using ethanol 50% then 70%??

[alone]

When we prepare a smear of some parasites sample >> why do we add 50% ethanol then 70% ethanole ???
what is the purpose of using ethanole with different concentration ???

[biohazard]

That's the way you dehydrate the sample. The protocols vary widely, but the idea of different concentrations is to remove water from the sample (or add water and remove ethanol/methanol/xylene etc if you want to rehydrate and do all this in reverse order).

If you used 70% ethanol straight away, it gets diluted too much and there's still water in the sample. But if you first use 50% and then 70%, you get much more water removed. And often the concentrations go as high as to 95% or even 100% in applications where virtually all the water needs be removed.

What I'm not sure, though, is why you must use different concentrations and not just several times of the highest concentration. This probably has to do with high concentrations of fixative (e.g. 96% ethanol) damaging the sample by dissolving lipid membranes of the cells but if this is done gradually, no such damage takes place. Initial lower concentrations may also facilitate the better infiltration of the fixative into the sample specimen.

I'll get back to this if I manage to figure out the exact reason :P