This branch of chemistry has for its province the determination of the
constituents of a chemical compound or of a mixture of compounds. Such a
determination is _qualitative_, the constituent being only detected or
proved to be present, or _quantitative_, in which the amount present is
ascertained. The methods of chemical analysis may be classified
according to the type of reaction: (1) _dry_ or _blowpipe analysis_,
which consists in an examination of the substance in the dry condition;
this includes such tests as ignition in a tube, ignition on charcoal in
the blowpipe flame, fusion with borax, microcosmic salt or fluxes, and
flame colorations (in quantitative work the dry methods are sometimes
termed "dry assaying"); (2) _wet analysis_, in which a solution of the
substance is treated with reagents which produce specific reactions when
certain elements or groups of elements are present. In quantitative
analysis the methods can be subdivided into: (a) _gravimetric_, in which
the constituent is precipitated either as a definite insoluble compound
by the addition of certain reagents, or electrolytically, by the passage
of an electric current; (b) _volumetric_, in which the volume of a
reagent of a known strength which produces a certain definite reaction
is measured; (c) _colorimetric_, in which the solution has a particular
tint, which can be compared with solutions of known strengths.
_Historical._--The germs of analytical chemistry are to be found in the
writings of the pharmacists and chemists of the iatrochemical period.
The importance of ascertaining the proximate composition of bodies was
clearly realized by Otto Tachenius; but the first systematic
investigator was Robert Boyle, to whom we owe the introduction of the
term _analysis_. Boyle recognized many reagents which gave precipitates
with certain solutions: he detected sulphuric and hydrochloric acids by
the white precipitates formed with calcium chloride and silver nitrate
respectively; ammonia by the white cloud formed with the vapours of
nitric or hydrochloric acids; and copper by the deep blue solution
formed by a solution of ammonia. Of great importance is his introduction
of vegetable juices (the so-called _indicators_, q.v.) to detect acids
and bases. During the phlogistic period, the detection of the
constituents of compounds was considerably developed. Of the principal
workers in this field we may notice Friedrich Hoffmann, Andreas
Sigismund Marggraf (who detected iron by its reaction with potassium
ferrocyanide, and potassium and sodium by their flame colorations), and
especially Carl Scheele and Torbern Olof Bergman. Scheele enriched the
knowledge of chemistry by an immense number of facts, but he did not
possess the spirit of working systematically as Bergman did. Bergman
laid the foundations of systematic qualitative analysis, and devised
methods by which the metals may be separated into groups according to
their behaviour with certain reagents. This subdivision, which is of
paramount importance in the analysis of minerals, was subsequently
developed by Wilhelm August Lampadius in his _Handbuch zur chemischen
Analyse der Mineralien_ (1801) and by John Friedrich A. Gottling in his
_Praktische Anleitung zur prufenden und zurlegenden Chemie_ (1802).
The introduction of the blowpipe into dry qualitative analysis by Axel
Fredrik Cronstedt marks an important innovation. The rapidity of the
method, and the accurate results which it gave in the hands of a
practised experimenter, led to its systematization by Jons Jakob
Berzelius and Johann Friedrich Ludwig Hausmann, and in more recent times
by K.F. Plattner, whose treatise _Die Probirkunst mit dem Lothrohr_ is a
standard work on the subject. Another type of dry reaction, namely, the
_flame coloration_, had been the subject of isolated notices, as, for
example, the violet flame of potassium and the orange flame of sodium
observed by Marggraf and Scheele, but a systematic account was wanting
until Cartmell took the subject up. His results (_Phil. Mag._ 16, p.
382) were afterwards perfected by Robert Wilhelm Bunsen and Gustav Merz.
Closely related to the flame-colorations, we have to notice the great
services rendered by the spectroscope to the detection of elements.
Rubidium, caesium, thallium, indium and gallium were first discovered by
means of this instrument; the study of the rare earths is greatly
facilitated, and the composition of the heavenly bodies alone
determinable by it.
Quantitative chemistry had been all but neglected before the time of
Lavoisier, for although a few chemists such as Tachenius, Bergman and
others had realized the advantages which would accrue from a knowledge
of the composition of bodies by weight, and had laid down the lines
upon which such determinations should proceed, the experimental
difficulties in making accurate observations were enormous, and little
progress could be made until the procedure was more accurately
determined. Martin Heinrich Klaproth showed the necessity for igniting
precipitates before weighing them, if they were not decomposed by this
process; and he worked largely with Louis Nicolas Vauquelin in
perfecting the analysis of minerals. K.F. Wenzel and J.B. Richter
contributed to the knowledge of the quantitative composition of salts.
Anton Laurent Lavoisier, however, must be considered as the first great
exponent of this branch of chemistry. He realized that the composition
by weight of chemical compounds was of the greatest moment if chemistry
were to advance. His fame rests upon his exposition of the principles
necessary to chemistry as a science, but of his contributions to
analytical inorganic chemistry little can be said. He applied himself
more particularly to the oxygen compounds, and determined with a fair
degree of accuracy the ratio of carbon to oxygen in carbon dioxide, but
his values for the ratio of hydrogen to oxygen in water, and of
phosphorus to oxygen in phosphoric acid, are only approximate; he
introduced no new methods either for the estimation or separation of the
metals. The next advance was made by Joseph Louis Proust, whose
investigations led to a clear grasp of the law of constant proportions.
The formulation of the atomic theory by John Dalton gave a fresh impetus
to the development of quantitative analysis; and the determination of
combining or equivalent weights by Berzelius led to the perfecting of
the methods of gravimetric analysis. Experimental conditions were
thoroughly worked out; the necessity of working with hot or cold
solutions was clearly emphasized; and the employment of small quantities
of substances instead of the large amounts recommended by Klaproth was
shown by him to give more consistent results.
Since the time of Berzelius many experimenters have entered the lists,
and introduced developments which we have not space to mention. We may,
however, notice Heinrich Rose[15] and Friedrich Wohler,[16] who, having
worked up the results of their teacher Berzelius, and combined them with
their own valuable observations, exerted great influence on the progress
of analytical chemistry by publishing works which contained admirable
accounts of the then known methods of analysis. To K.R. Fresenius, the
founder of the _Zeitschrift fur analytische Chemie_ (1862), we are
particularly indebted for perfecting and systematizing the various
methods of analytical chemistry. By strengthening the older methods, and
devising new ones, he exerted an influence which can never be
overestimated. His text-books on the subject, of which the _Qualitative_
appeared in 1841, and the _Quantitative_ in 1846, have a world-wide
reputation, and have passed through several editions.
The quantitative precipitation of metals by the electric current,
although known to Michael Faraday, was not applied to analytical
chemistry until O. Wolcott Gibbs worked out the electrolytic separation
of copper in 1865. Since then the subject has been extensively studied,
more particularly by Alexander Classen, who has summarized the methods
and results in his _Quantitative Chemical Analysis by Electrolysis_
(1903). The ever-increasing importance of the electric current in
metallurgy and chemical manufactures is making this method of great
importance, and in some cases it has partially, if not wholly,
superseded the older methods.
Volumetric analysis, possessing as it does many advantages over the
gravimetric methods, has of late years been extensively developed. Gay
Lussac may be regarded as the founder of the method, although rough
applications had been previously made by F.A.H. Descroizilles and L.N.
Vauquelin. Chlorimetry (1824), alkalimetry (1828), and the volumetric
determination of silver and chlorine (1832) were worked out by Gay
Lussac; but although the advantages of the method were patent, it
received recognition very slowly. The application of potassium
permanganate to the estimation of iron by E. Margueritte in 1846, and
of iodine and sulphurous acid to the estimation of copper and many other
substances by Robert Wilhelm Bunsen, marks an epoch in the early history
of volumetric analysis. Since then it has been rapidly developed,
particularly by Karl Friedrich Mohr and J. Volhard, and these methods
rank side by side in value with the older and more tedious gravimetric
methods.
The detection of carbon and hydrogen in organic compounds by the
formation of carbon dioxide and water when they are burned was first
correctly understood by Lavoisier, and as he had determined the carbon
and hydrogen content of these two substances he was able to devise
methods by which carbon and hydrogen in organic compounds could be
estimated. In his earlier experiments he burned the substance in a known
volume of oxygen, and by measuring the residual gas determined the
carbon and hydrogen. For substances of a difficultly combustible nature
he adopted the method in common use to-day, viz. to mix the substance
with an oxidizing agent--mercuric oxide, lead dioxide, and afterwards
copper oxide--and absorb the carbon dioxide in potash solution. This
method has been improved, especially by Justus v. Liebig; and certain
others based on a different procedure have been suggested. The
estimation of nitrogen was first worked out in 1830 by Jean Baptiste
Dumas, and different processes have been proposed by Will and F.
Varrentrapp, J. Kjeldahl and others. Methods for the estimation of the
halogens and sulphur were worked out by L. Carius (see below, S _Organic
Analysis_).
Only a reference can be made in this summary to the many fields in which
analytical chemistry has been developed. Progress in forensic chemistry
was only possible after the reactions of poisons had been systematized;
a subject which has been worked out by many investigators, of whom we
notice K.R. Fresenius, J. and R. Otto, and J.S. Stas. Industrial
chemistry makes many claims upon the chemist, for it is necessary to
determine the purity of a product before it can be valued. This has led
to the estimation of sugar by means of the polarimeter, and of the
calorific power of fuels, and the valuation of ores and metals, of
coal-tar dyes, and almost all trade products.
The passing of the Food and Drug Acts (1875-1899) in England, and the
existence of similar adulteration acts in other countries, have
occasioned great progress in the analysis of foods, drugs, &c. For
further information on this branch of analytical chemistry, see
ADULTERATION.
There exists no branch of technical chemistry, hygiene or pharmacy from
which the analytical chemist can be spared, since it is only by a
continual development of his art that we can hope to be certain of the
purity of any preparation. In England this branch of chemistry is
especially cared for by the Institute of Chemistry, which, since its
foundation in 1877, has done much for the training of analytical
chemists.
In the preceding sketch we have given a necessarily brief account of the
historical development of analytical chemistry in its main branches. We
shall now treat the different methods in more detail. It must be
mentioned here that the reactions of any particular substance are given
under its own heading, and in this article we shall only collate the
various operations and outline the general procedure. The limits of
space prevent any systematic account of the separation of the rare
metals, the alkaloids, and other classes of organic compounds, but
sources where these matters may be found are given in the list of
references.
_Qualitative Inorganic Analysis._
Dry methods.
The dry examination of a substance comprises several operations, which
may yield definite results if no disturbing element is present; but it
is imperative that any inference should be confirmed by other methods.
1. Heat the substance in a hard glass tube. Note whether any moisture
condenses on the cooler parts of the tube, a gas is evolved, a sublimate
formed, or the substance changes colour.
Moisture is evolved from substances containing water of
crystallization or decomposed hydrates. If it possesses an alkaline or
acid reaction, it must be tested in the first case for ammonia, and in
the second case for a volatile acid, such as sulphuric, nitric,
hydrochloric, &c.
Any evolved gas must be examined. Oxygen, recognized by its power of
igniting a glowing splinter, results from the decomposition of oxides
of the noble metals, peroxides, chlorates, nitrates and other highly
oxygenized salts. Sulphur dioxide, recognized by its smell and acid
reaction, results from the ignition of certain sulphites, sulphates,
or a mixture of a sulphate with a sulphide. Nitrogen oxides,
recognized by their odour and brown-red colour, result from the
decomposition of nitrates. Carbon dioxide, recognized by turning
lime-water milky, indicates decomposable carbonates or oxalates.
Chlorine, bromine, and iodine, each recognizable by its colour and
odour, result from decomposable haloids; iodine forms also a black
sublimate. Cyanogen and hydrocyanic acid, recognizable by their odour,
indicate decomposable cyanides. Sulphuretted hydrogen, recognized by
its odour, results from sulphides containing water, and
hydrosulphides. Ammonia, recognizable by its odour and alkaline
reaction, indicates ammoniacal salts or cyanides containing water.
A sublimate may be formed of: sulphur--reddish-brown drops, cooling to
a yellow to brown solid, from sulphides or mixtures; iodine--violet
vapour, black sublimate, from iodides, iodic acid, or mixtures;
mercury and its compounds--metallic mercury forms minute globules,
mercuric sulphide is black and becomes red on rubbing, mercuric
chloride fuses before subliming, mercurous chloride does not fuse,
mercuric iodide gives a yellow sublimate; arsenic and its
compounds--metallic arsenic gives a grey mirror, arsenious oxide forms
white shining crystals, arsenic sulphides give reddish-yellow
sublimates which turn yellow on cooling; antimony oxide fuses and
gives a yellow acicular sublimate; lead chloride forms a white
sublimate after long and intense heating.
If the substance does not melt but changes colour, we may have
present: zinc oxide--from white to yellow, becoming white on cooling;
stannic oxide--white to yellowish brown, dirty white on cooling; lead
oxide--from white or yellowish-red to brownish-red, yellow on cooling;
bismuth oxide--from white or pale yellow to orange-yellow or
reddish-brown, pale yellow on cooling; manganese oxide--from white or
yellowish white to dark brown, remaining dark brown on cooling (if it
changes on cooling to a bright reddish-brown, it indicates cadmium
oxide); copper oxide--from bright blue or green to black; ferrous
oxide--from greyish-white to black; ferric oxide--from brownish-red to
black, brownish-red on cooling; potassium chromate--yellow to dark
orange, fusing at a red heat.
2. Heat the substance on a piece of charcoal in the reducing flame of
the blowpipe.
([alpha]) The substance may fuse and be absorbed by the charcoal; this
indicates more particularly the alkaline metals.
([beta]) An infusible white residue may be obtained, which may denote
barium, strontium, calcium, magnesium, aluminium or zinc. The first
three give characteristic flame colorations (see below); the last
three, when moistened with cobalt nitrate and re-ignited, give
coloured masses; aluminium (or silica) gives a brilliant blue; zinc
gives a green; whilst magnesium phosphates or arsenate (and to a less
degree the phosphates of the alkaline earths) give a violet mass.
A metallic globule with or without an incrustation may be obtained.
Gold and copper salts give a metallic bead without an incrustation. If
the incrustation be white and readily volatile, arsenic is present, if
more difficultly volatile and beads are present, antimony; zinc gives
an incrustation yellow whilst hot, white on cooling, and volatilized
with difficulty; tin gives a pale yellow incrustation, which becomes
white on cooling, and does not volatilize in either the reducing or
oxidizing flames; lead gives a lemon-yellow incrustation turning
sulphur-yellow on cooling, together with metallic malleable beads;
bismuth gives metallic globules and a dark orange-yellow incrustation,
which becomes lemon-yellow on cooling; cadmium gives a reddish-brown
incrustation, which is removed without leaving a gleam by heating in
the reducing flame; silver gives white metallic globules and a
dark-red incrustation.
3. Heat the substance with a bead of microcosmic salt or borax on a
platinum wire in the oxidizing flame.
([alpha]) The substance dissolves readily and in quantity, forming a
bead which is clear when hot. If the bead is coloured we may have
present: cobalt, blue to violet; copper, green, blue on cooling; in
the reducing flame, red when cold; chromium, green, unaltered in the
reducing flame; iron, brownish-red, light-yellow or colourless on
cooling; in the reducing flame, red while hot, yellow on cooling,
greenish when cold; nickel, reddish to brownish-red, yellow to
reddish-yellow or colourless on cooling, unaltered in the reducing
flame; bismuth, yellowish-brown, light-yellow or colourless on
cooling; in the reducing flame, almost colourless, blackish-grey when
cold; silver, light yellowish to opal, somewhat opaque when cold;
whitish-grey in the reducing flame; manganese, amethyst red,
colourless in the reducing flame. If the hot bead is colourless and
remains clear on cooling, we may suspect the presence of antimony,
aluminium, zinc, cadmium, lead, calcium and magnesium. When present in
sufficient quantity the five last-named give enamel-white beads; lead
oxide in excess gives a yellowish bead. If the hot colourless bead
becomes enamel-white on cooling even when minute quantities of the
substances are employed, we may infer the presence of barium or
strontium.
([beta]) The substance dissolves slowly and in small quantity, and
forms a colourless bead which remains so on cooling. Either silica or
tin may be present. If silica be present, it gives the iron bead when
heated with a little ferric oxide; if tin is present there is no
change. Certain substances, such as the precious metals, are quite
insoluble in the bead, but float about in it.
4. Hold a small portion of the substance moistened with hydrochloric
acid on a clean platinum wire in the fusion zone of the Bunsen burner,
and note any colour imparted to the flame.
Potassium gives a blue-violet flame which may be masked by the
colorations due to sodium, calcium and other elements. By viewing the
flame through an indigo prism it appears sky-blue, violet and
ultimately crimson, as the thickness of the prism is increased. Other
elements do not interfere with this method. Sodium gives an intense
and persistent yellow flame; lithium gives a carmine coloration, and
may be identified in the presence of sodium by viewing through a
cobalt glass or indigo prism; from potassium it may be distinguished
by its redder colour; barium gives a yellowish-green flame, which
appears bluish-green when viewed through green glass; strontium gives
a crimson flame which appears purple or rose when viewed through blue
glass; calcium gives an orange-red colour which appears finch-green
through green glass; indium gives a characteristic bluish-violet
flame; copper gives an intense emerald-green coloration.
5. _Film Reactions._--These reactions are practised in the following
manner:--A thread of asbestos is moistened and then dipped in the
substance to be tested; it is then placed in the luminous point of the
Bunsen flame, and a small porcelain basin containing cold water placed
immediately over the asbestos. The formation of a film is noted. The
operation is repeated with the thread in the oxidizing flame.
Any film formed in the first case is metallic, in the second it is the
oxide. The metallic film is tested with 20% nitric acid and with
bleaching-powder solution. Arsenic is insoluble in the acid, but
immediately dissolves in the bleaching-powder. The black films of
antimony and bismuth and the grey mottled film of mercury are slowly
soluble in the acid, and untouched by bleaching-powder. The black
films of tin, lead and cadmium dissolve at once in the acid, the lead
film being also soluble in bleaching-powder. The oxide films of
antimony, arsenic, tin and bismuth are white, that of bismuth slightly
yellowish; lead yields a very pale yellow film, and cadmium a brown
one; mercury yields no oxide film. The oxide films (the metallic one
in the case of mercury) are tested with hydriodic acid, and with
ammonium sulphide, and from the changes produced the film can be
determined (see F.M. Perkin, _Qualitative Chemical Analysis_, 1905).
Wet methods.
Having completed the dry analysis we may now pass on to the _wet_ and
more accurate investigation. It is first necessary to get the substance
into solution. Small portions should be successively tested with water,
dilute hydrochloric acid, dilute nitric acid, strong hydrochloric acid,
and a mixture of hydrochloric and nitric acids, first in the cold and
then with warming. Certain substances are insoluble in all these
reagents, and other methods, such as the fusion with sodium carbonate
and potassium nitrate, and subsequent treatment with an acid, must be
employed. Some of these insoluble compounds can be detected by their
colour and particular reactions. For further information on this
subject, we refer the readers to Fresenius's _Qualitative Analysis_.
The procedure for the detection of metals in solution consists of
first separating them into groups and then examining each group
separately. For this purpose the cold solution is treated with
hydrochloric acid, which precipitates lead, silver and mercurous salts
as chlorides. The solution is filtered and treated with an excess of
sulphuretted hydrogen, either in solution or by passing in the gas;
this precipitates mercury (mercuric), any lead left over from the
first group, copper, bismuth, cadmium, arsenic, antimony and tin as
sulphides. The solution is filtered off, boiled till free of
sulphuretted hydrogen, and ammonium chloride and ammonia added. If
phosphoric acid is absent, aluminium, chromium and ferric hydrates are
precipitated. If, however, phosphoric acid is present in the original
substance, we may here obtain a precipitate of the phosphates of the
remaining metals, together with aluminium, chromium and ferric
hydrates. In this case, the precipitate is dissolved in as little as
possible hydrochloric acid and boiled with ammonium acetate, acetic
acid and ferric chloride. The phosphates of aluminium, chromium and
iron are precipitated, and the solution contains the same metals as if
phosphoric acid had been absent. To the filtrate from the aluminium,
iron and chromium precipitate, ammonia and ammonium sulphide are
added; the precipitate may contain nickel, cobalt, zinc and manganese
sulphides. Ammonium carbonate is added to the filtrate; this
precipitates calcium, strontium and barium. The solution contains
magnesium, sodium and potassium, which are separately distinguished by
the methods given under their own headings.
We now proceed with the examination of the various group precipitates.
The white precipitate formed by cold hydrochloric acid is boiled with
water, and the solution filtered while hot. Any lead chloride
dissolves, and may be identified by the yellow precipitate formed with
potassium chromate. To the residue add ammonia, shake, then filter.
Silver chloride goes into solution, and may be precipitated by dilute
nitric acid. The residue, which is black in colour, consists of
mercuroso-ammonium chloride, in which mercury can be confirmed by its
ordinary tests.
The precipitate formed by sulphuretted hydrogen may contain the black
mercuric, lead, and copper sulphides, dark-brown bismuth sulphide,
yellow cadmium and arsenious sulphides, orange-red antimony sulphide,
brown stannous sulphide, dull-yellow stannic sulphide, and whitish
sulphur, the last resulting from the oxidation of sulphuretted
hydrogen by ferric salts, chromates, &c. Warming with ammonium
sulphide dissolves out the arsenic, antimony and tin salts, which are
reprecipitated by the addition of hydrochloric acid to the ammonium
sulphide solution. The precipitate is shaken with ammonium carbonate,
which dissolves the arsenic. Filter and confirm arsenic in the
solution by its particular tests. Dissolve the residue in hydrochloric
acid and test separately for antimony and tin. The residue from the
ammonium sulphide solution is warmed with dilute nitric acid. Any
residue consists of black mercuric sulphide (and possibly white lead
sulphate), in which mercury is confirmed by its usual tests. The
solution is evaporated with a little sulphuric acid and well cooled.
The white precipitate consists of lead sulphate. To the filtrate add
ammonia in excess; a white precipitate indicates bismuth; if the
solution be blue, copper is present. Filter from the bismuth hydrate,
and if copper is present, add potassium cyanide till the colour is
destroyed, then pass sulphuretted hydrogen, and cadmium is
precipitated as the yellow sulphide. If copper is absent, then
sulphuretted hydrogen can be passed directly into the solution.
The next group precipitate may contain the white gelatinous aluminium
hydroxide, the greenish chromium hydroxide, reddish ferric hydroxide,
and possibly zinc and manganese hydroxides. Treatment with casutic
soda dissolves out aluminium hydroxide, which is reprecipitated by the
addition of ammonium chloride. The remaining metals are tested for
separately.
The next group may contain black nickel and cobalt sulphides,
flesh-coloured manganese sulphide, and white zinc sulphide. The last
two are dissolved out by cold, very dilute hydrochloric acid, and the
residue is tested for nickel and cobalt. The solution is boiled till
free from sulphuretted hydrogen and treated with excess of sodium
hydrate. A white precipitate rapidly turning brown indicates
manganese. The solution with ammonium sulphide gives a white
precipitate of zinc sulphide.
The next group may contain the white calcium, barium and strontium
carbonates. The flame coloration (see above) may give information as
to which elements are present. The carbonates are dissolved in
hydrochloric acid, and calcium sulphate solution is added to a portion
of the solution. An immediate precipitate indicates barium; a
precipitate on standing indicates strontium. If barium is present, the
solution of the carbonates in hydrochloric acid is evaporated and
digested with strong alcohol for some time; barium chloride, which is
nearly insoluble in alcohol, is thus separated, the remainder being
precipitated by a few drops of hydrofluosilicic acid, and may be
confirmed by the ordinary tests. The solution free from barium is
treated with ammonia and ammonium sulphate, which precipitates
strontium, and the calcium in the solution may be identified by the
white precipitate with ammonium oxalate.
Having determined the bases, it remains to determine the acid radicals.
There is no general procedure for these operations, and it is customary
to test for the acids separately by special tests; these are given in
the articles on the various acids. A knowledge of the solubility of
salts considerably reduces the number of acids likely to be present, and
affords evidence of great value to the analyst (see A.M. Comey,
_Dictionary of Chemical Solubilities_.) In the above account we have
indicated the procedure adopted in the analysis of a complex mixture of
salts. It is unnecessary here to dwell on the precautions which can only
be conveniently acquired by experience; a sound appreciation of
analytical methods is only possible after the reactions and characters
of individual substances have been studied, and we therefore refer the
reader to the articles on the particular elements and compounds for more
information on this subject.
_Quantitative Inorganic Analysis_.
Quantitative methods are divided into four groups, which we now pass on
to consider in the following sequence: ([alpha]) gravimetric, ([beta])
volumetric, ([gamma]) electrolytic, ([delta]) colorimetric.
([alpha]) _Gravimetric._--This method is made up of four operations: (1)
a weighed quantity of the substance is dissolved in a suitable solvent;
(2) a particular reagent is added which precipitates the substance it is
desired to estimate; (3) the precipitate is filtered, washed and dried;
(4) the filter paper containing the precipitate is weighed either as a
_tared_ filter, or incinerated and ignited either in air or in any other
gas, and then weighed.
(1) Accurate weighing is all-important: for details of the various
appliances and methods see WEIGHING MACHINES. (2) No general
directions can be given as to the method of precipitation. Sometimes
it is necessary to allow the solution to stand for a considerable time
either in the warm or cold or in the light or dark; to work with cold
solutions and then boil; or to use boiling solutions of both the
substance and reagent. Details will be found in the articles on
particular metals. (3) The operation of filtration and washing is very
important. If the substance to be weighed changes in composition on
strong heating, it is necessary to employ a tared filter, i.e. a
filter paper which has been previously heated to the temperature at
which the substance is to be dried until its weight is constant. If
the precipitate settles readily, the supernatant liquor may be
decanted through the filter paper, more water added to the precipitate
and again decanted. By this means most of the washing, i.e.freeing
from the other substances in the solution, can be accomplished in the
precipitating vessel. If, however, the precipitate refuses to settle,
it is directly transferred to the filter paper, the last traces being
removed by washing and rubbing the sides of the vessel with a piece of
rubber, and the liquid is allowed to drain through. It is washed by
ejecting a jet of water, ammonia or other prescribed liquid on to the
side of the filter paper until the paper is nearly full. It can be
shown that a more efficient washing results from alternately filling
and emptying the funnel than by endeavouring to keep the funnel full.
The washing is continued until the filtrate is free from salts or
acids. (4) After washing, the funnel containing the filter paper is
transferred to a drying oven. In the case of a tared filter it is
weighed repeatedly until the weight suffers no change; then knowing
the weight of the filter paper, the weight of the precipitate is
obtained by subtraction. If the precipitate may be ignited, it is
transferred to a clean, weighed and recently ignited crucible, and the
filter paper is burned _separately_ on the lid, the ash transferred to
the crucible, and the whole ignited. After ignition, it is allowed to
cool in a desiccator and then weighed. Knowing the weight of the
crucible and of the ash of the filter paper, the weight of the
precipitate is determined. The calculation of the percentage of the
particular constituent is simple. We know the amount present in the
precipitate, and since the same amount is present in the quantity of
substance experimented with, we have only to work out a sum in
proportion.
([beta]) _Volumetric._--This method is made up of three operations:--(1)
preparation of a _standard_ solution; (2) preparation of a solution of
the substance; (3) _titration_, or the determination of what volume of
the standard solution will occasion a known and definite reaction with a
known volume of the test solution.
(1) In general analytical work the standard solution contains the
equivalent weight of the substance in grammes dissolved in a litre of
water. Such a solution is known as _normal_. Thus a normal solution of
sodium carbonate contains 53 grammes per litre, of sodium hydrate 40
grammes, of hydrochloric acid 36.5 grammes, and so on. By taking
1/10th or 1/100th of these quantities, _decinormal_ or _centinormal_
solutions are obtained. We see therefore that 1 cubic centimetre of a
normal sodium carbonate solution will exactly neutralize 0.049 gramme
of sulphuric acid, 0.0365 gramme of hydrochloric acid (i.e. the
equivalent quantities), and similarly for decinormal and centinormal
solutions. Unfortunately, the term normal is sometimes given to
solutions which are strictly decinormal; for example, iodine, sodium
thiosulphate, &c. In technical analysis, where a solution is used for
one process only, it may be prepared so that 1 cc. is equal to .01
gramme of the substance to be estimated. This saves a certain amount
of arithmetic, but when the solution is applied in another
determination additional calculations are necessary. Standard
solutions are prepared by weighing out the exact amount of the pure
substance and dissolving it in water, or by forming a solution of
approximate normality, determining its exact strength by gravimetric
or other means, and then correcting it for any divergence. This may be
exemplified in the case of alkalimetry. Pure sodium carbonate is
prepared by igniting the bicarbonate, and exactly 53 grammes are
dissolved in water, forming a strictly normal solution. An approximate
normal sulphuric acid is prepared from 30 ccs. of the pure acid (1.84
specific gravity) diluted to 1 litre. The solutions are titrated (see
below) and the acid solution diluted until equal volumes are exactly
equivalent. A standard sodium hydrate solution can be prepared by
dissolving 42 grammes of sodium hydrate, making up to a litre, and
diluting until one cubic centimetre is exactly equivalent to one cubic
centimetre of the sulphuric acid. Similarly, normal solutions of
hydrochloric and nitric acids can be prepared. Where a solution is
likely to change in composition on keeping, such as potassium
permanganate, iodine, sodium hydrate, &c., it is necessary to check
or re-standardize it periodically.
(2) The preparation of the solution of the substance consists in
dissolving an accurately determined weight, and making up the volume
in a graduated cylinder or flask to a known volume.
(3) The titration is conducted by running the standard solution from a
burette into a known volume of the test solution, which is usually
transferred from the stock-bottle to a beaker or basin by means of a
pipette. Various artifices are employed to denote the end of the
reaction. These may be divided into two groups: (1) those in which a
change in appearance of the reacting mixture occurs; (2) those in
which it is necessary to use an indicator which, by its change in
appearance, shows that an excess of one reagent is present. In the
first group, we have to notice the titration of a cyanide with silver
nitrate, when a milkiness shows how far the reaction has gone; the
titration of iron with permanganate, when the faint pink colour shows
that all the iron is oxidized. In the second group, we may notice the
application of litmus, methyl orange or phenolphthalein in
alkalimetry, when the acid or alkaline character of the solution
commands the colour which it exhibits; starch paste, which forms a
blue compound with free iodine in iodometry; potassium chromate, which
forms red silver chromate after all the hydrochloric acid is
precipitated in solutions of chlorides; and in the estimation of
ferric compounds by potassium bichromate, the indicator, potassium
ferricyanide, is placed in drops on a porcelain plate, and the end of
the reaction is shown by the absence of a blue coloration when a drop
of the test solution is brought into contact with it.
([gamma]) _Electrolytic._--This method consists in decomposing a
solution of a salt of the metal by the electric current and weighing the
metal deposited at the cathode.
It is only by paying great attention to the current density that good
results are obtained, since metals other than that sought for may be
deposited. In acid copper solutions, mercury is deposited before the
copper with which it subsequently amalgamates; silver is thrown down
simultaneously; bismuth appears towards the end; and after all the
copper has been precipitated, arsenic and antimony may be deposited.
Lead and manganese are partially separated as peroxides, but the
remaining metals are not deposited from acid solutions. It is
therefore necessary that the solution should be free from metals which
may vitiate the results, or special precautions taken by which the
impurities are rendered harmless. In such cases the simplicity of
manipulation and the high degree of accuracy of the method have made
it especially valuable. The electrolysis is generally conducted with
platinum electrodes, of which the cathode takes the form of a piece of
foil bent into a cylindrical form, the necessary current being
generated by one or more Daniell cells.
([delta]) _Colorimetric._--This method is adopted when it is necessary
to determine minute traces (as in the liquid obtained in the
electrolytic separation of copper) of substances which afford
well-defined colour reactions.
The general procedure is to make a series of standard solutions
containing definite quantities of the substance which it is desired to
estimate; such a series will exhibit tints which deepen as the
quantity of the substance is increased. A known weight of the test
substance is dissolved and a portion of the solution is placed in a
tube similar to those containing the standard solutions. The
colour-producing reagent is added and the tints compared. In the case
of copper, the colour reactions with potassium ferrocyanide or ammonia
are usually employed; traces of ammonia are estimated with _Nessler's
reagent_; sulphur in iron and steel is determined by the tint assumed
by a silver-copper plate suspended in the gases liberated when the
metal is dissolved in sulphuric acid (Eggertz's test) (see W. Crookes,
_Select Methods in Analytical Chemistry_).
_Organic Analysis._
The elements which play important parts in organic compounds are carbon,
hydrogen, nitrogen, chlorine, bromine, iodine, sulphur, phosphorus and
oxygen. We shall here consider the qualitative and quantitative
determination of these elements.
_Qualitative._--Carbon is detected by the formation of carbon dioxide,
which turns lime-water milky, and hydrogen by the formation of water,
which condenses on the tube, when the substance is heated with copper
oxide. Nitrogen may be detected by the evolution of ammonia when the
substance is heated with soda-lime. A more delicate method is that due
to J. L. Lassaigne and improved by O. Jacobsen and C. Graebe. The
substance is heated with metallic sodium or potassium (in excess if
sulphur be present) to redness, the residue treated with water,
filtered, and ferrous sulphate, ferric chloride and hydrochloric acid
added. A blue coloration indicates nitrogen, and is due to the
formation of potassium (or sodium) cyanide during the fusion, and
subsequent interaction with the iron salts. The halogens may be
sometimes detected by fusing with lime, and testing the solution for a
bromide, chloride and iodide in the usual way. F. Beilstein determines
their presence by heating the substance with pure copper oxide on a
platinum wire in the Bunsen flame; a green coloration is observed if
halogens be present. Sulphur is detected by heating the substance with
sodium, dissolving the product in water, and adding sodium
nitroprusside; a bluish-violet coloration indicates sulphur (H. Vohl).
Or we may use J. Horbaczewski's method, which consists in boiling the
substance with strong potash, saturating the cold solution with
chlorine, adding hydrochloric acid, and boiling till no more chlorine
is liberated, and then testing for sulphuric acid with barium
chloride. Phosphorus is obtained as a soluble phosphate (which can be
examined in the usual way) by lixiviating the product obtained when
the substance is ignited with potassium nitrate and carbonate.
Carbon and hydrogen.
_Quantitative._--Carbon and hydrogen are generally estimated by the
_combustion_ process, which consists in oxidizing the substance and
absorbing the products of combustion in suitable apparatus. The
oxidizing agent in commonest use is copper oxide, which must be
freshly ignited before use on account of its hygroscopic nature. Lead
chromate is sometimes used, and many other substances, such as
platinum, manganese dioxide, &c., have been suggested. The procedure
for a combustion is as follows:--
[Illustration: FIG. 1.]
[Illustration: FIG. 2.]
[Illustration: FIG. 3.]
[Illustration: FIG. 4.]
A hard glass tube slightly longer than the furnace and 12 to 15 mm. in
diameter is thoroughly cleansed and packed as shown in fig. 1. The
space a must allow for the inclusion of a copper spiral if the
substance contains nitrogen, and a silver spiral if halogens be
present, for otherwise nitrogen oxides and the halogens may be
condensed in the absorption apparatus; b contains copper oxide; c is a
space for the insertion of a porcelain or platinum boat containing a
weighed quantity of the substance; d is a copper spiral. The end d is
connected to an air or oxygen supply with an intermediate drying
apparatus. The other end is connected with the absorption vessels,
which consist of a tube (e) containing calcium chloride, and a set of
bulbs (f) containing potash solution. Various forms of potash bulbs
are employed; fig. 2 is Liebig's, fig. 3 Mohr's or Geissler's, fig. 4
is a more recent form, of which special variations have been made by
Anderson, Gomberg, Delisle and others. After having previously roasted
the tube and copper oxide, and reduced the copper spiral a, the
weighed calcium chloride tube and potash bulbs are put in position,
the boat containing the substance is inserted (in the case of a
difficultly combustible substance it is desirable to mix it with
cupric oxide or lead chromate), the copper spiral (d) replaced, and
the air and oxygen supply connected up. The apparatus is then tested
for leaks. If all the connexions are sound, the copper oxide is
gradually heated from the end a, the gas-jets under the spiral d are
lighted, and a slow current of oxygen is passed through the tube. The
success of the operation depends upon the slow burning of the
substance. Towards the end the heat and the oxygen supply are
increased. When there is no more absorption in the potash bulbs, the
oxygen supply is cut off and air passed through. Having replaced the
oxygen in the absorption vessels by air, they are disconnected and
weighed, after having cooled down to the temperature of the room. The
increase in weight of the calcium chloride tube gives the weight of
water formed, and of the potash bulbs the carbon dioxide.
Liquids are amenable to the same treatment, but especial care must be
taken so that they volatilize slowly. Difficultly volatile liquids may
be weighed directly into the boat; volatile liquids are weighed in
thin hermetically sealed bulbs, the necks of which are broken just
before they are placed in the combustion tube.
The length of time and other disadvantages attending the combustion
method have caused investigators to devise other processes. In 1855 C.
Brunner described a method for oxidizing the carbon to carbon dioxide,
which could be estimated by the usual methods, by heating the
substance with potassium bichromate and sulphuric acid. This process
has been considerably developed by J. Messinger, and we may hope that
with subsequent improvements it may be adapted to all classes of
organic compounds. The oxidation, which is effected by chromic acid
and sulphuric acid, is conducted in a flask provided with a funnel and
escape tube, and the carbon dioxide formed is swept by a current of
dry air, previously freed from carbon dioxide, through a drying tube
to a set of potash bulbs and a tube containing soda-lime; if halogens
are present, a small wash bottle containing potassium iodide, and a U
tube containing glass wool moistened with silver nitrate on one side
and strong sulphuric acid on the other, must be inserted between the
flask and the drying tube. The increase in weight of the potash bulbs
and soda-lime tube gives the weight of carbon dioxide evolved. C.F.
Cross and E.J. Bevan collected the carbon dioxide obtained in this way
over mercury. They also showed that carbon monoxide was given off
towards the end of the reaction, and oxygen was not evolved unless the
temperature exceeded 100 deg.
Methods depending upon oxidation in the presence of a contact
substance have come into favour during recent years. In that of M.
Dennstedt, which was first proposed in 1902, the substance is
vaporized in a tube containing at one end platinum foil, platinized
quartz, or platinized asbestos. The platinum is maintained at a bright
red heat, either by a gas flame or by an electric furnace, and the
vapour is passed over it by leading in a current of oxygen. If
nitrogen be present, a boat containing dry lead peroxide and heated to
320 deg. is inserted, the oxide decomposing any nitrogen peroxide which
may be formed. The same absorbent quantitatively takes up any halogen
and sulphur which may be present. The process is therefore adapted to
the simultaneous estimation of carbon, hydrogen, the halogens and
sulphur.
Nitrogen.
Nitrogen is estimated by (1) Dumas' method, which consists in heating
the substance with copper oxide and measuring the volume of nitrogen
liberated; (2) by Will and Varrentrapp's method, in which the
substance is heated with soda-lime, and the ammonia evolved is
absorbed in hydrochloric acid, and thence precipitated as ammonium
chlorplatinate or estimated volumetrically; or (3) by Kjeldahl's
method, in which the substance is dissolved in concentrated sulphuric
acid, potassium permanganate added, the liquid diluted and boiled with
caustic soda, and the evolved ammonia absorbed in hydrochloric acid
and estimated as in Will and Varrentrapp's method.
[Illustration: FIG. 5.]
_Dumas' Method._--In this method the operation is carried out in a
hard glass tube sealed at one end and packed as shown in fig. 5. The
magnesite (a) serves for the generation of carbon dioxide which clears
the tube of air before the compound (mixed with fine copper oxide (b))
is burned, and afterwards sweeps the liberated nitrogen into the
receiving vessel (e), which contains a strong potash solution; c is
coarse copper oxide; and d a reduced copper gauze spiral, heated in
order to decompose any nitrogen oxides. Ulrich Kreusler generates the
carbon dioxide in a separate apparatus, and in this case the tube is
drawn out to a capillary at the end (a). This artifice is specially
valuable when the substance decomposes or volatilizes in a warm
current of carbon dioxide. Various forms of the absorbing apparatus
(e) have been discussed by M. Ilinski (_Ber._ 17, p. 1347), who has
also suggested the use of manganese carbonate instead of magnesite,
since the change of colour enables one to follow the decomposition.
Substances which burn with difficulty may be mixed with mercuric oxide
in addition to copper oxide.
_Will and Varrentrapp's Method._--This method, as originally proposed,
is not in common use, but has been superseded by Kjeldahl's method,
since the nitrogen generally comes out too low. It is susceptible of
wider application by mixing reducing agents with the soda-lime: thus
Goldberg (_Ber._ 16, p. 2546) uses a mixture of soda-lime, stannous
chloride and sulphur for nitro- and azo-compounds, and C. Arnold
(_Ber._ 18, p. 806) a mixture containing sodium hyposulphite and
sodium formate for nitrates.
_Kjeldahl's Method._--This method rapidly came into favour on account
of its simplicity, both of operation and apparatus. Various substances
other than potassium permanganate have been suggested for facilitating
the operation; J.W. Gunning (_Z. anal. Chem._, 1889, p. 189) uses
potassium sulphate; Lassar-Cohn uses mercuric oxide. The applicability
of the process has been examined by F.W. Dafert (_Z. anal. Chem._,
1888, p. 224), who has divided nitrogenous bodies into two classes
with respect to it. The first class includes those substances which
require no preliminary treatment, and comprises the amides and
ammonium compounds, pyridines, quinolines, alkaloids, albumens and
related bodies; the second class requires preliminary treatment and
comprises, with few exceptions, the nitro-, nitroso-, azo-, diazo- and
amidoazo-compounds, hydrazines, derivatives of nitric and nitrous
acids, and probably cyanogen compounds. Other improvements have been
suggested by Dyer (_J.C.S. Trans._ 67, p. 811). For an experimental
comparison of the accuracy of the Dumas, Will-Varrentrapp and Kjeldahl
processes see L. L'Hote, _C.R._ 1889, p. 817. Debordeaux (_C.R._ 1904,
p. 905) has obtained good results by distilling the substance with a
mixture of potassium thiosulphate and sulphide.
Halogens, sulphur, phosphorus.
The halogens may be estimated by ignition with quicklime, or by
heating with nitric acid and silver nitrate in a sealed tube. In the
first method the substance, mixed with quicklime free from chlorine,
is heated in a tube closed at one end in a combustion furnace. The
product is dissolved in water, and the calcium haloid estimated in the
usual way. The same decomposition may be effected by igniting with
iron, ferric oxide and sodium carbonate (E. Kopp, _Ber._ 10, p. 290);
the operation is easier if the lime be mixed with sodium carbonate, or
a mixture of sodium carbonate and potassium nitrate be used. With
iodine compounds, iodic acid is likely to be formed, and hence the
solution must be reduced with sulphurous acid before precipitation
with silver nitrate. C. Zulkowsky (_Ber._ 18, R. 648) burns the
substance in oxygen, conducts the gases over platinized sand, and
collects the products in suitable receivers. The oxidation with nitric
acid in sealed tubes at a temperature of 150 deg. to 200 deg. for
aliphatic compounds, and 250 deg. to 260 deg. for aromatic compounds,
is in common use, for both the sulphur and phosphorus can be
estimated, the former being oxidized to sulphuric acid and the latter
to phosphoric acid. This method was due to L. Carius (_Ann._ 136, p.
129). R. Klason (_Ber._ 19, p. 1910) determines sulphur and the
halogens by oxidizing the substance in a current of oxygen and nitrous
fumes, conducting the vapours over platinum foil, and absorbing the
vapours in suitable receivers. Sulphur and phosphorus can sometimes be
estimated by Messinger's method, in which the oxidation is effected by
potassium permanganate and caustic alkali, or by potassium bichromate
and hydrochloric acid. A comparison of the various methods for
estimating sulphur has been given by O. Hammarsten (_Zeit. physiolog.
Chem._ 9, p. 273), and by Holand (_Chemiker Zeitung_, 1893, p. 991).
H.H. Pringsheim (_Ber._ 38, p. 1434) has devised a method in which the
oxidation is effected by sodium peroxide; the halogens, phosphorus and
sulphur can be determined by one operation.