[ap90r]

Hello,
     I'm a grad student working in crop sciences and have been preparing an experiment to do with soil salinity. As part of my experiment, I'm comparing the effect of two different salts (sodium chloride and sodium sulfate) on plants in a greenhouse. To accurately compare the two, I need to calculate how much salt to add to reach isosmotic (solutions with equal osmotic pressure) solutions of both at several different concentrations. I think I can figure it out, but I'm stuck when it comes to measuring osmotic potential.

     It's been a while since I took college chemistry, but I broke open a chemistry textbook and relearned the van't Hoff equation. The problem I'm having is calculating the van't Hoff factor. From what I've found, the ideal solute that's completely water soluble would have a factor of how many ions it broken down into. So, sodium sulfate should have a van't Hoff factor of 3 (2 Na+ and one SO4- ion), and sodium chloride should have one of 2 (1 Na+ and 1 Cl-). However, the book also said that the ideal factor is not always accurate since a certain percentage of these ions form pairs and so they then count as a single particle.

    For my experiment, I need it to be as precise as possible, and although I've found a measured van't Hoff factor (not the ideal one but one actually measured out) for sodium chloride, I haven't been able to find any for sodium sulfate. I found somewhere where it said sulfates tend to have measured factors that are more different from their ideal factors - but no figures were given. So, good to know I'd screw up if I went with the ideal van't Hoff factor, but no help 

    So my question is, where could I find a measured van't Hoff factor for sodium sulfate? From my understanding, the actual non-ideal factors differ depending on temperature and molarity, so I guess it might not be that simple to just find one if I'm going to be using my own molar concentrations of sodium sulfate. Or might it be possible to calculate a non-ideal, adjusted van't Hoff factor for a certain concentration and temperature?

    Any help you could give me would be greatly appreciated. I'm planning on getting my experiment going next week, so if I'm unable to figure out isosmotic solutions for NaCl and Na2SO4, I might need to modify my experiment.


Thanks,
Adam

[Jorriss]

I don't know where there are data bases but you can make a quasi van't hoff factor from an equilibrium constant most likely.

[ap90r]

Do you know how I'd go about plugging the equilibrium constant into the van't Hoff factor? My guess would be to find out the concentration of sulfate ions and sodium ions, and then for whatever remained as sodium sulfate in solution would be counted as one particle. But I could very easily be wrong there.

[enahs]

Honestly, what I would do is just measure the Van't Hoff factor for the given conditions you expect.

I would do boiling point elevation, and freezing point depression, very accurately, and average the two. Do each experiment multiple times too. This can all be done in one day. Do proper controls (multiple times), etc.

Should be easy to find experimental setups and help with those on the internet or in books, but ask if you need help.


Using equilibrium is not going to work to well I would think. Ion pairs can very greatly based on temperature as well as other ions in solution; so experimental determination in similar conditions is probably the easiest and best way to go. 

[ap90r]

Thanks so much for the *delete me* I should have access to a lab and could easily do that.

Thanks again,
Adam