finding values from arrays

hi, trying to display some values but am not sure how to go about it. I want to do is; find Vb at half of Vb_ev and then find the corresponding pH value at that point. see last 2 lines. any help will be appreciated.

Ka = 1.755388e-5;
    Ca = 0.5;
    Cb = 0.1;
    Va = 100;
    Vb = 0.1:0.1:1200;
    Ma = (Ca * Va) / 1000;
    Mb = (Cb .* Vb) ./ 1000;
    for i = 1:length(Mb)
      M_excess = Ma - Mb(i);
      if abs(M_excess)<eps
          Hplus = Ka * ((Ma_final * 0.999999) ./ Mb_final);
          Vb_ev = Vb(i); 
          pH_ev = pH(end); 
      else
          Ma_final = (M_excess * 1000) ./ (Va + Vb(i));
          Mb_final = (Mb(i) * 1000) ./ (Va + Vb(i));
          Hplus = Ka * (Ma_final ./ Mb_final);
      end
      pH(i) = -log10(Hplus);
    end
            % 
            Vb_Mid_Pt1 = find half of Vb_ev
            pH_Mid_Pt1 = i want to find pH at Vb_Mid_Pt1;