Introduction:

Precipitation Gravimetry is a kind or form of gravimetric processes of analysis. Gravimetric analysis is a quantitative analysis that comprises estimation of the weight of an element or definite compound of the element. The procedure entails isolating and weighing an element or a definite compound of the element in as pure a form as possible. The element or compound is separated from the weighed portion of the substance being observed. The weight of the element or radical might then be readily computed from knowledge of the formula of the compound and the atomic weights of the constituent elements. However gravimetric analysis is a process of quantitative analysis, a separation is comprised and the methods employed are at times employed for preliminary separations. As weight can be evaluated by greater accuracy than nearly any other basic property, gravimetric analysis is potentially one of the most precise and accurate analytical methods available. There are four basic kinds of gravimetric analysis: particulate Gravimetry, volatilization Gravimetry, precipitation Gravimetry and electrodeposition Gravimetry.

Principle of Precipitation Gravimetry:

In precipitation Gravimetry, an insoluble compound forms whenever we add a precipitating reagent, or precipitant, to a solution having our analyte. In most of the methods the precipitate is the product of a simple metathesis reaction (that is, exchange reaction or double replacement reaction) among the analyte and the precipitant; though, any reaction producing a precipitate can potentially serve up as a gravimetric process. The precipitating reagent or precipitant is the reagent added, which reacts by the analyte in solution to make the precipitate, whereas the precipitate is the insoluble compound prepared. Precipitation Gravimetry is employed for the separation of elements from samples and for the determination of the weight of elements in a specified sample.

The entire precipitation gravimetric analysis share two significant attributes. Primary, the precipitate should be of low solubility, of high purity and of known composition if its mass is to precisely reflect the analyte's mass. Secondly, the precipitate should be simple to separate from the reaction mixture.

Solubility Considerations:

To give precise results, a precipitate's solubility should be minimal. The precision of a total analysis method generally is better than ±0.1% that signifies that the precipitate should account for at least 99.9 percent of the analyte. Extending this need to 99.99% makes sure that the precipitate's solubility doesn't limit the precision of a gravimetric analysis.

We can decrease solubility losses via carefully controlling the conditions beneath which the precipitate forms. This, in turn, needs that we account for each and every equilibrium reaction influencing the precipitate's solubility.

The other significant parameter which might influence a precipitate's solubility is the pH of the solution in which the precipitate forms. For illustration, hydroxide precipitates like Fe(OH)₃, are more soluble at lower pH levels at which the concentration of OH^- is small. It is significant thus, to adjust the pH of a solution to keep up low solubility of the precipitate.

Solubility can frequently be reduced by employing a non-aqueous solvent. The precipitate's solubility is usually more in aqueous solutions as the capability of water molecules to stabilize ions via Solvation. The poorer solvating capability of non-aqueous solvents, even those which are polar, leads to a smaller solubility product. For illustration, PbSO₄ consists of a KSP of 1.6 x 10^-8 in water, while in a 50:50 mixture of H₂O /Ethanol the k_sp is four orders of magnitude smaller.

How to obtain precipitate of high purity:

Moreover to having a low solubility, the precipitate should be free from impurities. As precipitation generally takes place in a solution which is rich in dissolved solids, the initial precipitate is generally impure. We should take out such impurities before finding out the precipitate's mass. 

The maximum source of impurities is the outcome of chemical and physical interactions occurring at the surface of precipitate. A precipitate is usually crystalline - even if only on a microscopic scale by a well-defined lattice of cations and anions. Such cations and anions at the precipitate's surface carry, correspondingly, a positive or a negative charge as they encompass incomplete coordination spheres. In a precipitate of AgCl, for illustration, each and every silver ion in the precipitate's interior is bound to six chloride ions. The silver ion at the surface, though, is bound to no more than five chloride ions and carries a partial positive charge (figure shown below). The presence of such partial charges forms the precipitate's surface an active site for the physical and chemical interactions that generate impurities.

Fig: Lattice structure of AgCl-Ball and stick diagram

Ball-and-stick diagram exhibiting the lattice structure of AgCl. Each and every silver ion in the lattice's interior binds by six chloride ions, and each and every chloride ion in the interior binds by six silver ions. Such ions on the lattice's surface or edges bind to fewer than six ions and carry a partial charge. A silver ion on the surface, for illustration, carries a partial positive charge. Such charges make the surface of a precipitate an active site for the physical and chemical interactions.

One general impurity is an inclusion. A potential interfering ion whose size and charge is identical to a lattice ion, might replace to the lattice structure, given that the interferent precipitates by the similar crystal structure. The probability of making an inclusion is greatest whenever the concentration of the interfering ion is substantially more than the lattice ion's concentration. The inclusion doesn't reduce the quantity of analyte that precipitates, given that the precipitant is present in adequate excess. Therefore, the precipitate's mass is for all time bigger than expected.

Fig: Examples of impurities during precipitation

Figure: Three illustrations of impurities that might form throughout precipitation. The cubic frame symbolizes the precipitate and the blue marks are impurities: (a) inclusions (b) occlusions and (c) surface adsorbates. Inclusions are arbitrarily distributed all through the precipitate. Occlusions are localized in the interior of the precipitate and surface adsorbates are localized on the exterior of the precipitates. For simplicity of viewing, in (c) adsorption is illustrated on simply one surface.

The inclusion is hard to take out as it is chemically part of the precipitate's lattice. The only manner to take out an inclusion is via reprecipitation. After isolating the precipitate from its supernatant solution (that is, the solution remaining after the precipitate is made), we dissolve it via heating in a small part of an appropriate solvent. We then let the solution to cool, reforming the precipitate. Since the interferent's concentration is less than that in the original solution, the amount of comprised material is smaller. We can repeat the procedure of reprecipitation till the inclusion's mass is unimportant. The loss of analyte throughout reprecipitation, though, can be an important source of error. 

Occlusions form whenever interfering ions become trapped in the growing precipitate. Dissimilar to inclusions that are arbitrarily dispersed in the precipitate, an occlusion is localized, either all along flaws within the precipitate's lattice structure or within aggregates of individual precipitate particles (Figure (b) above). An occlusion generally raises a precipitate's mass; though, the mass is smaller if the occlusion comprises the analyte in a lower molecular weight form as compare to that of the precipitate.

We can reduce occlusions via maintaining the precipitate in equilibrium with its supernatant solution for the extended time. This procedure is termed as a digestion. Throughout digestion, the dynamic nature of the solubility-precipitation equilibrium, in which the precipitate dissolves and reforms, makes sure that the occlusion is re-exposed to the supernatant solution. As the rates of dissolution and reprecipitation are slow, there are fewer prospects for making new occlusions.

After the precipitation is complete, the surface carries on attracting ions from solution. These surface adsorbates include a third kind of impurity. We can reduce surface adsorption via reducing the precipitate's available surface area. One advantage of digesting a precipitate is that it raises the average particle size. Since the probability of a particle fully dissolving is inversely proportional to its size, throughout digestion larger particles increase in size at the expense of the smaller particles. One effect of making a smaller number of larger particles is the total decrease in the precipitate's surface area. We as well can take out surface adsorbates via washing the precipitate; though the potential loss of analyte can't be ignored.

Inclusions, occlusions and surface adsorbates are the illustrations of coprecipitates-or else soluble species that form in the precipitate having the analyte. The other kind of impurity is an interferent which forms an independent precipitate in the conditions of the analysis. For illustration, the precipitation of nickel dimethylglyoxime needs a slightly basic pH. Under such conditions, any Fe³⁺ in the sample precipitates as Fe(OH)₃. Moreover, as most of the precipitants are hardly ever selective toward a single analyte, there is for all time a risk which the precipitant will react by both the analyte and an interferent.

We can reduce the formation of additional precipitates via cautiously controlling solution conditions. Whenever an interferent forms a precipitate which is less soluble than the analyte's precipitate, we can precipitate the interferent and eradicate it via filtration, leaving the analyte behind in solution. On the other hand, we can mask the analyte or the interferent to prevent its precipitation.

Illustration: Both of the above-illustrated approaches are described in Fresenius' analytical procedure for finding out Ni in ores having Pb²⁺, Cu²⁺ and Fe³⁺. Dissolving the ore in the presence of H₂SO₄ selectively precipitates Pb²⁺ as PbSO₄. Treating the supernatant by H₂S precipitates the Cu²⁺ as CuS. After eliminating the CuS via filtration, adding ammonia precipitates Fe³⁺ as Fe(OH)₃. Nickel that forms a soluble amine complex, remains in the solution.  

Controlling Particle Size:

Size matters whenever it comes to making a precipitate. Larger particles are simpler to filter, and as observed earlier, a smaller surface area signifies that there is less opportunity for surface adsorbates to form.  By cautiously controlling the reaction conditions we can considerably raise a precipitate's average particle size. 

Precipitation comprises of two different events: nucleation, the primary formation of smaller stable particles of precipitate and particle growth. Larger particles form whenever the rate of particle growth surpasses the rate of nucleation. Understanding the conditions preferring particle growth is significant whenever designing a gravimetric process of analysis. 

Von Wiermarn invented that the particle size of precipitates is inversely proportional to the relative super saturation of the solution throughout the precipitation procedure.

We define a solute's relative super saturation, RSS, as:

RSS = Q - S/S

Here, Q is the concentration of the mixed reagents prior to precipitation takes place and is the degree of super saturation, and 'S' is the solubility of the precipitate at equilibrium. A solution having a large, positive value of RSS consists of a high rate of nucleation, generating a precipitate having many small particles and high surface area. Whenever the RSS is small, precipitation is more likely to take place by particle growth than via nucleation, generating a precipitate having few larger crystals and low surface area.

High relative super-saturation ----- most of the small crystals

                                                     (High surface area)

Low relative super-saturation ------ some larger crystals

                                                     (Low surface area)

By observing the equation above, it shows that we can minimize RSS by reducing the solute's concentration, 'Q' or by increasing the precipitate's solubility, 'S'. Some steps are generally taken to keep 'Q' low and increase 'S'.

1) Precipitation from the dilute solution. This keeps 'Q' low.

2) Add dilute precipitating reagents gradually, with effective stirring. This as well keeps 'Q' low. Local surplus of the reagent are prevented via stirring.

3) Precipitation from hot solution. This raises S. The solubility must not be too great or the precipitation will not be quantitative. The mass of the precipitation might be performed in the hot solution and then the solution might be cooled to form the precipitate ion quantitative.

4) Precipitate at as low a pH as probable to still maintain quantitative precipitation. Most of the precipitates are more soluble in acid medium, and this slows the rate of precipitation. They are more soluble as the anion of the precipitate joins with protons in the solution.

Most of such operations as well frequently reduce the degree of contamination. The concentration of impurities is kept lower and their solubility is raised, and the slower rate of precipitation reduces their chance of being trapped. The larger crystals encompass a smaller particular surface area and therefore less chance of adsorption of impurities.

There are practical restrictions to minimizing RSS. Some of the precipitates, like Fe(OH)₃ and PbS, are thus insoluble that 'S' is extremely small and a large RSS is unavoidable. These solutes inevitably form small particles. Furthermore, conditions favoring a small RSS might lead to a relatively stable supersaturated solution which needs a long time to completely precipitate.

A visible precipitate takes longer to form whenever RSS is small both as there is a slow rate of nucleation and as there is a steady reduction in RSS as the precipitate forms. One solution to the latter problem is to produce the precipitant in situ as the product of a slow chemical reaction. This keeps the RSS at an efficiently constant level. Since the precipitate forms under conditions of low RSS, initial nucleation generates a small number of particles. As additional precipitant forms, particle growth supersedes nucleation, resultant in larger precipitate particles. This method is termed as homogeneous precipitation.

Steps involved in Gravimetric Analysis:

1) Preparation of the solution

2) Precipitation

3) Digestion

4) Filtration

5) Washing

6) Drying or Ignition

7) Weighing

8) Calculation

Preparation of the Solution: However some form of preliminary separation might be essential to remove interfering materials, in other examples the precipitation step in gravimetric analysis is adequately choosy that other separations are not needed. The substance to be removed should be in solution form. For this, precisely weigh an appropriate quantity of substance and dissolve it in distilled water or appropriate solvent, heat the solution if essential. The solution conditions should be adjusted to keep low solubility of the precipitate and to get it in a form appropriate for filtration. Proper adjustment of the solution conditions before precipitation might as well mask potential interferences. Factors which should be considered comprise the volume of the solution throughout precipitation, the concentration range of the test substance, the presence and concentration of other constituents, the temperature and pH. 

Precipitation: This step comprises reaction having a precipitant to provide precipitate. Whenever the precipitation is performed, a slight excess of precipitating reagent is added to reduce the solubility via mass action (common ion effect and to guarantee complete precipitation). Whenever the approximate amount of analyte is known, a 10 % surplus of the reagent is usually added. Completeness of precipitation is checked via waiting till the precipitate has settled and then adding some drops of precipitating reagent to the clear solution above it. If no new precipitate forms, then the precipitation is complete.

Digestion: If a precipitate is permitted to stand in the presence of the mother liquor (that is, the solution from which it was precipitated), the larger crystals grow at the expenditure of the small ones. This is termed as digestion or Ostwald ripening. The small particles are likely to dissolve and re-precipitate on the surfaces of the bigger crystals. Moreover, the individual particles agglomerate. This yields in an appreciable decrease in surface area. As well, imperfections of the crystals tend to disappear and adsorbed or trapped impurities tend to go to solution. Digestion is generally done at elevated temperatures to speed the procedure, however in several cases; it is done at room temperature. It enhances the filterability of the precipitate and its purity. 

Filtration: Subsequent to precipitating and digesting the precipitate, we separate it from solution via filtering. The most general filtration process employs filter paper that is categorized according to its speed, its size and its ash content on ignition. Speed or how fast the supernatant passes via the filter paper, is a function of the paper's pore size. A bigger pore lets the supernatant to pass faster via the filter paper, however doesn't retain small particles of precipitate. Filter paper is rated as fast (that is, retains particles bigger than 20 to 25 μm), medium-fast (that is, retains particles bigger than 16 μm), medium (that is, retains particles bigger than 8 μm), and slow (that is, retains particles bigger than 2 to 3 μm). The proper selection of filtering speed is significant. If the filtering speed is too fast, we might fail to retain some of the precipitate, causing a negative determinate error. On contrary, the precipitate might clog the pores if we make use of a filter paper which is too slow.

As filter paper is hygroscopic, it is not simple to dry it to a constant weight. Whenever accuracy is significant,  the  filter  paper  is  eliminated  before  finding out  the  precipitate's  mass.  After transferring the precipitate and filter paper to a covered crucible, we heat the crucible to a temperature which converts the paper to CO₂ (g) and H₂O (g), a procedure known as ignition.

Gravity filtering is achieved via folding the filter paper to a cone and putting it in a long-stem funnel (figure shown below). A seal between the filter cone and the funnel is made by dampening the paper by water or supernatant, and pressing the paper to the wall of the funnel. Whenever correctly prepared, the funnel's stem fills by the supernatant, raising the rate of filtration.

Fig: Preparing a filter paper cone

The filter paper circle in (a) is folded in half (b), and folded in half again (c). The folded filter paper is parted (d) and a small corner is torn off (e). The filter paper is opened up to a cone and put in the funnel (f).

The precipitate is transferred to the filter in some steps. The primary step is to decant the majority of the supernatant via the filter paper devoid of transferring the precipitate. This prevents the filter paper from clogging at the starting of the filtration procedure. The precipitate is rinsed as it remains in its beaker, by the rinsing decanted via the filter paper. Ultimately, the precipitate is transferred to the filter paper by using a stream of rinse solution. Any precipitate clinging to the walls of the beaker is transferred by employing a rubber policeman (that is, a flexible rubber spatula joined to the end of a glass stirring rod).

Fig: Procedure for transferring supernatant to the filter paper cone

The alternative process for filtering a precipitate is the filtering crucible. The most general is a fritted-glass crucible having a porous glass disk filter. Fritted-glass crucibles are categorized via their porosity: coarse (retaining particles bigger than 40 to 60 μm), medium (retaining particles bigger than 10 to 15 μm), and fine (retaining particles bigger than 4 to 5.5 μm). The other kind of filtering crucible is the Gooch crucible that is a porcelain crucible having a perforated bottom. A glass fiber mat is put in the crucible to keep the precipitate. For both kinds of crucibles, the precipitate is transferred in the similar way illustrated earlier for filter paper. Rather than employing gravity, the supernatant is drawn via the crucible having the assistance of suction from a vacuum aspirator or pump (figure shown below).

Fig: Procedure for filtering a precipitate through a filtering crucible

The trap prevents water from the aspirator from back-washing to the suction flask.

Washing: Co-precipitated impurities, particularly those on the surface, can be eliminated by washing the precipitates after filtering. The precipitate will be wet by the mother liquor that is as well eliminated by washing. Most of the precipitates can't be washed by pure water, as peptization takes place (that is, procedure of passing of a precipitate to colloidal particles on adding appropriate electrolyte). Prevention comprises in adding an electrolyte to the wash liquid. The electrolyte should be one which is volatile at the temperature to be employed for drying or ignition, and it should not dissolve the precipitate.

Drying or igniting the precipitate:

After extracting the precipitate from its supernatant solution, the precipitate is dried to take away residual traces of rinse solution and any volatile impurities. The temperature and process of drying based on the process of filtration and the precipitate's desired chemical form. Putting the precipitate in a laboratory oven and heating to a temperature of 110^oC is adequate whenever removing water and other simply volatilized impurities. Higher temperatures need a muffle furnace, a Bunsen burner, or a Meker burner and are required if we require to thermally decompose the precipitate prior to weighing.

As filter paper absorbs moisture, we should get rid of it before weighing the precipitate. This is achieved by folding the filter paper over the precipitate and transferring both the filter paper and the precipitate to a porcelain or platinum crucible. Gentle heating primarily dries and then chars the filter paper. Once the paper starts to char, we slowly raise the temperature till all traces of the filter paper are gone and any remaining carbon is oxidized to CO₂.

Fritted-glass crucibles can't withstand high temperatures and should be dried in an oven at temperatures beneath 200^oC. The glass fiber mats employed in Gooch crucibles can be heated to a maximum temperature of around 500^oC. To make sure that drying is complete, the precipitate is repeatedly dried and weighed till a constant weight is achieved.

Weighing and Calculation:

The residue after drying and ignition is weighed and the weight of the precipitate is achieved by employing the formula:

Weight of the precipitate = Weight of crucible all along with the precipitate - Weight of empty crucible.

From the weight of the precipitate one can find out the percentage of analyte present in the sample. Computations are generally made up on a percentage basis.

The general formula for computing the percentage of the substance sought (analyte) is:

% Sought (analyte) = [{Weight of precipitate (g) x gravimetric factor}/Weight of sample (g)] x 100%

Gravimetric factor = f. w of substance sought or analyte/f.w of substance weighed or sample weighed

This is for all time significant to write a balanced equation of the reaction between the sample and the precipitant.

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