Question

What are all of the equations involved in a group III cation analysis?

Thank you

Answer 1

Note that Group III cations are:

(Co2+, Ni2+, Fe3+, Mn2+, Cr3+, Al3+, Zn2+), therefore

we will need to precipitate them as Sulfates in basic medium, that's why we use (NH4)2S, since NH4+ will make it acidic and S-2 will help in precipitation

Co(NO3)2 + 2 (NH4)2S ----> CoS(s) + 2 NH4NO3 (reaction 1)

Ni(NO3)2 + 2 (NH4)2S ----> NiS(s) + 2 NH4NO3 (reaction 2)

Fe(NO3)3 + 3 (NH4)2S ----> Fe2S3(s) + 3 NH4NO3 (reaction 3)

Zn(NO3)2 + 2 (NH4)2S ----> ZnS(s) + 2 NH4NO3 (reaction 4)

Mn(NO3)2 + 2 (NH4)2S ----> MnS(s) + NH4NO3 (reaction 5)

2 Cr(NO3)3 + 3 (NH4)2S + 6 H2O ----> 2 Cr(OH)3(s) + 6 NH4NO3 + 3 H2S (reaction 6)

2 Al(NO3)3 + 3 (NH4)2S + 6 H2O ----> 2 Al(OH)3(s) + 6 NH4NO3 + 3 H2S (reaction 7)

.individual identification

Ni(NO3)2 + 2 HDMG -->  Ni(DMG)2 + 2 HNO3 (reaction 8)

Fe(NO3)3 + 6 KSCN -->  K3[Fe(SCN)6] + 3 KNO3 (reaction 9)

Co(NO3)2 + 4 NH4SCN -->  (NH4)2[Co(SCN)4] + 2 NH4NO3 (reaction 10)

2 Mn(NO3)2 + 5 NaBiO3 + 16 HNO3 -->  2 HMnO4 + 5 Bi(NO3)3 + 5 NaNO3 + 7 H2O (reaction 11)

2 Na[Cr(OH)4] + 2 NaOH + 3 H2O2 --> 2 Na2CrO4 + 8 H2O (reaction 12)

then, we add NaOH, strong base t favour precipitation once again:

Co(NO3)2 + 2 NaOH ----> Co(OH)2(s) + 2 NaNO3 (reaction 13)

Ni(NO3)2 + 2 NaOH ----> Ni(OH)2(s) + 2 NaNO3 (reaction 14)

Fe(NO3)3 + 3 NaOH ----> Fe(OH)3(s)+ 3 NaNO3 (reaction 15)

Mn(NO3)2 + 2 NaOH ----> Mn(OH)2(s) + 2 NaNO3 (reaction 16)

Zn(NO3)2 + 2 NaOH ---->  Zn(OH)2(s) + 2 NaNO3 (reaction 17)

Al(NO3)3 + 3 NaOH ---->  Al(OH)3(s) + 3 NaNO3 (reaction 18)

Cr(NO3)3 + 3 NaOH ---->  Cr(OH)3(s) + 3 NaNO3 (reaction 19)

then;

Zn(OH)2(s) + 2 NaOH ---> Na2[Zn(OH)4] (reaction 20)

Al(OH)3(s) + NaOH ---> Na[Al(OH)4] (reaction 21)

Cr(OH)3(s) + NaOH ---> Na[Cr(OH)4] (reaction 22)

and finally:

Mn(OH)2(s) + H2O2 --> MnO(OH)2(s) + H2O (reaction 23)

2 Co(OH)2(s) + H2O2 --> 2 Co(OH)3(s) (reaction 24)

2 Na[Cr(OH)4] + H2O2 --> 2 Na2CrO4 + 8 H2O (reaction 25)

which allows us to:

Na2CrO4 + BaCl2 --> BaCrO4(s) + 2 NaCl (reaction 26)

and

Na2[Zn(OH)4] + H2S --> ZnS(s) + 2 NaOH + 2 H2O (reaction 27)