Potassium (K)

Atomic number: 19
Atomic weight: 39.0983
Average concentration in ocean: 10.2 mmol/kg
Residence time: 12,000,000 years

Distribution in ocean
Potassium is the sixth most abundant element dissolved in seawater.  It has a conservative distribution that varies only due to differences in evaporation or precipitation in the water that sank to form intermediate or deep waters in the sea (data).   Concentrations are higher in the Atlantic because that ocean is saltier

Potassium is present in seawater as the K+ ion.

Residence time
The sodium residence time is from Tracers in the Sea (Broecker, W. S. and Peng, T.-H., 1982.  Eldigo Press, Palisades, NY).  It was calculated from the global average river input.

Potassium can be determined gravimetrically as potassium tetraphenylborate or by atomic absorption spectrometry.