Alfred Isaac Cohn.

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

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delicacy of the reaction, but the ammonium salts
have, most probably from the action of the am-
monia on the phenolphtalein, resulting in the
formation of phenol di-imidophtalein, which yields


colorless solutions with alkalies. The reaction oc-
curring may be expressed as follows :

GH,.0. + 2NH, = C..H..N ,O. + 2H,O.

phenolphtalein phenol di-imidophtalein

The indicator is, hence, useless for ammonia.
Phenolphtalein is, however, one of the best indi-
cators for titrating caustic alkalies and their carbon-
ates, and for the alkali earths. By its means also,
caustic alkalies may be titrated in the presence of
carbonates or bicarbonates, and carbonates may be
estimated in the presence of bicarbonates, which
are neutral to the indicator, and the solution
becomes colorless as soon as the alkali or carbonate
has been neutralized. The indicator is very
sensitive to carbonic acid, however, hence the
estimations must be conducted in boiling solutions.*
The residual bicarbonate may then be estimated, if
desired, with methyl orange. Caustic alkali in the
presence of carbonate may also be estimated by first
diluting the solution so that not more than about
0.5$ of total alkali is present, then precipitating the
carbonate with barium chloride in excess, and
rapidly titrating to avoid the formation of new
carbonate from the caustic alkali in the solution.
The solution containing the precipitated barium
carbonate need not be filtered, as the salt does not
interfere with the reaction.

Phenolphtalein may be used for titrating alkali


sulphides. These yield a red color with the in-
dicator, which is discharged when half the alkali
has been converted into sodium- (or potassium-)
hydrogen sulphide. The total alkali may also be
titrated, but in this case the operation must be
effected in boiling solutions, since the liberated
hydrogen sulphide destroys the color of the solu-

The alkali radical of borax cannot be estimated
with phenolphtalein, because the color produced on
adding acid fades away. The boric acid in borax
may be titrated accurately, however, by employing a
/5 sodium-hydrate solution, but it is necessary
to first add about 30$ of glycerin to the liquid to
be titrated.

Normal sulphites are neutral to phenolphtalein, in
both hot and cold solutions. Reddened phenol-
phtalein solutions are decolorized by boric and
arsenous acids, potassium bichromate and gum arabic.
For arsenites the indicator is not very serviceable,
as it only shows about 90^ of the combined sodium
or potassium. With the arsenates, however, a well-
defined change occurs when one-third of the alkali is
neutralized. With the bibasic phosphates, which are
almost, but not quite, neutral to the indicator, the
results are not very satisfactory.

Codeine, morphine, cinchonine, quinine, quinidine,
brucine, strychnine, aniline, quinoline, and paratolui-
dine are neutral to phenolphtalein, and may be


readily titrated by its means, their salts being dis-
solved or suspended in a few Cc. of alcohol and
then titrated with alkali to redness. The acid
radicals of all the alkaloids excepting coniine,
nicotine, and codeine, may also be estimated.

Phenolphtalein is excellent for estimating the
organic acids, such as oxalic, acetic, tartaric, citric,
lactic, etc., and particularly in mixtures of alcohol
and ether, in which many organic acids which are
insoluble in water may be readily titrated. It is
best to titrate the organic acids in cold or only
moderately warm solutions when using potassium-
or sodium hydrate as a titrating fluid. The indi-
cator is also exceedingly useful for titrating fatty
acids in alcoholic solution, and for free alkali in
soap where litmus is inapplicable.

The indicator can be used with only those metals
that yield white precipitates with sodium sulphide or
with a carbonate of an alkali. It is also useful in
many cases where the solutions to be titrated are
colbred, because in these the color-changes may be
quite readily observed.

It has 'been pointed out by Messrs. N. and C.
Draper that the color of phenolphtalein solutions
colored red by potassa or soda is partially discharged
on the addition of alcohol, the reason ascribed
being the formation of some bicarbonate by taking
up carbonic acid from the air. On heating, or on
dilution with water, the color is restored.

I 3 2 JND 1C A TO AS

Phenolphtalein is also used in the form of test-
paper (see Test-papers).


C,H 4 (NH a ) a
Synonym : Meta-diamido Benzene.

Preparation : Meta-phenylene-diamine is obtained
by the reduction of dinitrobenzene, or nitraniline,
with tin and hydrochloric acid.

Properties : The meta-phenylene diamine occurs as
a crystalline powder or mass, readily soluble in alco-
hol and in ether, and but little so in water. It is a
strong di-acid base, and capable of forming well-
defined salts with acids.

Tests : Meta-phenylene diamine is readily prone to
change on contact with air, when it loses its origi-
nally white color and becomes reddish. A I : 20
aqueous solution should be colorless, or only faintly
yellow ; if the color is deeper, the preparation
should be rejected. On account of its readiness to
decompose, the preparation must be kept in small,
well-closed bottles.

Application : Meta-phenylene diamine, as well as its
hydrochloride, C 6 H 4 (NH 3 ) 3 .2HC1, which occurs as
a white or faintly reddish-white crystalline powder
soluble in water, is very useful for detecting nitrites
in potable water. On adding sulphuric acid to the


suspected water, the nitrous acid liberated from any
nitrites that may be present affords a yellow color
even if the slightest traces of nitrites are present. For
this purpose a I : 200 aqueous solution of the base
or its salt is used. I Cc. of the solution is added to
IOO Cc. of the suspected water, and I Cc. of diluted
sulphuric acid (1:3) added. The color so obtained
is then compared with a known standard, by which
means the quantity of nitrites present may be esti-

Meta-phenylene diamine hydrochloride gives also
an exceedingly sharp reaction with aldehydes, and is
used for detecting the latter in alcohol.


ALKALIES Green, then Yellow, and finally almost

Synonyms : Pellagri's Indicator ; Phyllocyanic

Source : Phyllocyanin was discovered (and named)
by Frmy, as being one of the two constituents
(Filhol, and also Stokes, claim there are four) to
which chlorophyll owes, in part, its color. Fremy
separated it from the accompanying yello<v phyllo-
xanthin, also named by him, by shaking chlorophyll
with a mixture of two parts of ether and one part
of dilute hydrochloric acid. The latter dissolves


the phyllocyanin, and the former dissolves the
phylloxanthin. The immiscible solvents were then

Preparation : Phyllocyanin is prepared by Fremy's
process, or by boiling chlorophyll with a strong
alcoholic solution of potassium hydroxide, neutraliz-
ing the solution with hydrochloric acid, and then
filtering. On evaporating the filtrate the phyllo-
cyanin is obtained.

Properties : Phyllocyanin occurs as a dark blue mass
soluble in water and alcohol, the solutions having
an olive color with bronze-red or violet reflection.
It is acid in character, and yields brown or green
salts, those of the alkalies being soluble in water.
With sulphuric and hydrochloric acids it yields solu-
tions which are green, reddish, violet, or a hand-
some blue, according to the strength of the solu-

Application : For use a solution of phyllocyanin is
prepared to which just sufficient acid is added to
change the color of the solution to a purple.

The indicator is quite sensitive to alkalies, which
change the color of the acid solution to green, then
yellow, and finally almost black.



. carbonates : Blue . -..

ALKALIES = caustic . Red ACIDS = Blue

Preparation : Poirrier Blue, 46, is obtained by the
action of sulphuric acid on triphenylrosaniline, and
is closely allied to Gentian Blue in properties.

Properties : Poirrier Blue, 46, occurs as a blue
powder having a coppery luster. It is soluble in
water and in alcohol, yielding blue solutions. That
obtained with alcohol is lighter in color than the
aqueous solution, but the color is deepened on the
addition of water. Potassium and sodium hydrates
change the color to a red, the blue being restored
on adding an acid. Ammonia, however, decolorizes
the solution, but the blue is also restored in this
case by an acid.

Application : Poirrier Blue, 46, was first recom-
mended as an indicator by Engel and Ville. An
aqueous I : 500 solution of the indicator is employed,
of which one or two drops are added to 100 Cc. of

. the liquid to be titrated. In the presence of only
alkali carbonates the color remains blue, but if caustic
bases are present, the color changes to red. Ammonia,
ethylamine, and other amines do not change the
color. The indicator is exceedingly sensitive to
acids, and the results obtained by its means are
very good, compared with those obtained with


litmus or methyl orange. Borax and boric acid
sharply change the color to blue, the solutions only
becoming red when the total acid has been com-
pletely neutralized. Phosphates and arsenates act
similarly. This is a valuable property peculiar to
the Poirrier Blue; many indicators are affected by
alkalies before neutralization is complete. Bicar-
bonates of the alkalies afford a blue color which does
not change until the bicarbonates have been con-
verted into basic carbonates.

Morphine, phenols, resorcin, chloral, and hydrocy-
anic acid, act like acids towards the indicator, and
may be titrated by its means. Phosphoric and
arsenic acids are tribasic with the Blue, but as the
tribasic phosphates are very unstable, and the
change of color is not very marked in consequence,
the indicator is unserviceable for titrating them.
Carbolic acid is monovalent, and resorcin is bivalent
with the Blue ; monovalent alcohols are neutral, and
polyvalent alcohols are acid. The aldehydes are all

Poirrier Blue has the character of a very weak
acid, and its salts are so unstable that they are de-
composed by water alone when in very great dilu-
tion, hence readings with the indicator may be
misleading unless sufficient of the indicator is present.
The change of color in the latter case is very sharp,
and is facilitated if a little alcohol is added to the
solution to be titrated.



ALKALIES = No Precipitate ACIDS = Turbidity

Synonym . Bolton's Indicator.

Preparation : The polysulphide indicator is a very
concentrated solution of sublimed sulphur in a solu-
tion of an alkaline sulphide. The solution must be
concentrated in order to avoid the necessity of using
too large a quantity of the indicator, which would
vitiate the results because of the natural alkalinity of
the solution ; while at the same time sufficient dis-
solved sulphur must be present to afford a clearly
visible precipitate on neutralization.

Application : The polysulphide solution was recom-
mended as an indicator by Bolton. Its action de-
pends on the fact that sulphur is precipitated in
neutral or acid solutions, but is soluble in alka-
line sulphide solutions. For the application of
the indicator a drop or two of the solution is a suf-
ficient addition to 250 Cc. of the liquid to be,
titrated. . The liquid is then heated, the titration
being best effected in an Erlenmeyer flask, and
when it boils, the titrating acid is permitted to flow
in. So long as the liquid remains alkaline, the
liberated sulphur redissplves, but on the least ex-
cess of acid, the sulphur renders the liquid milky.
Of course the indicator can only be used in alkaline
solutions. The natural alkalinity of the polysul-


phide solution may be determined beforehand, and
the necessary corrections in the acid-readings be


K 2 Cr 2 7
ALKALIES Reddish-Yellow ACIDS = Pale- Yellow

Synonyms : Potassium Bichromate ; Acid Potassium
Chromate; Red Potassium Chromate ; Potassium
Anhydrochromate ; Richter's Indicator.

Preparation : A mixture of finely powdered chrome
ironstone (FeOCr 3 O 3 ), caustic lime, and potassium
carbonate is heated for several hours. Oxidation en-
sues, carbon dioxide, potassium chromate, calcium
chromate, and iron oxide resulting as follows:

2FeOCr a O, + 3K,CO, + CaO + ;O

= 3K a Cr0 4 + CaCr0 4 + Fe,O 3 + 3 CO 3 .

The greenish-yellow mass so obtained is exhausted
with a little boiling water which dissolves out the
soluble chromates. The calcium salt is then de-
composed by potassium hydroxide, and the potas-
sium chromate in solution converted into a bichro-
mate by the addition of sufficient sulphuric acid.

The salt is then crystallized, and purified by

Properties : Potassium bichromate occurs as large,


orange-red, translucent crystals having a bitter, me-
tallic taste, and soluble in 10 parts of water at
15 C. Its formula is K 2 Cr a O 7 .

Tests : Potassium bichromate should be tested for
sulphuric acid by dissolving 3 Gm. in 100 Cc. of
distilled water, and adding hydrochloric acid and
barium-chloride solution. No change or precipitate
should be observed within 12 hours. Too little
hydrochloric acid should be guarded against, other-
wise a misleading precipitate of barium chromate
may be formed.

It is important that the salt be tested for any
sulphuric acid that might be present, when intended
for use as an oxidizer. Fresenius tests it by adding
hydrochloric acid and alcohol, and after complete
reduction, adding the barium chloride and per-
mitting the mixture to stand for 12 hours.

The salt is tested for chlorides by adding some
nitric acid, warming the solution, and then adding a
few drops of silver-nitrate solution. No precipi-
tate should 'be formed. Calcium salts may be
detected by adding I Cc. of ammonia water to 10
Cc. of a I : 20 aqueous solution of the salt, and then
adding a little ammonium-oxalate solution. No
precipitate should be formed.

Application : Potassium bichromate was recommend-
ed as an indicator by M. Richter. The bichromate
is dissolved in water, a very little phenolphtaiein


is added, and potassium-hydrate solution added
cautiously until a very faint yellowish-red solution
results. The change of color from a pale-yellow
with acids to a reddish-yellow with alkalies is very
sharp. The indicator is useful for titrating caustic
alkalies and ammonia, the formation of monochro-
mates being readily observed.


K a CrO 4


Synonyms: Potassium Monochromate; Neutral Po-
tassium Chromate; Yellow Potassium Chromate.

Preparation : Potassium chromate is obtained by a
process exactly similar to that described in the
preparation of potassium bichromate, only the final
conversion into a bichromate is omitted, the yellow
liquid being filtered, concentrated by evaporation,
and set aside to crystallize. The crystals are then
purified by recrystallization.

The chromate is also obtained by passing chlorine
gas into a hot solution of caustic potassa containing
chromium hydroxide in suspension.

A pure, chlorine-free preparation is secured by
fusing 25 parts of potassium bichromate with 20
parts of pure potassium nitrate (or a corresponding
quantity of potassium carbonate). This yields 34
parts of pure neutral potassium chromate.


Properties : Potassium chromate occurs as yellow,
translucent crystals, permanent in air, and soluble
in 2.7 parts of water at 15.5 C. and in 1.57 parts at
100 C. It is insoluble in alcohol.

On adding an acid to the solution of the salt, the
yellow color is changed to red, due to the formation
of a bichromate.-

Solutions of the salt are faintly alkaline towards
litmus arid curcuma, but are neutral, according to
De Vrij, to phenolphtalein.

Tests : Potassium chromate should be tested for free
alkali by dissolving o. I Gm. of the salt in 25 Cc. of
water, and adding a few drops of phenolphtalein

Sulphates and chlorides are tested for as described
under potassium bichromate.

Alumina and alkaline earths are tested for by
adding ammonia and ammonium oxalate to a 1 : 20
solution of the chromate ; there should be no pre-

Application : Potassium chromate is used as an in-
dicator in the titration of halogen salts, using silver-
nitrate solution as a titrating fluid. The moment
when all the halogen radical of the salt has combined
with the silver is indicated by a red precipitate or
color, due to the formation of silver chromate.

It has also been employed in the estimation of


For use a solution of I Gm. in 10 Cc. of distilled
water is prepared.


K 8 Fe a Cy ia


Synonyms : Red Prussiate of Potash ; Potassium
Ferridcyanide ; Potassio-ferric Cyanide,

Preparation : Potassium ferricyanide 'is prepared by
passing chlorine gas into a I : 8 solution of potas-
sium ferrocyanide until a small sample of the solu-
tion no longer gives a blue reaction with ferric

Reichard proposes bromine as a substitute for
chlorine gas, and states that I part of bromine suf-
fices for 5 parts of potassium ferrocyanide.

Properties : Potassium ferricyanide occurs as red
crystals soluble in 4 parts of cold water and yield-
ing a brownish-yellow solution. The dilute solution
is lemon-yellow. The salt is but slightly soluble in

Tests : Potassium ferricyanide should be tested for
sulphuric acid by adding to a I : 20 solution a little
hydrochloric acid followed by barium-chloride solu-
tion. Any ferrocyanide present may be detected
by adding a little ferric-chloride solution, which
would give a blue color.

Chlorides may be detected by fusing 0.5 Gm. of


the permanganate with I Gm. of pure potassium ni-
trate, exhausting the fused mass with water, filtering,
acidulating the filtrate with nitric acid, and then
adding silver-nitrate solution.

Sodium salts are detected by dissolving I Gm. of
the permanganate in water, acidulating with hydro-
chloric acid, adding ferric chloride in excess to pre-
cipitate the excess of iron, filtering, and then
testing the filtrate with potassium metantimoniate.

Application: Potassium ferricyanide is employed as an
indicator in the titration of ferrous salts by means
of potassium bichromate. The latter oxidizes the
ferrous salts to ferric, the end of the reaction being
known when a blue color ceases to be yielded, on
adding the bichromate solution. The process does
not yield very satisfactory results, because the indi-
cator cannot be added directly to the ferrous-salt
solution, but a drop or two of the latter, after every
addition of bichromate, is brought into contact with
the indicator solution on a porcelain tile. The
method is thus very troublesome, and takes up too
much time.

The ferricyanide is also used in the volumetric
gas analysis of hydrogen dioxide, and also for
alkalies, and alkali earths. It is, besides, used in
estimating phosphoric acid by means of uranium

For use a recently prepared I : 10 solution is



ALKALIES = Carmine-red ACIDS =Yellow

Synonym : Oser and Kalmann's Indicator.

Preparation : On treating gallic acid with potassium
permanganate, hydrorufigallic (tetrahydroellagic)
acid, C 14 H 10 O 8 ', is obtained in the form of yellow
needles readily soluble in alcohol and in ether, but
difficultly soluble in water. It has been found,
however, that a sensitive indicator may be obtained
by fusing I part of the hydrorufigallic acid so ob-
tained with 5 parts of caustic potassa, until a
reddish-violet mass results, an isomeric compound
being thus formed, and which, when decomposed
by sulphuric acid, affords a greenish-yellow residue.
This is finally purified by recrystallization from boil-
ing water.

Properties : The preparation obtained as above oc-
curs as greenish-yellow, microscopic crystals, which
sublime at 200 to 220 C., decompose above 230
C., and dissolve in solutions of caustic alkalies and
yield a green solution, the color of which changes
to a carmine-red on contact with air. The red color
is changed to yellow by acids, and restored again
by alkalies.

Application : The indicator is useful in titrating
mineral waters, because it is reddened by carbonates


of the alkalies, magnesium, calcium, and iron, but
is unaffected by carbonic-acid gas. The combined
carbonic acid in well-water, springs, etc., is also
readily determined by its means.


KMn0 4


Synonyms : Potassium Hypermanganate ; Potassium
Oxymanganate ; Potassium Supermanganate.

Preparation : Potassium permanganate is prepared
by heating 10 parts of caustic potassa, 7 parts of
potassium chlorate, and 8 parts manganese dioxide
in a crucible until a test taken from the mass dis-
solves and yields a deep-green solution. The mass
is then boiled with water until the potassium man-
ganate has become converted in a permanganate,
evidenced by a change of color from green to red.
The solution is then concentrated by evaporation,
and set aside to crystallize.

According to an improved process proposed by
Boettger, 2 parts of caustic potassa and I part of
potassium chlorate are fused together, and then 2
parts of finely powdered manganese dioxide care-
fully added, the heat being continued for a short
time. The fused mass is then comminuted and
boiled with 40 parts of water, while a stream of car-


bon dioxide is forced through the liquid until a few
drops of the latter placed on white paper no longer
show a green, but red, color. The solution is then
concentrated and allowed to crystallize. Instead
of carbon dioxide, chlorine gas and bromine have
also been used, and with good results.
Properties: Potassium permanganate occurs as dark-
violet, anhydrous crystals presenting a metallic lus-
ter. It is soluble in 16 parts of water at 15 C.,
yielding neutral solutions having a deep-purple

Potassium permanganate is an exceedingly pow-
erful oxidizer, and is readily reduced by all organic
matter; hence, it must not be carelessly brought
into contact with readily oxidizable substances. On
being heated it yields 10.5$ oxygen.

Its solutions are decomposed by mineral acids
and by alkalies, manganous salts resulting with the
former and manganates with the latter.

Carbonates of the alkalies are indifferent toward
potassium permanganate, but ammonia decomposes
and decolorizes it. The dry salt, treated with con-
centrated sulphuric acid, yields ozone.

Tests : Sulphates and chlorides when present may be
detected by boiling 0.5 Gm. of the salt with 2 Cc.
of alcohol and 25 Cc. of distilled water, and acidu-
lating the colorless solution with nitric acid. On
now adding barium nitrate or silver nitrate, white
precipitates will form.


Nitrates are tested for by dissolving 0.5 Gm. of
the permanganate in 5 Cc. of hot water, and grad-
ually adding oxalic acid until decolorization is
complete. 2 Cc. of the filtrate are then mixed
with 2 Cc. of concentrated sulphuric acid, and
the mixture overlaid on I Cc. of a concentrated
ferrous-sulphate solution. If a nitrate is present, a
brown zone will form at the point of contact of the
two liquids.

Application : Potassium permanganate is not used
as an indicator per se. It does, however, indicate
the end of the reaction by being decolorized when
used as an oxidizer. It is thus useful in estimating
ferrous salts, which it oxidizes to ferric salts when
in acid solution. The end of the reaction is known
to be at hand when decolorization ceases to be com-
plete, the end point being reached when the liquid
acquires a faint, permanent pink tint.

Potassium permanganate is also used in the esti-
mations of sulphur, hypophosphites, thiosulphates,
oxalates, and hydrogen peroxide.


The reflecting galvanometer has but very recently
been employed by Prof. Kiister as a physico-chemical
substitute for the indicators commonly employed in
titrations. As is well known, many mixtures offer

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Online LibraryAlfred Isaac CohnIndicators and test-papers; their source, preparation, application, and test for sensitiveness . . → online text (page 8 of 14)