Analysis of 1st Group of Cations: Ag+ and Pb2+ Ions

 
Table 1.
 
Solubility products of 1
st
 group of cations
 
The cations in this group can be easily analyzed 
as their chloride
 precipitation
s 
with 0.3 M HCl. When the 
acid
concentration is increased, the 
precipitated salts in the medium re-soluble again. At this point, 
the
concentration of the acid is 
highly important parameter.
 
Ag
Ag
+
+
 ions assay
 ions assay
 
 Hydrochloric acid (HCl) solution:
Ag
+ 
ions (20 drops) 
precipitate as a white-colored chloride when 1 M HCl (4
drops) solution is added to the medium.
 
Ag
+
 + HCl                 AgCl
(k)  
 + H
+
        
(White)
 
The resulting white-colored solution is centrifuged, and the precipitate is taken
up from the media. NH
3 
(6 M) solution is added to the medium until AgCl is
dissolved. AgCl precipitates dissolve as diammine complexes with NH
3 
solution.
 
AgCI + 2NH
3               
         Ag(NH
3
)
2
+
 + Cl
-
 
 Potassium iodide (KI) solution:
Ag
+
 ions (20 drops) precipitate as yellow creamy silver iodide (AgI) when 1 M KI (4 drops) is added to the
solution.
Ag
+
 + KI                   AgI
(k)
   +  K
+
    
                                   (Yellow)
 
Separate
 the filtrate 
from the 
resulting 
solution by centrifugation
,
 and add 1 M KCN solution to dissolve the
remaining precipitate. These processes indicate the presence of 
Ag
+
 
ions in the 
medium
.
 
Potassium chromate (K
2
CrO
4
) solution:
Ag
+
 ions (20 drops) precipitate as tile red colored silver chromate (Ag
2
CrO
4
) when K
2
CrO
4
 (4 drops) is added to
the solution.
 
2Ag
+
 + CrO
4
2- 
                Ag
2
CrO
4 (k)
  (Tile red)
 
Sodium hydroxide (NaOH) solution:
Ag
+
 ions (20 drops) precipitate as darkish-brown colored silver hydroxide (AgOH) when 1 M NaOH (4-5 drops) is
added to the solution.
 However, the compound immediately decomposes into silver oxide
 (
Ag
2
O) 
due to it’s
unstable
.
 
Ag
+
 + OH
-
                  AgOH
                                 (Darkish-brown)
   2AgOH                  Ag
2
O  + H
2
O
                     (Black)
 
Pb
Pb
2+
2+
 ions assay
 ions assay
 
HCl
  
solution:
Pb
2+
 
ions (20 drops) 
precipitate as white-coloured chloride forms when 1 M HCl (4
drops) solution is added to the medium.
 
Pb
2+
 + 2HCl                PbCl
2 
  + 2H
+
                         (White)
 
The resulting white precipitate is separated from the solution by centrifugation, 2
mL of 
distilled
 water is added, and the mixture is heated in 
a
 water bath for 5
minutes. If the precipitate dissolves, there are 
Pb
2+
  ions in the
 media
.
 
KI solution:
 
Distilled water (20 drop) is added to the Pb
2+
sample solution (20 drops),
and then 1 M KI (3-4 drops) is added to the resulting mixture. The yellow
colored lead (II) iodide (PbI
2
) precipitate is obtained after the experimental
process.
 
Pb
2+
 + KI               PbI
2
  + K
+
 
   
   (Yellow)
 
Thioacetamide (CH
3
CSNH
2
)
 solution:
 
Distilled water (20 drops) is added to the Pb
2+ 
sample solution (20 drops), and thioacetamide solution (3-4
drops) is added to the mixture. Then, the solution is heated 
in the water bath for 5 minutes. 
Pb
2+
 ions in the
sample precipitate as black colored lead (II) sulfur (PbS).
 
Pb
2+
 + H
2
S                 PbS 
(k)
                                   (Black)
 
The 
resulting 
precipitate should dissolve in the 3
 
M HNO
3
 solution.
 
K
2
CrO
4
 solution:
Pb
2+ 
ions (20 drops) precipitate as yellow colored lead (II) chromate (PbCrO
4
) when K
2
CrO
4
 (3-4 drops) is added
to the solution.
 
 Pb
2+
 + CrO
4
2- 
                PbCrO
4(k)
                                          (Yellow)
 
Hg
Hg
2
2
2+
2+
 ions assay
 ions assay
 
HCl solution:
 
Hg
2
2+
 
ions (20 drops) precipitate as their white chloride salts when 3 M HCl
(3-4 drops) is added to the solution.
 The resulting precipitate is insoluble in
cold
 and
 hot water 
or
 dilute
d
 HNO
3
 solution, however, is soluble in 
NH
3
,
concentrated 
HNO
3 
or aqua regia solutions.
 
K
2
CrO
4
 solution:
 
Hg
2
2+
 
ions (20 drops) precipitate as reddish-brown mercury (I)
chromate (
Hg2CrO4) 
salt when 
K
2
CrO
4 
(3-4 drops) is added to the
solution.
 
 Hg
2
2+
 + CrO
4
2- 
                 Hg
2
CrO
4 (k)
                                       (Reddish-brown)
 
 Hg
2
2+
 + 2 Cl
- 
                 Hg
2
Cl
2(k)
                                        (White)
 
KI solution:
 
Distilled water (20 drops) is added to the Hg
2
2+
 sample solution (20 drops), and 1
M KI solution (4-5 drops) is added to the mixture. Yellow-green colored
precipitate of mercury (I) iodide (Hg
2
I
2
) is obtained after this process.
 
This
precipitate 
is soluble when 
the 
excess
ive 
KI is added
 to the precipitate
, 
however,
insoluble in dilute acids.
 
Hg
2
2+
 + 2KI                 Hg
2
I
2
    + 2K
+
                       (Green-Yellow)
 
Copper strip (Amalgam):
 
First
ly
, the copper wire (strip) piece is cleaned with 8-10 drops of concentrated
HN
O
3 
solution in the 
watch 
glass
, then it
 is dried 
by filter 
paper and 
Hg
2
2+
 sample
solution 
is added to the
 copper strip.
 
Hg
2
2+
 
ions coat the copper strip (bright gray
colored)
.
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The analysis of cations in the 1st group involving Ag+ and Pb2+ ions is carried out by precipitating insoluble chloride salts in the presence of diluted HCl solution. The solubility products of AgCl, PbCl2, and Hg2Cl2 play a crucial role in the precipitation reactions. Various reagents such as HCl, NH3, KI, KCN, K2CrO4, and NaOH are used to identify and separate Ag+ and Pb2+ ions through distinctive color changes and precipitate formations. The process involves the formation of complex compounds and demonstrates the unique properties of each cation in the group.


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  1. 1stGROUP OF CATIONS (Ag+,??? 2+ve Pb2+ions, and the main prominent feature of this group is that they precipitate as insoluble chloride salts in water in the presence of diluted HCl solution. ?+and Pb2+) This group contains Ag+,Hg2 However, when the solubility products (Ksp) of this group are taken into consideration as presented in Table 1, all lead ions will not be able to precipitate in this group because the solubility of lead (II) chloride (PbCl2) in water (more soluble in hot water) is high. Pb2+ions are still analyzed in the first group. Table 1. Solubility products of 1st group of cations Subtances Solubility products (Ksp) 1 x 10-10 AgCl 1.56 x 10-10 PbCl2 2.0 x 10-18 Hg2Cl2 The cations in this group can be easily analyzed as their chloride precipitations with 0.3 M HCl. When the acid concentration is increased, the precipitated salts in the medium re-soluble again. At this point, the concentration of the acid is highly important parameter.

  2. Ag+ ions assay Hydrochloric acid (HCl) solution: Ag+ ions (20 drops) precipitate as a white-colored chloride when 1 M HCl (4 drops) solution is added to the medium. Ag+ + HCl AgCl(k) + H+ (White) The resulting white-colored solution is centrifuged, and the precipitate is taken up from the media. NH3 (6 M) solution is added to the medium until AgCl is dissolved. AgCl precipitates dissolve as diammine complexes with NH3 solution. Ag(NH3)2+ + Cl- AgCI + 2NH3

  3. Potassium iodide (KI) solution: Ag+ ions (20 drops) precipitate as yellow creamy silver iodide (AgI) when 1 M KI (4 drops) is added to the solution. Ag+ + KI AgI(k) + K+ (Yellow) Separate the filtrate from the resulting solution by centrifugation, and add 1 M KCN solution to dissolve the remaining precipitate. These processes indicate the presence of Ag+ ions in the medium. Potassium chromate (K2CrO4) solution: Ag+ ions (20 drops) precipitate as tile red colored silver chromate (Ag2CrO4) when K2CrO4 (4 drops) is added to the solution. 2Ag+ + CrO42- Ag2CrO4 (k) (Tile red) Sodium hydroxide (NaOH) solution: Ag+ ions (20 drops) precipitate as darkish-brown colored silver hydroxide (AgOH) when 1 M NaOH (4-5 drops) is added to the solution. However, the compound immediately decomposes into silver oxide (Ag2O) due to it s unstable. Ag+ + OH- AgOH (Darkish-brown) 2AgOH Ag2O + H2O (Black)

  4. Pb2+ ions assay HClsolution: Pb2+ ions (20 drops) precipitate as white-coloured chloride forms when 1 M HCl (4 drops) solution is added to the medium. Pb2+ + 2HCl PbCl2 + 2H+ (White) The resulting white precipitate is separated from the solution by centrifugation, 2 mL of distilled water is added, and the mixture is heated in a water bath for 5 minutes. If the precipitate dissolves, there are Pb2+ions in the media. KI solution: Distilled water (20 drop) is added to the Pb2+sample solution (20 drops), and then 1 M KI (3-4 drops) is added to the resulting mixture. The yellow colored lead (II) iodide (PbI2) precipitate is obtained after the experimental process. Pb2+ + KI PbI2 + K+ (Yellow)

  5. Thioacetamide (CH3CSNH2) solution: Distilled water (20 drops) is added to the Pb2+ sample solution (20 drops), and thioacetamide solution (3-4 drops) is added to the mixture. Then, the solution is heated in the water bath for 5 minutes. Pb2+ ions in the sample precipitate as black colored lead (II) sulfur (PbS). Pb2+ + H2S PbS (k) (Black) The resulting precipitate should dissolve in the 3 M HNO3solution. K2CrO4 solution: Pb2+ ions (20 drops) precipitate as yellow colored lead (II) chromate (PbCrO4) when K2CrO4 (3-4 drops) is added to the solution. Pb2+ + CrO42- (Yellow) PbCrO4(k)

  6. Hg22+ ions assay HCl solution: Hg22+ ions (20 drops) precipitate as their white chloride salts when 3 M HCl (3-4 drops) is added to the solution. The resulting precipitate is insoluble in cold and hot water or diluted HNO3 solution, however, is soluble in NH3, concentrated HNO3 or aqua regia solutions. Hg22+ + 2 Cl- (White) Hg2Cl2(k) K2CrO4 solution: Hg22+ ions (20 drops) precipitate as reddish-brown mercury (I) chromate (Hg2CrO4) salt when K2CrO4 (3-4 drops) is added to the solution. Hg22+ + CrO42- (Reddish-brown) Hg2CrO4 (k)

  7. KI solution: Distilled water (20 drops) is added to the Hg22+ sample solution (20 drops), and 1 M KI solution (4-5 drops) is added to the mixture. Yellow-green colored precipitate of mercury (I) iodide (Hg2I2) is obtained after this process. This precipitate is soluble when the excessive KI is added to the precipitate, however, insoluble in dilute acids. Hg22+ + 2KI Hg2I2 + 2K+ (Green-Yellow) Copper strip (Amalgam): Firstly, the copper wire (strip) piece is cleaned with 8-10 drops of concentrated HNO3 solution in the watch glass, then it is dried by filter paper and Hg22+ sample solution is added to the copper strip. Hg22+ ions coat the copper strip (bright gray colored).

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