Constant A(T) used for buffer calculations: The pK_a value depends on the ionic strength.

!--------------------------------------------------------------!
$buffer-constant-A(T)                              optional    !
 constant-Centigrade-to-Kelvin    double           required    !
 T_A(T)                           double_array     required    !
$end_buffer-constant-A(T)                          optional    !
!--------------------------------------------------------------!

Syntax

!--------------------------------------------------------------!
$buffer-constant-A(T)                                          !
 constant-Centigrade-to-Kelvin = 273.15d0                      ! Kelvin = Celsius + 273.15
                                                               !
 !=============================================================!
 !                  first column: T[C]   second column: A(T)   !
 !=============================================================!
 T_A(T)                        =   0d0         0.4918d0        !   0° C = 273.15 K
                                  10d0         0.4989d0        !  10° C = 283.15 K
                                  20d0         0.5070d0        !  20° C = 293.15 K
                                  25d0         0.5114d0        !  25° C = 298.15 K
                                  30d0         0.5161d0        !  30° C = 303.15 K
                                  37d0         0.5321d0        !  37° C = 310.15 K
                                  40d0         0.5262d0        !  40° C = 313.15 K
                                  50d0         0.5373d0        !  50° C = 323.15 K
                                  60d0         0.5494d0        !  60° C = 333.15 K
                                  70d0         0.5625d0        !  70° C = 343.15 K
                                  80d0         0.5767d0        !  80° C = 353.15 K
                                  90d0         0.5920d0        !  90° C = 363.15 K
                                 100d0         0.6086d0        ! 100° C = 373.15 K
$end_buffer-constant-A(T)                                      !
!--------------------------------------------------------------!

The left   column of the specifier T_A(T) contains the temperature in degrees of Centigrade (Celsius) betwenn 0° C and 100° C.
The right column of the specifier T_A(T) contains the corresponding value of the constant A as a function of temperature T, i.e. A(T).

The values are taken from page 30 of:
R.J. Beynon, J.S. Easterby, "Buffer solutions: The basics", Oxford University Press (1996).

They can also be approximated by a second-order polynomial (R.J. Beynon, Comput. Appl. Biosci. 4 (4), 487 (1988)):

A(T) = 0.4918 + 0.0006614 T + 0.000004975 T²

Physical significance of this parameter

The ionic strength of an electrolyte influences the pK_a value of the buffer. This dependence can be described by the following equation (sometimes known as the Debye-Hückel relationship) where the constant A(T) enters.

 pK_a' =  pK_a + ( 2 z_a - 1 ) [ A I^1/2 / ( 1 + I^1/2 ) - 0.1 I ]

where I is the ionic strength and z_a is the charge on the conjugate acid species. pK_a' is the modified pK_a value.
The value of A (sometimes called Debye-Hückel parameter) is about 0.5 but it is temperature dependent.

Internally, the program takes the temperature T₀ that is given in the input file under the keyword $global-parameters (in units of Kelvin) and interpolates linearly between the two appropriate neighboring A(T) values to find the value for A(T₀).
The conversion between temperature in Kelvin and Centigrade is done by the constant: constant-Centigrade-to-Kelvin = 273.15d0

Example:

lattice-temperature = 288.15d0         ! 288.15 [K] = 15° [C] + 273.15 [K]

A(T = 10° C) = 0.4989d0
A(T = 20° C) = 0.5070d0

=> Internally the program calculates the value for A(T = 15° C) = (0.4989 + 0.5070)/2 = 0.50295.

The following interpolation formula is used:

A(T = x° C) = A(Ti) + slope * ('lattice-temperature' - 'constant-Centigrade-to-Kelvin' - Ti) =
            = A(Ti) + slope * ('lattice-temperature' - '273.15'                        - Ti) =

where slope = ( A(Ti+1) - A(Ti) ) / ( Ti+1 - Ti )
and it holds:  Ti  <  'lattice-temperature' - '273.15'Ti  <  Ti+1

Ti+1 and Ti are the closest temperature points above and below the specified temperature lattice-temperature.

If the lattice-temperature is smaller than the smallest value of A(T), the smallest A(T) value is taken.
If the lattice-temperature is larger   than the largest    value of A(T), the largest   A(T) value is taken.

The value of A always depends on temperature. This can only be switched off by specifying only one value of T and A(T) in the database or in the input file.

The values for T and A(T) that are specified in the database can be overwritten in the input file. For details, have a look at the input file keyword $buffer-constant-A(T).