Essay on Test on Cation in solution

Submitted By mihachew
Words: 1878
Pages: 8

Test on Cation in solution
Aim
The purpose of this experiment is to observe the characteristic precipitates formed when metal cations react with hydroxide solution and ammonium hydroxide solution.
Introduction
Cations are positively charged ions. They are present in inorganic salts along with the corresponding anions.
One way of differentiate them through tests in which precipitates and complex ions are formed when sodium hydroxide or aqueous ammonia is added to their salt solutions. The cation part of the salt will react with the hydroxide ions to form a hydroxide salt which it may or may not be soluble and may or may not be coloured.
This way of testing for cations is widely spread in qualitative analysis based on selective precipitation of different cations.
Apparatus
Test tubes, test tube stand, dropping pipette, cation solutions (ammonium chloride, lithium nitrate, sodium chloride, potassium iodide, magnesium chloride, calcium chloride, strontium chloride, barium chloride, aluminium chloride, chromium III chloride, manganese sulphate, iron II sulphate, ferric chloride, nickel chloride, copper sulphate, zinc chloride) and reagent solution (sodium hydroxide solution and respectively ammonium hydroxide).
Procedure
A set of four two tubes were placed in to test tube rack. Using a dropping pipette in each tube were placed 2 cm3 of the cation solution. Using another dropping pipette 1 cm3 of the reagent was added: sodium hydroxide solution in one of the tubes and ammonium hydroxide in the other test tube. And the observations were recorded in the given table.
Some more of the reagent was added in the test tubes and observations were recorded again in the same table.
The experiment was repeated for all the cations available.
The two solutions of sodium hydroxide and respectively ammonium hydroxide were chosen because: sodium hydroxide provides a high concentration of hydroxide being considered a strong base, ionises almost 100%. (100% split up into metal ions and hydroxide ions in solution)
Each mole of sodium hydroxide dissolves to give a mole of hydroxide ions in solution:

Explaining the term “strong base ” (Chemguide.co.uk, 2002) ammonium hydroxide, on the other hand, provides a low hydroxide concentration, making it a weak base. This is considered a typical weak base because ammonia doesn’t contain hydroxide ions, but reacts reversible with water to produce ammonia ions and hydroxide ions.

Explaining the term “weak base ” (Chemguide.co.uk, 2002)
Because the reversibility of the reaction at any given time about 99% of the ammonia is still present as ammonia molecules and as low as 1% has actually produced hydroxide ions.
Results
Cation
Initial colour
Addition of sodium hydroxide
Solubility in excess NaOH
Addition of ammonium hydroxide
Solubility in excess NH4OH
NH4+
Colourless
No ppt, colourless solution soluble No immediate ppt (Solution remain colourless)
Insoluble
Li+
Colourless
No ppt, colourless solution soluble Solution remain colourless
Insoluble
Na+
Colourless
No ppt, colourless solution soluble Solution remain colourless
Insoluble
K+
Colourless
No ppt, colourless solution soluble Solution remain colourless
Insoluble
Mg2+
Colourless
Gelatinous spearing white ppt
Insoluble
Gelatinous white ppt
Insoluble
Ca2+
Colourless
Gelatinous sparing white ppt
Insoluble
Sution remain colourless
Insoluble
Sr2+
Colourless
Gelatinous sparing white ppt
Insoluble
Sparing white ppt
Insoluble
Ba2+
Colourless
Sparing white ppt
Insoluble
Very sparing white ppt
Insoluble
Al3+
Colourless
Gelatinous white ppt
Soluble forming colurless solution
Gelatinous white ppt
Insoluble
Cr3+
Light Grey/ blue Blue/light green copious ppt
Soluble forming green solution
Blue/green copious ppt
Insoluble
Mn2+
Light green
Viscous, copious, strong buff ppt with brown ring at the surface and sides of the tube (oxidation)
Insoluble
Viscous, strong buff solution with brown ring at the surface…