Question

A derivative of the polysaccharide chitosan is being utilized in a spray to treat severe wounds. The chitosan aids in quickly stopping the flow of blood by forming a gel at the wound site. The behavior of the gel is strongly dependent upon the activity of ionic species in the spray. In one process to form the spray, an aqueous solution containing 0.002 M NaCl and 0.006 M CaCl2 is mixed with a solid powder of the chitosan derivative. (a) What is the ionic strength of the precursor solution, before the chitosan is added? (b) Use the Debye-Huckel expression to calculate the activity coefficient of Na+ , Cl- , and Ca2+ in the precursor solution at 25o C.

Answer 1

a) ionic strength before the chitosan added can be calculated as

Ionic strength = 1/2 ( concentration of NaCl X number of Na+ X (charge on Na)^2 + concentration of NaCl X number of Cl- X (Charge on Cl)^2 + Conc of CaCl2 X number of Ca X( charge on Ca)^2 + conc of CaCl2 X number of Cl- X (Charge on Cl)^2)

Ionic strength = 1/2 (0.002 X 1 X (1)^2 + 0.002 X 1 x (-1)^2 + 0.006 X 1 X (2)^2 + 0.006 X 2 X (-1)^2)

Ionic strength = 1/2 ( 0.002 + 0.002 + 0.024 + 0.012) = 0.02

b) According to Debye Huckel expression

log γ = -0.509|z(+)z(-)|X (Ionic strength)^1/2

For Na+

log γ =-0.509 |(1) (0.02)^1/2 = -0.509 X 0.1414 = -0.0719

taking antilog

γ =0.8474

For Cl-

log γ =-0.509 |(-1) (0.02)^1/2 = -0.509 X 0.1414 = -0.0719

taking antilog

γ =0.8474

For Ca+2

log γ =-0.509 |(+2) (0.02)^1/2 = -0.509 X 2 X 0.1414 = -0.1438

taking antilog

γ =0.7181