Platinum jewelry hides oily dirt and tarnish well. When it comes time to clean platinum, soak for a minute or so in plain rubbing alcohol and rinse. Dry and polish the jewelry using a clean, soft cloth. Don’t use brushes of any kind, which can scratch platinum. Avoid using bleach, which can weaken precious-stone mountings. As expensive as this precious metal may be, platinum scratches easily, so don't use any abrasive cleaning tools or gritty cleansers.
Reference:
Consumer Reports editors in How to Clean and Care for Practically Anything.