# $buffer-solutions¶ Buffer solutions: To control pH values in electrolytes. It is possible to overwrite the database entries of$buffer-solutions by this keyword and its specifiers in the input file. You can also define a completely new buffer which is not contained in the database. If you overwrite a buffer that is contained in the database, it is necessary that you include all specifiers, including the ones that you do not want to overwrite. The program then uses all the values given in the input file and ignores all database entries.

$buffer-solutions optional buffer-name character required number-of-ions integer required ion-valency double_array required ion-name-1 character required ion-name-2 character required ion-name-3 character optional ! only necessary for some buffers, e.g. PBS ion-name-4 character optional ! only necessary for some buffers, e.g. PBS pKa double_array required ! pKa at 25° C ( = 298.15 K) dpKa_dT double_array required ! d pKa / d T z_acid double_array required ! charge on the conjugate acid species$end_buffer-solutions                      optional


$${\text p}K_{\text a}$$ and $${\text d~p}K_{\text a}/{\text d~T}$$.

Example 1: MOPS

!----------------------------------------!
$buffer-solutions ! buffer-name = MOPS ! MOPS (C7H15NO4S) + NaOH number-of-ions = 2 ! ion-valency = -1.0 +1.0 ! ! (C7H14NO4S)^- Na^+ ! ion-name-1 = Mops^- ! C7H15NO4S <==> (C7H14NO4S)^- + H^+ ion-name-2 = Na^+ ! pKa = 7.31 ! pKa at 25° C ( = 298.15 K) ! Note: This pKa is thermodynamic value. ! The working pKa' is 7.20. dpKa_dT = -0.011 ! d pKa / d T z_acid = 0.0 ! charge on the conjugate acid species (0 = C7H15NO4S)$end_buffer-solutions                    !
!----------------------------------------!


Involved ions and molecules:

• buffer-name: MOPS $$({\text C}_7{\text H}_{15}{\text N}{\text O}_{4}{\text S})$$ + $${\text {NaOH}}$$

• ion-valency: $$({\text C}_7{\text H}_{14}{\text N}{\text O}_{4}{\text S})^-$$ and $${\text {Na}}^+$$

• ion-name-1: $${\text C}_7{\text H}_{15}{\text N}{\text O}_{4}{\text S}$$ <==> $$({\text C}_7{\text H}_{14}{\text N}{\text O}_{4}{\text S})^-$$ + $${\text H}^+$$

• ion-name-2: $${\text {Na}}^+$$

• z_acid: 0 = $${\text C}_7{\text H}_{15}{\text N}{\text O}_{4}{\text S}$$

Example 2: PBS (phosphate buffer)

The phosphate buffer is special (and thus more complicated) because it consists of three $${\text p}K_{\text a}$$ values (and it thus has four different ions).

!-------------------------------------------------------------!
$buffer-solutions ! buffer-name = PBS ! PBS (phosphate buffer) number-of-ions = 4 ! ion-valency = -1.0 -2.0 -3.0 1.0 ! ! (H2PO4)^- (HPO4)^2- (PO4)^3- Na+ ! ion-name-1 = H2PO4^- ! NaH2PO4 <==> (H2PO4)^- + Na^+ ion-name-2 = HPO4^2- ! Na2HPO4 <==> (HPO4)^2- + 2 Na^+ ion-name-3 = PO4^3- ! ion-name-4 = Na^+ ! pKa = 2.15 7.21 12.33 ! pKa,1 pKa,2 pKa,3 at 25° C ( = 298.15 K) dpKa_dT = 0.0044 -0.0028 -0.026 ! d pKa / d T z_acid = 0.0 -1.0 -2.0 ! charge on the conjugate acid species ! 0 = H3PO4 -1 = (H2PO4)^- -2 = (HPO4)^2- !$end_buffer-solutions                                         !
!-------------------------------------------------------------!


Involved ions and molecules:

• ion-valency: $$({\text H}_2{\text P}{\text O}_4)^-$$, $$({\text H}{\text P}{\text O}_4)^{2-}$$, $$({\text P}{\text O}_4)^{3-}$$ and $${\text {Na}}^+$$

• ion-name-1: $${\text {Na}}{\text H}_{2}{\text P}{\text O}_{4}$$ <==> $$({\text H}_2{\text P}{\text O}_4)^-$$ + $${\text {Na}}^+$$

• ion-name-2: $${\text {Na}}_2{\text H}{\text P}{\text O}_{4}$$ <==> $$({\text H}{\text P}{\text O}_4)^{2-}$$ + $$2~{\text {Na}}^+$$

• ion-name-3: $$({\text P}{\text O}_4)^{3-}$$

• ion-name-4: $${\text {Na}}^+$$

• z_acid: 0 = $${\text H}_3{\text P}{\text O}_4$$, -1 = $$({\text H}_2{\text P}{\text O}_4)^{-}$$, -2 = $$({\text H}{\text P}{\text O}_4)^{2-}$$

For the explanation of the specifiers, please also check the description in the database of the keyword \$buffer-solutions .