Franciz,
I’m sorry it took me so long to reply.
No matter whether you use Aqua Regia or HCl/Cl2, platinum will be slow to digest in massive form. As stated I like to avoid nitric because it can cause additional work further down the recovery/refining process with many types of scrap. For 10 grams if you started with 300 mL HCl and added sodium hypochlorite to generate your Cl2 gas in solution you would be fine. I do use heat with HCl/Cl2 to speed the reaction. The heat drives the gas out of solution though, so I reflux it back to conserve it.
But some of the questions you need to ask are what are you doing with this once it is in solution. If you wish to precipitate a platinum salt, you will have difficulty in concentrating your solution sufficiently because of excess sodium from your sodium hypochlorite additions. Ways around this are to use nitric to start, consume all nitric, then finish with Cl2 or H2O2. You can also generate your Cl2 gas externally to avoid adding salts. Often HCl/Cl2 is used for recovery then values cemented with zinc.
Knowing what form your platinum is in and what you wish to do once digested will help in what may be your best approach. As a reference point, a piece of pure platinum in hot AR the thickness of a US dime can take 4-5 hours to digest.
