Nitric acid does decompose in light to NO2,one of the reasons it is stored in amber or dark colored bottles, or dark places...
Light would not be an effective way to denox a gold chloride solution,(especially on with any excess of HNO3, or any base metals. just my humble opinion, unless your willing to sit there for a very long time
Just look how long it can take to rid excess HNO3 and NOx, gases in a fairly pure gold chloride solution, then if the gold chloride solution has much other metals involved like copper which will form nitrates, say for example copper nitrate in solution, we have to drive it under heat to evaporate it to a thick solution, then upon adding an acid, normally HCl, the metal salt Copper nitrate will form HNO3 again and NOx gases, and then we may have to repeat this several times (normally 2-3 times) depending on excess nitric used and base metals involved, and if we did not evaporate down far enough, even after that we may have to add sulfamic acid...
And all the while we can be doing this with the bright sunlight glowing into the vessel.
Note the sunlight or other light can decompose nitric acid to Nox gases...
Using only the amount of nitric acid you need (in small additions of nitric acid) (with the excess of calculated amount of HCl, for the weight of gold in solution). Not dissolving all of the fairly pure gold, making nitric acid, the limited reactant in our chemical reaction.
Then after the initial reaction is finished, using heat to dissolve more of that gold, as you are concentrating the solution, only adding nitric drop-wise to the fairly concentrated chloride solution, even then leaving only a small portion of our undissolved gold, under heat with the concentrated solution, (where you have no need for the pure gold button to be added, Harold's trick).
And then the small additions of HCl to convert any metal nitrates to HNO3, decomposing these with more heat as the fairly concentrated, solution is re-concentrated under heat to decompose the newly formed HNO3 to NOx gases. Repeating the process if noticeable NOx fumes (red gases of NO and NO2 in damp air) are form after the addition of HCl, to the concentrated solution.(repeated for good luck) To de-NOx the limited use of HNO3 we have used. Cooling before dilution, and adding a little sulfamic acid (and, or H2SO4) to insure nitric and NOx removal, with the benefit of precipitating the lead sulfate salts, along with any silver chloride salts. and letting this set lightly covered, to sit overnight and settle well, (or in the sunlight if you wish), before carefully decanting the cooled or chilled auric chloride solution from the precipitated salts, and filtering our gold chloride solution into our clean precipitation vessel, in preparation for the gold precipitation with our reactant of choice, I believe would be a better way to go than counting on the sun, of course this is just my opinion.
Beside what else would I have to do during the solar eclipse? Sit in the dark with some welding helmet on trying to see if the sun was out or being blocked out by all of the forest fire smoke around me?
