%
%	Calculate the pH of a Boric Acid Solution using the fsolve function - 
%   This subroutine contains the boric acid equations to be solved simultaneously
%
%	Variables are:  x1 = B(OH)3
%			        x2 = B(OH)4 -
%			        x3 = H3O +
%			        x4 = OH -
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function F = BAequation(X)

global Ka Kw CT
%
% Calculation of the difference functions
%
F(1,1) = -X(2)+X(3)-X(4);                   % electroneutrality
F(2,1) = X(1)+X(2)-CT;                      % mass balance
F(3,1) = X(2)*X(3)-Ka*X(1);                % boric acid equilibrium
F(4,1) = X(3)*X(4)-Kw;                     % water equilibrium