Soup – Why does brining work with dry heat methods and not wet heat methods

I suspect that the biggest problem here is that your brine isn't anywhere close to being strong enough. Cooks Illustrated has a good guide to the entire process but in a nutshell:

• Sea salt is expensive and inefficient for brining; the impurities actually make it more difficult to dissolve and disperse properly. Kosher salt is generally recommended, although table salt is also fine.

• A typical brine is 1/4 cup table salt and 1/2 cup sugar per quart, which translates to about 70 g and 140 g respectively per L. For very high-heat methods (grilling/broiling), you halve the amounts. Also, for kosher salt you need to double the volume (no change if measuring by weight). Even the lower, high-heat cooking concentration is almost twice as concentrated as what you're doing.

• You also need to scale the amount of brine with the weight of the bird itself. The rule of thumb is 1 quart or L per pound (2.2 kg) of meat. For a whole chicken, which is generally around 6 or 7 pounds, 1.5 L of brine is nowhere near enough, especially if you're brining in a pot as opposed to a bag (does your 1.5 L even cover the chicken?).

• It doesn't really matter if you butcher the chicken first (although most people don't). You're exposing slightly more surface area that way but not really enough to matter.

• Make sure you are actually dissolving all the crystals! From what you're describing, you're getting high concentrations of salt in some areas and none in others. That means you didn't get proper dispersion. You really need to make sure that all of the salt (and sugar, if you're using any) is completely dissolved, otherwise you don't have a "brine", you have water with a bunch of little piles of salt. Some people will suggest heating or even boiling your brine to ensure proper dissolution; just make sure you let it cool off afterward if you do this, before submerging the bird.

In answer to your specific questions:

1. The container should be well-sealed to prevent evaporation, not to mention off-odours in your fridge. However, I've used pots with loose-fitting lids and had no problems. It doesn't make a huge difference as far as the efficacy of the brine.

2. Fridge temperature is ideal. Do not even think about using room-temperature water, that is highly unsafe for storing raw meat for 6-8 hours at a time.

3. As long as you don't overcrowd the vessel and do disperse the crystals properly, the actual amount of space is not a major issue. If it's exposed, it's exposed.

4. Longer than 12 hours is not recommended. Actually, according to CI, longer than 8 hours is not recommended. Don't overdo it - you're brining, not marinating.

5. No matter how you cook any piece of meat, it will give up a certain amount of water and therefore a certain amount of salt (from the brine). Left unstated is why you would even consider boiling a brined chicken; brining is primarily a technique for dry-heat cooking (roasting/grilling), and if you want to boil/poach/braise/whatever then you should be focusing more on flavouring the cooking liquid than the meat itself. I wouldn't bother brining if you're making chicken soup, there are better ways to flavour that.

I'm not exactly sure why you're trying to brine then boil; you should be able to simply boil your chicken in salty stock and get plenty of salt into it. It won't take nearly as long as brining, because things happen faster in boiling water.

Assuming you boil for any significant length of time, much of the salt will indeed come out into the cooking liquid, and you'll end up with your soup approximately as salty as the chicken. (If the chicken is in large pieces, this of course only applies to the part the brining and cooking liquids can actually reach.)