Question

Sulfide ion was determined by indirect titration with EDTA. To a solution containing 25 mL of 0,04332 M Cu(ClO4)2 plus 15 ml of 1 M acetate buffer (ph 4,5) was added 25 mL of unknown sulfide solution with vigorous stirring. The CuS precipitate was filtered and washed with hot water. Then ammonia was added to the filtrate ( which contains excess Cu) until the blue color of Cu (NH3)4 was observed. Titration with 0,03927 M EDTA required 12,11 mL to reach the murexide end point . Calculate the molarity of sulfide in the unknown

Answer 1

Persamaan reaksinya : Cu(ClO4)2 + S2- ----> CuS + Cu(NH3)4

Dari persamaan diatas dapat diketahui bahwa mol Cu2+ di sebelah kiri tanda panah = mol Cu2+ pada CuS + Mol Cu2+ pada Cu(NH3)4

*) Mol Cu2+ dalam Cu(ClO4)2 :
    Mol = volume x M
          =  25 mL x 0,04332 M
          =  1,083 mmol

*) Mol Cu2+ dapat ditentukan dari titrasi dengan EDTA. Karena EDTA adalah ligan    bidentat, maka 1 mol Cu setara dengan 2 mol EDTA.
 
Mol EDTA = volume x M
                 = 12,11 mL x 0,03927 M  
                 =  0,476 mmol

* Mol Cu2+ pada CuS = mol Cu2+ pada Cu(ClO4)2 - mol Cu2+ pada Cu(NH3)4
                                   = 1,083 mmol - 0,476 mmol
                                   = 0,607 mmol.

Menentukan mol S :
Mol S2- = Mol Cu2+ dalam CuS
          = 0,607 mmol.

Sehingga, konsentrasi S2- dalam larutan awal = mol S2- / volume
                                                                         = 0,607 mmol / 25 mL
                                                                         = 0,02428 M 

*Semoga bermanfaat.