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 (I) = 1/2(0.002 x 1^2 + 0.002 x 1^2 + 0.006 x 2^2 + 0.012 x 1^2)

                                = 0.02 M

(b) Activity coefficient for,

log(y(Na+)) = -0.51 x 1^1 x sq.rt.(0.02)/1+3.3 x 0.45 x sq.rt.(0.02)

y(Na+] = 0.872    

log(y(Cl-)) = -0.51 x 1^1 x sq.rt.(0.02)/1+3.3 x 0.3 x sq.rt.(0.02)

y(Cl-) = 0.865   

log(y(Ca2+)) = -0.51 x 2^1 x sq.rt.(0.02)/1+3.3 x 0.6 x sq.rt.(0.02)

y(Ca2+) = 0.595