Elbert William Rockwood.

An introduction to chemical analysis, for students of medicine, pharmacy and dentistry online

. (page 13 of 18)
Online LibraryElbert William RockwoodAn introduction to chemical analysis, for students of medicine, pharmacy and dentistry → online text (page 13 of 18)
Font size
QR-code for this ebook


standard solutions are potassium permanganate, KMnO 4 ;
potassium dichromate, K^C^O?, and iodin.

The Preparation and Properties of Standard Oxidizing
Solutions

Potassium permanganate, KMnO^ can be obtained pure in
the crystalline form, though it is so often impure that the solu-
tion should be tested after making by titrating against a stand-
ard. The crystals dissolve to an intensely reddish-purple solu-
tion. The molecular weight of the substance is 158.03. As
stated before (page 154) when it acts as an oxidizing agent two
molecules yield five atoms of oxygen. The combined valences
of the five atoms of oxygen given by this double molecule is ten.
Therefore a normal solution would contain one-tenth the
double molecular weight of the salt in a liter or 3 1 .606 grammes.
The decinormal solution containing 31.606 is more commonly
employed. This solution is likely to undergo slight decom-
position on standing, although it will maintain its strength for
several weeks. Unless freshly prepared it should be standard-
ized before the solution is used, in the same manner as in the



1 68 INTRODUCTION TO CHEMICAL ANALYSIS

original preparation. Organic matter produces changes in
the solution, which should therefore not be brought into con-
tact with rubber. A pouring burette or one with a glass stop-
cock is suitable for its measurement. When the permanganate
gives up its oxygen, providing a free mineral acid is present, the
purple color disappears as a result of the formation of the color-
less manganese salt of the acid. That is, the manganese,
which was a part of the anion, becomes the cation. This
decolorization indicates the completion of the reaction. An
indicator is therefore unnecessary. If the crystals are known
to be pure the weighed amount can be dissolved in a liter of
water. In case of doubt as to their purity about 3.5 grammes
should be so dissolved, the concentration of the solution as-
certained by one of the following methods and the proper
volume of water added to reduce it to the desired concentration.

The substances most commonly used to ascertain the con-
centration of a permanganate solution are oxalic acid, ammo-
nium ferrous sulphate, and metallic iron.

A decinormal solution of oxalic acid can be taken as the
standard. The reaction is

2 KMnO 4 + 5H 2 C 2 4 + 3H 2 SO 4
= K 2 S0 4 + 2MnS0 4 + ioC0 2 + 8H 2 O.

The 10 c.c. of the oxalic acid is measured with a pipette and
after acidifying with dilute sulphuric acid, is placed in a flask
or beaker and the mixture warmed to about 60. Then the
permanganate solution is allowed to run in. At first the color
disappears slowly but afterward more rapidly. If it turns
brown the amount of sulphuric acid is insufficient. The per-
manganate should be added cautiously to avoid an excess,
stopping when the pink color is permanent. From the
average of several determinations calculate the concentration
of the permanganate solution, remembering that if it were
decinormal exactly 10 c.c. would be reduced by the oxalic acid.



ANALYSIS BY OXIDATION AND BY REDUCTION 169

Find the volume of water which will, if added, produce the
decinormal solution, as in the preparation of standard solutions
of sodium hydroxid (page 160) or hydrochloric acid (page 161).
After dilution test again to see if the result is correct. It is
well in this second test to make use of another standard solu-
tion such as one of decinormal ferrous ammonium sulphate.

Ferrous ammonium sulphate, (NH^fe^SO^^ 6# 2 0, can
be obtained pure in the form of greenish crystals. For this
purpose they must not have lost any of their water by efflores-
cence or be at all brown in color, which indicates that the iron
is changing to the ferric form. A decinormal solution con-
taining 38.41 grams to the liter should be used with sulphuric
acid in the same manner as in the titration by oxalic acid.
The reaction is



Here as before one cubic centimeter of the iron solution
should decolorize exactly one of the decinormal potassium
permanganate.

If metallic iron is the standard about o.i grm. of the purest
piano wire, accurately weighed, should be dissolved in a flask
by means of dilute sulphuric acid. Access of air can be pre-
vented by a Bunsen valve made of a short piece of slit rubber
tubing slipped over the exit tube and closed at the upper end,
which allows the hydrogen to escape (Fig. 17). When the
wire has dissolved, the solution can be diluted with recently
boiled water and immediately titrated with the permanganate.
The reaction is similar to that with the ammonium ferrous
sulphate because the iron in dissolving forms ferrous sul-
phate, FeSO 4 . Since all iron contains carbon, allowance
must be made for this. It it has not been determined in the
sample which was used, the wire may be estimated at 99.6 per
cent. pure.



170



INTRODUCTION TO CHEMICAL ANALYSIS



Of iodin the decinormal solution is used. This contains
1 2 .692 grms. in the liter. It can be made by dissolving about
1 8 grms. of pure potassium iodid in 200-300 c.c. of water in a
liter flask, adding 12.692 grms. of chemi-
cally pure iodin and when this has dis-
solved, filling to the mark with water. If
pure iodin cannot be obtained somewhat
more than this weight may be dissolved
and the concentration of the solution be
ascertained by titrating with a standard
solution of sodium thiosulphate (hypo-
sulphite) in the manner described below.
The iodin solution should be kept in a
cool, dark place, but even then it does
not maintain its concentration, so that
unless it has been recently prepared it
should be standardized before using.

The oxidizing action of the iodin is an
indirect one. It unites with the hydrogen
of the water present, leaving the oxygen
free to combine with oxidizable sub-
stances. This is illustrated by its action




FIG. 17. Flask
fitted with Bunsen
valve which allows
the gas to escape from



within but prevents on soc iium sulphite or arsenous oxid.

the access of air.

Na 2 S0 3 +I 2 +H 2 = 2HI+Na 2 S0 4 .



Although solutions of free iodin have a color and most
solutions of its compounds do not, the distinction is not great
enough to accurately mark the end reaction. Therefore a
few drops of a boiled starch solution are added as an indica-
tor. This gives a deep blue color as long as any free iodin is
present and is colorless when the iodin is in combination with
other elements.

Decinormal potassium dichr ornate, K^Cr^O?, which is often



ANALYSIS BY OXIDATION AND BY REDUCTION 171

used in oxidimetry, contains 4.907 grms. in a liter. The crys-
tallized salt can be obtained in a pure state and the solution
may be made by drying the salt at 100, then dissolving this
weight in water and diluting to a liter. The solution is much
more permanent than that of potassium permanganate or iodin
and is not affected by contact with rubber. An indicator must
be used with it in the volumetric tests of ferrous compounds
and this lessens its convenience. When the dichromate is
used as an oxidizing agent it loses three atoms of oxygen. It
is for this reason that its decinormal solution contains one-six-
tieth of its molecular weight in grammes per liter (page 1 54) .
The sulphuric acid combines with the potassium and chromium
forming the sulphates of these metals. Thus



The principal use of the dichromate solution in volumetric
analysis is in the quantitative determination of iron and its
ferrous compounds. If a ferrous compound is present with
an acid the oxygen unites with the hydrogen ion of the latter,
and the anion combines with iron, changing it to the ferric
state.



Analysis of Reduction

This is the opposite of oxidimetry. Soluble reducing
agents, or deoxidizers will remove oxygen from many of its
compounds when in solution and if the end of this reducing
action is definitely marked they may often be employed in the
preparation of standard volumetric solutions. Oxalic acid
and sodium thiosulphate are very commonly used. The
former has already been discussed.

Sodium thiosulphate, -/Va 2 S 2 3 ,5# 2 0,, contains in a liter of
the decinormal solution 24.822 grms. of the crystallized salt.
The solution can be prepared by dissolving in water this



172 INTRODUCTION TO CHEMICAL ANALYSIS

weight of the pure crystals which have been dried by pressing
in blotting-paper after pulverizing, then diluting to a liter.
The solution should be kept in the dark. It even then de-
composes slowly, however, so that old solutions cannot be
depended upon. Sodium thiosulphate is principally used in
volumetric analysis in the determination of free iodin. This
includes the determination of other substances like bromin
and chlorin, one atom of either of which sets free one atom
of iodin from an iodid. If, therefore, to a solution of free
bromin or chlorin a little potassium iodid is added, the bro-
min or chlorin frees the iodin. The amount of iodin thus
liberated indicates that of bromin or chlorin.

Practical Exercises in Analysis by Oxidation and Reduction

1 . From the normal solution of oxalic acid previously pre-
pared make a decinormal solution by diluting one volume
with nine of water. By the aid of this standardize a decinor-
mal solution of potassium permanganate by the method
described on page 168.

2. With the decinormal permanganate makes a determina-
tion of the amount of iron in a solution of ferrous sulphate,
calculating the weight of Fe and FeSO 4 present.

3. By the aid of a decinormal solution of potassium di-
chromate (made by the directions given) make a titration of
the same ferrous solution. Determine the end of the reaction
by removing a small drop of the solution on the end of a glass
rod and with this stirring a drop of a dilute, freshly prepared
solution of potassium ferricyanid as an indicator. As long
as there remains any of the unoxidized iron a blue color will
result. When sufficient dichromate is present only a brown-
ish-yellow appears. The tests are most conveniently made
by placing a number of drops of the indicator on a porcelain
plate and touching these with the stirring rod after the addi-
tion of each portion of the dichromate. The results obtained



ANALYSIS BY OXIDATION AND BY REDUCTION 173

by this method should agree with those by the permanganate.
They thus serve to confirm the correctness of the dichromate
solution as well as of the accuracy of the determination of the
amount of iron.

4. Use the decinormal permanganate to determine the con-
centration of a solution of hydrogen peroxid (dioxid). The
reaction is 5H 2 O2+2KMnO4 + 3H2SO4 = K 2 SO4+2MnSO4
-f 8H 2 O+5C>2. Measure into a beaker one cubic centime-
ter of the peroxid by means of a pipette and dilute with 10
to 20 times its volume of water acidified with H 2 SO 4 . From
a burette add the permanganate slowly until there is a per-
manent pink color. (In the U. S. P. process 10 c.c. of the
peroxid are diluted to 100 c.c., and 16.9 c.c. of this is titrated.
The Pharmacopoeia requires it to decolorize 30 c.c. of the
decinormal permanganate. This corresponds to a 3 per cent,
solution by weight.) Calculate the percentage strength of
the hydrogen peroxid solution by weight.

Instead of being expressed by weight the concentration of
the hydrogen peroxid is often referred to the volume of oxy-
gen which it will evolve when decomposed by heating. One
atom is thus set free from each molecule, or one-half the
amount that is given off when it is acted upon by potassium
permanganate, as represented by the above equation. For
each two molecules of the permanganate which are decolorized,
therefore, there are present five atoms of active oxygen in the
peroxid. Consequently i c.c. of the decinormal permanga-
nate corresponds to 0.0008 grm. of such active oxygen.
From the results obtained in the above determination of the
concentration of hydrogen peroxid by weight calculate its
oxygen volume of active oxygen, using 0.00143 grm. as the
weight of one cubic centimeter of oxygen.

5. Iron is determined quantitatively by permanganate or
dichromate solutions, but only when it is in the ferrous form.
Hence ferric compounds must be reduced to ferrous before



174 INTRODUCTION TO CHEMICAL ANALYSIS

they are titrated. This can be effected by a number of reduc-
ing agents. Place in a flask fitted with a valve as described
above (page 170) 10 c.c. of a ferric solution, acidify with sul-
phuric acid and add a few small fragments of granulated zinc
which is free from iron or in which the amount of iron is
known (using than a definite weight of zinc). Let it dissolve
completely when, if the iron is reduced, the liquid will be
colorless with no yellow tint. Then titrate immediately with
the decinormal permanganate solution and, after the amount
of iron has been found, calculate the weight and percentage
of the ferric compound in the original solution.

6. With the aid of a decinormal solution of sodium thio-
sulphate determine the concentration of a solution of iodin,
as follows : Into a measured volume of the iodin solution run,
from a burette, the standard thiosulphate until the brown
color has almost disappeared. Then add a few drops of a
starch solution and continue the titration until the blue is
just destroyed, leaving the liquid colorless. The reaction is
then completed, the thiosulphate being converted into a
sodium tetrathionate,

2Na 2 S 2 O 3 +I 2 = 2NaI+Na 2 S 4 O 6 .

7. Chlorin or bromin when brought into contact with potas-
sium iodid liberates an equal number of atoms. In conse-
quence of this action the concentration of chlorin or bromin
water is easily found. To 10 c.c. of the solution add about
half a gramme of potassium iodid in crystals or solution and
titrate with thiosulphate, using starch as an indicator, as in
the last operation.

8. In the same manner determine the amount of available
chlorin in calcium hypochlorite, CaOCl 2 .

CaOCl 2 +2HCl = CaCl 2 +H 2 0+Cl 2 .
Acidify a known quantity with hydrochloric acid after the



ANALYSIS BY OXIDATION AND BY REDUCTION 175

addition of potassium iodid and starch, and titrate with the
thiosulphate as in the preceding exercises.

The presence of chlorates lessens the accuracy of the last
determinations.

9. With starch as an indicator compare the concentration
of the decinormal thiosulphate with the decinormal iodin
solution to prove that both are correct.

10. When a ferric salt is warmed with potassium iodid it is
changed to the ferrous state, one atom of iodin being set free
for each atom of iron.

FeCl3+KI = FeCl 2 +KCH-I.

The quantitative method of determining ferric compounds
based upon this reaction is carried out in the following manner :

To 10 c.c. of the ferric solution add about a gramme of po-
tassium iodid and 2 c.c. of hydrochloric acid. By the above
reaction the iodin is slowly set free. The mixture should be
placed in a 100 c.c. glass-stoppered bottle and the whole
warmed in water two hours at 40. The temperature should
not be allowed to exceed this nor the stopper be removed be-
cause of danger of loss of iodin through volatilization. Cool,
and after the addition of a few drops of starch solution, titrate
with decinormal sodium thiosulphate. Calculate the amount
of the ferric salt, reckoning one atom of iron for each one of
free iodin.

11. Prepare a decinormal solution of iodin by weighing
(page 170), or take a larger amount than is necessary, dissolve
in the same manner, determine its concentration by titration,
and dilute to the standard.

1 2 . With the standard iodin solution make a determination
of a solution of arsenous acid, HsAsOs, or an arsenite, like
potassium arsenite, KsAsOs. In the case of the arsenous acid
pure sodium or potassium bicarbonate must be present to
neutralize the hydriodic acid formed in the titration. Five to



176 INTRODUCTION TO CHEMICAL ANALYSIS

ten times as much of this should be used as the estimated
weight of the arsenous acid. The titration is carried out as
before until the starch indicator is colored a faint blue. A
gentle heat may be used to aid in the solution of the bicarbon-
ate, but it cannot be heated high enough to decompose with
bicarbonate as the carbonate thus produced would interfere
with the action of the indicator. If a free alkali other than a
bicarbonate is present it must be neutralized by hydrochloric
acid and any acids must be neutralized with a bicarbonate
before titration. The reaction between the iodin and arsen-
ite is similar to that with the arsenous oxid already referred to.



ANALYSIS BY PRECIPITATION 177

CHAPTER IV

ANALYSIS BY PRECIPITATION

HERE the standard solution converts the compound of
which the amount is sought, or some constituent of it, into an
insoluble form, thus producing a precipitate. Knowing the
amount of the standard necessary to effect this result the
weight of the precipitate or of the substance from which it is
derived can be calculated. As the point where precipitation
is complete is, in almost all cases, indistinct, indicators are
usually required in this class of analysis. These are such
compounds as will not be acted upon by the standard until
the compound under investigation has been completely trans-
formed and which will then react with the excess of the stand-
ard solution producing a color or some other visible change.

Precipitation is also often used in combination with other
methods of volumetric determination. Thus soluble com-
pounds of barium, strontium, and calcium can be precipitated
as neutral carbonates by ammonia and ammonium carbonate.
If these precipitates are washed and suspended in water they
can be titrated by solutions of the normal acids, their amount
being calculated from the volume of acid necessary to produce
an acid reaction. The equations representing the chemical
change occurring during their solution may be represented by
the following:



The Preparation and Properties of Standard Solutions
Used in Analysis by Precipitation

Decinormal Silver Nitrate, AgNO^. As the crystals can

usually be obtained in the pure state, this can be made by dis-
12



178 INTRODUCTION TO CHEMICAL ANALYSIS

solving 16,989 grms. in sufficient water to make the volume of
the solution one liter. Or a somewhat greater weight may be
dissolved, if the purity is doubtful, and the concentration as-
certained by titration against decinormal sodium chlorid.
Instead of the sodium chlorid decinormal hydrochloric acid
can be used. In the latter case after the amount to be used
has been accurately measured by a pipette, it must be care-
fully neutralized with sodium carbonate before the titration,
avoiding an excess of the carbonate. The value of the silver
nitrate in analysis by precipitation is that it forms insoluble
compounds with the chlorids, bromids, iodids, and cyanids.
The indicator is generally normal (yellow) potassium chro-
mate. This forms an insoluble, dark red, silver chromate
with the silver nitrate, but not until the above-mentioned
compounds have been precipitated if they are present in the
solution. The end reaction is most clearly seen if instead of
daylight a yellow light, like that of illuminating gas, is used,
the titration being conducted in a rather dark place.

The silver nitrate solution is decomposed by the action of
light and by organic matter. It should therefore be pre-
served in an amber-colored bottle or in a dark place. It
should be protected from dust and not be used in burettes
which have rubber-tubing connections.

Decinormal Potassium Sulphocyanate, KSCN, contains in a
liter 9.718 grms. of the salt. It cannot well be prepared by
weighing the solid, since the latter is deliquescent. About
10 grammes of this should be dissolved in a liter of water and
10 c.c. of decinormal silver nitrate titrated with the solution
after acidifying with 5 c.c. of dilute nitric .acid. The reaction
is shown by the following equation:

AgN0 3 +KSCN = AgSCN+KN0 3 .

About ten drops of a solution of ammonium ferric sul-
phate (iron alum) is to be added for an indicator. The



ANALYSIS BY PRECIPITATION 179

silver sulphocyanate is precipitated first, giving the liquid
a milky appearance. When all the silver has been con-
verted into this compound the sulphocyanate acts on the in-
dicator producing red ferric sulphocyanate which indicates
the end reaction. With a solution prepared in this way less
than 10 c.c. of the sulphocyanate should at first precipitate
10 c.c. of the decinormal silver nitrate. The amount of water
which must be added to dilute it to correspond to the silver
solution can be calculated as in the preparation of standard
solutions of sodium hydroxid and hydrochloric acid. When
this has been added it should be tested again in the same man-
ner to ascertain its correctness. This sulphocyanate solution
may be used for the estimation of silver, even in the presence,
of many other dissolved metals, since most of these are un-
affected by the reagent. It can also be employed in connec-
tion with decinormal silver nitrate to determine the amount
of any substance which is completely precipitated by the lat-
ter compound. This includes all those mentioned as capable
of being determined by standard silver nitrate. In this case
the method is that of residual titration. It consists in adding
to the solution a measured quantity of the decinormal silver
nitrate, greater than is sufficient to precipitate the compound,
then titrating this with sulphocyanate to learn what the ex-
cess is. The difference in cubic centimeters between the
volumes of silver nitrate used and sulphocyanate used shows
the amount of standard silver solution which is taken up by
the substance under investigation. From this its weight
is obtained. The end reaction is rather easier to distinguish
than when potassium chromate is the indicator.

The standard solutions of silver nitrate and potassium sul-
phocyanate may be employed in the estimation of any com-
pounds of the metals which can be converted easily and without
loss into chlorids. This can be done with the carbonates,
hydroxids, 'oxids and nitrates of potassium, sodium, ammo-



l8o INTRODUCTION TO CHEMICAL ANALYSIS

nium, calcium, strontium, barium, magnesium, and some other
metals by the action of hydrochloric acid; also with the chlo-
rates, which by ignition set free oxygen and are changed to
chlorids. Carbonates may be decomposed with the evolu-
tion of carbon dioxid and nitrates with a setting free of nitric
acid. This is accomplished by adding to a weighed or meas-
ured amount of the substance under investigation an excess
of the acid (after evaporation to dryness in case of solutions)
using concentrated acid with nitrates. The excess of the
acid, that is, the part which has not united with the metal to
form chlorids, must be completely driven off by first evaporat-
ing to dryness on a steam-bath, then heating in an air-bath
.at 120 until a piece of blue litmus-paper laid across the dish
is no longer reddened. The chlorid is then dissolved in water
and this solution titrated as before, using a known fraction
of the liquid and making duplicate determinations. After
the determination of the chlorin the weight of the metal, and
hence of the original compound, can be calculated.

Practical Exercises in Analysis by Precipitation

1 . Prepare decinormal silver nitrate by one of the methods
given above.

2. With this solution determine the percentage concentra-
tion of a solution of sodium chlorid. Use only enough of the
potassium chromate to make the solution slightly yellow, which
will take but a few drops. With a large quantity it becomes
more difficult to tell when the silver chromate commences to be
permanent. Make the titration by yellow light or gas light if
convenient (not that of an incandescent burner, however).
One molecule of silver nitrate precipitates oneof the chlorid.

AgN0 3 +NaCl=AgCl+NaN0 4 .

3. In the same manner determine the concentration of a
solution of potassium bromid.



ANALYSIS BY PRECIPITATION l8l

4. A potassium cyanid solution upon the addition of the
silver ion forms at first no precipitate, but the soluble salt
KAg(CN) 2 .

AgN0 3 +KCN = KAg(CN) 2 +HN0 3 .

When all the potassium cyanid has been thus changed any
excess of silver nitrate decomposes the double salt and a pre-
cipitate appears. This reaction can be made use of as a quan-
titative method. Into 10 c.c. of the potassium cyanid solution
run from the burette decinormal silver nitrate, stirring
continually. The equation above shows that as soon as a
permanent precipitate appears, for each molecule of the silver
nitrate used two of the cyanid have entered into reaction.


1 2 3 4 5 6 7 8 9 10 11 13 15 16 17 18

Online LibraryElbert William RockwoodAn introduction to chemical analysis, for students of medicine, pharmacy and dentistry → online text (page 13 of 18)