An Introductory Course of Quantitative Chemical Analysis | Page 3

analysis are subdivided, according to their
nature, into those of !gravimetric analysis, volumetric analysis!,
and !colorimetric analysis!. In !gravimetric! processes the constituent to
be determined is sometimes isolated in elementary form, but more
commonly in the form of some compound possessing a
well-established and definite composition, which can be readily and
completely separated, and weighed either directly or after ignition.
From the weight of this substance and its known composition, the
amount of the constituent in question is determined.
In !volumetric! analysis, instead of the final weighing of a definite
body, a well-defined reaction is caused to take place, wherein the
reagent is added from an apparatus so designed that the volume of the
solution employed to complete the reaction can be accurately measured.
The strength of this solution (and hence its value for the reaction in
question) is accurately known, and the volume employed serves,
therefore, as a measure of the substance acted upon. An example will
make clear the distinction between these two types of analysis. The
percentage of chlorine in a sample of sodium chloride may be
determined by dissolving a weighed amount of the chloride in water
and precipitating the chloride ions as silver chloride, which is then
separated by filtration, ignited, and weighed (a !gravimetric! process);
or the sodium chloride may be dissolved in water, and a solution of
silver nitrate containing an accurately known amount of the silver salt
in each cubic centimeter may be cautiously added from a measuring
device called a burette until precipitation is complete, when the amount
of chlorine may be calculated from the number of cubic centimeters of
the silver nitrate solution involved in the reaction. This is a !volumetric!
process, and is equivalent to weighing without the use of a balance.
Volumetric methods are generally more rapid, require less apparatus,
and are frequently capable of greater accuracy than gravimetric
methods. They are particularly useful when many determinations of the
same sort are required.
In !colorimetric! analyses the substance to be determined is converted

into some compound which imparts to its solutions a distinct color, the
intensity of which must vary in direct proportion to the amount of the
compound in the solution. Such solutions are compared with respect to
depth of color with standard solutions containing known amounts of the
colored compound, or of other similar color-producing substance which
has been found acceptable as a color standard. Colorimetric methods
are, in general, restricted to the determinations of very small quantities,
since only in dilute solutions are accurate comparisons of color
possible.

GENERAL DIRECTIONS
The following paragraphs should be read carefully and thoughtfully. A
prime essential for success as an analyst is attention to details and the
avoidance of all conditions which could destroy, or even lessen,
confidence in the analyses when completed. The suggestions here given
are the outcome of much experience, and their adoption will tend to
insure permanently work of a high grade, while neglect of them will
often lead to disappointment and loss of time.
ACCURACY AND ECONOMY OF TIME
The fundamental conception of quantitative analysis implies a necessity
for all possible care in guarding against loss of material or the
introduction of foreign matter. The laboratory desk, and all apparatus,
should be scrupulously neat and clean at all times. A sponge should
always be ready at hand, and desk and filter-stands should be kept dry
and in good order. Funnels should never be allowed to drip upon the
base of the stand. Glassware should always be wiped with a clean,
lintless towel just before use. All filters and solutions should be
covered to protect them from dust, just as far as is practicable, and
every drop of solution or particle of precipitate must be regarded as
invaluable for the success of the analysis.
An economical use of laboratory hours is best secured by acquiring a
thorough knowledge of the character of the work to be done before
undertaking it, and then by so arranging the work that no time shall be
wasted during the evaporation of liquids and like time-consuming
operations. To this end the student should read thoughtfully not only
the !entire! procedure, but the explanatory notes as well, before any

step is taken in the analysis. The explanatory notes furnish, in general,
the reasons for particular steps or precautions, but they also
occasionally contain details of manipulation not incorporated, for
various reasons, in the procedure. These notes follow the procedures at
frequent intervals, and the exact points to which they apply are
indicated by references. The student should realize that a !failure to
study the notes will inevitably lead to mistakes, loss of time, and an
inadequate understanding of the subject!.
All analyses should be made in duplicate, and in general a close
agreement of results should be expected. It should, however, be