Kimia Analitik Mencuci Endapan Koloid Kristal Washing Precipitate

Washing of Precipitate Pencucian Endapan

Why it should be washed? • Most precipitate produced in the presence of one or more soluble substance • The soluble substances are not volatile at the temperature which the precipitate heated • The soluble substances are removed by washing

Ideal Washing Solution 1. No solvent action upon the precipitate but dissolve foreign substance easily 2. No dispersive action on precipitate 3. Form no volatile or insoluble substance with the precipitate 4. Easily volatile at the temperature of drying of the precipitate 5. Contain no substance which is interfere the subsequent operation to the filtrate

Caution! • Use minimum volume of washing solution since no precipitate is insoluble • Do qualitative test of removal the impurities • Wash with a number of small portion of washing solution with well drained between each washing than large portion of washing solution or adding a new portion while the solution still remain

Choosing the washing solution • Pure water should not be used unless it certain will not dissolve appreciable amount or peptise the precipitate • Colloidal/gelatinous precipitate is washed with electrolyte solution • Crystalline precipitate is washed with a solution contain common ion

Washing by decantation • If the precipitate settle rapidly (high density) or gelatinous (colloidal) • Clear solution upon the precipitate (supernatant) as much as possible is transferred to the filter medium, avoid to disturb the precipitate.

Washing by decantation • 20 t0 50 mL of the washing solution is added to the residue in the beaker, the solid stirred and allowed to settle • When the supernatant is clear, as much as possible of the liquid decanted to the filter medium. • This process is repeated as many times as necessary before the precipitate is transferred to the filter medium

Qualitative test for the completeness of washing • Collect a small volume (2-3 mL) of the last washing • Add the appropriate reagent • Do the same action to the unused washing solution as a comparative test

Qualitative test Ion

Cl-

SO4=

C2O4=

H+

Reagent

HNO3

HCl

H2SO4

Methyl red

Positive Product

AgNO3

BaCl2

KMnO4

AgCl

BaSO4

Mn2+ CO2

Methyl red molecule

White White Purple Red or Appearance precipit precipita color is reddish ate te disappear, Positive

gas

orange

Three classes of wash solutions : 1. Solutions which prevent the precipitate from becoming colloidal and passing through the Filter. 2. Solutions which reduce the solubility of the precipitate. 3. Solutions which prevent the hydrolysis of salts of weak acids and bases.