So, getting a bit OT and "after the fact" from the original post, but this still seemed the most appropriate place for the following.
A chemist, I am not. I recall sitting in high school chemistry daydreaming while the chemical equations floated across the blackboard.... covalent bonds, ionic bonds, blah, blah, blah. But one thing that stuck (or maybe it was from a Simpson's episode) was to NEVER mix an acid and a base.
Perhaps there are exceptions, or ways to do it well, but let me describe (with a rather bashful look on my face) what I've done and hopefully someone can tell me exactly what is _is_ that I've done, and if I was lucky - and maybe just how lucky. Maybe it just looked worse than it was.
So, "citric acid solution disposal".... that's where we find ourselves. I'm all done with the de-rusting described elsewhere on this forum, and in passing by the neglected barrel of acid tonight I noticed mold forming around the top and on the surface of the liquid. So, this is nothing strange in my basement. But I thought to myself: "if there's mold growing, it can't be THAT bad to dispose of it without additional precautions". I live rurally, so no city sewers. I did not want, nor was it convenient, to dispose into the septic tank. My basement has a french drain (basically into the ground with some gravel around it) in the corner, and this seemed quite appropriate to safely dispose a weak citric acid solution.
Do correct me if I'm wrong because I have more to go. I transferred about 1/4 of the barrel into the laundry tub where it slowly empties down the french drain. I added a stream of fresh water to further dilute things as it was draining.
So, here's where things get interesting....
I thought I'd make an effort to bring the pH back to neutral (7) by the addition of a very small amount of lye (sodium hydroxide). Into the full washtub (pH of about 4-5, to start) I sprinkled perhaps 2 tablespoons or so of lye across the surface. At first, nothing much happened. Then I noticed the bottom starting to change colour a bit. Then I began to see "dark clouds" forming at the bottom of the basin....which quickly grew into an evil looking thunderstorm of blackish/blue churning through the water! All the while, the tap continued to run diluting things. This is well water, so no chlorine but other minerals for sure including high iron and likely some sulfur compounds by the smell some days.
Anyway, the entire sink turned this dark blue/black colour but no noticeable odour nor extreme temperature change were noted. I checked and I overshot on the pH with just that little bit of lye, with a reading of about 9-10. I figured things were all good though, and proceeded to write this post before disposing of the remainder.....
I stand ready for education. I promise not to doze off, as long as you skip the heavy math equations.