Nathaniel Wright Lord.

Metallurgical analysis online

. (page 10 of 32)
Online LibraryNathaniel Wright LordMetallurgical analysis → online text (page 10 of 32)
Font size
QR-code for this ebook

remaining as oxides or carbonates. When the fused mass is boiled with
water till thoroughly disintegrated and then filtered off and washed, the
sulfate all passes into the filtrate.

Sulfides can be more or less completely oxidized to sulfates in the
"wet way" by treating them with hot concentrated HN0 3 or aqua
regia. Wet methods are not very satisfactory, as free sulfur is liable to
separate and fuse to globules, its melting-point being below the boil-
ing-point of HNO 3 . Once in this form it is very slowly oxidized by
boiling with ordinary oxidizing agents. Iron sulfide can be completely
oxidized, however, by heating with a large excess of concentrated
HNO 3 and adding a little powdered KC10 3 .

When iron sulfide, or even iron containing but little sulfur, is dissolved
in dilute HN0 3 , 1.2 sp. gr., a considerable proportion of the sulfur
separates as such and escapes oxidation.

Solutions containing ferric sulfate, on evaporation to dryness and



" baking," as is common in iron analysis, may lose S0 3 unless enough
potassium or sodium is added to hold it all in combination with the
alkali, as the sulfate of iron is easily decomposed by heat and S0 3


See FRESENIUS, "Quantitative Analysis;" also PHILLIPS, J. Am.
Chem. Soc., 1896, p. 1079.


This precipitation must be conducted under carefully regulated con-
ditions, if the results are to be satisfactory.

When the amount of sulfur present is very small the contamination of
the BaS(>4 is not apt to be important and the chief thing is to get the
sulfur completely precipitated; but when large amounts of sulfur are
to be precipitated, as when pyrite is to be analyzed, the case is very
different and great care must be taken to have the BaS04 pure as well
as completely precipitated.

When alkalies are present, all BaS0 4 precipitates carry down alkali
sulfates, the error being worst if alkali chlorides are present. If ammo-
nium salts are present, ammonium sulfate is carried down, and is of
course lost on ignition. In the presence of much alkali chloride, the
precipitate contains a certain amount of free sulfuric acid. All of these
errors make results run low, in some cases perhaps as much as several
per cent, of the total weight.

In addition to these minus errors, there are plus errors. All barium
sulfate precipitates contain barium chloride. If the precipitation is
made very slowly, the amount of this is very small. When the precipi-
tation is made rapidly, it is much larger. Nitrates are occluded by
barium sulfate, giving results which are high. When barium sulfate is
precipitated from solutions containing much ferric iron, iron salts will
adhere to the precipitate, making it reddish in color, unless considerable
HC1 is present. Some of the sulfuric acid appears to be in combination
with the iron instead of with the barium, and is driven off on ignition,
causing low results. Water alone will not wash out any of the above
salts occluded with the barium sulfate.

Barium sulfate while very insoluble in water is not so in dilute acids,
the amount dissolved increasing with the concentration of the acid,
though the presence of a considerable excess of BaCl 2 very largely
decreases the solubility of the barium sulfate in HC1. Acid solutions
of FeCl 3 , when hot, hold a little BaSO 4 in solution, which separates
when the liquid cools.

The precipitate of BaS0 4 is fine, liable to run through the filter paper,


if precipitated cold, and so should be precipitated hot and allowed to
stand on a warm plate with frequent stirring. In this way the precipi-
tate will grow dense and granular so that it can be easily filtered.

It will be seen from the above that the determination of sulfur when
present in large amounts is attended with many difficulties and for
accurate results correction must be made by analysis of the precipitate.
For good technical results perhaps the best way is to wash the precipi-
tate off the filter back into the beaker, add HC1 equal to the water
present and evaporate almost to dryness, add 100 c.c. of water and
5 c.c. of 10 per cent. BaCl 2 , let settle and filter.


HILLEBRAND, Bulletin 422, U. S. Geol. Survey, p. 195.

ALLEN and JOHNSON, J. Am. Chem. Soc., May, 1910, p. 588.

ARCHBUTT, J. Soc. Chem. Ind., IX, p. 25.

LUNGE, J. Soc. Chem. Ind., VIII, pp. 967 and 819.

AUCHY, J. Am. Chem. Soc., 1901, p. 147; Am. Chem. J., 1902, p. 495.

Process for Sulfur in Iron Ores. Mix 1 gram of the finely
pulverized ore with 5 grams of dry Na 2 CO 3 and 0.5 to 1 gram
of NaNO 3 , according to the amount of sulfur in the ore. Put
the mixture in a platinum crucible and fuse carefully. As soon
as it is well melted, chill the crucible by dipping the bottom into
water. This will usually loosen the cake so that it can be re-
moved from the crucible.

As ordinary gas contains sulfur, fusions made over it are likely
to absorb some SO 2 from the flame. Therefore an alcohol or
gasoline blast lamp should be used. If gas is used, the crucible
must be kept covered during the fusion and should be protected
by inserting it into a tightly fitting collar of sheet asbestos
nearly to the top. This will act as a shield to prevent the prod-
ucts of combustion from getting into the crucible. In accurate
work it is always necessary to make a blank analysis and deter-
mine the small amount of sulfur contained in the reagents or
absorbed from the gas flames. Deduct this from the amount
found when working on the ore.

Boil out the fusion with water until all the material is soft and
no hard lumps remain. If the solution is colored by Na2MnO 4 ,
add a few drops of alcohol. Filter and wash well with hot water.
Add HC1 to the cold solution which should have a volume of
about 100 c.c. The acid should be added until the solution is
just acid, then about 4 c.c. more, Now heat nearly to boiling


and add 5 to 10 c.c. of a 10 per cent, solution of BaCl 2 previously
diluted with 10 to 20 c.c. of water and heated. Stir and let the
precipitate of BaSO 4 settle. When clear, filter, wash with hot
water, ignite and weigh the BaS04. This weight multiplied
by 0.1374 gives the amount of S.

If it is suspected that the BaS0 4 is contaminated with SiO 2
it should be treated with HF. It is a good thing to do in any
case. Add to the ignited precipitate a drop of H 2 SC>4 and 2 c.c.
of HF. Evaporate off the acids and finally again ignite and

Notes on the Process. BaS0 4 is easily reduced to BaS by heating
with carbon. This may occur in the crucible and will make the results
come low; hence, in igniting the precipitate detach it as far as possible
from the filter, burn the paper carefully on a platinum wire, avoiding a
high heat. Add the ash to the precipitate in the crucible and heat gently
with the cover off until all the carbon is burned, finally igniting to a
bright red heat.

Instead of drying the paper, the wet filter and precipitate may be ig-
nited together by proceeding as follows : Put the filter paper containing
the precipitate into a good-sized platinum crucible. The paper should
be put in point down and open, just as it sets in the funnel. Now
set the uncovered crucible over a very low flame and dry out the paper
carefully. Then continue the heating to char the paper without
letting it ignite. Should it catch fire, extinguish the flame by momen-
tarily covering the crucible. When all the volatile matter is expelled,
slightly increase the heat which should not, however, exceed a dull red.
The carbon will now all burn away and the precipitate become white.
Finally raise the temperature to bright redness. Cool and weigh as
before. This process is called " smoking off" the filter and saves much
time. It can be used safely on all small BaS0 4 precipitates. After
weighing the precipitate, add a little water to it and test with turmeric
paper. If it reacts alkaline, the results are untrustworthy, as reduction
has occurred; in this case add a drop of H 2 S0 4 ; heat till dry, ignite and
weigh again, taking the second weight as the correct one.


This method fails to determine the sulfur in any BaS0 4 or PbS0 4
contained in the ore. Therefore it is not so generally applicable as the
fusion method unless the residue is separately treated by the fusion
method and any sulfur thus obtained added to that obtained by the wet


Process of Analyses. Weigh 1 to 5 grams of the very finely
pulverized ore. Put it in a covered casserole or beaker and add
20 c.c. of concentrated HNO 3 . Heat and add 1 gram of KC1O 3
in several portions. Now digest at a moderate heat till all action
ceases, then evaporate off most of the HNO 3 . Add an excess of
HC1 and warm until the iron is all dissolved. Evaporate to
dryness and proceed as with the dried residue in the determination
of sulfur in iron or steel.

When sulfur is present in large amounts, as for instance, in
pyrite, it is necessary that the BaS(>4 be precipitated from a solu-
tion free from iron. Dissolve 0.5 gram of the very finely ground
sample in 20 c.c. of aqua regia in a beaker with a watch-glass
cover. Heat until decomposition is complete, then evaporate
to dryness. If necessary use a little KC10 3 with the aqua regia
to dissolve the pyrite. Moisten the dry mass with 1 c.c. HC1
and 100 c.c. of water. Heat until all except the gangue is dis-
solved and filter. To the cold solution add ammonia until
alkaline, heat to boiling and filter off the Fe(OH) 3 and wash
thoroughly. Make the filtrate acid with HC1, heat to boiling
and add slowly BaCl 2 with constant stirring. After standing
some time, filter and wash, transfer the BaSO 4 back to the beaker,
add as much HC1 as there is water present, evaporate almost to
dryness, add 100 c.c. water and then 20 c.c. BaCl 2 , allow to stand
a half hour, filter and wash. Ignite very carefully to prevent
reduction of the BaS0 4 .


As the sulfur is usually present in these metals in very small percent-
ages only, its accurate determination demands great care.

Process of Analysis. Take from 2 to 5 grams according to the
percentage of sulfur. Add 25 to 40 c.c. concentrated HN0 3 .
Cool the dish if the action is too rapid, or heat it if it is too slow.
The rate of solution must not be too rapid or low results may

When the metal is nearly all dissolved, heat to boiling and add
2 to 3 c.c. of concentrated HC1 to complete the solution. Now
add about 0.5 gram KC10 3 free from sulfur. Boil to dryness
and bake on a hot plate 10 minutes. Add 10 to 20 c.c. con-


centrated HC1 to dissolve the residue and again dry down
thoroughly. Dissolve again in 15 to 40 c.c. of concentrated
HC1. Evaporate the solution until a skin begins to form on
the surface or until it becomes syrupy. Now add 5 to 10 c.c.
of concentrated HC1, according to the amount of iron taken.
When all the iron dissolves dilute the liquid with its own volume
of hot water and filter into a small beaker through a paper pre-
viously washed out with a little hot dilute HC1 (this facilitates
filtration). Wash the dish and insoluble residue with hot water.

The filtrate and washings must not exceed 75 c.c. Now warm
to about 60C. and add 10 c.c. of a 10 per cent, solution of BaC^.
Let stand till the precipitate settles, leaving the liquid perfectly
clear. (Two hours is sufficient if everything is right.)

Filter onto a small ashless filter, wash with water containing a
few drops of HC1, ignite and weigh the BaS04. Test the filtrate
by the addition of considerably more BaCl2 solution which must
give no additional precipitate.

The residue from which the solution for the determination of
sulfur is filtered must contain no basic iron salts, as these may
hold sulfur. . These are likely to form if the HC1 solution is con-
centrated too far and insufficient acid is added before dilution.


J. Anal, and App. Chem., VI, p. 318.

Notes on the Process. It is imperatively necessary that a blank be
run on all the reagents used in the process and the weight of a BaS04
obtained in this way deducted from that found in the analysis.

Certain high carbon steels and most ferro-silicons will resist the action
of concentrated HN0 3 almost entirely. When treating such metals add
some potassium chlorate with the nitric acid at the start, and also at
intervals add concentrated HC1, 1 c.c. at a time, until the metal is
dissolved; then add more KC10 3 and proceed as usual.

Ferro-silicons with over 10 per cent, of silicon will resist the action of
all the ordinary solvents. These and other insoluble alloys cannot be
treated by wet methods for the determination of sulfur.

Where the percentage of silicon does not exceed about 15 per cent., the
small addition of sodium fluoride to the HN0 3 as described under Phos-
phorus on page 46, will usually bring the metal into solution and the de-
termination can then be carried out as usual, by adding the chlorate,
evaporating, baking and taking up in HC1. In other cases the metal
must be fused. The fusion is best made in a platinum crucible with a


mixture of equal parts of NaN0 3 and Na 2 C0 3 using at least six parts
of the mixture to one of the metal. The fusion can be then soaked
out with water and the water solution treated as in the case of an ore.
The sulfur will all go into the water solution, provided the iron is
completely oxidized. It is essential that the metal be very finely pow-
dered. The peroxide of sodium can be substituted for the nitrate or may
be used alone, in which case at least 8 grams of the reagent must be
used for one of metal, and the fusion made in a nickel crucible. Blanks
must be run on all the reagents.


F. C. PHILLIPS, J. Am. Chem. Soc., 1896, p. 1079.
E. H. SANITER, J. Soc. Chem. Ind., 1896, p. 155.


The direct oxidation methods are accurate and the only ones that can
be relied upon to give with certainty the total sulfur in any material.
But they are too slow to answer for control work. For such purposes
the evolution methods are very generally used. They are either gravi-
metric or volumetric and can be made extremely rapid. For some
materials they will give reliable results.

These methods all depend upon the assumption that when iron is
dissolved in HC1 the whole of the S is evolved as H 2 S and passes off
with the excess of hydrogen. This is probably true or nearly so for steel
containing but little carbon and possibly for gray pig-iron; it certainly
is not true for white iron and mottled irons high in combined carbon;
and probably not true for high-carbon steels and some ferro-silicons,
especially those containing much sulfur.

In these latter materials part of the sulfur appears to be evolved as
more or less volatile liquid or possibly gaseous compounds of carbon,
hydrogen and sulfur and not as H 2 S. The proportion of the sulfur
evolved as H 2 S will vary in the same iron if the heat treatment has been
different; if slowly cooled, it may be gray and evolve most of the sulfur
as H 2 S; if suddenly cooled by chilling in water (shot samples) it will be
white and only a small portion of its sulfur may be evolved as H 2 S.
Hence by evolution methods the latter sample would show a much lower
percentage of sulfur than the former. The sulfur that is lost is in the
form of (CH 3 ) 2 S or some similar form. The higher the percentage of
carbon in solid solution (Martensite) in the iron the more sulfur is thus
lost. Part of this goes over with the gases and part stays behind in the
flask. If the evolved gases be passed with hydrogen through a glass



tube heated to dull redness the sulfur compounds are changed to H 2 S.
If the tube is filled with asbestos coated with platinum black the action
is more rapid. Care must be taken to exclude air or explosion will result.
The amount of sulfur evolved in combination as (CH 3 ) 2 S increases with
the manganese in the sample and decreases with the phosphorus. Of
that retained in the flask, part may be in this form and if any titanium
is present some will be held in combination with the titanium. Phillips
has shown that sulfur retained in the flask as difficultly volatile organic
compounds may be distilled off by prolonged boiling.

By taking certain precautions the amount of sulfur lost can be re-
duced to a very small amount and the results by the evolution process
can be made to check fairly well with the standard process even on pig-
irons. These precautions are as follows:

(1) The weighed sample should be annealed in a non-oxidizing
atmosphere according to a certain procedure. This changes the marten-
site to pearlite with cementite, graphite or ferrite according to the
amount of carbon. (2) The speed of solution should be as great as
possible. (3) The acid used should be concentrated acid, of sp. gr.
1.19. Under these conditions the use of a hot tube through which to
pass the evolved gases is not necessary.

Ordinary low-carbon steels do not have to be annealed, but high-
carbon steels, pig-irons and the alloy steels, such as self-hardening steels
and nickel-chromium steels must be annealed to get correct results.

For rapid work, as for furnace control, the annealing of the sample may
be omitted but it should be understood that this always gives low results,
sometimes as much as 20 per cent, low, when the sample is a pig-iron.

On the other hand, in the case of steels, the results for sulfur by the
evolution process may give too high results. Besides H 2 S other gases
are given off which may affect the results. These are hydrocarbons and
PH 3 and AsH 3 which are absorbed to a certain extent and affect iodine
used for titrating, but do not affect gravimetric results. According to
Elliot these are not absorbed in CdCl 2 solution containing acetic acid
and ammonium acetate as they are in alkaline solutions or in solutions
of lead, zinc, or copper.

The H 2 S evolved is very easily decomposed by comparatively feeble
oxidizing agents, water being formed and free sulfur deposited. Pro-
longed contact with air and sunlight, the presence of FeCl 3 , traces of
chlorine, all act on it in this way, and must be avoided in the process.
There is no necessity, however, of working in an atmosphere of hydrogen
or of carbon dioxide if the process is rapidly conducted. On the other
hand, slow evolution, or the use of HC1 containing traces of chlorine or
FeCl 3 will cause decomposition of the H 2 S and retention of sulfur in the


Rusting of 'the drillings previous to the addition of HC1 leads to the
formation of FeCl 3 and may cause a separation of sulfur from the gas
in the flask. Dilute HC1 (1 : 1) is usually used in these methods but
according to the writer's experience this sometimes fails to cause com-
plete evolution of the sulfur as H 2 S where the concentrated acid succeeds.
(See PHILLIPS, J. Am. Chem. Soc., 1895, p. 891.)

While the H 2 S is easily absorbed, the organic sulfur compounds are
only slowly and incompletely taken up. Long boiling is frequently
necessary to drive them completely out of the flask in which the iron
is dissolved. It is evident from what has been said that the evolution
processes are reliable only when they are checked by using the gravi-
metric process on the same kind of sample.

The H 2 S evolved may be determined in many ways. It may be
absorbed in an alkaline solution of lead acetate, then oxidized to 80s
and precipitated by BaCl 2 , or it may be absorbed in a solution of HC1
and bromine and the H 2 SO 4 formed precipitated by Bad 2, or in an al-
kaline solution of KMn0 4 and precipitated as before. Some chemists
absorb the H 2 S in a CuS0 4 solution, filter off the copper sulfide, ignite
and weigh as CuO. The most widely used method is, however, a
volumetric one in which the H 2 S is titrated with a standard iodine solu-
tion. It is very rapid and is quite as accurate as the other evolution


The H 2 S may be absorbed in NaOH or KOH or an ammoniacal solu-
tion of zinc or cadmium. The cadmium solution is preferred because it
fixes the sulfur in a visible form and is not easily altered on standing.
KOH and NaOH are liable to contain oxidizing impurities such as
nitrites or ferric hydroxide which would oxidize H 2 S.

The reactions involved in the process are as follows:

FeS(or MnS)+2HCl = FeCl 2 (or MnCl 2 )+H 2 S.
H 2 S+CdCl 2 = CdS+2HCl.

The HC1 liberated is neutralized by the NH 4 OH present. When the
H 2 S has all come over, the solution is diluted and strong HC1 is added in
excess; then the reverse reaction takes place,

CdS+2HCl = H 2 S+CdCl 2 .

A considerable excess of HC1 is necessary to completely dissolve all
of the CdS. The volume of the solution should be very large to prevent
the escape of the liberated H 2 S. Also it should not be hot.
The H 2 S is then titrated with iodine.

H 2 S+2I = 2HI+S.


The liberated sulfur causes the liquid to become curiously opales-
cent and show various colors, but this does not at all obscure the end
reaction which is very sharp. Before titration a little starch solution is
added to the liquid to be titrated. One drop of iodine in excess of the
amount required to titrate the EbS causes an intense blue color of " starch
iodide" to be formed.

Preparation of the Starch Solution. Stir 5 grams of starch into
200 c.c. of cold water. Heat the liquid to boiling with constant
stirring until the starch is thoroughly dissolved. Now dilute
the liquid with cold water to about a liter, and add 10 grams of
crystallized ZnCl 2 . Let the solution settle for some time, and
pour off for use the nearly clear supernatant liquid. This solu-
tion is very sensitive and keeps indefinitely.

A solution of 1 gram of starch in 200 c.c. of boiling water alone
may be used, but it must be made fresh every day.

For a more sensitive preparation of starch see page 225.

Preparation of the Iodine Solution. Weigh on a watch-glass
3.96 grams of pure resublimed iodine. Put it into a liter flask,
add about 6 grams of pure potassium iodide (free from iodate)
and 10 c.c. of water. Let stand in the cold until all the iodine
dissolves. Then dilute to 1 liter.

One cubic centimeter of this solution should be equivalent to
0.0005 gram of sulfur. If 5 grams of metal are taken for the
analysis each cubic centimeter of iodine solution consumed will be
equivalent to 0.01 per cent, of sulfur.

In the reaction H 2 S+2I = 2HI-fS, two atoms of I are
equivalent to one atom of S, or 253.84 grams of 1 = 32.06 grams
of S. To find the amount of I, which must be contained in 1 c.c.
to give a solution of the above value in sulfur, make the propor-
tion 0. 005 :x = 32.06: 253.84. This gives x = 0.003957 gram per
1 c.c. or 3.96 grams per liter.

Iodine is insoluble in water, but is easily and rapidly dissolved
in a very concentrated solution of KI, though very slowly in a dilute

The iodine solution is not constant; hence, its strength must be
determined frequently.

Standardizing the Iodine Solution. Prepare the following
solutions :


A. Eight grams crystallized sodium thiosulfate dissolved in
water and diluted to 1 liter.

B. 0.1531 gram of fused potassium dichromate is dissolved in
water and diluted to 100 c.c. If more convenient this solution
may be made by diluting 10 c.c. of the dichromate solution used
in the iron assay (of which 1 c.c. equals 0.01 Fe) to 57.42 c.c.;
10 c.c. of this solution will liberate iodine from KI equivalent
to 0.005 gram sulfur.

Measure with a pipette 10 c.c. of the thiosulfate solution A
into a beaker. Add 100 c.c. of water and 1 c.c. of starch solution.
Now run in the iodine solution from a burette until the last drop
gives a decided blue color not disappearing on stirring. Note
exactly the amount used. Repeat the determination two or
three times (the results should agree almost exactly), and take
the average as the amount of iodine solution equivalent to 10 c.c.
of the thiosulfate.

Measure 10 c.c. of the dichromate solution B into a beaker.
Add 50 c.c. of cold water and then about 0.5 gram of pure KI.
When the KI is dissolved add 5 c.c. of concentrated HC1.

The KI must be free from iodate. It may be tested by dis-
solving a portion in water, adding some starch solution and a

Online LibraryNathaniel Wright LordMetallurgical analysis → online text (page 10 of 32)