C. Remigius Fresenius.

Quantitative chemical analysis online

. (page 7 of 69)
Online LibraryC. Remigius FreseniusQuantitative chemical analysis → online text (page 7 of 69)
Font size
QR-code for this ebook


4

32.

It is important, in the first place, to observe that we embrace
in the following general analytical method only the separation and
determination of the metals and their combinations with the
metalloids, and of the inorganic acids and salts. With respect to
the quantitative analysis of other compounds, it is not easy to lay
down a universally applicable method, except that their constitu-
ents usually require to be converted first into acids or bases, before
their separation and estimation can be attempted ; this is the case,
for instance, with phosphorus sulphide, sulphur chloride, iodine
chloride, nitrogen sulphide, &c.

The quantitative analysis of a substance presupposes an
accurate knowledge of its properties, and of the nature of its
several constituents. These data will enable the operator at once
to decide whether the direct estimation of each individual constitu-
ent is necessary; whether he need operate only on one portion
of the substance, or whether it would be advantageous to deter-
mine each constituent in different portions. Let us suppose, for
instance, we have a mixture of sodium chloride and anhydrous
sodium sulphate, and wish to ascertain the proportion in which
these two substances are mixed. Here it would be superfluous to
determine each constituent directly, since the determination either
of the quantity of the chlorine, or of the sulphuric acid, is quite
sufficient to answer the purpose ; still the estimation of both the
chlorine and the sulphur trioxide will afford us an infallible con-
trol for the correctness of our analysis ; since the united weights
of these two substances, added to the sodium and soda respectively
equivalent to them, must be equal to the weight of the substance
taken.

These estimations may be made, either in one and the same
portion of the mixture, by first precipitating the sulphuric acid
with barium nitrate, and subsequently the hydrochloric acid from
the filtrate with solution of silver nitrate ; or a separate portion of
the mixture may be appropriated to each of these two operations.
Unless there- is some objection to its use (e.g., deficiency or hetero-
geneousness of substance), the latter" method is more convenient,



70 OPERATIONS. [ 33.

and generally yields more accurate results ; since, in the former
method, the unavoidable washing of the first precipitate swells the
amount of liquid so considerably that the analysis is thereby
delayed, and, moreover, loss of substance less easily guarded
against.

Before beginning all analyses, at least those of a more complex
nature, the student should write out an exact plan, and accurately
note on paper, during the entire process, everything that he does.
It is in the highest degree unwise to rely on the memory in a com-
plicated analysis. When students, who imagine they can do so,
come, a week or a fortnight after tjiey have begun their anah >is,
to work out the results, they find generally too late that they have
forgotten much, which now appears to them of importance to
know. The intelligent pursuit of chemical analysis consists in the
projecting and accurate testing of the plan ; acuteness and the
power of passing in review all the influencing chemical relations
must here support each other. He who works without a thor-
oughly thought-out plan, lias no right to say he is practising chem-
istry ; for a mere unthinking stringing together of a series of nitra-
tions, evaporations, ignitions, and weighings, howsoever well these
several operations may be performed, is not chemistry.

We will now proceed to describe the various operations consti-
tuting the process of quantitative analysis.

33.
1. WEIGHING THE SUBSTANCE.

The amount of matter required for the quantitative analysis of a
substance depends upon the nature of its constituents; it is, there-
fore, impossible to lay down rules for guidance on this point.
Haifa gramme of sodium chloride, and even less, is sufficient to
effect the estimation of the chlorine. For the quantitative analy-
sis of a mixture {' <-<>mm<>n salt and anhydrous sodium sulphate, 1
gramme will suffice; whereas, in the case of ashes of plants, com-
plex minerals. Are., 3 or 4 grammes, and even more, are required.
1 to 3 gnu. can therefore be indicated as the average quantity
suitable in most cases. For the estimation of eonstituents present
in very minute pro portions only, as, for instance, sodium and
potassium in limestones, phosphorus or sulphur in cast-iron, tv.e.,
much -n-ater quantities are often required 10, 20, or TM grammes.



33.] WEIGHING THE SUBSTANCE. 71

The greater the amount of substance taken the more accurate
will be the analysis ; the smaller the quantity, the sooner, as a rule,
will the analysis be finished. We would advise the student to
endeavor to combine accuracy with economy of time. The less
substance he takes to operate upon, the more carefully he ought to
weigh ; the larger the amount of substance, the less harm can
result from slight inaccuracies in weighing. Somewhat large
quantities of substance are generally weighed to 1 milligramme ;
minute quantities, to 0*1 milligramme.

If one portion of a substance is to be weighed off, we first
weigh two watch-glasses which* fit on each other, or else an empty
platinum crucible with lid, then we put some substance in, and
weigh again ; the difference between, the two weighings gives tho
weight of the substance taken.

o

This mode of weighing off, however, is advisable only when
the substance is to be further treated in the watch-glasses or
crucible, or when the substance is not of an adherent nature, or
when the adherent particles may be washed away with
water. If the substance is to be transferred to a flask
or beaker, and treated with a concentrated solvent, the
weighing is best done in a small tube sealed at one end.
In this case the approximate weight of the tube should
be known. After receiving the substance, the tube and
its contents are carefully weighed; then nearly the
whole, or a suitable quantity, of the substance is shaken
out into the flask or beaker, the weight again taken, and
the difference in weight, showing the quantity of sub-
stance taken, noted. When the substances handled are
Wroscopic, the tube must be closed; this is easily ac-
cmnplished by inserting the tube into one slightly larger, as

in Fig. 43.

If several quantities of a substance are to be open
the best way is to weigh off the several portions success
which may be accomplished most readily by weighing ma
tube, or oLr appropriate v,^ ^ **-,

ata^^A^fi * * -* -

""The work may often also be materially lightened by weighing
off a larger portion of the substance, dissolving Oil



Fig. 48.



72 OPERATIONS. [ 34, 35.

litre, and taking out for the several estimations aliquot parts, with
the 50- or 100-c. c. pipette. The first and most essential condition
of this proceeding, of course, is that the pipettes must accurately
correspond with the measuring flasks ( 18 and 20).

34.
2. ESTIMATION OF THE WATER.

If the substance to be examined after having been freed from
moisture by a suitable drying process ( 26-32) contains water,
it is usual to begin by determining the amount of this water. This
operation is generally simple ; in some instances, however, it has
its difficulties. This depends upon various circumstances, viz.,
whether the compounds intended for analysis yield their water
readily or not ; whether they can bear a red heat without suffering
decomposition ; or whether, on the contrary, they give off other
volatile substances, besides water, even at a lower temperature.

The correct 'knowledge of the constitution of a compound
depends frequently upon the accurate estimation of the water con-
tained in it; in many cases for instance, in the analysis of the
salts of known acids the estimation of the water contained in the
analyzed compound suffices to enable us to deduce the formula.
The estimation of the water contained in a substance is, therefore,
one of the most important, as well as most frequently occurring
operations of quantitative analysis. The proportion of water con-
tained in a substance may be determined in two ways, viz.. tt, from
the diminution of weight coiiM-qm-nt upon the expulsion of the
water; &, by weighing the amount of water expelled.

35.
a. ESTIMATION OF THE WATER FROM THE Loss OF WEIGHT.

This method, <,n account of its simplicity, is most frequently
employed. Tin- modus operandi depends upon the nature of the
substance under examination.

a. The *?>*/</,; "bears /<////'//'</< without losing other
besides I!'///,/-, ,/,,/



The sultance is weighed in a platinum or porcelain crucible,
nd placed over the gas- or spirit-lamp : the heat should be very



35.] ESTIMATION OF WATER. 73

gentle at first, and gradually increased. When the crucible has
been maintained some time at a red heat, it is allowed to cool a
little, put still warm under the desiccator, and finally weighed when
cold. The ignition is then repeated, and the weight again ascer-
tained. If no further diminution of weight has taken place, tin-
process is at at end, the desired object being fully attained. But
if the weight is less than after the first heating, the operation must
be repeated until the weight remains constant.

In the case of silicates, the heat must be raised to a very high
degree, since many of them (e.g., talc, steatite, nephrite) only begin
at a red heat to give off water, and require a yellow heat for the
complete expulsion of that constituent. (Tn. SCHEERER.*) Such
bodies are therefore ignited over a blast-lamp. The flame should
be observed ; if it be colored, it indicates some volatilization of
alkali.

In the case of substances that have a tendency to puff off, or to
spirt, a small fiask or retort may sometimes be advantageously sub-
stituted for the crucible. Care must be taken to remove the last
traces of aqueous vapor from the vessel, by suction through a glass
tube.

Decrepitating salts (sodium chloride, for instance) are put-
finely pulverized, if possible in a small covered platinum crucible,
which is then placed in a large one, also covered ; the whole is
weighed, then heated, gently at first for some time, then more
strongly ; finally, after cooling, weighed agaiu.

?. The substance loses on ignition other Constituents besides
' Water (Boric Acid, Sulphuric Acid, Silicon Fluoride, <&c.).

Here the analyst has to consider, in the first place, whether the
water may not be expelled at a lower degree of heat, which does
not involve the loss of other constituents. If this may be done,
the substance is heated either in the water-bath, or where a higher
temperature is required, in the air-bath or oil-bath, the tempera-
ture being regulated by the thermometer. The expulsion of the
water may be promoted by the co-operation of a current of air
(compare ' 29 and 30) ; or by the addition of pure dry san<
the substance, to keep it porous.f The process must be continued
under these circumstances also, until thevein at



*Jahresber. vonLiEBiG u. KOPP, 1851, 610.
| Ann. d. Chem. u. Pharm., LIII, 233.



74 OPERATIONS. [ 35.

In cases where, for some reason or other, such gentle heating
is insufficient, the analyst has to consider whether the desired end
may not be attained at a red heat, by adding some substance that
will retain the volatile constituent whose loss is apprehended.
Thus, for instance, the crystallized aluminium sulphate loses at a
red heat, besides water, also sulphuric acid; now, the loss of the
latter constituent may be guarded against by adding to the sul-
phate an excess (about six times the quantity) of finely pulverized,
recently ignited, pure lead oxide. But the addition of this sub-
stance will not prevent the escape of silicon fluoride from silicates
when exposed to a red heat (Lisx *).

Thus, again, the amount of water in commercial iodine may
be determined by triturating the iodine together with eight times
the quantity of mercury, and drying the mixture at 100 (BoL-
LEY, DINGLER'S Polytech. Journ., cxxvi, 39).

For the determination of water in silicofluorides magnesia is
added to the substance. For this purpose about twice as much
magnesia as is required for decomposing the silicofluoride is ig-
nited in a platinum crucible, weighed, stirred with warm water
to a thick paste, the weighed silicofluoride added, the whole
stirred with a platinum wire of known weight, more water added
if necessary to effect solution, and the mixture then carefully
dried and ignited. The loss of weight represents the water con-
tained in the silicofluoride, since the decomposition products
magnesium fluoride, silicic acid, and metallic oxide weigh as
much as the anhydrous silicofluoride plus the magnesia. A cor-
rection in this case would be necessary only when the separated
metallic oxide, e.g., ferrous oxide, takes up atmospheric oxygen
on ignition (F. STOLBA f ).

y. The substance contains several differently combined quantities
of Water which require different Degrees of Temperature
for Expulsion.

Substances of this nature are heated first in the water-bat li,
until their weight remains constant; they are then exposed in the
oil- or air-bath to 150, 200, or 250, &c., and finally, when



*Ann. d. Cftem. u. Pharm., LxXxi, 189.
\Zeitschr.f. analyt. Chem., vn, 93,



36.] ESTIMATION OF WATER. 75

practicable, ignited over a gas- or spirit-lamp. In such opera-
tions I prefer to use the apparatus illustrated in Fig. 38.

The bulb-tube may also be replaced, if desired, by a tube of
uniform width in which is slid a small porcelain boat containing
the substance. In order to prevent a desiccated substance from
attracting water during the weighing, the boat is weighed in a
cork-stoppered glass tube.

In this manner differently combined quantities of water may
be distinguished, and their respective amounts correctly esti-
mated. Thus, for instance, crystallized copper sulphate contains
28-87 per cent, of water, which escapes at a temperature below
140, and 7'22 per cent., which escapes only at a temperature
between 220 and 260.

It is frequently advisable to assist the action of heat by the
aid of a partial vacuum. Thus magnesium sulphate in vacuo
over sulphuric acid at 100 loses 5 equivalents of water; dried
in the air at 132 it loses 6, and at alow red heat 7, equivalents.

tf. When the substance has a tendency to absorb oxygen (from
the presence of ferrous compounds, for instance) the water is bet-
ter determined in the direct way than by the loss. ( 36.)

36.
"b. ESTIMATION OF WATER BY DIRECT WEIGHING.

This method is resorted to by way of control, or in the case of
substances which, upon ignition, lose, besides water, other con-
stituents, which cannot be retained even by the addition of some
other substance (e.g., carbon dioxide, oxygen), or in the case of
substances containing bodies inclined to oxidation (e.g., ferrous
compounds). The principle of the method is to expel the water
by the application of a red heat, so as to admit of the condensa-
tion of the aqueous vapor, and the collection of the condensed
water in an appropriate apparatus, partly physically, partly by
the agency of some hygroscopic substance. The increase in the
weight of this apparatus represents the quantity of the water ex-
pelled.

The operation may be conducted in various ways ; the follow-
ing apparatus, Fig. 44, is one of the most appropriate :



76



OPERATIONS.



[36.



B, Fig. 44, represents a gasometer filled with air ; I a flask
half-filled with concentrated sulphuric acid ; c and a o are calcium-
fhloride tubes ( 66, 7) ; dis a bulb-tube of highly infusible glass.




Fig. 44.

The sub&t&nce intended for examination is weighed in the
perfectly dry tube d, which is then connected with c and the
weighed calcium chloride tube ao, by means of sound and well-
dried perforated corks.

The operation is commenced by opening the stop-cocl< of the
gasometer a little, to allow the air, which loses all its moisture in 7>
and c, to pass slowly through d\ the tube d is then heated to !><>-
yond the boiling-point of water, by holding a lamp towards /*,
taking cure not to burn the cork; and finally, the bulb which con-
tains the substance is exposed to a low red heat, the temperature
at/" being maintained all the while at the point indicated. When
the expulsion of the water has been accomplished, a slow current
of air is still kept up till the bulb-tube is cold ; the apparatus is
then disconnected, and the calcium chloride tube a <>. weighed.
The increase in the weight of this tube represents the quantity of
water originally present in the sultance examined.

The empty hulh <t, in which the greater portion of the water
collect-. ha> not only for its object to prevent the liquefaction c f.
the calcium chloride, hut enaMes the analyst also to test the con
densed water as to its reaction and purity.



36.] ESTIMATION OF WATER. 77

The apparatus may, of course, be modified in various ways;
thus, tliQ chloride of calcium tubes may be U-shaped ; a U-tube,
filled with pieces of pumice-stone saturated with sulphuric acid,
may be substituted for the flask with sulphuric acid; and the gas-
ometer may be replaced by an aspirator (Fig. 34) joined to o. The
calcium-chloride tube c is, however, absolutely indispensable, be-
cause I have found * that air dried by sulphuric acid takes up a
slight quantity of moisture from well-dried calcium chloride, i.e.,
the sulphuric-acid dried air is converted into a calcium- chloride
dried air. Were the tube c omitted, the water-content of the
substance would hence be found to be too low, and by just the
quantity of moisture taken up by tho sulphuric acid from the
dried calcium-chloride tube. On interposing 0, however, the air
is dried by calcium chloride both before and after passing over
the substance, hence the increase in weight of ao will give the
exact water-content of the substance.'




Fig. 45.

The expulsion of the aqueous vapor from the tube containing
the substance under examination, into the calcium chloride tube,
may be effected also by other means than a current of air sup-
plied by a gasometer or aspirator ; viz., the substance under ex-
amination may be heated to redness in a perfectly dry tube, to-
gether with lead carbonate, since the carbon dioxide escaping
from the latter at a red heat, serves here the same purpose as a
stream of air. This method is principally applied in cases where
it is desirable to retain an acid which otherwise would volatilize
together with the water ; thus, it is applied, for instance, for
the direct estimation of the water contained in acid potassium
sulphate.

* Zeitsclir. f. analyt. Chem., iv, 177,



78 OPERATIONS. [ 36.

Fig. 45 represents the disposition of the apparatus.

a b is a common combustion furnace ; cf a tube filled as fol-
io^ : from c to d with lead carbonate,* from d to e the substance
intimately mixed with lead carbonate, and from e to /pure lead car-
bonate. The calcium-chloride tube g, being accurately weighed,
is connected with the tube cf, by means of a well-dried perfo-
rated cork,/.

The operation is commenced by surrounding the tube with red-
hot charcoal, advancing from f toward c ; the fore part of the
tube which protrudes from the furnace should be maintained at a
degree of heat which barely permits the operator to lay hold of it
with his fingers. All further particulars of this operation will be
found in the chapter on organic elementary analysis. The mix-
ing is performed best in the tube with a wire. The tube cf may
be short and moderately narrow. Of course, the charcoal furnace
may be replaced by a gas combustion furnace.

The volatilization of an acid cannot in all cases be prevented
by lead oxide ; .thus, for instance, we could not determine the
water in crystallized boric acid by the above process. This could
readily be done, however, by igniting the acid mixed with excess
of dry sodium carbonate in a glass tube drawn out behind in the
form of a beak, receiving the water in a calcium -chloride tube,
and transferring the final residue of aqueous vapor into the Ca Cl a
tube by suction, after the point of the beak has been broken off.
(See Organic Analysis.)

The foregoing methods for the direct estimation of water do
not, however, yet embrace all cases in which those described
in 35 are inapplicable ; since they can be employed only if the
substances escaping along with the water are such as will not
wholly or partly condense in the calcium -chloride tube (or in a
tube containing fused potassa, or one filled with pumice-stone satu-
rated with sulphuric acid, which might t>e used instead). Thus
they are perfectly well adapted for determining the water in the
basic zinc carbonate, but they cannot be applied to determine the
water in sodium-ammonium sulphate. "With substances like the
latter, we must either have recourse to the processes of organic
elementary analysis, or we must rest satisfied with the indirect
estimation of the water.

* The lead carbonate must have been previously ignited to incipient decom-
position, and cooled in a closed tube.



37, 38.] SOLUTION-. 79

37.
3. SOLUTION OF SUBSTANCES.

Before pursuing the analytical process further, it is in most
cases necessary to obtain a solution of the substance. This opera-
tion is simple where the body may be dissolved by direct treat-
ment with water, or acids, or alkalies, &c. ; but it is more compli-
cated in cases where the body requires fluxing as an indispensable
preliminary to solution.

When we have mixed substances to operate upon, the compo-
nent parts of which behave differently with solvents, it is not by
any means necessary to dissolve the whole substance at first ; on
the contrary, the separation may, in such cases, be often effected,
in the most simple and expeditious -manner, by the solvents them-
selves. Thus, for instance, a mixture of potassium nitrate, calcium
carbonate, and barium sulphate may be readily and accurately
analyzed by dissolving out, in the first place, the potassium nitrate
with water, and subsequently the calcium carbonate by hydrochloric
acid, leaving the insoluble barium sulphate.

38.
a. DIRECT SOLUTION.

The direct solution of substances is effected, according to cir-
cumstances, in beakers, flasks, or dishes, and may, if necessary, be
promoted by the application of heat ; for which purpose the water-
bath will be found most convenient. In cases where an open fire,
or the sand-bath, or an iron-plate is resorted to, the analyst must
take care to guard against actual ebullition of the fluid, since this-
would render a loss of substance from spirting almost unavoidable,
especially in cases where the process is conducted in a dish. Fluids
containing a sediment, either insoluble, or, at least, not yet dissolved,
will, when heated over the lamp, often bump and spirt even at
temperatures far short of the boiling-point.

In cases where the solution of a substance is attended witlj
evolution of gas, the process is conducted in a flask, placed in $
sloping position, so that the spirting drops may be thrown against
the walls of the vessel, and thus secured from being carried on*
with the stream of the evolved gas ; or it may be conducted in q



80 OPERATIONS, [ 39.

beaker, covered with a large-sized watch-glass, which, after the
solution is effected, and the gas expelled by heating on the water-
bath, must be thoroughly rinsed with the washing-bottle.

In cases where the solution has to be effected by means of con-
centrated volatile acids (hydrochloric acid, nitric acid, aqua regia),
the operation should never be conducted in a dish, but always in a
flask covered with a watch-glass, or placed in a slanting position,
and the application of too high a temperature must be avoided.
The operation should always be conducted also under a hood, with
proper draught, to carry off the escaping acid vapors. All such
arrangements for carrying off vapors as require the flasks to be
closed are not to be recommended ; at times they are even inad-
missible, because the vapors of nitric acid, nitrohydrochloric acid,
etc., powerfully attack cork and caoutchouc, and hence solutions
may become contaminated with organic substances, sulphuric acid
(from oxidation of the sulphur contained in the vulcanized caout-
chouc), and other substances.

It is often necessary, in conducting a process of solution, to
guard against the action of the atmospheric oxygen ; in such cases,



Online LibraryC. Remigius FreseniusQuantitative chemical analysis → online text (page 7 of 69)