Alfred Isaac Cohn.

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

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of 3 to 4 volumes of water and I volume of alcohol,
and decanting the clear solution. The whole cochi-
neal is directed to be used, because the liquid is
difficult to filter if the powdered insect is used.

The color of these solutions is a deep, ruby-red
which is changed to a violet by alkalies, carbonates
of the alkalies, alkaline earths, sodium phosphate,
acetates of the alkalies, and salts of the weak acids
generally. Acids restore the red color.

Cochineal is very serviceable for indicating the
carbonates of the alkaline earths for which litmus is
inapplicable, and also for the caustic alkalies, car-
bonates and sulphides. Although somewhat influ-
enced by the carbonic-acid gas or hydrogen sulphide
liberated, it is far less affected by these than is
litmus. It is also particularly useful for titrating the
ammonia or the excess of sulphuric acid in the Kjel-
dahl nitrogen estimation process, in which phenol-
phtalein is not very serviceable.

Cochineal may be used hot or cold for titrating
carbonates or bicarbonates, and also for alkali in
soaps.



COCHINEAL 53

With normal sulphites 50$ of the base may
be titrated with a strong acid. For organic acids
the indicator is inapplicable. With phosphoric acid,
the reaction is neutral when a monobasic salt is
formed, as acid phosphates are neutral to cochineal.

The indicator is excellently adapted for titrating
the alkaloids generally, and morphine, brucine, atro-
pine and cocaine in particular.

On account of the acid reaction of the cochineal
tincture (or carminic-acid solution), no harm is done
when alkalies are being titrated with acid. When,
however, acids are being titrated with alkalies, the
cochineal tincture must be first neutralized (colored
violet by a minute quantity of ammonia); the end
of the reaction is then indicated by the violet color.

Tincture of cochineal is best kept in an acid con-
dition, because the neutral or alkaline tincture soon
spoils. In use, the absence of iron, aluminum, or
copper salts must be assured from the solutions to
be operated on, as these salts yield a pink color,
even when solutions are acid. The tincture is also
particularly serviceable for titrations by artificial
light, as the change of color is very sharply defined,
and the depth of the colors enhanced. The sodium
flame is very useful with this indicator.



54 INDICA TCRS

COLEIN

C,oH 10 0.

ALKALIES = Yellow ACIDS =Red

Source : Colein is a resin-like substance isolated
from the stems and leaves of Coleus verschaffelti.
It was first obtained and described by A. H.
Church in 1877.

Preparation : Colein is obtained by crushing the
stems and leaves of Coleus verschaffelti with a small
quantity of alcohol faintly acidulated with sulphuric
acid, expressing the crimson pulp (avoiding contact
Nvith metals), shaking the liquid obtained with ba-
rium carbonate, and filtering. Most of the alcohol is
then distilled off, and at a certain stage a deep-
colored, resin-like substance sinks to the bottom of
the vessel beneath a supernatant rich-red liquid.
The precipitated matter is the colein, and it is puri-
fied by dissolving it in alcohol and reprecipitating it
by the addition of ether or water ; the purifying
process is repeated several times.

Properties : Pure colein is a brittle, resinous sub-
stance, having aconchoidal fracture, and yielding a
reddish-purple powder verging on crimson. It is
freely soluble in alcohol, very difficultly so in water,
and insoluble in ether. It yields red solutions, but
the alcoholic solution soon loses its color, owing,
probably, to union with a portion of alcohol. The



COA'CO RED SS

color may, however, be restored by the addition of
a drop of sulphuric acid. Concentrated sulphuric
acid dissolves it unchanged, but nitric acid con-
verts it into a brown, resinous mass. Concentrated
caustic alkalies (even ammonia) profoundly alter it.
Its composition, from analyses made by Church, is
C, H 10 0,.

Application : Boettger recommended colein as an
indicator, but it has found very little use, so far.

Colein Test-Paper is also used (see under Test-
Papers).



CONGO RED

(C 6 H 4 ) 9 .(N a )i.(CioHa) 9 .(NH,.SO,Na) 9

ALKALIES = Red ACIDS = Blue

Synonyms: Sodium Tetrazodiphenyl-naphtionate;
Sodium Diphenyl-diazobinaphtionate.

Preparation: Congo red was discovered by Boettger
in 1884. It is the sodium salt of diphenyl-diazo-
binaphtionic acid, and is obtained by dissolving
28.2 parts of benzidine sulphonate in 500 parts of
water, adding first 30 parts of hydrochloric acid
(20 Be.), and then 13 parts of sodium nitrite dis-
solved in 50 parts of water, in small portions.
The solution is cooled, and to it is then added a
solution of 55 parts of sodium naphtionate and 12
parts of sodium acetate in 100 parts of water.



$6 INDICA TOXS

The precipitate is collected, dissolved in a solution
of sodium carbonate, and the dye salted out, and
dried.

Properties: Congo red occurs as reddish-brown
lumps which are readily soluble in water and in
alcohol. Its solutions are very sensitive to free
acids, which yield a blue color and liberate the
base ; alkalies restore the red color.

Application : Congo red is very useful for estimating
free acids, more particularly as acid salts do not
affect it. It may, therefore, be employed for esti-
mating sulphuric acid in aluminum sulphate, as the
alum has no action on it. Free carbonic or acetic
acid, present to the extent of only 0.002^, suffices
to yield the blue color. But, if a drop of ammonia
water is added to the solution, a stream of car-
bonic-acid gas may be passed through it without
the blue color being produced. Hydrogen sulphide
also yields the blue color; nor does boiling, the
solution remedy the change effected by the hydro-
gen sulphide or carbonic-acid gas.

Congo red may be used for estimating mineral
acids in the presence of organic acids, as the latter
do not affect it.

Congo red is also useful for titrating aniline and
toluidine in the residual liquors from aniline -works.
For this purpose, the liquid is diluted with water
until a perfect solution is had, then an alcoholic



CONGO RED 57

Congo-red solution is added, and finally sufficient
sulphuric or hydrochloric acid carefully added until
a decided bluish-violet color is observed (a distinct
blue is seen only when a decided excess of acid is
added).

Sulphates, chlorides, and nitrates of the alkalies
seriously interfere with the end-reaction, hence
their absence should be assured.

Bisulphites and biphosphates are neutral to Congo
red. For organic acids the indicator is useless.

Phosphates may be titrated by means of Congo
red ; sulphites, however, cannot ; and bichromates
are neutral to the indicator, whereas the chromates
are alkaline.

Congo-red solution for titrimetric use is made by
dissolving I part of Congo red in 100 parts of 30-
per-cent. alcohol, about 10 drops being added to
100 Cc. of the liquid to be titrated. The sensitive-
ness of the solution is such that o. I Cc. added to
IOO Cc. of distilled water requires only 0.7 Cc.
of centinormal hydrochloric acid, and O.6 Cc. of
centinormal ammonia, to produce the changes of
color, but requires 2.5 Cc. of centinormal potassa.

Congo paper is also used as an indicator, and is
treated of under Test-Papers. ^



58 INDICA TORS

CORALLIN

C )8 H I4 3

ALKALIES = Violet-red ACIDS = Yellow

Synonyms: Aurine; Commercial Rosolic Acid.

Preparation and Properties: See Rosolic Acid.

Application: Corallin' test solution may be prepared
by dissolving I Gm. of Corallin in 10 Cc. of alco-
hol, and adding distilled water to make up to 100
Cc. The solution is colored violet-red by alkalies,
and yellow by acids. It is especially useful for
titrating free ammonia.

CORALLIN RED

C 19 H J3 0,.NH,

ALKALIES = Red ACIDS = Yellow

Synonyms: Paeonin; Aurine R. ; Impure Rosani-
line Rosolate; Diamidotriphenylcarbidride.

Preparation: Corallin red was discovered by Marnas.
It is obtained by heating one part of commercial
rosolic acid (corallin ; aurine) with three parts of
ammonia in a closed vessel at a temperature of
140 C. The semi-liquid mass is then treated with
hydrochloric acid, and the precipitate collected,
washed, and finally dried.

Corallin red is believed to be most probably a
product intermediate between aurine and para-



CURCUMIN 59

rosaniline, in which the hydroxyl is replaced by an
amido group.

Properties: Corallin red occurs as a reddish-brown
powder, which is readily soluble in alcohol, insol-
uble in cold water, and but slightly soluble in boil-
ing water. Its solutions have a beautiful red color,
which is changed, even by a trace of acid, to a
yellow.

The formula assigned to it is C 19 H 13 O 2 (NH,).

Application : Like Commercial Rosolic Acid (which
see).

CURCUMIN

ALKALIES = Reddish-brown ACIDS = Yellow

Synonyms: Turmeric Yellow; Curcummin.

Source : Curcumin is a coloring-matter obtained
from the roots of Curcuma longa L., one of the
Zingiberaceae, and found in the East Indies, China,
and Madagascar. It was first isolated by Vogel,
who also found in the root another, a brown col-
oring-matter.

Preparation : Curcumin is obtained by first exhaust-
ing curcuma roots with water to remove the brown
matter, extractive, etc., then drying, powdering,
and exhausting with alcohol, which takes up the
curcumin. The alcohol is next distilled off, the
dried residue treated with ether, and the filtered



60 INDICA TORS

ethereal solution evaporated. The impure cur-
cumin so obtained is dissolved in alcohol, precip-
itated by lead acetate, the precipitate collected
and washed with alcohol, and decomposed by a
current of hydrogen sulphide while suspended in
water. This precipitate is then collected, dried,
and exhausted with ether which, on evaporation,
deposits pure curcumin.

Gajewski, in 1870, obtained pure curcumin by
removing the fatty matter from the root with carbon
disulphide, then extracting the coloring-matter with
ether, and recrystallizing from ether or hot ben-
zene. In 1873 he modified the process by washing
the ethereal extract with weak ammonia, and pre-
cipitating the pure curcumin by means of a current
of carbonic-acid gas.

Daube also, in 1870, obtained the curcumin by
exhausting the roots with boiling benzene, and dis-
solving the crystals obtained on cooling, in cold
alcohol, precipitating the filtrate with alcoholic
lead-acetate solution, decomposing the precipitate
with hydrogen sulphide, and crystallizing from
alcohol.

The yield of curcumin is about 0.3 per cent.

Properties: Curcumin occurs as a cinnamon-colored,
translucent, foliaceous mass which yields a yellow
powder melting at 40 C., or as yellow prisms or
crystalline powder melting at 178 C. (183 C. Cia-
mician and Silber).



CURCUMIN 6 1

Curcumin is insoluble in water, is somewhat sol-
uble in hot benzene, and is readily soluble in ether,
alcohol, acetic acid, alkalies, fats, and in oils. On
exposure to sunlight it is bleached.

Curcumin yields with alkalies intensely reddish-
brown solutions ; and similar solutions are afforded
by the alkaline earths and borax. Acids restore
the yellow color. In concentrated sulphuric acid,
curcumin dissolves with a carmine color; in phos-
phoric and in hydrochloric acids it dissolves with-
out decomposition, and also with a carmine color.
Nitric acid decomposes it.

Boric acid added to an alcoholic solution of cur-
cumin does not change the color of the latter, but
on boiling and evaporating, a carmine-red residue
remains. This compound is decomposed, accord-
ing to Schlumberger, by long contact with water,
yielding a resinous yellow substance, known as
pseudo-curcumin, which does not yield a red color
with boric or hydrochloric acid, and dissolves in
alkalies with a greenish-gray color.

The curcumin-boric-acid compound yields with
alkalies, reddish-violet solutions which rapidly
become gray. On boiling an alcoholic solution of
the compound with sulphuric acid, a dark, blood-
red color develops, and on cooling, rosocyanine
deposits.

Curcumin solutions exhibit a green fluorescence,
and yield brown, insoluble lakes with lead, lime,
and baryta.



62 INDICA TOR '3

Application : The sensitiveness of curcumin appears
to be increased by the presence of most salts, hence
the indicator is applicable to a wide variety of
titrimetric determinations. It is also applicable in
the presence of ammonia, to which it is insensitive.
It is sensitive, however, to carbonic acid, and is
acid to bicarbonates of the alkalies.

With the sulphates and sulphides of the alkalies,
the indicator shows when half of the combined
alkali becomes a sulphate or sulphide; i.e., when
bisulphates or bisulphides are formed. With sul-
phites and phosphates the end-reaction is not def-
inite; hence, for these the indicator is useless. In
sodium silicate only 90$ of the sodium, and in
borax only 50$ of the sodium, is shown, and the
end-reaction is uncertain.

Sodium thiosulphate is neutral to the indicator.



CURCUMIN W.

CH 3 .(C a H,.SO 3 Na.N),O

ALKALIES = Red ACIDS = Greenish Yellow

Synonyms' Curcumin S. ; Sun Yellow; Sodium (or
Ammonium) Azoxystilbene-disulphonate.

Preparation: Curcumin W. is obtained by dissolving
5 parts of sodium (or ammonium) para-nitrotolu-
enesulphonate in 70 parts of water, adding 3 parts
of soda-lye (40 Be\), and warming the mixture



CYANINE 63

while stirring vigorously. When the color has
changed from a saffron to a yellowish-red the dye
is salted out and dried.

Properties : Curcumin W. forms a reddish-brown
powder, soluble in water, and yielding a yellow
solution, the color of which is changed to a red by
alkalies, and restored by acids.

Application : Curcumin W. was recommended by
Storch, who states that it is more sensitive than
phenolphtalein. Salts which ordinarily affect the
sensitiveness of indicators appear to even increase
that of Curcumin W. It is neutral to bicarbonates,
and is serviceable for ammonia. Carbonic-acid gas
affects it. The indicator may be used in those
cases where other indicators, such as phenolphta-
lein and curcuma, cannot be used for ammonia.

CYANINE

CeH 5 .CH 2 .C 9 H fl N.C.C 9 H a NCl.

ALKALIES = Deep Blue ACIDS = Colorless

Synonyms: Quinoline Blue; Diamylcyanine Iodide.

Preparation : Cyanine was discovered by Greville
Williams in 1856. It is prepared by heating a
mixture of equivalent quantities of quinoline and
lepidine (^-methylquinoline) with amyl iodide,
then heating the resulting product with potassa,
and finally crystallizing.



C>4 INDICA TORS

Properties : Cyanine is obtained in the form of shin-
ing prisms or plates having a green metallic luster.
The crystals are insoluble in cold water, difficultly
soluble in boiling water, with which they yield a
violet-blue solution having a quinoline odor, and
are soluble in alcohol with deep, blue color. Acids
discharge the color, which is restored by alkalies.
Cyanine dissolves in concentrated sulphuric acid,
and the colorless solution, on heating, yields iodine
vapors.

Application : Cyanine is very sensitive to acids, even
carbon dioxide sufficing to discharge the color. It
is capable of but limited use, however.



DIAZO-PARANITRANILINE-GUAIACOL

NO a C 9 H 4 N:NC 6 H3(OCH 3 )OH

ALKALIES = Red ACIDS = Greenish-yellow

Synonym: Riegler's Indicator.

Preparation : The indicator is prepared by adding a
solution of diazo-paranitraniline to an alkaline solu-
tion of guaiacol.

Properties: Diazo-paranitraniline forms a brown
substance insoluble in water but soluble in alcohol.
Its composition is represented by the formula
NO 3 C 6 H 4 N : NC 8 H 3 (OCH 3 )OH.



D IME TH YLA MID OA ZOBENZENE 6 5

Application : This indicator has but very recently
been introduced by E. Riegler, who states that a
solution of 0.2 Gm. in 100 Cc. of alcohol is much
more sensitive than phenolphtalein. One or two
drops of the solution added to an alkaline solution
suffice to color the latter a very handsome red,
changed by an excess of acid to a greenish-yellow
color.



DIMETHYLAMIDOAZOBENZENE

C 8 H 5 .N a :C 6 H 4 N(CH3) ! ,

ALKALIES Yellow ACIDS Red

Synonyms: Butter Yellow; Aniline-azodimethyl-
aniline ; Benzene-azodimethylaniline.

Preparation : Dimethylamidoazobenzene was dis-
covered in 1875 by O. N. Witt. The indicator is
prepared by dissolving 9.3 Gm. of aniline in 30 Gm.
of a 25-per-cent. hydrochloric acid, and after cool-
ing the solution and diluting it, adding 7 Gm. of
sodium nitrite previously dissolved in water, where-
by the aniline hydrochloride is converted into diazo-
benzene chloride. The mixture is now stirred well,
and poured into a solution of 12 Gm. of dimethyl-
aniline in 15 Gm. of hydrochloric acid, 30 Gm. of
sodium acetate being then added. The crystals,
which soon separate, are then recrystallized from
alcohol.



66 2ND 1C A TORS

Properties : Dimethylamidoazobenzene crystallizes
in the form of golden-yellow scales which melt at
115 C., and are soluble in alcohol, strong mineral
acids, alkaline solutions and oils, but insoluble in
water. In dilute solutions of dimethylamidoazo-
benzene, even traces of acids yield a handsome red
color, which is changed to yellow by alkalies.

Application : Dimethylamidoazobenzene was recom-
mended by B. Fischer and O. Phillip as an indica-
tor instead of methyl orange, because the transition
from the lemon-yellow to red is more easily ob-
served than the change from orange to red exhib-
ited by methyl orange, and also because the indi-
cator may be readily prepared by the operator,
whereby a constant and definite preparation may
always be at command (which is not the case with
methyl orange).

Lunge and Thompson state, however, that methyl
orange is more sensitive than the dimethylamido-
azobenzene, and that, in equal concentration, it
will exhibit similar shades of color.

For use, a 1:200 alcoholic solution of dimethyl-
amidoazobenzene is made, about 5 drops being
added to 100 Cc. of the liquid to be titrated. The
color of the liquid will be lemon yellow, and
changed sharply by acids to a handsome carnation
red. Carbonic-acid gas does not affect the indicator.



DIPHENYLAMINE



DIPHENYLAMINE

(C,H 6 ) 2 NH

Synonym : Phenylaniline.

Preparation: Diphenylamine is obtained by the
dry distillation 'of triphenyl-rosaniline (aniline blue),
and by heating aniline hydrochloride with aniline to
240 C.

Properties: Diphenylamine occurs as a grayish,
crystalline mass, or white crystals, slightly soluble
in water, easily soluble in alcohol and in ether, and
fairly so in acids, with which it forms salts. It is a
very weak base, however, its salts being decomposed
even by water. It has a pleasant odor, melts at
54 C., and boils at 310 C. A i-per-cent. solu-
tion in concentrated sulphuric acid, forming diphe-
nyl-sulphonic acid, is colored intensely blue by
nitric acid. It is also temporarily colored by
nitrous acid, the color gradually fading to a green-
ish yellow; hypochlorous, permanganic, molybdic,
and selenous acids likewise color it somewhat; and
so do certain ferric salts, barium salts, and hydrogen
dioxide. Ozone colors the alcoholic solution, or
test-paper prepared from the latter, a yellow to
brown.

Application : According to Longi, diphenylamine is
serviceable as an indicator in the volumetric deter-



68 2ND 1C A TORS

mination of nitric acid using a decinormal solution
of stannous potassium sulphate.

Diphenylamine is principally employed for the
detection of nitrates in potable water, or in wine or
milk to which any water containing nitrates has been
added. Traces of nitric acid in sulphuric acid may
also be detected by means of it.

In testing water, wine, or milk, a few drops of
diphenylamine solution are added to I Cc. of the
liquid, and then I Cc. of concentrated sulphuric
acid is added so as to form a lower layer. At the
zone of contact a blue color develops, and is so in-
tense that I part of nitric acid in 3,000,000 parts of
liquid is readily detected.

EOSINE

C a H 4 .(CO) 2 .O.(C 6 HBr a .NaO) a
ALKALIES = Green Fluorescence ACIDS = Yellow

Synonyms: Eosine Yellowish; Bromeosine; Tetra-
bromeosine; Tetrabromfluoresceine ; Tetrabromo-
resorcinphtalein.

Preparation : Eosine was discovered in 1874 by
Caro. It may be obtained by the action of bromine
on fluoresceine, C 6 H 4 .CO,.C.O.(C 6 H,) 2 (OH) 2 , in a
cold solution in glacial acetic acid, some potassium
chlorate being finally added to the mixture in order
to decompose the hydrobromic acid formed, where-
by a saving is effected in the quantity of bromine



EOSINE 69

required. Four hydrogen atoms of the fluoresceine
are thus replaced by an equal number of bromine
atoms, and the tetrabrorhfluoresceine gradually
separates in the form of yellowish-red crystals,
which are purified by recrystallization from alcohol,
and finally converted into a potassium or sodium
salt, which is the commercial form of the article.

Eosine is also obtained by heating in an enam-
elled steam-kettle a solution of 120 parts of soda
lye (36 Be".)' and 300 parts of water, and gradually
adding 64 parts of bromine. There results from
the reaction sodium bromide, sodium bromate, and
sodium hypobromite. The mixture is then heated
for half an hour to convert the last into bromate.
The solution is then cooled, and to it is added a
cooled solution of 32 parts of fluoresceine, 300
parts of water, and 50 parts of soda lye (36 Be.).
The mixture is briskly stirred, and 280 parts of
hydrochloric acid gradually added, the whole boiled
for 5 minutes and 600 parts of cold water added.
The precipitate is then collected and dried. The
boiling and addition of water are usually repeated
once or twice to remove all traces of hydrochloric
acid. The dye is then converted into a potassium
or sodium salt.

Properties : Uncombined cosine occurs as yellowish-
red crystals containing some alcohol of crystalliza-
tion. It is almost insoluble in water, but yields
readily soluble, bibasic salts with alkalies.



7O fNDICA TORS

The potassium salt of cosine forms bluish-red
crystals, or a brownish powder. It is soluble in
water and in alcohol, and yields solutions having a
magnificent green fluorescence. Acids destroy the
fluorescence, the solution becoming yellow. Mineral
acids cause a yellowish-red precipitate in the solu-
.tions. Organic acids only partially decompose the
salt. Eosine yields handsome, insoluble lakes with
salts of lead and tin, and with alumina.

Application : Eosine is peculiarly applicable for use
in dark-colored solutions, in which the ordinary
color-changes can be observed only with difficulty
or not at all. It is also useful for the titration of
soap, and for alkalies.

For use as an indicator a 3-per cent, aqueous
solution is prepared, of which about 10 drops are
added to 100 Cc. of the liquid to be titrated.

EOSINE-METHYLENE BLUE

ALKALIES = Red ACIDS = Blue

Preparation : The compound is prepared by mixing
stoichiometrical equivalents of basic cosine and
methylene blue dissolved in alcohol.

Properties : The alcoholic solution of the compound
has a bluish-violet color which is changed by organic
and other acids, as well as acid compounds, to a
pure blue or bluish-green ; and by alkalies, and



ETHYL ORANGE J I

alkaline substances, to a red color. The neutrality
point is always indicated by the restoration of the
original bluish-violet.

Application : Eosine-methylene blue is said by
H. Rosin to be exceedingly sensitive so much so
that glass, which is alkaline in spots, is colored red
in those places by the solution. The solution is
also applicable as a sensitive biochemical reagent, as
it affords characteristic colors with pathological
elements.

ETHYL ORANGE

C6H4.SO3Na.N a .C 8 H 4 N(C 2 H 6 ) 9

ALKALIES = Yellow ACIDS = Red

Synonyms: Diethylaniline Orange; Sodium (or
Ammonium) Diethylamidoazobenzene-sulphonate ;
Sodium (or Ammonium) Diethylaniline-azoben-
zene-sulphonate.

Preparation : Ethyl orange is prepared in exactly
the same manner as methyl orange (which see),
diethylaniline being substituted for the dimethyl-
aniline in the formula.

Properties: Ethyl aniline occurs as an orange-yellow
crystalline powder, soluble in alcohol, and almost
insoluble in water.

Application: Ethyl orange has been recommended
as being more delicate than methyl orange. It L



72 1NDICA TORS

sensitive to free acids, and is colored red even by
neutral aluminum sulphate. It may be used like
methyl orange, but is not, however, as sensitive as
the latter.

A solution of I part in 400 parts of 3<D-per-cent.-


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Online LibraryAlfred Isaac CohnIndicators and test-papers; their source, preparation, application, and test for sensitiveness . . → online text (page 4 of 14)