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Gravimetric methods of analysis

GravimetryGravimetric methods are quantitative methods that are based ondetermining the mass of a pure compound to which the analyte ischemically related.

Classifications of Gravimetric methods

1-Precipitation gravimetry, the analyte is separated from a solution ofthe sample as a precipitate and is converted to a compound of knowncomposition that can be weighed.

2-Volatilization gravimetry, the analyte is separated from otherconstituents of a sample by conversion to a gas of known chemicalcomposition. The weight of this gas then serves as a measure of theanalyte concentration.

3-Electrogravimetry, the analyte is separated by deposition on anelectrode by an electrical current. The mass of this product thenprovides a measure of the analyte concentration.

Features or properties of Gravimetric Analysis

· Traditional Method.· Cheap, easily available apparatus, simple to carry out.· Slow, especially when accurate results are required.· Wide range of sample concentrations (ng - kg).· No calibration required (except for the balance).· Accurate.

Precipitation Gravimetry

In precipitation gravimetry, the analyte is converted to a sparinglysoluble precipitate. This precipitate is then filtered, washed free ofimpurities, converted to a product of known composition by suitable heattreatment, and weighed. For example, a precipitation method fordetermining calcium in natural waters. The reactions are:

2NH3 + H2C2O4 2NH4++C2O4

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Ca+2 (aq) + C2O4-2 (aq) CaC2O4 (s)

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The precipitate is filtered using a weighed filtering crucible, then driedand ignited. The process converts the precipitate entirely to calciumoxide. The reaction is:

CaC2O4 (s) Δ CaO (s) + CO (g) + CO2

After cooling, the crucible and precipitate are weighed, and the mass ofcalcium oxide is determined by subtracting the known mass of thecrucible. The calcium content of the sample is then computed.

Procedure for gravimetric analysis (Precipitation gravimetry)

What steps are needed?

The steps required in gravimetric analysis, after the sample has beendissolved, can be summarized as follows:

1. Preparation of the solution2. Precipitation3. Digestion4. Filtration5. Washing6. Drying or igniting7. Weighing8. Calculation

Properties of precipitates and precipitating reagents

Properties precipitating reagents

Ideally, a gravimetric precipitating agent should reactspecifically or at least selectively with the analyte. Specific reagents,which are rare, react only with a single chemical species. Selectivereagents, which are more common, react with a limited number ofspecies.

precipitating agent

sample

dissolvedcomponents

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Properties of good precipitates

1. Easily filtered and washed free of contaminants.

2. Of sufficiently low solubility that no significant loss of the

analyte occurs during filtration and washing.

3. Unreactive with constituents of the atmosphere

4. Of known chemical composition after it is dried or, if

necessary, ignited.

Particle size and filterability of precipitates

Why we prefer precipitates of large particles.

Precipitates consisting of large particles are generallydesirable for gravimetric work because these particles are easy to filterand wash free of impurities. In addition, precipitates of this type areusually purer than are precipitates made up of fine particles.

Factors that determine the particle size of precipitates

The particle size of solids formed by precipitation varies enormously.

1-Colloidal suspensions,

· whose tiny particles are invisible to the naked eye (10-7 _ 10-4 cm indiameter).

· Colloidal particles show no tendency to settle from solution

· not easily filtered.

2-Crystalline suspension

· particles with dimensions on the order of tenths of a millimeter or greater.

· The temporary dispersion of such particles of a tend to settle spontaneously,

· easily filtered.

Scientists have studied precipitate formation for many years,but the mechanist of the process is still not fully understood. It is certain,however, that the particle size of a precipitate is influenced by suchexperimental variables as precipitate solubility, temperature, reactant

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concentrations, and rate at which reactants are mixed. The net effectof these variables can be accounted for, at least qualitatively, byassuming that the particle size is related to a single property of thesystem called the relative supersaturations, where:

in this equation, Q is the concentration of the solute at any instant and Sis tits equilibrium solubility.

Generally, precipitation reactions are slow, so that even when aprecipitating reagent is added drop by drop to a solution of an analyte,some s supersaturation is likely. Experimental evidence indicated thatthe particle size of a precipitate varies inversely with the average relativesupersaturation during the time when the reagent is being introduced.Thus, when (Q _ S)/S is large, the precipitate tends to be colloidal, when(Q _ S)/S is small, a crystalline solid is more likely.

High relative supersaturation many small crystals

(high surface area)

Low relative supersaturation fewer, larger crystals

(low surface area)

Obviously, then, we want to keep Q low and S high during precipitation.Several steps are commonly taken to maintain favorable conditions forprecipitation or the experimental control of particle size

1. Precipitate from dilute solution. This keeps Q low.2. Add dilute precipitating reagents slowly, with effective stirring, this

also keeps Q low. Stirring prevents local excesses of the reagent.3. Precipitate from hot solution. This increase S. the solubility should

not be too great or the precipitation will not be quantitative (withless than 1 part per thousand remaining). The bulk of theprecipitation may be performed in the hot solution, and then thesolution may be cooled to make the precipitation quantitative.

4. Precipitate at as low a pH as is possible to maintain quantitativeprecipitation. As we have seen, many precipitates are moresoluble in acid medium, and this slows the rate of precipitation.They are more soluble because the anion of the precipitatecombines with protons in the solution.

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Colloidal Precipitates

Individual colloidal particles are so small that they are notretained by ordinary filters. Moreover, Brownian motion prevents theirsettling out of solution under the influence of gravity. Fortunately,however, we can coagulate, or agglomerate, the individual particles ofmost colloids to give a filterable, amorphous mass that will settle out ofsolution.

Structure of Colloids

Colloidal suspensions are stable because all of the particlesof the colloid are either positively or negatively charged. Colloidalparticles are very small and have a very large surface-to-mass ratio,which promotes surface adsorption. (The process by which ions areretained on the surface of a solid is known as adsorption).

As a precipitate forms, the ions are arranged in a fixed pattern. InAgCl, for example, there will be alternating Ag+ and Cl- ions on thesurface. While there are localized (+) and (-) charges on the surface, thenet surface charge is zero.

However, the surface does tend to adsorb the ion of the precipitateparticle that is in excess in the solution, for example, Cl- if precipitatingCl- with Ag+; this imparts a charge. (With crystalline precipitates, thedegree of such adsorption will generally be small in comparison withparticles that tend to form colloids.) The adsorption creates a primarylayer that is strongly adsorbed and is an integral part of the crystal. It willattract ions of the opposite charge in a counter layer(counter-ionlayer) or secondary layer so the particle will have an overall neutralcharge. There will be solvent molecules interspersed between thelayers. Normally, the counter layer completely neutralizes the primarylayer and is close to it, so the particles will collect together to form larger-sized particles; that is, they will coagulate. However, if the secondarylayer is loosely bound, the primary surface charge will tend to repel likeparticles, maintaining a colloidal state.

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A colloidal silver chloride particle suspended in a solution of silver nitrate.

Coagulation of Colloids

Coagulation of a colloidal suspension can often be brought

1.by short period of heating to decreases the number of adsorbed ionsand thus the thickness, of the double layer.

2.Increase the electrolyte concentration of the solution.

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If we add ionic compound to a colloidal suspension. The concentration ofcounter – ions increases in the vicinity of each particle. The net effect ofadding an electrolyte is thus a shrinkage of the counter-ion layer.

Peptization of Colloids

Peptization is the process by which a coagulated colloid reverts toits original dispersed state. Peptization is the reverse of coagulation (theprecipitate reverts to a colloidal state and is lost). It is avoided by washingwith an electrolyte that can be volatized by heating.

Practical Treatment of Colloidal Precipitates

Colloids are best precipitated from hot. stirred solutions containingsufficient electrolyte to ensure coagulation. The filterability of acoagulated colloid frequently improves if it is allowed to stand for an houror more in contact with the hot solution from which it was formed. Thisprocess is known as digestion.

Digestion is a process in which a precipitate is heated for an hour ormore in the solution from which it was formed (the mother liquor).

Crystalline Precipitates

Crystalline precipitates are generally more easily filtered and purifiedthan are coagulated colloids. In addition, the size of individual crystallineparticles, and thus their filterability, can be controlled to a degree.

Mechanism of Precipitate Formation

The effect of relative supersaturation on particle size can beexplained if we assume that precipitates form in two ways, by:nucleation and by particle growth. The particles size of a freshlyformed precipitate is determined by the mechanism of predominates.

After the addition of the precipitating agent to the solution ofthe ion under analysis there is an initial induction period beforenucleation occurs. This induction period may range from a very short

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time period to one which is relatively long, ranging from almostinstantaneous to several minutes.

In nucleation, a few ions, atoms, or molecules (perhaps as few asfour or five) come together to form a stable solid.

Often, these nuclei form on the surface of suspended solidcontaminants, such as dust particles. Further precipitation then involvesa competition between additional nucleation and growth on existingnuclei (particle growth).

· If nucleation predominates, a precipitate containing a large numberof small particles results,

· If growth predominated, a smaller number of larger particles isproduced.

We can summarized the precipitation mechanism

1) Induction period.

2) Nucleation.

3) Particle growth to form larger crystal

4) Adsorption.

5) Electrostatic.

Impurities in Precipitates

Precipitates tend to carry down from the solution other constituentsthat are normally soluble, causing the precipitate to becomecontaminated. This process is called coprecipitation. In other wards,coprecipitation is a phenomenon in which otherwise soluble compoundsare removed from solution during precipitate formation.

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There are four types of coprecipitation:

1.surface adsorption,

2.mixed-crystal formation,

3.occlusion,

4. mechanical entrapment.

A B

C D

A B

C D

Types of coprecipitation:

A: surface adsorption

B: inclusion-isomorphic carrying (Mixed-crystal formation)

C: occlusion

D: mechanical entrapment in colloidal.

1- Surface adsorption

Adsorption is a common source of coprecipitation and is likely tocause significant contamination of precipitates with large specific surfaceareas, that is, coagulated colloids.

Although adsorption does occur in crystalline solids, its effects onpurity are usually imdetectable because of the relatively small specificsurface area of these solids.

The net effect of surface adsorption is therefore the carryingdown of an otherwise soluble compound as a surface contaminant. Inorder to minimizing adsorbed impurities on colloids:

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· Using digestion process to improve the purity.

· Washing a coagulated colloid with a solution containing a volatileelectrolyte. The adsorbed layers can often be removed by washing.

· Reprecipitation is effective way to minimize the effects of adsorp-tion.

2- Mixed-crystal formation

Mixed-crystal formation ,one of the ions in the crystal latticeof a solid is replaced by an ion of another element. For this exchange tooccur, it is necessary that the two ions have the same charge and that theirsizes differ by no more than about 5%. This problem occur with bothcolloidal suspensions and crystalline precipitates. Ex (Pb ion replace someof the barium ion). In order to minimizing this type of coprecipitation:

· the interfering ion may have to be separated before the finalprecipitation step.

· a different precipitating reagent that does not give mixed crystals withthe ions interested may be used.

3- Occlusion

Occlusion is a type of coprecipitation in which a compound istrapped within a pocket formed during rapid crystal growth. , materialthat is not part of the crystal structure is trapped within a crystal. Forexample, water may be trapped in pockets when AgN03 crystals areformed.

Occluded impurities are difficult to remove. Digestion may help somebut is not completely effective. The impurities cannot be removed bywashing. Reprecipitation that go on at the elevated temperature ofdigestion open up the pockets and allow the impurities to escape into thesolution.

4- Mechanical entrapment

Mechanical entrapment occurs when crystals lie closetogether during growth. Here, several crystals grow together and in sodoing trap a portion of the solution in a tiny pocket.

Mechanical entrapment can be minimumize when the rate of precipitateformation is low—that is under conditions of low supersaturation. Inaddition, digestion is often remarkably helpful in reducing this types ofcoprecipitation.

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Precipitation from Homogeneous Solution

Precipitation from homogeneous solution is a technique in which aprecipitating agent is generated in a solution of the analyte by a slowchemical reaction. Local reagent excesses do not occur because theprecipitating agent appears gradually and homogeneously throughoutthe solution and reacts immediately with the analyte. As a result, therelative supersaturation is kept low during the entire precipitation. Ingeneral, homogeneously formed precipitates, both colloidal andcrystalline, are better suited for analysis than a solid formed by directaddition of a precipitating reagent.

Drying and Ignition of Precipitates

After filtration, a gravimetric precipitate is heated until its massbecomes constant. Heating removes the solvent and any volatilespecies carried down with the precipitate. Some precipitates are alsoignited to decompose the solid and form a compound of knowncomposition. This new Compound is often called the weighing form.

Types of Precipitating Agents

1- Inorganic Precipitating Agents

These reagents typically form slightly soluble salts or hydrousoxides with the analyte. As you can see from the many entries for eachreagent, few inorganic reagents are selective.

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2- Reducing Agents

This type of reagents convert an analyte to its elemental form forweighing.

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3- Organic Precipitating Agents

Numerous organic reagents have been developed for thegravimetric determination of inorganic species. Some of these reagentsare significantly more selective in their reactions than the inorganicreagents.

We encounter two types of organic reagents. One forms slightlysoluble non-ionic products called coordination compounds; the otherforms products in which the bonding between the inorganic species andthe reagent is largely ionic.

Organic precipitating agents have the advantages of:

· Some of organic precipitating agents are very selective, and othersare very broad in the number of elements they will precipitate.

· giving precipitates with very low solubility in water.

· give a favorable gravimetric factor.

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Calculation of Results from Gravimetric Data

The results of a gravimetric analysis are generally computed fromtwo experimental measurements: the mass of sample and the mass ofa product of known composition.

The precipitate we weigh is usually in a different form than theanalyte whose weight we wish to report. The principles of converting theweight of one substance to that of another depend on using thestoichiometric mole relationships. We introduced the gravimetric factor(GF), which represents the weight of analyte per unit weight ofprecipitate. It is obtained from the ratio of the formula weight of theanalyte to that of the precipitate, multiplied by the moles of analyte permole of precipitate obtained from each mole of analyte, that is,

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In gravimetric analysis, we are generally interested in the percentcomposition by weight of the analyte in the sample, that is,

We obtain the weight of substance sought from the weight of theprecipitate and the corresponding weight/mole relationship

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Calculations are usually made on a percentage basis:

where gA represents the grams of analyte (the desired test substance)and gsampie represents the grams of sample taken for analysis.

We can write a general formula for calculating the percentagecomposition of the substance sought:

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Applications of Gravimetric Methods

Gravimetric methods have been developed for most inorganicanions and cations, as well as for such neutral species as water, sulfurdioxide, carbon dioxide, and iudine. A variety of organic substances canalso be easily determined gravimetri-cally. Examples include lactose inmilk products, salkylates in drug preparations, phenolphthalein inlaxatives, nicotine in pesticides, cholesterol in cereals, and benz-aldehyde in almond extracts. Indeed, gravimetric methods are amongthe most widely applicable of all analytical procedures