Gravimetry

advertisement
12/21/2018
Chapter 3. Gravimetry
Chapter 3, Gravimetry
Gravimetric methods of analysis are used where weights of reactants and products of chemical reactions are
reproducible, stable and reflect the presence of constituents which are important in the establishment of identity.
Two important methods deal with the trapping and weighing of products in the solid and gaseous phases. The
first of these falls into the category of a precipitation method.
3-1. Precipitation methods. Many metallic elements in their ionic forms react with negative counter ions to
produce stable precipitates. Silver ions form stable and highly insoluble salts with chloride, bromide and iodide.
Calcium precipitates quantitatively with oxalate and can be measured reproducibly at any of three temperature
dependent plateaus as the oxalate, the carbonate and the oxide. Barium precipitates quantitatively as the sulfate.
The reactions often follow the same patterns:
Positive and negative ions in an aqueous solution, otherwise soluble with the counter ions in their environment,
produce highly insoluble precipitates with certain added reagents.
3-2. Volatilization methods. An interesting volatilization method which is entirely gravimetric is the one shown
by the equations below.
The analyte can be bicarbonate as shown or a mixture of carbonate and bicarbonate. The total amount of
carbonate in whatever form is found by placing the analyte in a solution containing an excess of H2SO4. This
solution is in a flask connected to incoming nitrogen gas gently bubbled through the solution and an exit tube
first to a drying agent to absorb aerosolized water and water vapor and then to a mixture of NaOH and drying
agent to absorb the CO2 and
water subsequently produced by the absorption by NaOH:
The apparatus is shown below. The tube containing the NaOH on asbestos and the CaSO4 to absorb the final
water product is pre- and post-weighed to given the total amount of carbonate in the sample. Note that the
http://www5.csudh.edu/oliver/che230/textbook/ch03.htm
1/10
12/21/2018
Chapter 3. Gravimetry
nitrogen gas acts only as a carrier and does not take part in any reaction.
3-3. Considerations for the isolation of precipitates
Precipitates ought to be easy to wash free of contaminants without loss of the precipitate either in solution or
through the filter. The particle size of the precipitate ought to be large enough not to escape through the filter
pores. That the precipitate has a low solubility is paramount. The precipitate ought not to react with the
atmosphere and it must have a known composition which remains stable after ignition.
3-3a. Substances of low solubility have the nasty habit of forming colloidal suspensions. Colloidal particles have
diameters from 10-7 cm to 10-4 cm. That is on the order of from 10 atomic diameters to 10,000 atomic
diameters. Particles in this size range are still sufficiently jostled about by thermal molecular motion to remain in
suspension. Where they are the result of a process of precipitation brought about by the addition of ionic species,
the particles are surrounded by the excess ionic species. If Ba2+ is added in excess to SO42- the BaSO4
precipitate which is formed is considered to be surrounded by Ba2+ ions. If the opposite procedure were being
followed, the precipitate would be surrounded by SO42- ions. That these particles all have like charge and
therefore repel each other suggests that your technique must favor the formation of large rather than small
precipitate particles and to offer ways of encouraging the coagulation of particles after they have formed.
This can be done by carrying out the precipitation at a temperature close to the boiling point of water, in a dilute
solution of your analyte and with constant stirring for the reasons given below.
Although analytical chemists still have some disagreement as to the mechanism of precipitation, there is wide
agreement that a quantity called the relative supersaturation affects the particle size. Relative supersaturation is
given as
http://www5.csudh.edu/oliver/che230/textbook/ch03.htm
2/10
12/21/2018
Chapter 3. Gravimetry
where Q is the instantaneous concentration of the species added to effect precipitation and S is the equilibrium
solubility of the substance which precipitates. Particle size seems to be inversely proportional to Relative
Supersaturation because a high concentration of added reagent increases the probability that oppositely charged
ions will begin the precipitation process at late as well as early stages of the addition and the resulting particles
will be on the order of atomic dimensions, whereas the maintenance of a value of Q just slightly above S lowers
that probability but offers in any case a layer of the added reagent ions around existing particles for their further
growth.
3-4. The Electric Double Layer
If a particle of precipitate is surrounded
by the ion in excess, say Ba2+ in the
case of the determination of SO42-, any
negative ions in the immediate
surroundings will be attracted to that
primary positive layer. In the case of the
addition of a BaCl2 solution to a
Na2SO4 solution, the ions Cl- and
SO42- are available. As the sulfate is
used up in the precipitation process it is
the chloride which is left and which
forms the second layer. Thus we have
an electric double layer, made up first
of barium ions then of chloride ions.
This double layer keeps the colloidal
precipitate particles from coming into
contact with each other for further
coagulation.
There are two ways to bring the
particles closer together and to increase
the probability of coagulation: (1)
heating increases overall thermal
motion, affecting both the mobility of
adsorbed ions and of the colloidal
precipitate particles themselves. The
summary effect is that there are collisions of particles which result in the increase in particle size due to
increased coagulation; (2) increasing the electrolyte concentration of the solution, for reasons not entirely clear,
results in a decrease in the mean radius of the electric double layer and encourages further coagulation. Carrying
out both of these operations results in digestion of the precipitate, an unfortunate term because biological
digestion usually refers to the dissolving of food and absorption at the molecular level through the wall of the
intestine. Digestion in quantitative analysis refers to the coagulation of a precipitate into a filterable form.
Unfortunately after successful digestion, some of the primary electric layer is made up of Na+ ions which must
be washed away ultimately for quantitative results to be achieved. The Ba2+ ions as well will give a positive
error if not removed and end up being dried with the precipitate as excess BaCl2. Many coagulated precipitates
do not respond well to washing with distilled water because as the second electric layer is removed (excess Clfor example) the first remains on all particles with an electric charge of the same sign. The result is that there is a
return to the repulsive state and an effective increase in the radius of the particles which then begin once again to
separate as colloidal particles. The process is called peptization and is to be avoided if some of the precipitate is
not to be lost. One way around this for many precipitates is to encourage digestion by heating and also by
http://www5.csudh.edu/oliver/che230/textbook/ch03.htm
3/10
12/21/2018
Chapter 3. Gravimetry
increasing the electrolyte concentration by washing with a reagent which will go off as a gas during the drying
process. Dilute nitric acid, HNO3 , is effective for washing excess ions from AgCl. In choosing such a wash, it is
imperative that the procedure has been carried out and has been shown to yield reproducible, quantitative results.
Unexpected side reactions, complex formation and changes in solubility with added reagents are sufficiently
unpredictable to make intuition in the absence of experience unacceptable.
3-5. Other demons which can plague quantitative precipitate isolation.
During the precipitation procedure a number of other problems can arise to give erroneous positive or negative
results. Among these are surface adsorption, mixed crystal formation, occlusion and mechanical entrapment.
Any ions may be carried down during a precipitation as the result of surface adsorption. Na+ , or Cl- in the case
of the determination of SO42- by the addition of dilute BaCl2 solution to a NaSO4 solution. Both Ba2+ and Na+
can compete for lattice positions as the particles form. Likewise, the ions Cl- and SO42- can have the same
effect. In the quantitative determination of some transition metals, iron for example as Fe(OH)3, zinc, cadmium
and manganese may be present as impurities and all three form sparingly soluble hydroxides as well, though
each with greater solubility than the hydroxide of iron:
Compound
Solubility Product
Fe(OH)3
4 x 10-38
Cd(OH)2
2.5 x 10-14
Mn(OH)2
1.9 x 10-9
Zn(OH)2
1.2 x 10-17
Mixed crystal formation can occur if two ions have the same charge, if their ionic diameters are sufficiently
close to fit into the same crystal lattice. Ions which commonly interfere with each other are shown in the table
below with their ionic diameters in picometers given after each.
Interfering
Ions
K+, 133 pm
NH4+, 148 pm
Sr2+, 113 pm
Ba2+, 135 pm
Mn2+, 80 pm
Cd2+, 97 pm
In cases where one has a known interference of one ion with the other it is necessary to find methods of
removing one before carrying out a precipitation of the other, or using a precipitating reagent in which there is
no interference.
http://www5.csudh.edu/oliver/che230/textbook/ch03.htm
4/10
12/21/2018
Chapter 3. Gravimetry
Occlusion and mechanical entrapment. If a precipitation procedure is carried out too quickly, pockets of
solvent and spectator ions can form, trapping them within the precipitate particles and dashing one's hope of
removing them during the washing procedure. This is another reason why the relative supersaturation must be
kept as low as possible so that in principle at least, all precipitation occurs only at the surface of a growing solid
particle, devoid of solvent pockets.
All of these problems of coprecipitation of unwanted ions can lead to positive or negative errors. In the example
above where it pointed out that Na+ or Cl- may coprecipitate in the SO42- determination, surface adsorption will
produce a positive error. In the case of mixed crystal formation, the direction of the error depends on the relative
atomic weight of the ion which replaces that which is desired in the precipitate. In the case of the precipitation of
zinc hydroxide, mixed crystal formation with manganese would produce a negative error but with cadmium or
zinc a positive error.
Desired precipitate
Compound
Mn(OH)2
Zn(OH)2
Cd(OH)2
At. Wt. of M2+
54.94
65.39
112.41
Direction of error
negative
---
positive
3-6. The use of the technique of homogeneous solutions to effect precipitation.
A solution containing a reagent which produces a desired ion to effect precipitation, often by gentle heating of
the solution, offers an exquisite means for obtaining well-formed large crystal particles which lend themselves
splendidly to the technique of filtration.
The model we use to explain why this happens also uses the concept of relative supersaturation. The initial
nucleation of sparingly soluble particles offers a surface template which favors "locking" onto ions in the
vicinity which by the luck of the draw (and the kinetic molecular theory) find themselves at the right energy and
orientation to enter the crystal lattice. Ions isolated from a growing crystal are not favored to enter this process
because at least two are required, both at the right energy and orientation to start the growth of a new crystal. If
the concentration of one ion of a sparingly soluble salt increases gradually by slow homogeneous synthesis in a
solution, then as its concentration reaches the threshold of supersaturation for the ion pair, a relatively small
number of nucleated particles grows to larger size (because the probability of finding a place in an existing
crystal lattice for any single ion is greater than that of a spontaneous creation a new crystal from dissolved and
randomly arranged ions) rather than a large number of nucleated particles growing in constant competition with
the rest and thus remaining small. The result for the latter is a non-filterable precipitate, but one in the former
which filters quite well.
See the demonstration of this effect at
http://155.135.31.26/oliver/demos/prechomo/prechomo.htm
Here is a table of common reagents useful for the preparation of ions often needed for precipitation processes.
Reagent
Precipitating species
http://www5.csudh.edu/oliver/che230/textbook/ch03.htm
Precipitation reaction
Elements
5/10
12/21/2018
Chapter 3. Gravimetry
which yield to
this reaction
Urea
OH-
(NH2)2CO + 3H2O --->
Al, Ga, Th, Bi,
Fe, Sn
CO2 + 2NH4+ + 2OHTrimethyl phosphate
PO43-
(CH3O)3PO + 3H2O --->
3CH3OH + H3PO4
Zr, Hf
Ethyl oxalate
C2O42-
(C2H5)2C2O4 + 2H2O --->
2C2H5OH + H2C2O4
Mg, Zn, Ca
Dimethyl sulfate
SO42-
(CH3O)2SO2 + 4H2O --->
Ba, Ca, Sr, Pb
CO32-
Cl3CCOOH + 2OH- --->
Trichloroacetic acid
2CH3OH +SO42- + 2H3O+
La, Ba, Ra
CHCl3 + CO32- + H2O
Thioacetamide
H2S
CH3CSNH2 + H2O --->
Sb, Mo, Cu, Cd
CH3CONH2 + H2S
Dimethyl glyoxime
CH3(CNOH)2CH3
CH3COCOCH3 +
Ni
2H2NOH ---> DMG + 2H2O
8-Acetoxyquinoline
C9H6NOH
CH3COOQ + H2O --->
CH3COOH + HOQ
Al, U, Mg, Zn
3-7. Preparation of a dry weight of your precipitate.
The resulting precipitate must be heated until a stable dry state is reached. Some understanding of typical
precipitate properties is mandatory for repeatable results to be achieved.
Note in the figure at the right that whereas AgCl achieves a stable dry weight just above 100oC, BaSO4 does not
do so until it reaches a temperature in the vicinity of 700oC Aluminum oxide, Al2O3, loses water slowly as the
temperature rises to 1000oC at which point it achieves stability. Some compounds decompose in several stages,
reaching stable plateaus. Calcium oxalate, CaC2O4 H2O, loses all its water at around 200oC and remains stable
http://www5.csudh.edu/oliver/che230/textbook/ch03.htm
6/10
12/21/2018
Chapter 3. Gravimetry
as CaC2O4 until just above 400oC at which point it
decomposes to calcium carbonate, CaCO3 where it
remains stable up to 700oC. Between 700oC and
850oC it slowly decomposes to CaO where it remains
stable until its melting point at 2614oC.
A device not seen often in analytical laboratories but
useful for producing automatic plots of mass of
sample vs. temperature such as those at the right is the
thermobalance (below). Region A includes the
heating circuit, a temperature sensor, sample cup and
counter weight resting on one end of the balance arm,
B. A light wave is partially attenuated at C, giving a
negative feedback to the amplifier circuit at D,
designed to yield an output voltage which increases
with the force necessary to keep the balance in
equilibrium (and the attenuation at a constant value).
One can adjust the baseline voltage at E, the tare
adjuster, so as to produce the graph at the chart
recorder, F.
Example 3-1: A 0.3427 g sample of bronze-age
jewelry is analyzed for silver content by first
dissolving it in concentrated nitric acid and precipitating it as AgCl. The precipitate is transferred to a dry
sintered glass filter weighing 12.2347 g where it is separated from the filtrate and washed with dilute nitric acid.
The filter and precipitate are dried at 150oC, cooled, weighed and found to weigh 12.4373 g. Calculate the %Ag
in the jewelry.
(To be solved and discussed in class)
Example 3-2. A sample of iron ore weighing 0.4275 g
is dissolved in 12M HCl. The resulting solution is
slowly made basic with NaOH until the first hint of a
turbid solution is detected. 10.0 g urea are dissolved
and the solution heated just to the boiling point for 4
hours. The precipitate, Fe2O3 x H2O, is trapped using
ashless filter paper. The precipitate and filter paper are
fired in a porcelain crucible of empty weight 12.2837g
until the ashless filter paper is completely incinerated
and anhydrous Fe2O3 is left. The resulting weight of
crucible and precipitate is 12.4274 g. Determine the
%Fe, the %Fe2O3 and the %Fe3O4.
(To be solved and discussed in class)
http://www5.csudh.edu/oliver/che230/textbook/ch03.htm
7/10
12/21/2018
Chapter 3. Gravimetry
Example 3-3. A sample known to contain only KCl and NaCl and weighing 0.4263 g is dissolved in water and
treated to an excess of AgNO3, using standard methods of precipitation. The AgCl precipitate is caught on a
Gooch Crucible of original dry weight of 15.2748 g. The AgCl precipitate is dried at 150oC, cooled and the
crucible and precipitate are found to weigh 16.2872 g. Determine the %KCl and the %NaCl in this sample.
Solution is based on the difference in the %Cl in the pure salts:
Salt
%Cl
NaCl
60.66
KCl
47.55
(To be solved and discussed in class)
3-8. Preferred methods of gravimetric analysis. Most inorganic ions have yielded to gravimetric analytical
techniques, but one finds many interfering ions. The table below illustrates both the abundance of reagents
available for use as well as the problems which can be encountered by interfering ions:
Analyte
Precipitate
Measured form
Interferences
K+
KB(C6H5)4
KB(C6H5)4
NH4+,Ag+,Hg2+, Tl+,Rb+,Cs+
Mg2+
Mg(NH4)PO4.6H2O
Mg2P2O7
Many metals (none from Na+ and
K+)
Ca2+
CaC2O4.H2O
CaCO3 or CaO
Many metals (none from Mg2+,
Na+ and K+)
Ba2+
BaSO4
BaSO4
Na+,K+,Li+,Ca2+,Al3 +
,Cr3+,Fe3+,Sr2+,Pb2+
Ti4+
TiO(5,7-dibromo-8hydroxyquinoline)2
TiO(5,7-dibromo-8hydroxyquinoline)2
Fe3+,Zr4+,Cu2+,C2O4 2-, citrate,
HF
VO43-
Hg3VO4
V2O5
Cl-,Br-,I-,SO42- ,
CrO42-,AsO43-,PO43-< / sup>
Cr3+
PbCrO4
PbCrO4
NH4+,Ag+
Mn2+
Mn(NH4)PO4.H2O
Mn2P2O7
Interferences from numerous
metals
Fe3+
Fe(HCO2)3
Fe2O3
Interferences from numerous
metals
http://www5.csudh.edu/oliver/che230/textbook/ch03.htm
8/10
12/21/2018
Chapter 3. Gravimetry
Co2+
Co(1-nitroso-2naphtholate)3
CoSO4 (by reaction with
H2SO4 )
Fe3+,Zr4+,Pd2+
Ni2+
Ni(dimethylglyoximate)2
Ni(dimethylglyoximate)2
Pd2+,Pt2+,Bi3+,Au3+
Cu2+
CuSCN
CuSCN
NH4+,Pb2+,Hg2+,Ag+
Zn2+
Zn(NH4)PO4.H2O
Zn2P2O7
Interferences from numerous
metals
Ce4+
Ce(IO3)4
CeO2
Th4+,Ti4+,Zr4+
Al3+
Al(8-hydroxyquinolate)3
Al(8-hydroxyquinolate)3
Interferences from numerous
metals
Sn4+
Sn(cupferron)4
SnO2
Cu2+,Pb2+,As(III)
Pb2+
PbSO4
PbSO4
Ca2+,Sr2+,Ba2+,Hg2+, Ag+,HCl,
HNO3
NH4+
NH4B(C6H5)4
NH4B(C6H5)4
K+, Rb+, Cs+
Cl-
AgCl
AgCl
Br-, I-, SCN-, S2-, S2O32-, CN-
Br-
AgBr
AgBr
Cl-, I-, SCN-, S2-, S2O32-, CN-
I-
AgI
AgI
Br-, Cl-, SCN-, S2-, S2O32-, CN-
SCN-
CuSCN
CuSCN
NH4+,Pb2+,Hg2+,Ag+
CN-
AgCN
AgCN
Cl-, Br-, I- , SCN-, S2-, S2O32-
F-
(C6H5)3SnF
(C6H5)3SnF
Except alkali metals, many
interferences,
and SiO44- , CO32-
ClO4-
KClO4
KClO4
SO42-
BaSO4
BaSO4
Na+,K+,Li+,Ca2+,Al3 +
,Cr3+,Fe3+,Sr2+,Pb2+
PO43-
Mg(NH4)PO4.6H2O
Mg2P2O7
Many interferences except Na+,K+
NO3-
Nitron nitrate
Nitron nitrate
ClO4-, I-, SCN-, CrO42-,ClO3-,
NO2-, Br-, C2O42-
CO32-
CO2 (by addition of acid)
http://www5.csudh.edu/oliver/che230/textbook/ch03.htm
CO2
CO2 is trapped as Na2CO3 on
Ascarite
9/10
12/21/2018
Chapter 3. Gravimetry
There are a number of organic functional groups which precipitate with metal ions by one of two routes: (1)
chelating agents are organic compounds which "wrap around" a metal ion thanks to cationic side chains which
form coordinate covalent bonds with the ion, and (2) a straightforward ion-ion bond which produces a new
species that excludes water of solvation and thus precipitates. Good examples of chelating agents include
Ethylene Diamine Tetraacetic Acid (EDTA), oxalic acid, glycine, 8-hydroxyquinoline and dimethylglyoxime.
Some common organic precipitating agents:
Compound
Ions precipitated
Dimethylglyoxime
Ni2+,Pd2+,Pt2+
EDTA (Ethylenediamine tetraacetic acid)
Zn2+, Cu2+, Pb2+, Ca2+, Ni2+, Fe3+
Cupferron
Fe3+,VO2+,Ti4+, Zr4+,Ce4+,Ga3+,Sn4+
8-Hydroxyquinoline
Fe3+,Al3+,Mg2+,Zn2+, Cu2+,Cd2+,Pb2+, Bi3+,
Ga3+,Th4+, Zr4+, TiO2+, UO22+
Salicylaldoxime
Bi3+,Ni2+,Pd2+,Zn2+, Cu2+,Pb2+
1-Nitroso-2-naphthol
Fe3+,Co2+,Pd2+, Zr4+
Nitron (C20H16N4)
NO3-, ClO4-, BF4-, WO42-
Sodium tetraphenylborate
NH4+, organic ammonium, Ag+, Cs+, Rb+,
K+
Tetraphenylarsonium chloride
Cr2O72-, MnO4-, ReO4-, MoO42-,WO42-,
ClO4-
http://www5.csudh.edu/oliver/che230/textbook/ch03.htm
10/10
Download
Related flashcards
Create Flashcards