C. Remigius Fresenius.

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* Zeitsclir. f. analyt. CTiem., n, 49, and n, 341.



496 DETEUMINATION. [

o need not be refilled as a rule, but it is advisable to transpose these
two, i.e., put in the place of o the freshly charge* j n.

If it is preferred to decompose the carbonate in the dry way,
this may be accomplished by fusing the finely powdered carbonate
(alkali carbonates need not be powdered) with six- to ten -fold its
quantity of fused potassium dichromate. The operation should be
conducted in a piece of combustion tubing, slightly bent in the
middle, and one end of which is connected with an apparatus for
purifying the air, the other being connected with a calcium -chloride
tube for drying the carbonic acid, soda-lime tubes for absorption, a
guard tube, and an aspirator or hydraulic air-pump. After a gentle
current of air through the apparatus has been established, heat the
tube to decompose the carbonate, and produce a slow evolution of
carbonic-acid gas. As soon as the mass fuses calmly the operation
is completed. The current of air is continued a little longer, and
then the increase in weight of the absorption tubes determined by
weighing. The method requires no modification even when sul-
phites or thiosulphates are present (PERSOZ *).

The process as used by H. HOSE is as follows ; and the apparatus
employed is shown in Fig. 98.

The flask for decomposing the carbonate should be small (150
c.c.), in order to facilitate subsequent removal of carbonic acid by
aspiration, unless the substance froths strongly during its decom-
position, in which case a larger flask must be used. The end of
the funnel tube, after it is inserted in the rubber stopper which is
fitted to the flask, is drawn to a less diameter and bent upwards in
the form of a hook, to prevent the entrance of gas-bubbles. Above
the stop-cock its internal diameter should not be so small as to pre-
vent water when poured in from filling it, and this portion should
be so long that the pressure of the liquid filling it will suffice to
force gas through the apparatus. A piece of glass tube bent at a
right angle is fitted to the funnel by means of a piece of rubber
tube slipped over it.

The nearly horizontal glass tube (about 0'7 metre long) is of
thin glass, and of a diameter no; less than 12 millimetres. It is
inclined to such extent thai water condensing \\\ it may flow back.
Tin- upper half is filled with granulated dried calcium chloride,
secured in place by a little cotton or asbestos at each end. In the

* Compt. rend., LIII, 239.Zeitschr.f. analyt. Chem., i, 83.



139.]



CARBONIC ACID.



497



end of tb. large tube a small tube is fitted by means of a rubber
stopper, and" W^fiiis is joined "by a rubber tube the potash appara-
tus and soda-lime tube (weighable either jointly or separately)
charged with absorbents, as described 174. The flask is
removed to receive the weighed substance, and replaced without
disturbing the position of the rest of the apparatus. It can now
be ascertained whether the apparatus will leak gas by forcing a
little air (free from carbonic acid) through the funnel tube, closing
the stop-cock, and observing whether the unequal height of liquid
in the two limbs of the potash apparatus remains for a few minutes.
Introduce a little water through the funnel tube, and next acid
slowly by turning the stop-cock until evolution of CO 2 ceases.
The small right-angled tube, to which is attached a large tube
filled with fragments of caustic potassa, is now inserted in the glass
funnel, and a slow current of air (1 bubble per second) is drawn
through the apparatus by means of an aspirator (Fig. 100) con-
nected with the soda-lime tube. The aspirator should not be con-
nected directly to the soda-lime tube, but to a calcium -chloride
tube, which ought to be connected with the latter during the
whole operation. As soon as the current of air is established,




Fig. 98.

apply the smallest possible flame of a Bunsen lamp, best main-
tained constant by capping the burner with wire gauze until the
fluid just boils. Keep up the gentle boiling a few minutes until
water condenses in the tube, but not until condensed drops appear



498



DETERMINATION.



[ 139.



quite up to the calcium chloride. Remove then the lamp, and
aspirate a while longer somewhat faster. The volume of air neces-
sary to remove the carbonic acid depends upon the size of the
decomposing flask. When the operation is completed, disconnect
the absorbing apparatus, close the ends with caps of rubber tubing,
and weigh after lapse of half an hour.

For liberating the carbonic acid, sulphuric acid (the concen-
trated diluted with 4 or 5 times its volume of water) is best
adapted, provided it readily decomposes the substance without
formation of insoluble sulphates.

When there are objections to using sulphuric acid, dilute hydro-
chloric acid (containing about 10 per cent.) may be used, or more
rarely nitric acid. Nitric acid cannot be used when substances are
present which cause its decomposition ; e.g., ferrous salts and sul-
phides.

When sulphuric acid is used, the evolution of H a S from sul-




Fig. 100.

phides, if present, may be prevented by adding first a solution of
chromic acid or mercuric chloride. If sulphites are present, use



139.] CARBONIC ACID. 499

chromic acid or potassium chromate. When hydrochloric acid is
employed, the disturbing influence of compounds which cause evo-
lution of chlorine maybe prevented by allowing some concentrated
solution of stannous chloride to run into the flask before addition
of the acid. When hydrochloric acid is used, or even sulphuric in
the presence of chlorides, it is best to guard against the possibility
of carrying IIC1 gas into the potash apparatus by substituting
STOLBA'S preparation of anhydrous copper sulphate and pumice-
stone (see page 490) for that portion of the calcium chloride
which nils 10 to 15 cm. of the end of the tube.

A modification * of the above-described apparatus, possessing
some obvious advantages, is shown by Fig. 99. In place of the
empty part of the long glass tube shown in Fig. 98, there is sub-
stituted a smaller strong tube, provided with a cooling apparatus
through which water circulates. This is connected by a piece of
close-fitting rubber tube with the remaining part d. Some suitable
form of apparatus for absorbing CO a must, of course, be attached
to d in the manner shown by Fig. 98. The calcium- chloride tube,
used to prevent moist air from entering the absorbing apparatus,
is conveniently supported by attaching it to the aspirator (Fig.
100). The aspirator may be connected with the apparatus from
the beginning to the end of the operation, with its stop-cock so
adjusted that water flows from it drop by drop. In conducting
the operation, a little variation from the before described manipu-
lation is admissible on account of the presence of the condensing
apparatus. After enough acid has been admitted to effect decom-
position, the stop-cock of a is closed, a little liquid still being
allowed to remain above it.. Heat is then applied as before
directed, but continued longer until the CO 2 is almost or quite
expelled from the flask by steam. This point is indicated by
almost, or nearly, entire cessation of dropping of water from the
aspirator. Diminish now the heat, and immediately after open
the stop-cock of a and let air (free from CO a ) enter and replace
the condensing steam. Boil again to expel the air which has
entered, after which a small volume of air drawn through the
apparatus by the aspirator will ensure the bringing of all the CO a
into the absorbing apparatus.

* Devised by H. L. WELLS, of the Sheffield Laboratory.



600 DETEEMINATION, [ 139.

f. Separation from all Bases without Exception. (Determi-
nation of Acid ly Expulsion, Absorption , and Volumetric
Estimation.)

If the carbonic acid is disengaged in the evolution apparatus
described under e (which I consider the most satisfactory), or in a
similar one, the quantity of the carbonic acid expelled may be
also ascertained by one of the methods given above for estimating
free carbonic-acid gas, i.e., it may be passed into a carbonate-free
mixture of barium- or calcium chloride and ammonia, as described
in 139, I, 5, /?, and the analysis then made as in 139, I, J, /?, lb.
This method is, however, far more inconvenient, and much
slower, than that described in 139, II, , and affords good
results only when all the sources of error already pointed out are
avoided.

On the other hand, it is sometimes advantageous, particularly
when determining very small quantities of carbonic acid, to ab-
sorb this in a definite volume of standard baryta water, and to
p. )ceed with the analysis according to PETTENKOFER'S process
( 139, i, 5, y). As this method is employed in the determina-
tion of carbonic acid with atmospheric air, I refer to this section,
merely remarking that AL. MULLER,* E. ScHULZE,f and P.
WAGNER % have devised special apparatus and given rules for
carrying out the process in the most satisfactory manner.

g. Estimation by Measuring the Gas.

a. According to C. SCHEIBLER this process is applicable
in the case of all salts which are decomposed by hydrochloric
acid in the cold. It is distinguished for rapid and convenient
execution and very satisfactory results, but requires a special
apparatus. It is much employed in determining calcium carbon-
ate in bone black.

Fig. 101 shows the ingenious apparatus devised for this
method. The carbonate to be decomposed is introduced into
A. The decomposition is effected by simply lifting the bottle,
as shown, thus allowing the hydrochloric acid contained in the

* Zeitsclir. /. analyt. Chem., i, 47. t & > 1X 29 0.

J 76., ix, 445.

%Anleitung zum Gebrauch des Apparates zur Bestimmung der Kohlemaurtn.
Kalkerde in der Knochenkohle . etc. Dr. C. SCHEIBLEK. Printed iu manuscript,
Berlin, 1862.



139.]



CARBONIC ACID.



501



gutta-percha cylinder S to flow out. The glass stopper closing
A must be ground accurately to fit tightly, and must be greased,
so as to be perfectly air-tight; it should, further, be perfor-




Fig. 101.

ated, and have a short glass tube cemented into the perforation.
The carbonic- acid gas evolved passes through this tube and the
rubber tubing r, which is connected with it, then through a
glass tube cemented into one of the perforations of the flask JB,
and finally into a bladder of very thin caoutchouc, K, connected



502 DETERMINATION". [ 139.

air-tight with A. One of the two other perforations of the
stopper of B carries a short rubber tube closed by a pinch- cock ; in
the other a glass tube, u, is fixed, and is connected with the
measuring apparatus. This last consists of a 150-c. c. tube, (7,
graduated in 0'5 c. c., and connected with a similar, but not
graduated tube, D, as shown in the cut. In the rubber stopper
closing the lower end of this tube there is also fixed a short glass
tube closed by a piece of rubber tubing carrying a pinch-cock,
jP, in turn connected with a glass tube cemented into, and nearly
reaching to, the bottom of the flask E. In the second tubulure
of E\s cemented a short glass tube, to which the rubber tube, -y,
is connected. The flask E serves as a reservoir; on opening .P
the water contained in the tubes C and D flow into E\ on blow-
ing into v the water may be driven up through the open pinch-
cock back into the tube again. At the beginning E is nearly
filled with water through D.

All parts of the apparatus excepting the decomposing bottle
being permanently connected, it is advisable to fasten them to a
wooden stand by means of suitable brass fastenings. The stand
should also carry a thermometer.

Every experiment is begun by first filling C and D with water
to the zero point, this being accomplished by blowing into v, the
stopper of A being removed. As soon as the water level is
slightly above the zero point, P is closed, and then slightly opened
to allow water to drop out until the level is reached. Of course
a certain amount of caution is necessary in blowing into V, as well
as in managing the pinch-cock, for were water blown up intou and
B, the entire apparatus would have to be taken apart and the water
removed. While water is filling C, the air it displaces enters 13
and compresses the caoutchouc bladder. Should this compression
not be sufficiently accomplished, blow carefully into q until the
bladder has completely collapsed. When a number of consecu-
tive experiments are made, the bladder will always empty itself.
Should it happen that the bladder becomes empty before the
water in the tubes has reached the zero mark, then the water
in the tubes will not be in equilibrium ; in such a case open q for
a moment only. The apparatus should be used in a place where
the temperature is as constant as possible, and care should be
taken that neither the direct ravs of the sun nor the radiations



139.] CARBONIC ACID. 603

from a stove strike the apparatus, because sudden changes of
temperature during the experiment will naturally interfere with
the accuracy of the results.

To perform the analysis, introduce the finely triturated sub-
stance into the decomposition flask A, fill the gutta-percha cylinder
with 10 c. c. hydrochloric acid (sp. gr. 1*12) by means of a pipette,
carefully place the cylinder within the flask A, and tightly insert
the stopper previously greased with tallow. As this will cause
the water-level to somewhat fall in C and rise in D, open q for a
moment, when the equilibrium will be restored. Now note the
thermometer and barometer, grasp the flask with the right hand
around the neck in order to avoid warming the contents, tilt
slightly and allow the hydrochloric acid to flow out while the
pinch-cock is opened with the left hand, and in such a manner
that the water-level in both tubes is exactly at the same height ;
these operations are continued without intermission so long as any
disengagement of carbonic acid takes place and a lowering of the
water-level in C is observed. When the level remains constant
for a few seconds the experiment is finished. Care must be taken
that the levels in C and D are at the same height, then read off
the height and observe whether the temperature has remained
constant. If it has, the c. c. read off denote the volume of car-
~bonic acid disengaged since, however, a small quantity lias re-
mained behind dissolved in the hydrochloric acid, a small correc-
tion has to be made. SCIIEIBLEK has determined the quantity of
carbonic acid remaining behind dissolved in 10 c. c. of hydro-
chloric acid at a medium temperature, and he directs adding
.3*2 c. c. to the volume read off and then reducing the whole to
0, 760 mm., in the dry condition (see 198).* Every 1000 c. c.
of carbonic acid thus reduced to normal conditions weighs
1-96507 grin.

If it is desired to dispense with all corrections, begin each
series of experiments by establishing the relation between the car-



* This method of correction involves some uncertainty, since the quantity of
carbonic acid remaining absorbed depends upon the concentration of the saline
solution resulting, as well as upon the quantity of air with which the carbonic
acid is mixed ; hence rises and falls according to the total quantity of carbonic
acid evolved. Compare SCHEIBLEB'S later directions, and PIETKICH (Zeitavhr. /.
analyt. Chem., ni, 165).



504



DETERMINATION".



L 139.



bonic acid obtained (and with the correction 3-2 c. c., repre-
senting the carbonic acid remaining behind in solution) and a
weighed portion of finely triturated and dried pure calcium car-
bonate (or Iceland spar). The relation will, of course, depend
upon the conditions (temperature and pressure) prevailing on that
particular day. Suppose, for instance, from 0*2737 grm. of cal-
cium carbonate, containing 0-120307 grm. of carbon dioxide, we
had obtained, after adding the correction 3 '2 c. c. , 63 '76 c. c. ; sup-
pose, too, that an analysis of 0-2371 grm. of dolomite made under
similar conditions had yielded (including
the 3-2 c. c.) 57-3 c. c. ; the dolomite would
have contained 63-76 : 0-120307 : : 57-3 : x
= 0*10812 grm. of carbonic acid, or 45-62
per cent. Even this method, however, can
give accurate results only when the saline
solutions resulting and the quantities of
carbonic acid evolved are fairly alike in
the first experiment and in the analysis.

ft. B. E. DIETRICH * has devised a very
convenient apparatus in which the evolved
carbonic acid is measured over mercury.
lie has also calculated tables (given on
pages 506-508) showing the weight of 1 c. c.
of carbonic acid at pressures of 720 to 760
mm. and temperatures of 10 to 25, and
the quantity of CO, absorbed by 5 c. c. of
hydrochloric acid (sp. gr< 1*125) for evolved
quantities between 1 and 100 c. c. of gas.-
Using DIETRICH'S apparatus and tables,
estimations * of CO, may be very rapidly
and accurately made, and the method is particularly to be recom.
mended when a long series of experiments are to be made.

[The azotometer, Fig. 102, is employed, and the details of
the process are for the most part similar to those followed in the
estimation of ammonia as described on page 257. The weighed
carbonate is put in the bottle &, and the tube/" is charged with 5
c. c. of IIC1., sp. gr. 1 -125. "When the burette is adjusted to zero.




Fig. 102.



*Zeiteehr.f. analyt. Chem., in. 162, iv, 141, and v, 49.



140.] SILICIC ACID. 505

the acicUis poured at once upon the carbonate. The precautions
to be observed in the measurement of the gas are as detailed on
page 257. It is not needful to wait so long for the gas to cool.
The necessary corrections are applied by aid of the tables given
by DIETRICH, pages 506508. Their use is perfectly similar to
that of the tables given on pages 259-261.]

;/. G. RUMPF * describes a very simple apparatus which may
be readily constructed in the laboratory. Since, however, there
is no ad vantage in using it unless RUMPF'S tables are at hand, and
as lack of space forbids their insertion here, I must simply refer
to the original paper.

tf. Small quantities of carbonic acid in minerals can be
estimated as follows : By means of an air-pump fill with mer-
cury a graduated tube provided at its upper end with a well
greased glass cock. Fold up the mineral in blotting-paper and
send it up the tube; follow this by a measured quantity of
hydrochloric acid, sent up by means of a pipette having a bent-up
tip, and then measure the evolved gas. To this volume there
must, of course, be added also the quantity of gas absorbed by
the hydrochloric acid. For the method of calculating the weight
from the volume, see 198.

140.

2. SILICIC ACID.
I. DETERMINATION.

The direct estimation of silicic acid is almost invariably effected
by converting the soluble modification of the acid into the insol-
uble modification, by evaporating, and completely drying; the
insoluble modification is then, after removal of all foreign matter,
strongly ignited (over the bellows blowpipe) and weighed. For
the properties of silicic acid, see 93, 9.

For the guidance of the student I would observe here that, to
guard against mistakes, he should always test the purity of the
weighed silicic acid. The methods of testing will be found
below.

If you have free silicic acid in the state of hydrate, in an

* Zeitschr.f. analyt. Chcm., vr, 398.



606



DETERMINATION.



L 139.



TABLE OF THE WEIGHT OF A CUBIC

In Milligrammes from 720 to 770 mm. of press-

MILLIMETRES.





720


722


724


726


728


730


732


734


736


738


740


742


744


10


1.77446


1.77945


1.78445


1.78944


1.79443


1.79942


1.80441


1.80941


1.81440


1.81940


1.82438


1.82937


1.83437


11


1.76668


1.77165


1.77662


1.78160


1.78657


1.79155


1.79652


1.80149


1.80647


1.81144


1.81642


1.82139


1.82636


12


1.75881


1.76377


1.76873


1.77368


1.77864


1.78359


1.78855


1.79351


1.79846


1.80342


1.80838


1.81333


l.siss


13


1.75092


1.75587


1.76081


1.76576


1.77070


1.77565


1.78059


1.78554


1.79048


1.79543


1.80037


1.80532


1.81026


14


1.74301


1.74795


1.75288


1.75781


1.76275


1.76768


1.77261


1.77754


1.78248


1.78741


1.79234


1.79728


1.80221


15


1.73502


1.73993


1.74484


1.74974


1.75465


1.75955


1.76446


1.76937


1.77427


1.77918


1.78408


1.78899


1.79390


16


1.72699


1.73188


1.73677


1.74166


1.74655


1.75144


1.75633


1.76122


1.76611


1.77100


1.77590


1.78078


1.78567


17


1.71888


1.72376


1.72862


1.73349


1.73836


1.74322


1.74809


1.75296


1.75783


1.76269


1.76756


I.;:-.M:;


1.77729


18


1.71069


1.71554


1.72040


1.72525


1.73011


1.73497


1.73982


1.74468


1.74953


1.75439


1.75925


1.76410


1.76896


19


1.70239


1.70723


1.71207


1.71691


1.72175


1.72659


1.73143


1.73627


1.74111


1.74595


1.75078


1.75562


1.76046


20


1.69412


1.69894


1.70377


1.70859


1.71341


1.71823


1.72305


1.72788


1.73270


1.73725


1.74234


1.74716


1.75199


21


1.68571


1.69051


1.69532


1.70012


1.70493


1.70974


1.71454


1.71935


1.72415


1.72896


1.73377


1.7':JS.-,7


1.74338


22


1.67722


1.68201


1.68680


1.69151


1.69638


1.70117


1.70596


1.71075


1.71554


1.72033


1.72512


1.72991


1.73470


23


1.66862


1.67340


1.67817


1.IJS-J94


1.68772


1.69249


1.69727


1.70204


1.70681


1.71159


1.71636


1.72114


1.72591


24


1.65994


1.66470


1.66945


1.67421


1.67897


1.68372


1.68848


1.69324


1.69799


1.70275


1.70751


1.71227


1.71702


25


1.65113


l.fi.MH7


1.66061


i.f,i;:,:r,


1.67009


1.67484


1.67958


1.68432


1.68906


1.69380


1.69854


1.70329


1.70803




720


722


724


726


728


730


732


734


736


738


740


742


744



MILLIMETRES.



139.]



CAKBON1C ACID.



507



CENTIMETRE OF CARBONIC ACID.

ure of mercury, and from 10 to 25 Cent.

MILLIMETRES.



746


748


750 752


754


756


758


760


762


764


766


768


770




1.83936


1.84435


1.84934


1.85433


1.85933


1.86432


1.86931


1.87430


1.87930


1.88429


1.88928


1.89427


1.89926


10"


1.83134


1.83631


1.84129


1.84626


1.85123


1.85621


1.86118


1.86616


1.87113


1.87610


1.88108


1.88605


1.89103


11


1.82324


1.82820


1.83315


1.83811


1.84307


1.84802


1.85298


1.85793


1.86289


1.86785


1.87280


1.87776


1.88271


12


















1










1.81521


1.82015


1.82510


1.83004


1.83499


1.83993


1.84488


1.84982


1.85477


1.85971


1.86466


1.86960


1.87455


13


1.80714


1.81208


1.81701


1.82194


1.82687


1.83181


1.83674


1.84167


1.84661


1.85154


1.85647


1.86141


1.86634


14


1.79880


1.80371


1.60861


1.81352


1.81843


1.82333


1.82824


1.83314


1.83805


1.84296


1.84786


1.85277


1.85767


15


1.79056


1.79545


1.80034


1.80523


1.81012


1.81501


1.81990


1.82479


1.82968


1.83457


1.83946


1.84435


1.84924


16


1.78216


1.78703


1.79189


1.79676


1.80163


1.80650


1.81136


1.81623


1.82110


1.82596


1.83083


1.83570


1.84056


17


1.77381


1.77867


1.78353


1.78838


1.79324


1.79809


1.80295


1.80781


1.81266


1.81752


1.82337


1.82723


1.83209


18


1.76530


1.77014


1.77498


1.77982


1.78466


1.78950


1.79434


1.79917


1.80401


1.80885


1.81369


1.81853


1.82337


19


1.75681


1.76113


1.76645


1.77127


1.77610


1.78092


1.78574


1.79056


1.79538


1.80021


1.80503


1.80985


1.81467


20


1.74818


1.75299


1.75780


1.76260


1.76741


1.77221


1.77702


1.78183


1.78663


1.79144


1.79624


1.80105


1.80586


21


1.73949


1.74428


1.74907


1.75386


1.75865


1.76344


1.76823


1.77302


1.77781


1.78260


1.78739


1.79218


1.79697


22


1.73068


1.73546


1.74023


1.74501


1.74978


1.75455


1.75933


1.76410


1.76888


1.77365


1.77842


1.78320


1.78797


23


1.72178


1.72654


1.73129


1.73605


1.74081


1.74556


1.75032


1.75508


1.75984


1.76459


1.76935


1.77411


1.77886


24


1.71277


1.71751


1.72225


1.72699



Online LibraryC. Remigius FreseniusQuantitative chemical analysis → online text (page 45 of 69)