Well, HCl salt is an option, but surtenly not when it comes to amphetamine (european bees know what I'm talking about). Why? Becouse amphetamine.HCl is highly hydroscopic and the street name for the amph.HCl overhere is "goo-speed"... Anyways, phosphate salt is to prefer, this is a piece of information taken from rhodium.ws that can bee useful:
"....135 g (1 mol) of amphetamine freebase were stirred into 300ml of acetone in a 1000ml erlenmeyer flask. To the resultant solution there were slowly added under constant agitation 115.3 g of 85% phosphoric acid (containing 1 mol of H3PO4), care being taken to avoid any sudden rise in temperature or local overheating due to the considerable amount of heat that is evolved. During the addition of the phosphoric acid a fine, white, flocculent precipitate appears which becomes more and more dense and abundant, as the quantity of added acid increases.
When the entire quantity of the phosphoric acid has thus been added, agitation of the mixture is continued for about a half-hour or more to insure complete conversion. The precipitate is then allowed to settle, the supernatant liquid is drawn off, and the residue is filtered. The precipitate thus separated is washed with acetone and is then dried by evaporation to constant weight. It forms a fine, white, impalpable powder consisting of pure monobasic amphetamine phosphate.
Reference: Pharmaceutical Manufaturing encyclopedia (1988)..."
[pH]armacist - Never underestimate the power of retrosynthesis. Svenskar suger på fotboll!