loops for if, ifelse, end

i have the script and i want to add three condition, not sure how to do it.

Kw = 1e-14;
Ka = 1.755388e-5;
pKa = 4.756;
%
Ca = 0.5;
Cb = 0.1;
Va = 100;
Vb = 0.1:0.1:1500;
%
Ma = (Ca * Va) / 1000;
%
Mb = (Cb .* Vb) ./ 1000;
%
for i = 1:length(Mb) 
M_excess = Ma - Mb(i);

this is where im stuck!!!!!!!!!

if M_excess > 0
Ma_final = (M_excess * 1000) ./ (Va + Vb(i)); 
Mb_final = (Mb(i) * 1000) ./ (Va + Vb(i));
Hplus = Ka * (Ma_final ./ Mb_final);
if M_excess < 0
OH = (M_excess * 1000 * (-1)) ./ (Va + Vb(i));
Hplus = Kw ./ OH;
if M_excess = 0
Hplus = Ka * ((Ma_final * 0.999999) ./ Mb_final); ./ (Va + Vb(i));
end
pH(i) = -log10(Hplus);
end

Answer 1

[Azzi Abdelmalek]

if M_excess > 0
  Ma_final = (M_excess * 1000) ./ (Va + Vb(i));
  Mb_final = (Mb(i) * 1000) ./ (Va + Vb(i));
  Hplus = Ka * (Ma_final ./ Mb_final);
elseif M_excess < 0
  OH = (M_excess * 1000 * (-1)) ./ (Va + Vb(i));
  Hplus = Kw ./ OH;
else
  Hplus = Ka * ((Ma_final * 0.999999) ./ Mb_final);
end
pH(i) = -log10(Hplus);

[harley]

thanks Azzi, still skips the calc when M_excess = 0. Any other thoughts?

[harley]

Alternatively if M_excess = 0 is there a way to tell it to use the pH and Vb calculated in the iteration?

[Azzi Abdelmalek]

Because M_excess is never equal to 0, you can check it by

any(Ma - Mb==0)   % you will find 0

[harley]

i see, i that because no array is created for M_excess? any thoughts on getting around this would be greatly appreciated, don't mind if i have to re write the script.

[harley]

if this helps it calc Hplus when Ma = Mb

[Azzi Abdelmalek]

In your case Ma is never equal to Mb

[harley]

thanks for your time Azzi, very new to matlab Ma = 0.05 in my Mb array i have Mb = 0.05 at row 10000? sorry i am having trouble understanding matlab logic.

[Azzi Abdelmalek]

Ok, First, it's not row 10000, but 5000

Mb(5000)

Now type

a=Mb(5000)

The result is 0.0500

Now try

a==0.05

You will find 0, which means they are not equals.

You can also try this

   0.3-0.2==0.1  % the result is false

To understand what is going on look at this link

http://www.mathworks.com/matlabcentral/answers/69-why-does-1-2-3-1-3-not-equal-zero

[harley]

sorry for the confusion, yes you are right row 5000. I kind of understand what's happening now, so do i need to specfic a tolerance? Thanks for your time again.

[Azzi Abdelmalek]

You should use some tolerence

   find(abs(Mb-Ma)<eps)

[harley]

Thanks Azzi how have been extremely helpful, how do i apply that 'find(abs(Mb-Ma)<eps)' to your edited script.

[Azzi Abdelmalek]

if abs(M_excess)<eps
  Hplus = Ka * ((Ma_final * 0.999999) ./ Mb_final);
elseif M_excess > 0
  Ma_final = (M_excess * 1000) ./ (Va + Vb(i));
  Mb_final = (Mb(i) * 1000) ./ (Va + Vb(i));
  Hplus = Ka * (Ma_final ./ Mb_final);
elseif M_excess < 0
  OH = (M_excess * 1000 * (-1)) ./ (Va + Vb(i));
  Hplus = Kw ./ OH;
end
pH(i) = -log10(Hplus);

[harley]

thank you very much, all working now.

[harley]

a minor question. How do i display the pH and Vb value at the point of abs(M_excess)<eps. I also want to plot the pH on figure 1.