# Find pH-dependent d13C of solution, in contact with CO2(g), d13C = 0
SOLUTION 1
 pH 4.5; C 1
REACTION
 NaOH; 1.4e-3 in 20
USER_GRAPH
 -head pH eps; -axis_titles pH eps_co2/TIC
 -axis_scale y_axis -8 1 1; -axis_scale x_axis 4.5 10 0.5
# Find d13C in solute species 1= CO2aq, 2= HCO3-, 3=CO3-2
 -start
# Define enrichments (eps) relative to g= CO2_gas from Clark and Fritz, 1997, p. 121 ...
  4 T = TK
  10 eps_1_g = 0.19 - 373 / T
  20 eps_2_g = -24.1 + 9.552e3 / T
  30 eps_3_g = -3.4 + 0.87e6 / T^2
 # Find carbonate species...
  40 co2 = mol("CO2")
  50 hco3 = mol("HCO3-")
  60 co3 = mol("CO3-2")
  70 tic = co2 + hco3 + co3
# and d13 of the species...
  80 d13g = 0
  90 d13tot = (co2 * (eps_1_g + d13g) + hco3 * (eps_2_g + d13g) +\
               co3 * (eps_3_g + d13g)) / tic
# chart...
  100 graph_x  -la("H+")
  110 graph_y -d13tot
 -end
END