rusty said:
Now boil the solution down gently, to a syrup, being careful not to spatter. The purpose of the sulphuric acid will be explained later.
This evaporation is a tiresome process. It goes rapidly at first, but soon the liquid begins to spatter and you must lower the flame.
She also suggests that one generously throw gasoline on waste material for incineration, but you and I both know that's a recipe for disaster.
She screwed up. The false notion that you can, somehow, "boil gently" is misleading. If you achieve a boil, it's guaranteed, you will lose values. I had to learn that lesson the hard way, and it could have been avoided by the simple mention that once the boiling temperature is achieved, the bubbles burst on the surface with enough energy to carry off minute droplets of gold chloride.
My baptism by fire came quickly. I watched the solution carefully, and restricted heat once it started boiling. It was not a heavy boil. How much I lost will never be known, but the afternoon I spent hand rubbing the paint on my travel trailer, those many years ago, were very instrumental in making me understand the significance of keeping the solution under a boil. My travel trailer was parked a few feet away from the window in which I had installed a fan. I had no fume hood at this point in time. You likely understand that I had one shortly thereafter.
You can make determinations of occurring loss by placing a cold object in the vapors coming off when you're evaporating. If the condensate has any color, you're losing values. You likely understand the object should NOT be metallic. A porcelain spatula, or even a heavy glass stirring rod will work.
While we're on this subject, there's one other thing that needs to be addressed.
The idea that you can evaporate to a syrup three times and be assured that you have expelled all of the nitric
is not true. You can evaporate to a syrup a half dozen times and not succeed. Key to understanding is what constitutes a syrup. It's difficult to define, because contaminants in the solution alter how it behaves.
If you introduce HCl prematurely, nothing is accomplished. In such a case, you would most likely repeat the same mistake time and again, not understanding that the solution requires further evaporation before introducing the HCl. When it is introduced at the proper time, assuming the solution contains free nitric, the familiar brown fumes are expelled.
The ultimate goal is to use no more nitric than is necessary. We all know that. Problem is, there are instances where a small excess is very desirable. It is an excellent indicator that all values have been dissolved when processing filing from the jeweler's bench, for example. In such a case, I discovered, long ago, that the addition of a button of pure to the solution, when evaporation commenced, guaranteed the consumption of the free nitric. It beats, hands down, attempting to evaporate, in particular when you introduce the HCl prematurely, and achieve no results.
Harold
