MATLAB: Fitting model to titration data

curve fittingMATLAB

I'm trying to fit a model of titration to experimental data. The function "calcpH" calculate a pH value for each experimental point and I'm trying to obtain the value of Ca (and eventually Ka) that fit best the model to the experimental values.
When I uncomment the lines of the fitting procedure I get the following error: 
Error using ajuste_listado_1>sseval
Too many output arguments.
Error in ajuste_listado_1>@(x)sseval(x,vbr,pHr)
Error in fminsearch (line 200)
fv(:,1) = funfcn(x,varargin{:});
What is going wrong ????
THANK YOU
CODE OF THE SCRIPT
clear
clc
pHr=[3.15,3.30,3.48,3.65,3.73,3.86,3.93,4.03,4.10,4.20,4.28,4.35,4.44,4.51,4.59,4.66,4.75,4.84,4.94,5.03,5.18,5.34,5.54,5.91,6.10,6.40,8.66,9.32,9.88,10.16,10.39,10.49,10.64,10.97,11.16,11.28,11.37,11.45,11.51,11.56,11.60,11.67];
vbr=[0.00 ,0.20 ,0.40 ,0.60 ,0.80 ,1.00 ,1.20 ,1.40 ,1.60 ,1.80 ,2.00 ,2.20 ,2.40 ,2.60 ,2.80 ,3.00 ,3.20 ,3.40 ,3.60 ,3.80 ,4.00 ,4.20 ,4.40 ,4.60 ,4.70 ,4.80 ,4.90 ,5.00 ,5.10 ,5.20 ,5.30 ,5.40 ,5.50 ,6.00 ,6.50 ,7.00 ,7.50 ,8.00 ,8.50 ,9.00 ,9.50 ,10.00];
Ka=1.8e-5;
Ca=0.01
Va=50;
Cb=0.1;
Vb=0:0.2:10;
Kw=1e-14;
% FITTING 
% fun=@(x)sseval(x,vbr,pHr);
% bestx = fminsearch(fun,x0)
% Ca = bestx(1);
x=zeros(1)
pH = calcpH(x,Ka,Ca,Kw,Cb,Va,vbr);
plot(vbr,pHr,'o',vbr,pH,'x')
title('Titration of acetic acid')
xlabel('Vol NaOH [mL]')
ylabel('pH')
function sseval(x,vbr,pHr)
Ca=x(1)
pH = calcpH(x,Ka,Ca,Kw,Cb,Va,vbr)
sse = sum((pHr - pH').^2)
end
function pH=calcpH(x,Ka,Ca,Kw,Cb,Va,vbr)
VT=Va+vbr;
for i=1:length(vbr)
syms xh
h=vpasolve(Cb*vbr(i)/VT(i)-(Ca*Va/VT(i))*Ka/(Ka+xh)-Kw/xh+xh==0,xh);
pH(i)= -log10(h(3));
end
end

Best Answer

Thank you very much
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