Question

The following data on chlorine dioxide added as a disinfectant to water

Time (minutes)	Residual Concentration (mg/L)
5	0.90
10	0.88
20	0.72
25	0.61
60	0.28
235	0.08

Determine:  

a) The decay rate, k* of chlorine dioxide in this water

b) The initial dose of disinfectant C_o that pathogens in the water would experience

c) The immediate demand C_ID for chlorine dioxide if 3.0 mg/L is the dosage.

Answer 1

Ans a) Since, chlorine dioxide follows second order kinetics, to determine rate constant plot 1/C vs time plot as shown

T (min)	1/C
5	1.11
10	1.13
20	1.39
25	1.64
60	3.57
235	12.5

Now, rate constant (k*) = slope of line = (y2 - y1) /(x2 - x1)

=> k* = (12.5 - 3.57) / (235 - 60)

=> k* = 0.051 L / mg min

Ans b) To determine initial concentration extend the graph till it intersect y axis. The above graph intersect y axis at 1.10. So, at time equal to zero, 1/Co = 1.10

=> Co = 1/1.10 = 0.909 mg/L  

Ans c) We know,

Immediate demand ()= Dosage + Residual chlorine

To determine residual chlorine plot concentration vs time plot,

  

From concentration vs time plot we can see residual concentration (chlorine left) = 0.08 mg/l

=> = 3 + 0.08 = 3.08 mg/L