Introduction:

Gravimetric analysis is the analytical method which is based on the measurement of the weight of acknowledged composition. This is one of the most precise and accurate methods of macro quantitative methods. In gravimetric analysis, the substance of known composition should be associated chemically to the analyte.

This analytical method (that is, Gravimetry) was one of the main analytical methods used in the analysis of ores and industrial materials in the past.

Definition:

Gravimetric analysis is the analytical method that includes measurement of weight of components of recognized sample. This is a quantitative method.

Major Types of Gravimeter analysis:

• Precipitation gravimetric analysis
• Volatilization gravimetric analysis

1) Precipitation Gravimetric Analysis:

This kind of analysis is the most generally used. This briefly includes making the specie to be determined to chemically react by a reagent to yield a product of limited solubility; after filtration and other appropriate chemical treatment, the solid residue of recognized chemical composition is then weighed. One significant theory that requires to be well understood is precipitation equilibria.

Precipitation Equilibria:

If the substance encompasses limited solubility and their solubility is exceeded, the ions of dissolved part exist in equilibrium with the solid (that is, undissolved part). It doesn't signify that they are fully insoluble however instead, some dissolved part exists in equilibrium with the solid materials, that is, they are slightly soluble.

AgCl_(s) ↔ (AgCl)_aq ↔ Ag⁺ + Cl^-

The general rule is that for precipitation to be made the product of (Ag⁺) and [Cl^-] should be more than K_sp.

If the product is merely equivalent to K_sp, all the Ag⁺ and Cl^- would remain in the solution K_sp = [Ag⁺] [Cl^-]

The concentration of any solid like AgCl that is proportional to its density, is constant and is joined in equilibrium constant to provide K_sp.

K_sp for Ag₂CrO₄ is as shown below:

Ag₂CrO₄ ↔ 2Ag⁺ + CrO₄^2-

K_sp = [Ag⁺]² [CrO₄^2-]

NB: K_sp = S²

Steps of a Gravimetric Analysis:

The gravimetric analysis needs two main measurements namely the weight of the sample and the weight of the product of known composition derived from the sample (that is, Analyte). To accomplish this, the given steps are needed.

• Preparation of solution
• Precipitation
• Digestion
• Filtration
• Washing
• Drying
• Weighting
• Calculation

a) Preparation of Solution:

Solution whose condition improves the formation of precipitate is the first step. Different interfering substances which might obstruct this should be eliminated. The commonest manner of  eliminating  interferences is via  introducing  reagent  which  selectively  mask  the interfering  substances  thus,  eliminating  this   from  the  chemical  activity  in  the solution. The conditions of the solution which should be adjusted, so as to encourage the precipitation are pH, temperature, volume of the solution and concentration of other constituents.

b) Precipitation:

The precipitate must be adequately insoluble and must have larger crystals in such a way that they can be filtered.

Introducing precipitating agent for all time assist in making sure the formation of desirable precipitate. The ideal precipitating  agent would respond specifically with the analyte to produce solid which would (a) encompass an adequately low solubility to minimize the loss (b) be readily filtered and washed free of contaminants and (c) be unreactive and of acknowledged composition after drying.

Precipitation takes place via Super-saturation and Nucleation.  This is followed via crystal growth. The bigger the super-saturation, the more fast is the growth of crystal. 

Relative Super-saturation = (Q - S)/S

Here,

Q = concentration

S = Solubility

Higher relative Super-saturation → Numerous small crystals (high surface area)

Low relative Super-saturation → Fewer larger crystals (Low surface area)

Usually, to keep 'Q' low and 'S' high the given conditions should succeed.

• Making of Precipitate from the dilute solution to keep 'Q' low.
• Add dilute precipitating reagent slowly by efficient stirring
• Get Precipitate from the unit solution
• Carrying out precipitation at low pH

c) Digestion of Precipitate:

This is a significant step in the gravimetric analysis. The two kinds of crystals made are small crystals and large crystals. Though, whenever the precipitate is allowed to stand, the bigger crystals grow at the expenditure of small crystals, thus forcing the small crystal to dissolve and precipitate on the surface of the larger crystal. Individual then agglomerate to efficiently encompass a common counter ion   layer. A few insoluble crystalline precipitate termed as colloidal are made. Ions are arranged in a fixed pattern alternating the positive and negative charges. For illustration, in AgCl, Ag⁺ is alternated by Cl^-, in such a way that the total surface charge is zero. However surface tends to adsorb ion is in excess. The adsorption makes a primary layer which is strongly adsorbed and is an integral part of the crystal. This attracts ion of opposite charge in a counter layer or secondary layer, thus as to provide overall neutral particle. Particles coagulate whenever the counter layer neutralizes the primary layer. Whenever coagulated particles are  filtered and washed by water, the secondary layers become loosely bound  and  the  particles  revert  to  the  colloidal  state  via  a  procedure  termed as peptization.

The colloidal particles could be hydrophilic or hydrophobic. Precipitates tend to carry down from the   solution other constituent which are generally soluble, causing contamination of precipitate via a process termed as co-precipitation. The Co-precipitation can take place via the given known method:

• Occlusion
• Surface adsorption
• Post Precipitation

d) Washing and Filtration of the Precipitates:

Precipitates are washed out after filtration, so as to get rid of any co precipitated impurities. The mother liquor that wet the precipitation is as well eliminated. Though, peptization does take place whenever water is employed to wash the precipitation. This is frequently prevented via adding electrolyte to the wash liquid example: HNO₃ or NH₄NO₃ for AgCl.

A test is frequently conducted to make sure the washing is completed and efficient. This is completed by testing the filtrate for the presence of ion of precipitation.

e) Drying and Igniting of Precipitate:

The wash liquid and adsorbed electrolyte from the precipitate are further treated so as to encompass precipitation in a form appropriate for weighing. This is completed generally by heating (or drying) at 110 - 120^oc for one or two hours. Ignition is needed, whenever the precipitation is to be heated at much higher temperature so as to transform the precipitate to a more appropriate form for weighing. The drying method continues till a constant weight is accomplished (that is, successive weighing differs via the factor of 0.3 or 0.4 mg.)

f) Calculation:

This is for all time on the percentage basis.

If A is the analyte of interest, then

% A = (Weight of A/Weight of sample) x 100

More frequently, the weight of 'A' is not measured directly. Rather, the species which is generally isolated and weighed either contain 'A' or can be chemically associated to 'A'.

Gravimetric factor is required to convert the weight corresponding to A.

Gravimetric factor = (F.w substance Sought/Fw Substance weighed) x (a/b)

Here, a and b are integers which make the numerator and denominator chemical equivalent.  

Volatilization gravimetric analysis:

In this kind of gravimetric analysis, the substance to be found out is separated in a gas form from the reminder of the sample. The weight of volatile component is then compared by the weight of non-volatilized portion. This process is otherwise termed as gravimetric combustion analysis.

In this kind of quantitative analysis, partly combusted product is passed via catalyst like Pt gauze, CuO, PbO₂ or MnO₂ at elevated temperatures to complete the oxidation to be CO₂ and H₂O.

The product is then passed via chamber having P₄O₁₀ that absorb H₂O and the other chamber of NaOH on asbestos that absorbs CO₂. The rise in mass of each and every chamber states how much of H₂ and C are generated correspondingly.

Combustion analysis has presently undergone quick changes dissimilar in the past whenever changes are limited to the weight of combustion product. Modern instrument utilize thermal conductivity, infrared absorption or Coulometry (having electrochemical produced reagent) to measure the product.

Application of Gravimetry as a separation method of metals:

Gravimetric analysis is extremely accurate and precise, if it is carried out beneath the right experimental conditions.

These are a few factors which affect the solubility of precipitate. These comprise:

• Temperature
• Solvent
• Rate of precipitation formation

Usually, in the application of gravimetric method to separate metals from the material, varying precipitating agent has been developed to improve the precipitation. 

A)  Inorganic precipitating agent:

Precipitating Agent     Element separated

i) NH₃ (aq)            →    Be, Al, Fe, Sc

ii) H₂S                   →    Cu, Zn, As

iii) (NH₄)₂ MoO₄    →    Cd, Pb 

iv) HCl                   →    Ag, Na, Si

B) Reducing Agent:

This is better as it transforms the analyte to its elemental form for weighing. A few generally used reducing agents are illustrated below:

Reducing Agent         Analyte

SO₂                     →   Se, An

SnCL₂                 →   Ag

HCOOH               →   Pt

H₂                       →   Re, Ir

C) Organic Precipitating agents:

There are huge numbers of organic compounds which are very helpful as precipitating agent for metals. A few are selective, whereas some are very wide in the number of elements they precipitate.

Organic precipitating agents include benefits of giving precipitate which are of very low solubility in water and by favourable gravimetric factors.

Two kinds of organic precipitating agents are in use, as illustrated:

i) One form slightly soluble non-ionic complexes termed as coordination compound.

ii) The other forms ion bonding between the inorganic species and reagent.

Organic precipitation   agents are chelating agents that form slightly soluble uncharged chelate by metal ion.

pH adjustment controls the selectivity and nature of chelating substances.

Mⁿ⁺ + n HX → MXn + n H⁺

Some of the general illustrations of organic precipitating agents are:

Organic reagent          Metal precipitated

Dimethyglyoxime   →     Pb

8-hydroxyquinoline (oxine) → Al, Mg

Sodium diethyldithiocarbonate → K, Pb, Cs, Tl, Ag, Cu, Hg

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