_DETERMINING THE REAL VALUE OR PERCENTAGE OF COMMERCIAL SODAS,
CHLORIDE OF LIME, ETC._
Examination of Commercial Sodas.--Mohr's Alkalimeter.--Preparation
of the Test Acid.--Sampling Alkalies.--The Assay.--Estimation of
Chlorine in Bleaching Powder.--Fresenius' Method.--Gay-Lussac's
Method.--The Test Liquor.--Testing the Sample.--Estimation of
Alumina in Alum Cake, etc.
In a manufacture such as paper-making, which involves the
consumption of enormous quantities of materials of variable
quality, as soda ash, caustic soda, and bleaching powder, for
example, it will be readily seen that some means should be at the
command of the consumer who does not avail himself of the services
of a practical chemist at his works, by which he can ascertain the
_actual_ value of the various substances he uses. An art which, up
to a certain point in its progress, is mainly a chemical operation,
it would undoubtedly be more safely and economically conducted when
supervised by persons well acquainted with chemical principles
and reactions, and less dependent upon individual judgment, than
is, perhaps, too frequently the case. Under such supervision more
perfect uniformity of results--a consideration of the greatest
importance in a manufacture of this kind--would be ensured.
[Illustration: Fig. 83. Fig. 84. Fig. 85.]
=Examination of Commercial Sodas.=--The methods of determining the
percentage of real alkali in the commercial products which have
received the name of _Alkalimetry_ are fortunately of a simple
character, and such as a person of ordinary intelligence and
skill can readily manipulate and render thoroughly reliable by
exerting the necessary care. He must, however, be provided with a
few indispensable appliances, which will be described, and with
these he should make several trials upon various samples until
he finds that his results are uniform and his manipulation easy
and reliable. He will require a chemical balance,[34] capable of
weighing to the tenth of a grain; a few glass "beakers" (Fig.
83) of various sizes, capable of holding from four to eight or
ten ounces of fluid; several glass stirrers; a bottle of litmus
solution, made by dissolving litmus in hot water; books of litmus
and turmeric papers; and several glass flasks (Fig. 84) of various
sizes, capable of holding from four to eight ounces. Besides these
accessories, certain measuring instruments, termed _alkalimeters_
or _burettes_, are employed, of which either of the two following
may be employed. These instruments are of glass, and hold up to 0
or zero exactly 1,000 grains. The scale is graduated in a hundred
divisions, which are again subdivided into tenths. Bink's burette
is shown in Fig. 85, and Mohr's burette in Fig. 86. The latter,
being provided with a stand, enables the operator to add the test
liquor--with, which the burette is charged--drop by drop, when the
alkaline solution to be tested is near the point of saturation,
without engaging the hands.
[Illustration: Fig. 86. Fig. 87.]
=Mohr's Alkalimeter.=--This useful instrument (Fig. 86) and the
method of using it is thus described by Mohr:--"I have succeeded
in substituting for expensive glass stop-cocks an arrangement
which may be constructed by any person with ease, which remains
absolutely air and water-tight for an indefinite period, which
may be opened and regulated at will by the pressure of the
fingers, and which costs almost nothing. It consists of a small
piece of vulcanized indiarubber tube, which is closed by a clamp
of brass wire (Fig. 87). The ends of this clamp, which I call a
pressure-cock, are bent laterally at right angles in opposite
directions and furnished with knobs, so that when both ends are
pressed the clamp is opened, and a single drop or a continuous
current of liquid may be allowed to escape at pleasure. The
measuring-tube is a straight glass cylinder, as uniform as
possible, graduated to 0·2 or 0·1 cubic centimètres, and somewhat
contracted at its lower end, so as to fit into the indiarubber
tube. A small piece of glass tube inserted below the pressure-cock
forms the spout. The pressure-cock has the advantage of not
leaking, for it closes itself when the pressure of the fingers
is removed. The measure, furnished with the pressure-cock, is
fastened upon an appropriate stand, which can be placed at any
required height. When used, it is filled above the zero point with
test liquor, the cock opened for an instant, so as to let the
air escape from the spout, and the level of the solution is then
adjusted. This is done by bringing the eye level with the zero
point, and applying a gentle pressure to the cock until the liquid
has sunk so low that the inferior curve of the liquid touches the
graduation like the circle of a tangent; the cock is then closed,
and at the same moment the liquid remains at zero, and continues to
do so for weeks if evaporation is prevented. The test-measure being
normally filled, the experiment may be commenced; this is done
sitting, while the filling of the measure is done standing.
"The weighed sample of alkali is first placed in a beaker-glass,
and the test-liquor is allowed to flow into it by gently pressing
the cock. Both hands are set at liberty, for when the pressure-cock
is released it closes of itself. The volumetric[35] operation may
be interrupted at pleasure, in order to heat the liquid, shake it,
or do whatever else may be required. The quantity of liquid used
may be read off at any moment, and in repeating an experiment,
the limit of the quantity used before may be approached so near
that the further addition of liquid may be made drop by drop."
The test-acid to be used _volumetrically_--that is, with the
alkalimeter, has a specific gravity of 1·032 at 60° F., and 1,000
grains by measure contain exactly 40 grains of real or anhydrous
(that is, without water) sulphuric acid.
The chemical principles involved in the process of alkali-testing
may be thus briefly stated:--According to the laws of chemical
combination defined by the atomic theory of Dalton, all substances
combine in _definite_ proportions or "equivalents"; thus, 1 part by
weight of _hydrogen_ combines with 8 parts by weight of _oxygen_
to form water. The equivalent number of hydrogen, therefore, is
1, and of oxygen 8, and that of water 9. Again, 3 equivalents of
oxygen combine with 1 equivalent of sulphur (16) to form sulphuric
acid; thus, sulphur 16, oxygen 24, equals anhydrous sulphuric
acid 40; therefore 40 is the _equivalent_ or combining number of
this acid, and it cannot be made to unite with alkalies or other
bases in any other proportion. For example, 40 _grains_ by weight
of _pure_ sulphuric acid will neutralise exactly 53 grains of
_dried carbonate of soda_, 31 grains of _pure anhydrous soda_, or
40 grains of _hydrate of soda_ (caustic soda). This being so, it
is only necessary to have exactly 40 grains of _real_ sulphuric
acid in 1,000 grains of water to form a _test-acid_, which, when
employed to neutralise an alkaline solution, will show, by the
proportion of dilute acid used to saturate the alkali, the absolute
percentage present in the sample.
=Preparation of the Test-Acid or Standard Solution.=--As there is
some trouble involved in the preparation of the test-liquor, it
is advisable to prepare a sufficient quantity at a time to last
for many operations. It may be readily made by mixing 1 part of
concentrated sulphuric acid with 11 or 12 parts of _distilled
water_, the mixture being made in what is termed a "Winchester"
bottle, which holds rather more than half a gallon, and is provided
with a glass stopper. The acid solution must be _adjusted_ or
brought to the proper strength after it has cooled down to 60°
F.; and it should be _faintly tinged_ with litmus, which will
give it a pinkish hue. The acid, to be of the proper strength,
should _exactly_ neutralise 53 grains of pure carbonate of soda,
previously calcined at a red heat, or 31 grains of pure anhydrous
soda. To prepare the anhydrous carbonate of soda, a few crystals of
carbonate of soda are placed in a Berlin porcelain crucible, and
this must be heated over a spirit-lamp or Bunsen burner. When all
the water of crystallisation has become expelled, the calcination
is continued until the mass is at a bright red heat, when the
vessel may be allowed to cool. 53 grains of the calcined carbonate
are now to be carefully weighed, and next dissolved in a glass
beaker, in about 2 ounces of distilled water. The alkalimeter is
now to be charged with the test-acid to the level of zero, and (if
Mohr's burette be used) the beaker containing the alkaline solution
is to be placed upon the stand immediately beneath the exit-tube.
Now press the knobs of the pressure-cock, and allow a portion
of the liquor to flow into the beaker. When the effervescence
which immediately sets up subsides, make further additions of the
test-liquor from time to time, until the effervescence becomes
sluggish, at which period the acid must be added with greater
caution. When the solution approaches saturation it acquires a
purplish tint (due to the litmus with which the acid is tinged),
which it retains until the point of saturation is reached, when it
suddenly changes to a pink colour. After each addition of the acid
the solution should be stirred with a thin and clean glass rod;
and before the final change from purple to pink, the end of the
glass rod should be applied to a strip of blue litmus paper, when,
if the moistened spot touched assumes a red colour, the saturation
is complete; if, on the contrary, the paper is unchanged, or has a
violet or reddish hue, add the test-liquor, one or two drops at a
time, with continued stirring, until a drop of the solution applied
with a glass rod reddens litmus paper, when the saturation is
finished. If any test-liquor remain in the burette, this indicates
that there is excess of acid in the test-liquor; consequently more
distilled water must be added to the bulk, the burette emptied
and refilled with the reduced liquor, and another 53 grains of
anhydrous carbonate of soda treated as before, until 1,000 grains
of the acid liquor _exactly_ neutralise the solution. Should the
whole contents of the burette in the first trial be used before
saturation is complete, a little more sulphuric acid must be put
into the Winchester or test-acid bottle, and a 53-grain solution
of carbonate of soda treated as before. A very little practice
will enable the operator to adjust his test-liquor with perfect
accuracy; and, to prevent mistakes, the bottle should be labelled
"Test-acid," and always be kept closed by its stopper.
=Sampling Alkalies.=--Soda-ash of commerce is usually packed in
wooden casks, and in order to obtain a fair average sample from a
large number of these casks, which may represent one consignment,
it is important to take small samples, as near the centre of each
cask as possible, from as many of the casks as time will permit.
Each sample, as drawn from the cask, should be at once placed
in a rather wide-mouthed bottle furnished with a well-fitting
cork. Each sample should be numbered and marked with the brand
which distinguishes the cask from which it was taken. The duty
of sampling should be placed in the hands of a person of known
integrity and intelligence.
When about to test a sample of soda-ash, the contents of the bottle
should first be emptied upon a sheet of dry paper, and the larger
lumps then crushed to reduce the whole to a coarse powder, and
this must be done as quickly as possible to prevent absorption of
moisture from the atmosphere. 100 grains of the alkali must now
be accurately weighed and put into a glass flask (Fig. 84), and
the remainder of the alkali returned to the bottle and the vessel
securely corked. About half an ounce of distilled water is then to
be put into the flask and gentle heat applied, with an occasional
shaking, until the alkali is all dissolved. The flask is then to
be set aside for a few minutes, until any insoluble matter present
has subsided, when the clear liquor is to be carefully poured into
a beaker glass; the sediment must be washed several times with
small quantities of distilled water, and the washings added to
the solution in the beaker. This washing is of great importance
and must be performed several times, or until the last washing
liquor produces no effect upon yellow turmeric paper, which even
slight traces of alkali will turn a brown colour. So long as this
brown tint is given to the turmeric paper the presence of alkali
is assured, and the washing must be continued. It is important,
after each washing, to pour off the last drop of the liquor above
the sediment, by which the operation is more effectual, and is
effected with less water than when this precaution is not observed.
In order to ensure perfect accuracy in the result, every particle
of the washings must be added to the contents of the beaker-glass
in which the assay is to be made.
=The Assay.=--The alkalimeter is first to be filled with the
test-acid exactly to the line 0 or zero of the scale as described,
and the beaker containing the solution to be tested then placed
immediately beneath the dropping tube of the instrument; a thin
glass rod should be placed in the beaker as a stirrer. The acid
liquor is then allowed to flow gradually into the alkaline
solution (which should be repeatedly stirred with the glass rod),
by pressing the knobs of the pressure-cock, until the solution
assumes a purple tint, which it will retain until the exact point
of saturation has been arrived at, when, as before stated, it
will suddenly change to a pink colour. Before the latter stage is
reached the beaker should be placed over a spirit lamp or Bunsen
burner, and the liquid heated to expel the carbonic acid which is
evolved, and partly absorbed by the solution during the process of
saturation. When the neutralisation is complete, the alkalimeter
is allowed to repose for a few moments, so that the acid liquor
may drain from the interior of the glass tube into the bulk of the
fluid, and the quantity of test-acid used is then determined by
reading off the number of divisions of the alkalimeter that have
been exhausted, every one of which represents 1/100th part, or 1
per cent. of _alkali_, whenever the _equivalent weight_ is taken
for assay. Every 1/10th part of an alkalimeter division represents
1/10th of 1 per cent., and the result is thus obtained without
the necessity of any calculation. The following table shows the
_equivalent_ or combining proportions of soda with 40 grains of
real (that is, anhydrous) sulphuric acid:--
Are
equivalent
to
40 grains of sulphuric acid } 31 grains soda (anhydrous).
1,000 grains of dilute } 40 grains hydrate of soda (pure
sulphuric acid (sp. gr. 1·033) } caustic soda).
1,000 grains of dilute } 53 grains carbonate of soda
sulphuric acid (water-grain } (anhydrous).
measure) sp. gr. 1·032 } 143 grains crystallized
} carbonate of soda.
Mr. Arnot recommends the following method for alkali testing: "The
sample, which should be a fair average of the drum or cask from
which it is drawn, should, in the case of caustic soda, be quickly
crushed into small fragments, and returned to the stoppered bottle
in which it was collected for testing. It need not be finely
ground, and, indeed, should not be, as it very readily attracts
moisture from the air. The contents of the drum are usually pretty
uniform, and the crushing recommended will give the operator
a sample quite fit to work upon. Samples of soda-ash and soda
crystals will, of course, be fairly representative of the casks
from which they are drawn. One hundred grains of the prepared
sample must be weighed out upon a watch-glass or slip of glazed
paper, and transferred to a porcelain basin, with at least half a
pint of boiling water. The watch-glass is preferable for caustic
soda, and the weighing in the case of that agent must be done
expeditiously. While the sample is dissolving the burette will be
charged with the standard acid. To the soda solution a few drops
of solution of litmus, sufficient to colour it distinctly, will
be added. The acid will then be run into the blue soda liquor;
at first, within reasonable limits, this may be done rapidly,
but towards the close of the operation the acid must be added
cautiously, and the solution kept well stirred. In the case of
caustic, when the blue has distinctly changed to red, the operation
may be considered completed, and the measures may be read off the
burette; and this is, without calculation, the result required.
When the soda in the sample is a carbonate, the blue colour of the
litmus will be changed to pink before all the soda is neutralised,
owing to a portion of the liberated carbonic acid remaining in
the solution; this must be eliminated by placing the basin over a
Bunsen burner and boiling the solution. The blue colour will thus
be restored, and more acid must be added, repeating the boiling
from time to time, until the red colour becomes permanent. It is
sometimes necessary to filter the soda solution before testing;
this applies specially to recovered soda, and, although in a less
degree, to soda-ash." When the soda solution is filtered, it will
be necessary to thoroughly wash out the liquor absorbed by the
filtering paper, the washings being added to the bulk of the liquor
as before. The best plan is to allow the soda solution to stand
for some time until all the sediment has deposited, and then to
pour off as much of the liquor as possible, and then to wash the
sediment into a very small filter, in which it will receive further
washing, until no trace of alkali can be detected in the last wash
water.
=Estimation of Chlorine in Bleaching Powder.=--It is desirable that
the manager or foreman of a paper-mill should have at his command
some ready means by which he may test the percentage of chlorine
in samples of bleaching powder, or chloride of lime, delivered at
the mill, not alone to enable him to determine the proportions to
be used in making up his bleaching liquors, but also to ensure his
employers against possible loss in case of inferior qualities being
delivered at the mill. Bleaching powders being purchased according
to percentage, it is absolutely necessary that the purchaser should
have this determined to his own satisfaction before either using
or paying for the material. Good chloride of lime should contain
35 per cent. of available chlorine, but the powder should not be
accepted which contains less than 32 per cent. There are several
methods of estimating the percentage of chlorine in bleaching
powder, which is composed of hypochlorite of lime, chloride of
calcium, and hydrate of lime, the latter substances being of no
service in the bleaching process.
According to Fresenius, in freshly prepared and perfectly normal
chloride of lime, the quantities of hypochlorite of lime and
chloride of calcium present stand to each other in the proportion
of their equivalents. When such chloride of lime is brought into
contact with dilute sulphuric acid, the whole of the chlorine it
contains is liberated in the elementary form. On keeping chloride
of lime, however, the proportion between hypochlorite of lime
and chloride of calcium gradually changes: the former decreases,
the latter increases. Hence from this cause alone, to say nothing
of original difference, the commercial article is not of uniform
quality, and on treatment with acid gives sometimes more, and
sometimes less, chlorine. As the value of bleaching powder depends
entirely upon the amount of chlorine set free on treatment with
acids, chemists have devised very simple methods of determining the
available amount of chlorine in any given sample, these methods
having received the name of _chlorimetry_. The method of Fresenius
is generally considered both practicable and reliable.
=Fresenius' Method= of preparing the solution of bleaching powder
to be tested is as follows:--Carefully weigh out 10 grains of
the sample, and finely triturate it in a mortar with a little
cold water, gradually adding more water; next allow the liquor to
settle, then pour the liquid into a litre flask, and triturate
the residue again with a little water, and rinse the contents of
the mortar carefully into the flask, which should then be filled
with water up to the graduated mark. Now shake the milky fluid
and proceed to examine it while in the turbid state; and each
time, before measuring off a fresh portion, the vessel must be
again shaken to prevent the material from depositing. The results
obtained with the solution in its turbid condition are considered
more accurate and reliable than when the clear liquid alone is
treated, even though the deposit be frequently washed. This may be
proved, Fresenius says, by making two separate experiments, one
with the decanted clear liquor, and another with the residuary
turbid mixture. In an experiment made in his own laboratory
the decanted clear fluid gives 22·6 of chlorine, the residuary
mixture 25·0, and the uniformly mixed turbid solution 24·5. One
cubic centimètre of the solution of chloride of lime so prepared
corresponds to 0·01 gramme of chloride of lime.
=Gay-Lussac's Method.=--This method, which is known as the
_arsenious acid process_, has been much adopted for the
determination of chlorine in bleaching powders, and is conducted as
follows:--
_The Test-liquor._--This is prepared by dissolving 100 grains of
_pure_ arsenious acid in about 4 ounces of pure hydrochloric acid,
and the solution is to be diluted with water until, on being poured
into a graduated 10,000 grains measure-glass, it occupies the
volume of 700 grains measure marked on the scale. Each 1,000 grains
measure of this liquid now contains 14·29 grains of arsenious acid,
corresponding to 10 grains of chlorine, or 1/10 grain of chlorine
for every division or degree of the scale of the chlorimeter, for
which purpose a Mohr's burette of the above capacity may be used,
or a graduated tube of the form shown in Fig. 85 may be employed.
_Testing the Sample._--100 grains of the chloride of lime to
be tested are next dissolved in water, and poured into a tube
graduated up to 2,000 grains measure. The whole must be well shaken
in order to obtain a uniformly turbid solution, and half of it
(1,000 grains measure) transferred to a graduated chlorimeter,
which is, therefore, thus filled up to 0°, or the zero of the
scale, and contains exactly 50 grains of the chloride of lime
under examination, whilst each degree or division of the scale
contains only ½ grain. 1,000 grains measure of the arsenious acid
test-liquor are now poured into a glass beaker, and a few drops of
a solution of sulphate of indigo added, in order to impart a faint,
but distinct, blue colour to it; the glass is then to be shaken
so as to give a circular movement to the liquid, and whilst it is
whirling round the chloride of lime solution from the chlorimeter
is gradually and cautiously added until the blue tinge given to the
arsenious acid test-liquor is destroyed, care being taken to stir
the mixture well with a glass rod during the whole process, and to
stop as soon as the decoloration is complete. We will assume that
in order to destroy the blue colour of 1,000 grains measure of the
arsenious acid test-liquor 90 divisions or degrees of the chloride
of lime solution have been employed. These 90 divisions, therefore,
contained the 10 grains of chlorine required to destroy the colour
of the test solution; and since each division represents ½ grain
of chloride of lime, 45 grains of chloride of lime (10 grains of
chlorine) were present in the 90 divisions so employed, from which
the percentage strength may be ascertained:--
For 45 : 10 :: 100 : 22·22.
The chloride of lime examined, therefore, contained 22¼ per
cent. (nearly) of chlorine. This method is extremely simple and
trustworthy when properly employed, but to ensure accuracy certain
precautions must be adopted. Instead of pouring the test liquor
into the solution of the sample (as in alkalimetry), the solution
of the sample must be poured into the test-liquor. If the contrary
plan were adopted the hydrochloric acid of the test-liquor would
liberate chlorine gas so fast that much would be lost, and the
result rendered incorrect. By pouring, on the contrary, the
chloride of lime solution into the arsenious acid solution the
chlorine is disengaged in small portions at a time, and meets with
an abundance of arsenious acid to react on. The mixture of chloride
of lime should also be employed turbid.
=Estimation of Alumina in Alum Cake, etc.=--Mr. Rowland Williams,
F.C.S., in a paper read before the Chemical Society in June,
1888, describes a method of estimating the alumina in alums,
alum cakes, and sulphate of alumina, by which he obtained more
accurate results than are obtained by the ordinary ammonia method
of estimation. After pointing out several objections to the method
of precipitating the alumina by ammonia, he proceeds:--"There
is another method for the estimation of alumina which is not so
well known as the above. This is by means of sodium thiosulphate.
Having had a very extensive and successful experience of this
process, I can recommend it with confidence. Considerable practice
is, however, necessary in order to secure good results, as
certain conditions must be carefully attended to, otherwise the
precipitation will be incomplete. The estimation is made in a
moderately dilute solution. In the case of alum cake and sulphate
of alumina I dissolve 400 grains in water, filter, dilute to 10,000
grains. I use 1,000 grains of this solution (equal to 40 grains of
the sample) for estimating the alumina. If any free acid is present
it is neutralised by a few drops of carbonate of soda solution,
and the whole diluted to about 8 ounces measure. A large quantity
of crystallized thiosulphate of soda is then added, and the liquid
boiled for at least half-an-hour, constantly replacing the water
lost by evaporation. By the end of that time all the alumina will
be precipitated in a finely-divided form, along with more or less
free sulphur. The precipitate is then filtered off and washed well
with boiling water. The filtration and washing take place very
rapidly, and may generally be accomplished in about twenty minutes,
this being a great saving of time in comparison with the long and
tedious washing by decantation, which is necessary in the case of
gelatinous alumina. Before filtration, it is advisable to add a
drop or two of carbonate of soda solution, lest the liquid should
have become slightly acid during boiling."