Page 69 - Chemistry - XI

However, if the starting solution is fi ltrate, it is boiled for some time to remove all H S gas until it gives negative
2
test for H S with lead acetate paper.
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To the above solution, add few drops of conc. HNO , boil and cool. Now add a pinch of ammonium chloride
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and excess ammonium hydroxide solution till it smells of ammonia. If the precipitate is formed, it confi rms the
presence of Group III cations. Filter the precipitate. Preserve the fi ltrate for the subsequent group.

White gelatinous ppt. Al OH Reddish brown ppt. Fe OH

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Dissolve the ppt. in dilute HCl and divide into Divide the ppt. in minimum quantity of dil. HCl
two parts: and divide the solution into two parts:

(i) NaOH test: To the fi rst part, add NaOH (i) Potassium ferrocyanide KFeCN

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6
solution drop by drop till excess. test: To one part of the solution, add

● A white ppt. appears which dissolves KFeCN solution → Prussian blue
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in excess of NaOH solution. To this coloured solution or ppt.
solution, add a pinch of NH Cl and heat.
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● A white ppt. is formed.
(ii) Lake test: Add two drops of blue litmus (ii) Potassium sulphocyanide (KCNS) test:
solution and then NH OH in excess → To another part of solution, add KCNS
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Pale blue fl oating ppt. in a colourless liquid solution → Blood red colouration.
appears.
Al Confi rmed Fe Confi rmed
+3
+3

Chemical Reactions

Test for Al +3
Al +3NaOH +3Na +
Al OH
+3
3
White gelatinouus ppt.

Al OH +3HCl AlCl+3H O
3 3 2
1. NaOH test:
AlCl +3NaOH +3NaCl
Al OH
3 3

Al OH +NaOH NaAlO +2HO
3 2 2
Sooluble

NaAlO+ NH Cl+H O +NH +NaCl
AlOH
2 4 2 3 3
White ppt.

2. Lake test:
AlCl +3NH OH +3NH Cl
Al OH
3 4 3 4
W hhite ppt.

Al(OH) adsorbs blue colour of the litmus which fl oats in remaining colourless solution. adsorbs blue colour of the litmus which fl oats in remaining colourless solution.
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