function [pHr, pHAdjustment] = realpH(pHa, temp, is)
% Apparent glass electrode pH is not the same as real pH for thermodynamic calculations.
%
% Given the experimental glass electrode measurement of pH, this function returns
% the pH to be used for thermodynamic calculations, `pHc = -log10[H+]`,
% by subtracting the effect of the ion atmosphere around H+ which
% reduces its activity coefficient below unity.
% See `p49 Alberty 2003`.
%
% USAGE:
%
%    [pHr, pHAdjustment] = realpH(pHa, temp, is)
%
% INPUTS:
%    pHa:             apparent pH, measured by glass electrode experimentally
%    temp:            experimentally measured temperature
%    is:              estimate of ionic strength
%
% OUTPUTS:
%    pHr:             real pH to be used for thermodynamic calculations
%    pHAdjustment:    adjustment to pH
%
% .. Author: -Ronan M.T. Fleming

gibbscoeff = 1.10708 - (1.54508*temp)/10^3 + (5.95584*temp^2)/10^6; % p48 Alberty

%Adjust pH using Extended Debye-Huckle equation
pHAdjustment = ( (gibbscoeff*(is^(0.5))) / (log(10)*(1+1.6*(is^(0.5)))) );

% fprintf('%s\t%f\n','pH adjustment',phAdjustment);
pHr = pHa - pHAdjustment;
