# Find pH-dependent enrichment of 13C in CO2 gas compared with solution
SOLUTION 1
 pH 4.5; C 1
REACTION
 NaOH; 1.4e-3 in 20
USER_GRAPH
 -axis_titles pH eps_CO2/TIC
 -axis_scale y_axis -8 1 1; -axis_scale x_axis 4.5 10 0.5
 -head pH eps_CO2/TIC
 -start
# Define alpha's from Clark and Fritz, 1997, p. 121 ...
  4 T = TK
  10 aa_1_2 = exp(24.29e-3 - 9.925 / T)
  20 aa_3_2 = exp(20.7e-3 - 9.552 / T + 870 / T^2)
  30 aa_cc_2 = exp(26.561e-3 - 17.2183 / T + 2988.0 / T^2)
  32 aa_0_2 = exp(24.1e-3 - 9.552 / T)
# Find H12CO3- and H13CO3-...
  50 aH = act("H+")
  60 K1 = aH * act("HCO3-") / act("CO2") / act("H2O")
  70 K2 = aH * act("CO3-2") / act("HCO3-")
  80 m12C = (tot("C(4)")) / (aH/K1 + 1 +  K2/aH)
  82 Rst = 0.011237
  90 m13C = Rst * 1e-3 / (aa_1_2 * aH/K1 + 1 + aa_3_2 * K2/aH)
# the ratio 13C/12C in HCO3-...
  100 R2 = m13C / m12C
# Fractionate 13C into CO2(g)...
  110 R_co2 = aa_0_2 * R2
# Find d13C in CO2(g), equals eps with d13C = 0 in solution...
  120 d13C_co2 = 1e3 * (R_co2 / Rst - 1)
# and graph...
  130 graph_x -la("H+")
  140 graph_y d13C_co2
 -end
END