Potassium was estimated from the formula 530.2*Salinity/1.80655/(1000-1.81578*Salinity/1.80655) where 1.80655 is the Salinity/Chlorinity ratio and 530.2 is the number of moles of K/Chlorinity -

Millero, F., 1974. Seawater as a multicomponent electrolyte solution, in The Sea, Vol. 5, E. Goldberg, ed., Wiley, pp. 3-80.

An original reference on potassium in seawater is:

Culkin, F., and Cox, R. A., 1966.  Sodium, potassium, magnesium, calcium and strontium in sea water.  Deep-Sea Res., 13: 789-804.