In Tibetan buddhism, there are some mantras are specially used to clease away the impurity of our newly bought chanting rosary.
If you are a chinese mahayana practitioner, then there are no specific mantras to purify your mala. However, u can use some fresh flower water or obtain some water offered at the buddha's shrine to wash your new mala. If your mala is sadal wood, then u need not perform this. Merely reciting the any mantra or Buddha's name is already a good cleansing of your mala. Outside ritual is not important as the crucial practise is your mind.
