Introduction and Methods

Introduction

Chemical Composition and Reactivity

Methods and Techniques for Dealing with the Chemistry of Seawater

 


Introduction

Breathe in, breathe out. Like a giant lung, oceans absorb vast amounts of carbon dioxide (CO2) from the atmosphere, and release it once again as cold water currents reach warmer areas of the globe. Indeed, CO2 solubility varies with temperature, together with other factors such as salinity and pressure.

Chemically speaking, why does seawater so readily absorb carbon dioxide, thereby buffering the anthropogenic emission of this gas?

The oceans cover about 71% of the earth’s surface and gaseous exchange occurs through the ocean’s surface. But the answer to this question lies deeper, in what is a widely underestimated fact: the pH (acidity level) of seawater is substantially alkaline, ranging from 8.0 to 8.7. This means that the balance of positive and negative ions is reached through a higher concentration of hydroxide ions (OH -) compared to hydrogen ions (H+).

Having a pH value greater than 7 enables seawater to react with and dissolve huge amounts of CO2, absorbing atmospheric excess and thus affecting its concentration. However, there is a reason behind the alkalinity of seawater, its current chemical composition. While different salts are present in seawater, the primary one is sodium chloride. As with any salt, when it dissolves in water, positive charges (cations) and negative charges (anions) are generated.


Chemical Composition and Reactivity

Let’s explore the mean composition of seawater in greater detail: summing up all the positive charges (Na+, K+, Mg2+, Ca2+, Sr2+) one obtains 605.85 mmol/Kg of solution. Carrying out the same operation for negative charges (Cl -, Br -, F -, SO42-) the result is slightly less: 603.25; 2.50 millimoles are clearly missing! As with all ionic solutions, seawater must obey the law of electro neutrality, so evidently some negative charges (anions) have been ruled out: they are indeed HCO3 -, to a minor extent OH and finally, to a far lesser extent, CO32-. The last three ions all react with atmospheric CO2, and are therefore designated as reactive. On the contrary, the former cations and anions are classified as spectator ions (see table 1.1). Reactive ions have an active role in chemical equilibria, as shown in the same table.

The presence of OH (hydroxide ions) is the reason for a pH>7. Their concentration (due to the logarithmic nature of pH scale) is at pH = 8.0 equal to 0.001 mmol/L (in pure water). Under the same conditions, the H+ ion concentration is 100 times less. OH ions alone are insufficient to fill the gap: other negative ions are required; these are mainly HCO3 ions and also some CO32- .

This has enormous repercussions on the equilibrium of CO2 between the atmosphere and oceans. Compared to the atmosphere, which contains around 850 Gt (gigatons) of carbon (in the form of CO2), the oceans hold 38,000 Gt of carbon. That’s nearly 45 times more.

So when we talk about CO2 ppm in the atmosphere that is only the “top of the iceberg”! CO2 dissolves in seawater like O2 and N2. However, being a reactive gas, there is an almost immediate reaction with the water itself (N2 and O2 do not) yielding HCO3 and CO32-. After completion of these reactions, yet a third slowly takes place (one which is nearly always disregarded): the formation of solid calcium carbonate, CaCO3.

Table 1.1 Seawater composition: spectator and reactive species


 

 

 

 

 

 

 

 

 

 

 

 

 

In chemistry, this is known as precipitation. CaCO3 usually has a calcite structure; aragonite, the other polymorphic structure, is slightly more soluble. Seawater is oversaturated, both in terms of calcite and aragonite, due to its relatively high Ca++ ion concentration (10.28 mmol/Kg-solution). However, this reaction requires nucleation and the growth of crystal nuclei, and is usually sluggish (it may speed up in the cells of calcifying organisms like invertebrates). In other words, it is a heterogeneous reaction between a liquid phase and a solid one.

The destiny of this salt is to eventually sedimentate on the ocean floor (if very deep, it may fail to reach the bottom, dissociating again into ions due to the extremely high pressure, and recycle). In any case, CO2 removed from the atmosphere will eventually form limestone.


Methods and Techniques for Dealing with the Chemistry of Seawater

Every year there are hundreds of publications and articles on this topic: some fearing ocean 'acidification' (a lowering of pH values, remaining in the alkaline range) and the consequence on calcifying organisms, and some stressing a possible increase in the ocean's ability to uptake anthropogenic CO2. Indeed, several groups of scientists have employed computer-aided modelling and complex models to simulate the chemical/physical behaviour of ocean water and predict the effects of man-made activities such as fossil burning.

These models cover a host of variables, and in the absence of deep insight into the structure of the complex codes used, one has no choice but to take the results at face value.

Obviously, the effects of temperature, salinity and pressure on seawater are accounted for, but the codes are far from user-friendly, and even other scientists are unable to draw clear conclusions regarding the behaviour of seawater and related chemical equations.

In this context, this handbook offers simple routines with clearly described codes for solving the various chemical equilibria in seawater, nothing concealed and everything accurately referenced. Anyone with a little chemical knowledge will be able to follow them. The routines and codes, which are also present on my website, can be downloaded and modified.

The aim of this book is to examine the chemical reactions that occur in seawater, using a simple and intuitive computer approach. Despite quite frequent discussion and examination in scientific papers and the press of the relationship between ocean chemistry and environmental issues (such as CO2 uptake, ocean acidification and carbonate sediment), the basic underlying chemistry is poorly understood.

On the other hand, with computer codes just a few hundred lines long, basic chemistry can offer a variety of simple and extremely interesting results for anybody curious about reactions in seawater. Well, let’s not oversimplify! Seawater solution has a high ionic strength (high density of oppositely charged ions), a fact that hinders the direct usage of equilibrium constants taken from standard thermodynamic databases. For the same reason, the temperature, pressure and salinity dependence of the above constants is not at all straightforward and must be carefully modelled.

Consequently, simple chemical equilibrium constants are of limited use in the numerical solution of equilibria. On the contrary, employing the parametrisation taken from literature, and using codes for the resolution of simultaneous reactions, results can be obtained in a matter of seconds.

Before tackling seawater reactions, some introductory concepts need to be clarified, like how a chemical reaction evolves in time (kinetic systems) and how one or multiple simultaneous reactions can reach a state of equilibrium (equilibrium calculations).

Instead of using the classical set of differential equations which describe the time-evolution of the kinetic systems, a computer aided, iterative procedure will be applied.

Using the same logical approach, simultaneous equilibria will be solved by iterative procedures, rather than through a complex mathematical approach.

The routines employ simple, basic language that’s easy to use, whether directly or transferred into other languages. The specific basic language is JustBASIC, and it can be downloaded for free at www.justbasic.com. I am aware that much more sophisticated languages exist, but the performances of JustBASIC are perfectly aligned with the needs and difficulty level of the algorithms in this book.