C. Remigius Fresenius.

Quantitative chemical analysis online

. (page 3 of 69)
Online LibraryC. Remigius FreseniusQuantitative chemical analysis → online text (page 3 of 69)
Font size
QR-code for this ebook

3. As to the shape best adapted for weights, I think that of
short frusta of cones inverted, with a handle at the top, the most-
convenient and practical form for the large weights ; square pieces
of foil, turned up at one corner, are best adapted for the small
weights. The foil used for this purpose should not be too thin,
and the compartments adapted for the reception of the several
smaller weights in the box should be large enough to admit of their
contents being taken out of them with facility, or else the smaller
weights will soon become crumpled and defaced. Every one of
the weights (with the exception of the milligramme) should be dis-
tinctly marked.

4. So far as the material most suitable for weights is con-
cerned, rock crystal, though best adapted for normal weights, is
unsuitable for ordinary weights because of its high cost and the
inconvenient form the weights would have. Platinum, were it not
so costly, would be surely adopted generally, on account of its
unchangeability. As a rule, however, platinum is used for weights

* It were desirable that makers of analytical weights endeavor to procure
normal weights. It is very annoying, in many cases, to find notable differences
between weights of the same denomination, but coming from different makers,
as I have frequently observed.



smaller than 1 gramme or 0*5 gramme, while brass is used for a
the higher denominations. Brass weights must be carefully
shielded from the action of acid or other vapors, or their correct-
ness will be impaired ; nor should they ever be touched with the
fingers, but always with small pincers. It is an erroneous notion
to suppose that weights slightly tarnished are unfit for use. In
fact, it is scarcely possible to keep weights for any very great
length of time from becoming slightly tarnished. I have carefully
examined many weights of this description, and have found them
to correspond as exactly with one another in their relative propor-
tions as they did when first used. The tarnishing coat is so
extremely thin that even a very delicate balance will generally fail
to point out any perceptible difference in the weight. It will
nevertheless be found advantageous to gild the brass weights pre-
vious to their final adjustment.

The following is the proper way of testing the weights :
One scale of a delicate balance is loaded with a
weight, and the balance is then completely equipoised by taring
with small pieces of brass, and finally tinfoil (not paper, since this
absorbs moisture). The weight is then removed, and replaced suc-
cessively by the other gramme weights, and afterwards by the same
amount of weight in pieces of lower denominations.

The balance is carefully scrutinized each time, and any devia-
tion from the exact equilibrium marked. In the same way it is
seen whether the two-gramme piece weighs as much as two single
grammes, the five-gramme piece as much as three single grammes
and the two-gramme piece, etc. In the comparison of the smaller
weights thus among themselves, they must not show the least dif-
ference on a balance turning with 0*1 milligramme. In comparing
the larger weights with all the small ones, differences of 0*1 to 0*2
milligramme may be passed over. If you wish them to be more
accurate, you must adjust them yourself. In the purchase of
weights chemists ought always to bear in mind that an accurate
weight is truly valuable, whilst an inaccurate one is absolutely
worthless.* It is the safest way for the chemist to test evei
weight he purchases, no matter how high the reputation of the

* Compare W. CROOKES, on adjusting chemical weights (Zeitschr.f. Analyt
Chem., vi, 431), and K. L. BAUER (ibid., vm, 390).



There are two different methods of determining the weight of
substances ; the one might be termed direct weighing, the other is
called weighing l>y substitution.

In direct weighing, the substance is placed upon one scale, and
the weight upon the other. If we possess a balance, the arms of
which are of equal length, and the scales in a perfect state of
equilibrium, it is indifferent upon which scale the substance is
placed in the several weighings required during an analytical pro-
cess ; i.e., we may weigh upon the right .or upon the left side, and
change sides at pleasure, without endangering the accuracy of our
results. But if, on the contrary, the arms of our balance are not
perfectly equal, or if the scales are not in a state of perfect equili-
brium, we are compelled to weigh invariably upon the same scale,
otherwise the correctness of our results will be more or less materi-
ally impaired.

Suppose we want to weigh one gramme of a substance, and to
divide this subsequently into two equal parts. Let us assume
our balance to be in a state of perfect equilibrium, but with
unequal arms, the left being 99 millimeters, the right 100
millimeters, long ; we place a gramme weight upon the left scale,
and against this, on the right scale, as much of the substance to be
weighed as will restore the equilibrium of the balance.

According to the axiom, " masses are in equilibrium upon a
lever, if the products of their weights into their distances from the
fulcrum are equal," we have consequently upon the right scale 0*99
grm. of substance, since 99xl'00 100 X 0'99. If we now, for the
purpose of weighing one half the quantity, remove the whole
weight from the left scale, substituting a 0'5 grm. weight for it,
and then take off part of the substance from the right scale, until
the balance recovers its equilibrium, there will remain 0*495 grm. ;
and this is exactly the amount we have removed from the scale :
we have consequently accomplished our object with respect to the
relative weight ; and as we have already remarked, the absolute
weight is not generally of so much importance in scientific work.
But if we attempted to halve the substance which we have on the
right scale, by first removing both the weight and the substance


from the scales, and placing subsequently a 0*5 grin, weight upon
the right scale, and part of the substance upon the left, until the
balance recovers its equilibrium, we should have 0-505 grm. of
substance upon the left scale, since 100 X 0-5 = 99 X 0-505;
and consequently, instead of exact halves, we should have one
part of the substance amounting to 0*505, the other only to 0-485,

If the balance is equal-armed, but the scale-pans are not in a
state of absolute equilibrium, we are obliged to weigh our sub-
stances in vessels to insure accurate results (although the arms of
the balance be perfectly equal). It is self-evident that the weights
in this case must likewise be invariably placed upon one and the
same pan, and that the difference between the two scale-pans
must not vary during the course of a series of experiments.

From these remarks result the two following rules :

1. It is, under all circumstances, advisable to place the sub-
stance invariably upon one and the same pan most conveniently
upon the left.

2. If the operator happens to possess a balance for his own
private and exclusive use, there is no need that he should adjust it
at the commencement of every analysis ; 'but if the balance be used
in common by several persons, it is absolutely necessary to ascer-
tain, before every operation, whether the state of absolute equili-
brium may not have been disturbed.

Weighing ~by substitution yields not only relatively, but also
absolutely accurate results ; no matter whether the arms of the
balance be of exactly equal lengths or not, or whether the scales be
in perfect equipoise or not.

The process is conducted as follows : The material to be
weighed say a platinum crucible is placed upon one scale, and
the other scale is accurately counterpoised against it. The plati-
num crucible is then removed, and the equilibrium of the balance
restored by substituting weights for the removed crucible. It is
perfectly obvious that the substituted weights will in variably
ex press the real weight of the crucible with absolute accuracy.
\\ c weigh by substitution whenever we require the greatest pos-
sible accuracy; as, for instance, in the determination of atomic
weights. The process may be materially shortened by first placing
a tan; ('which must of course be heavier than the substance to be
weighed) upon one scale, say the left, and loading the other scale


with weights until equilibrium is produced. This tare is
retained oh the left scale. The weights after being noted are re~
moved. The substance is placed on the right scale, together with
the smaller weights requisite to restore the equilibrium of the
balance. The sum of the weights added is then subtracted from
the noted weight of the counterpoise : the remainder will at once
indicate the absolute weight of the substance. Let us suppose, for
instance, we have on the left scale a tare requiring a weight of fifty
grammes to counterpoise it. We place a platinum crucible on the
right scale, and find that it requires an additional weight of 10
grammes to counterpoise the tare on the left. Accordingly, the
crucible weighs 50 minus 10, i.e., 40 grammes.

The following rules will be found useful in performing the
process of weighing :

1. The balance should be kept in a dry place, protected from
acid vapors, etc., and, if possible, not exposed to direct sunlight.
It should be placed on a firm support and in a level position ; nor
must it be too near the source of heat, should the room be heated,
otherwise it may be unequally heated.

2. The safest and most expeditious method of ascertaining the
exact weight of a substance, is to avoid trying weights at random ;
instead of this, a strictly systematic course ought to be pursued in
counterpoising substances on the balance. Suppose, for instance,
we want to weigh a crucible, the weight of which subsequently
turns out to be 6*627 grammes; we first place 10 grammes on the
other scale against it, and we find this is too much ; we place the
weight next in succession, i.e., 5 grammes, and find this too little;
next 7, too much; 6, too little; 6*5, too little; 6*7, too much ; 6*6,
too little; 6*65, too much; 6*62, too little; 6-63, too much; 6*625,
too little; 6*627, right.

For the sake of illustration* a most complicated case has been
selected ; this systematic way of laying on the weights will, how-
ever, in most instances lead to the desired end, in half the time
required when weights are tried at random. After a little practice
a few minutes will suffice to ascertain the weight of a substance to
within 0*1 milligramme, provided the balance does not oscillate too


3. The milligrammes and fractions of milligrammes are deter-
mined by a centigramme rider (to be placed on or between the
divisions on the beam) far more expeditiously and conveniently
than by the use of the weights themselves, and at the same time
with equal accuracy.

4. Particular care and attention should be bestowed on enter-
ing the weights in the book. The best way is to write down the
weights first by inference from the blanks, or gaps in the weight
box, and to control the entry subsequently by removing the weights
from the scale, and replacing them in their respective compartments
in the box. The student should from the commencement make it
a rule to enter the number to be deducted in the lower line" thus,
in the upper line, the weight of the crucible -f- the substance ; in
the lower line, the weight of the empty crucible.

5. The balance ought to be arrested every time any change is
contemplated, such as removing weights, substituting one weight
for another, etc. etc. , or it will soon be spoiled.

6. Substances (except, perhaps, pieces of metal, or some other
bodies of the kind) must never be placed directly upon the scales,
but ought to be weighed in appropriate vessels of platinum, silver,
glass, porcelain, etc., never on paper or card, since these, being
liable to attract moisture, are apt to alter in weight. The most
common method is to weigh in the first instance the vessel by itself,
and to introduce subsequently the substance into it, to weigh again,
and subtract the former weight from the latter. In many instances,
and more especially where several portions of the same substance
are to be weighed, the united weight of the vessel and of its con-
tents is first ascertained ; a portion of the contents is then shaken
out, and the vessel weighed again ; the loss of weight expresses the
amount of the portion taken out of the vessel.

7. Substances prone to attract moisture from the air must be
weighed invariably in closed vessels (in covered crucibles, for
instance, or between two watch -glasses, or in a closed glass tube);
fluids are to be weighed in small bottles closed with glass stoppers.

8. A vessel ought never to be weighed while warm, since it
will in that case invariably weigh lighter than it really is. This is
owing to two circumstances. In the first place, every substance
condenses upon its surface a certain quantity of air and moisture,
the amount of which depends upon the temperature and hygro-

10.] WEIGHING. 25

scopic state of the air, and likewise on its own temperature. Now
suppose a crucible lias been weighed cold at the commencement
of the operation, and is subsequently weighed again while hot,
together with the substance it contains, and the weight of which
we wish to determine. If we subtract for this purpose the
weight of the cold crucible, ascertained in the former instance,
from the weight found in the latter, we shall subtract too much,
and consequently we shall set down less than the real weight for
the substance. In the second place, bodies at a high temperature
are constantly communicating heat to the air immediately around
them; the heated air expands and ascends, and the denser and
colder air, flowing towards the space which the former leaves, pro-
duces a current which tends to raise the scale, making it thus
appear lighter than it really is.

9. If we suspend from the end edges of a correct balance
respectively 10 grammes of platinum and 10 grammes of glass, by
wires of equal weight, the balance will assume a state of equili-
brium ; but if we subsequently immerse the platinum and glass
completely in water, this equilibrium will at once cease, owing to
the different specific gravity of the two substances ; since, as is
well known, substances immersed in water lose of their weight
a portion equal to the weight of their own bulk of water. If
this be borne in mind, it must be obvious to every one that
weighing in the air is likewise defective, inasmuch as the bulk
of the substance weighed is not equal to that of the weight
used. This defect, however, is so very insignificant, owing
to the trifling specific gravity of the air in proportion to that
of solid substances, that we may generally disregard it alto-
gether in analytical experiments. In cases, however, where
absolutely accurate results are required, the bulk both of
the substance examined, and of the weight, must be taken
into account, and the weight of the corresponding volume
of air added respectively to that of the substance and of
the weight, making thus the process equivalent to weighing
in vacuo.

26 OPERATIONS. [ 11.



The process of measuring is confined in analytical researches
mostly to gases and liquids. The method of measuring gases has
been brought to such perfection by BUNSEN, REGNAULT and REISET,
that it may be said to equal in accuracy the method of weighing.
However, such accurate measurements demand an expenditure of
time and care which can be bestowed only on the nicest and most
delicate scientific investigations.*

The measuring of liquids in analytical investigations was resorted
to first by DESCROIZILLES (Alkalimeter, 1806). GAY-LUSSAO
materially improved the process, and indeed brought it to the
highest degree of perfection (measuring of the solution of sodium
chloride in the assay of silver in the wet way). More recently
F. MOHR f has bestowed much care and ingenuity upon the pro-
duction of appropriate and convenient measuring apparatus, and
has added to our store the eminently practical pinch-cock burette.
The process is now resorted to even in most accurate scientific
investigations, since it requires much less time than the process of

The accuracy of all measurings depends upon the proper con-

* A detailed description of BUNSEN'S method is to be found in the Iland-
worterbuch der Chemie, by LIEBIG, POGGENDORFF, and WOHLER, n, 1053
(KOLBE'S Eudiometer), and i, 2, 2d edit., 930 (Volumetric Analysis of Gases, by
KOLBE and FRANKLAND). BUNSEN, further, wrote a valuable monograph on
this subject under the title Gasometric Methods, and published by FR. VIEWEG
& SON, Brunswick, 1857, and which was translated by ROSCOE. The methods
of gas measurement employed by REGNAULT and REISET, as well as by FRANK-
LAND and WARD, differ from the improved BUNSEN method in that in the
former the measuring tubes stand in cylinders filled with water, whereby the
temperature of the gas is brought in a few minutes to that of the water, thus
materially shortening the time required in gas analysis. In the FRANKLAND-
WARD method the gasometric determination is also independent of the atmos-
pheric pressure. Both methods, as a matter of course, require complicated and
expensive apparatus. These are minutely described and figured in the above-
mentioned article by FRANKLAND. The WILLIAMSON-RUSSELL gasometric
apparatus is described in the Jour. Cham. Soc., xvn, 238; and RUSSELL'S modi-
fication, ibid. (2), vi, 128, also in Zeitsc7tr. f. anatyt. Chem., vn, 454.

t LehrbucTi far Titrirmethode, Dr. Fr. MOHR.


struction pf the measuring vessels, and also upon the mariner in
which the process is conducted.


We use for the measuring of gases graduated tubes of greater
or less capacity, made of strong glass, and sealed at one end, which
should be rounded. The following tubes will be found sufficient
for all the processes of gas measuring required in organic elementary
analyses :

1. A bell-glass capable of holding from 150 to 250 c. c,, and
about 4 centimetres in diameter, and divided into cubic centimetres.

2. Five or six glass tubes of about 12 to 15 millimetres bore
diameter, and capable of holding from 30 to 40 c. c. each, divided
into 0*2 c. c.

The sides of these tubes should be fairly thick, otherwise they
will be liable to break, especially when used to measure over mer-
cury. The sides of the bell-glass should be about 3, of the tubes
about 2, millimetres thick.

The most important point, however, in connection with meas-
uring instruments is that they be correctly graduated, since upon
this of course depends the accuracy of the results. For the method
of graduating consult BERZELIUS' " Lehrbuch der Chemie^ 4th
ed., x, under Messenj also GEEVILLE WILLIAMS' " Chemical
Manipulation. ' '

In testing the measuring tubes we have to consider three
questions :

1. Do the divisions of a tube correspond with each other?

?/. Do the divisions of each tube correspond with those of the
other tubes ?

3. Do the volumes expressed by the graduation lines corre-
spond with the weights used by the analyst ?

These three questions are answered by the following experi-
ments :

a. The tube which it is intended to examine is placed in a per-
pendicular position, and filled gradually with accurately measured
small quantities of mercury, care being taken to ascertain with the
utmost precision whether the graduation of the tube' is proportion-
ate to the equal volumes of mercury poured in. The measuring-

28 OPERATIONS. [ 12.

off of the mercury is effected by means of a small glass tube, sealed
at one end, and ground perfectly even and smooth at the other.
This tube is filled to overflowing by immersion under mercury,
care being taken to allow no air bubbles to remain in it; the
excess of mercury is then removed by pressing a small glass plate
down on the smooth edge of the tube.*

5. Different quantities of mercury are successively measured
off in one of the smaller tubes, and then transferred into the other
tubes. The tubes may be considered in perfect accordance with
each other, if the mercury reaches invariably the same divisional
point in every one of them.

Such tubes as are intended simply to determine the relative
volume of different gases, need only pass these two experiments ;
but in cases where we want to calculate the weight of a gas from
its volume, it is necessary, also to obtain an answer to the third
question. For this purpose

c. One of the tubes is accurately weighed and then filled with
distilled water of a temperature of IT '5 to the last mark of the
graduated scale ; the weight of the water is then accurately deter-
mined. If the tube agrees with the weight, every 100 c. c. of
water at 17*5 must weigh 99 '78 grammes, f Should it not agree,
no matter whether the error is due to faulty weights or incorrect
graduations, we must apply a correction to the volume observed
before calculating the weight therefrom. For instance, if 100 c. c.
had been found to weigh 100 grammes assuming our weight to be
perfect then the c. c. divisions would be too large, and to convert
100 c. c. into normal c. c. the following calculation would have to
be made :

99-78 : 100 :: 100 : x.

In gas analysis proper by BUNSEN'S methods (the simplest and
most accurate) a suitable eudiometer is indispensable. BUNSKN'S
eudiometer, Fig. 3, is a glass tube 500 to 600 mm. long, with a
bore of 20 mm., as uniform as possible throughout, and the thick-
ness of the glass not exceeding 2 mm. At the upper, closed end

* As warming the metal is to be carefully avoided in this process, it is .-id vis-
able not to hold the tube with the hand in immersing it in the mercury, but to
fasten it in a small wooden holder.

f A gramme is the weight of 1 c. c. of water in vacuo at 4.



of the tube^ there are sealed in at opposite sides two fine platinum
wires. These wires are _ bent internally to lie close to the walls of

Fig. 3.

Fig. 4.

the tube, and approach each other at the apex of the tube until
separated by a distance of 1 to 2 mm.

The tube is graduated in millimetre divisions by means of an
ingenious divider. The volumes corresponding to the several divi-
sions are then determined by measurement with equal volumes of
mercury, and noted down in a table'. This method of dividing and
adjusting is unquestionably the most accurate.

Besides this large eudiometer there is required also a short one,
Fig. 4, similarly graduated in millimetres, and slightly curved at
the lower end. Its length is 250 mm., and its bore 20 mm. in
diameter ; the glass should be 2 mm. thick.

BUNSEN'S method of gas analysis requires a laboratory with a
northern exposure and uniform temperature, and consumes much
time because of the slow cooling of the gases. In order to adapt
the method for the use of those who do not possess a suitable labo-
ratory, and to shorten the time, O. KERSTEN * recommends that
the BUNSEN eudiometer be provided with a screw stopper like that
in BUNSEN'S absorptiometer tube,f and that the readings should be
taken after immersing the eudiometer in water. The same result
is obtained in another manner in the eudiometer recommended by

In measuring gases attention must be given to the following
points: 1. Correct reading of the results; 2, the temperature of
the gas; 3, the pressure under which the gas is confined; and 4,
the circumstance whether the gas is dry or moist. The three last
points will be readily understood when it is remembered that a

*Zeitschr.f. analyt. Ghem., i, 281.
f BUNSEN, Gasometr. Meth., p. 147.
\Zeitschr.f. analyt. Ghem., vn, 86.

30 OPERATIONS. [ 13.

given weight of gas undergoes considerable alteration in volume by
changes in temperature or pressure, as well as from greater or less
tension of the admixed aqueous vapor.


1. CORRECT READING OF RESULTS. When mercury is introduced
into a tube, it exhibits a convex surface, because of its cohesion,
the phenomenon being particularly striking in a narrow glass tube.
On the other hand water, under similar circumstances, exhibits a
concave surface, owing to the attraction between the tube- wall and
the water. These circumstances render accurate reading-oft' diffi-

Online LibraryC. Remigius FreseniusQuantitative chemical analysis → online text (page 3 of 69)