An Introductory Course of Quantitative Chemical Analysis | Page 5

of all material; but it should be remembered that this can be
accomplished better by the use of successive small portions of
wash-water (perhaps 5-10 cc.), if each wash-water is allowed to drain
away for a few seconds, than by the addition of large amounts which
unnecessarily increase the volume of the solutions, causing loss of time
in subsequent filtrations or evaporations.
All stirring rods employed in quantitative analyses should be rounded
at the ends by holding them in the flame of a burner until they begin to
soften. If this is not done, the rods will scratch the inner surface of
beakers, causing them to crack on subsequent heating.
EVAPORATION OF LIQUIDS
The greatest care must be taken to prevent loss of solutions during
processes of evaporation, either from too violent ebullition, from
evaporation to dryness and spattering, or from the evolution of gas
during the heating. In general, evaporation upon the steam bath is to be
preferred to other methods on account of the impossibility of loss by
spattering. If the steam baths are well protected from dust, solutions
should be left without covers during evaporation; but solutions which
are boiled upon the hot plate, or from which gases are escaping, should
invariably be covered. In any case a watch-glass may be supported
above the vessel by means of a glass triangle, or other similar device,
and the danger of loss of material or contamination by dust thus be
avoided. It is obvious that evaporation is promoted by the use of
vessels which admit of the exposure of a broad surface to the air.
Liquids which contain suspended matter (precipitates) should always
be cautiously heated, since the presence of the solid matter is frequently
the occasion of violent "bumping," with consequent risk to apparatus

and analysis.


PART II
VOLUMETRIC ANALYSIS
The processes of volumetric analysis are, in general, simpler than those
of gravimetric analysis and accordingly serve best as an introduction to
the practice of quantitative analysis. For their execution there are
required, first, an accurate balance with which to weigh the material for
analysis; second, graduated instruments in which to measure the
volume of the solutions employed; third, standard solutions, that is,
solutions the value of which is accurately known; and fourth, indicators,
which will furnish accurate evidence of the point at which the desired
reaction is completed. The nature of the indicators employed will be
explained in connection with the different analyses.
The process whereby a !standard solution! is brought into reaction is
called !titration!, and the point at which the reaction is exactly
completed is called the !end-point!. The !indicator! should show
the !end-point! of the !titration!. The volume of the standard solution
used then furnishes the measure of the substance to be determined as
truly as if that substance had been separated and weighed.
The processes of volumetric analysis are easily classified, according to
their character, into:
I. NEUTRALIZATION METHODS; such, for example, as those of
acidimetry and alkalimetry.
II. OXIDATION PROCESSES; as exemplified in the determination of
ferrous iron by its oxidation with potassium bichromate.
III. PRECIPITATION METHODS; of which the titration for silver
with potassium thiocyanate solution is an illustration.

From a somewhat different standpoint the methods in each case may be
subdivided into (a) DIRECT METHODS, in which the substance to be
measured is directly determined by titration to an end-point with a
standard solution; and (b) INDIRECT METHODS, in which the
substance itself is not measured, but a quantity of reagent is added
which is known to be an excess with respect to a specific reaction, and
the unused excess determined by titration. Examples of the latter class
will be pointed out as they occur in the procedures.
MEASURING INSTRUMENTS
THE ANALYTICAL BALANCE
For a complete discussion of the physical principles underlying the
construction and use of balances, and the various methods of weighing,
the student is referred to larger manuals of Quantitative Analysis, such
as those of Fresenius, or Treadwell-Hall, and particularly to the
admirable discussion of this topic in Morse's !Exercises in Quantitative
Chemistry!.
The statements and rules of procedure which follow are sufficient for
the intelligent use of an analytical balance in connection with processes
prescribed in this introductory manual. It is, however, imperative that
the student should make himself familiar with these essential features
of the balance, and its use. He should fully realize that the analytical
balance is a delicate instrument which will render excellent service
under careful treatment, but such treatment is an essential condition if
its accuracy is to be depended upon. He should also understand that no
set of rules, however complete, can do away with the necessity for a
sense of personal responsibility, since by carelessness he can render
inaccurate not only his own analyses, but those of all other students