Arthur,
Check this chart on page 788. The Be' columns are degrees baume'
http://books.google.com/books?id=H0TOAAAAMAAJ&pg=PA786&lpg=PA786&dq=36+baume%27+percentage+nitric&source=bl&ots=QFJE8eQwq2&sig=aQJLHy_qe1QoAEpfpkLUWYcNwVA&hl=en&ei=186eTr2YM-PisQKzvY3lCQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBkQ6AEwADgK#v=onepage&q=36%20baume%27%20percentage%20nitric&f=false
According to that chart, 36 degree baume' HNO3 is about 52.3%, by weight.
Then, go to this chart.
http://www.handymath.com/cgi-bin/nitrictble2.cgi?submit=Entry
At 20C, the specific gravity of 52% nitric is 1.3219. Therefore, 1 liter weighs 1321.9g. Multiply 1321.9 x .52 and this gives 687g of nitric per liter of 52% nitric. Do the same for the 70% and you come up with 989g/liter.
Therefore, 36 baume' nitric is about 687/989 = about 69.5% as strong as 42 baume nitric. The ratio of strength is 989/687 = about 1.44:1
Notes:
I rounded off the 52.3 to 52 when I calculated. You can get more exact numbers by plugging in the 20C and 52.3% in the calculator at the top of the chart on the 2nd link. However, since the acids you buy are probably not that exact, I would just assume that the 36 baume' nitric is about 70% as strong as the 42 baume nitric - easy number to remember. Also, I didn't use the specific gravity given in the chart on the 1st link because of adjustments needed for temperature. I usually use 20C (68F) in these calculations, since that's close to room temperature.
Another important point: To make 50/50 nitric from 70% nitric, you add 1 liter of acid to 1 liter of water. To make this same exact strength from the 36 baume' (52%) nitric, you add 554ml of water to 1 liter of this acid. The calculation of this requires a little algebra, so I won't go into the math unless you want me to.
You can also use this 36 Be' nitric to make up aqua regia. You'll have a little extra water in there, which might slow it down a bit but, all in all, it should work fine.
Actually, according to the 1st chart, 42 baume' is 67% and not 70% (70% is 43 degrees baume'). If 67%, this will throw off my numbers a bit, but not enough to affect any practical refining work. This isn't rocket science.
The use of the Baume' scale of density or specific gravity is fairly antiquated and I'm surprised it's still used. When I first started out in plating, they used it for some things and we had to keep a set of special Baume hydrometers on hand in order to measure it. Either that, or measure the specific gravity and use a conversion chart. I always though it was stupid to use it instead of the simpler, and much more intuitive, specific gravity.
I hope I did all this math right. I think I did but, if someone catches a glitz, please let me know. The math on this is fairly simple, but it is confusing and not intuitive at all. You have to use the charts. There may be simpler methods but I don't know what they are.
Chris
