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Walter Scott Hendrixon.

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of filtrate dil. sulfuric acid and let stand. A white precipitate shows
lead sulfate. To the other add potassium chromate. A yellow preci-
pitate is lead chromate. -

Treat the remaining precipitate on the filter with about 5 c.c. of
NH\OH. Pass it through a second time. Now add to this filtrate an
excess of dil. nitric acid, making sure of an excess by mixing and
testing. AgCl is obtained if silver is present. The blackening of the
residue left on filter when ammonia was added shows Hg 2 present, the
black substance being Hg and HgNH 2 Cl.

Write equations for all reactions that occur in the analysis of
Group I.

149. Group II; As, Sb, Sn, Hg, Pb, Bi, Cu, Cd: Heat filtrate (1) from
group (I) nearly to boiling and pass in H 2 S for about 10 minutes keep-



68 LABORATORY MANUAL OF GENERAL CHEMISTRY

ing the temperature near the boiling point so as to precipitate arsenic
from arsenates if present. Now let cool, add an equal volume of wa-
ter and pass in the gas for several minutes to insure the complete
precipitation of cadmium and tin. Even then it is well to let the pre-
cipitate settle, pour off a few drops of the liquid, add water and more
gas. Disregard a white precipitate which might be ZnS. When as-
sured that the precipitation is complete let the precipitate settle and
pour off the solution through a filter. At once boil it to expel H 2 S and
set it aside as filtrate (2) for group III.

Wash the precipitate four times by decantation; that is, pour upon
it hot water, boil, let settle completely and pour off the wash water.
Drain carefully the last time since much water present will dilute the
nitric acid to be used.

If it is known that As, Sb and Sn are absent omit the bracketed di-
rections, and boil with dil. nitric acid as directed after the bracketed
lines. If not known whether they are present or not proceed as di-
rected within the brackets.

[Now heat a small portion of the washed precipitate with about 3
c.c. of yellow ammonium sulfide, best in a dish on a water bath with
stirring. In any case do not boil. Filter off the solution and add to fil-
trate dil. HC1 till acid throughout, and boil. If only white finely di-
vided sulfur is precipitated As, Sb and Sn are absent. In this case pro-
ceed to boil the remainder of the precipitate with nitric acid as after
the brackets. If, however, a flocculent yellowish precipitate is ob-
tained one or more of the above metals are present. Treat the re-
mainder of the washed precipitate with ammonium sulfide as directed
for the small portion, filter and set aside to be examined for As, Sb
and Sn, as described within the brackets below.

The residue consisting of sulfides undissolved by ammonium sul-
fide must be well washed on the filter. Punch through the tip of the
filter, wash the sulfides into a test tube and let settle.] Pour off as
much as possible of the water, add about 10 c.c. dil. nitric acid and
boil persistently. All the sulfides except that of Hg are dissolved. A
floating residue of S often remains and is to be disregarded. A heavy
residue which settles at once to the bottom must be tested for Hg. Fil-
ter off the HgS, add to the nitrate 10 cc. dil. sulfuric acid and begin its
evaporation in a porcelain dish. To confirm the presence of Hg dis-
solve the sulfide on the filter by pouring upon it about 3 c.c. of hot
con. HC1 to which a little potassium chlorate has been added. Heat and
pass through again if necessary. Add to the solution bromine water
till the color of Br persists, boil out the excess and add a few drops of
a solution of stannous chloride. A white precipitate turning to gray
and perhaps to black shows Hg present.

Evaporate the nitric acid solution containing the other metals till



LABORATORY MANUAL OF GENERAL CHEMISTRY 69

heavy white fumes of sulfuric acid freely come off. The evaporation
must be carried far enough to expel all the nitric acid. (See 131) Let
the dish and contents cool, then add 10 c.c. of water and let stand 5
minutes. A white precipitate shows lead sulfate. Filter it off and add
to filtrate an excess of ammonium hydroxide, making sure of an ex-
cess by shaking to mix and then testing with turmeric paper. A white
precipitate shows Bi but its presence should be confirmed thus: Fil-
ter it off. Dissolve on the filter in a few drops of con. HC1, evaporate
off nearly all of the acid. (Why?) Pour into much water. A white
cloud is BiOCl.

The filtrate from the bismuth hydroxide is blue if copper is pres-
ent. If copper is absent add hydrogen sulfide to the filtrate which will
give yellow CdS if Cd is present. If the blue color shows Cu present
test for Cd by one of the following:

(a) Add to the blue solution a solution of potassium cyanide (very
poisonous) till the blue color disappears, then hydrogen sulfide which
will give yellow CdS if Cd is present.

(b) Add to the blue solution dil. sulfuric acid till colorless then
card teeth or steel wool and boil for some time. The iron removes the
Cu. Why? Filter, be sure that the filtrate is still acid. If not add a
slight excess of sulfuric acid and then H 2 S which will give yellow CdS
if Cd is present.

[To the (NH 4 ) 2 S solution containing As, Sb and Sn, add dilute HC1 in
excess, which re-precipitates the sulphides if present. A precipitate
is always produced owing to the separation of sulphur. If the sul-
phides are present, however, the precipitate is more highly colored and
somewhat flocculent. Filter and wash the sulphides, carefully remove
to a test tube and heat with strong HC1. Sb 2 S 3 and SnS 2 are dissolved,
while AsaSs is not. Dissolve the As 2 S 3 in hot, strong HC1 and KCIO*,
boiling if necessary, best in a small flask placed in the evaporating
closet. Add to the solution NEUOH in excess then NH 4 C1, filter if not
clear and add MgSO 4 , which will give on standing a precipitate of
NH 4 MgAsO 4 if As is present.

Boil persistently the filtrate from the arsenic sulfide with card
teeth. The Sb is deposited on the card teeth from which it can be de-
tached in scales by stirring with a glass rod, and the tin remains in so-
lution as SnCl 2 . Filter off the antimony and test the filtrate for tin
with HgCl 2 . Wash the antimony thoroughly, dissolve on the filter with
a very little aqua regia, dilute with 10 to 20 times its volume of water
and add H 2 S which will give orange yellow Sb 2 S 3 L

Write equations for all reactions involved in Group II assuming
that all metals are present in the beginning as chloride.

150 Group III: ..Fe, Al, Cr, Co, Hi, Mn, Zn: To a small portion
of filtrate 2 from Group II add ammonia in excess as proved by shak-



70 LABORATORY MANUAL OF GENERAL CHEMISTRY

ing and testing. If a precipitate is formed one or more of the metals,
Fe, Al, Cr are present. If no precipitate is formed add H 2 S; or, if a
precipitate was formed by ammonia filter it off and add H 2 S to the fil-
trate. If H 2 S causes a precipitate which is not black Co, and Ni are
absent. A black precipitate means either Co or Ni or both and they
may mask Mn and Zn. By making these preliminary tests and atten-
tion to the following much work and time may be saved.

If no precipitate was formed by either ammonia or H 2 S proceed
with the rest of the filtrate to Group IV. If a precipitate was formed
only with ammonia there is no need to test for Co, Ni, Mn, Zn. In this
case add ammonia to all the filtrate, filter off the hydroxides of Fe,
Al, Cr, and save the filtrate for Group IV, omitting the addition of H 2 S
below and also that part of the directions applying to Co, Ni, Mn, Zn.
If H 2 S made a precipitate while ammonia did not Al and Cr are ab-
sent but Fe may be present if the precipitate was black.

The following scheme provides of course for the presence of all
these metals:

To filtrate (2) from Group II add ammonia in slight excess test-
ing after shaking, then add about 2 c.c. more. Heat the solution in a
flask or large test tube nearly to boiling and pass in H 2 S. Shake and
heat frequently to make the precipitate granular and more easily filt-
ered and washed. When the precipitation is apparently completed fil-
ter a small portion and pass into it more gas. If more precipitate forms
add more H 2 S to the whole and thus proceed till complete precipita-
tion is attained.

Filter through a fluted filter and wash with hot water. Boil the
filtrate (3) till all hydrogen sulfide is expelled, filter it and set aside
for Group IV. As soon as the precipitate produced by H 2 S is suffi-
ciently washed, (about two funnels full of hot water), punch through
the tip of the filter and wash most of the precipitate into a small
flask with a fine stream of water from the wash bottle. At once add
considerable excess of dil. HC1. Only NiS and CoS remain undis-
solved. Filter, reserve filtrate for B and test for Ni and Co in A.

A. Dissolve the NiS and CoS in the filter in about 3 c.c. of aqua
regia and evaporate the solution nearly to dryness in a dish. Add about
4 c.c. water then NaOH drop by drop with shaking till a permanent
precipitate is obtained. Avoid a large excess of NaOH. Divide the li-
quid and suspended precipitate into two portions, and test for Co and
Ni as follows:

1. To one portion add about 2 cc. acetic acid, 30 per cent, and 15
c.c. of a 25 per cent solution of potassium nitrite and let the solution
stand half an hour. A yellowish, white granular precipitate is



LABORATORY MANUAL OP GENERAL CHEMISTRY 71

2. To the other half add a solution of potassium cyanide (dan-
gerous) drop by drop till the precipitate just dissolves. It is essen-
tial to avoid a large excess. Warm the solution, add about 5c.c. NaOH
and then bromine water till the color of Br persists. A black precipi-
tate often forming after some time is Ni(OH) 8 .

B. Evaporate the filtrate from the CoS and MS nearly to dryness,
add about 5 c.c. water, make strongly alkaline with NaOH and add
gradually with stirring about 2 grams of sodium dioxide. Boil a few
minutes, add water and filter off the Fe and Mn hydroxides. Save the
nitrate for C and wash the precipitate. To test for Mn make a
bead of sodium carbonate, while hot touch to powdered KC1O 3 . Let
cool, take up with it a little of the precipitate and fuse with the blow-
pipe. A green bead shows Mn present. To confirm the presence of
Mn and test for Fe remove a small portion of the precipitate from the
filter, dissolve it in dilute nitric acid and add a solution of potassium
ferrocyanide which will give a dark blue color if Fe is present. Now
pour upon the remaining precipitate about 3 c.c. of hydrogen dioxide
and then add 10 c.c. nitric acid, sp. gr. 1.2, which means practically 1
vol. of pure concentrated acid diluted with 2 parts of water. Warm
and pass through again if necessary. To this solution add a little at a
time 2-4 grams of lead dioxide or red lead and heat to gentle boiling
for a few moments. Let the suspended matter settle when the liquid
above it will be colored red by permanganic acid if Mn is present.

C. To the nitrate from the Fe and Mn hydroxides which may con-
tain Al, Cr and Zn, add con. nitric acid in slight excess testing with
litmus paper. Add 5 c.c. ammonium chloride, heat and add ammonia
in slight excess as shown by test after stirring. A white flocculent
precipitate shows aluminium hydroxide. Filter it off and acidify the
filtrate with acetic acid, mixing and testing with litmus paper. The
liquid is yellow if chromate radical is present. Add barium chloride
which will give yellow BaCrO4. The BaCl 2 must be added in excess to
remove all the CrO4 which would interfere with the test for Zn. Filter
and pass through several times if necessary. The clear filtrate should
not be yellow. If it is add more BaCl 2 and filter again. To the clear,
colorless filtrate add H 2 S which will give an evident, white precipi-
tate if Zn is present. Hydrogen sulfide always produces an opales-
cence at this point and this is to be disregarded.

GROUP IT.

151- If the analysis is being carried through all the groups the
solution used will be nitrate (3), in which case it should be made
slightly acid with dilute HC1, evaporated to two-thirds of its volume,
and ammonium hydroxide added till alkaline then ammonium carbon-
ate. If the solution is an original one, add ammonium chloride, am-



72 LABORATORY MANUAL OP GENERAL CHEMISTRY

monium hydroxide till alkaline then ammonium carbonate till the pre-
cipitation is complete. To determine this heat the solution nearly to
boiling, let settle and add a few drops of the carbonate to the clear so-
lution. Filter and to a small portion of the filtrate add sodium phos-
phate which will give (NH 4 )MgPO 4 if magnesium is present. Reserve
the remainder of the filtrate (4) for Group V. Wash the carbonates
on the filter and dissolve by pouring upon the filter not more than 5
c.c. of acetic acid, letting it run through into a clean tube. Run through
several times if necessary to dissolve all the carbonates. To a small
portion of the acetic acid solution add twice its volume of a saturated
solution of calcium sulfate. Note carefully whether the precipitate is
formed at once or only after a few seconds. Proceed according to one
of the following as required, using the rest of the acetic acid solution:

(a) If no precipitate was formed, to the remainder of the acetic
acid solution add ammonia in excess and ammonium oxalate which will
precipitate calcium oxalate.

(b) If the precipitate was slowly formed, only Sr and Ca can be
present. To the acetic acid solution add a solution of ammonium sul-
fate and let stand. Filter off the strontium sulfate, and to the fil-
trate add ammonium hydroxide till alkaline then ammonium oxalate
which will precipitate CaC 2 O4, if Ca is present.

(c) If the precipitate with CaSO 4 was immediate Ba is present,
and the other two may be. To the acetic acid solution add an excess
of pure potassium chromate solution. Filter off the barium chromate.
Make the filtrate alkaline with ammonia and add ammonium carbon-
ate, which will precipitate Ca and Sr carbonates if they are present.
Filter off the carbonates, wash till most of the excess of chromate is
removed, and dissolve on the filter with acetic acid. Test a small por-
tion of the filtrate with CaS0 4 . If no precipitate forms on long stand-
ing Sr is absent and Ca should be tested for as in (a). If Sr is pres-
ent remove it from the remainder of the acetic acid solution with am-
monium sulfate, let stand 5 minutes, filter and test filtrate for Ca as
in (b).

GROUP V.

152. Evaporate a part of filtrate (4) to dryness in a porcelain
crucible; heat till all ammonium salts are expelled, and test with the
spectroscope as directed in lecture for Na, K and Li. If uncertain as
to the spectrum lines and flame colors, compare with spectra of the
known substances.

To test for NH 4 , place in a porcelain crucible a small amount of
lime, add enough of the original solution to moisten it, cover with a
watch glass with a strip of moist turmeric paper on the under side,
and warm gently. If NH 4 be present, NH 8 will be evolved and can be
recognized by its action on the turmeric paper and by its odor.



APPENDIX



153. Atomic Weights of the Common Elements:



Aluminum


Al


27.1


Lead


Pb


207.20


Antimony


Sb


120.2


Lithium


Li


6.94


Arsenic


As


74.96


Magnesium


Mg


24.32


Barium


Ba


137.37


Manganese


Mn


54.93


Bismuth


Bi


208.0


Mercury


ffff


200.6


Boron


B


11.0


Molybdenum


Mo


96.0


Bromine


... Br


79.92


Mckel


IVi


58.68


Cadmium.. .


....Cd


112.40


Nitrogen


IV


14.01


Calcium


...Ca


40.07


Oxygen





16.00


Carbon


...C


12.00


Phosphorus


....p


31.04


Chlorine


....Cl


35.46


Platinum


Pt


195.2


Chromium


.... Cr


52.0


Potassium


K


39.10


Cobalt


...Co


58.97


Silicon....


Si


28.3


Copper


Cu


63.57


Silver


Ag


107.88


Fluorine.. .


F


19.0


Sodium..


...Na


23.00


Gold


Au


197.2


Strontium


Sr


87.62


Hydrogen..


..H


1.008


Sulphur


s


32.06


Iodine . .


I


126.92


Tin .. . .


Sn


118.7


Iron. .


Fe


55.84


Zinc


...Zn


65.37















154. Vapor Pressure of Water in Millimeters of Mercury:



Degrees C.


Pressure


Degrees C.


Pressure


15


12.7


26


25.0


16


13.6


27


26.5


17


14.5


28


28.1


18


15.4


29


29.8


19


16.4


30


31.6


20


17.4


31


33.4


21


18.5


32


35.4


22


20.0


33


37.4


23


20.9


34


39.6


24


22.2


35


41.9


25


23.5







iTendrixon



387354



Lab orate



ry manual of



H4



general che




387354



UNIVERSITY OF CALIFORNIA LIBRARY





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Online LibraryWalter Scott HendrixonA laboratory manual of general chemistry → online text (page 8 of 8)