You need to also know either the alkalinity or dissolved inorganic C. You can approximate the alkalinity if you know the salinity and temperature. For example:

http://cdiac.ornl.gov/ftp/oceans/KitackLee_Alk_Climatology/GRL_Surface_alk.pdf

If you know the pH and can estimate the alkalinity from salinity and temperature, then you can use CO2SYS to solve for the pCO2 and other properties of the CO2 system. Here is a link to download CO2SYS:

http://cdiac.ornl.gov/oceans/co2rprt.html

Here is a link with another Excel version of CO2SYS:

http://www.ecy.wa.gov/programs/eap/models.html

Thans so much for Greg Pelletier! I will read these data! I found J. F. McClendon determined the pH in seawater which was close to the predicted value from CO2-H2CO3 Model during pH in 7.5~8.8. They also gave the equation as logC(CO32-(T))=-7.30+0.81pH. Could you find this original paper or other similar literatures?

thank u so much Greg Pelletier sir