Amount of red bull to use to test heart rate of daphnia.?

I'm assuming you're doing this for a science class, so you will want to follow the scientific method and be as scientifically rigorous as you (reasonably) can.

On that point, are you required to use red bull? If you're not, and can easily obtain caffeine tablets, I'd suggest using them instead. Reasons being is that Red Bull contains many other substances other than caffeine, such as dyes, sugar, calcium, sodium, et cetera, so you can't be CERTAIN it's the caffeine providing any effect unless you ran controls on everything else in the red bull (which is totally impractical to do!). You almost completely eliminate that issue with caffeine pills/tablets, meaning your experiment is much more reliable and you can safely conclude that the observed result is actually due to the caffiene. Another advantage of using caffiene tablets is it'll simplify your dilution calculations, because you're not adding a solution with some concentration of caffiene to another solution, you're just dissolving a tablet in a certain volume of water.

Another thing to keep in mind is that you should ABSOLUTELY use controls! For Red Bull, I'd try and use at least a control with water, and if possible do controls on water with dissolved sodium, calcium, and sugar, since those are some things in Red Bull that wouldn't be too hard to obtain to use as controls... For caffiene pills, I'd look at what is in them (typically the only listed ingredients are a bit of calcium and caffiene), and use appropriate controls for that, and of course still use a control that's just water. Having at least one control is VERY important, because how do you know the change was due to something you did unless you've got something to compare it to?

Finally, since you don't know what concentrations will have what effect, I suggest using several logs of concentration. As an example, let's say we're using caffiene tablets that contain 200 mg of caffiene. Dissolve one in 100 mL of water, and you've got a solution with a concentration of 2 mg/mL or 2 g/L. So, your next solution should say, have a concentration of 20 mg/mL, and the next with 200 mg/mL and so on (obviously you can dissolve more than 1 tablet for higher concentrations). Since you have NO IDEA what the effect will be, you may want to start with a pretty low concentration, like 1 microgram per milliliter and go up from there (and don't go so high that's it is impractical for you to do, obviously). As a suggestion, for each concentration you could have more than 1 solution at that same concentration so as to increase the reliability of your results (I'd suggest 3, since that way you can even see if your trials are clustered together or if one is WAY different than the other 2).

You could even generate a dose-response curve (google if you don't know what this is--the expected shape would be sigmoidal for this I'm pretty sure) from the data! Things from that which would be interesting would be the lowest concentration you saw any effect over the control(s), the concentration at which half of the maximal effect is observed (for a sigmoidal curve, this is typically the inflection point of the graph), and the concentration at which everything dies, then the one at which everything dies pretty much instantly (assuming you reach it). For some basically all of those you'll just be able to say it was between concentration A and B for example (say, that it became lethal between 10 mg/mL and 100 mg/mL) and, but that's still pretty interesting!

Hope this helps. Obviously, you probably won't take all these suggestions into account, but hopefully they at least make you think!

I mean you should use enough to where the daphnia is basically swimming in the red bull. It wont die immediately, the caffeine intake isn't based upon how much you use, but how much is consumed by the daphnia. Usually people just add concentrated caffeine (like 15%), so I imagine red bull doesn't have more then that.

If you use alcohol to test slowing heart rate i think you can even go up to like 10%+ ethanol.

*edit - if for some reason they do die, you can try using less (I mean its a daphnia, they aren't expensive), you can figure this out by trial and error, but it may be the result of other additives in red bull. Try using other caffeine products if it doesn't work.

Full strength, in my opinion. I did this experiment freshman year of college and we used caffeine drops (I think over 10%). Red bull isn't going to have that much, so I would use full strength if you're going to do it. Good luck!

3 cans of Red Bull

Don't drink too much. Everything good goes to be bad for your health when it's too much