Gravimetric Methods

Gravimetric Methods_Chem-Eng.21

Gravimetry based on measurement of weight of an analysed species or a compound containing the analysed species. Measuring mass is the most fundamental of all analytical measurements, and gravimetry is unquestionably the oldest analytical technique. Described above examples illustrate the four gravimetric methods of analysis. If the signal is the mass of a precipitate, the method calls precipitation gravimetry. There are four fundamental types of gravimetric analysis: physical gravimetry, thermogravimetry, precipitative gravimetric analysis, and electrodeposition.

SYSTEM IN QUANTITATIVE ANALYSIS

Moles = grams/formula weight

Molar (M) = moles of solute/liters of solution

Molal (m) = moles of solute/kilograms of solvent

Formal (F) = gram formula weight of solute/liters of solution

Mole Fraction(mf) = moles of solute/( moles of solvent + moles of solute)

Normal (N) = equivalent of solute/liters of solution

Gram per volume = grams of solute/liters of solution

Weight percebt (wt%) = 100% x grqms of solute/(grams of solvent + gram of solute)

Volume percent (vol%) = 100 x liters of solute/liters of solution

Part per million (ppm) = milligrams of solute/kilograms of solution

= milligrams/liters

EVAPORATION

Analyte weighed, then evaporated at a certain temperature, weighed again, and the missing part is determined.

Example:
A total of 1 gram of hydrated copper (II) sulphate, CuSO4.xH2O, the crystal is heated so that all the water evaporates. Mass of solids left behind is 0,64gram. How much water content results in the crystal?

ELEKTROGRAVIMETRIC

The substances analyzed was placed in the electrolysis cell. After electrolysis, the metal deposited on the cathode, and then weighed.

Example:

Determination of Cu in the solution at acidic condition using Cathode Pt.

Cathode : Cu2+(aq)   +   2e    àCu(s)
                H+(aq)    +   e          à½H2(g)
Anodes : H2O(l)   +   ½O2      à 2H+(aq)   +   2e

PRECIPITATION

“Change the shape of the desired components into a soluble form."

Reaction : aA + rR -> AaRr

a molecule analytes A, reacts with reagent R molecules r, produce AaRr

example:
Calcium can be determined gravimetrically by precipitation as calcium oxalate. Furthermore, the roasting until the calcium oxalate into calcium oxide.