Alfred Isaac Cohn.

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

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RHUBARB PAPER

ALKALIES = Red ACIDS = Yellow

Preparation: A red and a yellow rhubarb paper may
be made.

Yellow Rhubarb Paper is obtained by impregnat-
ing white filtering paper in an alcoholic extract of
the rhubarb root, then passing through water
faintly acidulated with phosphoric acid, and finally
drying.

Red Rhubarb Paper is obtained by passing the
paper, after impregnation with the rhubarb ex-
tract, through water rendered alkaline with am-
monia.

Application : The yellow paper yields a fine red color
with alkalies, whereas the red paper is changed to
yellow by acids. The yellow paper is stated by
Dieterich to be^ sensitive enough -to detect I part of
potassium hydrate in 8000 of water; and I part of
ammonia in 20,000 parts of water. The paper is
unaffected by boric acid.



SCHIFF'S PAPER 213

ROSE PAPER

ALKALIES = Bright-green ACIDS = Pink

Preparation: Rose paper is prepared by dipping
paper into an infusion of the petals of the red rose
(Rosa gallica), and drying.

Application: Rose paper is but little used, and pos-
sesses no advantage over other test-papers. The
paper is colored a bright-green by alkalies, and by
acids a pink.

ROSOLIC-ACID PAPER

ALKALIES^ Red ACIDS = Yellow

Preparation : Rosolic-acid paper is prepared by im-
pregnating filtering-paper with a i-per-cent. solution
in 6o-per-cent. alcohol.

Application : The paper has a yellow color, and is ap-
plicable to all purposes as the rosolic-acid test solu-
tion. It is sensitive to a I : 20000 potassium-
hydrate solution.

SCHIFF'S PAPER

GLUCOSE AND CARBOHYDRATES = Red

Preparation : SchifFs paper is prepared by immersing
strips of filtering-paper in a mixture of equal vol-
umes of glacial acetic 'acid and xylene, and a very
little alcohol.



214 TES T-PA PERS

Application : Schiff's paper is applied for the detection
of glucose and other carbohydrates. The substance
to be tested is heated with sulphuric acid, and if
carbohydrates are present, furfurol is evolved and
colors the test-paper held in the vapors a fine red.

SERUM PAPER
Synonym : Richardson's Paper.

Preparation : Serum paper is prepared by impreg-
nating filtering-paper with serum obtained from a
typhoid-fever subject, and then drying carefully in
an unwarmed place.

Application: Serum paper is used instead of fresh
typhoid serum for carrying out Widal's typhoid
reaction.

SILVER-NITRATE PAPER

PHOSPHORUS = Black ARSENIC = Yellow

CHROMATES = Red URIC ACID = Brown

Synonym: Schiff's Paper.

Preparation : Fitter-paper is impregnated with a solu-
tion of silver nitrate, and dried in air free from hy-
drogen sulphide.

Application : Silver-nitrate paper is employed for the
detection of chromic and arsenous acids, and for
detecting phosphorus, by the vapors of which it is
blackened. It is an exceedingly sensitive means of
detecting arsenic.

Silver-nitrate paper is also used for detectiug uric



STARCH- AND A MMONIUM-PER SULPHA TE PAPER 2 1 5

acid in urine. The paper is stained brown by uric
acid.

STARCH PAPER

IODINE = Blue

Preparation: Starch paper is prepared by applying
with a broad, camel's-hair brush a solution ob-
tained by triturating 10 parts of starch with 10
parts of cold water, and then adding 890 parts of
boiling water. Care should be taken that no spot
on the paper be touched twice, otherwise fibers of
the paper are carried off mechanically. The paper
should be dried in an unwarmed place.

Application : Starch paper is exceedingly sensitive to
traces of free iodine, with which it strikes a blue
color owing to the formation of starch iodide. One
part of iodine in 25000 may be detected by means
of this paper.

STARCH- AND AMMONIUM-PERSUL-
PHATE PAPER

IODIDES = Intensely Blue

Preparation: According to Bourget, filtering-paper is
impregnated with a 5-per-cent. starch solution,
dried, and then cut up into squares of 5 Cm. each.
Two or three drops of a 5-per-cent. ammonium-per-
sulphate solution are dropped in the center of each
square, and the pieces dried in the dark.



2l6 TEST PAPERS

Application : Paper so prepared, and immersed in a
fluid containing even traces of iodine, develops an
intensely blue color, a distinct reaction being visible
in solutions containing 0.00005 per cent, of potas-
sium iodide. Paper prepared with sodium- or po-
tassium-persulphate may also be used, but is de-
cidedly less sensitive. Bourget recommends the
paper especially for the investigation of saliva and
urine at the bedside.

SULPHANILIC-ACID AND ALPHA-NAPH-
TYLAMINE-SULPHATE PAPER

NITRITES = Red

Synonym : Griess's Red Test-Paper.

Preparation : White, unsized paper is impregnated
with an alcoholic solution of sulphanilic acid and
alpha-naphtylamine sulphate (or hydrochlorate),
and dried.

Application : The paper is employed in the detection
of nitrous acid and nitrites, which change the color
of the paper to a deep red. It is also used to detect
bilirubin and aldehydes, in urine.

TANNIN PAPER

ALKALIES = Brown FERRIC SALTS Black

Preparation : Tannin paper is prepared by immersing
filter-paper in a concentrated solution of tannin,
and drying in a warm place.



THALLOUS-HYDRATE PAPER 21?

Application: Tannin paper is employed for caustic
alkalies, alkaline earths, ammoniacal salts, and iron
salts.

TETRAMETHYL-PARAPHENYLENEDI-
AMINE PAPER

OZONE Violet

Synonyms: Wurster's Blue Ozone Paper; Tetra Paper.

Preparation : White, unsized paper is impregnated
with a solution of tetramethyl-paraphenylenedia-
mine, and dried.

Application: The paper is employed for detecting
ozone and ozonizers, as turpentine, pine wood, and
also hydrogen dioxide. Traces of ozone or nascent
oxygen in neutral or acetic-acid solutions color the
paper intensely violet or bluish-violet.

THALLOUS-HYDRATE PAPER

OZONE ^ Brown

Synonyms: Boettger's Ozone Paper; Thallium-
Hydroxide Paper.

Preparation: Paper is impregnated with a lo-per-cent.
solution of thallous hydrate recently prepared by
precipitating a solution of thallous sulphate by the
proper stoichiometrical quantity of baryta water.
The paper should be rapidly dried, and is colorless.



2 1 8 TES T-PA PERS

Application: The paper is applied for the detection
of ozone, which colors it brown from the formation
of thallium oxide. The absence of nitrous acid
must be, however, assured, as this may partly, or
even entirely, prevent the reaction.

TROP^EOLIN PAPER

ALKALIES = Yellow ACIDS = Red

Synonyms: Boas' Paper; Lutke's Paper.

Preparation : Tropseolin paper is prepared by dipping
white filtering-paper in a I : 1000 solution of tro-
paeolin OO and then drying.

Application : The paper may be applied like tro-
paeolin-OO solution. It is reddened by free acids,
and the yellow color is restored by alkalies. It is
excellent for oxalic and organic acids, and is not
affected by carbonic-acid gas, bicarbonates, or acid
salts.

ULTRAMARINE PAPER

ACIDS = Colorless

Preparation: A convenient quantity of ultramarine
is carefully washed with water, and then mixed
with a decoction of Irish moss in water (i : 30 or
40). The mixture so obtained is then painted on
unsized paper as evenly and thinly as possible, by
means of a flat brush.



ZINC-IODIDE AND STARCH PAPER 21$

Application: Ultramarine paper is applied for the
detection of free acids in alum, aluminum sul-
phate, etc. The acids decolorize the paper.

WHITE-LEAD PAPER

METALLIC SALTS = Color of Sulphides

Synonyms : Schott's Paper; Polka Paper.

Preparation : The paper is prepared by coating sized
paper with a mixture of white lead and starch
paste.

Application : White-lead paper is used as an indicator
in titrating solutions of metallic salts with sodium
sulphide.

ZINC-IODIDE AND STARCH PAPER

IODINE = Blue

Preparation : Paper is impregnated with the solution
prepared as detailed under Zinc-Iodide and Starch,
and dried.

Application : The paper is sometimes employed in
iodimetric determinations. It is rendered blue by
traces of free iodine.



PART IV
TABLES AND TABULAR SUMMARY



H. TROMMSDORFF'S TABLE, SHOWING THE SENSITIVENESS OF
INDICATORS TO ACIDS AND ALKALIES



Indicators


Strength of
Indicator
Solution


Cc.of
Indicator
Solution
added to
ioo Cc. of
titrated
Fluid


Cc. of titrating Fluid required


Azolitmin ....


i : loo water


0.5


1.2 HC1 ; 3 KOH

IOO IOO


Carminic acid


1:100 water


0-5


0.7 HC1; 0.8 NH, ;

100 IOO

1.2 NaOH

IOO


Cochineal. . . .


1:80 water


0.5


3 HC1; 2.8 NaOH

IOO IOO


Congo red. . . .


1:100, 30/6-
alcohol


O.I


0.7 HC1; 0.6 NH, ;

' IOO IOO

2.5 KOH

IOO


Corallin ....


I "ioo water


Oe


o 6 H HC1 o 8 n NH








IOO IOO

2.8 NaOH

IOO



220



TABLES AND TABULAR SUMMARY
H. TROMMSDORFF'S T ABLE- Continued



221



Indicators


Strength of
Indicator
Solution


Cc. of

Indicator
Solution
added to
100 Cc. of
titrated
Fluid


Cc. of titrating Fluid required


Ethyl orange.


1:400, 30$-
alcohol


0.2


^ HC1


Fluorescein . .


1:100 alco-
hol


O.I


Fluorescence neutralized by
0.5 HC1

10


Gallein (com-
mercial). ...


1:2 alcohol


O.I


1.2 HC1 brownish;

IOO








1.6 -NH 3 violet.

IOO









1.2 HC1 to last restores

IOO

color, which is changed
again by 0.7 KOH


Litmus (puri-
fied)...


i :io water


O.2


0.05 HC1- o.i KOH


Methyl orange


1:200 water


i 0.05 j
Jo.i I

(0.2 \


10 10

i HC1 ; restored by

IOO

0.9 NHs ; again acid by








I HC1, and restored by

IOO








i KOH ; quantity of in-

100

dicator immaterial


Orange-peel
tincture. . . .


1:5 water


I


o.i -KOH

10


Phenacetolin.


1:200 alco-
hol


( O.I
( 0.2


o.i KOH : with 0.2 Cc. the
10
change is more distinct



222



TABLES AND TABULAR SUMMARY
H. TROMMSDORFF'S



Indicators



Strength of
Indicator
Solution



Cc. of

Indicator

Solution

added to

100 Cc. of

titrated

Fluid



Cc. of titrating Fluid required



Phenolphtal-
ein .



Poirrier blue,
C 4 B



1:100 alco-
hol



1:200 water



o.5



O.I



Rosolic acid. .



Tropaeolin



alcohol



1:400, 30$-
alcohol



0.5



O.2



0.5 KOH necessary for dis-
100

tinct redness



0.7 KOH colorless when

100

titrated hot ; restored by

1.2 HC1 to violet

100



0.7 HC1; 0.8 NH 3



100



2.5 -HC1
10



100

4.1 KOH
100



TABLES AND 7*ABULAR SUMMARY



223



R. T. THOMSON'S TABLE, SHOWING THE HYDROGEN ATOMS
REPLACED BY N A OH OR KOH WHEN A COMPOUND NEUTRAL
TO THE INDICATOR IS FORMED. THE BLANK SPACES INDI-
CATE THAT THE END-REACTION IS OBSCURE



Acid


Formula


Methyl
Orange


Phenolphtalein


Litmus


Cold


Cold


Boiling


Cold


Boiling




H a S0 4
HC1
HNO,
H 2 S 2 3
H a C0 3
H a SO,
H a S
H,P0 4
H 3 AsO 4
H 3 As0 3
HN0 2
H 4 Si0 4
H 3 B0 3
H 2 CrO 4
H a C 2 4
HC a H 3 O 2
HC 4 H 7 O 2
H 2 C 4 H 4 4
HC 3 H 6 3
H a C 4 H 4 O,
H 3 C 6 H 6 O 7


2

I
I
2
O

I
O

I
I

4

indicator
destroyed



I


2
I
I
2
I dilute
2
I dilute
2
2

I

2
2
I

2

I
2

3


2

I
I
2
O



2
2


2

I
I
2

O
I
O

2

I nearly
Inearly
2nearly

I
2


2
I

I
2


O

O
O

2


Hydrochloric . .


Nitric


Thiosulphuric


Sulphurous
Hydrosulphuric. . .
Phosphoric


Arsenic


Arsenous .


Nitrous


Silicic . .


Boric




Oxalic


Acetic


Butyric


Succinic . .


Lactic. . .


Tartaric


Citric





22 4



TABLES AND TABULAR SUMMARY



DIETERICH'S TABLE SHOWING SENSITIVENESS OF VARIOUS
TEST-PAPERS



Test-papers


SO,


HCl


KOH


NH,












Red










Brazil Wood




















Cochineal. . .








35 ooo






























35 ooo




o ooo








HaGmatoxylin .










Huckleberry* ....






35 o




Litmus Blue .










" Red










Phcnolphtalein










Rhubarb










Rosolic Acid .












IO OOO


1 5 ooo

















APPENDIX

ALIZARIN GREEN B.

O.HO.C 10 H4:NO:C.oH 6 .S03H

ALKALIES = Green ACIDS =Carmine-red

Synonym: Formanek's Indicator.

Properties: Alizarin Green B., as furnished by Dahl
& Co. of Barmen, belongs to the group of oxazines
and thiazines, and forms a blackish-green powder
quite readily soluble in water and yielding a solution
having a dirty-green color. It is obtained by the
action of beta-naphtoquinosulphonic acid on amido-
naphtolsulphonic acid in alkaline solution at a high
temperature. It is more difficultly soluble in alco-
hol, yielding a flesh- colored solution. On diluting
the concentrated aqueous solution, a flesh-colored
solution is also afforded. The green color of the
alizarin-green solution is changed by diluted acids
to a carmine-red, and restored by alkalies to green,'
the color changes being very sharp. The neutral
point is indicated by a flesh color.

Application: According to J. Formanek, Alizarin
Green B. is equal to litmus and azolitmin in sensi-
tiveness. The titration of carbonates must be car-
ried out in boiling liquids, as with litmus. In the

225



226 IN DIC A TORS

presence of salts of ammonium, aluminium, and other
weak bases, the color-change is not sharp, just as is
the case with litmus.

The indicator is also adapted for use in the "dip"
method in the form of test-papers, which may be
prepared by impregnating filter-paper either with
an aqueous solution of the indicator, in which case
the paper, after drying, is flesh-colored ; or by im-
pregnating the paper with a solution rendered a
pure green color by the cautious addition of an al-
kali ; the paper so obtained has a green color.

The indicator is said to be also well adapted for
use by artificial light.

Alizarin Green G. exhibits properties similar to
those of Alizarin Green B., but it is almost insoluble
in alcohol.

AMMONIACAL COPPER SOLUTION

A precipitate of copper oxide indicates the end of the reaction

Preparation: Dissolve 10 Gm. of copper sulphate in
about 500 Cc. distilled water, -and add ammonia wa-
ter, under constant stirring, until the precipitate
first formed is almost just dissolved; then add suf-
ficient distilled water to make up the volume to
1000 Cc. Filter the solution, and standardize against
decinormal sulphuric acid.

Application: Ammoniacal copper solution is recom-
mended by Elie Falieres for the titration of alkaloids.
It is used as follows: o. I Gm. of the alkaloid is dis



CORALL2N-MA LA CHITE-GKEEN 227

solved in 20 Cc. of decinormal sulphuric acid, and
the solution introduced into a cylindrical jar placed
on a black under ground. The copper solution is
then allowed to flow in just until the formation of a
persistent, slight cloudiness. The number of Cc. of
copper solution used up represents only the free
acid ; this number subtracted from 20, represents
the acid combined with the alkaloid, from which
the weight of the latter may hence be directly cal-
culated.

In applying this method to the titration of cin-
chona barks, the author has found that it permits of
the titration of the total alkaloid in the primary ex-
tract, as the extractive and other matters present
are entirely without influence on the visibility of the
precipitate of copper oxide. The method is, hence,
useful, as it obviates the necessity for the successive
purification of the extracts required when using
indicators giving color reactions. The method is
stated to be applicable for all alkaloids.

CORALLIN-MALACHITE-GREEN *

ALKALIES = Purple ACIDS = Green

Preparation: According to Lachaux, who first intro-
duced corallin-malachite-green as an indicator in
1892, the mixture is prepared as follows:

* The double zinc-chloride of tetramethyl-diparamido-triphenyl-
carbinol,



228 INDICATORS

3.1 Gm. of corallin or commercial rosolic acid are
dissolved in 150 Cc. of 9O-per-cent. alcohol, neutral-
ized, and mixed with 0.5 Gm. malachite-green dis-
solved in 50 Cc. of alcohol. With this mixture
alkalies give a purple color, changed to green by
acids.

Application : Malachite-green dissolves in alcohol,
yielding a greenish-blue solution, which possesses
no value as an indicator by itself, but when mixed
with corallin it blends with the colors of that in-
dicator and renders the end reaction more distinct.
As only the corallin in this mixture acts as an indi-
cator, it follows that this corallin-malachite green
mixture can be used in titrating only those acids
and bases which give a distinct end reaction with
corallin alone.

E. S. Runyon (Jour. Amer. Chem. Soc., XXIII,
p. 402) recommends the indicator for determining
the alkali in highly colored beet molasses, as well
as for determining the total acidity of highly colored
wines, vinegars and ciders.

The method of procedure is as follows : Transfer 10
Cc. of the sample to a beaker, dilute with about 300
Cc. of boiling distilled water, heat the mixture to
boiling for a moment to expel all CO 2 , cool to about
75 C., add 10 drops of the corallin malachite-green
solution, then add an excess of decinormal NaOH,
indicated by a purple color, titrate the excess of



DIA ZOPA RA NITKAN1LINE 22Q

alkali with decinormal acid solution, adding the acid
slowly until the appearance of a distinct green color.

The change in color is best observed by trans-
mitted light. It is easier to detect the transition
from the alkali to the acid side than the reverse.

The results obtained with corallin-malachite-green
are invariably lower than those obtained with the
other indicators, but this is to be expected, as with
this indicator the titration is made toward the acid
reaction, and with the other indicators it is made
toward the alkali side.

In testing the sensitiveness of the corallin mala-
chite-green, five drops of the mixture were added
to 100 Cc. of distilled water, when o. I Cc. of centi-
normal HC1 or centinormal NaOH was sufficient
to give a distinct acid or alkali reaction. In the
presence of other coloring matters, slightly more of
the standard solutions was required.

DIAZOPARANITRANILINE-PROPYL-
METACRESOL

C,H 6 N : NC 6 H a .CH3(OH)CH(CH,) a .CH 3 : OH : CH^CH,),

ALKALIES = Pink ACIDS = Faint Yellow

Properties : Diazoparanitraniline - propylmetacresol
forms a powder insoluble in water, but soluble in a
dilute alcohol containing 30 per cent, or more of the
latter.

Application : Diazoparanitraniline-propylmetacresol
is stated by A. E. Sunderland and A. E. Rhodes to



230 2ND 1C A TORS

be especially sensitive to alkalies, and to be far
more sensitive than phenolphtalein, while equal to
methyl orange in the estimation of ammonia. It
can replace lacmoid for estimating the hardness of
waters by the alkalimetric test, and can serve as
an indicator in the titration of organic acids. Like
phenolphtalein, however, it is sensitive to carbon
dioxide and carbonates.

It is stated to surpass other indicators in sharp-
ness of color-change (pink when alkaline, to faint-
yellowish when acid), as well as in sensitiveness.

IRON ISOPYROTRITARATE

Fe(C 7 H 7 O 3 ) 3 + 2H 2 O

ALKALIES = Orange-red Acms = Violet ; Colorless
to Yellow with excess

Synonym: Simon's indicator.

Preparation: The products of the action of heat and
potassium bisulphate on tartaric acid are distilled
under reduced pressure. The brown viscous residue
is esterified by prolonged boiling with alcohol, the
pyruvic- and pyrotartaric-acid esters distilled off
under reduced pressure, and the residue finally dis-
tilled at atmospheric pressure. A yellowish oil
passes over, which partly solidifies on cooling. The
crystalline product so obtained (about 25 Gm. from
25 kilos of tartaric acid) is isopyrotritaric acid, hav-
ing a melting-point of 164 C., and resolidifying at



IKON ISOPYKOTR1TARATE 23!

156 C. It readily sublimes in the form of white
needles, which are easily soluble in ether and in al-
cohol, and less so in water (1:25 in boiling water)
or acetic acid, are feebly acid, and have the formula
C 7 H 8 O 3 . The potassium salt, C 7 H 7 O 3 K.2H a O, is
readily soluble in water, and the solution gives with
silver nitrate a yellowish, gelatinous precipitate
which quickly blackens. Its main characteristic,
however, is the intense violet color it yields with
ferric salts, and which is of very stable nature, being
affected neither by heat nor time. The iron com-
pound is readily prepared from the acid by digest-
ing freshly precipitated ferric hydrate with a hot
saturated solution of the acid, and evaporating the
solution to dryness in vacuo. It has the formula
Fe(C,H,0 8 ),+ 2H,0.

Properties: Iron isopyrotritarate dissolves in water
affording a solution having a red color which, on the
addition of an acid, changes to violet, but which is
discharged by an excess, and becomes orange-red
to yellow on the addition of an alkali.

Application: The substance is highly recommended
as an indicator by L. J. Simon, who states that the
color-changes are very sharp. The acid itself may
be advantageously used to indicate the presence of
ferric salts, the reaction being shown in neutral solu-
tions of i : 100000, and is as delicate as that afforded
by potassium sulphocyanide.




232 INDICATORS

IRON SALICYLATE

ALKALIES - Yellow ACIDS - Red .

Synonym: Wolff's indicator.

Preparation : The indicator is, practically, a solution of
iron and sodium salicylate, and it may be prepared
thus : Dissolve 2 Gm. of salicylic acid in 10 Cc. of
lO-per-cent. caustic-soda solution, and add 90 Cc.
water and 5 Cc. of a solution of ferric chloride (sp.
gr. 1.280) I part and water 19 parts. The solu-
tion is now divided into two equal portions, to one
of which just sufficient caustic soda is added to de-
velop a deep-orange color; to the other just
enough sulphuric acid. is added to yield a red tint.
The two portions are then mixed, and .4 or 5 Gm.
of sodium salicylate dissolved in the solution.

Another method is to dissolve 5 to 6 Gm. of
sodium salicylate in 25 Cc. of water, add solution
of ferric chloride by drops until a permanent, slight
turbidity appears, then filter, divide into two por-
tions and treat as above with caustic-soda solution
and sulphuric acid, and finally dissolve 10 Gm. of
sodium salicylate in the mixed portions.

Still another method is to dissolve freshly precip-
itated ferric hydrate to saturation in a concentrated,
neutral solution of sodium salicylate, by heating to
a temperature of 80 C., and then filtering. Of
the solution, from 0.5 to I Cc. is used for each,
titration.



IRON SALJCYLATE 233

Application: This indicator is recommended by Jules
Wolff, who states that it is very sensitive to alkalies,
as well as to sulphuric, hydrobromic, hydriodic, and
hydrochloric acids. It is unaffected by phosphoric
and hydrofluoric acids, as well as by carbonic, sul-
phurous, arsenous, oxalic, and boric acids. Ammonium
salts, when present, have no disturbing action on
the indicator. The indicator is suitable for esti-
mating the alkali in potassium and sodium carbonates,
since it is not affected by carbonic acid. The alka-
line solution is yellow, but in proportion as car-
bonic acid is evolved, it reddens, and when the acid
is present in slight excess, becomes reddish-violet
to violet. The titration is considered to be com-
plete when only the violet supervenes, indicating
that all the alkali has been saturated.

The indicator is particularly useful for estimating
boric acid in borates. For instance, 20 Cc. of a so-
lution of 7.387 Gm. of borax in 200 Cc. water are
treated with a known volume of decinormal sul-
phuric acid in excess, some of the salicylate indi-

* cator added, and the free acid titrated with caustic-
soda solution until the violet tint is replaced by
orange-red. The end reaction is very sharp, and
takes place the moment when the sulphuric acid is
neutralized by the caustic soda. The orange-red
persists in an excess of soda. If a large excess of
sulphuric acid is present, the liquid remains almost
colorless on the addition of the indicator, and only



234 IND1CA TORS

as neutralization proceeds does the violet color fi-
nally develop and persist. The difference between
the acid used and that found, gives the equivalent
of the sodium present. The boric acid is then
found by adding about 20 Cc. of glycerin and a few
drops of phenolphtalein solution. The liquid at
first acquires a pink color, due to the acidity caused
by the glycerin, but soon becomes lighter in pro-
portion as the caustic-soda solution is added, until
the phenolphtalein end-reaction is observed. The
acidity due to the glycerin must be determined by
making a blank titration using a volume of water
similar to that used in the titration, and deducting


1 2 3 4 5 6 7 8 9 10 12 14

Online LibraryAlfred Isaac CohnIndicators and test-papers; their source, preparation, application, and test for sensitiveness . . → online text (page 12 of 14)