Equilibrio en el agua de mar

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Chemical Equilibrium in Seawater
R. M. PYTKOWICZ, E. ATLAS, and C. H. CULBERSON School of Oceanography, Oregon State University, Corvallis, Ore. 97331

Downloaded by 200.77.100.234 on October 19, 2009 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch001

The concept of chemical equilibrium can be applied to the oceans in three ways; it enters the study ofthe geochemical control of the oceanic composition, for fast reactions equilibrium constants are used to calculate the concentrations of species present in seawater, and for slow reactions departures from equilibrium are useful for kinetic studies. Seawater differs from the solutions that are usually examined by chemists because of the large number of solutes and, at times, of suspended particlesthat are present in the oceans. Also, one must consider the concurrent effects of purely chemical, hydrographic, biological, and geological processes upon the composition of seawater. One must consider as well the gravitational field and pressure, temperature, and compositional gradients that result from the extent and depth of the oceans. The study of the equilibrium chemistry of the oceans isfacilitated because the major ions are present in almost constant proportions. This makes seawater an ionic medium which can be characterized by one compositional parameter, the chlorinity or the salinity. The constancy of the relative composition is not always present in estuaries because river water, which mixes with seawater, has its major ions in proportions which differ from those in seawater.Still, it will be shown that equilibrium data obtained for seawater can often be applied to estuarine waters. In this work we will first examine briefly the composition of seawater. Then, we will outline some major aspects of equilibria as applied to the oceans. Next, we will consider to what extent these equilibrium considerations are relevant to estuaries and finally, we will examine sometopics on the control of the oceanic composition. We will not attempt a compre1

In Marine Chemistry in the Coastal Environment; Church, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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MARINE CHEMISTRY

h e n s i v e c o v e r a g e of the s u b j e c t but w i l l e m p h a s i z e b a s i c c o n cepts a n d methods. T h e C o m p o s i t i o n of S e a w a t er A l l the n a t u r a l l y o c c u r r i n g e l e m e n t s a n d m a n y of t h e i r c o m p o u n d s f i n d t h e i r w a y i n t o the o c e a n s t h r o u g h r i v e r s , g r o u n d w a t e r s , a e r i a l t r a n s p o r t , a n d s u b m a r i n e v o l c a n i s m . A few m a j o r c o n s t i t u e n t s , shown i n T a b l e I, a c c o u n t f o r o v e r 90% Y weight o fthe s o l u t e s present i n seawater ( 1 ) .
b

Downloaded by 200.77.100.234 on October 19, 2009 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch001

T a b l e I.

M a j o r C o n s t i t u e n t s of S e a w a t e r of 34. 3%o S a l i n i t y (19%o C h l o r i n i t y )

Constituent Cl" Na
+

ppm

Constituent Ca K
+

ppm

Constituent B(OH) SrF"
2 +

ppm

18,971 10,555
2

2+

403.9
391

25. 6
7.7 1. 3

so
Mg

4

"

2,657 1,268

HCO ~ Br"

142 65.9

T h e s e m a j o r constituents a r e p r e s e n t i n a l m o s t constant p r o p o r t i o n s i n the o c e a n s (1_,2) i n d i c a t i n g that the m i x i n g t i m e of the o c e a n s , w h i c h i s of the o r d e r of 1,000 y e a r s , i s fast r e la t i v e to the i n p u t r a t e s a n d to the r e a c t i v i t y of the c o n s t i t u e n t s (3.-5). T h e c o n s t a n t r e l a t i v e c o m p o s i t i o n a l s o l e d to the d e f i n i t i o n s of chlorinity and salinity which a r e presented i n standard oceanographic texts, e. g. , R i l e y a n d C h e s t e r (6). It i s i m p o r t a n t to The defined r e a l i z e that...