Where in the code, and how should I define y? (A clearer presentation of the question)

Hi all, this serves to present the question in a detailed manner. In the attached code, I am trying to solve 7 ODEs,

dydt(1) ... dydt(7).

These ODEs have variable parameters:

CH, CSO2_bulkslurry , E, NSO2, CCO2_bulkslurry, E_CO2, NCO2, CSO3, RCaSO3 and RCaCO3.

In turn, these variable parameters consist of constant parameters and variables. Of which the constant parameters are:

V_Headspace F CSO2_in R Temp HSO2 kga kLa DCa2 DSO2 kLa_CO2 HCO2 DCO2 KSPCaSO3 BETCaSO3 ktot BETCaCO3 MWCaCO3 Kad  KCO2 KHCO3 KSO2 KHSO3 Kw  CH0

The seven variables are

SO2_g, CO2_g, S_total, C_total, Ca_total, CCaCO3 and CCaSO3.

All I have to do is to solve the 7 ODEs over a period of 183000 seconds.

The equation to evaluate one of the variable parameters, CH ,is rather complicated, and requires fsolve.

CH = fsolve(@(CH) CH + 2.* Ca_total - ((S_total.* KSO2.*CH)/(CH^2 + KSO2.*CH + KSO2.*KHSO3))- 2.*((S_total.*KSO2.*KHSO3)/(CH^2 
                + KSO2.*CH + KSO2.*KHSO3))-((C_total.*KCO2.*CH)/(CH^2 + KCO2.*CH + KCO2.*KHCO3))- 2.*((C_total.*KCO2.*KHCO3)/(CH^2 + KCO2.*CH + KCO2.*KHCO3))- Kw/CH, CH0);

As a result, I thought of just using a vector of experimental values.

CH = [7.41E-06;9.33E-06;1.20E-05;1.05E-05;1.74E-05;3.72E-05;3.55E-05;1.00E-04;4.07E-02;2.45E-01; 6.17E-01;1.32E+00;2.29E+00;2.34E+00;2.40E+00;1.82E+00;1.38E+00;2.09E+00;1.82E+00;1.58E+00; 2.29E+00;1.62E+00;1.12E+00;8.91E-01;8.51E-01;7.59E-01;8.71E-01;1.12E+00;1.00E+00;8.51E-01];

I hope I unpacked the question sufficiently.

When I run the attached code, I get the error message

Undefined function or variable 'y'.

Error in eulerian1 (line 47)
y = y0 + odesfun(t,y,Parameter).'*delta;

Which alerts me to define y. My question is where in the code, and how should I define y?