James Freeman Sellers.

An elementary treatise on qualitative chemical analysis online

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until they are precipitated." Bailey and Cady's Qualitative Analysis.

91 1 NH 4 MgAsO 4 is soluble in acids, and hence it is necessary to
neutralize with NH 4 OH.

941 Experience shows that many of the sulphides of this group are
soluble in warm dilute HNO 3 . Hence students frequently fail to pre-
cipitate completely all the sulphides of the group if HNO 3 is not com-
pletely removed. It is always safe to evaporate the solution to small
volume to expel IIN0 3 , and then dilute with water properly acidified
with HC1. 2 The quantity of hydrochloric acid in this connection is
important. This group includes eight rather widely different metals
whose sulphides vary both in their solubility and in their chemical
conduct towards reagents. The order of the precipitation of their
sulphides from cold dilute hydrochloric acid solution is that of the
metals mentioned : lead, cadmium, tin, bismuth, antimony, copper,
mercury, arsenic. If, however, the quantity of the acid is increased
and the solution heated to nearly boiling, the order is partly changed.
The sulphides of arsenic are rendered more insoluble, while those of
the other metals become more soluble. In certain quantitative deter-
minations of arsenic its separation is effected by the use of hot



NOTES 167

concentrated hydrochloric acid. As stated above, the arsenic sul-
phides are colloidal in character and, as is the habit of such com-
pounds, are rendered more insoluble by heat and acids. The degree
of acidity of the solution should be about 1 of concentrated acid to 10
of water, for the precipitation of the arsenic sulphides, and an equal
volume of water added on cooling to precipitate the other sulphides.
3 Should the solution contain nitric acid, ferric salts, chromates,
and other oxidizing agents, a white precipitate of sulphur would
appear here. This should not be mistaken for the sulphides of the
group, all of which are colored. 4 Dilute a portion of filtrate (a)
and pass in H 2 S again to make sure of complete removal of Group II.
The filtrate should be thoroughly tested for remnants of the group
with H 2 S. Either an excess of hydrochloric acid or an insufficiency
of hydrogen sulphide might prevent the complete precipitation of the
group.

95 1 This reagent should be carefully inspected and made fresh at
frequent intervals. On standing, either it is destroyed by oxidation,
or it runs to the higher sulphides by the loss of ammonia. In the
former case it becomes colorless, and in the latter case the yellow
color changes to red. The reagent should be yellow. 2 Though CuS is
practically insoluble in Na 2 S x , HgS is more soluble in this reagent than
in (NH4) 2 S X . Hence if Hg" is suspected, it is better to avoid the greater
evil and use (NH 4 ) 2 S X , even though copper may be present. 3 For
detecting traces of arsenic, neither the mirror nor the ammonium
molybdate test is very reliable. For this purpose, Fleitman's test,
which is more delicate, is given : In a large test-tube, hydrogen is
generated by heating a concentrated solution of sodium hydroxide
with zinc or aluminum (Gatehouse) nearly to boiling. Introduce the
arsenic solution, and above the liquid near the mouth of the tube
insert a loose plug of absorbent cotton to absorb the moisture.
Spread over the mouth of the tube a cup of filter paper on which is
placed a crystal of silver nitrate. On warming the tube, arsine is
evolved, which first forms a yellow coating on the crystal, quickly
turning black. Another delicate test is Gutzeit's, which is elaborately
described in the U. S. Pharmacopoeia, 1900, p. 521.

96 1 Drs. Emerson and Boggs suggest tin for zinc, and remark :
"We find the precipitation of antimony by means of metallic tin is
better than zinc, as it avoids the co-precipitation of tin with anti-
mony." 2 In this separation the zinc must not be allowed to dis-
solve completely, lest the tin also dissolve and oxidize before testing



168 CHEMICAL ANALYSIS

with HgCl2. On account of the rapid oxidation of SnCl2, it should be
tested quickly with HgCl 2 , after filtering and dissolving with HC1.

981 Only a small quantity of nitric acid should be used, lest some
mercury be dissolved. Should this happen, the metallo-cyanide of
mercury would be decomposed by hydrogen sulphide, and thus the
test for cadmium would be obscured (cf. p. 86).

99 1 Kesidue (a') may also contain sulphur and PbS0 4 . 2 It is
preferable to add H2S04 before evaporating, which would drive out
HNOs. 3 Lead should be entirely precipitated here, lest the remnant
again appear as PbS in the test for cadmium. 4 In filtering this
mixture care should be taken to pour on the liquid slowly with a
glass rod so as to collect all of the precipitate at the apex of the filter.
When carefully washed, a few drops of boiling dilute hydrochloric
acid are added to the residue and filtered into a beaker with about
200 c.c. of water. A solution of sodium chloride hastens the precipita-
tion. 5 For further confirmation of the presence of bismuth, the
white precipitate is allowed to settle and is filtered. Then a solution
of sodium stannite is poured on the filter. The residue turning black
confirms bismuth. Noyes and Bray prescribe the following prepara-
tion of sodium stannite solution : ' ' Add a 10 per cent solution of NaOH
solution to a 10 per cent SnCl 2 solution until the Sn(OH) 2 first formed
is dissolved. The solution must be freshly prepared." 6 Another
method for the detection of cadmium in the presence or absence of
copper is as follows : Whether the ammoniacal solution is blue or not,
add hydrochloric acid just to acid reaction, then some iron filings ;
boil, and filter. Test the filtrate for cadmium by passing hydrogen sul-
phide gas. 7 Should the precipitate be black, CdS would be masked
by the black HgS or PbS. In this event, boil the precipitate in about
25 c.c. dilute H 2 S0 4 , add 50 c.c. water, and pass H 2 S. A yellow pre-
cipitate confirms cadmium.

101 1 For further discussion of this subject, see Ostwald's Founda-
tions of Analyt. Chem. and Alexander Smith's General Inorganic
Chemistry. 2 Drs. Emerson and Boggs comment: "Boettger, fol-
lowing Ostwald, attributes the solubility of Zn(OH) 2 in NH 4 OH to the
formation of the complex ion Zn(NH 3 ) n n varying with the concen-
tration of ammonia. The solubility of Mn(OH) 2 is attributed to the
same cause as that of Mg(OH) 2 , while that of the Co(OH) 2 and
Ni(OH) 2 is attributed to both causes, excess of NH 4 + ions especially
tending to form the complex ions. The explanation of the decreased
solubility of Al, Fe'", and Cr hydroxides is also a little different;



NOTES 169

for example, when A1(OH) 3 dissolves in the alkalies, it gives the ion
A10 3 ~. Water reacts with it thus : A1O 3 plus 3 H 2 O give A1(OH) 3 plus
3 (OH). Therefore, any suppression of the OH~ ions (as by addition
of NH 4 C1, which is a product of the reaction) tends to cause it to go
to completion, with the formation of insoluble A1(OH) 3 ." These
writers then facetiously conclude: "It looks as though there are
enough theories for everybody to be suited."

103 1 Theory of solution, p. 18. 2 Love"n, Zeit.f. anorgan. Chem.,
2, 404 (1896).

104 1 When ammonium hydroxide is first added in excess in the
cold a pink solution is formed, but on boiling chromium hydroxide is
reclaimed.

107 1 Single ferrous salts oxidize so quickly in the air that it is
deemed expedient to use the double ammonium ferrous salt. This,
however, has the disadvantage of introducing an ammonium ion
which retards the precipitation by ammonium hydroxide and so-
dium hydroxide.

108 1 Kriiss and Moraht give this compound as Fe(CNS) 3 9 KCNS.
Ber. d. chem. Ges., 22, 206. 2 Borates and fluorides are so rarely
encountered in ordinary analysis that they will not be considered here.

110 1 Phosphoric acid is tested for at this stage in order to deter-
mine which procedure to follow, In Absence of Phosphates or In
Presence of Phosphates. 2 The action of ammonium chloride with
ammonium hydroxide on the solubility of certain hydroxides may be
better understood by this additional note to the discussion of the sub-
ject, Influence of Ammonium Salts, pp. 101-104. Using more recent
lettering, let Ci, C 2 , C 3 , respectively, represent a, 6, c in the equation
a b = c - k (cf. p. 18). In order that any hydroxide may be precipi-
tated, the product C 3 K in the equation Ci C 2 = C 3 K must reach
its maximum or so-called solubility product. In the cases of various
hydroxides, averages of the solubilities of classes of them are given
for comparison :

1. Na, K 20 moles a liter

2. Ba, Sr, Ca 0.1

3. Mn, Mg 0.0002 "

4. Fe, Cr, Al, Zn, Co, Ki . . . 0.00004 "

Now if an ammonium salt say, ammonium chloride is added to a
weak ammoniacal mixture of these classes, what will happen ? Class 1
is highly ionized and very soluble ; Class 2 is less ionized and less



170 CHEMICAL ANALYSIS

soluble than Class 1 ; Class 3 is less ionized and less soluble than
Class 2 ; Class 4 is poorly ionized and sparingly soluble. Classes 1,
2, and 3 are all better ionized than ammonium hydroxide, and when
ammonium chloride is added, by the common ion principle, the excess
of NH 4 + ions will suppress the OH~ ions from the better dissociated
hydroxides, and drive them back from the solubility product, thus
preventing precipitation. In Class 4, however, the members are so
poorly ionized and so insoluble in water that NIU+ ions of ammonium
chloride have no opportunity to suppress the OH~ ions of the hydrox-
ides. The results are that ammonium chloride in the presence of
slight excess of ammonium hydroxide redissolves manganese and
magnesium hydroxides, but does not affect those of ferric iron, chro-
mium, aluminum, zinc, cobalt, and nickel. All of these conditions
obtain only when the excess of ammonium hydroxide is slight. When
a large excess of ammonium hydroxide is used, the conditions and
results are different from those just discussed. Ferric hydroxide is
practically unaffected, but all the other metallic hydroxides consid-
ered are more or less redissolved. The re-solution of aluminum
hydroxide is caused by its amphoteric or dual nature. It is both a
weak acid and a weak base, and dissociates into two systems of ions.
As an acid it loses water and dissociates into H+ and the aluminate
anion A10 2 ~; as a base it dissociates into the aluminum cation A1+
and 3 OH~. Now when ammonium hydroxide is added, its OH~ ions
unite with the 11+ ions to form water. But the system C H CA1O2 ' s
constant, and the suppression of its H+ ions causes more A1(OH) 3 to
be dissociated into H+ and A10 2 ~ until, if sufficient ammonium hydrox-
ide is added, the whole of the A1(OH) 3 runs into H+ and A1O 2 ~.
During the progress of these changes the free NH 4 + ions unite with
A1O 2 ~ ions to form C N H 4 C A io 2 of high solubility product. Should a
better ionized base say, sodium hydroxide be used, solution will
be effected more quickly and with less of the reagent. As regards
chromium, zinc, manganese, cobalt, nickel, and magnesium, an excess
of ammonium hydroxide unites with them to form complex metallo-
ammonia cations, thus requiring more molecules of the hydroxides of
the simple cations to reach concentration. The effect, of course, is to
redissolve these hydroxides in varying degrees, nickel being affected
most and chromium least. The addition of ammonium chloride to
an ammonium hydroxide solution of the hydroxides in question
weakens the ionization of the ammonium hydroxide by suppressing
the OH~ ions. This prevents the re-solution of aluminum hydroxide.



NOTES 171

On the other hand, the increase in the number of NH 4 + ions increases
the number of metallo-ammonia ions of the other metals, and thus
prevents their precipitation. 3 Barium carbonate is added to insure
a complete separation of the iron group from manganese. In the
presence of ammonium salts, ammonium hydroxide does not pre-
cipitate manganese hydroxide, but the tendency of the latter is to
oxidize quickly to an insoluble basic oxide, which frequently precipi-
tates it out of due course. Barium carbonate forms with the members
of the iron group insoluble basic carbonates, but produces no precip-
itate from neutral or slightly acid solutions of manganese salts.
4 Filtrate (6) contains barium which was added as a reagent, and, of
course, it will be found in Group V. But a test for its presence was
made before adding BaCO 3 . Phosphates of barium, strontium, cal-
cium and magnesium are insoluble in alkalies, but are soluble in acids.
Hence if these salts are present, they are precipitated by NH 4 OH and
redissolved by HC1. For this reason filtrate (&) should be combined
with filtrate (a).

1141 Unless care is taken to perform the experiment in the cold,
the compound ZnS0 4 4 NH 3 will be formed instead of (NH 4 ) 2 ZnO 2 .
2 Green and more stable manganous sulphide may be formed by
boiling the pink variety with an excess of the reagents, ammonium
sulphide and ammonium hydroxide.

118 1 This washing should be done with warm water containing a
little (NH 4 ) 2 S. The appearance of manganese in the filtrate is due to
oxidation or the formation of a colloidal precipitate on account of
having no "salt" in the wash water. Also, we find that the NiS may
appear in the filtrate where (NH 4 ) 2 S is used as a precipitant. This
may be precipitated by boiling with acetic acid. 2 If the NaOH is
not added in considerable excess, the Zn(OH) 2 will reprecipitate on
boiling. Hence, according to Fresenius it is best not to boil, but to
stir with cold NaOH solution.

119 1 In case manganese is not found in residue (a), this brown
precipitate is to be examined for manganese. After filtering, if the
brown color persists, it is probably nickel sulphide, which can be pre-
cipitated by boiling with acetic acid. 2 Possibly a better way of
confirming manganese is to evaporate the solution to dryness and
fuse with dry Na 2 CO 3 and KN0 3 to quiet fusion on a platinum foil.
A green mass on cooling confirms manganese.

120 1 A large excess of KCN should be avoided, as the excess
must be neutralized with NaBrO. On the other hand, a small excess



172 CHEMICAL ANALYSIS

must be added both to precipitate cobaltous cyanide and to redissolve
it as a double cyanide.

126 1 NH 4 OH only partly precipitates Mg(OH) 2 . 2 Sodium co-
baltic nitrite, Na 3 Co(N0 2 ) 6 , may be substituted for H 2 PtCl 6 in testing
for ammonium and potassium compounds. In this event, substitute
acetic acid for alcohol.

133 1 For complete descriptions of this method, see Fresenius'
Quantitative Chem. Analysis, 16th ed., p. 117, and Crookes' Select
Methods in Chem. Analysis, p. 26.

146 1 Edward Hart gives the following excellent method for test-
ing chlorides, bromides, and iodides in presence of each other : The
iodide is oxidized with ferric sulphate and the iodine distilled off and
tested with starch paper ; the bromide is then oxidized with potassium
permanganate, and the bromine removed and tested by distilling in
chloroform ; and the liquid is finally tested for the chloride with
silver nitrate. Amer. Chem. Journ., 6, 346.

1511 Professor Slangier adds this instructive supplement to prelimi-
nary tests for solids : "If the substance is a solid, apply the follow-
ing tests : (1) If it is a paint or pigment, extract for several hours
with ordinary ether, and use residue for subsequent tests. (2) If it
contains arsenic or other volatile constituents, add concentrated sul-
phuric acid to decompose and nitric acid repeatedly to oxidize, heat-
ing over a small flame. Continue until the liquid is clear and colorless.
Then evaporate until fumes of S0 3 appear. Dilute, filter, and proceed
as usual. (3) If it contains aluminum or is largely mineral matter, as
a baking powder, add concentrated nitric acid and heat at first gently
and then more strongly. Repeat the operation several times until
carbonaceous matter is consumed. Then add hydrochloric acid and
boil. Proceed as usual."

152 1 Noyes and Bray prefer to treat the residue with concentrated
sulphuric acid and hydrofluoric acid in lieu of alkali carbonates.
In some respects this has its advantages, but the method requires the
frequent use of expensive platinum crucibles by inexperienced stu-
dents ; and also in the presence of barium (and to some extent stron-
tium and lead) difficultly soluble sulphates are formed. Journ. Amer.
Chem. Soc., 29, 140. 2 In the event the two solutions produce a
precipitate or the fusion does not effect solution, the aqueous extract
or mixture from the crucible is evaporated to dryness. The mass is
then taken up with boiling water and filtered. The filtrate contains
the chlorides of sodium and potassium, and those of barium and



NOTES 173

strontium may be present. The filtrate can be tested directly for
barium and strontium (cf. p. 124). The residue may be silica and the
chlorides of silver and lead. Boil with ammonium acetate and some
dilute acetic acid. Filter and wash thoroughly with hot water. Test
filtrate for lead with potassium chromate. Warm residue with potas-
sium cyanide solution. Filter, and test filtrate for silver by adding
excess of nitric acid. (This must be done in a hood to avoid breathing
the deadly prussic acid.) Transfer the residue to a platinum crucible,
and test for silica by adding solution of hydrofluoric acid and a few
drops of sulphuric acid.

1531 In order to guard against accidents and as a reserve for tests
for ammonium, a portion of this original solution should be set aside
for future examination. 2 If the solution is alkaline, it may contain
various solutes, namely, silver salts, sulpho-salts, silicic acid, metallic
hydroxides, etc. In this condition the solution should be acidified
with nitric acid, and if a precipitate forms, more acid should be added
and the solution warmed. If the precipitate does not redissolve, the
mixture is filtered and the residue examined as other substances insol-
uble in water. The filtrate is evaporated to expel nitric acid, diluted
with water, and examined as other acid or neutral solutions. 3 No
matter what the valence of iron was originally, at this stage of the
analysis it must necessarily be bivalent or ous. See reactions, bottom
of p. 107.

1541 If, on standing, a brown precipitate is formed, filter, and test
residue for manganese ; if the filtrate remains brown, boil with acetic
acid, filter, and test residue for nickel. Use filtrate for Group IV.



INDEX



PAGE

Acid, acetic 150

" boric 135

" carbonic 130

" chloric 149

" chromic 130

" hydrobromic . . . 142
" hydrochloric . . .141
" hydrocyanic . . . 143
" hydroferricyanic . . 144
u hydroferrocyanic . . 143
" hydrofluoric . . . .137

" hydriodic 142

" hydrosulphocyanic . 144
" hydrosulphuric . . .144

" nitric 148

" oxalic 136

" phosphoric . . . .135

" silicic 131

" sulphuric 134

" sulphurous .... 134

" tartaric 136

Acids, analytical classification

of 75

" reactions for .... 130
" analysis for . . . .155

Alcohol 72

Aluminum 100

Ammonium 126

Ammonium salts, reagents . 69
Analytical classifications . 73
Analytical groups . . . . 75
Antimony 91



PAGE

Aqua regia 68

Arsenic, arsenic .... 90

" arsenious . . . . 88
Barium \ 121

" chloride .... 69

Bismuth 83

Blowpipe 40

Borax . . 72

" beads .... 46, 105

Bromine 70

Cadmium 87

Calcium ....'... 123

" hydroxide ... 70
Carbon disulphide .... 72

Chlorine 70

Chromium . . . . . . 104

" salts, reagents . 71
Cobalt 116

" nitrate 70

Copper 84

Crucibles 41

Decantation 26

Dissociation . . . . . . 13

Ether 72

Evaporation 38

Filtration 27

Flame 40

Fusion 43

Fusion mixture 43

Group I, acids .... 130

"II, " 141

" III, " . . . . . 148



175



176



CHEMICAL ANALYSIS



PAGE

Group I, metals .... 7^

"II, " .... 82

" III, " .... 100

" IV, " .... 114

"V, " .... 121

" VI, " .... 126

Ignition 39

Ions 15

Iron, ferric 107

" ferrous 107

" salts, reagents ... 70

Lead 80

" acetate 70

" peroxide 72

Magnesium 126

** sulphate ... 70

Manganese 114

" peroxide ... 72

Marsh's test 89

Mercuric chloride .... 70
Mercury, mercuric .... 82
" mercurous ... 78
Metals, analytical classifica-
tion of 75



Metals, reactions for . . . 77

" analysis of. ... 153

Nickel 117

Potassium 127

" hydroxide ... 71

" salts, reagents . . 71

Precipitation 33

Separations .26

Silver 77

" nitrate 71

Sodium 127

" hydroxide . . . . 71

" peroxide .... 72

" salts, reagents . . 71

Solution 6

Spectra 54

Spectroscope 53

Spectroscopy 52

Strontium 122

Tin chloride 71

" stannic 93

" stannous 93

Washing 29

Zinc . 114






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