It depends on the nature of  accuracy you want. For rough measure you can do it by iodometry :

 H3AsO4 + 2 H+ + 2 I−    →  H3AsO3 + I2 + H2O  The liberated iodine can be extracted in organic solvent say by carbon tetrachloride or by indirect titration as in strong acid thiosulfate titration may not be possible. For removal  if interested you may write to me in my e-mail address

You can consult the work of X. Chris Le at the University of Alberta for As speciation analysis. Basically the determination of different oxidation states (esp.) in natural water systems need more advanced analytical techniques. Here is the link:

http://www.ualberta.ca/~xcle/publications.html

You can determine the species of As by ion chromatography.  They are essentially going to be anions.  Removal of Arsenite is easier than arsenate.  If you need more information, there are many review papers.  If you can specify you specific need, I may be able to help

I propose very easy and chipper method: you can use standard chemical  analysis method of As with  UV-VIS spectrometer when they use reduction of full containing As by iodide ion following formation of arsine and complexation with Ag -complexe.

1 step analysis without iodne- analysis just As(III)

 2 step analysis with reduction by iodine of As(V) to As(III)