Aquatic Chemistry Introduction & Conservation Principles

Transcription

1 Aquatic Chemistry Introduction & Conservation Principles A survey of the chemical composition of natural waters, elements, compounds, dissolved and particulate components. Please read Chapter 1 in the book.

2 Chemical Composition of Natural Waters Dissolved Components Major Ions in Freshwater and Sea Water: 1. Cations: Na +,K +,Ca 2+,Mg Anions: HCO 3,CO= 3,Cl,SO = 4 3. Neutral Species: SiO 2,orH 4 SiO 4 Minor Ions and Trace Compounds: 1. Sr 2+,Li +,Rb +,Fe (II), Fe (III),... 1

3 2. F,Br, Al(OH) 4, Zn 2+,Cd 2+,Pb 2+, As(Arsenate & Arsenite), Se(Selenate & Selenite), Cr Organic Molecules: Natural Compounds (Amino Acids; low molecular weight organic acids,...) Particulate Matter Organic Matter: Biogenic debris and various organisms. Thousands/millions/... of organic compounds

4 C, H, O, N, S, P + oligo- Organic Matter Composition: elements (metals in particular) Environmental Particles: Minerals such as clays and oxides resulting from the physical weathering of rocks. Particles range in size from nano-meter size to 100 of µm or more. Chemical Analyses Analytical chemistry is a cornerstone of aquatic chemistry. However, it will not be discussed here, there is another class devoted to that topic: Environmental Analytical Chemistry.

5 Distinguishing between particulate matter and dissolved compounds: Separation method most often: filtration. It is controversial method, that can lead to artifacts, it is only an operational definition. Chemical Accounting Mass Balance Equations Account the various chemical species of the elements of interest, and also the solvent: water. In 1 liter (L) of water a T = 25 o C 55.4 moles of H 2 O Because the number of moles of the solvent water are always large compare to the number of moles of the dissolved

6 species, we shall thereafter use a more convenient reference frame for describing the chemical composition of water. Units We need at least one currency. Ideally, it should be independent of temperature and pressure! Molality moles per kilogram of solvent (mol.kg 1 w ). This is the only legal unit, but it is rarely used to report analytical data. All thermodynamic calculations however should be performed on this scale. Formality moles per kilogram of solution (mol.kg 1 s ) Molarity moles per liter (mol.l 1, or M). The most practical and used units. Because one works with volumetric

7 flasks in the laboratory, this unit is used extensively. The interactions of the solutes (ions) with the solvent (water) makes it different from the molality unit. Also the volume of a certain mass of water will change with temperature (find variation of density with respect to temperature). Weight or Volume fractions : %, per mil, ppm, ppb, ppt unit-less! Convenient for solids primarily, but used again extensively by analytical chemists to express aqueous concentrations, primarily ppm, ppb, and ppt for trace elements. Quite often used as equivalent to milli-g/l (mg/l), micro-g (µg/l), and nano-g/l (ng/l), but strictly speaking they are not since one needs to know the density of the solution (not the solvent, water) at the corresponding temperature. The difference is usually small, smaller than

8 the analytical error. Therefore, ppm are almost equivalent to mg/l, at least for dilute solutions. The same holds with respect to ppb and mg/l, ppt and ng/l. BUT these units can be quite confusing! Without specifying the species/element considered it is useless. Example: 3 mg/l of phosphate! Does this refer to the mass of P or to the mass of PO 4? Also, because it involves the mass of each species it is a lot less practical to look at stoichiometric relationships between species.

9 THE TABLEAU METHOD Objectives: Define a chemical basis made of components to describe the chemical reactions occurring in aquatic systems. Components They need to be independent, similar to a vectorial basis in math. We want to describe our chemical system using this reference frame. Elements (Na, K, Ca, Mg, S, O, H, C, Cl,...) could form an appropriate basis, but this will not be always an easy basis to use. We need to choose the best components. Reactions They must also be independent, i.e., the stoichiometry of one reaction can not be expressed as the linear combination of two or more other reactions. 2

10 Rule As a rule of thumb, a necessary but not sufficient condition is that the number of components be equal to the number of species minus the number of independent reactions. Advantages: Using this method, we shall transform our chemical problem into a mathematical set of equations that can be solved by a numerical scheme or by simple approximations. Respecting the notion of basis, i.e., independent components, we can modify easily the basis, swap components, using simple linear algebra principles.

11 Rules for Selecting Components In dilute solutions the number of moles of water is large compared to the number of moles of solutes, therefore it is best to always choose H 2 O as a component. However, since it entails solving a trivial equation we shall omit it from the tableaux after getting used to the notation. The proton H + needs to be selected as a component systematically. As we shall see, the proton conservation equation will be a key expression. All chemical species will be expressed as a linear combination of the components species: The Stoichiometry of the Chemical Reactions.

12 Each chemical species will have a unique decomposition on the components basis. Electroneutrality All aqueous solutions are electro-neutral, i.e., they do not carry any net electrical charge, although they are composed of charge species (ions). The electro-neutrality equation expresses the balance of positive and negative charges: z i m i =0 with z i being the charge of the ion i, and m i its concentration. By making the appropriate choice of components we can express this equation directly in the tableau. One just needs to select the neutral species.