Alfred Isaac Cohn.

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

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Synonym: Neutral Red.

Preparation : Toluylene red was discovered in 1879
by O. N. Witt. It is prepared by simply boil-
ing an aqueous solution of toluylene blue. It is,
also obtained by the oxidation of a mixture of equal
molecular weights of dimethylparaphenylenediamine
and metatoluylenediamine at the boiling point.

Properties: Toluylene red forms orange-red needles
containing 4 molecules of water of crystallization,
and having the composition


At a temperature of 150 C. it is converted into the
blood-red anhydrous modification. It also occurs
as a greenish-black powder, easily soluble in water
with a crimson color, and in alcohol with a magen-
ta-red, having a faint brownish-red fluorescence.
With alkalies it yields an intense, yellow color,
restored to red by acids.

Application : Toluylene red is rarely used ; it pos-
sesses no advantages whatever over other indicators.



C.H 4 .SO,Na.N a .C,H 4 NH.C 8 H
ALKALIES = Yellow ACIDS = Yellowish-red to Red

Synonyms: Orange GS. ; Orange N. ; Orange iv;
New Yellow; Fast Yellow; Acid Yellow D. ;
Diphenylamine Orange; Orange M. ; Diphenyla-
mine Yellow; Von Muller's Indicator.

Preparation : Tropaeolin OO is prepared by heating
5 parts of fuming sulphuric acid (20$ anhydride)
and I part of phenylamido-azobenzene for four
hours at a temperature of 60 to 70 C., cooling,
and pouring the mass into 50 parts of water. The
precipitate is then collected, washed, and converted
into a sodium salt, which is the commercial form of
the article.

Properties : The base, phenylamido-azobenzene-
sulphonic acid, forms graphite-like needles, difficultly
soluble in water and yielding a reddish-violet solu-

The sodium salt occurs in the form of orange-
yellow scales or powder, soluble in water and
yielding an orange-yellow solution. Its composition
is NaSO s .C fl H 4 .N a .C 6 H 4 . = NH.C 6 H 6 . When hydro-
chloric acid is added to the aqueous solution, a
violet precipitate is formed. With concentrated
sulphuric acid it affords a violet-colored solution.

OO 169

Application : Tropaeolin OO was recommended by
von Miiller as an indicator. For use a solution of
o. 5 Gm. of tropaeolin OO in a liter of water, or a cold
saturated alcoholic solution, is prepared; 2 Cc. of
the aqueous, or a few drops of the alcoholic, solution
are added to 50 Cc. of the liquid to be tested. The
light-yellow solution is colored yellowish-red by
acids, even by oxalic and other organic acids ; on
adding an excess of acid a red color is obtained.
Alkalies restore the yellow color.

The indicator is not affected by free carbonic-acid
gas, bicarbonates, or by acid salts, in which respect
it is superior to litmus, because normal sodium-
carbonate solution may be used instead of caustic-
soda solution for titrating.

The color of tropaeolin OO is changed to red
only by free acids, and not by solutions of metallic
salts. Hence small quantities of added acids may
be readily determined. Hydrogen sulphide also
does not affect the indicator.

Tropaeolin OO must be employed only in cold

Tropaeolin OO is a good indicator, but its quality
cannot IDC depended on ; hence, readings given by
it may be unreliable. It behaves very much like
methyl orange, but is less sensitive to acids. It
may be distinguished from methyl orange, accord-
ing to Engel, by means of gold chloride, as this
affords a violet, then green color with tropaeolin


OO, whereas with methyl orange it yields a red

The indicator is also used in the form of a' test-
paper (see under Test-Papers).



ALKALIES = Brown ACIDS = Brownish-yellow

Synonyms : Orange II ; Betanaphtol Orange ; Man-
darin G. Extra ; Chrysaurein ; Gold Orange ;
Orange Extra ; Atlas Orange ; Orange A ; Sulpho-

Preparation : Tropaeolin OOO No. 2 is obtained by
the action of para-diazobcnzene-sulphonic acid on
beta-naphtol in alkaline solution, the process being
quite similar to that whereby tropaeolin OOO No. 2
is prepared. The product is converted into the
sodium salt, which is the commercial form of the

Properties: The base, C.H 4 .HSO 3 .N a .C 10 H 6 .HO,
occurs as orange-yellow scales insoluble in water ;
on drying they lose water and are converted into a
red powder.

The sodium salt occurs as a bright orange powder,
soluble in water and yielding an orange-colored
solution. On adding hydrochloric acid to the solu-
tion it occasions a brownish-yellow flocculent pre-


cipitate. Caustic alkalies change the color to a
dark brown. With concentrated sulphuric acid it
yields a dark-red solution which, on dilution with
water, becomes a reddish-brown.

Application: Like Tropaeolin OOO No. 2.

C 8 H 4 .S03Na.N 2 .C 10 H 6 (OH)


Synonyms: Orange I; Sodium <*-Naphtolazoben-
zene Sulphonate ; Alpha-Naphtol Orange ; Orange
B. ; Naphtol Orange; Alpha-Naphtol Yellow; Sul-
phoazobenzene-alpha-naphtol; Von Muller's Indi-

Preparation : Tropaeolin OOO No. 2 is prepared as
follows: 25 parts of caustic soda are dissolved in
450 parts of boiling water, 100 parts of sulphanilic
acid added, and the solution cooled, when 64 parts
of sulphuric acid, and finally, 41 parts of sodium
nitrite dissolved in 200 parts of water added. To
the solution so obtained, a solution prepared by
boiling 80 parts of alpha-naphtol and 30 parts
of caustic soda in 400 parts of water, is slowly
added with constant agitation. The precipitate
is then collected, dried, and converted into the
sodium salt, which is the commercial form of the


Properties: The acid base forms almost black scales
having a greenish shimmer, and the composition
C.H 4 (SO,H).N a . = C :o H e (HO). The sodium salt
occurs as reddish-brown powder, readily soluble in
water and yielding an orange-red solution, the color
of which is changed to red or brownish-red by
alkalies. In concentrated sulphuric acid it dissolves
with a magenta color. Its solutions yield with
hydrochloric acid a brownish-yellow coloration or
flocculent precipitate.

Application : Tropaeolin OOO was recommended as
an indicator by W. von Miiller. For use a cold,
saturated, aqueous solution is most suitable. A few
drops of the solution are added to the liquid to be
titrated. This colors it a scarcely perceptible
yellow, which is sharply and suddenly changed to
red by alkalies. Ammoniacal salts do not affect it,
nor does carbonic-acid gas ; it is, hence, applicable in
the presence of the latter.


Preparation: Zinc-iodide and starch solution is pre-
pared by heating 4 parts of starch, 20 parts of zinc
chloride, and 100 parts of water over a free flame
for an hour or more, until a clear solution is ob-
tained, the evaporated water being replaced from
time to time. Sufficient water is then added to
make the whole measure 1000 Cc., and the mixture


is then set aside for a week or two, when the solu-
tion is filtered, and 2 parts of zinc-iodide then

Application : The solution is used in iodometric de-
terminations as an indicator. It is colored blue by
the slightest traces of free iodine.



FOR the preparation of test-papers only the finest
quality of filtering-paper should be used, and even
then, before impregnation with the indicator-sub-
stance, it should be treated with very dilute hydro-
chloric acid, then with very dilute ammonia, then
again with the hydrochloric acid, and finally
thoroughly washed, to remove all traces of im-
purities, with plenty of recently distilled, freshly
boiled water, and finally dried in an ammonia-free

The paper so treated is immersed in the indicator-
solution, the surplus of the latter is removed by
passing the paper over glass rods held horizontally,
and the paper then hung over stretched cords, or
suspended by two corners by means of clips. When
the dripping has ceased, the paper should be re-
versed, i.e., hung by the two corners that had been
lowermost. If this is not done, the paper, when dry,
will have received a surcharge of the indicator-sub-


stance at its lower end, because of a greater quan-
tity of the solution having accumulated there dur-
ing the hanging. It is well, therefore, to reverse
the position of the paper several times during the
period it is drying, in order to insure uniformity of
color and sensitiveness.

During the drying care must also be taken that the
paper be not exposed to ammoniacal, acid, or sul-
phurous fumes, because many test-papers are ex-
ceedingly sensitive to them, and may in consequence
.suffer an impairment of delicacy after exposure to

Where the substance to be applied is in the form
of a paste, as in making ultramarine-, starch-, etc.,
papers, the paste is best applied by means of a
broad, flat, camel's-hair brush, care being taken to
touch every part of the paper but once.

Test-papers are used either by dipping directly
into the liquid to be tested, and observing the color-
changes, if any, that occur; or better, by applying
a drop or two of the solution taken up on a glass
rod to the paper. For titrimetry proper, test-
papers are not very serviceable, as a rule, because
very many trials must be made before the end re-
action can be determined with precision. This oc-
casions a great loss of time, besides rendering the
final result slightly inaccurate on account of the
withdrawal of repeated appreciable quantities of the
liquid being titrated.


For the qualitative detection of metals, etc,,
however, the test-papers are excellent, and are very

All test-papers should be preserved in dark, or
amber-colored, carefully stoppered bottles, and be
as little exposed to light and air as possible, as these
have a disturbing effect on a very large number of
the dyes of which the indicators for the most part


ALKALIES = Green or Blue ACIDS = Red

Synonym: Anchusin Paper; Boettger's Test-Paper.

Preparation : Paper is impregnated with a 3-per-
cent, alcoholic solution of alkanin, or with an
ethereal extract of alkanet-root bark. The paper
so obtained possesses a rose-red color when dry,
and should be very carefully preserved in an am-
monia-free atmosphere.

Application : Alkanin paper is extremely sensitive
to free alkalies and alkaline salts, and particularly
to ammonia, even the slightest traces of which color
the paper green. It is sensitive to a 1:25000
solution of potassium hydrate, and to a 1:80000
solution of ammonia.

Blue Alkanin Paper may be obtained by immers-
ing the red paper, prepared as above, in a i-per-


cent, solution of sodium carbonate, and drying.
This paper is sensitive to a 1:60000 solution of
sulphuric acid, and to a 1 : 80000 solution of hydro-
chloric acid.



Preparation : Azolitmin paper is prepared by im-
pregnating filter-paper with a solution obtained by
dissolving I part of azolitmin and 0.5 part of
crystallized sodium carbonate in 1000 parts of
water, and adding to the solution just sufficient
phosphoric acid to cause a faint redness. The
paper when dry has a reddish-violet color which
is changed to a red by acids and to a blue by

Application : Azolitmin paper is very sensitive, and
it is used like litmus. Before use, its sensitiveness
should be invariably tested. It is sensitive to sul-
phuric acid i : 40000; and to hydrochloric acid
I : 5 ooo, according to Dieterich.


ALKALIES = Brownish-red ACIDS = Bluish-violet

Preparation : A solution of Benzopurpurin B. is
made violet with a small quantity of acid, and white


filtering-paper impregnated with the solution and

Application : Benzopurpurin paper affords a very
sensitive means of detecting ammonia, the slightest
traces of which change the bluish-violet color of the
paper to a red on drying.


ALKALIES = Blue ACIDS = Yellow

Synonyms: Pernambuco Paper; Fernambuco Paper.

Preparation : Unsized, white paper is impregnated
with a solution of brazilin, and 'dried; or, the paper
may be immersed in a 12.5-per-cent. infusion o
Brazil wood, to which ammonia has been added
until a bluish-red color is had, and dried.

Application : Brazilin paper is very sensitive to acids,
and may be used in their estimation. The paper
should be very carefully preserved in the dark, and
from air as much as possible.

Concentrated or gaseous hydrofluoric acid, and
also phosphoric and oxalic acids, have no action on
the paper until water has been added to the con-
centrated acids.

The paper is sensitive to I : 80 coo of ammonia,
and to a i : 30 ooo solution of potassium hydrate.




Preparation : Filter-paper is impregnated with an
aqueous, concentrated solution of brucine sulphate,
and dried in a warm place.

Application : Brucine paper is applied for the detec-
tion of chlorine and nitric acid.


ALKALIES = Greenish-yellow AciDS Red

Preparation : Paper is immersed in a decoction of
buckthorn berries (Rhamnus cathartica), and dried.

Application: Buckthorn paper is used similarly as
georgina paper. Acids change its color to red ;
alkalies to a greenish-yellow.

The paper is sensitive to a i : 15 ooo solution of
potassium hydrate, and to a I : 3$ ooo solution of


ALKALIES = Purplish-red ACIDS = Yellowish-red

Preparation : A red carmine paper is prepared by
immersing pure filter-paper in an ammoniacal solu-
tion of carmine.

Application : The carmine paper is at times used in
analytical work, although infrequently. It is used
like the carmine tincture.



ALBUMIN Precipitate
Synonym : Geissler's Test-Papers.

Preparation : Geissler's test-papers are prepared by
saturating strips of paper with a concentrated citric-
acid solution and drying; other strips of paper are
impregnated with a solution containing 3 per cent,
of mercuric chloride and from 12 to 15 per cent, of
potassium iodide.

Application : The papers are employed for the detec-
tion of albumin in urine, and as follows: A strip of
the acid paper is first dipped into the urine to be
tested, and stirred ; then a mercury-potassium-iodide
paper is immersed. If albumin is present a precip-
itate forms.


MOISTURE = Decolorization
Synonym : Mann's Paper.

Preparation: One part of molybdic acid and two
parts of citric acid are fused together, and then dis-
solved in water. With the solution filtering-paper
is impregnated, and dried at a temperature of 100 C.

Application: The paper is blue when dry, and is
applied for the detection of water in alcohol, etc.,
as exposure to moisture discharges the blue color.



ALBUMIN = Precipitate

Synonym : Oliver's Albumin Papers.

Preparation : Some strips of filtering-paper are im-
pregnated with a concentrated solution of citric
acid, while other strips are impregnated with a
concentrated solution of potassium ferrocyanide.
The strips are then dried.

Application : The papers are used for detecting the
presence of albumin in urine. For this purpose, a
strip of the citric-acid paper is first stirred in the
suspected urine, and then a strip of the ferrocyanide
paper. Albumin, if present, occasions a precipitate.


ALBUMIN = Precipitate

Synonym : Oliver's Tungstate Paper.

Preparation: Paper is immersed in a strong solution
of sodium tungstate and citric acid, and then dried.

Application : The paper is used for detecting the
presence of albumin in urine. A small piece im-
mersed in the suspected fluid occasions a precipitate
if albumin is present.




Preparation : Paper is immersed in a solution of
cobaltous chloride colored with some methylene

Application : Cobalt- and mythelene-blue paper is
employed as an indicator in estimating zinc, copper,
and nickel, by means of sodium sulphide. The
paper may also be made so that only one half of
the strip is impregnated with the solution, the other
half being left white. In using this paper, a drop
of the mixture to be tested for zinc, copper, etc.,
is placed, after the addition of the sodium sulphide,
on the white end so that the solution may diffuse
to the colored end, which will -be colored black if
any excess of sulphide is present.



Synonym : Stahl's Paper.

Preparation : Paper is impregnated with an almost
neutral solution of cobaltous chloride, representing
0.3 Gm. of CoO in 100 Gm. of solution, and then


Application : The paper is used in the estimation of
zinc by means of sodium sulphide; and also for
hygroscopic determinations.



Preparation : Cochineal paper is prepared by im-
mersing strips of good white filtering-paper in an
aqueous cochineal solution, and drying.

Application : The paper is used like the tincture.
Its sensitiveness has been determined by Dieterich
to be such as to indicate the presence of I part of
sulphuric acid and of hydrochloric acid in -8000 and
10,000 parts of water, respectively.



Synonym : Boettger's Test-Paper.

Preparation : Cole'in paper is prepared by immersing
sheets of filtering-paper in an alcoholic solution of
cole'in, and then drying. Care must be taken in mak-
ing it, however, that the color be not too dark or
too light, as on the one hand it will not be suffi-
ciently sensitive, and on the other hand it will not
yield a decided and sharp change of color. Paper
made too weak may be remedied by immersing it
in water containing a mere trace of ammonia.


Application : Colein paper is said to constitute a very
sensitive indicator for titrimetric determinations.
It is colored yellow by alkalies, and red by acids.

The paper is sensitive to a I : 8000 solution of
sulphuric acid ; to 1:10000 hydrochloric acid ; to
I : 8000 potassium hydrate, and to 1:20000 am-
monia solution.



Synonyms: Herzberg's Paper; Riegel's Paper.

Preparation : Both a red paper and a blue are pre-
pared for use. The red paper is prepared by im-
pregnating suitable paper with a solution obtained
by dissolving I part of Congo red in 1000 parts of
3O-per-cent. alcohol. The blue paper is obtained
by passing red paper, prepared as above, through a
faintly acidulated bath.

The blue paper must be carefully preserved, be-
cause of its tendency to become red. It is al,so
reddened by aniline and alkaloids generally.

Application : The red paper is useful for testing paper
for free acids, because of its extreme sensitiveness
to these. The paper may also be used for estimat-
ing organic bases, such as aniline, quinine, brucine,
morphine, etc. It is also serviceable in many cases
in which litmus and phenolphtalein are useless. It
is not as sensitive as litmus paper, however, and is


of no special value, except that the dye adheres
very tenaceously to the paper, and is not washed
out readily on prolonged immersion in water, as is
the case with litmus paper.

The papers are sensitive to acids and alkalies to
the extent of about I : 3000.


Synonym: Fliickiger's Sublimate Paper.

Preparation : Paper is immersed in a concentrated so-
lution of corrosive sublimate, and dried.

Application: Corrosive-sublimate paper is chiefly em-
ployed for the detection of arsenic. With arseni-
uretted hydrogen it affords a, at first yellow, then
brown, color, which is blackened by silver nitrate.
The paper is sufficiently sensitive to detect 0.00005
Gm. of arsenous acid.


ALKALIES = Brown ACIDS = Yellow

Preparation : Curcuma paper is prepared by im-
pregnating white, unsized paper in an alcoholic solu-
tion of curcumin. It may be prepared from the
root directly, also, but in this case the powdered
root must first be exhausted with water to remove a
water-soluble coloring-matter which interferes with


the sensitiveness of the color-change; then dry-
ing the root in the dark, and finally exhausting
it with alcohol, the paper being then imbued with
this solution.

Tests : Curcuma paper should always be tested
before use as to its sensitiveness towards potassium
hydrate and ammonia. A good paper will res-
pond to a 1 : 18000 potassium-hydrate solution,
and to a 1:35 ooo ammonia solution.

Application : Curcuma paper has a yellow color,
which is changed to brown by alkalies. The paper
is useful for estimating alkali hydrates and earths,
but is unsuitable generally for alkalimetry proper,
because the change of color - is scarcely sharp
enough. It may be successfully used, however,
for titrating citric, acetic, tartaric, oxalic, lactic,
and succinic acids by means of fixed alkalies, and
especially in dark colored solutions.

With ammonia the end-reaction is indefinite, and
only 97$ of NH S is shown ; with ammoniacal salts,
a similarly indefinite reaction is obtained ; hence
curcuma paper is inapplicable for ammonia or its
salts. The paper is also unserviceable for sodium
and potassium carbonates and sulphides, and for a
similar reason.

With boric, molybdic, or phosphoric acid, the
paper, after drying, is brown. Borax, after the ad-
dition of an acid, yields a similar result, and paper


browned by boric and hydrochloric acids is colored
a handsome blue by alkalies.

In silicates and borates of sodium and potassium,
90$ of the combined alkali is shown in the former,
and 50$ in the latter, the reactions being good.

Thiosulphates are neutral to curcuma paper ;
normal sulphites and monophosphates are slightly

F<;r fatty acids, when ascertaining the saponifica-
*ion numbers of fats, oils, etc., the paper is of no
value, because the change of color is not sharp
enough, and the soaps formed are distinctly alkaline
to the paper. The paper is very useful, however,
for ascertaining the close of the alkaline reaction in
the* Pettenkofer process of air analysis.

Thompson recommends also a reddish-brown
paper obtained by impregnating suitable paper with
an alcoholic solution of curcumin or curcuma ex-
tract rendered alkaline by the addition of caustic
soda. Paper so prepared becomes darker in color
on moistening with pure wate-r, neutral, or alkaline
solutions, but when immersed in an acid solution,
the immersed part becomes a bright yellow, while
the part outside of the liquid draws up some of the
water only from the solution by capillary action, and
appears much darker by comparison. This paper is
very useful for ascertaining acidity in alcohol.





OZONE = Bluish-violet

Synonym : Wurster's Red Ozone Paper.

Preparation: White, unsized paper is charged with a
solution of dimethyl-paraphenylenediamine, and
then dried.

Application : The paper is employed for the detec-
tion of ozone, sulphuretted hydrogen, hydrogen
dioxide, turpentine, colophony, etc., and also for
wood-pulp in paper.


ALKALIES Blue to Green ACIDS = Red

Preparation : Elderberry paper is prepared by im-
pregnating filtering-paper with the juice of the el-
derberry, Sambucus nigra L. and vS. canadensis L.

Application : The elderberry paper is used like geor-
gina paper, though it is somewhat less sensitive. It
will react with I part of potassium hydrate in 5000
parts of water, and I part of ammonia in 10000
parts of water.



UREA = Brown

Synonym : Musculus' Paper.

Preparation : Ferment paper is prepared by filtering
decomposing urine through white filtering-paper,
then washing the latter, and coloring it with

Application : Ferment paper is applied for the detec-
tion of urea, with which it yields a brown color, due
to the decomposition of the urea induced by the
ferment in the paper, ammonium carbonate being

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Online LibraryAlfred Isaac CohnIndicators and test-papers; their source, preparation, application, and test for sensitiveness . . → online text (page 10 of 14)