The basic method of gravimetric analysis

gravimetric analysis.
[size=12]The basic method of gravimetric analysis is fairly straightforward. A weighed sample is dissolved after which an excess of a precipitating agent is added. The precipitate which forms is filtered, dried or ignited and weighed. 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 Ag⁺ is precipitated out by addition of Cl-
Ag⁺ + Cl^- =<-> AgCl(s)
the (low) solubility of AgCl is reduced still further by the excess of Cl^- which is added, pushing the equilibrium to the right.
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.

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 the almost instantaneous to several minutes (for dilute solutions of barium sulphate). 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 these per mole of precipitating product. As these nuclei form so ions from solution (which at this point are in excess) congregate around them. For example if silver nitrate were added very slowly to a solution of sodium chloride, silver chloride nuclei would form and chloride ions (which would be in excess relative to Ag+ ions) would congregate around them.
In addition to the primary adsorbed chloride ion, there are some sodium ions aggregating further from the AgCl nucleus. These are counter ions and tend to aggregate around the [AgCl:Cl]- centre because these centres have a net negative charge (excess Cl-) and additional positive 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.
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) and is encouraged by the analyst.
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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Practical Aspects

You will be shown in the laboratory sessions the important practical aspects of gravimetric analysis (e.g. handling of sinters, transfer of solid etc.). One aspect that should be emphasised is weighing. All weighings must be done at room temperature, you must not weigh your sinters straight from the oven - they must first be cooled in a desiccator. You must also weigh to constant mass. This means drying your product, cooling to room temperature, weighing and returning the product to the oven. The process of heating, cooling and weighing should be repeated until a constant mass is obtained. Generally, you can reduce the drying times by about a third each time, so if you dried the product for two hours the first time, you can cut this to one and a half hours the second time and so on. You must report all weighings.

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
Where Q is the molar concentration of the mixed reagents before any precipitation occurs and S is the molar solubility of the product (precipitate) when the system has reached equilibrium. 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.
The following methods are used to approach these criteria;
Precipitation from dilute solution. 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.
Precipitation from hot solution. The solubility S of precipitates increases with temperature and so an increase in S decreases the supersaturation.
Digestion of the precipitate. See earlier. (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).

Precipitation from homogenous solution

In this method, the precipitating ion is not added to the solution but is generated throughout the solution by a homogenous chemical reaction. This is the ultimate in precipitation technique. (!) Since the precipitating agent is generated evenly throughout the solution, local excess of the precipitating agent are avoided. In precipitation from homogenous solution the supersaturation (Q-S) is kept extremely low at all times resulting in a very pure, dense precipitate. Often substances which form only as amorphous (if you don’t know what that means - look it up !) solids will precipitate as well formed crystalline solids using this technique. The major techniques of homogenous precipitation may be classified as follows;

1. Increase in pH.
Usually the pH is made more alkaline by hydrolysis of urea (NH₂CONH₂) in boiling aqueous solution. The ammonia slowly liberated raises the pH of the solution homogeneously, causing metal ions that form insoluble hydroxides or hydrous oxides to precipitate. In the precipitation of aluminium from homogenous solution, urea is added to an acidic solution of aluminium containing some sulphuric or succinic acid. No precipitation occurs until the solution has been boiled long enough for the ammonia to raise the pH to the necessary value (about one hour or so). In this procedure aluminium precipitates as the basic sulphate or the basic succinate, not as aluminium hydroxide. The precipitate obtained in the way is much denser and freer from impurities than are aluminium precipitates formed by the conventional addition of ammonia to aluminium solutions.

Another example is the precipitation from homogeneous solution of barium chromate. Chromate is added to barium in solution which is acidic enough to prevent precipitation. Urea is added and the solution boiled. The ammonia released raises the solution pH and barium chromate slowly precipitates out.
2. Anion release
3. Cation release
4. Precipitation from mixed solvents
5. Valency change
These last four, although useful, are less common and so we shall not consider other than to note their existence. (Further information available in the set book).
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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. Coprecipiation occurs to some degree in every gravimetric analysis (especially barium sulphate and those involving hydrous oxides). You cannot avoid it - all you can do is minimise 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.

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.

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.

[b]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 sulphamic acid added. The mixture was then heated and held at boiling for 45 minutes. The precipitate (barium sulphate) was filtered off, washed and dried. Mass of precipitate obtained = 0.2533 g. Determine the identity of X.

Answer:

The precipitate is barium sulphate. The first stage is to determine the number of moles of barium sulphate produced, this will, in turn give us the number of moles of barium in the original sample.
Relative Molecular Mass of barium sulphate = 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 moldm^-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 %)
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The basic method of gravimetric analysis 163870

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