Analytical Chemistry: Action of Sodium and Ammonium Hydroxide on Salts

Vocabulary flashcards covering the principles of analytical chemistry, the colours of ions and salts, and the precipitation and complex-formation reactions of metal cations with sodium hydroxide and ammonium hydroxide.

Analytical Chemistry

The branch of chemistry that deals with experimental study both by qualitative and quantitative means.

Qualitative analysis

The identification of an unknown substance by chemical tests, including the use of sodium and ammonium hydroxides and observation of the colour and solubility of the precipitates formed.

Sodium hydroxide ($NaOH$)

A strong alkali which dissociates completely to furnish $OH^-$ ions.

Ammonium hydroxide ($NH_4OH$)

A weak alkali which dissociates partially to furnish sufficient $OH^-$ ions to precipitate insoluble metal hydroxides.

Salts of normal elements

Salts of elements belonging to Groups 1, 2, and 13 to 17 in the periodic table, which are generally colourless.

Salts of transition elements

Salts of elements belonging to Groups 3 to 12 in the periodic table, which are generally coloured.

Permanganate ion ($MnO_4^{1-}$)

A coloured anion that imparts a pink [purple] colour.

Dichromate ion ($Cr_2O_7^{2-}$)

A coloured anion that imparts an orange colour.

Copper(II) ion ($Cu^{2+}$)

A Group 11 [IB] coloured cation characterized by a blue/green colour.

Iron(II) ion ($Fe^{2+}$)

A Group 8 [VIII] coloured cation characterized by a green colour.

Iron(III) ion ($Fe^{3+}$)

A Group 8 [VIII] coloured cation characterized by a yellow/brown colour.

Amphoteric nature of metals

The property shown by aluminium, zinc, and lead, as well as their oxides and hydroxides, when they react with caustic alkalis ($NaOH$, $KOH$).

Action of $NaOH$ on $Ca(NO_3)_2$

Produces a white precipitate of calcium hydroxide ($Ca(OH)_2$) that is almost insoluble in excess $NaOH$ via the reaction: $Ca(NO_3)_2 + 2NaOH \rightarrow 2NaNO_3 + Ca(OH)_2$.

Action of $NH_4OH$ on $Ca(NO_3)_2$

Produces no precipitate of $Ca(OH)_2$ because the concentration of $OH^-$ ions is low and cannot precipitate calcium hydroxide.

Action of $NaOH$ and $NH_4OH$ on $MgCl_2$

Forms a dull white precipitate of magnesium hydroxide ($Mg(OH)_2$) that is insoluble in excess of either alkali.

Action of $NaOH$ and $NH_4OH$ on $FeSO_4$

Forms a dirty green precipitate of iron [II] hydroxide ($Fe(OH)_2$) that is insoluble in excess of either alkali.

Action of $NaOH$ and $NH_4OH$ on $FeCl_3$

Forms a reddish brown precipitate of iron [III] hydroxide ($Fe(OH)_3$) that is insoluble in excess of either alkali.

Action of $NaOH$ on $CuSO_4$

Forms a pale blue precipitate of copper [II] hydroxide ($Cu(OH)_2$) that is insoluble in excess $NaOH$.

Tetrammine copper sulphate ($[Cu(NH_3)_4]SO_4$)

A complex salt formed as a deep blue or inky blue solution when the pale blue precipitate of copper [II] hydroxide dissolves in excess $NH_4OH$.

Sodium zincate ($Na_2ZnO_2$)

A soluble compound formed as a colourless solution when the gelatinous white precipitate of zinc hydroxide dissolves in excess $NaOH$: $Zn(OH)_2 + 2NaOH \rightarrow 2H_2O + Na_2ZnO_2$.

Tetrammine zinc sulphate ($[Zn(NH_3)_4]SO_4$)

A soluble complex salt formed as a colourless solution when the white gelatinous precipitate of zinc hydroxide dissolves in excess $NH_4OH$.

Sodium plumbite ($Na_2PbO_2$)

A soluble compound formed as a colourless solution when the chalky white precipitate of lead [II] hydroxide dissolves in excess $NaOH$: $Pb(OH)_2 + 2NaOH \rightarrow 2H_2O + Na_2PbO_2$.

Action of $NH_4OH$ on $Pb(NO_3)_2$

Forms a chalky white precipitate of lead [II] hydroxide ($Pb(OH)_2$) that is insoluble in excess $NH_4OH$.