C. Remigius Fresenius.

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5 b. Me asuring
vessels which serve to
measure out quantities
of fluid at will.

4r. The Burette.

Of the various
forms and dispositions
of this instrument, the
following appear to me
the most convenient : *

21.
I. Holies Burette.

This excellent
measuring apparatus is
represented in Fig. 14.
It consists of a cylin-
drical tube, narrower towards the lower end for about an inch,

* In regard to other forms see F. Moim, Lehrbuch der Titrirmethode, 3d edit.,
2 ; G. C. WITTSTEIN, Vierteljahresschr. /. prakt. Pharm., xvi, 567, and Zeitschr.
f. analyt. Chem., VH, 84; A. GAWALOWSKY, Zeitschr. /. Chem. [N. F.], vi, 129,
and Zeitschr. f. analyt. Chem., ix, 369 ; GONDOLO, Rev. hebdomad. deChim., Nov.
1869, and Zeitschr. f. analyt. Chem. t ix, ;!7o.







21.] MEASURING OF FLUIDS. 43

with a slight widening, however, at the extreme point, in order
that the caoutchouc connector may take a firm hold. I only use
burettes of two sizes, viz., of 30 c. c., divided into 0*1 c. c. ; and
of 50 c. c., divided into 0'5 c. c. The former are employed
principally in scientific, the latter chiefly in technical, investiga-
tions. The usual length of my 30 c. c. burette is about 50 cm. ;
the graduated portion occupies about 49 cm. The diameter of the
tube is accordingly about 1 mm. in the clear ; the upper orifice
is, for the convenience of filling, widened in form of a funnel,
measuring 20 mm. in diameter; the width of the lower orifice is
5 mm. For very delicate processes, the length of the graduated
portion may be extended to 50 or 52 cm., leaving thus intervals
of nearly 2 mm. between the small divisional lines. In my 50 c. c.
burettes the graduated portion of the tube is generally 40 cm. long.
To make the instrument ready for use, the narrowed lower end
of the tube is warmed a little, and greased with tallow ; a caout-
chouc tube, about 30 mm. long, and having a diameter of 3 mm.
in the clear, is then drawn over it ; into the other end of this is
inserted a tube of pretty thick glass, about 40 mm. long, and
drawn out to a tolerably fine point; it is advisable to slightly
widen the upper end of this
tube also, and to cover it with
a thin coat of tallow ; and also
to tie linen thread, or twine,
round both ends of the con-
nector, to insure perfect tight-

ness - Fig. 15.

The space between the

lower orifice of the burette and the npper orifice 'of the small
delivery tube should be about 15 mm. The India-rubber tube is
now pressed together between the ends of the tubes by the pinch-
cock (or clip). This latter instrument is usually made of brass
wire; the form represented in Fig. 15 was devised by MOHR.

A good clip must pinch so tightly that not a particle of fluid
can make its way through the connector when compressed by it; it
must be so constructed that the analyst may work it with perfect
facility and exactness, so as to regulate the outflow of the liquid
with the most rigorous accuracy, by bringing a greater or less
degree of pressure to bear upon it.




44



OPERATIONS.



[21.



MOHR* has also devised very practical clips of glass (or horn)
and rubber, which are to be highly recommended.

Figs. 16 and 17 show the construction of these clips, which are
so simple that any one can readily
make them by following MOHR'S
instructions, as follows:

Bend two pieces of flat ther-
mometer-tubing, 80 to 90 cm. long,
into a very obtuse angle, and place
between them, in the middle, a
thin piece of cork, about 1 to 2
mm. thick; pass a rubber ring,





Fig. 16.



Fig. 17.



cut from a somewhat wide rubber tube, over the part inclosing the
cork. After placing the rubber tube of the burette between the
two glass tubes, press these together and slip another rubber ring
over the ends of the glass tubes. These rings serve to tightly com-
press and close the rubber tube on the burette. On pressing together
the divergent ends of the glass tubes, however, the pressure on the
rubber tube is relieved, and the liquid flows through the delivery
tube. On releasing the pressure, the elastic bands again completely
close the connecting tube.

For supporting MOIIR'S burettes, use is made of the holder
shown in Fig. 14; this instrument, whilst securely confining the
tube, permits its being moved up and down with perfect freedom,
and also its being taken out, without interfering with the pinch-
cock. The position of the burette must be strictly perpendicular,
to insure which, care must be taken to have the grooves of the cork
lining, which are intended to receive the tube, perfectly vertical,
with the lower board of the stand in a horizontal position.

The arm bearing the burette I now have made movable around
the upright, so that first one, then another burette may readily be



MOHR, LeJirbuch der Titrirmethode, :M rdit., p. 7.



21.]



MEASURING OF FLUIDS.



45



used. If it is desired to fix the arm, a screw (wanting in the illus-
tration) serves the purpose. A similar holder, with a brass clamp,
is shown in Fig. 18.

To best charge the burette for a volumetric operation, the
point of the instrument is immersed in the liquid, the pinch-cock
opened, and a little liquid, suf-
ficient at least to reach into the
burette tube, drawn up by ap-
plying suction to the upper end ;
the cock is then closed, and the *
liquid poured into the burette
until it reaches up to a little
above the top mark. The burette
having, if required, been duly
adjusted in the proper vertical
position, the liquid is allowed to
drop out to the exact level of
the top mark. The instrument
is now ready for use. When as
much liquid has flowed out as is
required to attain the desired
object, the analyst, before pro-
ceeding to read off the volume
used, should wait a few minutes,
to give the particles of fluid ad-
hering to the sides of the emp-
tied portion of the tube proper
time to run down. This is an
indispensable part of the opera-
tion in accurate measurements,
since, if neglected, an experiment
in which the standard liquid in the burette is added slowly to the
fluid under examination (in which, accordingly, the minute particles
of fluid adhering to the glass have proper time afforded them during
the operation itself to run down), will, of course, give slightly dif-
ferent results from those arrived at in another experiment, where
the larger portion of the standard fluid is applied rapidly, and the
last few drops alone are added slowly.

The manner in which the reading-off is effected, is a matter of




Fig. 18.



46



OPERATIONS.



[ 21.



groat importance in volumetric analysis; the first requisite is to
bring the eye on a level with the top of the fluid. A\ r e must
Consequently settle the question "What is to be considered the top?
If you hold a burette, partly filled with water, between the eye
and a strongly illumined wall, the surface of the fluid presents the
appearance shown in Fig. 19; if you hold close behind the tube a
sheet of white paper, with a strong light falling on it, the surface
of the fluid presents the appearance shown in Fig. 20.





Fig. 19.



Fig. 20.



Fig. 21,



In the one as well as in the other case, you have to read off at
the lower border of the dark zone, this being the most distinctly
marked line. FR. MOIIR recommends the following device for
reading off : Paste on a sheet of white paper a broad strip of black
paper, and, when reading-off, hold this close behind the burette, in
a position to place the border line between white and black from 2
to 3 mm. below the lower border of the dark zone, as shown in
Fig. 21 ; then read-off at the lower border of the dark zone.

Great care must be taken to hold the paper invariably in the
same position, since, if it be held lower down, the lower border of
the black zone will move higher up.

I prefer to read-off in a light which causes the appearance rep-
resented in Fig. 19.

By the use of ERDMANN'S float * all uncertainties in reading-off

* Journ. f. prakt. Chem., LXXI, 194.



21.] MEASURING OF FLUIDS. 47

may be avoided. Fig. 22 represents a burette thus provided. In
this case we always read-off the mark on the burette which coin-
cides with the circle in the middle of the float. The float must be
so fitted to the width of the burette that when placed in the filled
burette, it will, on allowing the fluid to run out gradually, sink
down without wavering ; and when it has been pressed down into
the fluid of the closed burette, it will slowly rise again. The weight
of the float must, if necessary, be so regulated by mercury that
when placed in the filled tube, the surface of the liquid will coin-
cide uniformly all around with the upper shoulder of the float. A
further important condition of the float is that its axis should
coincide as nearly as possible with that of the burette tube, so that
the division-mark on the burette may be always parallel with the
circular line on the float.

The correctness of the graduation of a burette is tested in the
most simple way, as follows : Fill the instrument up to the highest
division with water at 17*5, then let 10 c. c. of the
liquid flow out into an accurately weighed flask, and
weigh; then let another quantity of 10 c. c. flow
out, and weigh again, arid repeat the operation
until the contents of the burette are exhausted.
If the instrument is correctly graduated, every
10 c. c. of water at 17*5 must weigh 10 grm.
Differences up to O'Ol grm. may be disregarded,
since even with the greatest care bestowed on the
process of reading-off, deviations to that extent
will occur in repeated measurements of the upper-
most 10 c. c. of one and the same burette. With
the float-burettes the weighings agree much more
accurately, and the differences for 10 c. c. do not
exceed! 0-002 grm.

MOHE'S burette is unquestionably the best and
most convenient instrument of the kind, and ought
to be employed in the measurement of all liquids
which are not injuriously affected by contact with
caoutchouc. Of the standard solutions used at
present in volumetric analysis, that of potassium Fig. 22.
permanganate alone cannot bear contact with caoutchouc. Excel-





48



OPERATIONS.



[22.



lent directions for calibrating MOHR burettes Lave been given by

SCHEIBLER.*

22.
II. Gay-Lussatfs Burette.

Fig. 23 represents this instrument in, as I believe, its most
practical form.

I make use of two sizes, one of 50 c. c. graduated in 0'5 c. c.,
the other of 30 c. c. graduated in 0*1 c. c. The former is about
33 cm. long; the graduated portion occupies about
25 cm. ; the internal diameter of the wide tube meas-
ures 15 mm. ; that of the narrow tube 4 mm., which
in the upper bent- end gradually decreases to 2 mm.
The graduated portion of the smaller burette is about
28 cm. long, and has accordingly an internal diameter
of about 11 mm.

In use this burette is held in the left hand, with
the lower end resting lightly against the chest. Held
in this manner, and with an occasional turn of the spout
sideways, the outflow of liquid may be regulated at
will. The fluid is not allowed, as a rule, to run back
into the narrow tube during the course of an opera-
tion, as it is frequently difficult to renew the flow of the
fluid because of the formation of an air-bubble be-
tween the fluid and the drop remaining in the mouth
of the spout.

To provide a substantial stand for the burette, a
solid disk of wood 10 to 12 cm. in diameter and from
Fig. 23. 5 to 6 cm. thick has a suitable hole bored in its centre,
in which the burette may be inserted. This seems to me more
convenient than to cement the burette in a wooden foot.

To overcome the difficulty of renewing the flow of liquid when
an air-bubble has become enclosed between the fluid and the drop



40



ISO



remaining in the tip of the spout, MOHR has proposed closing the
wider tube with a perforated cork bearing a short glass tube bent
at right angles. On slipping a piece of rubber tubing over this



* Jour. f. prakt. Chemie, LXXVI, 177.



23.] MEASURING OF FLUIDS. 49

short tube, a^nd blowing into it more or less strongly, the outflow
of liquid may be regulated at will. Instead of blowing with the
mouth, a rubber bulb may be used, but the latter must be provided
with a small hole through which air may enter after the bulb has
been compressed, and which is closed by the finger during compres-
sion (HERVE-MANGON).*

The readings with this burette are taken just as with the MOHR
burette. The instrument is preferably held firmly against a per-
pendicular wall, a strongly illuminated white door, or a window
pane, to insure its being held perfectly vertical. When operating
with concentrated, and hence opaque, solutions of potassium per-
manganate, the method of reading requires modification, the upper
border of the liquid being then observed, arid the readings best
taken by reflected light against a white background.

The GAY-LUSSAC burette is tested as to its correctness just as is
the MOHR burette.

23.
III. Geissler's Burette.

In this instrument, figured in Fig. 24, the narrow tube, which
is outside in the GAY-LUSSAC burette, is placed within the wide
tube. The glass of that part of the inner tube which projects from
the wide tube is very thick, w r hile the part within the wide tube is
very thin.

This burette is very convenient to use, and is but little liable to
fracture. I am very partial to it.

What has been stated above regarding reading-off and testing
applies to this burette as well.

*Rep. chim. appliquee, i, 68.



50 OPP.RATIoN-. [ 24.



PRKLIMINAKY OIM-:KATIN<. TiiKrAKATioN- OF Sur.-r.v.xcr.s FOH
Tin: I'K- - - OF iJrAMiTATivi: AXALY? s,






24.
1. THE SELECTION OF THE SAMPI.K.

Before the analyst proceeds to make the quantitative analysis

fof a body, he cannot too carefully consider
whether the desired result is fullv attained if he

simply knows the respective quantity of every
individual constituent of that body. This pri-
mary point is but too frequently disregarded, and
thus false impressions are made, even by the
most careful analysis. This remark applies both
to scientific and to technical investigations.
Therefore, if the constitution of a mii
is to be determined, take the greatest pos-
sible care to remove in the first place every
particle of gangue, and disseminated impuri-
ties ; remove any adherent matter by wipin_
washing, then wrap the substance up in a sheet
of thick paper, crush it to pieces on a steel
anvil, and pick out with a pair of small pii.
the cleanest pieces. Crystalline substai.
prepared artificially, ought to be purified by re-
crystallization ; precipitates by thorough wash-
ing .V.-., Ye.

Ill technical investigations, when called
upon, for instance, to determine the amount *f
peroxide present in a mar ore, or the

amount of iron present in an iron ore, the first
point for consideration ought to he whether the
samples selected correspond as much as possible
the average quality of the ore. AVhat would
it serve, indeed, to the pure: I manga-

mine to know the amount of peroxide present
in a selei ly particularly rich, sample?

These few observations will suffice to show that no universally
applicable and valid rules to guide the analyst in the seleetu>'
the sample can bo laid down ; he nu^t in every individual c



MECHANICAL DIVISION. 51

no hand, examine the substance carefully, and more par-
ticularly alsv under the microscope, or through a lens ; and, on the
r hand, keep clearly in view the object of the investigation,
*nd then take his measures accordingly.



2. MECHANICAL Dmsiox.

In order to prepare a substance for analysis, i.e., to render it

accessible to the action of solvents or fluxes, it is generally indis-

, in the first place, to divide it into minute parts, since

I create numerous points of contact for the solvent, and

so far as practicable, remove the adverse

: the power of cohesion, thus fulfilling all the condi-

*:ect a complete and speedy solution.
.ns employed to attain this object vary according to the
different bodies we have to operate upon. In many
- v - nple crushing or pounding is sufficient; in other cases it
-xiry to reduce the powder to the very liighest degree of
-ifting or by elutriation.

oration of powdering is conducted in mortars. The

and absolutely indispensable condition is, that the material of

the mortar be considerably harder than the substance to be pulver-

ized prevent, so far as practicable, the latter from being

-.uninated with any particles of the former. Thus, for crush-

salts and other substances possessing no very considerable

degree of hardness, porcelain mortars may be used, whilst the

f harder substances (of most minerals, for instance,)

f agate, chalcedony, or flint. In such cases, the

- are first reduced to a coarse powder, best effected by

;m up in several sheets of writing-paper, and striking

with a hammer upon a steel or iron plate ; the coarse powder

dned is then pulverized, in small portions at a time, in an

agate mortar, until reduced to the state of an impalpable powder.

If v . i all portion of a mineral to operate upon, and

ed in afl cases where we are desirous of avoiding loss, it is

>able to -el mor g _o)for the preparatory rednc-

ineral to coarse powder.

ab and &? represent the two parts of the mortar; these in;
readily taken asunder. The substance to be crushed (havini..




62 OPERATIONS. [ 25.

practicable, first been broken into small pieces), is placed in the
cylindrical chamber ef ; the steel cylinder, which iits somewhat
Jooselyinto the chamber, serves as pestle. The mortar is placed
upon a solid support, and perpendicular blows are repeatedly

struck upon the pestle with a hammer
until the object in view is attained.

Minerals which are very difficult
to pulverize may be strongly ignited,
and then suddenly plunged into cold
water, and subsequently again ignited.
This process is of course applicable only
to minerals which lose no essential con-
stituent on ignition, and are perfectly
insoluble in water.

In the purchase of agate mortars,
especial care ought to be taken that they
v have no palpable cracks or indentations ;

very slight cracks, however, that cannot

be felt, do not render the mortar useless, although they impair its
durability.

Minerals insoluble in acids, and which consequently require
fusing, must especially be finely divided, otherwise we cannot calcu-
late upon complete decomposition. This object may be obtained
either by triturating the crushed mineral with water, or by elutri-
ation, or by sifting; the two former processes, however, can be
resorted to only in the case of substances which are not attacked
by water. It is quite clear that analysts must in future be much
more cautious on this point than has hitherto been the case, since
we know now r that many substances which are usually held to be
insoluble in water are, when in a state of minute division, strongly
affected by that solvent; thus, for instance, water, acting upon
some sorts of finely pulverized glass, is found to rapidly dissolve
from 2 to 3 per cent, of glass even in the cold. (PELOUZE.*)
Thus, again, finely divided feldspar, granite, trachyte and porphyry
give up to water both alkali and silica. (II. LunwiG.f)

Lemgation (trituration with water). Add a little water to
the crushed mineral in the mortar, and triturate the paste until all
crepitation ceases, or, which is a more expeditious process, transfer

* Compt. Rend., XLIII, 117-123. \ Archiv der Pharm., xci, 147.



I

I

I



25.] MEASURING OF FLUIDS. 53

the paste from the mortar to an agate or flint slab, and triturate
it thereon with a muller. Rinse the paste off, with the washing-
bottle, into a smooth porcelain basin of hemispherical form,
evaporate the water on the water-bath, and mix the residue most
carefully with the pestle. (The paste may be dried also in the
agate mortar, but at a very gentle heat, since otherwise the mortar
might crack.)

To perform the process of elutriation, the pasty mass, having
first been very finely triturated with water, is washed off into a
beaker, and stirred with distilled water ; the mixture is then allowed
to stand a minute or so, after which the supernatant turbid fluid is
poured off into another beaker. The sediment, which contains the
coarser parts, is then again subjected to the process of trituration,
etc., and the same operation repeated until the whole quantity is
elutriated. The turbid fluid is allowed to stand at rest until the
minute particles of the substance held in suspension have subsided,
which generally takes many hours. The water is then finally
decanted, and the powder dried in the beaker.

The process of sifting is conducted as follows : A piece of fine,
well-washed, and thoroughly dry linen is placed over the mouth of
a bottle about 10 cm. high, and pressed down a little into the mouth,
so as to form a kind of bag ; a portion of the finely triturated sub-
stance is put into the bag, and a piece of soft leather stretched tightly
over the top by way of cover. By drumming with the finger on the
leather cover, a shaking motion is imparted to the bag, which
makes the finer particles of the powder gradually pass through the
linen. The portion remaining in the bag is subjected again to
trituration in an agate mortar, and, together with a fresh portion
of the powder, sifted again ; the process is repeated until the
entire mass has passed through the bag into the glass.

When operating on substances consisting of different com-
pounds it would be a grave error indeed to use for analysis the
powder resulting from the first process of elntriation or sifting,
since this will contain the more readily pulverizable constituents in
a greater proportion to the more resisting ones than is the case
with the original substance.

Great care must, therefore, also be taken to avoid A loss of
substance in the process of elutriation or sifting, as this loss is
likely to be distributed unequally among the several component
parts. It is safer in such cases to effect the subdivision by patiently
triturating the dry substance, and to avoid elutriation or sifting.



64 OPERATIONS. [ 26.

In cases where it is intended to ascertain the average composi-
tion of a heterogeneous substance, of an iron ore for instance, a
large average sample is selected, and reduced to a coarse powder ;
the latter is thoroughly intermixed, a portion of it powdered more
finely, and mixed uniformly, and finally the quantity required for
analysis is reduced to the finest powder. The most convenient
instrument for the crushing and coarse powdering of large samples
of ore, &c., is a steel anvil and hammer. The anvil in my own
laboratory consists of a wood pillar, 85 cm. high and 26 cm. in
diameter, into which a steel plate, 3 cm. thick and 20 cm. in
diameter, is let to the depth of one-half of its thickness. A brass
ring, 5 cm. high, fits round the upper projecting part of the steel
plate. The hammer, which is well steeled, has a striking surface
of 5 cm. diameter. An anvil and hammer of this kind afford,
among others, this advantage, that their steel surfaces admit most
readily of cleaning. To convert the coarse powder into a finer, a
smooth-turned steel mortar of about 130 mm. upper diameter and
74 mm. deep is used the final trituration is conducted in an agate
mortar.

26.
3. DRYING.

Bodies which it is intended to analyze quantitatively must be,
when weighed, in a definite state in a condition in which they
can be always obtained again.

Now, the essential constituents of a substance are usually accom-
panied by an unessential one, viz., a greater or less quantity ot
water, enclosed either within its lamellae, or adhering to it from
the mode of its preparation, or absorbed by it from the atmosphere,
It is perfectly obvious that to estimate correctly the quantity of a
substance, we must, in the first place, remove this variable quantity
I' water. Mo#t tiolid bodies, therefore, require to be di'!i l><
////// can /><' quantitatively mml //-;/.

The operation of drying is of the very highest importance for
the correctness of the results ; indeed it may safely be nvcnvd that
many of the differences observed in analytical researches proceed
entirely from the fact that substances are analyzed in different
states of moisture.

Many bodies contain, as is well known, water which is proper



26.] DESICCATION. 55

to them either as inherent in their constitution, or as so-called water
of crystallization. In contradistinction to this,, we will employ the
term moisture to designate that variable adherent or mechanically



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