Gravimetric summary
Dr Harold Braustein The Jerusalem Engineering College
Analytical Chemistry
Gravimetry
Summary
Principles
Solution reaction between analytes and reagents to give sparingly soluble products;
filtration, drying or ignition of precipitates; electrolytic deposition of metals;
weighing.
Apparatus
Flasks, beakers, filter funnels, pipettes, filter crucibles, filter papers, oven, muffle
furnace, chemical balance, desiccator.
Applications
Extensive numbers of inorganic ions are determined with excellent precision and
accuracy; widely used in routine assays of metallurgical and mineralogical samples.
Relative precision 0.1–1%.
Disadvantages
Requires careful and time-consuming procedures with scrupulously clean apparatus
and very accurate weighings. Coprecipitation cannot always be avoided.
Gravimetry includes any analytical method in which the ultimate measurement is by
weight. The simplest form may merely be the drying or heating of a sample in order
to determine its volatile and non-volatile components, or possibly the sample might be
distilled and the residue and fractions of distillate weighed. Metals may be deposited
electrolytically and weighed. Of far greater scope and importance is the controlled
precipitation of an analyte from solution, followed by the weighing of the precipitate.
Subsequent attention will be restricted to these precipitation methods.
To be of gravimetric value, a precipitation process must fulfil certain conditions. The
precipitate must be formed quantitatively and within a reasonable time. Its solubility
should be low enough for a quantitative separation to be made. It must be readily
filterable and, if possible, have a known and stable stoichiometric composition when
dried so that its weight can be related to the amount of analyte present. Failing this, it
must be possible to convert the precipitate to a stoichiometric weighable form
(usually by ignition). In both cases the weighed form should be non-hygroscopic.
Precipitation Reactions
If it is remembered that a chemical reaction can be displaced by changing the state of
the products then, provided the solubility product of a precipitate is small, quantitative
reaction can be obtained. Practically, the precipitate may be formed directly on the
addition of a reagent or on the subsequent adjustment of solution conditions, e.g. pH.
Precipitates may be of different chemical types, e.g. salts, complexes, hydroxides,
hydrous oxides, and precipitating agents are conveniently divided into inorganic
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Gravimetric summary
Dr Harold Braustein The Jerusalem Engineering College
Analytical Chemistry
(Table 5.14) and organic (Table 5.15). The major areas of applications usually employ
inorganic precipitants for inorganic analytes.
Table 5.14 Some inorganic precipitants
Reagent Analyte and form precipitated Analyte form weighed
NH3(aq) Be hydrous oxide BeO
Al hydrous oxide Al2O3
Sc hydrous oxide Sc2O3
Fe hydrous oxide Fe2O3
In hydrous oxide In2O3
(NH4 )2U2O7 U3O8
H2S ZnS ZnO
GeS GeO2
As2S3 As2O3
(NH4)2HPO4 MgNH4PO4 Mg2P2O7
H2SO4
Sr, Cd, Pb and Ba sulphates sulphates
HCl AgCl AgCl
Si (silicic acid) SiO2
AgNO3
AgCl AgCl
AgBr AgBr
AgI AgI
BaCl2 BaSO4 BaSO4
The Solubility of Precipitates
For the solubility equilibrium
where g is the activity coefficient and C is the concentration of the species. The
activity coefficient may
be calculated from the Debye-Hückel equation for a temperature of 298 K,
where Z is the charge on A and μ (mol kg–1) is the ionic strength of its solution. Thus,
gA, Ksp and in turn the solubility of AB will increase with the ionic strength of the
solution environment. Figure 5.7 shows the effect of potassium nitrate on the
solubility of barium sulphate.
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Gravimetric summary
Dr Harold Braustein The Jerusalem Engineering College
Analytical Chemistry
The common ion effect is a further important factor affecting solubilities.
Addition of A or B to the above system (equation (5.28)) will shift the equilibrium to
the left and reduce the solubility of AB. In practice, this situation would arise when an
excess of a precipitating reagent has been added to an analyte solution. Such an
excess leads to the possibility of complexation reactions occurring which will tend to
increase the solubility of AB. For example, when aluminium or zinc is precipitated by
hydroxyl ions, the following reactions with excess reagent can occur
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Gravimetric summary
Dr Harold Braustein The Jerusalem Engineering College
Analytical Chemistry
The net result of the three factors discussed above is that frequently an optimum
(minimum) solubility is obtained when a small excess of the reagent is added. Figure
5.8 which is a solubility curve for AgCl illustrates this pattern.
The effect of pH changes on precipitate solubilities merits special consideration. The
dependence of solubility on pH may derive from the common ion effect where OH–
or H3O+ ions are generated by the dissolution of the precipitate, e.g.
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Gravimetric summary
Dr Harold Braustein The Jerusalem Engineering College
Analytical Chemistry
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Gravimetric summary
Dr Harold Braustein The Jerusalem Engineering College
Analytical Chemistry
oxalates of calcium and magnesium for instance, both compounds have fairly low
solubility products (2.3 × 10–9 mol2 dm–6 for calcium oxalate and 8.6 × 10–5 mol2
dm–6 for magnesium oxalate). However, in the case of the calcium salt, precipitation
is complete within a few minutes whilst the magnesium salt takes several hours. This
difference in rates may even be used as a basis for the quantitative removal of calcium
from solution without interference from magnesium. In order to expedite precipitate
formation it is necessary first to look more closely at the processes by which
precipitates are produced.
When a reagent is added to an analyte solution forming a sparingly soluble
compound, the solubility product for that compound is immediately exceeded and the
solution is said to be supersaturated. The relative supersaturation Rs is given by
where Q is the actual concentration of the solute and S is the equilibrium
concentration. The rate at
which Q reduces to S determines the rate of precipitation. Precipitate formation takes
place first by the
aggregation of small groups of ions or molecules, a process known as nucleation. It
may occur either by
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Gravimetric summary
Dr Harold Braustein The Jerusalem Engineering College
Analytical Chemistry
the chance aggregation of molecules or ions in a homogeneous nucleation process, or
by heterogeneous
nucleation when aggregation is initiated by particulate impurities within the solution.
Whilst the former process depends exponentially on the relative supersaturation of the
solution, the latter is largely independent of it. After nucleation the precipitation
continues by particle growth, with further ions or molecules adding to the aggregates.
The rate of particle growth will also be dependent upon the relative supersaturation
and on the surface area of the particles, but will not vary so dramatically as the rate of
homogeneous nucleation. The relation between these rates and the relative
supersaturation is summarized schematically in Figure 5.9.
In a particular system, the nature of the precipitated particles will be determined by
the relative rates of nucleation and particle growth. Where nucleation predominates,
small particles are produced and a colloid may
result, whereas a predominance of particle growth will form a more tractable
precipitate. From the practical standpoint, the difference (Q–S) should be controlled
and kept to a minimum. On mixing reagent and analyte solutions it is difficult to
avoid a momentarily high (Q–S) value (in the region where the two first make
contact), especially if S is small. Furthermore, a low value of S is necessary
for quantitative precipitation, so that a situation often arises in which the rate of
homogeneous nucleation vastly exceeds that of particle growth. Consequently, the
analyst frequently has to cope with colloidal precipitates or suspensions. Where S is
larger, crystalline precipitates are more readily obtained. Colloidal suspensions occur
as a result of electrical repulsions between particles which prevent agglomeration.
These repulsive forces develop from ions adsorbed onto the surface of the
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Gravimetric summary
Dr Harold Braustein The Jerusalem Engineering College
Analytical Chemistry
precipitate which cause the formation of an electrical double layer. The adsorbed
layer will contain an excess of the precipitate ion which predominates in solution
whilst the diffuse counter ion layer will contain an excess of oppositely charged ions
to maintain the overall electrical neutrality of the solution.
In the familiar example where Cl– has been precipitated as AgCl by the addition of an
excess of silver nitrate, the adsorbed layer will contain an excess of Ag +, and the
counter ion layer an excess of and OH–.
Adsorption can be diminished by heating or by the addition of a highly charged strong
electrolyte. This allows coagulation of the precipitate to proceed, but it must be
remembered that if a filtered precipitate of this type is being washed the particles can
be dispersed again as a colloid and pass through the filter.
This effect, known as peptization, may be prevented by using hot washing solutions
containing a suitable electrolyte. Precipitates formed by colloid agglomeration are
amorphous and porous with very high surface areas. In almost all cases, precipitates
are improved by heating them in contact with the solution before filtration. This is
known as digestion and will promote the formation of larger particles with a reduced
surface area, and more ordered arrangements within crystals. Thus both the surface
adsorption and occlusion of impurities will be reduced.
Purity of Precipitates
Steps are normally taken to prevent the simultaneous precipitation of materials other
than the desired analyte species. Incorporation of impurities into the precipitate may
however occur by coprecipitation or post-precipitation. The former arises during the
formation of the precipitate, and the latter after it has been formed. The various modes
of coprecipitation are summarized in Table 5.16.
Post-precipitation involves the deposition of a sparingly soluble impurity of similar
properties to the precipitate on the surface of that precipitate after it has been formed.
It is particularly a problem where similar materials are being separated on the basis of
their different rates of precipitation, e.g. calcium and magnesium oxalates or zinc and
mercury sulphides. Copreci
Table 5.16 The major types of coprecipitation and their relation to the precipitate type
Type of coprecipitation Mode of contamination Precipitate type most affected
isomorphic inclusion substitution of the precipitate lattice with impurity ions
of similar crystallinity crystalline non-isomorphic inclusion solid solution of the
impurity within the precipitate crystalline occlusion physical trapping of impurities
within precipitate particles crystalline and colloidal surface adsorption chemisorption
of impurities from the solution onto the precipitate surface colloidal precipitation can
be reduced by the normal practical procedures of precipitation from hot dilute analyte
solutions, followed by digestion of the precipitate. Inclusion phenomena when they
occur are very difficult to overcome and it may be necessary to remove the impurity
ion before precipitation. At the same time the degree of post-precipitation will be
increased by digestion and where this form of contamination is paramount rapid
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Gravimetric summary
Dr Harold Braustein The Jerusalem Engineering College
Analytical Chemistry
filtration is essential. The extent to which coprecipitation may affect an analysis is
well illustrated by the familiar precipitation of barium sulphate.
Substances that may appear as impurities in this precipitate include sulphuric acid,
alkali metal sulphates, ammonium sulphate, iron(III) sulphate, barium phosphate,
barium carbonate, barium chloride, barium nitrate and barium chlorate. The excellent
precision and accuracy often claimed for analyses based on this precipitate
undoubtedly result from a compensation of errors.
Practical Gravimetric Procedures
A typical gravimetric analysis procedure may be divided into five stages:
sample pretreatment;
precipitation;
filtration;
drying and ignition;
weighing.
The operations involved in the various stages are summarized by Table 5.17. Two
aspects, however, need slight amplification. Firstly, the possible
generation of the precipitating agent within the solution in a homogeneous
precipitation procedure should be considered. This method, in which the precipitation
is generated within the solution, has the advantage of preventing local high
concentrations of reagent and thus promoting particle growth as well as minimizing
the occlusion of impurities. Some homogeneous precipitation reactions are shown in
Table 5.18. Secondly, the filtration method must be selected to fit the treatment of the
precipitate. Where the material is merely to be dried and weighed, a sintered glass
crucible is generally the most satisfactory. If an ignition step is to be used, however,
the precipitate may be collected on a filter paper and transferred to a silica or platinum
crucible for ignition or filtered on an asbestos pad in a Gooch crucible. Sintered silica
crucibles are also used for these precipitates.
Table 5.17 Practical gravimetric procedures
Stage Practical manipulations Remarks
sample pre-treatment dissolution of sample, separation or masking of
interfering ions prevents simultaneous precipitation and reduces inclusion of
impurities precipitation from hot dilute solution, careful addition or
homogeneous generation of precipitating agent in small excess, with stirring:
digestion promotes particle growth and reduces occlusion. N.B. digestion can increase
post-precipitation filtration cooled solution filtered, precipitate washed with dilute
electrolyte solution decreases solubility, reduces adsorbed impurities and prevents
peptization
drying and ignition careful drying at 110–140°C access of air during
ignition prevents sputtering losses, ensures complete oxidation to a stoichiometric
product
weighing weighings carried out to nearest 0.1 mg reheating and reweighing until
constant weight is obtained; samples stored in a desiccator special care required for
hygroscopic solids
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Gravimetric summary
Dr Harold Braustein The Jerusalem Engineering College
Analytical Chemistry
Magnesium may be precipitated from solution as MgNH4PO4 . 6H2O, a compound
which has been widely used as a basis for the gravimetric determination of the
element over many years. An examination of its use will serve to illustrate some of
the problems associated with inorganic precipitations. The initial precipitation is made
from a solution at pH = 11–12 by the addition of ammonium phosphate in excess.
Some of the problems encountered in the analysis are:
(a)—
Precipitate Stoichiometry
The precipitate is not stoichiometric and it is necessary to convert it to the
pyrophosphate by ignition (1100°C) to obtain the most precise results.
(b)—
Solubility
For a precipitate used in gravimetric analysis magnesium ammonium phosphate has a
rather high solubility (0.1 g dm–3 in water at 20°C). Hence solution volumes should
be kept as low as possible and the precipitate must be filtered from cold solution.
Furthermore, the solution composition must be carefully controlled to ensure the
maintenance of conditions of precipitation
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Gravimetric summary
Dr Harold Braustein The Jerusalem Engineering College
Analytical Chemistry
minimum solubility. The presence of ammonium salts and hydrogen ions increases
the solubility so that the precipitant (NH4)3PO4 must be added in only small excess,
and the pH must be maintained high (pH = 11–12). Supersaturation is an additional
problem, and quantitative precipitation is only achieved after the mixture has been
allowed to stand for an extended time, e.g. overnight.
(c)—
Precipitate Purity
H3PO4, NH4H2PO4, (NH4)2HPO4, (NH3)3PO4, Mg(H2PO4)2, MgHPO4,
Mg3(PO4)2, basic Mg phosphates, Mg(OH)2, MgCl2 and NH4Cl are all possible
contaminants. Of these only NH4Cl which is volatile and MgHPO4 which ignites to
Mg2P2O7 are of no consequence. Amongst the rest are a number of compounds
(including the precipitant itself) which have crystallinity similar to the precipitate, and
are coprecipitated by isomorphic inclusion. The magnitude of the coprecipitation
problem is such that, to overcome it, dissolution and reprecipitation of the initial
precipitate is required. This is done by using the minimum amount of dilute
hydrochloric acid and a very small amount of ammonium phosphate.
Concentrated ammonia solution is then added to complete the reprecipitation.
The use of an organic precipitant may be exemplified by reference to the employment
of 8- hydroxyquinoline (Oxine), to determine aluminium. Aluminium oxinate,
Al(C9H6NO)3, can be quantitatively precipitated fromaqueous solutions between pH
= 4.2 and pH = 9.8. Due to the non-selective nature of Oxine as a reagent
many other metals are also precipitated within the same range. To achieve a
separation of aluminium from these other elements pH control may be employed.
Thus an acetic acid solution, buffered to pH =5 by sodium acetate, will provide a
medium for the separation of aluminium from troublesome contaminants such as Be,
Mg, Ca, Sr and Ba. Ammoniacal solution (pH = 9) will enable a separation
from As, B, F and P to be made, and if used in conjunction with H2O2 from Cr, Mo,
Nb, Ta and V as well. Selectivity is further enhanced by the use of masking agents
such as cyanide, tartrate or EDTA.
By careful control of these parameters aluminium may be separated, and few
interferences are observed if the precipitation is carried out from an ammoniacalcyanide-EDTA solution. When large amounts of Ca or Mg are present the
homogeneous precipitation procedure (Table 5.18) is usefully employed at pH
= 5. The precipitate is readily filtered and may be weighed after drying at 150°C as
the anhydrous compound.
Applications of Gravimetry
Gravimetric methods provide precise and accurate results and have found a wide
utility in chemical analysis for many years. They are best suited to the determination
of major constituents in samples because of the practical limitations in accurately
weighing quantities of less than 0.1 g. The analysis of rocks, ores, soils, metallurgical
and other inorganic samples for their major components has depended very much on
gravimetric methods. Gravimetric procedures are, however, usually time consuming
and demanding, with the result that there is a steady trend towards the use of quicker,
instrumentally based methods. Notable for its impact in recent years on these
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Gravimetric summary
Dr Harold Braustein The Jerusalem Engineering College
Analytical Chemistry
traditional areas has been X-ray fluorescence analysis . Nevertheless gravimetric
methods are still very much needed to calibrate these newer procedures. Table 5.14
and 5.15 give a good indication of the scope of gravimetry.
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