Our lab uses a sodium hydroxide solution of a (theoretically) known normality (0.8000 - 1.0000) to titrate product samples for quality control. However, we suspect that the normality of the $\ce{NaOH}$ left in the burette/tubing connected to our container is changing if we let it sit (our tests come out wrong, and the answers seem to get better if we run a couple of burettes worth of $\ce{NaOH}$ out first). I expect it's absorbing moisture from the air, but I don't know for sure.

We've tried different container types and we've put a desiccant in line with the tubing to absorb stray moisture (which it seems to be doing, there's been an observable color change), but the situation remains that we need to just throw out up to 200mL of $\ce{NaOH}$ any time we need to run a test. Does anyone on here have any recommendations for countering this normality drift?

  • $\begingroup$ As @iad22agp said, it's most definitely absorption of CO2; NaOH is known to do this. Maybe instead of throwing it out, you could titrate the NaOH before using it? $\endgroup$
    – orthocresol
    Jul 30 '15 at 19:21

I would be more worried about carbon dioxide (rather than water) being absorbed into the NaOH solution. A 1 molar NaOH solution is not going to be appreciably hygroscopic. But it will absorb CO2: two moles of hydroxide yield one mole of carbonate ion, which, depending on what you are titrating, may act as only deprotonate one mole of acid per mole base - reducing the effective normality of the NaOH solution. You could try protecting any air vents with a drying tube filled with sodium or calcium hydroxide, or use a regulated supply of chemically pure nitrogen as a blanket.

  • $\begingroup$ Thanks! We're going to try switching out the desiccant in our drying tube for sodium hydroxide pellets next week, and I'll let you know how it goes. We're also going to place a small watch glass or something over the open top of the burette, and hopefully stop some absorption from there too. Do you have any advice regarding using sodium hydroxide in the drying tube, or can you think of anything we should be aware of? $\endgroup$ Jul 31 '15 at 17:42
  • $\begingroup$ The NaOH seems to be okay in the burette now, and the stuff in the container is fine. We're still having trouble with the caustic that's left in the tubing though (so the first run is fine, but doing the test again with the caustic that was in the tubing gives bad results). We're going to try changing the tubing material. I'm accepting your answer because it was spot on, and the tubing wasn't something I asked about in my original question. $\endgroup$ Aug 11 '15 at 18:12

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