Chromatography Forum

Hi there i am sruggling with pH , i search the net but not found information

pH of aqous solution of methylene blue 3.1x10^-5 M ?

I love to learn and I am very weak at these calculations and concentration , is there is book u recommend it to me that can help .

Thanks for ur help

A general chemistry book wiht a chapter on acid base equilibrium should do. Or an introductory analytical chemistry text will surely have it.

Is your question because you need to know an expected value when you measure a solution or is this an attempt to answer a question in a book?

For weak acids or bases you need to convert the concentration of stuff into the conc of hydrogen ions then work out the pH. For this you need the pKa or pKb value.

Easiest would be to measure the pH.
As you ask for a pretty low concentration and the component is an organic salt, I would expect to get a pH around 7.

I read many books but to find the pH for this complex compound is difficult .Yes it is easy for acid or base but this is complex contains lots of hydrogen ions goes into solution and struggle I tried but i would love to see explanation .

It is not exam , it is nothing just i want to learn as i see this question in the net but no answer

Methylene blue can be protinated http://pubs.acs.org/doi/abs/10.1021/ja01246a036. So, you would need a pKb - and I've not run into a value. There is a trick to this in that the methylene blue cation also forms dimers and trimers in solution and this chemistry apparently plays into the acid/base equilibrium. Note http://pubs.acs.org/doi/abs/10.1021/ja00725a005. (I don't have access to the full article from home, but this looks like it is worth digging up for more information.)

Another paper has some references to addition of acid to methylene blue that may be helpful: http://www.mtf.stuba.sk/docs/internetov ... pustko.pdf

None of the hydrogen atoms you see on the structure of methylene blue will enter into acid/base chemistry in water. They are sigma bonded hydrgens either in methyl groups or to aromatic rings.

What's the reason for trying to calculate it?