PERHITUNGAN DAPAR

CONTOH PERHITUNGAN DAPAR

1. Persamaan Handerson-Hasselbach (persamaan untuk buffer) 

                         

    garam

pH = pKa + log 

                            asam

2. Persamaan Van Slyke untuk kapasitas dapar

        Ka [H₃O⁺]

ß = 2,3 C                          

       ( Ka [H₃O⁺] )²

                                          

Keterangan:

            ß          = Kapasitas dapar, ß = 0,01 – 0,1

C          = Konsentrasi total dapar (mol/L)

            Ka        =  Konstanta asam = antilog (-pKa)

            [H₃O⁺] = Konsentrasi ion Hidrogen = antilog (-pH)

Contoh perhitungan dapar

pH stabilitas sediaan    = 6,0

pKa H₂PO₄                         = 7,12

Persamaan Handerson-Hasselbach:

                        [HPO₄ ^2-]

6 = 7,12 + log                   

                        H₂PO₄^-

[HPO₄^2-]

 log                        = -1,12

            H₂PO₄^-

[HPO₄^2-]

                              = 0,076 → [HPO₄^2-] = 0,076 [H₂PO₄^-]

            H₂PO₄^-

Persamaan Koppel-Spiro-Van Slyke :

Ka                    = antilog (-pKa)           = antilog  (-7,12) = 7,6.10^-8

[HPO₄^2-]           = antilog (-pH) = antilog (-6)        = 1.10^-6

                              (7,6.10^-8) ( 1.10^-6)

0,1       = 2,3 C

                            [(7,6.10^-8) + ( 1.10^-6)]²

            = 2,3 C (6,55.10^-2) → C= 0,66 mol/L

C          = [garam] + [asam]

0,66     = [HPO₄^2-] + [H₂PO₄^-]              

            = 0,076 [H₂PO₄^-] + [H₂PO₄^-]

            = 1,076 [H₂PO₄^-]

[H₂PO₄^-] = 0,61

[HPO₄^2-] = 0,076 x 0,61

                = 0,046

Jadi, [H₂PO₄^-] = 0,61 M;  [HPO₄^2-]    = 0,046 M

BM KH₂PO₄ = 136,10

BM KNaHPO ₄ = 158,10

Dapar yang diperlukan untuk 1 L sediaan :

[KH₂PO₄] = [H₂PO₄^-] = 0,61 mol/L

                  = 0,061 x 136,10

                  = 83,02 gram/L

[KNaHPO ₄] = [HPO₄^2-] = 0,046 mol/L

                      = 0,046 x 158,10

                      = 7,27 gram/L