Henry P. Talbot.

An Introductory Course of Quantitative Chemical Analysis With Explanatory Notes online

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[Note 1: The powder must be triturated until it is fine, otherwise the
lumps will inclose calcium hypochlorite, which will fail to react with
the arsenious acid. The clear supernatant liquid gives percentages
which are below, and the sediment percentages which are above, the
average. The liquid measured off should, therefore, carry with it its
proper proportion of the sediment, so far as that can be brought about
by shaking the solution just before removal of the aliquot part for
titration.]

[Note 2: Bleaching powder is easily acted upon by the carbonic acid in
the air, which liberates the weak hypochlorous acid. This, of course,
results in a loss of available chlorine. The original material for
analysis should be kept in a closed container and protected form the
air as far as possible. It is difficult to obtain analytical samples
which are accurately representative of a large quantity of the
bleaching powder. The procedure, as outlined, will yield results which
are sufficiently exact for technical purposes.]




III. PRECIPITATION METHODS




DETERMINATION OF SILVER BY THE THIOCYANATE PROCESS


The addition of a solution of potassium or ammonium thiocyanate to one
of silver in nitric acid causes a deposition of silver thiocyanate as
a white, curdy precipitate. If ferric nitrate is also present, the
slightest excess of the thiocyanate over that required to combine with
the silver is indicated by the deep red which is characteristic of the
thiocyanate test for iron.

The reactions involved are:

AgNO_{3} + KSCN - > AgSCN + KNO_{3},
3KSCN + Fe(NO_{3})_{3} - > Fe(SCN)_{3} + 3KNO_{3}.

The ferric thiocyanate differs from the great majority of salts in
that it is but very little dissociated in aqueous solutions, and the
characteristic color appears to be occasioned by the formation of the
un-ionized ferric salt.

The normal solution of potassium thiocyanate should contain an amount
of the salt per liter of solution which would yield sufficient
(CNS)^{-} to combine with one gram of hydrogen to form HCNS, i.e.,
a gram-molecular weight of the salt or 97.17 grams. If the ammonium
thiocyanate is used, the amount is 76.08 grams. To prepare the
solution for this determination, which should be approximately 0.05
N, dissolve about 5 grams of potassium thiocyanate, or 4 grams of
ammonium thiocyanate, in a small amount of water; dilute this solution
to 1000 cc. in a liter bottle and mix as usual.

Prepare 20 cc. of a saturated solution of ferric alum and add 5 cc. of
dilute nitric acid (sp. gr. 1.20). About 5 cc. of this solution should
be used as an indicator.


STANDARDIZATION

PROCEDURE. - Crush a small quantity of silver nitrate crystals in a
mortar (Note 1). Transfer them to a watch-glass and dry them for an
hour at 110°C., protecting them from dust or other organic matter
(Note 2). Weigh out two portions of about 0.5 gram each and dissolve
them in 50 cc. of water. Add 10 cc. of dilute nitric acid which has
been recently boiled to expel the lower oxides of nitrogen, if any,
and then add 5 cc. of the indicator solution. Run in the thiocyanate
solution from a burette, with constant stirring, allowing the
precipitate to settle occasionally to obtain an exact recognition
of the end-point, until a faint red tinge can be detected in the
solution.

From the data obtained, calculate the relation of the thiocyanate
solution to the normal.

[Note 1: The thiocyanate cannot be accurately weighed; its solutions
must, therefore, be standardized against silver nitrate (or pure
silver), either in the form of a standard solution or in small,
weighed portions.]

[Note 2: The crystals of silver nitrate sometimes inclose water which
is expelled on drying. If the nitrate has come into contact with
organic bodies it suffers a reduction and blackens during the heating.

It is plain that a standard solution of silver nitrate (made by
weighing out the crystals) is convenient or necessary if many
titrations of this nature are to be made. In the absence of such a
solution the liability of passing the end-point is lessened by setting
aside a small fraction of the silver solution, to be added near the
close of the titration.]


DETERMINATION OF SILVER IN COIN

PROCEDURE. - Weigh out two portions of the coin of about 0.5 gram
each. Dissolve them in 15 cc. of dilute nitric acid (sp. gr. 1.2) and
boil until all the nitrous compounds are expelled (Note 1). Cool the
solution, dilute to 50 cc., and add 5 cc. of the indicator solution,
and titrate with the thiocyanate to the appearance of the faint red
coloration (Note 2).

From the corrected volume of the thiocyanate solution required,
calculate the percentage of silver in the coin.

[Note 1: The reaction with silver may be carried out in nitric acid
solutions and in the presence of copper, if the latter does not exceed
70 per cent. Above that percentage it is necessary to add silver in
known quantity to the solution. The liquid must be cold at the time of
titration and entirely free from nitrous compounds, as these sometimes
cause a reddening of the indicator solution. All utensils, distilled
water, the nitric acid and the beakers must be free from chlorides,
as the presence of these will cause precipitation of silver chloride,
thereby introducing an error.]

[Note 2: The solution containing the silver precipitate, as well as
those from the standardization, should be placed in the receptacle for
"silver residues" as a matter of economy.]




PART III

GRAVIMETRIC ANALYSIS




GENERAL DIRECTIONS


Gravimetric analyses involve the following principal steps: first, the
weighing of the sample; second, the solution of the sample; third, the
separation of some substance from solution containing, or bearing a
definite relation to, the constituent to be measured, under conditions
which render this separation as complete as possible; and finally,
the segregation of that substance, commonly by filtration, and the
determination of its weight, or that of some stable product formed
from it on ignition. For example, the gravimetric determination of
aluminium is accomplished by solution of the sample, by precipitation
in the form of hydroxide, collection of the hydroxide upon a filter,
complete removal by washing of all foreign soluble matter, and the
burning of the filter and ignition of the precipitate to aluminium
oxide, in which condition it is weighed.

Among the operations which are common to nearly all gravimetric
analyses are precipitation, washing of precipitates, ignition of
precipitates, and the use of desiccators. In order to avoid burdensome
repetitions in the descriptions of the various gravimetric procedures
which follow, certain general instructions are introduced at this
point. These instructions must, therefore, be considered to be as much
a part of all subsequent procedures as the description of apparatus,
reagents, or manipulations.

The analytical balance, the fundamentally important instrument in
gravimetric analysis, has already been described on pages 11 to 15.


PRECIPITATION

For successful quantitative precipitations those substances are
selected which are least soluble under conditions which can be easily
established, and which separate from solution in such a state that
they can be filtered readily and washed free from admixed material.
In general, the substances selected are the same as those already
familiar to the student of Qualitative Analysis.

When possible, substances are selected which separate in crystalline
form, since such substances are less likely to clog the pores of
filter paper and can be most quickly washed. In order to increase the
size of the crystals, which further promotes filtration and washing,
it is often desirable to allow a precipitate to remain for some time
in contact with the solution from which it has separated. The solution
is often kept warm during this period of "digestion." The small
crystals gradually disappear and the larger crystals increase in size,
probably as the result of the force known as surface tension, which
tends to reduce the surface of a given mass of material to a minimum,
combined with a very slightly greater solubility of small crystals as
compared with the larger ones.

Amorphous substances, such as ferric hydroxide, aluminium hydroxide,
or silicic acid, separate in a gelatinous form and are relatively
difficult to filter and wash. Substances of this class also exhibit
a tendency to form, with pure water, what are known as colloidal
solutions. To prevent this as far as possible, they are washed with
solutions of volatile salts, as will be described in some of the
following procedures.

In all precipitations the reagent should be added slowly, with
constant stirring, and should be hot when circumstances permit.
The slow addition is less likely to occasion contamination of the
precipitate by the inclosure of other substances which may be in the
solution, or of the reagent itself.


FUNNELS AND FILTERS

Filtration in analytical processes is most commonly effected through
paper filters. In special cases these may be advantageously replaced
by an asbestos filter in a perforated porcelain or platinum crucible,
commonly known, from its originator, as a "Gooch filter." The
operation and use of a filter of this type is described on page 103.
Porous crucibles of a material known as alundum may also be employed
to advantage in special cases.

The glass funnels selected for use with paper filters should have an
angle as near 60° as possible, and a narrow stem about six inches in
length. The filters employed should be washed filters, i.e., those
which have been treated with hydrochloric and hydrofluoric acids, and
which on incineration leave a very small and definitely known weight
of ash, generally about .00003 gram. Such filters are readily
obtainable on the market.

The filter should be carefully folded to fit the funnel according to
either of the two well-established methods described in the Appendix.
It should always be placed so that the upper edge of the paper
is about one fourth inch below the top of the funnel. Under no
circumstances should the filter extend above the edge of the funnel,
as it is then utterly impossible to effect complete washing.

To test the efficiency of the filter, fill it with distilled water.
This water should soon fill the stem completely, forming a continuous
column of liquid which, by its hydrostatic pressure, produces a gentle
suction, thus materially promoting the rapidity of filtration. Unless
the filter allows free passage of water under these conditions, it is
likely to give much trouble when a precipitate is placed upon it.

The use of a suction pump to promote filtration is rarely altogether
advantageous in quantitative analysis, if paper filters are employed.
The tendency of the filter to break, unless the point of the filter
paper is supported by a perforated porcelain cone or a small "hardened
filter" of parchment, and the tendency of the precipitates to pass
through the pores of the filter, more than compensate for the possible
gain in time. On the other hand, filtration by suction may be useful
in the case of precipitates which do not require ignition before
weighing, or in the case of precipitates which are to be discarded
without weighing. This is best accomplished with the aid of the
special apparatus called a Gooch filter referred to above.


FILTRATION AND WASHING OF PRECIPITATES

Solutions should be filtered while hot, as far as possible, since
the passage of a liquid through the pores of a filter is retarded by
friction, and this, for water at 100°C., is less than one sixth of the
resistance at 0°C.

When the filtrate is received in a beaker, the stem of the funnel
should touch the side of the receiving vessel to avoid loss by
spattering. Neglect of this precaution is a frequent source of error.

The vessels which contain the initial filtrate should !always! be
replaced by clean ones, properly labeled, before the washing of a
precipitate begins. In many instances a finely divided precipitate
which shows no tendency to pass through the filter at first, while the
solution is relatively dense, appears at once in the washings. Under
such conditions the advantages accruing from the removal of the first
filtrate are obvious, both as regards the diminished volume requiring
refiltration, and also the smaller number of washings subsequently
required.

Much time may often be saved by washing precipitates by decantation,
i.e., by pouring over them, while still in the original vessel,
considerable volumes of wash-water and allowing them to settle. The
supernatant, clear wash-water is then decanted through the filter,
so far as practicable without disturbing the precipitate, and a new
portion of wash-water is added. This procedure can be employed to
special advantage with gelatinous precipitates, which fill up the
pores of the filter paper. As the medium from which the precipitate
is to settle becomes less dense it subsides less readily, and it
ultimately becomes necessary to transfer it to the filter and complete
the washing there.

A precipitate should never completely fill a filter. The wash-water
should be applied at the top of the filter, above the precipitate.
It may be shown mathematically that the washing is most !rapidly!
accomplished by filling the filter well to the top with wash-water
each time, and allowing it to drain completely after each addition;
but that when a precipitate is to be washed with the !least possible
volume! of liquid the latter should be applied in repeated !small!
quantities.

Gelatinous precipitates should not be allowed to dry before complete
removal of foreign matter is effected. They are likely to shrink and
crack, and subsequent additions of wash-water pass through these
channels only.

All filtrates and wash-waters without exception must be properly
tested. !This lies at the foundation of accurate work!, and the
student should clearly understand that it is only by the invariable
application of this rule that assurance of ultimate reliability can
be secured. Every original filtrate must be tested to prove complete
precipitation of the compound to be separated, and the wash-waters
must also be tested to assure complete removal of foreign material. In
testing the latter, the amount first taken should be but a few
drops if the filtrate contains material which is to be subsequently
determined. When, however, the washing of the filter and precipitate
is nearly completed the amount should be increased, and for the final
test not less than 3 cc. should be used.

It is impossible to trust to one's judgment with regard to the washing
of precipitates; the washings from !each precipitate! of a series
simultaneously treated must be tested, since the rate of washing will
often differ materially under apparently similar conditions, !No
exception can ever be made to this rule!.

The habit of placing a clean common filter paper under the receiving
beaker during filtration is one to be commended. On this paper a
record of the number of washings can very well be made as the portions
of wash-water are added.

It is an excellent practice, when possible, to retain filtrates and
precipitates until the completion of an analysis, in order that, in
case of question, they may be examined to discover sources of error.

For the complete removal of precipitates from containing vessels, it
is often necessary to rub the sides of these vessels to loosen the
adhering particles. This can best be done by slipping over the end of
a stirring rod a soft rubber device sometimes called a "policeman."


DESICCATORS

Desiccators should be filled with fused, anhydrous calcium chloride,
over which is placed a clay triangle, or an iron triangle covered with
silica tubes, to support the crucible or other utensils. The cover of
the desiccator should be made air-tight by the use of a thin coating
of vaseline.

Pumice moistened with concentrated sulphuric acid may be used in place
of the calcium chloride, and is essential in special cases; but for
most purposes the calcium chloride, if renewed occasionally and not
allowed to cake together, is practically efficient and does not slop
about when the desiccator is moved.

Desiccators should never remain uncovered for any length of time. The
dehydrating agents rapidly lose their efficiency on exposure to the
air.


CRUCIBLES

It is often necessary in quantitative analysis to employ fluxes to
bring into solution substances which are not dissolved by acids. The
fluxes in most common use are sodium carbonate and sodium or potassium
acid sulphate. In gravimetric analysis it is usually necessary to
ignite the separated substance after filtration and washing, in order
to remove moisture, or to convert it through physical or chemical
changes into some definite and stable form for weighing. Crucibles
to be used in fusion processes must be made of materials which will
withstand the action of the fluxes employed, and crucibles to be used
for ignitions must be made of material which will not undergo any
permanent change during the ignition, since the initial weight of the
crucible must be deducted from the final weight of the crucible and
product to obtain the weight of the ignited substance. The three
materials which satisfy these conditions, in general, are platinum,
porcelain, and silica.

Platinum crucibles have the advantage that they can be employed at
high temperatures, but, on the other hand, these crucibles can never
be used when there is a possibility of the reduction to the metallic
state of metals like lead, copper, silver, or gold, which would alloy
with and ruin the crucible. When platinum crucibles are used with
compounds of arsenic or phosphorus, special precautions are necessary
to prevent damage. This statement applies to both fusions and
ignitions.

Fusions with sodium carbonate can be made only in platinum, since
porcelain or silica crucibles are attacked by this reagent. Acid
sulphate fusions, which require comparatively low temperatures, can
sometimes be made in platinum, although platinum is slightly attacked
by the flux. Porcelain or silica crucibles may be used with acid
fluxes.

Silica crucibles are less likely to crack on heating than porcelain
crucibles on account of their smaller coefficient of expansion.
Ignition of substances not requiring too high a temperature may be
made in porcelain or silica crucibles.

Iron, nickel or silver crucibles are used in special cases.

In general, platinum crucibles should be used whenever such use is
practicable, and this is the custom in private, research or commercial
laboratories. Platinum has, however, become so valuable that it is
liable to theft unless constantly under the protection of the user. As
constant protection is often difficult in instructional laboratories,
it is advisable, in order to avoid serious monetary losses, to use
porcelain or silica crucibles whenever these will give satisfactory
service. When platinum utensils are used the danger of theft should
always be kept in mind.


PREPARATION OF CRUCIBLES FOR USE

All crucibles, of whatever material, must always be cleaned, ignited
and allowed to cool in a desiccator before weighing, since all bodies
exposed to the air condense on their surfaces a layer of moisture
which increases their weight. The amount and weight of this moisture
varies with the humidity of the atmosphere, and the latter may change
from hour to hour. The air in the desiccator (see above) is kept at
a constant and low humidity by the drying agent which it contains.
Bodies which remain in a desiccator for a sufficient time (usually
20-30 minutes) retain, therefore, on their surfaces a constant weight
of moisture which is the same day after day, thus insuring constant
conditions.

Hot objects, such as ignited crucibles, should be allowed to cool in
the air until, when held near the skin, but little heat is noticeable.
If this precaution is not taken, the air within the desiccator is
strongly heated and expands before the desiccator is covered. As the
temperature falls, the air contracts, causing a reduction of air
pressure within the covered vessel. When the cover is removed (which
is often rendered difficult) the inrush of air from the outside may
sweep light particles out of a crucible, thus ruining an entire
analysis.

Constant heating of platinum causes a slight crystallization of the
surface which, if not removed, penetrates into the crucible. Gentle
polishing of the surface destroys the crystalline structure and
prevents further damage. If sea sand is used for this purpose, great
care is necessary to keep it from the desk, since beakers are easily
scratched by it, and subsequently crack on heating.

Platinum crucibles stained in use may often be cleaned by the fusion
in them of potassium or sodium acid sulphate, or by heating with
ammonium chloride. If the former is used, care should be taken not
to heat so strongly as to expel all of the sulphuric acid, since the
normal sulphates sometimes expand so rapidly on cooling as to split
the crucible. The fused material should be poured out, while hot, on
to a !dry! tile or iron surface.


IGNITION OF PRECIPITATES

Most precipitates may, if proper precautions are taken, be ignited
without previous drying. If, however, such precipitates can be dried
without loss of time to the analyst (as, for example, over night), it
is well to submit them to this process. It should, nevertheless, be
remembered that a partially dried precipitate often requires more care
during ignition than a thoroughly moist one.

The details of the ignition of precipitates vary so much with the
character of the precipitate, its moisture content, and temperature to
which it is to be heated, that these details will be given under the
various procedures which follow.




DETERMINATION OF CHLORINE IN SODIUM CHLORIDE


!Method A. With the Use of a Gooch Filter!

PROCEDURE. - Carefully clean a weighing-tube containing the sodium
chloride, handling it as little as possible with the moist fingers,
and weigh it accurately to 0.0001 gram, recording the weight at once
in the notebook (see Appendix). Hold the tube over the top of a beaker
(200-300 cc.), and cautiously remove the stopper, noting carefully
that no particles fall from it, or from the tube, elsewhere than into
the beaker. Pour out a small portion of the chloride, replace the
stopper, and determine by approximate weighing how much has been
removed. Continue this procedure until 0.25-0.30 gram has been taken
from the tube, then weigh accurately and record the weight beneath the
first in the notebook. The difference of the two weights represents
the weight of the chloride taken for analysis. Again weigh a second
portion of 0.25-0.30 gram into a second beaker of the same size as the
first. The beakers should be plainly marked to correspond with the
entries in the notebook. Dissolve each portion of the chloride in 150
cc. of distilled water and add about ten drops of dilute nitric acid
(sp. gr. 1.20) (Note 2). Calculate the volume of silver nitrate
solution required to effect complete precipitation in each case,
and add slowly about 5 cc. in excess of that amount, with constant
stirring. Heat the solutions cautiously to boiling, stirring
occasionally, and continue the heating and stirring until the
precipitates settle promptly, leaving a nearly clear supernatant
liquid (Note 3). This heating should not take place in direct sunlight
(Note 4). The beaker should be covered with a watch-glass, and both
boiling and stirring so regulated as to preclude any possibility of
loss of material. Add to the clear liquid one or two drops of silver
nitrate solution, to make sure that an excess of the reagent is
present. If a precipitate, or cloudiness, appears as the drops fall
into the solution, heat again, and stir until the whole precipitate
has coagulated. The solution is then ready for filtration.

Prepare a Gooch filter as follows: Fold over the top of a Gooch funnel
(Fig. 2) a piece of rubber-band tubing, such as is known as "bill-tie"
tubing, and fit into the mouth of the funnel a perforated porcelain
crucible (Gooch crucible), making sure that when the crucible is
gently forced into the mouth of the funnel an airtight joint results.
(A small 1 or 1-1/4-inch glass funnel may be used, in which case the
rubber tubing is stretched over the top of the funnel and then drawn
up over the side of the crucible until an air-tight joint is secured.)

[ILLUSTRATION: FIG. 2]


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Online LibraryHenry P. TalbotAn Introductory Course of Quantitative Chemical Analysis With Explanatory Notes → online text (page 8 of 17)