you may visit the following link

http://www.seafriends.org.nz/oceano/seawater.htm

Also see the attached file

This is really a question of carbonate equilibria.  Ocean water is pretty well buffered, but increasing atmospheric CO2 can cause a decline in the pH of the ocean.  Perhaps the best discussion of this I have found is in the following articles:

Freely et al., 2004, Impact of anthropogenic CO2 on the CaCO3 system in the oceans: Science, v. 305, p. 363-366.

Heogh-Guldberg et al., 2007, Coral reefs under rapid climate change and ocean acidification: Science, v. 318, 1737-1742.

Royal Society, 2005, Ocean acidification due to increasing atmospheric carbon dioxide. 

I've also published an article about measuring alkalinity - might not be of value, but here it is anyway:

https://www.researchgate.net/publication/201169276_Understanding_carbonate_equilibria_by_measuring_alkalinity_in_experimental_and_natural_systems

I've attached the Royal Society article.  I wouldn't think that variation in salinity would make too much of a difference.  The bigger impact will be mixing of river water with seawater in the estuary, assuming the pH of the river water is likely lower than that of seawater.  These articles, combined with the recommendations of Prof. Karbassi, should really get you started.  Carbonate equilibria is a deceptively complex topic!

Article Understanding carbonate equilibria by measuring alkalinity i...