function g=sb(x,t)
Vi=200;
v0=0.05;
ca0=7.72;
cb0=10.93;
na0=ca0*Vi;
fb0=0.05*cb0;
k=9e-5;
kc=1.08;
up=na0^2*x^2;
t=100;
down=na0*(1-x)*(fb0*t-na0*x);
g = kc*down - up;
