with lead acetate it yields white
precipitates; with copper salts, greenish-gray pre-
cipitates, which soon become blue on contact with
air; and with stannous chloride a red precipitate is

Haematoxylin is extremely sensitive to ammonia,
the most dilute solutions being rapidly affected
by it. It must be kept in well-closed bottles,
from air and light.


Tests : Haematoxylin should always be tested before
use, so far as its solubility is concerned. A small
quantity heated on platinum foil should leave no
residue. The crystals must also be clean and not

Application : Haematoxylin is not well adapted for
alkalimetry or acidimetry, because, during warming
with an alkali carbonate, it undergoes a certain
change, and upon neutralization exhibits a color
entirely different from what it originally had, which
may, hence, be the cause of uncertainty in observa-
tion. It may be used for certain substances, such
as copper salts, alum, chalk, etc.

Haematoxylin is one of the best of indicators for
general use for alkaloids, and particularly for atro-
pine and emetine.

Haematoxylin Test-paper, when recently made,
is also very serviceable. (See Test-papers).



Synonyms : Logwood ; Campeachy Wood.
Source : Haematoxylon is the name usually applied to
the wood of Hcematoxylon campcchianum L., log-
wood, one of the Leguminosae native to Cam-
peachy, Yucatan, Honduras Bay, and several of
the West Indies.


Properties : Haematoxylon is heavy, hard, and
coarsely grained, brown or reddish-brown within,
and blackish-red or bluish-black externally. The
constituents of the wood are hsematoxylin (upon
which its colorific properties depend), a volatile oil,
tannin, resinous substances, various salts of acetic
acid, calcium oxalate, potassium chloride, alumina,
iron and manganese oxides, and silica. The wood
yields to water its coloring-matter, and affords a
dark-red extract which affords blue precipitates with
lead acetate, lime-water, and alkali carbonates;
with alum a violet one; with nutgalls a black, and
with iron and chromium salts a violet-blue to black

Preparation: Haematoxylon is generally applied as
an indicator in the form of a tincture or decoction,
and is exceedingly sensitive. Since the wood is
very easily affected by both light and air, shavings
from the inner portion of a billet are used, and the
tincture prepared by treating the shavings with a 45-
per-cent. pure alcohol for 24 hours in a well-closed
vessel in a warm place. The tincture so obtained
is filtered through glass wool, carefully avoiding any
exposure to ammoniacal vapors, and is preserved in
small vials with closely fitting glass stoppers.

Application : A tincture so prepared is one of the
most sensitive of reagents for free alkalies and alka-
line salts. It is particularly sensitive to ammonia,


the faintest traces of which suffice to afford a blue
color, a fact that renders the application of the
haematoxylon as an indicator somewhat unsatisfac-
tory because of the difficulty of securing an abso-
lutely ammonia-free atmosphere in which to

Haematoxylin is excellently adapted for titrating
the alkaloids, and particularly for quinine and spar-

A few drops of the tincture added to a solution
containing alkalies or alkaline earths affords a red,
violet, or blue color, according to the quantity of
alkali present. One part of ammonia in 500,000
parts of water affords a distinct red color. Many
metals, particularly iron and copper, yield blue or
bluish-violet precipitates with haematoxylon tinc-
ture, even when other reagents fail to show the
minute traces present.

An aqueous decoction of haematoxylon is simi-
larly sensitive, and its color is rapidly changed to a
violet even by pure calcium carbonate, chalk, and
marble dust. Both the tincture and the decoction
should be kept only in an acidulated condition be-
cause of their exceeding proneness, when in neutral
or alkaline solution, to become colored.

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



(C.H,)2.(CO),.C a .(NH),.(HSO,) a

Synonyms: Sulphindigotic Acid; Indigosulphuric

Preparation : Solution of indigosulphonic acid for
indicating purposes is prepared by dissolving pow-
dered indigo in fuming sulphuric acid, then neutral-
izing with calcium carbonate, diluting the solution
with 16 times its volume of water, and filtering the
blue liquid so obtained.

Application : The blue color of the indigcsulphonic-
acid solution is not disturbed by carbonates of the
alkalies, but caustic alkalies change the color to
yellow, hence these latter may be determined in the
presence of carbonates. For this purpose, I or 2
drops of the indicator solution are added to the
solution to be titrated, and the acid then added.
When the free alkali is neutralized, a green color
forms, which gives place to a blue. With a white
dish, the color-change is seen to take place very
sharply, and good results may be had. The indi-
cator was recommended for use by Engel and



ALKALIES Yellowish-brown ACIDS = Blue

Synonyms : Soluble Indigo; Indigo Extract; Blue
Carmine; Indigotine ; Sodium Indigotin-disul-
phonic Acid.

Source : Indigo carmine is the blue pigment pre-
pared from indigo, an oxidation product of indigo
white, or indican, a constituent of the sap of
various species of the Indigoferae, chiefly Indigofera
tinctoria L., /. disperma, I. anil L., and 7. argentea
L., Nat. Ord. Leguminosae. These are found in
the East Indies, Africa, West Indies, Brazil,
China and Hindostan.

Preparation : According to Joclet, indigo carmine
is prepared by dissolving i part of powdered indigo
in 5 parts of concentrated sulphuric acid heated to
50 C., and adding to the solution gradually 3.3
parts of sodium carbonate dissolved in 30 parts of
water. The mixture is stirred, and, after 12 hours,
strained through a woolen cloth, the precipitate of
indigo carmine being finally washed with a very
small quantity of water.

Indigo carmine is also obtained by adding I part
of powdered and dried indigo, in two portions, to a


mixture of 4.5 parts of fuming sulphuric and con-
centrated sulphuric acids, stirring constantly and
avoiding a rise in temperature. When the mix-
ture is homogeneous, it is kept at a temperature of
50 C. for a week, and is then poured into 10 parts of
water, after which a solution of 10 parts of sodium
chloride having a sp. gr. of 1.170 is next added.
The precipitated indigo carmine is then collected by
filtration, washed and finally dried.

Properties : Indigo carmine is the sodium salt of
indigotin-disulphonic acid, and has the composition
C 18 H 8 N 3 O,(SO 8 Na) 2 . It occurs both as a blue,
pasty mass, and as a blue, dry powder soluble in
from 140 to 150 parts of water. It yields a blue
solution, the color of which is changed by caustic
soda to a yellowish brown, and restored by acids.

Tests: The fitness of indigo carmine for use is
recognized, according to Mierzinski, by placing a
small quantity of the preparation on a piece of
white filtering-paper. If the preparation is im-
pure, a green ring soon forms around the indigo
carmine. When the powdered indigo carmine is
to be tested, it is first stirred with a little boiling
water before placing on the filter-paper.

Application: Indigo-carmine solution is used in the
estimation of nitrites and of chlorine, both of which
decolorize it on boiling. It is also employed for
the estimation of oxygen and of tannin, the latter


particularly. For tannin, Lowenthal and Schroder
recommend the following solution :

30 Gm. of indigo carmine, in dry form, are dis-
solved in 3 liters of diluted sulphuric acid (i : 5 by
volume), then 3 liters of distilled water added, the
whole thoroughly shaken and filtered.

Care must be exercised that only a good quality
of indigo carmine be used, because, when a poor

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Online LibraryAlfred Isaac CohnIndicators and test-papers; their source, preparation, application, and test for sensitiveness . . → online text (page 5 of 14)