C. Remigius Fresenius.

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cult. The tube must invariably be placed perpendicularly, with
the eye on a level with the surface of the fluid. This is effected
by the aid of two plummets suspended a short distance from the
tube and at a proper distance from each other, or by the aid of any
convenient perpendicular door- or window-edge. A small mirror
is then pressed against the back of the tube, and the eye taxed on
it across the surface of the liquid. When the eye has thus been
placed in the proper position, the mirror is removed, and the
reading taken.

Instead of a mirror, BUNSEN generally employs a horizontal
telescope, movable vertically, and placed at a distance of four to
six paces from the eudiometer. This arrangement, while very
greatly facilitating the reading, is of especial advantage in the
measurement of gases, in that the observer is placed some distance
from the eudiometer, thus avoiding any expansion of the gases such
as is to be apprehended by the close proximity required in using
the mirror.

In taking the reading over water, the middle of the dark zone
formed by the water drawn up the walls of the tube, is to be taken
as the actual surface. When operating with mercury, however,
we have to place the real surface in a plane exactly in the middle
between the highest point of the surface of the mercury, and the
points at which the latter is in actual contact with the walls of the
tube. However, the results obtained in this way are only approx-

Absolutely accurate results cannot be arrived at, in measuring
over water or any other fluid that adheres to glass. But over mer-



cury they may be arrived at if the error of the meniscus be deter-
mined and the mercury be read off at the highest point. The
determination of the error of the meniscus is performed for each
tube, once for all, in the following manner : Some mercury is
poured into the tube, and its height read -off right on a level with
the top of the convex surface exhibited by it ; a few drops of solu-
tion of mercuric chloride are then poured on the top of the metal ;
this causes the convexity to disappear. The height of the mercury
in the tube is now read-off again and the difference noted. In the
process of graduation, the tube stands upright, in that of measur-
ing gases, it is placed upside down ; the difference observed must
accordingly be doubled, and the sum added to each volume of gas

The mercury used in measuring gases must be as pure as pos-
sible, and must more particularly be free from lead and tin, as
these impart to it the property of adhering to glass. Should they
be present, they may.be most readily removed by subjecting the
mercury for a day to the action of diluted nitric acid, in a shallow
porcelain dish, with frequent 'stirring. Dust, etc., maybe removed
by filtration through a cloth.

As a pneumatic trough, that devised by BUNSEN, Fig. 5, will
be found convenient. A is a piece of pear-wood
310 to 350 mm. long, 80 to 86 mm. wide,
with a cavity chiseled in it 240 to 250 mm.
long, 50 mm. wide, and 50 mm. deep. The
bottom of the cavity is round, except at one
end, where a flat surface 32 mm. wide and 50
mm. long is prepared. On this surface is ce-
mented a sheet
of vulcanized
caoutchouc 3
mm. thick. To
A are joined two
end pieces, BB,
each 19 mm.
thick, 100 tol 10
mm. wide, and
150 to 155 mm. Fig. 5.

high. They serve as supports for A, as well as ends for a further



trough, the side walls of which are formed of stout pieces of glass
cemented in grooves in A. and BB. The glass plates are each 310
to 320 mm. long, and 55 mm. high. They are placed not quite
parallel, the lower edges being separated by a distance of 67 to
70 mm., the upper 85 mm. The trough stands on a board, DD,
to which it is fastened by strips of wood, e e. A vertical pillar, F,
screwed into D, carries the inclined channel, G, lined with felt
it serves to support the measuring tube, h
a round, slanting cut in B, for the reception
of the tube; i is an incision for the reception
of the lower end of the tube ; it prevents th<
latter from falling into the lower part of the
trough. In use the trough is filled to within
an inch of the upper edges of the glass plates
with mercury, of which 30 [to 35 Ib. will be
necessary. In order that the mercury may
adhere to the wooden walls, the latter are first
rubbed moist then dry with mercury and
mercuric-chloride solution. In order to
transfer gases which have been collected in
large bottles, a similar but larger trough is

Lastly, in order to accurately determine the
volume of a gas collected over mercury, it is
above all necessary that the tube be first com-
pletely filled with mercury, and with the entire
exclusion of air bubbles, before introducing the
gas. To this end the tube is first washed with
water and dried with filter paper by aid of a
wooden rod, Fig. 6, the upper end of which
is provided with 10 to 20 little spikes. Care
must be taken that no filaments of paper be
left behind. The filling with mercury is accomplished by means of
;i funnel, Fig. 7, kept filled with mercury, and having a long stem,
with narrow exit, reaching to the bottom of the tube to be filled.
The metal thus introduced from below presents a mirror-like surface
on the sides of the tube. If such a funnel as described is not at
hand, a glass tube drawn out at the lower end may be fused to a
small funnel.

' i


rig. 6. Fig. 7.



2. INFLUENCE OF TEMPERATURE . The temperature of gases
to be measured is determined either by reducing it to the same
temperature as the confining fluid, or, using a closed eudiometer, to
that of the water in the cylinder provided for that purpose, and
then measuring the temperature of the liquid ; or by suspending a
sensitive thermometer by the side of the gas, and noting its varia-

If the construction of the pneumatic apparatus permits the total
immersion of the cylinder in the confining fluid, uniformity of
temperature between the latter and the gas which it is intended to
measure is most readily and speedily obtained ; bat in the reverse
case, the operator must always, after every manipulation, allow
half an hour, or, in operations combined with much heating, even
an entire hour, to elapse, before proceeding to observe the state of
the mercury in the cylinder and in the thermometer.

Proper care must also be taken, after the temperature of the gas
has been duly adjusted, to prevent re-expansion during the -reading-
off ; all injurious influences in this respect must accordingly be care-
fully guarded against, and the operator should, more especially,
avoid laying hold of the tube with his hand (in pressing it down,
for instance, into the confining fluid) ; making use, instead, of a
wooden holder.

On account of the necessity of bringing the gas and surround-
ing air to the same temperature, every sudden change in the latter
is prejudicial, hence it is advisable to select for gas analysis a room
having a northern exposure, and sheltered so far as possible from
the direct influence of solar heat.


3. INFLUENCE OF PRESSURE. When a gas is confined by a fluid,
and the level of the latter is the same inside the tube as outside,
then the gas is under the prevailing pressure only, that of the atmos-
phere ; and this may be directly found by a barometric reading.
On the other hand, when the confining fluid within the tube is
higher than that without, then the gas is under less pressure ; if
lower, it is under greater pressure than that of the atmosphere. In




the latter case, the level may be equalized by raising the tube ; in
the former, by sinking it, if the trough be deep enough. When

operating over water, the level
may be usually secured withou
difficulty ; when operating ove
mercury, however, this is very
often impossible, particularly
with wide tubes, as in Fig. 8.

In this case the gas is under
the pressure of the atmosphere
minus the pressure of a column
of mercury equal in height to
the line ab. The pressure is
accordingly ascertained by accu-
rately measuring the line J,
and subtracting its length from
the observed height of the
barometer. For instance, if the latter is 758 mm., and the line
ab measures 100 mm., the actual pressure upon the gas will
be 758 100 = 658 mm. mercury.

If there is water or some other' fluid, e.g., potassa solution, con-
fined over the mercury, we as a rule proceed just as if there were
no fluid present but mercury, but either bring the mercury inside
and outside the tube to the same level, or else measure the differ-
ence between the two surfaces of mercury. The pressure of the
additional column of water, etc. , is usually so insignificant that it
may be disregarded. Properly speaking, the fluid ought to be
measured, and, according to its specific gravity, reduced to mercu-
rial pressure, and this subtracted from the barometric reading.
This correction may, however, be omitted as a rule, since, as
already stated, an absolutely accurate measurement is impossible
under such circumstances.


Fig. 8.


4. INFLUENCE OF MOISTURE. When a gas saturated with
aqueous vapor is measured, its actual volume is not ascertained, since
the aqueous vapor, because of its tension, exerts a pressure on the
confining fluid. As, however, the tension of aqueous vapor is


known foy various temperatures, the necessary corrections may be
readily made. This is, however, only then possible when the gas
is firlly saturated with vapor. Care must be exercised, hence,
when measuring gases, that the gas be either fully saturated with
aqueous vapor, or else is absolutely dry.

The drying of gases confined over mercury is effected by means
of a ball of fused calcium chloride fixed on a platinum wire. This
may be prepared by inserting the end of a platinum wire, bent in
the form of a hook, into a bullet mold of about 6 mm. diameter,
and then filling the latter with fused calcium chloride free from
caustic lime. After cooling, the neck of calcium chloride adhering
to the wire is removed by means of a knife. In order to dry a gas,
the ball is pushed, by means of the wire, up through the mercury
into the gas, in which it is allowed to remain for about an hour,
when it is removed from the now fully dried gas. While the ball
is in contact with the gas, care must be taken that the end of the
platinum wire is entirely submerged in the mercury in the trough,
otherwise there will inevitably occur a diffusion of the confined gas
and outside air through the exposed wire.

It is more convenient to measure the gas in the moist condition,
when this may be done. BUNSEN effects the saturation with moisture
by introducing a drop of water the size of a lentil into the empty
tube, by means of a glass rod, taking care to deposit it at the top
of the tube, and without touching the sides of the tube. This
quantity of water is amply sufficient to fully saturate 'with aqueous
vapor at the ordinary temperature the gas subsequently introduced.

From the preceding remarks, it will be quite obvious that the
volumes of gases can be compared only when reduced to the same
temperature, pressure, and degree of moisture. As a rule the
reductions are made to 0, 760 mm. barometric pressure, and
absolute dryness. The methods by which this is effected, as well
as the manner of ascertaining the weights of gases from their vol-
umes, will be found under the calculation of analyses.

36 OPERATIONS. [ 17, 18.



In consequence of the vast development which volumetric
analysis has of late undergone, the measuring of fluids has become
an operation of very frequent occurrence. According to the dif-
ferent objects in view, various kinds of measuring vessels are
employed. The number proposed has gradually increased to such
an extent that all the forms and arrangements recommended can not
be discussed here, but only those will be described that have been
found most practical, and that have given the best results in my

The operator must, in the case of every measuring vessel, care-
fully distinguish whether it is graduated for holding or for deliver-
ing the exact number of c. c. marked on it. If you have made use
of a vessel of the former description in measuring off 100 c. c. of a
fluid, and wish to transfer the latter completely to another vessel,
you must, after emptying your measuring vessel, rinse it, and add
the rinsings to the fluid transferred; whereas, if you have made
use of a measuring vessel of the latter description, there must be no



aa. Measuring vessels which serve to measure out one d<ji,,
quantity of fluid.

"We use for this purpose

1. Measuring Flasks.

Fig. 9 represents a measuring flask of the most practical and
convenient form.

Measuring flasks of various sizes are obtainable, holding re-
spectively 200, 250, 500, 1000, 2000, etc., c. c. A> ;i ircneral
rule, they have no ground-glass stoppers; it is, however, very
desirable, in certain cases, to have measuring llusks with ground
stoppers. The flasks must be made of well-annealed glass of uni-


form thickness, so that fluids may be heated in them. The line-
mark should be placed within the lower third, or at least within
the lower half, of the neck.

Measuring flasks, before they can properly be employed in
analytical operations, must first be carefully
tested. The best and simplest method of
effecting this is to proceed thus: Put the
flask, perfectly dry inside and outside, on
the one scale of a sufficiently delicate balance,
together with a weight of 1000 grm. in the
case of a litre flask, 500 grm. in the case of.
a half -litre flask, etc., restore the equilibrium
by placing the requisite quantity of shot and
tinfoil on the other scale, then remove the
flask and the weight from the balance, place
the flask on a perfectly level surface, and
pour in distilled water at 17- 5 until the lower border of the dark
zone formed by the top of the water around the inner walls corre-
sponds with the line-mark. After having thoroughly dried the
neck of the flask above the mark, replace it upon the scale ; if this
restores the perfect equilibrium of the balance, the water in the
flask weighs, in the case of a litre measure, exactly 1000 grm. If
the scale bearing the flask sinks, the water in it weighs as much
above 1000 grm. as the additional weights amount to which you
have to put in the other scale to restore the equilibrium ; if it rises,
on the other hand, the water weighs as much less as the weights
amount to which you have to put in the scale with the flask to effect
the same end.

If the water in the litre measure weighs 1000 grm., in the
half-litre measure 500 grm., etc., the measuring flasks are correct.
Differences up to O'l grm. in the liter measure, up to 0'07 grm.
in the half -litre measure, and up to 0*05 grm. in the quarter-litre
measure, are not taken into account, as one and the same measuring
flask will be found to offer variation to the extent indicated, in
repeated consecutive weighings, though filled each time exactly up
to the mark with water of the same temperature.

Though a flask should, upon examination, turn out not to hold
the exact quantity of water which it is stated to contain, it may yet
possibly agree with the other measuring vessels, and may accord-

38 OPERATIONS. [ 18.

ingly still be perfectly fit for use for most purposes. Two meas-
uring vessels agree among themselves if the marked numbers of
c. c. bear the same proportion to each other as the weights found ;
thus, for instance, supposing your litre measure to hold 998 grm.
water at 17 -5, and your 50 c. c. pipette to deliver 49-9 grin,
water of the same temperature, the two measures agree, since

1000:50 = 998:49-9.

To prepare or correct a measuring flask, tare the dry litre, half-
litre, or quarter-litre flask, and then weigh into it, by substitution
( 9), 1000 grin., or, as the case may be, the half or quarter of
that quantity of distilled water at 17*5. Put the flask on a per-
fectly horizontal support, place your eye on an exact level with the
surface of the water, and mark the lower border of the dark zone
by two little dots made on the glass with a point dipped into thick
asphaltum varnish, or some other substance of the kind. Now
pour out the water, place the flask in a convenient position, and cut
with a diamond a fine distinct line into the glass from one dot to
the other.

Measuring vessels are sometimes graduated also for delivery.
These, however, can only be used where very accurate measuring
is unnecessary, because the quantity of water remaining in the flask
varies not inconsiderably, and hence in repeated measurings with
the same flask notable differences may arise. The graduation or
testing of such flasks is effected by filling the flask with water, then
emptying it and allowing it to drain for a minute, and then weigh-
ing into it the weight of distilled water at 17'5 corresponding to
the number of c. c.

In none of these weighings, as will be seen, have the operations
been conducted at 4, or reduced to vacuum, hence the measuring
vessels will not accurately conform to the standard. If, however,
this system is carried out with all the vessels used for measuring
fluids, as proposed by FR. MOHR, the measures will correspond per-
fectly among themselves, and this will suffice for all the purposes
of volumetric analysis. In the exceptional case where such a
measuring vessel is used in measuring a gas, the proper correction
may be made by multiplying the c. c. by 1*0022, as pointed out
by FR. MOHR (Zeitschr. f. Analyt. Chem., vii, 287).

19, 20.]



55. Measuring vessels which serve to measure out any quan-
tities of fluid at will.

2. The Graduated Cylinder.

This instrument, represented in Fig. 10, should be from 2 to 3
cm. wide, of a capacity of 100 to 300 c. c., and divided into single
c. c. It must be ground at the top, so that it may
be covered closely with a ground-glass plate. The
measuring with such cylinders is not quite so ac-
curate as with measuring flasks, as in the latter
the volume is read off in a narrower part. The
accuracy of measuring cylinders may be tested in
the same way as in the case of measuring flasks,
viz., by weighing into them water at 17*5; or,
also, very well, by letting definite quantities of
fluid flow into the cylinder from a correct pipette
or burette graduated for delivering, and observ-
ing whether or not they are correctly indicated by
the scale of the cylinder.








aa. Measuring vessels which serve to measure
out one definite quantity of fluid.

3. The Graduated Pipette.



Fig. 10.

This instrument serves to remove a definite volume of a fluid
from one vessel and to transfer it to another ; it must accordingly
be of a suitable shape to admit of its being freely inserted into
flasks and bottles.

We use pipettes of 1, 5, 10, 20, 50, 100, 150, and 200 c. c.
capacity. The proper shape for pipettes up to 20 c " capacity



[ 20,

is represented in Fig. 11 ; Fig. 12 shows the most practical form
for larger ones. To fill a pipette suction is applied
to the upper aperture, either directly with the lips
or through a caoutchouc tube, until the fluid
stands above the mark ; the upper orifice which is
somewhat narrowed and ground) is then closed
with the first finger of the right hand (the point of
which should be a little moist) ; the outside is then
wiped dry, if required, and, the pipette being held
in a perfectly vertical direction, the fluid is allowed
to drop out, by lifting the finger a little, till it has
fallen to the required level ; the loose drop is care-
fully wiped off, and the contents of the tube are then
finally transferred to the other vessel. In this pro-
cess it is found that the fluid does not run out
completely, but that a small portion of it remains
adhering to the glass in the point of the pipette ; after
a time, as this becomes increased by other minute
particles of fluid trickling down from the upper part
of the tube, there gathers at the lower orifice a drop
which may be allowed to fall off from its own
weight, or may be made to drop off by a slight
shake. If, after this, the point of the pipette be
laid against a moist portion of the inner side of the
vessel, another minute portion of fluid will trickle
out, and, lastly, another trifling droplet or so may
be got out by blowing into the pipette. Now,
supposing the operator follows no fixed rule in this
respect, letting the fluid, for instance, in one
operation simply run out, whilst in another operation he lets it
drain afterwards, and in a third blows out the last particles of it
from the pipette; it is evident that the respective quantities of fluid
delivered in the several operations cannot be quite equal. I prefer
in all cases the second method, viz., to lay the point of the pipette,
whilst draining, finally against a moist portion of the side of the
vessel, which I have always found to give the most accurately cor-
responding measurements.

The correctness of a pipette is tested by filling it up to the
mark with distilled water at 17'5, letting the water run out, in


Fig. 11. Fig. 12.


the manner just stated, into a tared vessel, and weighing; the
pipette may be pronounced correct if 100 c. c. of water at 17*5
weigh 100 grm.

Testing in like manner the accuracy of the measurements made
with a simple hand pipette, we find that one and the same pipette
will, in repeated consecutive weighings of the contents, though
filled and emptied each time with the minutest care, show differ-
ences up to O'Ol grm. for 10 c. c. capacity, up to 0*04 grm. for 50
c. c. capacity.

The accuracy of the measurements made with a pipette may
be heightened by giving the instrument the form and construction
shown in Fig. 13 and fixing it in a holder.

It will be seen from the drawing that these pipettes
are emptied only to a certain mark in the lower tube, and
that they are provided with a pinch-cock, a contrivance
which we shall have occasion to describe in detail when on
the subject of burettes. This contrivance reduces the dif-
ferences of measurements with one and the same 50 c. c.
pipette to 0'005 grm. '

Pipettes are used more especially in cases where it is in-
tended to estimate different constituents in separate por-
tions of a substance : for instance, 10 grm. of the sub-
stance under examination are dissolved in a 250 c. c.
flask, the solution is diluted up to the mark, shaken, and
2, 3, or 4 several portions are then taken out with a 50 c. c.
pipette. Each portion consists of \ part of the whole, and
accordingly contains 2 grm. of the substance. .Of course
the pipette and the flask must be in perfect harmony.
Whether they are may be ascertained by, for instance,
emptying the 50 c. c. pipette 5 times into the 250 c. c.
flask, and observing if the lower edge of the dark zone of
fluid coincides with the mark. If it does not, you may
make a fresh mark, which, no matter whether it is really
correct or not, will bring the two instruments in question
into conformity with each other.

Cylindrical pipettes, graduated throughout their entire
length, may be used also to measure out any given quanti-
ties of liquid; however, these instruments can properly be^.
employed only in processes where minute accuracy is not
indispensable, as the limits of error in reading off the divisions in


42 OPERATIONS. [ 21.

the wider part of the tube are not inconsiderable. For smaller
quantities of liquid this inaccuracy may be avoided by making the
pipettes of tubes of uniform width, having a small diameter only,
and narrowed at both ends. (Fit. MOHK'S measuring pipettes.)

When a fluid runs out of a pipette, drops sometimes remain
here and there adhering to the tube ; this arises from a film of fat

on the inside ; it may
be removed by filling
the instrument with a
concentrated solution
of potassium bichro-
mate mixed with sul-
phuric acid, or with
potassa solution, and,
after allowing sufficient
time for action, thor-
oughly washing.

Online LibraryC. Remigius FreseniusQuantitative chemical analysis → online text (page 4 of 69)