I'm no expert on IVing drugs, but from a common sense perspective, shooting lemon juice just seems like a disaster, or at least infection, waiting to happen.
Alprazolam is not water soluble, so creating a safe and effective solution is going to be nearly impossible. It's only slightly soluble in ethanol, which isn't a good solvent for injections, especially since you would have to utilize such a highly concentrated ethanol to even hope to get the alprazolam in solution at all.
I know you dont want to hear "don't do it" type responses, but this is a harm reduction focused forum and it's the answer you're likely to get. Please reconsider, this is a fool's errand. Just eat them or dissolve them sublingually if you want more rapid effects.