[The_Simpsons]

Okey this is my thought. I have an espresso machine. On the side there is a thing you open once in a while to empty which contain solid compact powder of coffe residue. People have also used this to put in the earth together with plants to kill bugs, so appaerently it contains caffeine. Compared to the caffeine transferred to the cup, does anybody know roughly how much caffeine it contains?

Im thinking of either mix it with HCl to make caffeine.hcl salt(not sure if it is produced, maybe anybody here knows?) or just subliming it in order to extract caffeine, will these methods work with the amount of caffeine present in the coffe residue?

[AWK]

HPLC or UV are usually used

[The_Simpsons]

What is HPLC and UV (apart from ultraviolet radiation)?

[AWK]

HPLC - high pressure liquid chromatography

[Dude]

Interesting thought.

The cheapest way to be to check SciFinder Scholar at your local public university and type in key words similar to what you might do in a Google search to find academic work done on it.  I'm reasonably certain that this type of work has been done and published.  If not, this would be an interesting exercise if several different brewing methods were used and several different brands of coffee were used.

Some things to consider:

1.  If bugs are killed with used coffee grinds, is it really caffeine doing the killing?  Intuitively, one would speculate that most of the caffeine would have been extracted out of the coffee grinds after brewing since caffeine is highly water soluble.  However, I am sure that there is some caffeine left in the dried used grinds (I would guess a very small amount of the original concentration - estimate 1/1000th of the original caffeine content) in addition to many other compounds which may be somewhat toxic to certain bugs.

2.  Prior to the supercritical carbon dioxide decaffeination process, methylene chloride (dichloromethane) was used to decaffeinate coffee (a chemical that will remove primarily caffeine and leave the other ingredients in the coffee grinds).  Methylene chloride is widely available at labs, but probably isn't something you would be able to get at the hardware store. 

3.  Referring back to point 1, any analytical method that you use will first extract the caffeine from the grinds (unless some NIR approach is available using the grinds directly) and then measure the caffeine content using probably the methods listed by AWK, which is kind of reciprocal since the brewing process is similar to the sample pretreatment process.

[The_Simpsons]

maybe its because of the acidity of the grind that it kills bugs.

[billnotgatez]

I Know of several people who put spent coffee grinds in their compost and have heard of no negative effects.

[hmx9123]

I believe that caffeine is extracted from coffee commercially with supercritical CO₂.  Not that it really helps you in your quest.

Also, the soly of caffeine in water is 2.2g/100mL water at 25 C, and 67g/100mL at 100 C.  You can probably try to extract the caffeine using that and some other organic solvents as a liquid-liquid extraction in order to separate the caffeine from the tannins, or at least mostly so.  Might be interesting to compare the amount of caffeine in the leftovers that you described with regular coffee on a weight vs. weight basis.

[billnotgatez]

This may be of interest

http://www.chemicalforums.com/index.php?topic=7094.0