I did something similar but then I decided it was a bad idea:
I bought some nice glass bottle with sealing glass stops, like I had used them in the chemistry labs. So I premixed the MMS, so I don't have to wait each morning for 3 minutes until I could use it. I had it diluted and used a dropper to extract the appropriate amount and diluted this further.
So far so good.
Now the level in the bottle started to lower, as I was using the activated semi-concentrated MMS solution and there was a gap or gas volume forming that would grow day by day. Each time the heating would go on in the morning, the gas would expand and I could smell the chlorine dioxide in my bathroom - so that wasn't so great. I used some vasiline and then the glass stop would seal better.
But then I wondered:
The volume of chlorine dioxide that is trapped in the bottle is explosive! A flash of light can cause an explosion and then the content of the bottle will disperse on everything in the bathroom, including myself.
Great, I thought, that was a stupid idea!
Probably the gas is diluted enough to not cause an explosion but I am not taking any chance. So I filled the bottle up to the neck and I am going to slowly get rid of it.
It may also not be good to make any amount that you don't want to use right away. After reading more about chlorine dioxide - and discussions with silverfox helped too - I learned that chlorine dioxide will decompose over time, so you always want to make a fresh dose, just as much as you need at a time.
I have the 50% citric acid and even with a couple of drops, the MMS will almost instantly turn brownish, so waiting 20 seconds is probably plenty of time. If I let it stand for 3 minutes, it will smell really strong from the chlorine dioxide. By making this time shorter I think you will loose less into the air. For the 10% citric acid you may have to wait 3 minutes, I don't know.
Oops, just read another post by silverfox. In his message he says that the industry standard is to wait 10 minutes - but I wonder what the concentration of their citric acid solution is.