Bleach + Drain Cleaner = Poison Gas

Dave,
Here are some observations that I've made from reading your post and some replies to your post: 1) You aren't particularly knowledgeable about chemistry. Your brief post indicates knowledge well below the norm. This isn't a problem if you are very careful about gathering extremely good advise before you start your project. 2) You are getting advice from folks, some of whom are great contributors to this group, who are also not chemistry experts. 3) You may injury or kill yourself or your family if you fail to invest the effort necessary to get good advice.
Here is what I would do in your situation, based upon the limited explanation that you have given for your problem: 1) I would set up a 2-window ventilation system for my basement during the project.
Ideally, I'd set up a "push-pull" system. I would have a fan installed at one window pushing air out of the basement and I'd have a second fan, in the furthest window possible, set up to pull in outside air.
I use rather huge 3 speed fans and I actually place them outside leaning on the window wells in such a way that the base of each fan is on the metal perimeter of the well and the top of the fan is leaning against the house. If the windows are located on the same wall, then good cross-ventilation may not be achieved. In that case, additional fans on the floor of the basement may be needed to create a circular flow of air - in from the "push" fan, through the floor fans and then out via the "pull" fan. Cheap window "box fans" may be ok for the fans on the basement floor, but I use much more powerful fans for the "push-pull" window installation.
2) I would also turn off the furnace and AC whenever working on this project so that basement fumes aren't spread throughout the home by the whole-house blower fan.
3) I would treat the basement walls with muriatic acid to provide efflorescence resolution and concrete etching. I would treat this acid with consider caution. It is about 39% hydrochloric acid and it produces nasty fumes. I would research this step carefully and thoroughly.
4) I would treat the walls for mold using boric acid or copper sulfate. Both of these products are acidic and both are very good at treating mold and fungus. Neither product should have any adverse interaction with muriatic acid. I would do this step after the muriatic acid treatment since the anti-mold treatment is the more desirable product to leave on the walls. I would research this step carefully and thoroughly.
5) I would definitely avoid using bleach when I intend to also use muriatic acid.
6) I would treat all newsgroup advise as doubtful (including this post!) and I would double and triple-check all advise.
7) I would remember that I am working with chemicals which are very dangerous. They are easy to obtain, but still very dangerous.
Please realize that even the most knowledgeable posters on this newsgroup may give you poor advice when they are making guesses about solving your problem.
You have been advised that bleach can be applied and rinsed prior to treating with the hydrochloric acid. I would never trust this UNLESS the basement where completely empty and I could very, very thoroughly rinse the walls. Even then, there is the danger of tracking bleach residue throughout the house. And remember that any bleach allowed to dry on the walls is still bleach. What dried is just the water in which the bleach was dissolved.
As I've said above, the bleach is very dangerous around any acid, but the other two chemicals, boric acid & copper sulfate, are acidic and safe around acids.
You've been told that all chlorine sources (eg: bleach and hydrochloric acid) are equivalent. This is not correct. Muriatic acid will attack mold and fungus, but not nearly as well as bleach, boric acid or copper sulfate.
Hydrochloric acid is very powerful and very corrosive, but it is a "nonoxidizing acid" and the chlorine in the HCl does not oxide and kill mold/fungus the same as the chlorine (AND the oxygen) available in sodium hypochlorite (bleach). All chlorine is not the same, which is why we don't bleach our cloths or kill mold on walls with sodium chloride (table salt), which is another non-oxidizing form of chlorine.
(Salt will kill mold and bacteria very slowly in total immersions situation such as preserving pickles. But this is a very different chemistry based upon the slow death of mold and bacteria cells through the depletion of cell water via osmosis. This is why we can cure foods over time using salt, sugar and other safe chemicals which will cause osmosis.)
Once again, please do not rely upon any free newsgroup advice (including this post) without double and triple checking every detail of the post. If not, then you could be pissing away your time, money, effort or health. I believe that the boric acid or copper sulfate in conjunction with the muriatic acid is a probable safe and effective method for you, but this is JUST MY opinion and you should treat the advise as just some doubtful free newsgroup advise until you have verified it.
Once again, be extremely careful about using bleach in conjunction with any other chemical. Chlorine gas is often produced, as well as possible nitrogen trichloride, hydrazine, phosgene and other toxic gases. All are highly poisonous and some are extremely explosive. Bleach with any acid is a deadly combination, as well as bleach with ammonia products.
Good luck, Gideon
=================== Dave wrote I've always heard that this combination produces something like cyanide gas. Now what if I want to clean a basement wall for painting? Bleach for the mildew and then the acid etch-wash for the efflorescense deposits. Do I end up with a risk of poison gas?
Thanks