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Ch. 5. ACTIVITY COEFFICENTS OF DISSOLVED SPECIES. 5-1. Introduction What is activity of a dissolved species? Effective concentration Equivalent to the concentration acting in effect Why do we need activity (or have activity)? Interactions among the dissolved matter

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ch 5 activity coefficents of dissolved species
Ch. 5. ACTIVITY COEFFICENTS OF DISSOLVED SPECIES
  • 5-1. Introduction
    • What is activity of a dissolved species?
      • Effective concentration
      • Equivalent to the concentration acting in effect
    • Why do we need activity (or have activity)?
      • Interactions among the dissolved matter
      • Interference among the dissolved matter
      •  causes partial influences on the solution properties
    • Where can we observe the activity effect?
      • Boiling point increase, freezing point decrease
      • Conductivity
      • Others
slide2
5-2. Activity Coefficient & Ionic Strength
    • Activity coefficient: A function of the ionic strength of the solution
      • ai = gimi
    • Ionic strength: A measure of the ionic characteristics of the solution
      • Lewis & Randall (1921)
      • I = ½ S zi2mi
      • Examples
        • Mono-monovalent salts: KCl I=mKCl
        • Mono-divalent salts: K2SO4  I=3mK2SO4
        • Di-divalent salts: CaSO4  I = 4mCaSO4
slide3
Approximate estimation of I from TDS
    • Eqn (4.4) to (4.6) on p.124
  • Approximate estimation of I from SpC
    • Eqn (4.7) to (4.9) on p.124
  • Otherwise?
    • Should be calculated from the chemical composition
slide4
5-3. Mean Ion-Activity Coefficients
    • The coefficients measured for a solutions which is due to the net effect of both cations and anions
      • g± =[gn+gn-]1/n , n = (n+ + n-)
    • Fig. 4.1 on p.125
    • McInnes convention (McInnes 1919)
      • g±KCl = gK = gCl
      • Obtain gK and gCl fromg±KCl
      • Then other gusing the above relation
    • Example 4.1 on p.127
slide5
5-4. Theoretical Calculation of the Activity Coefficients

Debye-Hückel limiting law (DHLL)

When I <0.001: eqn (4.30) on p.129

Extended Debye-Hückel equation (EDHE)

When I<0.1: eqn (4.28) on p.128

A=1.824928*106ro1/2(eT)-3/2, 0.5092 (at 25C)

B=50.3(eT)-1/2, 0.3283 (at 25C)

Table 4.1 for effective ionic radii on p.130

Fig. 4.3 on p.132

Other equations for higher I

Davies et al.; eqn (4.31) on p.132

Trusdell & Jones (1974)

Bronsted-Guggenheim-Scatchard specific ion interaction theory (SIT) equaton: eqn (4.32) p.133

Fig. 4.4. p.135.

Pitzer model; eqn (4.49) p.138

slide6
5-5. Limitation of Debye-Huckel Theory

All interactions are not purely ionic

Ions are not point charges

Ion size varies with I

Ions do interact w/ other ions and even with the same species

slide7
5-6. Activity Coefficients of Molecular Species

In most cases, approximately g=1

Generally follow Setchenoweqn (Lewis & Randall, 1961)

Log gi = KiI, where Ki = 0.02 ~ 0.23

Table 4.5 on p.144.