Alfred Isaac Cohn.

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

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quality of indigo carmine is employed when titrat-
ing with potassium permanganate solution, the end
of the reaction is not clear, as the greenish, tint
does not change sharply to yellow, but changes
through reddish and brownish tints which render
it impossible to note the end of the reaction.


C 8 H 4 (CO.C 6 H a I 2 0) 2

ALKALIES = Rose-red ACIDS = Yellowish

Synonyms: Tetraiodofluoresceine; Erythrosin B. ;
Pyrosine B. ; Dianthine B.

Preparation: lodeosine is obtained by dissolving 6
parts of fluorescein in a hot mixture of 8 parts of
soda-lye and 60 parts of water, and adding to this a
solution of 24 parts of iodine in 27 parts of soda-
lye mixed with 60 parts of water; to this mixture
are then added 25 parts of glacial acetic acid. The
whole is boiled, then neutralized with caustic
soda, and 25 parts of hydrochloric acid are added.


The precipitate is then collected and purified by
dissolving it in aqueous ether, shaking out with
weak caustic-soda solution and reprecipitating by
adding concentrated soda-lye. The brick-red pre-
cipitate is then washed with alcohol, and crystal-
lized from hot alcohol. The pure sodium salt thus
obtained is then decomposed by hydrochloric acid,
and the precipitate thoroughly washed and dried at
a temperature of about 120 C.

Properties: lodeosine occurs as a brick-red powder,
almost insoluble in water and in cold water, but
soluble in hot alcohol and in ether.

Application: Mylius and Forster recommend iodeo-
sine for the estimation of very minute quantities of
alkali ; for instance, such as may be dissolved out
from glass on contact with water. For this purpose
they recommend an ethereal solution containing
0.002 Gm. per liter (i : 500,000).

Titration with this indicator is carried out by
introducing from 50 to 100 Cc. of the liquid to be
operated on into a stoppered bottle and adding
from 10 to 20 Cc. of the ethereal solution. If de-
sired, 4 or 5 drops of a I : 10,000 aqueous solution
may be added to the liquid, and ether then added.
If free alkali is present, even in mere traces, the
aqueous layer soon acquires a rose-red tint, whereas
if acids are added the ethereal layer becomes colored


lodeosine is one of the best of indicators for use
in alkaloidal estimations, and for detecting traces
of acids or of acid salts in so-called neutral chem-

H. A. Cripps gives the following process for us-
ing the indicator for alkaloids: 20 to 25 Mg. of
alkaloid (or alkaloidal residue from a drug assay)
are dissolved in 5 Cc. of /2O hydrochloric acid,
and the solution diluted to measure 20 Cc. with
perfectly neutral water. Four drops of a 1 : 10,000
aqueous iodeosine solution are now added, and 10
Cc. of neutral ether. w/2OO barium-hydrate solution
is now added from a burette, with frequent agitation
until the aqueous layer has assumed a rose color.
A previous titration will have determined the value
of the baryta solution ; and the difference between
the two titrations will indicate the alkaloid.


C,H 4 (CO.CH a IONa) 2 O
ALKALIES = Cherry-red ACIDS Brownish-yellow

Synonyms: Pyrosine J. ; Dianthine G. ; Jaune d'Ori-
ent ; Erythrosine G. ; Diiodofluoresceine-Sodium
(or Potassium).

Preparation : lodeosine G. is obtained by mixing
solutions of 6 parts of fluoresceine and 16 parts of
iodine in 20 parts of soda lye each, and precipita*-


ing with 20 parts of glacial acetic acid. The pre-
cipitated diiodofluoresceine is then converted into a
sodium (or potassium) salt, which is purified by
recrystallization from hot alcohol.

Properties: lodeosine G. forms a yellowish-brown
powder or plates, soluble in water, and yielding a
cherry-red solution free from fluorescence. The
color of the solutions is changed to a brownish-
yellow by acids, and is restored to red by alkalies.

Application : lodeosine G. or lodeosine (which see)
is particularly useful in alkaloidal assays, and is also
serviceable for alkalies.



= Red = Red

Fe 2 (S0 4 )3.(NH 4 ) 2 S0 4 .24H 2

Synonym: Ammonio-Ferric Alum.

Preparation: 240 Gm. of solution of ferric sulphate
(sp. gr. 1.43) are added to a solution of 28 Gm. of
ammonium sulphate in 100 Gm. of warm water.
On slowly cooling the solution crystals are ob-
tained which are subsequently freed from the
mother liquor, washed with a little water and

Properties: Ammonio-ferric alum occurs as lilac or
violet efflorescent crystals having a sour, styptic


taste, and soluble in 3 parts of water and in 0.8
parts of boiling water. The salt is affected by
light, by which it is reduced to a ferrous condition,
hence it should be kept in amber bottles or in a
dark place, and also from contact with air. Its
formula is Fe 1 (SO 4 ) i .(NH 4 ) 1 SO 4 .24H,O.

Application: Ammonio ferric alum is employed as
an indicator in titrating silver salts by means of
decinormal ammonium- or potassium-sulphocyanate
solution, and similarly for titrating ferrous halogen
salts, using silver nitrate and a sulphocyanate.


C 13 H 9 N0 4 (?)


Synonyms: Resorcin Blue.

Preparation: Lacmoid was first prepared by Wesel-
sky and Benedikt, in 1880. It is obtained by heat-
ing 20 parts of resorcin, I part of sodium nitrite,
and I part of distilled water on an oil-bath to 110
C., at which temperature the reaction becomes quite
active. When the reaction moderates, the heat is
raised to from 115 to 120 C. ; ammoniacal vapors
are freely disengaged and the raspberry-red mass be-
comes reddish-violet, bluish-violet, and finally blue.
When the vapors are no longer disengaged, the
mass is dissolved in a little water, and precipitated
by hydrochloric acid. The precipitate is then col-


lected, washed and dried, and constitutes the
lacmoid in an impure form.

Schaerges recommends preparing the lacmoid by
the reaction of resorcin on sodium nitrite without
the addition of any water, and at a temperature not
exceeding no C., dissolving the product in a little
water, and removing the undecomposed resorcin
still present by shaking out with ether. Schaerges
claims that lacmoid so prepared is clearly soluble
in all proportions in water and in dilute alcohol, but
is insoluble in ether.

For obtaining a perfectly pure lacmoid, Forster
recommends treating the commercial article in fine
powder with boiling water, acidulating the blue
solution after cooling and filtering, with hydro-
chloric acid, collecting the precipitate after a few
hours, washing it with a little cold water, and dry-
ing it at not too high a temperature; or, dissolving
it in alcohol and evaporating the solvent.

Lacmoid solutions, no matter how carefully pre-
pared, are more or less prone to exhibit a violet
tinge which may prove very deceptive to other than
normal eyes. To remedy this defect, Forster rec-
ommends the addition of 5 Gm. of naphtol green
(Cassella & Co.) to 3 Gm. of purified lacmoid ob-
tained as above, and dissolving both in 700
Cc. of water and 300 Cc. of alcohol. Malachite
green had formerly been proposed, but the naphtol
green has been found to be more permanent, and to


yield no precipitate with lacmoid (malachite green
does), while it imparts a pure blue color to neutral
and alkaline fluids, which is changed to an onion-
red with acids. The sharpness of the change of
color is greatly enhanced by the addition of the
naphtol green.

Properties: Lacmoid occurs as a glistening brown
or dark-violet powder, amorphous granules, or as
blue-black scales. It consists principally of diazo-
resorcin, or resazurin, also known as " Weselsky's
indicator." Lacmoid is soluble in alcohol, acetone,
wood-alcohol, pyroligneous acid, acetic acid, phenol,
and amyl alcohol; it is less soluble in ether and in
water, and insoluble in chloroform, benzene, and

Its constitution is unknown, and the empirical
formula assigned it is C 12 H 9 NO 4 .

With neutral and alkaline liquids lacmoid yields
handsome blue solutions having a faint violet tinge,
changed by a mere trace of acid to red. The solu-
tions should be preserved in dark bottles, as they
are .very sensitive to light.

Tests: The fitness of lacmoid as an indicator is tested
by treatment with water. If but little or no color
is imparted to water, the sample should be rejected.
According to Traub, a few drops of decinormal
ammonia suffice to clearly show the blue color in a
solution containing I 12,000,000 of the indicator.


Application: The application of lacmoid as an indi-
cator is very similar to that of litmus. It is useful
for titrating both caustic alkalies and acids. Aque-
ous solutions of calcium or magnesium bicarbonate
affect it very sharply. Normal alkali sulphites are
strongly alkaline, and bisulphites are neutral, to
lacmoid. . The indicator is affected by the hydrogen
sulphide liberated in titrating sulphides, hence it is
unserviceable for them, but the latter, as well as
carbonates, sulphites, phosphates, arsenates, and
borates, may be readily and accurately titrated by
means of lacmoid paper, a drop of the liquid being
dropped on the paper, which is best washed, pre-
.vious to use, with pure distilled water.

To acid phosphates and arsenates the lacmoid is
neutral ; nevertheless, the paper is most service-
able, and shows 50$ of the alkali. Towards arsen-
ous acid the solution is neutral, and good results
are obtainable with both solution and paper. Lac-
moid paper may be used for borax, sodium and
ammonium carbonates, effervescent preparations of
magnesium, lithium, and other carbonates, etc., as
well as for the alkali of the alkali silicates (for these
latter the lacmoid solution answers equally well).

Lacmoid may be used hot for titrating the alkali
in carbonates or bicarbonates, but the change of color
does not take place until all the carbonic-acid gas
has been replaced. Barium, magnesium, and calcium
carbonates may be titrated by first adding an excess
of acid, and then titrating back with alkali.


Lacmoid solution is useless for sulphites and
sulphides, but is applicable for borates. Towards
thiosulphates it is neutral. Borax, silicic acid,
and arsenic acid do not affect the indicator in the
cold. Arsenic and phosphoric acids behave like
monobasic acids towards it.

Potassium and sodium bichromates are neutral to
lacmoid, but the neutral chromates are alkaline;
hence a mixture of bichromate with chromate, or of
chromic acid and a bichromate, may be readily
titrated by means of lacmoid.

Oxalic, acetic, lactic, tartaric, and nitric acids, as
well as organic acids in general, cannot be titrated.

The sulphates and chlorides of iron, copper, and
zinc, which are all more or less acid to litmus, are
neutral towards lacmoid ; hence free acids present
in solutions of these salts may be estimated with it.
For alumina, lacmoid will not answer.

Lacmoid is very useful for titrating atropine, bru-
cine, cocaine, codeine, coniine, emetine, morphine,
narcotine, nicotine, papaverine, pelletierine, quinine,
strychnine, thebaine, and veratrine.



Synonym : Lacmus.

Source : Litmus is a pigment obtained principally
from Rocella tinctoria Acharius, but also from


Lecanora tartarea Ach., R. fuciformis Ach., Vario-
laria dealbata, etc., lichens found growing some-
times on trees, sometimes on cliffs, on the coasts of
the Mediterranean Sea, Spain, France, Holland,
England, and Sweden, and also on the Canary
Islands, Orkney Islands, and Corsica.

Preparation : Litmus does not exist as such in the
plants. By the action of ammonia and air alone on
the lichens, a peculiar substance, orcein, discovered
by Robiquet in 1829 is formed ; but when potassa
is permitted to act in conjunction with the other
substances, a new coloring-matter is the result, a
compound of chiefly azolitmin and potassa. Hence,
the lichens are ground, and mixed with ammonia
and potassa ; they are then permitted to ferment
until the mixture acquires a violet color, when lime,
potassa, and generally urine also, are added, and the
whole allowed to stand for 2 or 3 weeks, or until a
blue color has been acquired. Chalk and gypsum
in variable quantities are then added, and the
mixture is strained through a hair sieve, and
finally formed into little cakes or cubes.

Properties : Litmus occurs in the form of small,
light, friable, finely granular cakes or cubes poses-
sing a handsome, violet-blue color. It has a
peculiar, characteristic odor recalling that of indigo,
and has a pungent and somewhat saline taste. It
is partly soluble in water with a blue, and in dilute


alcohol with a purplish-blue, color, which is changed
to red by acids. Its principal coloring-matter)
and the one on which its value as an indicator
depends, is azolitmin, which was discovered by
Kane in 1841 (see Azolitmin).

Tests : The quality of the litmus of the market is
apt to vary greatly, hence the article should in-
variably be tested when a new lot is purchased.
For the test, equal quantities of the litmus to be
tested and an article known to be good are weighed,
and both treated with equal volumes of distilled
water for 12 to 24 hours, shaking every now and
then. When the mixtures have settled, they are
filtered, and the colors then compared.

A still better plan is to heat the cold aqueous
extract so obtained to boiling, and then to add
hydrochloric acid by drops until a permanent onion-
red color is had on boiling for 6 to 8 minutes.
The solution is then rapidly cooled, and mixed
with an equal volume of alcohol. The tincture
so obtained is then tested as to its sensitiveness to
acids and alkalies. This should be such that when
sufficient of the tincture is added to 250 Cc. of
recently boiled and cooled distilled water to color
the latter a distinct violet (indicating neutrality), a
solution is had which is sharply changed to an
onion-red or pure blue by one drop of decinormal
hydrochloric acid or decinormal alkali respectively.
On standing for a minute or so, either solution will


acquire a violet tint. In these solutions two drops
of the decinormal acid will give a permanent onion-
red with the blue solution ; whereas two drops of
the decinormal alkali suffice to give a permanent
blue with the red solution.

0.2 Cc. of a ic-per-cent. solution of purified
litmus added to IOO Cc. of distilled water should
require about 0.05 Cc. of decinormal hydrochloric
acid, or o. I Cc. of decinormal potassa to effect
changes of color.

Application : Since the value of the litmus as an
indicator depends entirely on the azolitmin present,
and since the latter is accompanied by three other
coloring-matters which detract from its sensitive-
ness because yielding violet tints, it is desirable, in
preparing solutions for titrimetric observations, to
effect a separation of the coloring-matters so that
only azolitmin will be present in the solution. This
separation is effected by exhausting the coarsely
powdered litmus with successive portions of hot
alcohol, in which azolitmin is insoluble, but in which
the others are soluble. The residue is then
digested with cold dilute acid, in order to remove
the excess of free alkali present, and then with five
times its weight of boiling water, which dissolves
out the azolitmin. The solution is then filtered.

Of the solution made as above detailed 1 , about 5

drops are added to 100 Cc. of liquid to be titrated.

Litmus solution is best kept in comparatively


wide-mouthed, half-filled bottles, closed by a plug
of cotton to keep out dust, but not air. In the
absence of the latter, the solution undergoes a
peculiar fermentation, evidenced by a disagreeable
odor and loss of color. An apparently spoiled
solution recovers its original color and valuable
properties, however, on exposure to air in a shallow
dish ; it does not require filtering or boiling,
simple exposure, with exclusion of dust, sufficing.
The solution should not be kept in an acidified con-
dition, as it is then very prone to become mouldy.
Many substances have been proposed for the pur-
pose of preserving the solutions, among others,
carbolic acid. No special advantages have, how-
ever, been found to accrue from their use, and the
practice of adding preservatives is not general.

Litmus yields a deep blue color with alkalies, and
an onion-skin red with acids. Carbonates of the
alkalies also produce the blue color, and the indica-
tor may therefore be used for titrating hydroxides
of the alkalies only when no carbonates are present.
With carbonates and bicarbonates, it can only be
used when the liberated carbonic-acid gas is ex-
pelled from the solution by boiling, as the dissolved
gas would otherwise cause the red color to persist
even though the liquid is alkaline, and thus prove a
source of error.

When a diluted mineral acid is added drop by
drop to a solution of a carbonate of an alkali to



which litmus has been added, no change of color
takes place until from 6o# to 64$ of the carbonate
has been neutralized, when the solution assumes a
purplish or violet color. On now heating the solu-
tion, carbonic-acid gas is copiously evolved, and on
the continued addition of acid to the boiling solu-
tion, the latter acquires an onion-red color as soon
as the least excess of acid is added. The color is
permanent, not being affected by the boiling; and
the end of the reaction is readily recognized when
the color, at the point where the dropping acid
reaches the solution, cannot be distinguished from
that possessed by the solution in general.

The end of the reaction is very sharp with caus-
tic alkalies, alkaline earths, and their carbonates ;
and also with sulphides, but with these the hydro-
gen sulphide liberated must also be driven off by
boiling, as with carbonic-acid gas.

Litmus is fairly good for titrating ammonia, and,
under certain conditions, for ammonium carbonate
also. With silicates of the alkalies the reaction is
very sharp. With borates of the alkalies and of
magnesium, as well as with the sulphates and phos-
phates of the alkalies, and also arsenates, the end
of the reaction is uncertain, as the change of color
is too gradual. With arsenites, however, the
change is sharp.

With sulphites the colors yielded are somewhat


indefinite, but the change is very sharp and certain
the moment saturation is complete.

Litmus is useless for titrating tartaric and citric
acids, but is serviceable for benzoic and oxalic
acids. The indicator may also be employed in ti-
trating the acids in the normal salts of quinine,
strychnine, morphine, narceine and papaverine, as
these are neutral to litmus. Caffeine, narcotine, and
theobromine alkaloids are neutral to litmus, but their
salts behave like a corresponding quantity of free
acid. In titrating quinine, oxyacids are unsuitable
for use, because they cause too much fluorescence,
hence decinormal hydrochloric acid is most suitable.
Toward emetin, litmus is neutral.

Litmus is most serviceable for indicating the
neutrality of milk when the acidity of the latter is
neutralized previous to the estimation of fat. It is
also useful for estimating the acidity of urine.

Aniline, toluidine, and quinoline are neutral to
litmus, and so are also anhydrides and concentrated
anhydrous acids. With these latter the change of
color occurs only when water is present. Litmus
is not well adapted for use by gas- or lamp-light,
hence when it is desired to perform titrations at
night with it, the use of a sodium-flame will be
found to be necessary. The sodium-flame may be
obtained by placing on a retort ring a circular piece
of asbestos, in the center of which a small hole
has been cut out. Through this hole the flame


from a Bunsen-burner is allowed to pass, while it
touches the margin of the hole. Around the mar-
gin some sodium chloride or borax is sprinkled ;
or, the margin may be impregnated with a concen-
trated solution of sodium chloride. In the mono-
chromatic light so obtained, the onion-red color of
the litmus appears perfectly colorless, while the
blue is distinct and appears black.


CeH,.OH a (NC).(CeH 6 ) 3 Cl

ALKALIES = Yellow ACIDS = Colorless

Synonyms: Oxychlordiphenylquinoxaline; Auten-
rieth's Indicator.

Preparation : Luteol is prepared by heating one
molecular weight of ethoxyphenylenediamine with
one molecular weight of benzile in alcoholic solu-
tion, and recrystallizing the precipitated ethoxy-
phenylquinoxaline from alcohol. This is then heated
with phosphorus pentachloride to a temperature of
from 70 to 90 C. in a paraffin bath, when some of
the chlorine enters into the compound, while phos-
phorus trichloride and hydrochloric acid distil off.
The ethoxychlordiphenylquinoxaline so obtained
is next heated to from 180 to 200 C. with hydro-
chloric acid in a sealed tube, when the alkyl,
ethylene chloride, is split off. The residue, which


constitutes the luteol, is then purified by repeated
recrystallization from alcohol.

Properties : Luteol pccurs as fine, woolly, yellowish
needles that melt at 246 C., and sublime without
decomposition at higher temperatures. It is insol-
uble in water, and sparingly soluble in cold alcohol,
but is readily soluble in hot alcohol and in ether.
In concentrated sulphuric acid it dissolves with a red
color, but is reprecipitated by the addition of water.
It is sparingly soluble in concentrated hydrochloric
acid, but is perfectly insoluble in the diluted acid.
It is readily soluble in alkaline solutions, yielding a
yellow color. Chemically, luteol is a phenol, its
formula being: C.H,.OH i (NC).(C a H ) 1 Cl.

Application : Luteol was recommended as an in-
dicator by W. Autenrieth. In point of sensitiveness
it far exceeds both litmus and phenolphtalein ; 5 to
10 Cc. of a solution containing I drop of dilute
soda lye in a liter are distinctly colored yellow by
a few drops of an alcoholic luteol solution. To
ammonia, luteol is even more sensitive than is
Nessler's solution, as 5 to 10 Cc. of a solution con-
taining I drop of ammonia water per liter are
colored yellow immediately, whereas with Nessler's
test a reaction was obtained only after some time.

Luteol expels carbonic acid from carbonates, and
the acid character of the phenol is greatly increased
by the introduction of the chlorine atom into the


It is superior to phenolphtalein in that it may be
used in the presence of ammonia ; and better than
litmus because no intermediate colors appear during
the change, hence insuring greater accuracy. The
yellow color produced by alkalies is discharged by
acids, the solution becoming colorless.

Solution of luteol for indicating is prepared by
dissolving I part of the substance in 300 parts of
alcohol. Of this solution from 4 to 8 drops are
added to 100 Cc. of the solution to be titrated.



Source : The purple flowers of several species of
Mallow yield a pigment that has been applied as
an indicator. The species from which the pigment
is most generally obtained are Althcea rosea Cava-
nilles (Alcea rosea Linn.), hollyhock; Malva sylves-
tris, common mallow; and Malva vulgaris. The
mallows are biannual plants, and are indigenous to
the Orient, but are found cultivated over all of
southern and central Europe.

Preparation: The pigment of the mallow flowers is
extracted by exhausting the latter with hot water,
or with alcohol, and carefully evaporating the


Properties : The aqueous extract of the hollyhock is
violet-red in color, the margins of the solutions
presenting a dark-violet color, whereas the alco-
holic extract is purplish-red, and deposits the pig-
ment, on evaporation of the alcohol, in the form
of a blackish, nitrogenous substance. Solutions of
the pigment are colored a fine red by acids, and
green by alkalies, in which respect it is closely
allied to the pigment obtained from the common
Dahlia, and with which it is, in fact, believed to be
identical. Alum yields with the pigment-solution
a lilac color, and baryta-water causes the gelatiniza-
tion of the solution with a green color.

The flowers of Malva vulgaris yield, when ex-
tracted with water, a reddish-brown infusion which
is colored a wine-red by acids ; alkalies, however,

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Online LibraryAlfred Isaac CohnIndicators and test-papers; their source, preparation, application, and test for sensitiveness . . → online text (page 6 of 14)