GRAVIMETRIC ANALYSIS

    Table of Contents

        Introduction
Mechanism of Precipitation
Conditions for Analytical Precipitation
Impurities in Precipitates
Washing and Filtering
Drying the solid
Worked Examples and Problems

Introduction

           Gravimetric analysis, which by definition is based upon the measurement of mass, can be generalized into two types; precipitation and volatilization. The quantitative determination of a substance by the precipitation method of gravimetric analysis involves isolation of an ion in solution by a precipitation reaction, filtering, washing the precipitate free of contaminants, conversion of the precipitate to a product of known composition, and finally weighing the precipitate and determining its mass by difference.  From the mass and known composition of the precipitate, the amount of the original ion can be determined.

            For successful determinations the following criteria must be met: The desired substance must be completely precipitated. In most determinations the precipitate is of such low solubility that losses from dissolution are negligible. An additional factor is the "common ion" effect, this further reduces the solubility of the precipitate. When Cl- is precipitated out by addition of Ag+

Ag+ + Cl- AgCl(s)

            The (low) solubility of AgCl is reduced still further by the excess of Ag+ which is added, pushing the equilibrium to the right.  We can further decrease the solubility by decreasing the temperature of the solution by using an ice bath.  The weighed form of the product should be of known composition. The product should be "pure" and easily filtered. It is usually difficult to obtain a product which is "pure", i.e. one which is free from impurities but careful precipitation and sufficient washing helps reduce the level of impurity.
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Gravimetric methods are 
quantitative methods that are
based upon determining the 
mass of a pure compound to
which the analyte is related
Theodore W. Richards (1868 - 
1928) and his graduate students at 
Harvard developed or  refined many
 of  the  techniques  of gravimetric 
analysis of silver and chlorine. 
These techniques were used  to 
determine  the  atomic weights of 
25 of the elements by preparing 
pure  samples  of  the chlorides 
of  the  elements, decomposing 
known weights of the compounds,
and determining the chloride 
content  by gravimetric methods.
 From this work  Richards  became
 the  first American to receive the 
Nobel Prize in Chemistry in 1914.

Mechanism of Precipitation

              After the addition of the precipitating agent to the solution of the ion under analysis there is an initial induction period before nucleation occurs. This induction period may range from a very short time period to one which is relatively long, ranging from almost instantaneous to several minutes. 
             After induction, nucleation occurs, here small aggregates or nuclei of atoms form and it is from these "clumps" of atoms that the crystals which form the filtrate will grow. These nuclei may be composed of just a few atoms each so there may be up to 1010 of the nuclei per mole of precipitating product. As these nuclei form ions from the solution (which at this point are in excess) congregate around them. For example if hydrochloric acid were added very slowly to a solution of silver nitrate, silver chloride nuclei would form and silver ions (which would be in excess relative to Cl- ions) would congregate around them. 
             In addition to the primary adsorbed silver ion, there are some nitrate ions aggregating further from the AgCl nucleus. These are counter ions and tend to aggregate around the [AgCl:Ag]+ center because these centers have a net positive charge (excess Ag+) and additional negative charge is required to maintain electrical neutrality. The counter ions are less tightly held than the primary adsorbed ions and the counter ion layer is somewhat diffuse and contains ions other than those of the counter ions. These layers of charges are known as the electric double layer.

 
 
 
 
Nucleation is a process in which
a minimum number of atoms, ions,
or molecules join together to give
a stable solid.
Adsorption is a process in which a 
substance (gas, liquid, or solid) is 
held on the surface of a solid.  In 
contrast, absorbtion involves the
retention of a substance within the
pores of a solid.
The electric double layer of a colloid
consists of a layer of charge absorbed
on the surface of the particles and a 
layer with a net opposite charge in the
solution surrounding the particles
A colloid is a solid made up of 
particles having diameters less 
than 10-4 cm.
After nucleation growth occurs, large nuclei grow at the expense of smaller nuclei which dissolve. This process helps produce more easily filtered crystals (since it produces larger crystals).

Growth of larger nuclei or crystallites can be encouraged by digestion, a process which involves heating the solid and mother liquor for a certain period of time. During digestion, small particles dissolve and larger ones grow. Digestion of the product is an important practical process and you will find that most if not all gravimetric analysis involve a digestion period.

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Conditions for analytical precipitation

In an ideal world, an analytical precipitate for gravimetric analysis should consist of perfect crystals large enough to be easily washed and filtered. The perfect crystal would be free from impurities and be large enough so that it presented a minimum surface area onto which foreign ions could be adsorbed. The precipitate should also be "insoluble" (i.e. be of such slight solubility that loses from dissolution would be minimal).

Without going into detail, it has been shown (Von Weimarn) that the particle size of precipitates is inversely proportional to the relative supersaturation of the solution during precipitation; 

                   relative supersaturation = (Q-S)/S

For the best possible results, conditions need to be adjusted such that Q will be as low as possible and S will be relatively large.
 

A supersaturated solution is an 
unstable solution that contains 
more solutes than a saturated 
solution.  with time, supersatur-
ation is relieved by precipitation
of the excess solute.  To increase
the particle size of a precipitate, 
minimize the relative supersatur-
ation during the precipitate 
formation.

relative supersaturation = (Q-S)/S

where Q is the concentration at 
any instant and S is the equilibrium 
solubility.
 

The following methods are used to approach these criteria, (we will employ the first);

Precipitation from hot solution. The solubility S of precipitates increases with temperature and so an increase in S decreases the supersaturation.

Precipitation from dilute solution. (We will employ this method next semester for the crystallization of a metal oxalate)  This keeps Q low. Slow addition of precipitating reagent with effective stirring. This also keeps Q low, stirring prevents local high concentrations of the precipitating agent.

Precipitation at a pH near the acidic end of the pH range in which the precipitate is quantitative. Many precipitates are more soluble at the lower (more acidic) pH values and so the rate of precipitation is slower.

Digestion of the precipitate. (Also the digestion period results in some improvement in the internal perfection of the crystal structure [sometimes called ripening], here some internal foreign atoms may be expelled).
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Impurities in Precipitates No discussion of gravimetric analysis would be complete without some discussion of the impurities which may be present in the precipitates.

Coprecipitation

This is anything unwanted which precipitates with the thing you do want. Coprecipitation occurs to some degree in every gravimetric analysis (especially barium sulfate and those involving hydrous oxides). You cannot avoid it - all you can do is minimize it by careful precipitation and thorough washing.

Surface adsorption

Here unwanted material is adsorbed onto the surface of the precipitate. Digestion of a precipitate reduces the amount of surface area and hence the area available for surface adsorption. Washing can also remove surface material.

Occlusion

This is a type of coprecipitation in which impurities are trapped within the growing crystal.

Postprecipitation

Sometimes a precipitate standing in contact with the mother liquor becomes contaminated by the precipitation of an impurity on top of the desired precipitate.
Washing and Filtering

Problems with coprecipitation and 
surface adsorption may be reduced 
by careful washing of the precipitate.
With many precipitates, peptization
occurs during washing. Here part of 
the precipitate reverts to the colloidal 
form e.g.

AgCl(colloidal)  AgCl(s)

This results in the loss of part of the 
precipitate because the colloidal form
may pass through on filtration.  By 
washing with ice cold water this can be minimized

Drying the solid

Generally the solids are dried at 
about 120oC but conditions for 
drying can vary considerably. To 
determine the correct drying regime,
a thermogravimetric balance may be used.
Jons Jacob Berzelius (1779 - 1848), considered the leading chemist of his time, developed much of the apparatus and many of the techniques of 19th century analytical chemistry.  Examples include the use of ashless filter paper in gravimetry, the use of hydrofluoric acid to decompose silicates, and the use of the metric system in weight determinations.  He performed thousands of analyses of pure compounds to determine the atomic weights of most of the elements known then.  Berzelius also developed our present system of symbols for elements and compounds.

Calculations

You may find reference to the gravimetric factor in some texts - this is the ratio of RMM of substance sought to that of substance weighed.

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Worked Examples and Problems

Worked Example

A certain barium halide exists as the hydrated salt BaX2.2H2O, where X is the halogen. The barium content of the salt can be determined by gravimetric methods. A sample of the halide (0.2650 g) was dissolved in water (200 cm3) and excess sulfuric acid added. The mixture was then heated and held at boiling for 45 minutes. The precipitate (barium sulfate) was filtered off, washed and dried. Mass of precipitate obtained = 0.2533 g. Determine the identity of X.

Answer:

The precipitate is barium sulfate. The first stage is to determine the number of moles of barium sulfate produced, this will, in turn give us the number of moles of barium in the original sample.

Relative Molecular Mass of barium sulfate 
= 137.34 (Ba) + 32.06 (S) + (4 x 16.00) (4 x O)
= 233.40

Number of moles 
= mass / RMM
= 0.2533 / 233.40
= 1.09 x 10 -3

This is the number of moles of barium present in the precipitate and, therefore, the number of moles of barium in the original sample. Given the formula of the halide, (i.e. it contains one barium per formula unit), this must also be the number of moles of the halide. From this information we can deduce the relative molecular mass of the original halide salt:

RMM = mass / number of moles
= 0.2650 / 1.09 x 10-3
= 244.18

The relative atomic mass of 2 X will be given by the RMM of the whole salt - that of the remaining components; So RAM of 2 X = 244.18 - 173.37 = 70.81

2 X = 70.81, so X = 35.41.

The RAM of chlorine is 35.45 which is in good agreement with the result obtained and hence the halide salt is hydrated barium chloride and X = Chlorine

Problems

1. A sample (0.203 g) of hydrated magnesium chloride (MgClm.nH2O) was dissolved in water and titrated with silver nitrate solution (0.100 mol dm-3), 20.0 cm3 being required. Another sample of the hydrated chloride lost 53.2 % of its mass when heated in a stream of hydrogen chloride, leaving a residue of anhydrous magnesium chloride. Calculate the values of m and n

(Answer: m = 2, n = 6)

2. When an sample of impure potassium chloride (0.4500g) was dissolved in water and treated with an excess of silver nitrate, 0.8402 g of silver chloride was precipitated. Calculate the percentage KCl in the original sample.

(Answer: 97.12 %)