Gravimetric Analysis

Gravimetric Analysis or Quantitative Analysis by Weight

Process of isolating and weighing an element or a definite compound of the element in as pure a form as possible. 

Precipitation

A gravimetric method in which the signal is the mass of a precipitate.

Electrogravimetry

A gravimetric method in which the signal is the mass of an electro deposit on the cathode or anode in an electrochemical cell.

Volatilization Gravimetry

A gravimetric method in which the loss of a volatile species gives rise to the signal (remove the volatile species).

Particulate Gravimetry

A gravimetric method in which the mass of a particulate analyte is determined following its separation from its matrix.

Selective

Reacts with a few analytes.

Specific

Reacts with 1 analyte only.

Colloidal Suspensions

• 10-7 to 10-4 cm diameter

• Normally remains suspended

• Very difficult to filter

• Divided into two groups: lyophobic and lyophilic

Crystalline Suspensions

• Tenths of mm diameter

• Normally settle out spontaneously

• Readily filterable

Nucleation

Precipitation is initiated by the deposition of particles on the surface of suspended surface.

Nucleation

Individual ions/atoms/molecules coalesce to form “nuclei”.

Large number of small particles result.

What happens when nucleation predominates?

Particle Growth

Ions/atoms/molecules are added to the nucleus to form larger particles.

Smaller number of large particles result

What happens when particle growth predominates?

Particle Growth

This process is only moderately enhanced by high relative supersaturation and results in crystalline suspension.

Colloidal Suspension

Colloidal particles remain suspended due to adsorbed ions giving a net positive or negative charge.

Coagulation or Agglomeration

Colloidal particles aggregate into larger, filterable particles when suspended, aided by an inert electrolyte that enables them to come closer together.

Peptization

Re-dissolution of coagulated colloids by washing and removing inert electrolyte.

Peptidization

During the procedure, the precipitate is washed and filtered, but some of it reverts to its colloidal form due to the absence of supporting electrolytes.

Coprecipitation

May occur by the formation of mixed crystals or by the absorption of ions during the precipitation products.

Crystalline Precipitates

Adsorbs impurities when the particles are small (impurities are occluded in the crystal).

Curdy Precipitate

Do not enclose or occlude foreign ions.

Gelatinous Precipitates

Do not occlude impurities but adsorbs them on the tiny surface of the tiny particles.

Precipitation for Homogenous Solution

The precipitate forms as a result of the reaction in the solution. This process can yield both large and pure particles of the precipitate.

Post Precipitation

The longer time the desired precipitate is left in contact with the mother liquor, the greater the amount of contamination.

Occlusion of Water Molecule

For removing water that is trapped or strongly adsorbed in precipitates, or for completely converting some precipitates into the desired compound, it may be necessary to ignite them at higher temperatures.

Precipitation of AgCl

Reduced chlorine (in oxidation states other than  1-) to Cl- and precipitate as AgCl. Similar process can be done with other halides such as Br- and I-.

Precipitation of BaSO

Employed in the determination of sulfur in sulfides, sulfites, thiosulfates and tetrathionate.

Precipitation from Metal Hydrous Oxides by Ammonia

Used for the quantitative separation of Fe and the foregoing elements from the alkali and alkaline earth cations in the analysis of rocks, such as limestone.

Lyophobic Colloids

Dispersion are only slightly viscous.

Lyophilic Colloids

Dispersions are very viscous.

Peptization

The process of dispersing a gel or a flocculated solid to form a solution is called?

Flocculation or Coagulation Value

Colloidal particles clump together into larger clusters or flocs due to the addition of electrolytes, which neutralize their charges and reduce repulsion.

Peptization

Breaking down larger clusters or flocs of colloidal particles into individual particles, typically achieved by adding a peptizing agent that stabilizes the particles and prevents re-aggregation.

Supersaturated Solution

Is one that contains a greater concentration of solute than corresponds to the equilibrium solubility at the temperature under consideration.

Phosphates

Trimethyl or triethyl phosphate, when hydrolyzed with 1.8M sulfuric acid containing zirconyl ions, forms a dense precipitate of variable composition. This precipitate, when ignited, is weighed as ZrP2O7, a dipolyphosphate (pyrophosphate).

Oxalates

Urea raises the pH of an acid solution containing hydrogen oxalate ion (HC2O4^-), slowly generating oxalate ion and enabling dense precipitation of calcium oxalate.

Sulphates

Sulphate ions may be generated by the hydrolysis of aminosulphonic (sulphamic) acid.

Thermogravimetry

A technique in which a change in the weight of a substance is recorded as a function of temperature or time.

Static Air

Air from surroundings flows through the furnace.

Dynamic Air

Where compressed air from a cylinder is passed through the furnace at a measured flow rate.

Nitrogen Gas (Oxygen-Free)

Provides an inert environment.

Humidified Air

Used in the study of the decomposition of such compounds as hydrated metal salts.

Fractional Precipitation

To understand separations involving sulfide ions, we begin by examining the quantitative aspects of saturated hydrogen sulfide solutions, which are key for sulfide precipitation.

Dimethylglyoxime

Discovered by L. Tschugaeff and used by O. Brunck for nickel determination in steel, this reagent forms a red precipitate (Ni(C4H7O2N2)2) with nickel salts in ammoniacal or buffered solutions.

Furil-a-dioxime (III)

Furil-a-dioxime is used for nickel determination, forming a red precipitate with nickel salts in ammoniacal solutions. It offers the advantage of water solubility, preventing contamination of the nickel precipitate, but it is expensive to use.

Cyclohexane-1,2-dione dioxime (nioxime) (IV)

Is more soluble in water.

Cupferron (ammonium salt of N-nitroso-N-phenylhydroxylamine) (V)

An ammonium salt, precipitates various metals from acidic solutions, effectively separating them from others like aluminum, chromium, and zinc, with no interference from tartrate or oxalate.

N-Benzoyl-N-phenylhydroxylamine, C6H5CO(C6H5)NOH

This reagent, similar to cupferron in its reactions but more stable, is moderately soluble in hot water and readily soluble in ethanol and other organic solvents.

8-Hydroxyquinoline (oxine) (VI) - Oxine (C,H ,ON)

Forms sparingly soluble derivatives with metallic ions, which have the composition M(C,H,ON).

Benzoin-a-oxime (cupron) (VII)

This compound forms a green precipitate with copper in dilute ammoniacal solution, allowing specific separation from other metals like cadmium, lead, nickel, cobalt, zinc, aluminum, and iron.

Nitron (VIII)

The strong organic base 1,4-diphenyl-3-phenylamino-1H.

1,2,4-triazolium hydroxide

Nitron forms a sparingly soluble crystalline nitrate in acidic solutions, and similar insoluble salts can be determined for perchlorate, perrhenate, tetrafluoroborate, and tungstate.

Anthranilic acid (IX)

Anthranilic acid sodium salt precipitates zinc, cadmium, cobalt, nickel, copper, lead, silver, and mercury in neutral or weakly acidic solutions. Its cadmium, zinc, nickel, cobalt, and copper salts (M(CH602N)) are used for quantitative gravimetric determination of these elements.

Quinaldic Acid (X)

This reagent forms insoluble complexes with copper, cadmium, zinc, manganese, silver, cobalt, nickel, lead, mercury, iron (II), palladium (II), and platinum (II), and basic insoluble salts with iron (III), aluminum, chromium, beryllium, and titanium, depending on the solution's pH.

8-Hydroxyquinaldine (XI)

This reagent precipitates various metals in acetic acid-acetate solution and ammoniacal solution, excluding aluminum which requires tartrate to prevent precipitation.

Sodium tetraphenylborate Na+[B(CBH,),]

Useful reagent for Potassium.