Wilfred W. (Wilfred Welday) Scott.

Standard methods of chemical analysis; a manual of analytical methods and general reference for the analytical chemist and for the advanced student online

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cooled to room temperature, electrolysis is carried out under the same conditions as
that of the first deposit and on the same foil, if arsenic or antimony is the interfering
impurity; on a fresh foil if selenium or tellurium has been the contaminating element.
The undeposited copper is determined colorimetrically in the mixture of the first and
final electrolytes and added to the weight of the copper deposited.

If the sample contains a large percentage of arsenic or antimony, a portion represent-
ing 2 grams is drawn from a pipette into a Kjeldahl flask, 10 cc. of sulphuric acid added,
and the liquid boiled until nitric acid has been expelled. From this solution cuprous
sulphocyanate is precipitated according to the method described on page 153. The
funnel containing the filter is placed in a 500 cc. flask with long neck, the filter is punc-
tured and the precipitate washed into the flask with the least quantity of water possible,
the adherent precipitate is dissolved from the filter with warm dilute nitric acid, added
cautiously to avoid violent evolution of gases from the dissolving precipitate in the
flask. The washed filter is incinerated and the solution of its ash by nitric acid reserved
for addition to the electrolyte after completion of electrolysis. When solution of the
precipitate is complete, the liquid is boiled to small volume, neutralized, and 5 cc.
ammonium nitrate solution and 3 cc. excess free nitric acid added. The liquid is
transfered to an electrolytic jar and electrolysis carried out in the manner already
described.

The amounts of bismuth, arsenic, antimony, selenium or tellurium usually found
in blister copper may be precipitated together with iron present by addition of ammonia
to a pipetted portion. The filtered precipitate is purified of copper by solution with
nitric acid and reprecipitation. The combined filtrates are neutralized, 3 cc. of free
nitric acid added and the solution electrolyzed under the conditions already described.
The nitric acid solution of the incinerated filter, carrying the iron, etc., is added to the
electrolyte after electrolysis is complete. The undeposited copper is determined
colorimetrically by one of the procedures outlined on pages 165, 166 or 167.

The deposited copper is never absolutely pure. The total impurities seldom
exceed 0.03%. Ag from 0.000 to 0.18%; As from 0.000 to 0.003%; Sb from 0.000



162 COPPER

to 0.004%; Se and Te from 0.001 to 0.027%; Bi from 0.000 to 0.0003%. Periodical
complete analyses may be made and corrections applied to the analysis when ex-
ceedingly accurate percentages are required.

Too low a current density or excessive oxidizing power of the electrolyte may pro-
duce high results, due to the oxidation of the deposited copper. Too high a current
density or a deficiency of oxidizing power in the electrolyte, by causing a deposition
of impurities, will give high results.

The electrodes used by the Nichols Copper Co. are straight platinum wires for
the positive ends and cylinders If in. long, 1 in. in diameter of 0.004 in. irido-
platinum foil, ll sq. in. depositing surface, tor the cathodes.

A uniform current is essential.

The nitric acid used should be free of iodic acid.

The presence of oxide of nitrogen gases, or a chloride in an acid solution, will cause
a coarsely crystalline or brittle deposit, under conditions which in their absence would
produce a good plating. The deposit moreover may contain platinum from the anode
if the electrolyte contains a chloride salt.

Silver may be removed from the electrolyte by nitration, upon precipitation as a
chloride, or it may be deposited with the copper and correction made for its presence
from the result of a separate assay. In the latter case the copper deposits in poor
form, unless the silver be first plated out at a very low current density.

Solid matter, unless removed, will contaminate the deposit mechanically.

Arsenic, antimony, selenium or tellurium have an influence on the physical character
of the deposit which may affect the copper result beyond the sum of such impurities
deposited.

In the process of preparing an electrolyte, arsenic may be eliminated as arsenious
fluoride in the decomposition of silicious material by hydrofluoric acid. Selenium is
expelled by evaporation to dryness of a hydrochloric acid solution or by fuming a sul-
phuric acid solution. All impurities may be removed by occlusion with ferric hydroxide;
several times their weight of iron being added and the hydroxide then precipitated with
ammonia. In the handling of copper solutions account is to be taken of the retention
of copper in the ferric hydroxide precipitate and the combination of copper in ammo-
niacal solution with cellulose.

Whether impurities are deposited or not, appreciably high results are obtained by
continuing electrolysis for some time after the electrolyte has become impoverished
of copper.

Overheating of the copper deposit, in the process of ignition of the alcohol clinging
to the cathode, will cause oxidation of the copper. As much as possible of the alcohol
must be shaken off before passing the electrode rapidly through the flame. It is advis-
able to weigh the copper shortly after deposition, as prolonged contact with air is unde-
sirable, if extreme accuracy is desired.

The copper deposits may be removed by plunging the electrode, for a few moments,
in hot nitric acid. After washing with water, the foil is ignited to a cherry red in a
direct colorless flame. The ignition removes any grease which would be objectionable,
that may contaminate the platinum. Alcohol frequently contains oily matter which
will cling to the electrode in spite of the rapid ignition for drying the deposit,



OTHER METHODS
Determination as Cuprous Sulphocyanate

The procedure has been outlined under Separations on page 153.

CuCNSX0.5226=Cu.

Determination as Copper Oxide l

The solution, free from ammonium salts and organic matter, is heated to
boiling in a porcelain dish and pure potassium hydroxide solution added, drop
" Analytical Chemistry," Treadwell and Hall.



COPPER 163

by drop, until a permanent precipitate, dark brown in color, is formed. The
solution is alkaline to litmus-paper. The precipitate is washed by decantation,
transferred to the filter and washed with hot water free of alkali. The precipi-
tate and filter are ignited in a porcelain dish, first gently and finally with the
full heat of a Bunsen burner. The residue is weighed as CuO.

CuOX0.7989=Cu.

VOLUMETRIC METHODS FOR THE DETERMINATION

OF COPPER

Potassium Iodide Method

The procedure depends upon the fact that cupric salts when heated with
potassium iodide liberate iodine, the cuprous iodide formed being insoluble in
dilute acetic acid is thus removed, no reversible reaction taking place.

Reactions. 2CuS0 4 +4KI = 2CuI+2K 2 S0 4 +I 2 .
The liberated iodine is titrated with standard thiosulphate.
2Na 2 S 2 3 +2I =Na 2 S 4 6 +2NaI.

This method is exceedingly accurate. Very few metals interfere. Bismuth,
selenium, trivalent arsenic, antimony or iron should not be present. Lead, mer-
cury, and silver increase the consumption of iodide, but do not otherwise interfere.

Solutions. Sodium Thiosulphate. 7.5 grams of the salt, Na 2 S 2 3 -5H 2 0,
are dissolved and made to 2 liters with water. The solution is standardized against
a copper solution containing 1 gram of pure copper per liter, 1 cc. =0.001 gram
Cu. Approximately the same amount of copper is taken as will be determined
in the ores. For high-grade copper ores and crude copper, etc., it is advisable
to prepare a standard thiosulphate solution ten times the above strength. The
copper solution is made slightly ammoniacal and then acid with acetic acid.
Potassium or sodium iodide crystals, free from iodate, are added and the liberated
iodine titrated with the standard thiosulphate. (See Procedure.)

Weight of copper taken . . .

. . : . = value of 1 cc. of the thiosulphate solution.

cc. thiosulphate required

Standard Copper Solution. One gram of purest electrolytic copper is dis-
solved in 20 cc. of dilute nitric acid, sp.gr. 1.2, and the solution diluted to 1000
cc. For standardizing the thiosulphate to be used with high-grade copper
ores, crude copper, blister copper, etc., a copper solution containing ten times
the above amount of metallic coppers is prepared.

The following additional reagents are required: starch solution, solid potas-
sium iodide, 50% acetic acid solution, and other common laboratory reagents.

NOTE. Sodium thiosulphate is apt to change in strength upon standing, so
that restandardization is necessary.

Procedure. The solution containing the copper, separated from inter-
fering elements, by precipitation with aluminum powder or potassium sulpho-
cyanate, is evaporated to about 30 cc. and the free acid neutralized with sodium
carbonate, or ammonia, and then made slightly acid with acetic acid, 1 : 3,
the solution becoming clear, about 3 grams of potassium iodide, or the equivalent



164 COPPER

of a saturated solution, are added and the liberated iodine titrated with standard
thiosulphate, the reagent being added until the brown color changes to light
yellow and after the addition of starch solution until the blue color fades out.
The end-point is very sharp.

Cc. thiosulphate multiplied by value of reagent gives weight of copper in
sample.

NOTES. Nitrous oxides should be expelled before neutralizing with alkalies. A
large excess of acetic acid should be avoided. The solution should be cool and con-
tain at least 6 parts of KI for 1 of Cu, e.g., 1 Cu = 5.2231 KI = 1.9965 1 = 3.9034
NfteSgOs -5H 2 O. The solution should be concentrated, 40 to 50 cc.

Prof. Gooch recommends a volume of 100 cc., containing no more than 3 cc.
nitric, sulphuric, or hydrochloric acids, or 25 cc. of 50% acetic acid, with 5 grams
potassium iodide. Two to 3 grams more of potassium iodide are added if the titra-
tions are large. "Methods in Chemical Analysis."

When ferric iron is the only disturbing impurity and no nitrates are present, the
necessity of separation of copper may be avoided by fixing the free mineral acid by
use of sodium acetate and then adding a clear, 4^ per cent solution of sodium fluoride
until the red color of ferric acetate has bleached and then an excess of 10 cc. (Jour.
Sci. Chem. Ind., May 15, 1915, p. 462; Mott, Chemist Analyst, July, 1912.)

Arsenic or antimony when present in trivalent form may be oxidized by treatment
with bromine, chlorine, hydrogen peroxide or potassium permanganate, care being
taken to expel or reduce any excess of the oxidizing agent before titration.

Potassium Cyanide Method

This procedure is largely employed on account of its simplicity, although
it does not possess the degree of accuracy of the Iodide Method. The procedure
depends upon the decoloration of an ammoniacal copper solution by potassium
cyanide.

The operations of the standardization of potassium cyanide and of making
the assay should be as near alike as possible. If iron is present in the assay
it should be added to the standard copper solution titrated, in order to become
accustomed to the end-point in its presence.

Silver, nickel, cobalt, cadmium, and zinc interfere and should be removed
if present in appreciable quantities. Precipitation of metallic copper by alumi-
num powder, as directed under Separations, is recommended as a procedure
for iron ores and briquettes. In presence of smaller amounts of iron, the titra-
tion may be made in presence of iron suspended in the solution. It is not
advisable to filter off this precipitate, as it invariably occludes copper. With
practice, the shade of color the iron precipitate assumes at the end of the reac-
tion serves as an indicator, so that the operator is assisted rather than retarded
by its presence. l

2Cu(NH 3 )4S0 4 -H 2 O+7KCN =

K 3 NH4Cu 2 (CN) 6 +NH 4 CNO+2K 2 S04+6NH3+H 2 0.

Standard Potassium Cyanide Solution. Thirty-five grams of the salt are
dissolved in water, then diluted to 1000 cc.

Standardization. 0.5 gram of pure copper is dissolved in a flask by warming
with 10 cc. of dilute nitric acid (sp.gr. 1.2), the nitrous fumes expelled by boiling,
the solution neutralized, diluted and titrated as directed under Procedure.

Button, " Volumetric Analysis." Davies, C N., 58, 131. J. J. and C. Beringer,
C. N., 49, 3. Dr. Steinbeck, Z. a. C., 8, 1; C. N., 19, 181.



COPPER 165

If iron is present in the samples titrated, it is advisable to add iron to the
standard copper solution as directed above.

* CC.KCN Solution =Wt " Cu per cc ' of standard KCN '

Procedure. The solution containing the copper is neutralized with sodium
carbonate or hydroxide, the reagent being added until a slight precipitate forms.
One cc. of ammonium hydroxide is now added and the solution titrated with
standard potassium cyanide solution. The blue color changes to a pale pink;
finally a colorless solution is obtained. In presence of iron, when the copper
is in excess of the cyanide, the iron precipitate possesses a purplish-brown color,
but, as this excess lessens, the color becomes lighter until it is finally an orange
brown, the solution appearing nearly colorless. The reagent should be added
from a burette drop by drop as the end-point is approached.

Cc. KCN X factor per cc. = weight Cu in assay.



COLORIMETRIC DETERMINATION OF SMALL AMOUNTS

OF COPPER
Potassium Ethyl Xanthate Method

The method is based upon the fact that potassium ethyl xanthate produces
a yellow-colored compound with copper. The reagent added to a solution
containing traces of copper will produce a yellow color varying in intensity in
direct proportion to the amount of copper present. Larger amounts of copper
with the reagent produce a bright yellow precipitate of copper xanthate. Small
quantities of iron, lead, nickel, cobalt, zinc, or manganese do not interfere. The
procedure is especially valuable for determination of the purity of salts crys-
tallized in copper pans.

Special Solutions. Stock Solution of Copper Sulphate. 3.9283 grams
CuS0 4 -5H 2 are dissolved in water and made up to a volume of 1000 cc. One
cc. is equivalent to 0.001 gram Cu.

Standard Copper Sulphate. Ten cc. of the stock solution are diluted to 1000
cc. with distilled water. One cc. =0.00001 gram Cu.

Potassium Ethyl Xanthate Solution. One gram of the salt is dissolved in
1000 cc. of water. The solution is kept in an amber-colored glass-stoppered
bottle.

Procedure. Five grams of the substance are dissolved in 90 cc. of water
(see note) and the solution poured into 100-cc. Nessler tube; 10 cc. of the potas-
sium xanthate reagent are added and the solution mixed by means of a glass
plunger. To a similar tube containing 50 or 60 cc. of water are added 10 cc.
of the xanthate reagent and then gradually drop by drop the standard copper
solution from a 10-cc. burette (graduated in ^ cc.) until the colors in both
tubes match.

If a = grams of the substance taken for analysis, 6= number of cc. standard
copper solution required to match the sample; then 6X0.00001 X 100 +a = % Cu.

NOTES. The amount of the substance to be taken varies according to its copper
content. The greater the copper contamination of the salt, the less sample required.
The solution should be neutral or only very slightly acid.



166 COPPER

In place of the Nessler tubes the special colorimetric apparatus desciibed under
Titanium and under Lead may be used. A very weak copper standard will be
required for the comparison tube.

If the substance is insoluble in water the copper is rendered soluble by treat-
ment with nitric acid. Hydrochloric acid is added and the nitric expelled by evapo-
ration. The substance is taken up with water and the insoluble residue filtered off.

Starch and organic matter are destroyed by addition of 10 cc. 10% sodium
hydroxide +10 cc. of saturated sodium nitrate solution, then evaporating to dryness
arid igniting. Hydrochloric acid is now added to expel the nitric acid as directed
above.

Ferrocyanide Method for Determination of Small Amounts

of Copper

By this colorimetric method it is possible to detect one part of copper in
2,500,000 parts of water. The procedure depends upon the purplish to chocolate-
brown color produced by potassium ferrocyanide and copper in dilute solutions.
The procedure is applicable to the determination of copper in water and may
be used in presence of a number of elements that occur in slags. Iron also
produces a colored compound with ferrocyanide (1 part Fe detected in 13 million
parts H 2 0), so this element must be removed from the solution before testing
for copper.

Solutions. Standard Copper Solution. 0.393 gram CuS0 4 -5H 2 per liter.
1 cc. =0.0001 gramCu.

Ammonium Nitrate. 100 grams of the salt per liter.

Potassium Ferrocyanide. Four grams of the salt per 100 cc. of solution.

Procedure. A volume of 5 to 20 drops of potassium ferrocyanide, accord-
ing to the amount of copper present in the solution, is placed in a tall, clear,
glass cylinder or Nessler tube of 150 cc. capacity, 5 cc. of ammonium nitrate
solution added and then the whole or an aliquot portion of the neutral x solution
of the assay. The mixture is diluted to 150 cc. The same amount of ferrocyanide
and ammonium nitrate solutions are poured into the comparison cylinder, placed
side by side with the one containing the sample, on a white tile or sheet of white
paper. The standard copper solution is now run from a burette into the
comparison cylinder, stirring during the addition, until the color matches that
of the assay. The number of cc. required multiplied by 0.0001 gives the weight
of copper in the sample contained in the adjacent cylinder.

Amount of Cu XI 00

c^r- 7 - = % Cu in the sample.

Wt. of sample compared

NOTES. The solution must be neutral, as the copper compound is soluble in ammo-
nium hydroxide and is decomposed by the fixed alkalies. If the solution contains free
alkalies, it is made slightly acid and then the acid neutralized with ammonia, added
in slight excess. This is boiled to expel the excess of ammonia, and then tested accord-
ing to the directions under " Procedure." Solutions containing free acids are neu-
tralized with ammonia.

Iron may be removed by precipitation with ammonia. As this hydroxide occludes
copper, the precipitate should be dissolved and reprecipitated to recover the occluded
copper.

Determination of copper in water is accomplished by evaporating a quantity
of water to dryness, taking up the residue with a little water containing 1 cc. nitric
acid, the residue having been ignited to destroy organic matter, precipitating iron
with ammonia, as directed above, and determining copper in the filtrate.

The colorimeter used in determination of traces of lead and for the colorimetric
determination of titanium may be employed in place of the Nessler tubes.



COPPER 167

Ammonia Method for Determining Small Amounts of Copper

^ In the absence of organic matter, nickel and elements giving a precipitate
with ammonia, copper to an upper limit of 10 milligrams can be determined by
comparison of the depth of the blue tint of its ammonium solution with a tem-
porary or permanent standard copper solution of equal volume. Permanent
standard solution of copper sulphate, free of nitrate, if kept cool and away from
the direct sunlight, lasts for a long time, 1

Hydrogen Sulphide Method

In the absence of elements precipitated by hydrogen sulphide, copper to the
limit of about 1 milligram, in a solution not too strongly acid with sulphuric or
hydrochloric acid, may be determined by comparison of its sulphide with that of
a known quantity of copper in equal volume and similarly treated. The liquid
should be cold and the passage of the hydrogen sulphide stopped before the
compound coagulates.

NOTE. Either the ammonia or the hydrogen sulphide method is applicable to the
determination of the copper not deposited in the operation of the electrolytic method.

DETERMINATION OF IMPURITIES IN BLISTER AND
REFINED COPPER

Introduction. In the complete analysis of copper the following impurities
are generally estimated: silver, gold, lead, bismuth, arsenic, antimony, selenium,
tellurium, iron, zinc, cobalt, nickel, oxygen, sulphur, and less commonly, tin
and phosphorus. In high grades of blister and in refined copper the percentage
of these impurities is very low, the blister copper usually averaging over 99.0%
copper with silver and the refined copper over 99.93% of the metal. The principal
impurity in the refined element is oxygen, which may be present to the extent
of .02 to .15%, the remaining impurities being in the third decimal place.
From this it is readily seen that large samples are required for the accurate
determination of these constituents. The amount of sample taken in blister
copper depends upon the grade of copper analyzed. The impurities in this
vary from tenths of a per cent to thousandths, as the metal from one locality
may contain quite appreciable amounts of a constituent, which may be present
only in extremely small quantities or not at all in copper from a different section.
In usual practice it is customary to take from 10 to 50 grams of blister and 50
to 500 grams of refined copper for analysis, depending upon the purity of the
material. If a larger sample than 50 grams is taken, it is necessary to divide
the material into several lots, and, after removal of the bulk of copper and isola-
tion of the impurities, to combine the filtrates or residues containing the con-
stituents sought.

In the procedures the smallest amount of sample, 10 grams, is taken as
the basis of calculation for amounts of reagents used. For larger samples, in
the initial treatment for removal of copper, proportionately larger amounts of
the reagents are required, i.e., multiples of from 2 to 5 times the amount stated.
A 50-gram sample is the largest amount of material handled in one lot.

Scrupulous care must be exercised throughout the analysis to prevent con-

1 Heath, Jour. Am. Chem. Soc., 19, 21.



168 COPPER

lamination of the sample or reagents, and to avoid loss of constituents. The
reagents used should be free from the substance sought or from interfering sub-
stances. It is the practice to carry blank tests of the reagents through under
conditions similar to a regular analysis for iron, lead, zinc, arsenic antimony and
sulphur.

It is found best to determine the impurities in several portions, i.e., gold
and silver by assay; bismuth and iron in one portion; lead, zinc, cobalt, and
nickel in a second; arsenic, antimony, selenium, and tellurium in a third; and
separate portions for sulphur, oxygen, phosphorus and tin, when these are
occasionally required.

Determination of Bismuth and Iron

Separation of Copper. Amount of Sample. Blister copper 10 to 25 grams,
refined copper 100 to 500 grams. The drillings are dissolved in a large beaker
in 40 cc. of nitric acid per 10-gram sample and the free acid expelled by boiling.
The solution should not become basic during the evaporation. Water is added
to make the volume 130 cc. per 10 grams or proportionately more for larger
samples. Ammonia is now added in sufficient excess to hold the copper in
solution and 5 cc. of saturated ammonium carbonate solution and the sample
diluted to 200 cc. (25 cc. (NH 4 ) 2 C0 3 per 50 grams, and dilution to 1000 cc.).
The beaker is placed on the steam bath for several hours, preferably over night.
The solution is filtered hot (to avoid crystallization of the copper salt), the first
100 cc. being refiltered, and the residue washed with hot water containing a
little ammonia. By this procedure the copper passes into the filtrate and
bismuth and iron remain in the residue on the filter.

Separation of Iron and Bismuth. The precipitate is dissolved in warm,
dilute hydrochloric acid (1 : 3), ammonia added to the solution in sufficient
amount to almost neutralize the acid and the solution then saturated with hydro-
gen sulphide. After settling some time, the precipitate containing bismuth sul-
phide is filtered off, iron passing into the solution.

Determination of Iron. Hydrogen sulphide is expelled by boiling the
filtrate, and iron oxidized by addition of hydrogen peroxide, or potassium
chlorate (nitric acid should not be used). The solution is evaporated to dry-
ness and iron then determined in the residue by the stannous chloride method,
details of which may be found in the chapter on Iron, page 221.

Determination of Bismuth. The sulphides remaining on the filter are
dissolved in nitric acid, the solution evaporated with sulphuric acid to S0 3 fumes



Online LibraryWilfred W. (Wilfred Welday) ScottStandard methods of chemical analysis; a manual of analytical methods and general reference for the analytical chemist and for the advanced student → online text (page 23 of 111)