Are you using sterile equipment? Are you using bacostatis solution?
Yes, you can do this, but without sterile equipment and bacostatic water it isn’t a great idea. Especially if you’ll be using it 12+hrs after cooking it up.
And if you’re using sterile equipment and bacostatic solution you might as well also use a proper micron filter.
Granted I’ve know plenty of people who have cooked up speed/goofballs hours in advance and not had much issue, but ymmv.
The bigger concern is that when heroin is stored in H2O it will begin to degrade into morphine.
I left a bit of tar in some water overnight to let it dissolve and the next day it was essentially useful - it was enough heroin to get me high, but once it degraded into morphine it wasn’t nearly enough (esp because I had been planning to insulfate the solution, and morphine it pretty useless up the snozz).
So if you want to avoid bacteria and want heroin and not morphine don’t leave it in water too long.
I imagine there is some solution you could dissolve the heroin in that would both be bacostatis and prevent it degrading into morphine, but I’m not sure what it would be off the top of my head. If anyone knows how to keep heroin in solution stable over long periods of time please don’t hesitate to share.