Here it is:
#showtooltip
/cast [mod:alt,@player][nomod,help] Purify;[nomod,harm] Dispel magic; [@player] Purify
And here is a bonus:
#showtooltip
/cast [mod:alt,@player][nomod,help] Purify;[nomod,harm] Dispel magic; [mod:shift] Mass dispel; [@player] Purify
This last macro would do everything your old macro did but also casts "mass dispel" if you are holding the "shift" key.
I myself prefer this other macro, that adds a mouseover condition for dispelling/purifying:
#showtooltip
/cast [@mouseover,nomod,help][nomod,help][mod:alt,@player] Purify;[@mouseover,nomod,harm][nomod,harm] Dispel