Methylene blue used as funguscide in cichlid reproduction - concentration measurement?

I have an aqueous solution of methylene blue. How do I determine the concentration of this solution, so I can duplicate it? 




The concentration of an aqueous solution of methylene blue can be determined using a UV-VIS spectrometer. 


Measure the absorption at $\lambda = 664\, \mathrm{nm}$, use the molar extiction coefficient of $95000\, \mathrm{dm^{3}\cdot mol^{-1}\cdot cm^{-1}}$ 
([Reference](http://www.clays.org/journal/archive/volume%2036/36-3-214.pdf)) and calculate the concentration using the **Lambert-Beer** equation.


 3

If in addition to a UV-VIS spectrometer you also have access to a fluorimeter, I would recommend using fluorescence instead of absorbance. It will be more sensitive and less prone to interference. However, fluorimeters are more expensive and harder to come by than UV-VIS spectrometers which measure absorbance only.


When I measured methylene blue by fluorescence, I used 630 nm for excitation and 700 nm for emission. I found fluorescence to be much better than absorbance at 670 nm for the same samples, but the absorbance measurement also worked. The fluorescence properties of methylene blue are less well-known than the absorbance. [Other papers](http://www.ncbi.nlm.nih.gov/pubmed/20117811) often use different values for excitation/emission than what I used, but if you have a fluorimeter you can determine what's best for you.


 2