Spot Tests in Inorganic Analysis

673 Pages·1972·81.521 MB·English

by  F. Feigl and V. Anger (Auth.)

SPOT TESTS
IN
INORGANIC ANALYSIS
by
FRITZ FEIGL,Eng.,D.Sc.
Laboratorio da Produçâo Mineral, Ministéno da Agricultura,
Rio de Janeiro; Professor at the University of Brazil;
Member ot the Austrian, Brazilian and Gotenburg Academies of Sciences
and
Dr. VINZENZ ANGER
Research Laboratory, Lobachemie, Vienna, Austria
Translated by
RALPH E. OESPER,Ph.D.
Professor Emeritus, University of Cincinnati
Sixth English edition, completely revised and enlarged
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FOREWORD TO THE SIXTH EDITION
When I took over the task three years ago of completely revising the
previous edition of this work I had no idea that the task of writing the
Foreword for this edition would also devolve on me. Regrettably, Professor
Feigl did not live to see the completion of the sixth edition ; he died in Rio
de Janeiro on January 23, 1971 after a long illness.
I was, however, able to discuss with him before his illness became critical
the changes and improvements that were needed. Professor Feigl gave me a
completely free hand in this, and I believe that this new edition would have
had his full approval.
The arrangement of the text in the previous edition had to be altered
substantially and new chapters added. One of the new chapters, for example,
is that on preliminary and orientational tests whereby general chemical
information about samples can be obtained before actual analysis. It re-
places to some extent the chapter on systematic analysis, which has been
omitted in this edition. Chapters 3, 4 and 5 of the fifth edition have been
combined into one chapter, which is divided into sections devoted to the
various elements, (in alphabetical order), and these are further divided into
sub-sections according to the form in which the element is present - either
as free element, cation, anion, non-ionic compound or organic compound.
This change was felt to be of advantage as it makes the book easier to
use. All tests have been arranged according to the element to which they
relate and are no longer dispersed through various chapters. The tests cited
are sufficient to establish the presence or absence of a given element or ion.
This can be achieved without the chemical separations that used to be
necessary in qualitative analysis.
Another innovation is the inclusion of tests for non-ionic compounds of
some elements as well as tests for organometallic ions and tests to detect
elements in organic compounds.
Also new is the provision of a reference list for each subsection, followed
in some cases by a bibliography of the appropriate quantitative methods.
VI FOREWORD
Fourteen years have elapsed since the previous edition was published.
During this period many new reagents have been discovered, and the litera-
ture contains many tests relating to these newly discovered reagents. This
information has of course been included. It is difficult to compare the fifth
and sixth editions. Chapters have been amalgamated and new ones added,
the number of tests has increased from 649 to 899 and in the chapter on
"Applications" the number of sections from 95 to 129.
I sincerely hope that all these changes have improved the usefulness of
the volume.
Vienna, Austria VINZENZ ANGER
March 1972
Chapter 1
Development, Present State and Prospects of Inorganic
Spot Test Analysis1
Analytical chemists have long used single chemical tests carried out in
drops of solution on filter paper or on impermeable surfaces. A familiar
example is the use of indicator papers to detect rapidly the presence of
hydrogen or hydroxyl ions in a drop of a solution. Likewise, the endpoint of
certain reactions used in titrimetric analysis, or the completion of electrolytic
depositions, can be established by removing a drop of the test solution and
bringing it into contact with a suitable reagent on filter paper, on a watch
glass, or on a porcelain plate. It has not been definitely established who
made the first use of spot reactions for analytical purposes. Probably the
earliest published instance was given by F. Runge, who in 1834 used
potassium iodide-starch paper to detect free chlorine.2 In 1859 Schiff3 em-
ployed filter paper impregnated with silver carbonate to reveal uric acid in
urine. A drop of the specimen produced a brown fleck of free silver. This
appears to be the earliest precise description of a spot test, because the great
sensitivity of this reaction was determined in the same manner as at present,
i.e. by testing a series of dilute solutions of uric acid. The fundamental work
for that division of spot test analysis in which filter paper functions as the
reaction medium is a study by Schönbein.4 He showed that when aqueous
solutions rise in strips of filter paper, the water precedes the dissolved
material, and the relative height of ascent of solutes can differ enough to
make it possible to detect the cosolutes in separate zones. These observations
gave the impetus for the classic studies (1861-1907) of Fr. Goppelsroeder,
which were compiled in his "Kapillaranalyse', published at Dresden in 1910.
He made a very extensive study of the capillary rise of solutions and the
capillary spreading of drops of solutions in filter paper and investigated the
analytical use of these effects, particularly in the examination of organic
liquids, dissolved compounds, and dyes. His publications also contain
references to the capillary spreading of inorganic salts, effects which later
were studied by other chemists, especially Skraup and his associates δ and
Krulla.8 The findings suggested the problem of discovering whether an
References pp. 28-30
2 INORGANIC SPOT TESTS 1
inorganic capillary analysis is possible in which the primary objective is
the feasibility of carrying out color reactions in the form of spot tests in
the various zones of the paper to detect the materials which had been
separated by capillarity. Investigations along this line, which required,
above all, the determination of the minimum quantities of substances that
can be detected by spot reactions on paper, were conducted (1917-21) by
Feigl and Stern 7 with solutions of salts of the metals of the ammonium
sulfide group. These studies, and a continuation8 dealing with the detection
of metals of the hydrogen sulfide group, yielded observations which set the
course for further studies of spot reactions. Many tests, well known from
their accepted use in the classic procedures of qualitative inorganic analysis,
where they were executed in test tubes, displayed an unexpected great
sensitivity when they were tried as spot reactions on filter paper, since under
these latter conditions the picture of a reaction may be quite different from
that seen in a test tube. The appearance of the fleck frequently is very
different according to the concentration of the reacting partners, the quality
of the paper, and other experimental conditions. Not only were sensitive
individual tests found to be possible through spot reactions, but sometimes
several materials could be detected in a single drop of a solution, provided
the reagents were properly chosen. However, the most important finding
was that the amounts that can be detected by means of spot reactions on
filter paper are so small that microanalytical goals are reached.
In order further to develop the observations gleaned from the first com-
prehensive studies of spot reactions, with the objective of detecting at least
the great majority, if not all, of the anions and cations by means of spot
tests, and with the avoidance, wherever possible, of the usual separation
procedures, two things were essential : (a) the extension and refinement of the
technique of working with drops, and (b) the employment of familiar and/or
new sensitive detection reactions. The attention given to spot reactions not
only by the author and his school but subsequently by others (especially
N. A. Tananaeff) led in the course of time to great progress in both of these
directions and so much material was assembled that there was ample justifi-
cation for designating this new division of chemical analysis: spot test
analysis.
It has now become customary to speak of spot reactions or, more correctly,
of spot or drop tests, when in a chemical test by the wet method at least one
reactant—usually the material being detected or identified—is used in the
form of a drop of a solution.* The most usual kind of spot test consists in
bringing together drops of the test (sample) solution and the reagent solution
* The usual German designation is Tüpfelreaktion. The French use réaction à la touche, or
réaction à la goutte, or stilliréaction.9
References pp. 28-30
DEVELOPMENT, PRESENT STATE, PROSPECTS 3
on porous substrates such as filter paper, on impermeable media such as
spot plates, in microcrucibles, on watch glasses, or in micro test tubes.
Another version employs one of the reactants in the solid form, i.e. a little
of the material being studied is spotted with a drop of a suitable reagent
solution, or a drop of the test solution is brought into contact with a solid
reagent. Sometimes, a drop of a solution or a pinch of a solid can be made
to evolve a gas, which can then be detected by its action on a reagent
paper or on a drop of an appropriate reagent solution. Chapter 2 gives a
description of the apparative aids commonly used in these tests and discusses
the techniques employed.
It scarcely needs to be stressed that all improvements of the inherently
simple technique of spot test analysis were and continue to be of the highest
importance in its development. However, it is not its technique alone which
has led to systematic spot test analysis. Equally important, in fact even
more so in many respects, has been the need of finding appropriate reactions
to which the technique can be applied. Even the earliest high-quality spot
reactions indicated the lines along which spot test analysis would develop.
These guiding principles were: (a) to use reactions of the highest possible
sensitivity and reliability; (b) to employ all possibilities of enhancing sensi-
tivity and reliability. With respect to the former, spot test analysis has
filled a very useful function. In this effort to adapt chemical reactions
which had already been described, many tests that were scattered through
the literature and which in part had been forgotten, were revived, tried out
again, and improved in certain instances. It frequently became necessary
to unravel their chemical basis or to correct erroneous notions. New facts
were thus assembled which later proved of value in the search for new
analytically useful reagents and reactions. Many reactions to which spot
test analysis had recourse, and others which were first used for spot testing,
were later employed in qualitative macroanalysis, and sometimes were
introduced even into quantitative macro- and microanalysis. It is charac-
teristic of studies intended to extend spot test analysis that they frequently
deliver findings of use to analytical chemistry in general, and in many
cases they prove to have value also in other provinces of chemistry. The
reason for this is the fact that the prime factors in spot test analysis,
namely sensitivity and reliability, are important to all provinces of chemical
analysis. Therefore experimental chemistry in its widest sense must be
pursued in the scientific treatment of these questions.
Soon after the earliest detailed description of spot tests, it was proposed,10 on
the basis of a comparative study of the efficiency of spot tests and the tests em-
ployed in the classical procedures of qualitative inorganic macro- and microana-
lysis, that the type of procedure as well as the sensitivity of the tests be expressed
References pp. 28-30
4 INORGANIC SPOT TESTS 1
in a symbolic form. This seemed all the more necessary since the previous use of
the term "sensitivity" had not been uniform and frequently was not even exact.
Sometimes sensitivity was taken to mean the smallest quantity of a material that
could be detected by a test, and sometimes it was used to indicate the dilution
prevailing at a test with no reference to the quantity of material. The expression
x[S]y
refers to tests conducted in solutions. It contains information of interest to the
characterization of tests, to the comparison of the efficiency of tests, and for the
statistic of tests. In this expression, the symbols denote:
5 = the technique employed, i.e., reactions in micro test tubes, on filter paper,
spot plates, etc. ;
y = the volume (in ml) of the test (sample) solution taken for the test;
x = the quantity of material dissolved in y, which is revealable by the partic-
ular technique and expressed in gamma (γ) (1 γ = 1 microgram =
0.001 mg).
The term Limit of Identification, which was first suggested by the author, has
found almost universal acceptance as x. When the values of x and y are known,
the concentration (or dilution) of the test solution with respect to the material
being sought or identified is easily stated. This leads to the Concentration Limitu
or better its reciprocal the Dilution Limit. 12 The following simple relation
exists between the dilution limit, the volume of the test solution, and the iden-
tification limit:
y-106 Volume of the test solution (in ml) · 10e
Dilution limit = 1 : = 1 : —
Limit of Identification (in γ)
It follows from this expression that the identification limit of a test can be
calculated if the dilution (concentration) of a solution and the particular volume
of the solution used in the test are known. This computation is used in the experi-
mental determination of the identification limits of tests in which initial solutions
of known concentration are systematically diluted.
Hahn (loc. cit.) has quite properly pointed out that the numerical value of the
identification limit of a test expresses its quantity sensitivity, whereas the dilu-
tion (concentration) limit expresses the concentration sensitivity of a test. There-
fore, the complete appraisal of a test requires a knowledge of both its quantity-
and concentration-sensitivity. In the strictest sense, a test is highly sensitive,
if, when conducted in the smallest feasible volume of the test solution, the
identification limit is small and the dilution limit large. However, cases are
known in which a knowledge of the quantity sensitivity of a test (identification
limit) has to suffice. This situation is encountered when practically insoluble
materials are to be identified by means of dissolved, or melted, or gaseous
reagents. Accordingly, the term Limit of Identification is very convenient because
it retains its meaning for the characterization of the sensitivity of tests without
regard to the solubility or insolubility of the test material.
References pp. 28-30
DEVELOPMENT, PRESENT STATE, PROSPECTS 5
At the time the symbolic expression x[S]y was proposed for the charac-
terization of a test, there was little or no appreciation of one fact which
accumulated experience in spot test analysis has now shown to be of the
highest importance. If it is stated that 5 is a drop reaction, or a precipitation
or a color reaction, occurring in a given volume y, this designation of the
technique and volume employed is not adequate for the complete charac-
terization of the test. By taking proper measures it is possible to enhance
the discernibility of the occurrence of chemical reactions and thus enor-
mously improve the sensitivity of tests. In such cases, we speak of the con-
ditioning of tests. However, there is no symbolic designation of this bene-
ficiation which obviously is very important to the certainty of tests, a factor
which will be discussed later. Despite this limitation, the symbolic charac-
terization of tests has had a lasting influence. It has brought clarification
to the term "sensitivity"; it has led to new and critical reviews of the
analytical assessment of chemical reactions.
In this text, the limits of identification and the dilution limits which can
be attained by the respective spot tests are given immediately after the
outline of the various procedures. A tabular summary (Chapter 6) presents
the identification limits of a large number of the more important spot
reactions. If the drop volume, which ordinarily is around 0.05 ml, is taken
into account, the dilution limits corresponding to the identification limits
can be readily computed.
However, spot reactions are often conducted with microdrops, i.e. in
volumes of 0.03-0.001 ml, and much smaller identification limits are at-
tained then. An inspection of the tabular summary will disclose that few
identification limits are greater than 2 γ per 0.05 ml drop (dilution limit
= 1 : 25,000). In most cases this value lies below 0.2 y (dilution limit =
1 : 250,000). Far lower identification limits and correspondingly higher
dilution limits are reached in many instances. A comparison of the identi-
fication limits and dilution limits with those attainable by the classical
methods of qualitative microanalysis (particularly precipitations of crystals
that are then examined under the microscope) shows that the spot reactions
are generally superior with respect to concentration sensitivity (dilution
limit). As to quantity sensitivity (identification limit), spot reactions are
sometimes as good as micro precipitations, sometimes better, never inferior.
Consequently, if it is accepted that the goal of qualitative microanalysis is
not the use of a particular technique or sample size, but the detection of
absolute and relatively (with respect to the dilution) small quantities of
material, then spot reactions should undoubtedly be regarded as "micro-
chemical tests" in the great majority of cases. Sometimes, especially in the
Anglo-Saxon countries, spot reactions are called semimicro reactions, and
References pp. 28-30
6 INORGANIC SPOT TESTS 1
spot test analysis is referred to as semimicroanalysis. In the light of what has
just been said, such terms are incorrect and lead to error by such generali-
zation, since logically a spot test which is more sensitive than a classical
microchemical test can never be semimicrochemical. * It is permissible
and correct to distinguish between semimicro- and micro tests in order to
draw the limits between these two classes and to contrast them with macro
tests. Semimicro tests may refer to quantities (identification limits) of
10-200 y\ micro tests involve identification limits below 10 γ.**
The numerical values for the sensitivity of a test, i.e. its identification
limit and dilution limit, are not constants of the underlying chemical
reaction per se. This is plainly shown by precipitation reactions in which
the numerical value of the solubility product of the solid is a characteristic
and genuine reaction constant, and yet the sensitivities are by no means
dependent on the smallness of the particular solubility products. The
decisive factor for the sensitivity of a reaction is the discernibility of a
reaction product. This was first pointed out by Böttger 18 in a study which
was of high importance to reaction sensitivity. The fact that the sensitivity
of a precipitation reaction does not depend solely on the solubility product
of the solid product indicates that other processes besides a reaction which
can be represented stoichiometrically must also play a part. This is actually
the case. When precipitates appear, quite a few significant process supple-
* In this connection it must be noted that the widely used terms: microchemical identifica-
tion, determination and the like, cannot withstand close scrutiny. There is no "microchemistry"
as a separate branch. All special provinces of chemistry must justify their existence either by
dealing with certain groups of material or with special phenomena and regularities. A special field
of microchemistry would exist if it were concerned with phenomena and modes of behavior diffe-
rent from those dealt with in "macrochemistry". However, this is not the case, and the term
"microchemistry" is just as erroneous as the familiar term "technical chemistry", which has now
been replaced by "chemical technology**. Of course there is "microtechnique of general chemical
experimentation**l8 which is not confined to chemical analysis but is applicable likewise to
physico-chemical measurements, preparative studies, demonstration experiments, etc. In the
author's opinion, neither the use of a special technique nor the "study of techniques permitting
the chemical handling of quantities of material so small that the usual equipment is not longer
suitable" 14 can be called microchemistry. The situation is different with respect to the term
"microanalysis", which because of its goal (detection and determination of tiny amounts of ma-
terials) is actually a special branch of chemical analysis. However, a special microtechnique is
not always necessary to reach microanalytical objectives. This fact is clearly demonstrated by
spot test analysis and by trace analysis,1δ which both have become important parts of micro-
analysis, even though they do not require a special microtechnique. See, however, ref. 1β.
** There is as yet no generally accepted nomenclature for and differentiation between macro-,
semimicro- and microtests based on values of identification limits. Kirk 17 used the appropriate
terms "milligram analysis" and "microgram analysis**, and in quantitative work he distinguishes
between macro-, micro-, ultramicro-, and submicrogram methods, based on sample sizes of
100 mg, 10-20 mg, 1 mg, 1 γ, and 0.001 γ respectively. Gillis {loc. cit.), has adopted the same
classification for qualitative tests. It is obvious that there is no present agreement among
chemical analysts as to whether the term "micro" should refer to sample size and the correlative
special technique or—as the author believes—to the quantity of the material to be identified or
determined, independent of any special technique. In this connection compare ref.18.
References pp. 28-30