geothermal resource potential of the safford-san simon basin, arizona

APPENDIX 2
.pH· Correction for·· the·· Silica Geothermometers
Dissolved silica (Si(OH)4 concentrations in ground water are largely
temperature dependent.
Quartz or chalcedony (Si02) is dissolved in water
to form the Si(0H)4 species:
Equation 1
However, when pH is high, the silica analyzed in the laboratory includes
dissociated silicic acid:
Equation 2
Si(OH)4 = SiO(OH)a + H+
The~~fore,
excess silica introduced by ionization or dissociation of
silicic acid must be subtracted from analyzed silica before geothermometer
calculations are performed.
Such a correction may be called a pH correction
since the ionization of silicic acid is dependent largely upon pH.
The equilibrium constant (K)
for the dissociation of Si(OH) 4 in
water is 10- 9 • 7
at 2S o C (29S o K):
Equation 3
Equation 3 reduces to:
Equation 4
-9.70 = 10g[SiO(OH)3J + 10g&+] - 10g [Si(OH)J
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