Regarding levels:

• http://reference.wolfram.com/language/ref/Level.html
• http://reference.wolfram.com/language/guide/ExpressionStructure.html
• http://reference.wolfram.com/language/tutorial/Expressions.html (Look for the section on "Levels in Expressions")

In short, each expressions in Mathematica is ultimately a tree. A level is all of the sub-expressions at a particular depth in the tree (usually). You can use Level to extract a level (or levels). Just be careful, because the documentation can sometimes be "loose" with the term "level"--sometimes it means a literal Level and sometimes it's more like depth. Also be careful about negative levels, which count up from the bottom of the expression tree (so for unbalanced trees the sub-expressions at the same negative level won't be at the same depth). Anyway, it's too much info to put into a reply. You'll need to read the documentation and play around with examples.

I have checked the documentation but I am still not clear why ",2" is needed in Reverse[dataPairs1,2] I know it means reverse the elements at level 2 but what is level 2 ?

Also, in the same document I have

Plot[Vb[pH, Ca, Cb, pKa, pKb, Kw, Va], {pH, 2.9, 12.5}]

but again the axes are the "wrong" way round. Vb comes from

  Vb[pH_, Ca_, Cb_, pKa_, pKb_, Kw_, Va_] := 
   Va*((Ca/(1 + H[pH]/Ka[pKa])) - H[pH] + 
       Kw/H[pH])/((Cb/(1 + Kw/(H[pH]*Kb[pKb]))) + H[pH] - Kw/H[pH])

Is this enough or should I attach my document? Thank you.

Plot will always be arranged with the independent variable on the horizontal axis Blockquote

I suspected as much. Finding the inverse function is what I am effectively doing in a second document. The first and simpler document derives Vb from pH (i.e. input pH values, output Vb values). The second document does the opposite. I'll stick with generating a list of points ...

Life is too short for me even to consider your last suggestion - that would indeed be a desperate last resort.

Thanks again!

That's exactly what I wanted - many, many thanks. You haven't misinterpreted anything, it just looks strange because I only gave you the start of the data, In practice the pH values range from 2.9 to 12.5 and then it looks more sensible

Presumably, I can now modify ListPlot to do things like

• Adjust line and marker styles and colours
• Add axis titles
• Add gridlines
• Add a plot title
• Add a legend
• Change axis labels fonts and sizes

and anything else I can think of? If so, it will then just be a case of finding where all these options are in the documentation - shouldn't be too difficult.