No Mark, not at all.
The situation for diluting nitric acid depends entirely on what you are doing with it. By diluting the nitric, you get more bang for your buck. If I am dissolving silver, the process goes more quickly and with less waste if I dilute the nitric to 30-35% because more of the nitrogen oxides are redissolved into water. Furthermore, there is less of a concentration gradient involved with the whole dissolution process, meaning that there is less stirring needed. By this I mean that there is less Ag+ per unit volume in the perimeter of a dissolving object in a 35% solution than say a 70% concentration solution. This means less stirring and heat is needed. This is also true of aqua regia in some circumstances: the aqua regia will work longer and with less waste of reagent if used at more dilute concentration.
A good example of this is the dissolution of mercury: mercury will not readily dissolve in highly concentrated nitric acid, but goes readily into nitric acid of lower concentrations. There are several factors at play, namely the passivation effect, and the concentration gradient of Hg+2 being too high, basically shielding the mercury from further dissolution.
Aqua regia need not be made from concentrated acids, in fact, you can use dilute HCl and dilute nitric and, in the proper proportions, make an aqua regia solution. Generally, it is far easier to make the aqua regia from 1 part 16M nitric acid to 4 parts 12M HCl and then dilute with distilled water to your working concentration. There are many situations where using concentrated aqua regia might not be the best option, or when using it at a certain temperature range is preferable to others.
Yes, I realize that my explanation may have done more harm than good...
Really, there are many factors involved in using acids. There are occasions when you should use them out of the bottle, and then there are times when that is too much or just plain wasteful.
Lou
