Alfred Isaac Cohn.

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

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MEDICAL >ClnI<0>L




tl OF PHARMACY



CHEMISTRY
DEPARTMENT



Oafffornia College of Pharma



WORKS OF ALFRED I. COHN

PUBLISHED BY

JOHN WILEY & SONS, Inc.



Indicators and Test-papers.

Their Source, Preparation, Application, and Tests
for Sensitiveness. With Tabular Summary of the
Application of Indicators. Second Edition, Revised
and Enlarged, ix -f- 267 pages. 5 by j^. Cloth,
$2.50 net.

Tests and Reagents.

Chemical and Microscopical, known by their
Authors' Names; together with an Index of Sub-
jects, iii 4-383 pages. 6 by 9. Cloth, $3.50 net.

TRANSLATIONS.

Fresenius's Quantitative Chemical Analysis.

New Authorized Translation of the latest German
Edition. In two volumes. By Alfred I. Cohn,
Phar.D. Recalculated on the basis of the latest
atomic weights, and also greatly amplified by the
Translator. 2 vols., upwards of 2000 pages. 6 by 9.
280 figures. Cloth, $12.50, net.

Techno-Chemical Analysis

By Dr. G. LUNGE, Professor at the Eidgenossische
Polytechnische Schule, Zurich. Authorized Trans-
lation by Alfred I. Cohn, Phar.D. vii +136 pages.
5 by 7/4- *6 figures. Cloth, $1.00, net.

Toxins and Venoms and Their Antibodies.

By EM. Pozzi-EscoT. Authorized Translation by
Alfred I. Cohn, Phar.D. vii-l-ioi pages. 5 by 7^.
Cloth, $1.00, net.



INDICATORS AND TEST-PAPERS

THEIR SOURCE, PREPARATION,

APPLICATION, AND TESTS

FOR SENSITIVENESS

A Resume of the Current Facts regarding the Action
and Application of the Indicators and Test-
papers which have been Proposed from
Time to Time, and are in Present
Use in Chemical Manipulations

WITH A

TABULAR SUMMARY OF THE APPLICATION OF INDICATORS



DESIGNED FOR THE USE OF CHEMISTS,
PHARMACISTS, AND STUDENTS

California CoHege of Pharmac



BY



ALFRED I. JIOHN, PH.G.



SECOND REVISED AND ENLARGED EDITION.



NEW YORK

JOHN WILEY & SONS, INC.
LONDON: CHAPMAN & HALL, LIMITED



Copyright, 1899, 1902

BY

ALFRED I. COHN



PRESS OF

BRAUNWORTH & CO.

BOOK MANUFACTURERS

BROOKLYN. N. Y.



PREFACE



MUCH has been written on indicators within the last
twenty years in the various journals, etc., devoted to
chemistry and pharmacy. No systematic arrange-
ment, however, appears to have been made of the
facts recorded by a host of observers, which would
present the busy worker with the widely scattered in-
formation in the form of a succinct yet comprehensive
compilation.

The purpose of this work is, hence, to present an
up-to-date resum6 of the knowledge regarding the
various substances which have been proposed for use
from time to time as indicators, so far as their prep-
aration, properties, application, etc., are concerned.
Such a resume appears all the more desirable, because
of the increasing value of titrimetric methods, in
consequence of their ease and rapidity of application,
and more particularly because, with the increase of
knowledge regarding the action of the various acids,
alkalies, and reaction-products on indicators, arises

iii



IV PREFACE

the recognition of the necessity for a correct choice
of the indicator most suitable for a given case.

The subject-matter has been largely compiled from
the files of the various chemical journals, reports of
the meetings of chemical and pharmaceutical societies,
and works on chemistry, in the English, French, Ger-
man, Spanish, and Italian languages ; and it has been
arranged in the form believed to best meet the re-
quirements of the chemist, pharmacist, and student.

In the section devoted to Indicators only those sub-
stances have been discussed which are used purely and
simply as such ; under Test-papers, however, it has
been deemed desirable to include, all regarding which
any reference could be found.

ALFRED I. COHN

NEW YORK, July, 1899



PREFACE TO THE SECOND EDITION



THE flattering reception accorded the first edition of
the Indicators and Test-Papers has encouraged the
author to publish a second. In this the text has been
carefully gone over, and the few errors incident to a
first edition corrected. The entire work has been
brought up to date by the addition of an appendix
embodying the information on the indicators intro-
duced since the appearance of the first edition.

A. I. C.

NEW YORK, January, 1902



California CoHege of Pharmac"



TABLE OF CONTENTS



PART I

INTRODUCTION

PAGE

General Considerations I

Correct Choice of Indicators 5

Application of Indicators 8

Action of Indicators in other than Aqueous Liquids 12

Theory of the Action of Indicators 15

PART II
IN DIC A TORS

Alizarin 20

Alkanin 25

Alpha-Naphtolbenzein 29

Azolitmin 30

Benzaurin 34

Benzopurpurin B 35

Brazil Wood , 37

Brazil in 38

Cabbage, Red .'.... 41

Carmine.... 42

Carminic Acid 45

Cobaltous Nitrate 48

Cochineal 49

Coleln 54

Congo Red 55

Corallin 58

Corallin Red 58

v



VI TABLE OF CONTENTS

PACK

Curcumin 59

Curcumin W 62

Cyanine 63

Diazo-paranitraniline-Guaiacol 64

Dimethylamidoazobenzene 65

Diphenylamine 67

Eosine 68

Eosine-Methylene Blue . '. 70

Ethyl Orange * 71

Eupittonic Acid < ... 72

Flavescin 74

Fluorescein 75

Fuchsine 78

Gallein 80

Haematoxylin. . . , 82

Haematoxylon 85

Indigosulphonic Acid 88

Indigo Carmine. 89

lodeosine 91

lodeosine G 93

Iron and Ammonium Sulphate 94

Lacmoid 95

Litmus 99

Luteol 1 06

Mallow 108

Mesitylene Quinone no

Methyl Orange in

Methyl Orange and Phenolphtalein 118

Methyl Violet 118

Orange-peel Extract 120

Para-Nitrophenol 120

Phenacetolin 122

Phenol Nitrosylsulphonate 125

Phenolphtalein 126

Phenylenediamine, Meta- 132

Phyllocyanin 133

Poirrier Blue 48 135

Poly sulphide Solution 137

Potassium Bichromate 138

Potassium Chromate 140



TABLE OF CONTENTS Vll



Potassium Ferricyanide 142

Potassium Hydrorufigallate (?) 144

Potassium Permanganate 145

Reflecting Galvanometer 147

Resazurin 149

Resorcinbenzein 151

Rose Coloring-Matter 152

Rosolic Acid, Commercial 153

Salicylic Acid 157

Santalin 159

Sodium Salicylate 162

Starch 162

Tetrabromophenolphtalein. . . . . , 163

Thy molphtalein 166

Toluylene Red 167

Tropaeolin OO 168

Tropaeolin OOO No. i 170

Tropaeolin OOO No. 2 171

Zinc-Iodide and Starch 172

PART III
TEST-PAPERS

Alkanin 176

Azolitmin 177

Benzopurpurin 177

Brazilin 178

Brucine 179

Buckthorn 179

Carmine 179

Citro-Mercuric-Chloride 180

Citro-Molybdic- Acid 180

Citro-Potassium-Ferrocyanide 181

Citro-Sodium-Tungstate 181

Cobalt- and Methylene-Blue 182

Cobalt 182

Cochineal 183

Colein 183

Congo-Red 184

Corrosive-Sublimate 185



Vlll TABLE OF CONTENTS

PAGE

Curcuma 185

Dimethyl- Paraphenylenediamine 188

Elderberry .- 188

Ferment 189

Ferrous-Sulphate 189

Fuchsine 190

Georgina 190

Gold-Chloride 191

Guaiac 192

Guaiac- and Copper-Sulphate 192

Haematoxylin 193

Houzeau's Ozone 194

Huckleberry 195

Hyacinth 195

Indigo , 196

Indigo-Carmine 196

Indigo-Carmine and Sodium-Carbonate 196

lodate 197

Iris 197

Lacmoid 198

Lead 200

Litmus 200

Mallow 202

Manganous-Chloride 203

Manganous-Acetate 203

Metaphenylenediamine 203

Methyl-Orange 204

CEnokrine 204

Palladious-Chloride 205

Phenolphtalein 206

Phloroglucin- Vanillin 206

Picro-Citric-Acid 207

Pole-Reagent 207

Potassio-Mercuric-Iodide 208

Potassium-Chromate 208

Potassium-Ferricyanide. , 209

Potassium-Ferrocyanide 209

Potassium-Iodide 210

Potassium-Iodide and Starch 210

Potassium Sulphocyanate 211



TABLE OF CONTENTS IX

PACE

Resazurin 21 1

Rhubarb 212

Rose 213

Rosolic-Acid . 213

Schiff s 213

Serum 214

Silver-Nitrate ' 214

Starch 215

Starch- and Ammonium-Persulphate 215

Sulphanilic-Acid and Alphanaphtylamine Sulphate 216

Tannin 216

Tetramethvl-Paraphenylenediamine 217

Thallous- Hydrate 217

Tropaeolin 218

Ultramarine 218

White-Lead 219

Zinc-Iodide and Starch 219

PART IV

TABLES AND TABULAR SUMMARY

Trommsdorff' s Table, showing the Sensitiveness of Indicators to
Acids and Alkalies 220

Thomson's Table, showing the Hydrogen Atoms replaced by
NaOH or KOH when a Compound Neutral to the Indicator is
formed 223

Dieterich's Table showing Sensitiveness of Various Test-Papers. 224

Tabular Summary of Principal Indicators.

APPENDIX

Alizarin Green B 225

Ammoniacal Copper Solution 226

Corallin- Malachite-Green 227

Diazoparanitraniline- Propylmetacresol 229

Iron Isopyrotritarate 230

Iron Salicylate 232

Patent Blue L 235

Perezol 235

Potassium Ferrocyanide with Ammonium Molybdate 238

Sodium Alizarinsulphonate 239



PART I



GENERAL CONSIDERATIONS

An indicator, in chemistry, is a substance used for
the purpose of affording ocular evidence regarding
the condition of acidity, alkalinity, or neutrality ex-
isting in a liquid ; or of indicating the termination of
a reaction, whereby the quantity of the reagent, the
titrating liquid, used, enables the quantity of the re-
acting substance present in the liquid to be stoichio-
metrically determined. In titrimetric determinations
the indicator effects its object when, without taking
any immediate part in the reaction going on, it be-
comes affected the moment the liquid becomes neu-
tral, acid, or alkaline, and indicates this moment by
a change of color or the formation of a precipitate,
because of its having also entered into a reaction with
the reagent, but only after that between the reagent
and the substance under investigation is complete and
at an end.

The value of an indicator depends on its sensitive-
ness to the action of acids and alkalies, and on the dif-



2 INDICATORS AND TEST-PAPERS

ference in the intensity and sharpness of change in
the colors afforded by them. Many plants yield col-
oring-matters which are sensitive to the action of
acids and alkalies, and many synthetically produced
dyes are similarly sensitive. The number of sub-
stances that may serve as indicators is, hence, a com-
paratively large one. Not all of them are, however,
equally serviceable. Some are not sufficiently sen-
sitive ; in some the color-changes are not sufficiently
pronounced ; in others, the color-changes cannot be
observed in colored fluids; the action of others is
interfered with, or even neutralized, by products of
the reaction going on, or by certain chemicals which
may be present ; many, again, are only applicable to
special purposes, etc. To be of practical value for
general use, therefore, an indicator must not only be
sufficiently sensitive to sharply and decidedly show
the moment when the liquid passes into a neutral,
acid, or alkaline state, but the -change of color must
also be rapid and pronounced, the difference in the
tints afforded must be as extreme as possible, and the
color-changes must be as free as possible from inter-
mediate tints, and be afforded by a minimum quan-
tity of the indicator.

An ideal indicator that will answer for every pur-
pose has not as yet been found. Every one so far
used has some drawback, and no one can be relied on
absolutely in all cases. Litmus, for instance, which
may be considered as being of most general utility,



GENERAL CONSIDERATIONS 3

may be used with organic acids or ammonia, and also
for caustic alkalies in hot or cold liquids; but with
carbonates or bicarbonates the titrations must be car-
ried out in boiling liquids ; methyl orange, on the other
hand, is useful only in cold liquids, and when organic
acids and nitrites are absent ; phenolphtalein may be
used in hot or cold liquids, but ammonia or ammoni-
acal salts must be absent; rosolic acid, again, while
useful in hot or cold liquids, cannot be employed
when ammonia or organic acids are present; while
cochineal tincture is applicable in both hot and cold
liquids, but only when acetic acid and metallic salts
are absent, etc.

When the chemical characteristics of the indicators
are closely studied, it soon becomes apparent that the
latter possess more or less acid properties, as a rule,
and fall naturally into three classes, which, though
fairly distinct from each other, are not separated by
decided lines of demarcation, but tend to merge, each
into its next nearest neighbor. These three groups
may be defined as follows :

1. Indicators capable of forming fairly stable salts.
The members of this group are most sensitive to alka-
lies, and comprise such indicators as cochineal, congo-
red, dimethylamidoazobenzene, iodeosine, lacmoid,
methyl orange, etc.

2 . Indicators possessing very faint acid properties,
and yielding salts which are very unstable, and which
are readily decomposed by the hydrolytic action oj



4 INDICATORS AND TEST-PAPERS

water. The individuals of this class are particularly
sensitive to acids, and comprise such indicators as
curcuma, flavescin, phenolphtalein, etc.

3. Indicators occupying a place midway between
the other two classes. The indicators of this class
are somewhat stronger acids than those of the second
class, and are fairly sensitive to both acids and alka-
lies. The class comprises such indicators as fluores-
cein, gallein, hematoxylin, litmus, phenolphtalein,
rosolic acid, etc.



CORRECT CHOICE OF INDICATORS

THE division into groups, or classes, enables a cor-
rect judgment, which is of great importance, to be
formed with facility regarding the proper choice of
the indicator most suitable for the work in hand,
according to the acid or basic radical to be deter-
mined. For instance, for titrating weak acids, a
glance at the various classes will show that the indi-
viduals of the second class are best adapted for this
purpose; again, weak bases, it will be seen, would be
best titrated by the indicators of the first class ;
whereas strongly acid or basic substances would be
titratable by means of any of the indicators.

The majority of the indicators are, chemically con-
sidered, either weak acids or salts of weak acids, the
remainder being more or less basic. In some the free
acid is colorless, whereas the salt is colored, as, for
instance, in phenolphtalein ; in some the color of the
salt lacks intensity to such an extent as to be com-
paratively inappreciable in the dilution in which titra-
tions are ordinarily effected, whereas the free base
is very highly colored, as, for instance, in methyl
orange ; in others, again, both the free acid and free

5



O INDICATORS AND TEST-PAPERS

base have high tinctorial power and decided, but
different, colors, as, for instance, litmus or lacmoid.

The reasons governing the selection of an indicator
may be found in the statements just made. For
instance, the indicators of class i, which form quite
stable compounds, require strong acids to decom-
pose these compounds, i.e., to replace the rather
strong acids combined with the bases. For example,
methyl orange, which is, chemically, the sodium salt
of dimethylamidoazobenzene-sulphonic acid, is quite
stable, and the fairly strong acid radical can only be
partially replaced by organic acids, like acetic, tar-
taric, lactic, etc., whereas still weaker acids, like car-
bonic, boric, oleic, hydrocyanic, etc., are entirely
unable to replace it. It is for this reason that the
basic radicals of salts of the latter acids can be readily
titrated just as though only bases were present; salts
of the former acids could not, however, be titrated
at all, for, since their acid radicals can only partially
replace the acid radical of the indicator, the end reac-
tion will be indefinite because of a mixture of colors.

On the other hand, the indicators of the second
class, which possess but feeble acid properties, and
whose salts, as already stated, are very unstable, are
decomposed by almost all acids, even the weakest.
Hence, phenolphtalein, a representative of this class,
would be serviceable for many alkaloids, such as
quinine, brucine, cocaine, cinchonine, and quinidine,
and bases like aniline and pyridine, because, as these



CORRECT CHOICE OF INDICA TORS ^

do not yield a red color with phenolphtalein, any acid
combined with the alkaloids or bases may be titrated
directly with alkali, just as if no alkaloid or base were
present, but only free acid.

It will thus be seen that the correct choice of an
indicator is of some importance.



APPLICATION OF INDICATORS

In employing an indicator, care should be exercised
that a minimum quantity of the indicator be used
consistent with the production of a decided color and
sharp change of tint. It is needful, however, that
not too little be taken for the volume of liquid to be
titrated, because in too great dilutions a decided
difference will be observed in the readings, due to the
dissociation in the liquid of the compound formed
between the indicator and the titrating acid or alkali.
This dissociation will occasion intermediate tints, and
more acid or alkali than is normally necessary will
have to be added in order to secure a pure monotint.

On the other hand, an excess of the indicator must
also be avoided, because then the sensitiveness is
lessened. If too much is present, more acid or alkali
must be added than would be otherwise necessary,
because a minimum would not convert all of the indi-
cator into a salt, or liberate its free base, and in such
a case intermediate tints would also be produced ; and
could only be overcome by adding more titrating
fluid, thus again rendering the readings inaccurate in
very fine work.

8



A P PLICA TION OF IN DIG A TORS 9

In titrimetric operations, therefore, definite quanti-
ties of indicator and titrated solution should be em-
ployed ; moreover, at least three blank experiments
should be made to determine the quantities of acid
or alkali, or even of both, required to effect the color-
changes. The mean of the readings so obtained
should be noted on the labels of the bottles holding
the indicator-solutions, and should be deducted from
the readings obtained in practice. New blank deter-
minations should be made with every fresh lot of
indicator-solution prepared, because in many cases
considerable variation exists between different lots of
indicators. It is advisable also to test from time to
time the sensitiveness of the indicator-solutions, be-
cause some are prone to deteriorate on keeping for a
length of time. By attention to these details greater
accuracy is attained, and the errors due to personal
equation are largely minimized.

In performing titrations, shallow porcelain dishes
will be found most generally useful in practice. In
these the color-reactions are most clearly observed,
and the end of the reactions most readily determined.
The close of the reaction may be considered to be
reached when the color occasioned by a drop of the
acid- or alkali solution, as it falls into the liquid being
titrated, no longer changes, but remains permanent,
and can no longer be distinguished from that of the
liquid in general.

Glass beakers may also be used, but they should be



10 INDICA TORS AND TEST-PAPERS

placed on a white sheet of paper, and a similar sheet
placed behind, in order to facilitate the observation of
the color-changes. The beakers have the advantage
in cases where fluorescent indicators are used; i.e.,
where the fluorescence is made use of in determining
the end of the reaction. On the other hand, where it
is necessary to boil the titrated liquid to expel carbon
dioxide or hydrogen sulphide, the porcelain dishes are
most suitable (although the beakers are used by
many), because they are less likely to break, and,
which is more important, are far less likely to yield
traces of alkali to the boiling liquid, as many kinds of
glass are very prone to do and thereby introduce a
source of error in the readings. It is, hence, very
important that this point be not forgotten or over-
looked when titrating boiling liquids.

The color-changes of indicators are generally best
observed by daylight. There are cases, however,
when artificial light is the more suitable for use, as,
for instance, with cochineal, of which the change of
color from a yellowish-red to a violet-red is most
clearly observed by gaslight or the sodium flame
(see page 53).

Another point to which due regard should be paid
is the sensitiveness of the operator to certain colors.
It is often found that in the case of some operators
their eyes become much more rapidly fatigued by one
color than by another. Hence an indicator should be
chosen, so far as is consistent with the work to be done,



APPLICATION OF INDICATORS II

which the operator is best able to see well without
undue fatigue. Where an extended series of titra-
tionsis being carried out it is even advisable to change
the indicator from time to time in order that the eye
should not lose its sensitiveness to the color-changes.



ACTION OF INDICATORS IN OTHER
THAN AQUEOUS LIQUIDS

ALL the foregoing considerations have been based
on the employment of aqueous fluids for obtaining
the various color-reactions. Different phenomena
are observed, however, when other fluids are used.
It had long ago been observed that when highly alco-
holic fluids were used great discrepancies were noted
between the readings afforded in these and in aqueous
liquids. It has been found, in fact, that the influence
exerted by alcohol is sufficient to be the cause of
serious errors in volumetric work, unless due attention
be given to the variation in the readings caused by
it. The variation increases practically uniformly with
the increase in alcoholic strength, being greatest in
strongly alcoholic liquids, and smallest in liquids con-
taining but a little alcohol.

The variation in the readings may be readily
proved. For instance, when cochineal, brazilin,
hematoxylin, lacmoid, or litmus are used with alco-
hol, an increased quantity of alkali is required beyond
what would ordinarily suffice to afford the change in
color; i.e., alcohol appears to act the part of an acid.
On the other hand, with methyl orange, dimethyl-
amidoazobenzene, and the various tropaeolins, a large

12



ACTION OF INDICATORS 1 3

excess of acid is required above what usually suffices
to yield the color-changes; i.e., toward these indi-
cators the alcohol appears to act like so much alkali.

These phenomena have been recently studied by
John Waddell, who showed also that the addition of
ether, benzene, or chloroform, and in some cases of
acetone and alcohol, to aqueous or alcoholic solutions
in which fluorescein, phenacetolin, cyanine, phenol-
phtalein, paranitrophenol, methyl orange, turmeric,
lacmoid, and corallin were present in the form of
their salts, caused a change in color to that of the
free acid or base. This change of color was shown
to take place more readily when the indicator was
present in the form of its acetate than when in that of
its hydrochlorate; and when in the form of its ammo-
nium salt than when in that of its potassium salt.

For instance, a concentrated acetone solution of
phenolphtalein is scarcely colored by a large excess of
strong ammonia water; but the addition of water
immediately determines the appearance of the intense
red characteristic of alkaline solutions.

Methyl orange dissolved in acetic acid affords a red
solution, the color of which is changed to yellow on
the addition of alcohol or acetone, and restored to red
again on adding water.

Cyanine yields a colorless aqueous solution, but its
acetic-acid solution is changed to blue on the addition
of alcohol or acetone, and the color is discharged on
adding water.



14 INDICATORS AND TEST-PAPERS

The other indicators were found to yield similar
reactions. The probable reason of these color-changes
is to be found in the assumption that the very weak
acids and bases, of which indicators are constituted,
are very much less dissociated in organic solvents than
in water, and that their salts become almost entirely
hydrolyzed when these organic solvents are added to
aqueous solutions of the salts, in spite of the fact that
the concentration of the ions of the water is also reduced
somewhat. Hence when these solvents are added
they liberate the free indicator-acid or base, which ex-
hibits its usual color; and they will the more readily
liberate the indicator-acid or base if these are com-
bined with a weak base or acid. By observing whether
the change of color occurred on adding the solvents to
the acid or to the alkaline solution of the indicator, or
to both solutions, it was possible to determine whether
the indicator acts as an acid or base, or if as both.
From the observations made, it was determined that
phenolphtalein, paranitrophenol, and turmeric act like
acids; that cyanine, methyl orange, and lacmoid act


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Online LibraryAlfred Isaac CohnIndicators and test-papers; their source, preparation, application, and test for sensitiveness . . → online text (page 1 of 14)